PORTLAND CEMENT (TOTAL DUST) IN WORKPLACE
||Air filter and bulk material|
|OSHA Permissible Exposure Limits
Portland Cement (PC)
Final Rule Limits:
Expressed as Time-Weighted Averages (TWA)
mg/m3 Total Dust
mg/m3 Respirable Dust (See Section
||Total dust air samples are collected on tared 5-µm
pore size, 37-mm diameter polyvinyl chloride (PVC)
filters. The air samples are post-weighed and submitted
to the laboratory when total dust exposures exceed 10
mg/m3. A bulk sample consisting of 10 to
20 mL of Portland cement is collected to serve as reference material
for laboratory quantitation.|
|Recommended Air Volume:
|Recommended Sampling Rate:
|1 L/min for total dust air samples|
||Samples are quantitatively analyzed and qualitatively identified
by X-ray techniques. Standards are prepared from the
bulk material. Suspensions of the samples are deposited onto Ag
membrane filters and quantitatively analyzed by measuring
X-ray fluorescence (XRF) intensities for Ca, Si, Fe,
and Sr in PC and by measuring the attenuation of X rays from the
fluorescing Ag membrane filter. For qualitative confirmation, the
common PC crystalline solid phases in the bulk and air sample dusts
are compared using X-ray diffraction.|
||11 to 640 µg PC
[Detection limits depend upon interferences
present (See Sections 1.3.1.
|Precision and Accuracy
|0.6 to 7.7 mg total dust|
(See comments in Section
||Validated Analytical Method|
||Mike C. Rose|
Commercial manufacturers and products mentioned in this method
are for descriptive use only and do not constitute endorsements by
Similar products from other sources can be
Branch of Inorganic Methods Development
OSHA Salt Lake
Salt Lake City, Utah
This method describes the sampling and analysis of industrial hygiene
total dust air samples for unfinished Portland cement (PC).
General industry often expresses the composition of materials derived
from minerals as simple oxides. This convention is useful in accounting
for the elemental composition of PC, but it does not describe the actual
compounds present. Using this convention, the four major PC components are
Fe2O3. In terms of
these hypothetical components, the approximate composition of PC is as
||59 to 75%|
||17 to 28%|
|| 4 to 20%|
(See Reference 5.1.
for a ternary phase diagram that more clearly portrays the stoichiometric
oxide composition of PCs.)
Typical PC compositions also include the minor elements Cr, Zn, F, Mn,
P, Sr, Na, Ti, K, Mg, and S with concentrations ranging from 0.01 to
several percent, respectively (5.2.).
In chemical terms, the two essential major components in PC are
Additional compounds commonly present in PC include alumina
iron oxide (Fe2O3) and
calcium aluminum ferrite
The manufacture of PC involves heating together specifically selected
ground minerals to give the desired composition of PC. The heating process
produces chunks of PC called clinker. To produce commercial grade PC, the
PC clinker is usually ground with a small amount of the
calcium-containing mineral gypsum
(CaSO42H2O) to control
the rate of setting (5.1.-5.3.).
The PC composition and particle-size distribution determine
how the material sets up after water is added.
In crystallographic terms, Portland cement is a complex mixture; the
amount of each solid phase present depends on the starting composition,
thoroughness of mixing, firing conditions, and the thermal history of the
material. At least 22 different solid phases have been identified.
Unstable solid phases can persist at room temperature; for example, PC has
six distinct alite polymorphs (i.e., different
solid phases) - each stable at a different temperature range; the presence
of impurities in PC allow three alite polymorphs to coexist at room
Portland cement has negligible vapor pressure, negligible aqueous
solubility and a variable composition. Typical bags of commercial
"Portland cement" also contain additives. Because Portland cement is a
mixture and not a single compound, a representative pure bulk sample of
the PC material must be collected at the time of air sampling for
laboratory use to resolve this variability in composition. The bulk sample
defines the particular PC composition to which the employee is exposed.
Air samples are collected to determine personal exposures. Due to the
variable elemental composition of PC, this method analyzes several
elements by X-ray fluorescence (XRF) to quantitatively
evaluate PC exposures. Due to the variable crystalline composition this
method qualitatively assesses whether crystalline PC phases are present by
X-ray diffraction (XRD).
The following flow chart indicates the recommended treatment of these
Portland cement air samples
This method concentrates on total dust sampling; respirable samples
can also be taken to assess compliance to the OSHA 5
mg/m3 respirable dust PEL. Respirable PC
exposures are determined gravimetrically in the field by the compliance
officer or industrial hygienist. The respirable dust PEL for PC is
identical for most respirable dusts, and any exceptions (e.g., quartz and
vanadium pentoxide) are lower than 5 mg/m3. At
the present time, there is no need to verify that respirable dust air
samples consist only of PC. However, these respirable PC dust samples can
be submitted to the laboratory for quartz analysis because quartz
is a potential contaminant in PC. Refer to the quartz method (5.7.)
for the appropriate respirable quartz dust sampling procedure.
If the laboratory analyzing the samples has a controlled environment
where the relative humidity (RH) is less than 80%, the XRD qualitative
verification step may be performed prior to XRF quantitation. See Section
3. for procedural variations.
As previously mentioned, commercial PCs are complex mixtures. Each PC
component has different physical properties. Portland cement is ground in
the final stage of manufacture; therefore, one can expect different
particle-size distributions for each component. When
suspended in the air, the resulting differential settling of components
may significantly contribute to variation in the analytical results. To
compensate for the expected variation in sampling, several different
analyses are performed. Samples and standards are quantitatively analyzed
for PC based on Ca, Si, Fe, and Sr by XRF. The Ca and Si are major
constituents occurring in the two essential calcium silicates present in
PC. The trace elements Fe and Sr are in the raw materials used to produce
the PC clinker; therefore, assessments of these elements also provide
useful estimates for PC content. In addition, the X-ray
fluorescence from the Ag membrane filter is used for a mass absorption
(MA) analysis of the samples. By measuring the attenuation of X rays from
the fluorescing Ag membrane filter, the overall mass of the sample can be
determined. Analysis by MA is sensitive to the mean composition of the
sample. Due to the potential for outliers among the five quantitative
analyses, the median result of the five is the most representative measure
of PC content.
Because of the potential for interferences, qualitative confirmation is
a necessary step. After the XRF analyses, the prepared total dust air
samples and bulk reference samples are resubmitted for qualitative
verification using XRD to confirm the presence of PC crystalline solid
phases. The total dust and bulk samples are also screened for quartz using
OSHA Method ID-142
Quartz is usually a minor but common component In PC. The results for
quartz analyses of total dust and bulk samples are approximate. If the PC
material contains more than 1% quartz, the PEL for quartz should be
considered and respirable dust samples should be obtained (5.5.).
The elements Ca, Si, Fe, and Sr are widely distributed in the
environment; therefore, the identification of these elements does not
provide conclusive confirmation of the presence of PC. Also, mass
absorption analysis by XRF provides no qualitative information. For these
reasons, analytical results are reported only when there is both
general quantitative agreement and qualitative XRD verification of
the PC crystalline solid phases.
1.1.1. The previous PEL for PC was 50 million particles per
cubic foot (mppcf) which required an impinger sampling method for
collection. Analysis was performed by particle counting using light
microscopy. The 50 mppcf PEL was equivalent to approximately 15
mg/m3. This level is the same as for
particulates not otherwise regulated; therefore, the exposure could
be evaluated gravimetrically. The PEL was changed in 1989 to 10
mg/m3 due to the irritant status of PC and
so that gravimetric methods could be used by industrial hygienists
to monitor employee exposures (5.5.).
The 10 mg/m3 level, however, is less than
the nuisance dust level of 15 mg/m3 and
entails identification to verify PC content. The PC phases can not
be identified either gravimetrically or by light microscopy.
1.1.2. Classical wet-chemical techniques have been
applied to the elemental analysis of PC (5.2.,
Due to limited sensitivity, these techniques are not optimal for the
quantitation of low-mass air samples obtained in
industrial hygiene monitoring. These wet-chemical
techniques are destructive to the sample and do not offer phase
identification. Although it is possible to prepare samples first for
phase identification and then destructively analyze the sample by a
wet method, this would involve tedious sample preparation.
1.1.3. Atomic absorption spectroscopy (AAS) offered a potential
alternative method that might be adapted for low-mass
air samples (5.9.).
However, the identification of PC phases introduces the same
complications noted in Section 1.1.2.
1.1.4. Another quantitative method investigated was a
collaborative study with West Germany's Saarbergwerke
Aktiengesellschaft (SA) (5.10.)
using their XRD technique. Well-characterized reference
materials were provided by SA. The SA XRD method required reference
materials that have particle-size distributions
comparable to the air samples. Potential interferences had been
thoroughly identified. It was determined that the SA XRD method was
appropriate for respirable dust samples, but problems may arise when
attempting to quantify PC content in total dust samples. However,
the XRD method was considered a suitable technique to confirm the
presence of PC in total dust samples.
1.1.5. The XRD and XRF portions of this method were evaluated
using the equipment described in Section
1.2.1. X-ray fluorescence relies upon the
excitation of atoms in a sample by the application of X rays of
sufficient energy to cause the promotion of inner orbital electrons
and subsequent decay accompanied by characteristic
In an energy-dispersive X-ray
fluorescence (EDXRF) spectrometer, the selected energy scale (or
energy span) is divided up into smaller divisions called channels in
order to provide adequate energy resolution. Each channel represents
an X-ray energy having a small range of values. For
example, in a multichannel analyzer (MCA) having 1,000 channels, the
central channel would represent X-ray energies between
5.000 and 5.010 kV when the MCA is calibrated 10 kV full scale.
During an analysis, the energy of each detected X-ray
photon is measured. Those photons having energies within the
selected energy scale are counted by incrementing the channel in the
MCA that corresponds to the measured energy. The line spectra
resulting from X-ray fluorescence are broadened into
peaks by the limited ability of the spectrometer to resolve
X-ray photon energies. The peak shape approximates a
normal (so-called Gaussian) distribution. When plotted
as a function of energy, the count data result in a spectrum after
sufficient counts are accumulated. Profile-fitting
(deconvolution) computer software can be used to determine net
counts from peak areas or to resolve instances of interferences due
to peak overlaps.
The approximate relationship between an element's atomic number
and the energy of individual emission lines for each specific
X-ray line series (e.g., the Ka line or the Lb line) is given by Moseley's law:
E = a(Z - s)2
E = energy of X ray
Z = atomic number
s = constant for each line series
Moseley's law indicates that an element's spectral lines are a
smooth function of the atomic number. The spectral lines for
elements with low atomic number (light elements) occur at lower
energies than the corresponding lines for elements with high atomic
number (heavy elements). The peak energies and spectral group
patterns provide for qualitative identification.
1.2.2. Quantitation by XRF is tailored for thin layer dust
samples redeposited on Ag membrane filters. For uniform thin
deposits, EDXRF produces signal intensities that are proportional to
the amount of analyte present. Matrix effects are normally minimal
for such samples; however, non-linear calibration
curves can be used to correct for the fluorescing Ag support medium,
shadowing of particles in Ag membrane filter pores, and
particle-size effects. Calibration curves are further
discussed in Appendix
Low-energy X rays are more strongly absorbed in thin layer
samples than are high-energy X rays. The MA analysis in
this method uses the attenuation of low-energy X rays
(L series) arising from the fluorescing Ag membrane filter to
determine sample mass. The fluorescence X rays from the Ag membrane
filter blanks are unattenuated by sample; therefore, blanks have
higher counts than the air samples in this kind of analysis and are
more prone to counting error. To improve the analytical accuracy in
the MA analysis, three blanks (two field blanks and one lab blank)
1.2.3. The X-ray diffraction technique is based on
the Bragg equation:
nl = 2d
||order of diffracted beam (usually 1)|
||wavelength of the monochromatic X-ray beam
||distance between diffracting planes (Å)|
||angle between incident X-rays and the
diffracting planes (in degrees)|
||Most X-ray diffractometers are designed to
scan using degrees two-theta (2q). All 2q angle
references in this method assume a copper anode
X-ray tube and a wavelength of ~1.54 Å.
Each crystalline substance has a unique repeating structure that
extends in various directions throughout the crystal. As a result of
this periodicity, crystalline substances have atoms that lie in
planes parallel to one another which can participate in diffracting
incident X rays. The unique repeating structure can thus be probed
with X rays to determine the set of diffraction spacings
(d-spacings) that uniquely characterize each substance.
During X-ray diffraction analyses, the angle (measured
as 2q) is stepped in small increments. At
each increment, the diffractometer pauses to count the diffracted X
rays for a specified period of time. A diffraction pattern is
produced by stepping the diffraction angle over a wide range.
Because powder samples of crystalline substances present essentially
all orientations to the X-ray beam, diffraction peaks
from each different diffracting plane are reproducibly represented
in the diffraction pattern.
1.2.4. Two examples of experimentally determined PC XRD patterns
are included in Appendix
B. Each PC has a different mixture of crystalline phases and
each Ca-containing phase is subject to differing
amounts of ion-exchange with Mg and Sr. Because Mg and
Sr have ionic radii that differ from Ca, the
ion-exchange results in variations in
d-spacings that are evidenced by peak shifts in the
pattern. For this reason, the experimentally determined patterns may
differ from those found in published pattern libraries for pure
crystalline phases. The pattern for each PC is unique and may be
used like a fingerprint to identify it.
1.3. Method Performance
Two different PC standard reference materials, SRM 635 and SRM 637,
were obtained from the National Institute of Standards and Technology
(NIST). These were used to evaluate method performance. The detection
limit (DL), recovery (R), coefficients of variation (CV and
CVT), and overall error (OE) data were
1.3.1. Method detection limits
The detection limits for Si-, Ca-, and MA-based analyses
reported below were obtained for XRF instrumentation that was
optimized for the analytes Si and Ca in a typical PC having a
composition of 45% Ca and 10% Si. Due to the variability of
trace-element composition in PC, the detection limits
for Fe- and Sr-based analyses may deviate
from the estimates shown. Detailed comparisons of the PC SRMs are
shown in Section
For PC samples collected on polyvinyl chloride (PVC) membranes
and transferred to 25-mm Ag membrane filters, the PC
detection limits are shown below. These limits are based on the
analytical peaks (AP) for the major PC elements. The oxide
compositions shown were obtained from the NIST Certificates of
||------ SRM 637 -------
||------ SRM 635 -------
|DL (µg PC)
|DL (µg PC)
The range of detection limits based on trace-element
analyses of PC is very susceptible to composition. The detection
limits for PC from these trace elements are as follows:
||------ SRM 637 -------
||------ SRM 635 -------
|DL (µg PC)
|DL (µg PC)
Detection limits for mass absorption analyses are relatively
unaffected by composition. The detection limit for simple mass
absorption (MA) analysis of PC using Ag L radiation from Ag support
membrane filters are as follows:
DL (µg PC)
DL (µg PC)
1.3.2. Recovery, coefficients of variation, and overall error:
Combined results from air sampling at low humidity (SRM 635 and
637) and at 80% RH (SRM 637):
Mass range (mg)
0.6 to 7.7
1.3.3. Instrument response to the analyte:
The practical analytical range for the XRF analysis of PC
elements on Ag membrane filters extends from the detection limit up
to about 4 mg. For thin-film analyses by XRF, the upper
end of the analytical range depends primarily upon
self-absorption of X rays within the sample. In the
case of PC analyses, where the matrix is held constant, the more
energetic fluorescence lines have larger useful ranges. However, the
same sample is analyzed using all five lines, and thereby limits the
maximum sample weight to what can be practically analyzed by the
low-energy lines. During the method evaluation,
examination of the PC calibration curves gave the following
estimates of the upper analytical limits:
Ag membrane *
* Uses fluorescence from Ag membrane (L series X rays for Ag)
A more conservative estimate for the upper mass per analytical
sample is 4 mg. Samples exceeding 4 mg should be split into aliquots
to be analyzed separately.
1.4.1. The method provides for non-destructive
1.4.2. The sample preparation is compatible for both XRF and XRD
1.4.3. The method provides qualitative evidence of PC by
comparing XRF and XRD spectral patterns of air samples to those of
1.4.4. The method can provide additional qualitative information
for a large number of elements including any unexpected elements.
1.4.5. The method requires no sampling reagents and sample
preparation is minimal.
1.4.6. Samples are screened gravimetrically in the field. Samples
showing low exposures need not be submitted for laboratory analysis.
1.5.1. Samples with high sample weights (>4 mg) must be split
into aliquots during analysis.
1.5.2. The method requires expensive instrumentation and support
1.5.3. The method requires the collection and analysis of pure PC
bulk samples for quantitative analysis and qualitative confirmation.
1.5.4. The method is not suitable for finished PC materials
because comparable reference materials are usually unavailable.
Preliminary studies indicate that dry commercial mixtures containing
PC cannot be analyzed by this method. These include concrete,
sandcrete, fast-cure leak patching mix, and mortar mix.
1.5.5. The standards and samples prepared on Ag membrane filters
should be stored prior to analysis in a desiccator containing a
drying agent in order to prevent sample hydration. (See Sections 4.7.
Information contained in this section is a synopsis of current
knowledge of the physiological effects of PC and is not intended to be
used as a basis for OSHA policy.
Risks of exposure include eye, skin, and mucous membrane
irritation, and may include more severe respiratory effects, all of
which constitute material health impairments (5.5.)
- Eye exposure (5.11.):
of PC in vater, when splashed into the eye, cause smarting and
corneal edema. Aqueous suspensions are sufficiently alkaline to
injure the corneal epithelium and conjuctiva.
- Skin exposure (5.11.):
irritant dermatitis from repeated skin contact with PC includes
symptoms of xerosis, eczematous lesions with vesicles, erythema,
fissures, and mild scaling. Portland cement occasionally contains
hexavalent chromium which may produce secondary contact sensitivity
in some individuals.
- Respiratory tract exposure (5.11.):
include cough, expectoration, exertional dyspnea, wheezing, and
chronic bronchitis. Exposure can also cause chronic conjunctivitis,
blepharitis, and ulcers of the nose.
Aside from irritation, PC is eventually eliminated from the tissue
and is generally not considered harmful when ingested (5.5.).
However, PC added to grain has been used as a rat poison; death is
probably due to physical blockage of the gastrointestinal tract.
1.7. Sources of Exposure (5.11.)
Portland cement, hydraulic cement, CAS: 65997-15-1
||Source of exposure
||Breaking-up and grinding clinker, mixing, packaging and
|Commercial dry mixtures; concrete, mortars, grouts, and
||Mixing and on-site uses for highway paving,
domestic and commercial construction: structural support in
||During manufacture of building blocks, bricks, stone,
terrazzo, stucco, foamed concrete, and pre-cast
items; as a moisture sealant on pre-cast
Portland cement should not be confused with refractory cements
composed primarily of calcium aluminates.
2.1. Safety Precautions
2.1.1. The sampling equipment should be attached to the worker
in such a manner that it will not interfere with work performance or
2.1.2. All safety practices that apply to the work area being
sampled should be followed.
2.2.1. Air sampling equipment
- Low ash PVC membrane filter, 37-mm,
5-µm pore size [part no. 625413, Mine Safety
Appliances (MSA), Pittsburgh, PA or cat. no.
P-503700, Omega Specialty Instrument Co., Chelmsford,
Note: During the preparation for analysis, the PVC
membrane filter is dissolved in tetrahydrofuran (THF). Certain
acrylic copolymers added to PVC filters are insoluble in THF. If
the membrane filter composition is unknown, a laboratory test
should be conducted with THF to determine suitability before use.
- Cellulose back-up pads (support pads), (MSA,
- Polystyrene 37-mm diameter
closed-face cassette, (3-section, SKC
part no. 225-3, SKC, Fullerton, CA).
- Gel bands (omega Specialty Instrument Co., Chelmsford, MA) for
- Sampling pump:
For personal samples, use a personal
sampling pump that can be calibrated to within ±5% of 1 L/min with
the sampling device attached.
- Assorted flexible tubing.
- Stopwatch and bubble tube or meter for pump calibration.
- Analytical balance capable of 10 µg precision.
2.2.2. Bulk sampling equipment
Scintillation vials, 20-mL, part no. 74515 or 58515, (Kimble,
Div. of Owens-Illinois Inc., Toledo, OH) with
polypropylene or Teflon cap liners. If possible, submit bulk samples
in these vials. Tin or other metal cap liners should not be used
because chemical reaction with the sample can occur.
2.3. Sampling Technique
Note: Because PC composition is highly variable, a
representative bulk sample of the pure PC material must
be submitted with total dust PC air samples in order to define the
particular PC composition to which the worker is exposed. Total dust
PC air samples can not be quantitatively analyzed without an
appropriate pure bulk sample. If a pure bulk sample can not be
obtained, the industrial hygienist should sample for nuisance dust,
and should not submit the air samples for PC analysis.
Note: Respirable dust samples are normally not sent to the
laboratory for PC analysis; measure these gravimetrically in the
field. If quartz is suspected, any air samples should be submitted for
quartz analysis. [See OSHA Method ID-142
for further information.]
2.3.1. Air sample collection
Measure PC total dust air samples gravimetrically.
- Desiccate and then weigh the PVC filter before sampling.
- Place a cellulose backup pad in a cassette. Place the
preweighed PVC filter on top of the backup pad. If large loadings
are expected and the membrane filter has a smooth and a rough
side, place the membrane filter in the cassette with the smooth
side against the backup pad and use a
3-section cassette to help produce a more adherent
deposit. Assemble the cassette.
- Attach tubing between the pump and a flow calibration cassette
so that the air will be drawn through the membrane filter. Do not
place any tubing in front of the cassette.
- Calibrate each sampling pump to within ±5% of the recommended
sampling rate of 1 L/min with the calibration cassette attached
- Attach a prepared cassette to the calibrated sampling pump and
place in the employee breathing zone.
- If possible, take two half-shift samples at the
recommended sampling rate.
- Place plastic end caps on each cassette after sampling.
- Carefully remove the filter from the cassette, desiccate, and
weigh the PVC filter sample to determine total dust exposures.
Place each filter back in its cassette. Calculate the TWA exposure
for each employee using the sample weight(s) collected and air
volume(s) used. If the TWA exposure for any employee exceeds 10
mg/m3, submit the sample(s) to the
2.3.2. Bulk sample collection
Always collect a sample of powdered PC from the workplace.
Concrete mix or other PC mixtures with substantial additions
of gravel, sand, gypsum, or lime are generally not appropriate.
Sampling these types of mixtures should be performed to determine
quartz content, pH, etc.
- Collect between 10 to 20 mL of dry homogeneous PC dust
reference material representative of the PC used in the workplace.
Samples from bagged PC are preferred. Accurate analytical results
are dependent on a close match of the PC bulk sample to the PC
dust component of the air samples.
- Transfer the bulk material into a 20-mL
scintillation vial, seal with a cap having an inert plastic liner,
and wrap the cap with vinyl or electrical tape.
2.3.3. Wipe sample collection
Wipe samples are not appropriate for this method.
2.4. Sample Shipment
2.4.1. On the OSHA 91, state the type of operation sampled. List
the bulk samples and cross-reference these to the
appropriate air sample(s).
2.4.2. Document the operation and indicate any known or suspected
substances present in the area sampled.
2.4.3. Request Portland cement analysis.
2.4.4. Submit at least two blank samples with each set of air
2.4.5. Attach an OSHA-21 seal around each bulk, air, and blank
sample in such a way as to secure the end caps.
2.4.6. Ship air and blank samples to the laboratory with
2.4.7. Ship bulk samples separately from air samples. They should
be accompanied by material safety data sheets if available. Check
current shipping restrictions and ship to the laboratory by the
This method is optimized for the analysis of PC collected on PVC
filters using an EDXRF system for quantitation. A wavelength dispersive
X-ray fluorescence (WDXRF) system may be used in place of an
The user must decide upon the applicability of available equipment
and software for this method.
As mentioned in Section
1., the XRD portion may be performed prior to the XRF portion. Because
the sequence of analysis is flexible, improved laboratory efficiency may
be obtained. Circumstances arise where subsequent analyses may be
- Subsequent XRD verification of PC is not needed if the preliminary
XRF quantitation indicates that exposures are less than the PEL.
(Screening for quartz may still be advisable.)
- Subsequent XRF quantitation is not needed if the preliminary XRD
analyses do not qualitatively identify PC crystalline phases as present
in the air samples.
3.1. Safety Precautions
- Tetrahydrofuran has a low flash point, -14 °C (6 °F), and is
extremely flammable. Always use THF in a hood. THF is an ether
which can form explosive peroxides upon exposure to air;
therefore, it should be stored in closed metal containers. Always
use THF-resistant gloves, a lab coat, and safety
glasses when handling THF.
- Parlodion and isopentyl acetate are flammable and should be
used in a hood.
- Handle reagents and bulk samples carefully. Use protective
equipment such as: gloves, laboratory coats, safety glasses, and
an exhaust hood. Wear a fit-tested respirator if
necessary. Clean up spills immediately.
- Follow established laboratory safety guidelines. Modern
X-ray fluorescence spectrometers and
X-ray diffractometers have built-in
safety devices and interior to prevent X-ray
exposure. WARNING: These devices should not be adjusted
removed, or overridden for any reason.
- X-ray operators should wear radiation monitors.
These monitors consist of badges and finger rings which are
periodically analyzed to detect cumulative exposure to
- There should be a red or yellow warning light which, when lit,
indicates that power is supplied to the X-ray tube.
The instrument may be checked for radiation leaks using a
sensitive radiation survey meter. Radiation leaks, if present,
will be most easily detected when the X-ray tube is
operated at the highest power design specification.
- Periodically, have safety mechanisms checked to determine
satisfactory operation. A sensitive, fixed position radiation
alarm may be used as an area monitor; however, damaging radiation
exposures can occur in collimated beams that do not intersect the
- Avoid inserting fingers into the sample compartment; use
forceps to change samples.
3.2.1. X-ray fluorescence spectrometer
The specific equipment used in this evaluation is described in Section
4. The spectrometer should be equipped with appropriate
monitors, collimators, and secondary targets. The spectrometer used
to evaluate this method included the following items:
target with silver filter
NaCl secondary target (see Section
3.6.3. step 3.)
3.2.2. X-ray diffractometer
The specific equipment used in the evaluation is described in Section
4. The automated powder diffractometer (APD) should be equipped
with an output device that can provide hardcopies of scans. Other
useful features include:
Spinning sample holders
Support software for:
Storing, recalling, and comparing spectrum files
Locating peaks and determining peak intensity
Powder diffraction file
3.2.3. The following equipment can be used:
- Standard and sample preparation
- Sieve, nylon, 41-µm (Spectra/Mesh N sieve, Cat.
no. 08-670-202, Fisher Scientific, Springfield, NJ)
or (Cat. no. 146502, Spectrum Medical Industries, Inc., Los
Angeles, CA). A mini-sieve may be constructed by
sandwiching the nylon mesh between the two spacer rings of a
four-piece 37-mm air sampling
cassette. Similar non-contaminating sieves may be
- Low ash PVC membrane filters, 37-mm,
5-µm pore size [part no. 625413, Mine Safety
Appliances (MSA), Pittsburgh, PA or cat. no.
P-503700, Omega Specialty Instrument Co.,
- Analytical balance capable of 10-µg precision.
- Centrifuge tubes, round bottom, 40-mL (Pyrex
- Gloves, THF-resistant [such as latex gloves
(Cat. no. 8852, American Pharmaseal Lab., Glendale, CA)].
- Silver membrane filters: 25-mm diameter, 0.45-µm pore size
(Cat. no. FM25-0.45, Osmonics, Inc., Minnetonka,
- Ultrasonic bath.
- Filtration apparatus: 25-mm (Filter Holder Hydrosol
Manifold, cat. no. XX25 047 00, filtering clamps,
cat. no. XX10 025 03, fritted glass bases with
stoppers, cat. no. XX10 025 02, and glass funnels,
cat. no. XX10 025 11, Millipore Corp., Bedford,
- Liquid nitrogen cold-trap system for THF
collection (dewar, polypropylene vacuum flask, liquid nitrogen,
- Eyedropper and glass plate.
- Hot plate, intrinsically safe (Model HP-11515B,
Sybron/Thermolyne, Dubuque, IA).
- Teflon sheet, 0.3- to 1-mm thick (cut to fit
top of hot plate).
- Plastic petri dishes (Product no. 7242, Gelman Sciences, Ann
- Vacuum desiccator with anhydrite
(CaSo4) or other drying agent.
- Vacuum system.
- Accessories for XRF analyses
- Laboratory press, 12-ton (Cat. no. A14-100,
Kevex, San Carlos, CA.).
- Pellet die set for preparing Cl secondary target,
13-mm diameter (Cat. no. A10-401,
- Powder secondary target holders, 13-mm diameter
(Cat. no. A00-205, Kevex).
- Sample holders for 25-mm diameter Ag membrane filters (Cat.
no. A00-213, Kevex). Note: These holders may
require light machining in order to center the Ag membrane
filter over the most sensitive spot.
- Pellet die set for preparing MCA energy calibration sample,
31-mm diameter (Cat. no. AI0-403,
Kevex). Note: The 13-mm diameter die set may be
- Sample holder for 31-mm diameter MCA energy
calibration sample (Cat. no. A00-214, Kevex). Note:
If a 13-mm diameter pellet is used, substitute a
13-mm diameter sample holder (Cat. no.
- Radiation safety monitor (S.E. International Instrumentation
Model Radiation Alert Monitor 4, S.E. International
Instrumentation Division, Summertown, TN).
- Accessories for XRD analyses
- Sample holders for 25-mm diameter Ag membrane filters (Model
no. PW1813/26, Philips Electronics Instruments Co., Mahwah, NJ).
- Radiation safety monitor, see item [b)7)] above.
3.3. Reagents (use at least reagent grade chemicals)
3.3.1. XRF MCA energy calibration
- Titanium dioxide
- Zinc oxide
- Yttrium oxide
- Boric acid
3.3.2. Sample preparation
- Tetrahydrofuran (dry).
- Parlodion (Pyroxylin).
- Isopentyl (Isoamyl) acetate.
- Parlodion in isopentyl acetate, 1.5% (w/v):
Dissolve 1.5 g
of parlodion in isopentyl acetate and dilute to 100 mL with
3.3.3. XRF secondary target
- Sodium chloride
- Pellet binder (Chemplex Liquid Binder, Cat. no. D12-400,
3.4. Instrument Calibration
3.4.1. XRF calibrations
Use appropriate materials and manufacturer recommendations
when calibrating specific Instrumentation and software. Examples
of the calibrations performed on specific EDXRF equipment in Section
4. are given in the Standard Operating Procedure (5.12.).
- If one is not already available, prepare an appropriate
standard(s) for MCA energy calibration of the EDXRF spectrometer.
For the instrumentation described in Section
4. prepare a sample consisting of a
pellet containing 5 to 10% of an equimol mixture of the oxides of
Ti, Zn, and Y. Place this sample in a sample holder, and then
place in the EDXRF spectrometer.
- Perform an MCA energy calibration.
- Use excitation conditions for the Ag secondary target as
described in Section
3.6.3. step 3b. Use an analysis time adequate for 2,000
count peak height on the shortest calibration peak.
- Make a two-point MCA energy calibration using
the Zn and Y Ka lines (8.631 and 14.933 kV).
- Determine the peak-width at
half-maximum for calibrating the peak
profile-fitting (deconvolution) software. This
calibration is not routinely needed; it is typically performed
when the spectrometer is installed or when indicated by
preventative maintenance checks.
3.4.2. XRD calibrations
- Hardware 2q calibration:
of instrument calibration should be performed only by trained
personnel. The calibration should be checked periodically using a
stable reference standard consisting of polycrystalline quartz
(Arkansas stone) or polycrystalline silicon.
- Software 2q calibration:
displacement errors may result in a systematic bias in 2q peak locations. The Ag membrane filter may be
used as an internal standard to correct for the sample
displacement errors of thin samples. Scan either the primary or
secondary Ag diffraction peaks at 38.15° and 44.33° 2q, respectively. Offset the 2q values for the remainder of the scan for each
sample by the same amount that the observed Ag line was offset
from its theoretical 2q value. For
example, if the secondary Ag line that should occur at 44.33°
2q is found at 44.45°, correct all
observed lines by subtracting 0.12°.
3.5. Sample Preparation
Note: The PC standards, air samples, and blanks should be
prepared using the same lot of silver membrane filters in order to
reduce variability in mass absorption analyses.
3.5.1. Air samples and blanks
- Note which air sample weights are greater than 4.0 mg.
- Examine the filter and backup pad to determine if any
breakthrough to the backup pad has occurred. If there is
significant breakthrough, the sample is either not analyzed or
results are reported with a disclaimer (see Section
3.10.3. for reporting results).
- Assemble the filtering apparatus and liquid nitrogen cold
trap. Connect the cold trap to the filtering apparatus to collect
the waste THF. Any waste vapors should not enter the vacuum pump.
- Center a Ag membrane filter on the fritted-glass
base of the filtering apparatus. Also center the glass chimney on
top of the base and secure it with a clamp.
- Carefully transfer the respirable air sample (PVC filter) from
the cassette to a round-bottom 40-mL
- Add 10 mL of THF to each centrifuge tube to dissolve the
filter and suspend the sample. Use an additional 10 mL for air
samples greater than 4.0 mg. Sonicate the sample suspension for 15
- With the vacuum turned off, place 2 to 5 mL of THF in the
chimney of the previously assembled vacuum filtering apparatus.
- For samples <4.0 mg: Quantitatively transfer the
suspension with rinses of THF to the glass chimney of the vacuum
filtering apparatus. The total volume in the chimney should not
exceed 20 mL.
For samples >4.0 mg: Divide up suspensions by delivering
appropriate aliquots of the suspension to additional glass
chimneys. Each aliquot should contain less than 4 mg of sample
- Apply vacuum to the filtering apparatus, drawing the THF
through it. This should result in a thin, even layered deposition
of the sample on the Ag membrane filter. Do not rinse the chimney
after the material has been deposited on the membrane filter.
Rinsing can disturb the thin layer deposition just created. Vacuum
should be applied for sufficient time to dry the membrane filter.
- Carefully disassemble the chimney and clamp. Remove the Ag
membrane filter from the firitted-glass base using
forceps. Place 2 drops of 1.5% parlodion solution on a glass
plate. Fix the dust to the membrane filter by placing the bottom
side of the membrane filter in the parlodion solution. By
capillary action, the membrane filter draws the parlodion solution
to the analyte surface. Place a Teflon sheet on top of an
intrinsically-safe hot plate which is set at the
lowest setting. Place the Ag membrane filter on top of this heated
- When dry, place the fixed sample in a labeled Petri dish.
- Store Petri dishes holding the prepared samples and standards
in a desiccator containing a drying agent.
3.5.2. Standard preparation
- Do not grind the bulk sample. Size a representative
portion of the bulk sample, using a 41-µm sieve. This
results in a sample particle size of less than 41 µm
(fines). Place the fines in a scintillation vial.
- Re-sieve the material remaining on the sieve to obtain
additional fines. Add the fines to the material obtained in step
1. Repeat this step until no additional fines are obtained. Cap
- Rotate and shake the vial to mix the combined PC fines
thoroughly. This is the PC stock material from which standards are
- Weigh out six different-size aliquots of PC stock
material in the range of 0.5 to 6 mg on PVC filters. Useful
nominal weights are 0.5, 1.0, 2.0, 3.5, 5.0, and 6.0 mg. Carefully
place each PVC filter containing the weighed PC stock material in
separate, labeled, round-bottom centrifuge tubes.
Standard amounts are selected to bracket the expected analytical
range of the air samples. The higher standards (>3.5 mg) are
useful to determine the analytical range for particular PCs.
- Place a blank PVC filter in a labeled
round-bottom centrifuge tube.
Note: This blank and the blanks supplied with the
air sample set are used to check for contamination of sampling
media and as reagent blank standards in the calibration. They may
also be used to estimate the analytical detection limit for a
particular PC (See Section
- Prepare the six standards and blank as air samples using steps
5 through 12 in Section
3.5.1. DO NOT split standards greater than 4.0
mg; they are used to calibrate above 4.0 mg.
- Inspect the deposit for uniformity; clumping indicates that
insufficient sonication was used. Remake the standard if a
significant amount of clumping occurs.
- Place the THF waste in an explosion-resistant
metal container and dispose appropriately.
3.6. Analysis - XRF
3.6.1. Assemble sample holders for the Ag membrane filters on a
clean dust-free surface. Load each sample in a sample
holder. Precise sample positioning is critical for reproducible
instrument response and reliable intensity data.
3.6.2. The most sensitive location for the sample in the XRF is
determined by trial-and-error using copper peak
intensities from a small ring of fine copper wire resting on a
polypropylene film. Mark tie location of the ring center with a
felt-tipped pen, and reposition the sample on the
polypropylene film until a maximum signal is reached. Sample holders
can be customized to center the Ag membrane filters at the most
Note: Si and Ca Ka lines are low energy X rays;
sample holders prepared using polypropylene film will give reduced
sensitivity for these lines. For this reason, sample holders without
any support film are preferred.
3.6.3. Analytical conditions for quantitative
(For typical analytical sensitivities see Section
4.3.) Use X-ray excitation conditions appropriate
for the system and software. If calculations are not performed
immediately after each scan, the spectra should be saved for later
use. Conditions selected for analysis should match those used during
calibration. For the spectrometer, specified in Section
4., the following conditions are recommended (Refer to Reference
for additional details.):
- Use a Ta collimator.
- Use a Lucite monitor so calibration and analytical data may be
collected and used over a period exceeding one day. Correct the
count data for analyses performed on subsequent days to comparable
first-day intensities by using a ratio of monitor
data obtained on the different days. Optimize the spectrometer to
obtain a maximum counting rate which does not exceed a 50%
deadtime using the Lucite monitor. Samples normally will give
lower count rates than the monitor.
- Two different instrumental conditions are used to obtain data
for the monitor, samples, blanks, and standards:
- Si, Ca, and MA
If a Cl secondary target is unavailable,
prepare one by pressing a 13-mm pellet at 8 tons
under vacuum from a mixture of 0.58 g powdered NaCl and 7 drops
of liquid binder. Glue the pellet into a secondary target holder
and install it in the secondary target wheel.
Use the Cl secondary target (as NaCl) to quantitatively
analyze PC for Si, Ca, and the sample mass absorption of the Ag
L lines from the fluorescing Ag membrane filter. Chlorine X rays
from the secondary target efficiently fluoresce Si. Sufficient
white (broad non-monochromatic) X-ray
intensity is available from the NaCl to excite Ca and Ag.
Analyze Si, Ca, and MA samples as follows:
- Operate the primary Rh target at 15 kV and 3.3 mA.
- Samples are analyzed with the MCA set to the 10 kV energy
- Analyze for 200 s counting time
- Use a vacuum.
- Fe and Sr
Use a Ag secondary target for the quantitative analysis of
the trace elements Fe and Sr. Analyze Fe and Sr samples as
- Operate the primary Rh target at 35 kV and 0.5 mA.
- Analyze samples with the MCA set to the 20 kV energy
- Analyze for 1,000 s counting time.
- Analyze samples in air.
3.7. Analysis - XRD
Note: Do not store any Ag membrane filter samples on
aluminum sample holders. (See Section
4.7. for further details.)
3.7.1. Use X-ray power levels appropriate for the
target and hardware in use. For the instrumentation described in Section
4., the following conditions are recommended:
- Power the tube to 40 kV and 40 mA.
- Use a sample spinner.
- For each sample, perform scans consisting of 0.02° 2q steps with 10-s duration. For
|PC peaks (2q)
27 to 36°
50 to 54°
Either multiple narrow-angle scans or a single
wide-angle scan can be used.
3.7.2. Desirable XRD analyte sensitivities
(For typical analytical sensitivities see Section
4.3.2.) A minimum desirable sensitivity for integrated peak
areas should approach 10 counts/µg (10,000 counts/mg).
3.7.3. Assessment of the presence of PC
An automated pattern search of a powder diffraction library such
as the Joint Committee on Powder Diffraction Standards (JCPDS)
powder diffraction file of the International Centre for Diffraction
Data may be useful in identifying other constituents.
3.8.1. Positive XRF interferences include background signals
arising from either:
- instrument artifacts
- or -
- target and filter fluorescence
- target and filter Compton and Rayleigh scatter peaks
- escape peaks
- sum peaks
- sample displacement towards the secondary target and/or
- sample matrix interferences
- overlapping sets of M, L, and K spectral lines (MLK lines
from elements other than those of interest)
- matrix specific enhancement such as the additional
florescence of Ca resulting from excitation by Ti fluorescing in
The effect of most interferences can be minimized or resolved.
For example, a potassium Kb line that overlaps the calcium
Ka line can be
resolved through profile-fitting. Interferences arising
from sample preparation materials may be resolved using blanks. Sum
and escape peaks can be resolved by software. Alternate analytical
lines are often available to resolve interferences. (Note: Lines
from the Ta collimator can potentially interfere with the analysis
3.8.2. XRD interferences
Other diffraction peaks may be present in the sample that may
present positive interferences:
Gypsum (CaSO4 ·
|28.0, 32.2° |
29.2, 31.2, 33.5,
34.7, 50.5, 51.3°
3.9.1. XRF result calculations
The sequence of steps in calculating results from the XRF data
depends on software requirements. Alternate sequences may be
necessary when using different software.
- Perform escape peak corrections.
- Perform sum peak corrections, if available.
- Perform background modeling and subtraction.
- Identify the elements and interferences present using software
tools provided with the XRF system in use.
- Decnvolute (profile-fit) the identified elements to obtain
integrated (area) counts for the analytical peaks.
- Check for residual peaks, and repeat steps 4 and 5 until all
potentially interfering peaks are accounted for.
- If the analyses are performed over a period of more than one
day, use monitor data to correct for changes in X-ray
- Calculate the mean blank Ag L peak intensity (Iblank) for the
field and reagent blanks (Section
3.5.2. step 5). Count data for three blanks are averaged in
order to provide a more accurate estimate of Iblank.
- Calculate the sample mass-absorption Ag L counts
for all air samples, blanks, and standards by subtracting the net
Ag L peak intensity of each from the mean blank Ag L peak
intensity (i.e., Iblank - Isample).
Note: The MA calculation is not the same as blank
subtraction; the order of subtraction is reversed because the Ag L
peak intensity of the blank is greater than that of the samples.
Another consequence of MA analysis is that blanks have a larger
counting error than other samples; therefore, negative net counts
are possible for blanks or lightly loaded samples.
- For each of the five analyses (Si, Ca, MA, Fe, and Sr),
tabulate the data for the theoretical µg PC and integrated peak
areas for the blanks and standards. A typical example follows:
- Construct a concentration-response calibration
curve for each of the five different analyte elements. (For
further suggestions and examples see Appendix
- Use the curves from step 11 to calculate the µg PC in each air
sample for each analyte element. Negative weights are possible for
non-detected samples near the intercept of the
calibration curve. Round negative µg PC results to zero for use in
- For each analyte element, sum together the µg PC determined
for aliquots of split samples. If blanks supplied with the sample
set are contaminated, subtract the µg PC contamination for the
specific element from each sample result in the set.
- For each sample, rank the µg results from the five analyses.
For example, sample A4 gave the following results:
These results are
ranked as follows:
- Select the median (middle) value from the five ranked PC
results. In the example given in step 14 above, the result 1,958
µg is selected. (The actual mass of PC aerosol spiked on PVC
filter A4 was 1,910 µg. Additional examples of rankings and
recoveries are given in Tables 1
4.4.2.) For air samples, calculate the PC exposure
(mg/m3) by dividing the median value
(µg) by the sample air volume (L). If sample A4 had resulted from
sampling a total air volume of 230 L, it would represent an
exposure of 1,958 µg / 230 L = 8.51
mg/m3. Exposure results for PC are not
final until PC is qualitatively confirmed.
3.9.2. XRD calculations
Deconvolute the XRD peak intensities and compare the diffraction
patterns of standards with air samples having similar weights.
X-ray diffraction peak intensities can be very
sensitive to preferred orientations and particle-size
distributions, and intensities may not always be comparable between
samples and standards.
3.10. Reporting Results
Both XRD and XRF data are considered before reporting the results
to the IH. Interpretation of XRF or XRD results requires experience
and analyst interaction.
3.10.1. XRF results
- Compare the experimental and theoretical XRF line energies to
qualitatively confirm the presence of analyte elements., They
should compare within ±0.01 kV or be comparable within the range
of variability exhibited by standards. When instrument resolution
and sensitivity permit, seek qualitative confirmation on secondary
Good agreement of the middle three ranked results (Section
3.9.1. step 14) provides quantitative XRF verification. Too
wide a spread may indicate significant contamination by substances
other than PC.
- Compare the result by this method (Section
3.9.1. step 15) with the gravimetric result measured by the
3.10.2. XRD results
- Match the 2 peak locations of air samples to standards in
order to establish a qualitative XRD "fingerprint" of PC
components. Peak locations of air samples are expected to be
within ±0.1° 2q) of the peak locations
exhibited by standards.
Note: Due to ion exchange, the peak locations for
the major PC phases often differ from data obtained from pure
phases or patterns listed in the literature.
After PC is identified by XRD, report the result determined in
- The XRD intensities should be comparable for standards and air
samples of equivalent weight having similar
Particulate present on the backup pad constitutes some sample
loss. Occasionally this may be seen and can be due to a poor
cassette seal on the filter, improper positioning of the filter in
the cassette, or poor quality control of the filter and/or cassette.
If this type of contamination occurs, relay a note to the compliance
officer indicating that some of the sampled material was found on
the backup pad and the reported value may be lower than the actual
4. Validation-Backup Data
The EDXRF spectrometer used in the validation was a Kevex 770/8000
Delta system (Kevex Instruments Inc., San Carlos, CA) consisting of: Kevex
770 X-ray generator, its associated satellite box, vacuum
system, helium flush system, firmware-based 8000 keyboard
console, computer monitor, Digital Equipment Corporation (DEC) 11/73
computer, graphics memory, Kevex spectrum analyzer, and Toolbox II
software. A wavelength dispersive X-ray fluorescence (WDXRF)
system may be used in place of an EDXRF system.
The APD used in the validation was an APD 1800 (Philips Electronics
Inc., Mahwah, NJ). It included an X-ray generator, long
fine-focus copper target X-ray tube,
proportional gas counter detector, pulse-height analyzer,
graphite monochromator, 2q compensating slit, 1°
receiving slit, sample spinner, sample changer, recirculating cooling
system for the X-ray tube, and associated software (Version
3.5) using the laboratory DEC VAX 750 computer.
The backup data validation contains the following protocol:
Detection limit estimates
Recovery and coefficients of variation
Storage tests and 100% RH tests of prepared samples
4.1. Experimental Considerations
4.1.1. A 41-µm nylon sieve is used in the method (Section
4.4.1.). This effectively excludes particles larger than 41 µm
from the analysis. The selection of the 41-µm nylon
sieve for the preparation of the PC bulk reference materials is
based on the following considerations and experiments:
- Nylon is non-contaminating.
- Portland cement dust is reported to range in size from 0.2 to
100 µm (5.1.).
- Most of the major tricalcium silicate (alite) phases are in
the range of 15 to 40 µm (5.1.).
- There is a significant decrease in XRF sensitivity to the
major phase elements Si and Ca for larger particles. Material
passing through 400 or 325 mesh sieve (37 to 44 µm) has been
considered generally applicable for XRF powder analysis of
elements as light as aluminum (5.14.).
- In a preliminary study, five different Portland cement
materials were sized using the 41-µm nylon sieve. In
all cases, most of the material passed through the sieve:
|% <41 µm
Four SRMs (from NIST) and Wülfrather PC material (from SA) were
The SRM 1880 was not sized in this study. However, the NIST
documentation describing both SRM 1880 and SRM 1881 indicates
maximum diameters of approximately 43 µm and 50 µm, respectively.
A graphic representation of particle distributions (in reference
show that SRM 1880 has a much smaller percent of particles above
41 µm than does SRM 1881.
- Gravel, sand, and other large particles are often added to
commercial mixtures to decrease the void space in the finished
material. Other PC-related commercial mixtures were
sized with the 41-µm nylon sieve. Only a small
fraction of each passed through the sieve:
|% <41 µm
The sample preparation procedure in this method avoids many
potential interferences from commercial mixtures by not grinding
the bulk reference material.
- The respirable range is 0.5 to 10 µm. Particles larger than 10
µm generally are stopped in the nasal passages. The
41-µm limit is set well above 10 µm in order to
support the total dust PEL (10 mg/m3).
- Particles <41 µm include all respirable PC dust and
would include most total PC dust. It can be shown that PC
particles >41 µm rapidly settle out of the air leaving only the
finer particles (See Appendix
4.1.2. Selection of representative SRM materials for sample
spiking and detection limit determinations:
Certificates of analysis and preliminary experiments were used to
determine which of four different SRM materials would be most
suitable to evaluate. The table below summarizes the factors
||------------------ % of Component
------------------ Calibration Curve
For calibration curves, standards were prepared from PC SRM stock
material as described in Section
3. SRMs 635 and 637 were selected because they exemplified the
worst and best cases overall and were the best compromise in
representing the widest composition.
4.1.3. The selection of experimental conditions and appropriate
SRM for study of humidity effects on samples was based on the
- The hydration process of poured PC stops when the RH is below
- Portland cement chemically combines with water. The amount of
water normally retained after strong drying ranges from 20 to 25%
Samples of the sieved SRMs were weighed and then exposed to air at
100% RH for 48 h at room temperature. They were reweighed, placed
in a vacuum desiccator, and dried to constant weight. They gave
the following irreversible weight gains:
The SRM 637 was selected to perform the humidity study because it
had the higher irreversible weight gain.
4.2. Detection Limit Estimates
4.2.1. Experimental design
The following approach was used to estimate detection limits for
the two PC SRMs (SRM 635 and SRM 637):
- X-ray tube currents were set to values that gave
a maximum of 50% dead time on a Lucite monitor. Sample analysis
time was 200 s for the MA analysis and for the analysis of the
elements Si and Ca; and 1,000 s for the trace elements Fe and Sr.
The same analysis time was used for both blanks and samples.
- The approach used to calculate detection limits is attributed
to Birks (5.15.)
and is given in Bertin (5.14.).
[Note: Although widely used as an estimate of the qualitative
detection limit, this theoretical approach may not be appropriate
for samples containing significant interferences. The PC DL
estimates calculated may be lower than would be obtained
experimentally by other approaches. For example, at low jig
levels, dust may penetrate into the Ag membrane filter with
resulting shadowing and reduced sensitivity.]
The following equation (based on Poisson counting statistics)
was used to estimate µg detection limits:
B = blank background counts
= sensitivity (analyte counts/µg PC)
Portland cement detection limit estimates were determined for all
five analyses (Si Ka, Ca Ka, MA, Fe Ka and Sr Ka). The sensitivity for each
analysis can vary depending on sample composition. Standards
(2-mg) of the two SRMs were used to obtain the net
counts for each analyte. These spectra were
background-subtracted prior to
profile-fitting each analyte peak. Except in the case
of MA analyses, the net counts for the fit peak and the analyte
counts were the same. The analyte counts data used for the MA
analyses were obtained by subtracting the net Ag counts of each
standard from the mean net profile-fit Ag counts for
Energy spans used in profile-fitting the standards
were comparable to those used to integrate the background of the
In profile-fitting, the peak intensity affects the selection of
the integration limits. The software generally selects narrower
integration limits for smaller peaks. Using such narrow limits would
give unrealistically low estimates for the variation in background
counts. Therefore, the integration limits used to determine the
background counts for blanks were based on the
profile-fit integration limits used in the analysis of
a 3-mg SRM 637 sample exposed to 80% RH for 4 h. (This
sample had been used in the humidity study and is listed as sample
T8 in Table
The data for blank samples T1, T2, and B11 were used to calculate
mean blank background count data:
||------------ Blank background counts
|1.560 to 1.940
3.500 to 3.880
6.180 to 6.620
13.860 to 14.400
||The MA data are derived primarily from the Ag L peak and
are not background counts; however, the MA blank data are used
in the analysis to blank-correct the
mass-absorption analytical data in a manner
comparable to blank subtraction.|
||Blank sample B11 was contaminated with Fe as evidenced by
the presence of a significant Fe Ka peak (~800 counts above
background.) The Fe contaminant may have come from the forceps
used in preparing the sample. This outlier was not used in
determining the mean.|
The XRF DLs derived from these data are compared and listed with
sensitivities in Section
The PC DLs for the Fe and Sr analyses can vary considerably
because the amounts of these trace elements are highly variable.
Because the fluorescence X rays of these elements are penetrating,
particle-size effects are not expected to significantly
complicate matters; the PC DL of these elements are expected to be a
linear function of the composition of PCs encountered in the
The PC DLs for the Si and Ca major-element
compositions are less variable because PC
SiO2 and CaO compositions are expected to
be in the range of the two SRMs studied. However, the PC DLs of
these elements are not expected to be simple linear functions of
composition because the fluorescence X rays of these elements
(particularly Si) are less penetrating than those of Fe and Sr and
are therefore more prone to significant particle-size
The PC DL tends to be large for the MA analyses because the
counting error is greater for large peaks than for the low
backgrounds. Matrix composition does not appreciably affect the
sensitivity for the MA analyses; therefore the PC DL for the MA
analyses should not vary significantly between different PCs.
4.3. Analyte Sensitivities
4.3.1. XRF PC sensitivities
Analytical sensitivity (counts/µg PC) is matrix dependent. For
the elements Fe and Sr, which produce relatively penetrating X rays,
the sensitivities are roughly proportional to the concentration of
analyte element; materials with smaller amounts of analyte element
tend to give lower sensitivities. The Fe and Sr
trace-element composition of PC varies considerably
from one PC to another resulting in different sensitivities for
Although the Si and Ca composition of PC is not as variable as
that of Sr and Fe, the X rays from Si and Ca are not as penetrating
as those of Fe and Sr; therefore PC analyses using the Si and Ca X
rays are more susceptible to particle-size effects. In
homogeneous materials, large particles generally yield sensitivities
that are lower than those of small particles.
The analysis least affected by the PC composition is that based
on mass absorption of the Ag L lines from the Ag membrane filter
For the analytical conditions described in Section
3.6., the following analyte sensitivities were found:
||---- Counts/µg PC ---
||--------- Composition ---------|
As indicated in the equation in Section 4.2., DL and sensitivity
are related. Using the analytical conditions described in Section
3.6., the two SRMs in the evaluation gave the following
correspondences between sensitivities and DLs:
||----- SRM 635 -----
||----- SRM 637 -----|
The data show that a low sensitivity translates as a high
4.3.2. XRD sensitivities
Using the analytical conditions described in Section
3.7., the following are approximate sensitivity ranges observed
for five different PCs (Wülfrather and SRMs 635, 636, 637, and
|Integrated peak sensitivity
|23 to 79|
28 to 76
42 to 80
4.4. Analytical Data
Due to the lack of a sophisticated aerosol generation/particle
sizing system and the high cost of NIST SRM materials, certain
economies had to be undertaken for the collection of PC in air during
4.4.1. Experimental procedure - spiked
- In the industrial environment, only a small fraction of the PC
in use would become suspended in the air. The analytical portion
of this method requires that the standards be prepared from
exhaustively sieved PC bulk. For the purpose of this experiment,
the SRMs used in spiking samples were not exhaustively
sieved in order to produce a variability in particulate sizes and
matrix similar to what might occur in industrial hygiene sampling.
Simulated air samples were prepared directly from SRM material as
received from NIST; aliquots of the unsieved material were briefly
shaken from a 41-µm nylon sieve into an air stream
and deposited onto PVC filters. Only a small fraction of the
material passed through the sieve during the 1 to 2 min of
shaking. The following diagram shows the device used in preparing
the simulated air samples:
Note: The diagram shown above describes the
apparatus used in spiking validation samples with PC aerosol. This
diagram is not intended to represent proper field sampling.
- Sample collection in dry air was evaluated by collecting the
suspended dust SRMs in laboratory air on tared PVC membrane
filters using the device shown above (2-min sampling,
2 L/min, 30 to 40% RH). Sampling was followed by weighing to
determine the net weight of PC collected. These samples are
referred to as "Dry" in the tables below.
- Any affect from humid air was evaluated by first collecting
suspended dust samples of SRM 637 in dry air (as above). Next,
humid air was drawn through the samples (4-h
exposure, 1 L/min, 80% RH). This was followed by reweighing. The
net weight gain during exposure to humid air was 0.2%. These
samples are referred to as "80% RH" in the tables below.
- As stated above, only a small portion of each aliquot of the
NIST SRMs was actually delivered to each PVC filter. The
proportions delivered to the filters are listed below:
|SRM 635 (Dry)
3.5% to 10.4%
|SRM 637 (Dry)
6.7% to 21.5%
|SRM 637 (80% RH)|
7.0% to 17.2%
4.4.2. Analytical results
Samples were prepared and then analyzed by XRF following the
procedure in Section
- The tables which follow contain ranked lists of PC weights
found using analyses based on the five analytes. The theoretical
(gravimetric) weight of PC for each sample is also shown.
- The recovery (R) is based on the reported median and is
calculated from Found/Theoretical. The coefficient of variation
(CV) is shown for each experimental group. Additional samples were
analyzed along with the standards and simulated air samples in
order to provide a check on reproducibility and to evaluate
storage stability. These additional samples are referred to as
quality assurance samples.
- In assessing the agreement between theory and experiment, the
symbol "" locates the
position of the theoretical weight in the ranked lists. When the
theoretical weight is the same as one of the found weights, the
"" replaces the
decimal point (e.g., the "high" value for sample All in Table
The analyses that gave results that bracket the theoretical
weight are indicated in the column labeled "[Ana]".
SRM 637 - Recoveries
SRM 637 Dry Air Spiked Samples:
||------------------ µg PC Found
Mean recovery A1 to A9
CV = 0.126
* Samples A10 and A11 were blanks. Negative µg PC results can
occur for samples with counts near the intercept of the calibration curve.
They are presented here for descriptive purposes only.
Quality assurance (QA) samples:
The following six QA samples
SRM 637 standards tat had been stored 60 days on a laboratory bench:
||---------------- µg PC Found --------------
Mean recovery QA =
CV = 0.065
SRM 637 - Recoveries
SRM 637 Humid Air (240 min, 80% RH)
||------------------ µg PC Found
Mean recovery T3 to T9
CV = 0.113
* Samples T1 and T2 were blanks. Negative µg PC results can
occur for samples with counts near the intercept of the calibration curve.
They are presented here for descriptive purposes only.
Quality assurance samples:
The following four QA samples
consisted of re-analysis of two recent SRM 637 standards used
in the calibrations above, a re-analysis of sample A4 above,
and a new 2204 µg calibration standard TT4:
||---------------- µg PC Found
Mean recovery QA =
CV = 0.016
SRM 635 - Recoveries
SRM 635 Dry Air Spiked Samples:
||----------------- µg PC Found
Mean recovery B1 to B10
CV = 0.060
* Samples B11 and B12 are blanks. Negative µg PC results can
occur for samples with counts near the intercept of the calibration curve.
They are presented here for descriptive purposes only.
Quality assurance samples:
The following six QA samples are
SRM 635 standards that had been stored 60 days on a laboratory bench:
||---------------- µg PC Found ---------------
Mean recovery QA =
CV = 0.124
Note: The "High" result for sample BB6 is above the useful
This outlier is automatically excluded by use of the
- An evaluation of relative bias among the five different
analyses was performed. The tally or count of samples having a
particular analysis (Si to Sr) at each of the five rank locations
(Low to High) was determined. These are listed in the first six
columns of Table
3 below. The apparent bias suggested by these tallies is
listed in the "Apparent Bias" column.
An assessment was made to determine whether any of the five
analyses tended to be more reliable than the others. This was
performed in the following manner: For each sample, the identity
of the two analyses which bracketed the theoretical result (shown
in "[Ana]" columns of Tables 1 and 2 above) was determined. The
tally for each of the analyses that bracketed the theoretical
result was increased by one for each sample. For example, the Si
tally was increased by one for each sample having a Si result
ranked adjacent to the theoretical result. The process was
analogous for the Ca, MA, Fe, and Sr analyses. There were
instances where two results did not bracket the theoretical value;
for these samples, the tally for the closest analysis was
increased by two so that each sample would be given equal overall
weighing in the tallies. The number of times a particular analysis
bracketed the theoretical value is shown in the "Tally Bracket"
column of Table 3 below:
32 Air Samples and Blanks:
||------------------ Tally -----------------
16 QA samples:
Total 48 Air, Blank, and QA Samples:
* Reported value
As the "Tally Bracket" results in Table 3 indicate, no
single element was optimum for quantitating PC. Individually,
the analyses for an element may show an apparent bias in the
rank, but the median results of all the analytes taken together
gave the most representative estimate of the quantity of PC
present in the samples.
4.5. Recovery and Coefficients of Variation
For the simulated "air" samples, the recovery R (Found/Taken) and
CV data were calculated based on the medians of the five different
analyses described above. (CV for quality assurance samples are also
listed separately below.) The theoretical weight of each simulated
"air" sample was determined based on the net weight. Because air
samples were simulated and the sampling pump was used for movement of
particulate rather than for delivery of a specific volume of air, a 5%
pump error estimate was included in the calculation of the total
coefficient of variation (CVT) using the
CVT = [
Spiked "Air" Samples:
|Mass range (mg)
|0.9 to 7.3
0.6 to 7.7
0.9 to 2.9
|Combined results from above:
|Mass range (mg)
|635 & 637
||0.6 to 7.7
Quality assurance samples analyzed:
Re-analysis of standards stored for two months in petri dishes on
laboratory bench (These were not stored in a desiccator.):
|Mass range (mg)
|0.6 to 5.8
0.7 to 5.8
Analysis of a spiked membrane filter sample,
re-analysis of an 80% RH sample, and
re-analysis of two standards:
|Mass range (mg)
||1.0 to 3.4
Grand summary of all "air" and quality assurance sample results:
|Mass range (mg)
|635 & 637
||0.6 to 7.7
4.6. Overall Error
Overall error (OE) is defined as a combination of the contributions
of bias and imprecision in recovery and is expressed as a percentage:
OE = ± [ | R - 1 |
+ 2(CVT) ] 100%
For the "air" samples:
|Mass range (mg)
|0.9 to 7.3
0.6 to 7.7
0.9 to 2.9
Combined results from above:
|Mass range (mg)
|635 & 637
||0.6 to 7.7
4.7. Storage Tests and 100% RH Tests of Prepared
- Standards deposited on Ag membrane filters were reanalyzed
after being stored in petri dishes on a laboratory bench for 60 d
(AA and BB samples shown in Tables 1 and 2, Section
- High humidity experiments were also performed to determine the
effect of 48-h exposure to 100% RH on SRM 637
standards. These were 6-mg samples on Ag membrane
filters (fixed and unfixed with parlodion). During the exposure,
the membrane filters were kept in upright positions in XRD sample
holders (PW1813/26 with aluminum back plates, Philips) so that any
change in intensity would not be due to sample handling.
- The re-analysis of standards stored on a
laboratory bench for 60 d did not indicate storage problems.
- The experiments performed on SRM 637 standards exposed to 100%
RH resulted in large decreases in XRD intensity (about 30 to 40%),
pitting and corrosion of the aluminum back plates of the sample
holders, and the appearance of a light-colored dust
adhering to the backs of the Ag membrane filters. Presumably,
Ca(OH)2, leached from the sample to
react with the aluminum back plates. Although 100% RH is not
common in controlled environments such as analytical labs, this
experiment points out that prepared samples should not be stored
in aluminum sample holders unless placed in a desiccator.
Portland cement dust consists of a variable and complex mixture of
component phases. At the time of this writing, the total dust PC PEL
is not limited to a legally defined particle-size
distribution. Industrial exposures to PC aerosols vary with respect to
both compositions and particle-size distributions.
Thin-film XRF techniques are sensitive to both these
effects. As a consequence, the compositional variability is echoed in
the analytical results. In order to minimize variability in the
reported result, the following approach was taken:
- A representative bulk sample of PC from the workplace being
sampled is sieved to a particle-size range that is
typical of PC aerosols and is used to prepare standards.
- The median result of five different analytes measured by XRF is
used to provide a consensus estimate in order to resolve the effect
of varying aerosol composition during sampling.
- The presence of PC is confirmed qualitatively by matching the
XRD patterns of the air sample to the XRD "fingerprint" for PC
Impurities such as CaSO4 and uncombined
CaO readily absorb moisture from the air. This evaluation indicated
that minimal humidity effects are expected during sampling at
<80% RH. Storage of prepared samples for 60 d in petri
dishes on a laboratory bench also indicated no significant sensitivity
to the ambient conditions in the laboratory; however, prepared samples
exposed to 100% RH for 48 h were significantly affected. These latter
samples were on Ag membrane filters and were contained in sample
holders. Therefore, it is prudent to take reasonable precautions to
minimize sample storage in humid air.
OSHA Method ID-207 is suitable as an adjunct to gravimetric field
evaluations for the determination of PC exposures in workplace
5.1. Kirk, R.E. and D.F. Othmer:
Encyclopedia of Chemical Technology. 3rd ed., Vol. 5. New York:
John Wiley & Sons, 1979. pp. 163-193.
5.2. National Institute Standards and
Technology: Standard Reference Materials: Portland Cement
Chemical Composition Standards (Blending, Packaging, and Testing)
(National Bureau of Standards, Special Publication
260-110, 103 pages CODEN:XNBSAV) by R. Keith Kirby and
Howard M. Kanare. Washington, D.C.: U.S. Government Printing Office,
5.3. American Society for Testing and
Materials: 1978 Annual Book of ASTM Standards; Part 14.
Philadelphia, PA: ASTM, 1978. pp. 104-110.
5.4. American Conference of Governmental
Industrial Hygienists: Documentation of the Threshold Limit
Values and Biological Exposure Indices, 5th ed. Cincinnati, OH:
American Conference of Governmental Industrial Hygienists (ACGIH),
1986. p. 494.
5.5. "Air Contaminants; Final Rule": Federal
Register 54:12 (19 Jan. 1989). pp. 2598-2599.
5.6. Sisler, H.H., C.A. VanderWerf, and A.W.
Davidson: College Chemistry A Systematic Approach, 2nd ed.
New York: The Macmillan Co., 1961. p. 548.
5.7. Occupational Safety and Health
Administration - Salt Lake Technical Center: OSHA Analytical
Methods Manual (USDOL-OSHA-SLTC Method and Backup
Report No. ID-142). Cincinnati, OH: American Conference
of Governmental Industrial Hygienists (In publication).
5.8. Jugovic, Z.T.: Spectrophotometric and
EDTA Methods for Rapid Analysis of Hydraulic Cement. ASTM Special
Technical Publication: 985:15-25 (1988).
5.9. Scott, E.H.: Atomic Absorption
Methods for Analysis of Portland Cement. ASTM Special Technical
Publication: 985:57-72 (1988).
5.10. Occupational Safety and Health
Administration - Salt Lake Technical Center: Personal
communication to West Germany's Saarbergwerke Aktiengesellschaft
regarding "Report on Saarberg Portland Cement Analyses." January 30,
1990, Salt Lake City, UT: USDOL-OSHA-SLTC, 1990.
5.11. National Institute for Occupational
Safety and Health and Occupational Safety and Health Administration:
Occupational Safety Guidelines for Chemical Hazards,
Occupational Health Guideline for Portland Cement (September, 1978)
[DHHS (NIOSH) Publication No. 81-123, 19811.
5.12. Occupational Safety and Health
Administration - Salt Lake Technical Center: Standard Operating
Procedure, Inorganic Analysis by X-ray Fluorescence
Spectrometry (Semiguant-XRF). Salt Lake City, UT:
5.13. Silverman, L., C.E. Billings, and H.W.
First: \ Particle Size Analysis in Industrial Hygiene, New
York, Academic Press, 1971. pp. 13-16.
5.14. Bertin, B.P.: Principles and
Practice of X-ray Spectrometric Analysis. 2nd ed. New
York: Plenum Press, 1975. pp. 529 and 738.
5.15. Birks, L.S.: X-ray
Spectrochemical Analysis. 2nd ed. New York: Interscience
Suggestions for XRF Calibration Curves for PC
(The software features described below were incorporated into custom
analytical software used at the OSHA Salt Lake Technical Center.)
Least squares curve fits are commonly used to obtain calibration
curves. In X-ray calibrations, the count intensity is the
dependent variable, and the mass is the independent variable. The least
squares approach minimizes the sum of the squares of the differences
between the dependent variable data and the curve. To find the best fit
for PC analytes, the analyst should evaluate power series fits for the
first through fourth order curves. It is often desirable to weight the
data according to the reliability of the dependent variable. The
calibration data may be assigned weights inversely proportional to the
intensity variance. The random error in gross intensity obeys Poisson
statistics and is a significant fraction of the measurement error. The
Poisson variance statistic is simply the gross intensity. For intensity
measurements with a low background, the Poisson variance statistic may be
approximated by the net intensity.
Caution: When creating PC calibration curves, do not
over-weight the lower standards; the counting error is not
the only source of error in the measurement. In the evaluation of the PC
4.), the adjusted Poisson weights varied from 1,000 (maximum) for
blanks to about 2 for the highest standard.
For the MA analysis of PC using the fluorescence X rays from the Ag
membrane filter, the weighting is not simply related to the net counts;
net counts are calculated from silver line intensities (i.e., Iblank -
Isample). In the method evaluation, the Ag data were weighted
For a wide variety of PC SRMs and building construction materials
encountered in the preliminary evaluation, a monotonic positive S-shaped
(sigmoidal or ogival) curve often best described the calibration data for
each analytical line. An example sigmoidal curve is shown below:
Three regions of interest are shown above:
Region 1 (low sample weights) shows reduced sensitivity as the
result of two causes:
- Fine particulates may penetrate into the surface pores of the Ag
membrane filter. The Ag acts to shield particulates from a portion
of the incident X rays and also absorbs a portion of the
fluorescence X rays from the particulates (shadowing).
- Profile-fitting (deconvolution) software tends to
compute smaller peak widths for low intensity peaks.
Region 2 (moderate sample weights) shows an approximately linear
response to sample mass. For layers that are only a few particles
deep, the sensitivity is dependent upon the particle-size
distribution and the concentrations of all elements in the particles
Region 3 (high sample weights) shows reduced sensitivity due to
partial absorption of both incident and fluorescence X rays within
thick dust layers.
When preparing PC calibration curves, the analyst must decide upon the
most appropriate fit. The fit selected should be monotonic (have a
single-valued inverse) in the analytical range. If a curve
maximum is found in the range of standards suggested, the analyst should
consider reducing the analytical range, replicating standards, or trying
another curve fit.
The simplest mathematical models are represented by first-
or second-order fits. First- and second-order
models often provide adequate fits for analyses experiencing small
deviations from linearity. Two such instances are analyses based on the
higher-energy fluorescence X rays or analyses based on
material having a large weight fraction of small particles.
The simplest power series function that may describe a sigmoidal curve
is third order. Occasions may arise when a fourth-order fit
may be the best selection.
Caution: Additional small random (non-systematic) errors
due to variations in sample preparation, etc. are expected. Although a
fourth-order fit may intersect all the data points, a lover
order fit is generally preferred in order not to bias the curve fit by
these small irreproducible errors. It is a good policy to keep the
mathematical model as simple as possible.
The weight of PC is determined from the inverse function of the
selected power series. This is a simple calculation for first and
second-order fits. Determining the inverse of
higher-order equations is a more complex task; however,
computer programs are available to numerically solve
higher-order fits in the monotonic analytical range.
Solutions to higher-order (polynomial) equations are easily
determined by a successive approximation process called binary search. In
a binary search for this application, a succession of estimates of the
analyte mass is used to calculate corresponding count intensities (using
the calibration equation) until there is negligible difference between the
calculated and measured intensity. Each iteration of the search narrows
the subsequent search by halving the previous search domain. The binary
search procedure is as follows (an example follows the procedure):
- The initial analytical search range must be monotonic.
Determine the mass domain (initially, about 0 to 6 mg) and calculate the
mass at the center of the mass domain (M½). The value for
M½ is initially about 3 mg.
- Evaluate the polynomial at M½ in order to obtain the
calculated counts (I½).
- Determine if the measured intensity (Im)
is less than I½.
- If Im is not less than I½, the
upper half of the current mass domain becomes the new current mass
domain. Proceed to step 6 below.
- If Im is less than I½, the
lower half of the current mass domain becomes the new current mass
- The mass midway in the current mass domain becomes the new
- Repeat steps 2 through 6 until there is no significant difference
between Im and I½. This takes fewer
than 23 iterations.
- The last M½ obtained is the inverse function of the
polynomial that gives Im.
Note: For cases where the measured counts decrease with
increasing mass (e.g., Ag counts versus PC mass), first transform the
measured count data for the standards and samples by subtracting the
mean of the blank count data.
Consider the following example of the binary search procedure to find
the mass corresponding to a measurement of 5,500 counts (shown as a "+" in
the figure below):
The calibration curve above approximates the following calibration
The experimental data was fit by the third-order equation
I = ƒ(mass):
599.4118(mass)2 + 802.421(mass)
The monotonic analytical range is for the mass domain 0.0000 to 6.0248
Each iteration halves the span of the mass domain; this results in the
convergence to a mass that corresponds to a calculated count intensity
equal to the measured count intensity.
||-------- Mass ----------
||----------- Counts --------
I½ = ƒ (M½)
|0.0000 to 6.0248
1.5062 to 3.0124
2.6358 to 3.0124
2.8241 to 2.9183
2.8477 to 2.8712
2.8477 to 2.8535
This process can be continued to any desired precision of calculation.
During 23 iterations, the value of I½ approaches 5,500 counts
and the value of M½ approaches 2.8487 mg (to five figures).
PC X-ray Diffracton Patterns
Size Considerations in Sampling for PC
Particles <100 µm achieve terminal velocity very quickly in still
air. The terminal velocity may be estimated from a derivation using
Stokes' relation for streamline flow (5.13.):
FR = 3p µƒ V Dp
||viscous resistance force of the air (gm •
||viscosity of the fluid (g/cm • sec)|
||velocity of particle relative to air (cm/sec)|
||spherical particle diameter
In free fall, this force of resistance is balanced by the gravitational
force acting on the particle. (One may neglect the force due to buoyancy
because the density of air is much less than that of PC particles.)
FR = rp p/6
||density of particle (g/cm3)|
||acceleration due to gravity
3p µƒ V
Dp = rp p/6
Solving for V:
V = rp
(Dp)2 G /
Note: G / (18 ƒ) = 3.02 ×
105 for ambient air at the surface of the
Earth when Dp is expressed in cm. Use 3.02
× 10¯3 for Dp
expresses in µm.
PC phases have densities in the range 3.0 to 3.8
g/cm3. The calculated settling rate range for
41-µm spherical particles with this density range is 15 to 19
PC particles are not spherical (5.1.).
Most nonspherical particles settle at a slightly slower rate. The
worst-case deviation is about 33% (5.13.).
For nonsherical 41-µm particles of PC, the slowest velocity
range for settling out of the air would be 10 to 13 cm/sec. A stationary
volume of air 6 m high would become free of PC particles larger than 41 µm
in less than 1 min.
In the case of dust exposures involving dust that is continually
replenished from a source, the particle-size distribution
changes over time; the concentration of smaller particles increases until
a steady state is achieved. Particles <41 µm would
represent the greatest contribution to the dust suspended in the air.
The foregoing discussion considers only particles subject to streamline
flow, but non-streamline flow does not alter the conclusion.
The largest spherical particle falling in the streamline flow range can be
estimated from (5.13.):
Dp <100 / (rp)1/3
For the density range of PC phases, the largest particles that exhibit
streamline flow are in the range of 64 to 69 µm (corresponding to 43 to 47
cm/sec). Particles larger than this are subject to turbulent flow and
clear the air at velocities faster than 47 cm/sec.