NITROGEN DIOXIDE BACKUP DATA REPORT - (Inorganic Method #182

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NITROGEN DIOXIDE BACKUP DATA REPORT (ID-182)

This backup report was revised May, 1991

Introduction

The general procedure for the air sample collection and analysis of nitrogen dioxide (NO₂) is described in OSHA Method No. ID-182 (ID-182 examines the use of the combination tube, and a three-tube sampling device at a flow rate of approximately 0.2 L/min. The four-fold increase in sample flow rate during collection should assist in detecting low levels of NO₂ and help minimize any impact from sorbent contamination. Analysis is performed by ion chromatography (IC).

This method was evaluated when the Permissible Exposure Limit (PEL) was a 5 ppm Ceiling. The OSHA Final Rule PEL for NO₂ is currently 1 ppm. A 15-min sampling time was used. Short descriptions of the components used for the evaluation are listed below.

Generation System

All generation of nitrogen dioxide test atmospheres, and hence all experiments, with one exception, were performed using the equipment shown in Figure 1. The detection limit study did not use a test atmosphere generation for sample spiking and collection. Instead, samples were spiked with solutions of sodium nitrite.

Nitrogen dioxide permeation tubes (Thermedics Inc., Woburn, MA) were used as the contaminant source for all of the generation experiments except the conversion factor experiment. A cylinder of nitric oxide (NO) in nitrogen and oxidizer tubes were used to determine conversion factors. Permeation rates during the other experiments were determined by measuring the weight loss of three permeation tubes over a given period of time. A constant temperature of 35 °C was used. As shown below, the calculated overall NO₂ permeation rate for the three tubes was 89.09 µg/min.

Time Elapsed (min)	Weight Loss (µg)	Diffusion Rate (µg/min)

4,335 5,700 10,105 10,095	387,600 513,400 904,200 882,100	89.41 90.07 89.48 87.38

Ave. 89.09 ± 1.17 µg/min

The NO₂ produced from the permeation source was diluted with a small amount of filtered air and then mixed, using a glass mixing chamber, with filtered, tempered air. A flow, temperature and humidity control system (Miller-Nelson Research Inc., Model HCS-301) was used to condition the diluent air for mixing. A Teflon sampling manifold was attached to the mixing chamber. Flow rates for the diluent air were determined using a dry test meter. Contaminant gas flows were measured using mass flow controllers and soap bubble flowmeters.

Sampling Media

Three different TEA-IMS sampling systems were commercially available for NO₂ sampling at the beginning of the evaluation. The three devices are designed to simultaneously collect NO₂ and nitric oxide (NO). Preliminary studies conducted on the three different systems indicated the SKC collection device (1) listed below was the most suitable sampling device to collect NO and NO₂. A short description of each device is listed:

NO₂-NO collection device (Cat. No. 226-40, SKC, Eighty Four, PA):
The sampling device consists of three separate glass tubes, two TEA-IMS tubes and an oxidizer tube. Each glass tube is flame sealed. Both sample collection tubes consist of 400 mg TEA-IMS. The oxidizer contains approximately 1 g of a chromate compound. Either TEA-IMS tube can be used separately to monitor NO₂. When sampling for both NO and NO₂, the three tubes are connected with Tygon tubing such that the oxidizer tube is placed between the two TEA-IMS sampling tubes. The tubes used during the experiments were from lot no. 374.
SKC combination tube (Cat. No. 226-40 discontinued, SKC):
This combination tube contained all three sections in a single tube. Two 400 mg sections of TEA-IMS were separated by a 800 mg oxidizer section. This tube has been discontinued by SKC and replaced with the device mentioned above. The tubes used were from lot no. 306.
Supelco combination tube (Supelco, Bellefonte, PA):
This tube is similar in construction to the SKC lot no. 306 sampling tube (2) listed above with one exception. The Supelco tube uses a smaller mesh size of molecular sieve. Lot no. 582-99 was used for a Sampling and Analysis experiment.

SKC collection devices (1) and (2) listed above are identical except device (1) has a physical segregation of sorbents and oxidizer.

Due to low recoveries found during a preliminary study with Supelco sampling tubes, these tubes were excluded from the experiments.

Sample Collection

Air samples were collected from the Teflon manifold using calibrated Du Pont model P125 low flow pumps (flow rates of 0.175-0.200 L/min) for all experiments except for the Desorption Efficiency (DE) (Analysis - Section 1) determination. This experiment used low flow pumps to spike the TEA-IMS material with gaseous NO₂. The analysis experiment was designed to determine the amount of gas collected and not necessarily the sampling capability at this flow rate. A flow rate of 0.010 L/min with SKC Model 222-3-10 low flow pumps were used for the DE study.

Sample Analysis

Samples prepared for all experiments were analyzed by IC using the conditions specified in the method (11.2.) to use in result calculations. Later experiments revealed an average C.F. of 0.63 (11.3.-11.5.). A C.F. of 0.63 was used to for all air sample experiments in this evaluation which were performed below 10 ppm NO₂.

Evaluation

The following experiments were performed for the evaluation of Method No. ID-182:

Analysis - (DE) of spiked samples
Sampling and Analysis - generation and analysis of NO₂ samples
Collection efficiency and breakthrough of TEA-IMS sampling tubes
Storage stability of sampling tubes
Sampling at different humidities
Analytical method comparison
Analytical detection limit determinations
Determination of conversion factor for NO₂ concentrations of 10 to 200 ppm.
The preliminary sampling and analysis experiment using Supelco tubes is discussed in Section 9.

A statistical protocol (11.6.) was used to evaluate results. Data were subjected to the Bartlett's (11.7.) and an Outlier test (11.8.) to determine homogeneity of variance and identify any extraneous data.

1. Analysis (Desorption Efficiency, DE)

Procedure: A total of 20 spiked samples (8 samples at 0.5 and 6 samples at 1 and 2 times the Transitional PEL) were prepared and analyzed. Samples were prepared by spiking known amounts of NO₂ gas into TEA-IMS solid sorbent tubes. The spiked concentrations were approximately 2.5, 5.0, and 10 ppm of nitrogen dioxide. These concentrations are about 0.5, 1, and 2 times the OSHA Transitional PEL. Recoveries at these levels represent the analytical DE. Results also provide information regarding the extent of variability for the analytical portion of the method. Details for this experiment are discussed below:

1.1. SKC lot no. 374 sampling tubes were used.

1.2. Known NO₂ gas concentrations were prepared by using a ten-fold dilution of the NO₂ permeation source with tempered air (50% RH and 25 °C). Samples were dynamically spiked using calibrated SKC low flow rate pumps. The pumps slowly drew the diluted NO₂ contaminant gas into the TEA-IMS tubes. Samples were taken for measured time periods at a flow rate of approximately 0.010 L/min.

Results: The results of the analysis study are presented in Table 1. All data passed the Bartlett's test. One result tested as an outlier and was omitted. Results were pooled. The data (Table 1) indicates acceptable precision and accuracy (11.6.) for the analytical portion of the method and does not indicate a need for a desorption correction factor. The coefficient of variation for analysis (CV₁) was 0.021 and the average analytical or spiked recovery was 106%.

2. Sampling and Analysis

Procedure: A total of 18 samples (6 samples at each of the three test levels) were collected from dynamically generated test atmospheres and analyzed. Generation and analysis of NO₂ was the same as mentioned in the Introduction. Sample results from the dynamic generation provide the overall error and precision of the sampling and analytical method. Overall error should be ±25% and was calculated using the following equation (11.6.):

Overall error = ± [ | mean bias | + 2CV_T ] × 100%

2.1. SKC sampling tubes, lot no. 306, were used for this experiment.

2.2. Samples were taken for 15-min sampling periods at concentrations of approximately 0.5, 1, and 2 times the OSHA Transitional PEL. The relative humidity and temperature of the generation system were set at 50% and 25 °C.

Results: The results of the sampling and analysis experiment are shown in Table 2. The sampling and analysis data also show acceptable precision and accuracy (11.6.). All data passed both the outlier and Bartlett's test and results were pooled. The pooled coefficients of variation for spiked CV₁ (pooled), generated CV₂ (pooled) samples, as well as the overall CV_T (pooled), are as follows:

CV₁ (pooled) = 0.021 CV₂ (pooled) = 0.033 CV_T (pooled) = 0.034

The overall bias was 13% high. Overall error was acceptable (< ±25%) and was ±19.8%.

3. Collection Efficiency and Breakthrough

3.1. Collection Efficiency

Procedure: Samples were generated to measure the sorbent collection efficiency at about 9.5 ppm NO₂.

3.1.1. SKC sampling tubes, lot no. 306, were used to collect the NO₂ at 50% RH and 25 °C. These were the combination tubes; each glass tube contained two sections of TEA-IMS separated by an oxidizer section.

3.1.2. Using the same generation system described in the Introduction, six samples were collected at 2 times the OSHA Transitional PEL for 15 min.

3.1.3. The amount of NO₂ vapor collected in the first and second sections of the tubes was measured. The collection efficiency was calculated by dividing the amount collected in the first solid-sorbent section by the total amount of NO₂ collected in both sections.

Results: Results are reported in Table 3. Collection efficiency was adequate at two times the Transitional PEL with an average recovery of 97%.

3.2. Breakthrough

Procedure: Samples were generated at a concentration greater than the evaluation levels to determine the extent of NO₂ breakthrough from the first solid sorbent tube into a second tube. The calculated breakthrough should be less than 5%.

3.2.1. Four sampling tubes (SKC lot no. 374) were connected to backup tubes and then to sampling pumps. Air samples were collected for 15 min at a concentration of approximately 4 times the Transitional PEL. The generation system was set at 30% RH and 25 °C. The low humidity level was used as a "worst case" test since the presence of water is necessary for the conversion reaction of NO₂ to NO₂¯ to proceed (11.3.).

3.2.2. Breakthrough was assessed by analyzing both tubes and dividing the amount collected in the second solid-sorbent tube by the total amount collected in both sections.

Results: The amount of breakthrough is shown in Table 3. Breakthrough studies indicate the sorbent tube capacity for NO₂ is adequate for air concentrations at least to 21 ppm (using air volumes and flow rates described). Small amounts of NO₂ were detected on the backup tubes during both collection efficiency and breakthrough studies. This could be from contamination rather than actual breakthrough. Although sample results are blank corrected, blank readings can be variable (see Section 7 and Table 7 for further information regarding blanks). Regardless of blank contamination or breakthrough, the breakthrough recoveries for both studies are less than 5% and are considered acceptable.

4. Storage Stability

A study was conducted to determine any effects on storage of TEA-IMS samples containing known amounts of NO₂. A storage period of approximately 1 month was used. The procedure used is discussed below:

4.1. The determination was performed using SKC lot no. 306 tubes.

4.2. Twenty-four samples were generated at the OSHA Transitional PEL as described in the Introduction.

4.3. These samples were stored at 20 to 25 °C and were placed laboratory bench for the duration of the storage period.

4.4. Six samples were analyzed after 1, 5, 15, and 29 days.

Results: The results of the storage stability study are shown in Table 4. Collected samples are stable at room temperature. The mean of samples analyzed after 29 days was within ±5% of the mean of samples analyzed after one day. Samples may be stored in normal environmental conditions found in a laboratory setting for a period of 29 days after sampling without producing a significant change in results. 5. Humidity Study

Procedure: A study was conducted to evaluate any effects on recovery when sampling at different humidities. A contaminant flow conditioned at different relative humidities and a constant temperature of 25 °C was generated using the system described in the Introduction. Relative humidities of 30, 50, and 80% were used. SKC lot no. 374 tubes were used and six samples were generated at each humidity level.

Results: Results are shown in Table 5. Data from sampling at different humidities displayed no apparent effect on sampling efficiency. As shown in Table 5, an analysis of variance (F test) was performed on the data to determine if any significant difference existed in different humidity group results. The average recovery across the three different humidity levels was also considered. The calculated F value is below the critical value and a significant effect from humidity does not appear to exist. Evidence of a slight increase in average recovery is apparent with an increase in humidity. However, the increase is within the variability of the method and also does not appear as significant. Therefore, the humidity study did not reveal a significant difference in recoveries or variance when sampling at 30, 50, and 80% RH (25 °C).

6. Comparison of Analytical Methods

The IC method was compared to a reference method to determine if any significant disagreement existed between the 2 methods. The previous analytical method, the differential pulse polarographic (DPP) procedure (11.9.), was used as the reference analytical method. TEA-IMS samples were taken using the generation system described in the Introduction.

Procedure: Eighteen samples were generated and analyzed by IC. Since both analytical procedures use the same desorbing solution [(1.5% triethanolamine (TEA)], an aliquot was taken from each sample and analyzed by the polarographic method.

Results: A linear regression comparison of the two methods is shown in Figure 2 (the dotted line shown in Figure 2 represents ideal agreement between the two methods. The solid line represents the observed agreement). Results of the comparison between the IC and DPP method are also shown in Table 6. The comparison of the DPP and IC analytical methods show excellent correlation and agreement. The correlation coefficient (r) of 0.99 and a slope value of 1.0194 ± 0.0295 are very close to ideal values. An r and slope value equal to 1 would indicate ideal correlation and agreement between the two analytical methods. Over the concentration range tested the IC method results show an increase of 1.9% when compared to polarographic method results. The slightly higher recoveries of the IC procedure indicate that some of the bias noted (Section 2) can be attributed to the analytical portion of the method. The background levels inherent in the treated sorbent and erratic blank readings probably contribute to the positive bias also.

7. Analytical Detection Limits

Procedure: Qualitative and quantitative detection limits were determined by analyzing low concentration samples and blanks. The samples were prepared by spiking solutions containing 3 mL of 1.5% TEA with sodium nitrite solutions. The spiking was performed using a calibrated micropipette. Samples and blanks were analyzed using a 50 µL sample injection loop and a conductivity cell sensitivity range setting of 3 microsiemens.

7.1. Qualitative detection limit: The Rank Sum Test (11.10.) was used for the determination of the qualitative detection limit of the IC analysis of NO₂ (as nitrite).

7.2. Quantitative detection limit: The International Union of Pure and Applied Chemistry (IUPAC) detection limit equation (11.11.) was used to calculate the detection limit.

Results: The results are listed in Table 7 and graphically displayed in Figure 3. The qualitative detection limit is 0.07 ppm NO₂. The quantitative detection limit is 0.19 ppm NO₂. A 50 µL sample injection loop was used for all analyses in this evaluation. If necessary, a larger sample loop can be used to achieve a lower limit of detection. In the past, blank contamination was a serious problem and consequently caused high detection limits; blank levels were occasionally 0.5 to 1 times the Transitional PEL when using a 0.05 L/min flow rate for calculations. Soluble chloride salts can also elevate the detection limit. If the amount of chloride in the sample is large (>5 µg/mL), the nitrite ion appears as a shoulder on the chloride peak during IC analysis. Using the data reduction system described in Section 2 of the method (11.1.), the proposed factor for the conversion of NO₂ gas to the nitrite ion is concentration dependent. If the reaction is stoichiometric, a C.F. of 0.5 would be seen experimentally. In practice, however, this is not the case. For concentrations below 10 ppm, the average C.F. is 0.6 to 0.7 as reported by. Morgan et. al. (11.12.), in a previous study (11.9.), and by numerous others (11.5.). For concentrations of 0 to 10 ppm NO₂, a factor of 0.63 was adopted by OSHA (11.9.) and NIOSH (11.13.). The factor was not well defined at higher concentrations and needed further evaluation. The following procedure was used to experimentally determine the C.F. for concentrations greater than 10 ppm:

8.1. A cylinder of NO in nitrogen (Air Products Co., 1.05% NO) was used as the contaminant source. The rapid depletion of the NO₂ permeation tubes precluded their use for this experiment. The same generation system shown in Figure 1 was used with the gas cylinder replacing the permeation tubes as the contaminant source. The NO₂ was produced by flowing a diluted NO mixture through oxidizer sections, which converted the NO to NO₂ before collection.

8.2. The generation system was set at 50% RH and 25 °C.

8.3. Samples were taken using impingers containing 1.5% TEA solutions for variable time periods at different concentration ranges. These TEA solutions were used in an attempt to avoid any extraneous background contribution from solid sorbent desorption or intrinsic contamination from the tubes. Samples were taken at a flow rate of 0.025 L/min to assure complete oxidation of the NO and to provide sufficient residence time of NO₂ in the TEA solutions.

Results: The results for C.F. calculations from 10 to 200 ppm are listed in Table 8. Data in Table 8 show the conversion factors for NO₂ concentrations from 10 to 200 ppm. The conversion factor for the 10 to 100 ppm concentration range averaged 0.50; at about 200 ppm the factor was 0.37. Further work may be necessary to determine why the factor decreased at the 200 ppm level. Another study indicated no breakthrough of NO at this concentration (11.14.). Previous sample results and the toxicology of NO₂ indicate a 200 ppm NO₂ sample collected in an industrial setting is unlikely. A correction factor and further work at this concentration level was not pursued for these reasons. The conversion factor is further discussed in reference 11.14.

9. Sampling and Analysis - Supelco Tubes

A preliminary evaluation of the combination tube manufactured by Supelco was conducted using the same conditions and equipment mentioned in the Introduction. Samples were collected using the procedure mentioned in Section 2. Results are listed in Table 9. This data indicates a sample loss of approximately 30% when sampling at approximately 0.2 L/min. The loss could be associated with a difference in mesh size (Supelco tubes contain a smaller mesh molecular sieve than SKC tubes), flow rate differences or a poorly prepared lot. The original methodology for sampling NO₂/NO with this type of tube specified a flow rate of less than or equal to 0.05 L/min. The four-fold increase in flow rate may be causing premature breakthrough. The residence time of the sampled gas may not be sufficient at 0.2 L/min for this tube.

10. Discussion

Two different lots of SKC tubes were used for the evaluation. The combination tube consisting of all three sections in a single tube (lot no. 306) was commercially available at the beginning of the evaluation. This tube was used for the sampling and analysis, collection efficiency, and storage stability experiments. Design changes were instituted and a three tube collection device was produced to offer greater convenience when sampling NO₂ or both NO and NO₂ simultaneously. The three-tube collection device, lot no. 374, was used for the remaining studies. The two SKC devices are identical except the sorbent and oxidant are contained in three separate glass tubes for the three tube device.

The data generated during the evaluation of the method indicates an acceptable alternative to the polarographic method. The ion chromatographic method offers an accurate and precise determination of NO₂ exposures. A concentration-dependent conversion factor is required in calculations and the molecular sieve solid sorbent must be water-washed before impregnation and tube packing.

11. References

11.1. Occupational Safety and Health Administration Technical Center: Nitrogen Dioxide in Workplace Atmospheres (Ion Chromatography), by J.C. Ku (USDOL/OSHA-SLTC Method No. ID-182). Salt Lake City, UT. Revised 1991.

11.2. Saltzman, B.E.: Colorimetric Microdetermination of Nitrogen Dioxide in the Atmosphere. Anal. Chem. 26:1949 (1954).

11.3. Gold, A.: Stoichiometry of Nitrogen Dioxide Determination in Triethanolamine Trapping Solution. Anal. Chem. 49:1448-50 (1977).

11.4. Blacker, J.H.: Triethanolamine for Collecting Nitrogen Dioxide in the TLV Range. Am. Ind. Hyg. Assoc. J. 34:390 (1973).

11.5. Vinjamoori, D.V. and Chaur-Sun Ling: Personal Monitoring Method for Nitrogen Dioxide and Sulfur Dioxide with Solid Sorbent Sampling and Ion Chromatographic Determination. Anal. Chem. 53:1689-1691 (1981).

11.6. Occupational Safety and Health Administration Analytical Laboratory: Precision and Accuracy Data Protocol for Laboratory Validations. In OSHA Analytical Methods Manual. Cincinnati, OH: American Conference of Governmental Industrial Hygienists (Pub. No. ISBN: 0-936712-66-X), 1985.

11.7. National Institute for Occupational Safety and Health: Documentation of the NIOSH Validation Tests by D. Taylor, (DHEW/NIOSH Pub. No. 77-185). Cincinnati, OH, 1977.

11.8. Mandel, J. In Treatise on Analytic Chemistry. 2nd ed. Kolthoff, I.M. and Elving, P.J., ed. New York: John Wiley and Sons, Inc., 1978. p 282.

11.9. Occupational Safety and Health Administration Analytical Laboratory: OSHA Analytical Methods Manual (USDOL/OSHA-SLCAL Method No. ID-109). Cincinnati, OH: American Conference of Governmental Industrial Hygienists (Pub. No. ISBN: 0-936712-66-X), 1985.

11.10. Dixon, W.J. and F.J. Massey, Jr.: Introduction to Statistical Analysis. 2nd ed. New York: McGraw-Hill Book Co., 1957. pp 289-292.

11.11. Analytical Methods Committee: Recommendations for the Definition, Estimation and Use of the Detection Limit. Analyst 112(2):199-204 (1987).

11.12. Morgan, G.B., C. Golden, and E.C. Tabor: "New and Improved Procedures for Gas Sampling and Analysis in the National Air Sampling Network" Paper presented at the 59th Annual Meeting of the Air Pollution Control Association, San Francisco, CA, 1966.

11.13. National Institute for Occupational Safety and Health: NIOSH Manual of Analytical Methods by D. Taylor, (DHEW/NIOSH Pub. No. 78-175). Cincinnati, OH, 1978. Method no. S320.

11.14. Occupational Safety and Health Administration Technical Center: Nitric Oxide Backup Data Report (ID-190) by J.C. Ku. Salt Lake City, UT. Revised 1991.

Table 1
Analysis*
Nitrogen Dioxide

Level**	------0.5 × PEL-----			------ 1 × PEL-----			------ 2 × PEL-----
	µg taken	µg found	DE	µg taken	µg found	DE	µg taken	µg found	DE

	12.35 15.48 12.59 15.72 13.26 12.28 12.08 14.97	12.50 16.35 13.11 16.80 13.90 13.10 12.55 16.46	1.01 1.06 1.04 1.07 1.05 1.07 1.04 1.10	23.78 29.82 27.44 25.25 24.88 31.10	24.38 24.12 29.31 27.00 26.33 33.36	1.03 *** 1.07 1.07 1.06 1.07	58.36 53.06 52.14 65.68 57.08 52.33	60.68 54.42 54.95 68.36 61.73 54.91	1.04 1.03 1.05 1.04 1.08 1.05

	n Mean Std Dev CV₁		8 1.06 0.027 0.025			5 1.06 0.017 0.016			6 1.05 0.017 0.016

			CV₁ (pooled) = 0.021 Ave. DE = 1.06

DE = Desorption efficiency * SKC tubes, lot no. 374, were used Transitional PEL of 5 ppm NO₂ was used * Excluded from statistical analysis as an outlier

Table 2
Sampling and Analysis*
Nitrogen Dioxide

Test Level**	Found µg	Air Vol (L)	Found ppm	Taken ppm	Recovery (in %)

0.5 × PEL	14.46 12.32 10.59 14.37 16.03 15.02	2.59 2.23 1.94 2.62 2.81 2.67	2.97 2.94 2.90 2.92 3.03 2.99	2.64 2.64 2.64 2.64 2.64 2.64	113 111 110 111 115 113

			n Mean Std Dev CV₂		6 112 1.8 0.016

1 × PEL	28.77 23.61 21.01 27.59 28.24 29.17	2.59 2.23 1.94 2.62 2.81 2.67	5.90 5.63 5.76 5.60 5.34 5.81	5.06 5.06 5.06 5.06 5.06 5.06	117 111 114 111 106 115

			n Mean Std Dev CV₂		6 112 3.9 0.035

2 × PEL	56.83 46.99 38.21 53.06 55.53 54.39	2.59 2.23 1.94 2.62 2.81 2.67	11.66 11.20 10.47 10.76 10.50 10.83	9.45 9.45 9.45 9.45 9.45 9.45	123 119 111 114 111 115

			n Mean Std Dev CV₂		6 115 4.8 0.042

CV₂ (pooled) Ave. Recovery	= 0.033 = 113%	CV_T (pooled) Overall Error	= 0.034 = ±19.8%

* SKC tubes, lot no. 306, were used ** Transitional PEL of 5 ppm NO₂ was used

Table 3
Collection Efficiency
(25 °C and 50% RH)

	------ µg NO₂ Found -----------
Sample No.	First Section	Second Section	% Collection Efficiency

1 2 3 4 5 6	56.83 46.99 38.21 53.06 55.53 54.39	2.32 ND ND 2.19 1.98 2.30	96.1 100.0 100.0 96.0 96.6 95.9

Average			97.4

Note:	(1)	SKC tubes, lot no. 306, were used
	(2)	Sampling rate Sampling time	= =	0.2 L/min 15 min
	(3)	Concentration	=	approximately 2 times OSHA Transitional PEL
	(4)	ND	=	None detectable < 0.24 µg NO₂¯ (3-mL sample volume)

Breakthrough Study
(25 °C and 30% RH)

	------ µg NO₂ Found -----------
Sample No.	1st Tube	2nd Tube	% Breakthrough

1 2 3 4	103.8 104.5 105.1 103.2	3.34 ND 3.31 ND	3.1 0 3.1 0

Average			1.6

Note:	(1)	1st and 2nd tube	=	SKC tubes, lot no. 374, were used
	(2)	Sampling rate Sampling time	= =	0.175 L/min 15 min
	(3)	Generation concentration	=	21 ppm NO₂
	(4)	ND	=	None detectable < 0.24 µg NO₂¯ (3-mL sample volume)

Table 4
Storage Stability Test*
Nitrogen Dioxide

Storage Day	Found µg	Air Vol (L)	Found ppm	Taken ppm	Recovery (%)

Day 1	28.77 23.61 21.01 27.59 28.24 29.17	2.59 2.23 1.94 2.62 2.81 2.67	5.90 5.63 5.76 5.60 5.34 5.81	5.06 5.06 5.06 5.06 5.06 5.06	117 111 114 111 106 115

			n Mean Std Dev CV	6 112 3.9 0.035

Day 5	25.74 23.56 20.69 26.52 29.32 28.41	2.61 2.23 1.93 2.60 2.72 2.61	5.24 5.61 5.70 5.42 5.73 5.79	5.04 5.04 5.04 5.04 5.04 5.04	104 111 113 108 114 115

			n Mean Std Dev CV	6 111 4.2 0.038

Day 15	24.56 22.64 20.56 28.27 29.50 28.69	2.61 2.23 1.93 2.60 2.72 2.61	5.00 5.40 5.66 5.78 5.76 5.84	5.04 5.04 5.04 5.04 5.04 5.04	99.2 107 112 115 114 116

			n Mean Std Dev CV	6 111 6.4 0.058

Day 29	27.34 23.95 23.66 27.58 28.41 31.92	2.61 2.23 1.93 2.60 2.72 2.61	5.57 5.71 6.52 5.64 5.55 6.50	5.04 5.04 5.04 5.04 5.04 5.04	111 113 129 112 110 129

			n Mean Std Dev CV	6 117 9.2 0.079

* SKC tubes, lot no. 306 were used

Table 5
Relative Humidity Test (25 °C)*
Generated NO₂ Concentration = 2.64 ppm

RH, %		34	50	80

NO₂ Found, ppm		2.74 2.73 2.65 3.11 2.73 2.77	2.79 2.94 2.90 2.92 3.03 2.99	2.88 2.81 2.79 2.91 3.10 2.86

	n Mean, ppm Std Dev, ppm CV Recovery	6 2.79 0.16 0.058 106%	6 2.93 0.083 0.028 111%	6 2.89 0.11 0.038 109%

F test results: F_calc = 2.078, F_crit = 6.36, p < 0.01 * SKC tubes, lot no. 374, were used

Table 6
Comparison of Methods*
[Ion Chromatographic (IC) vs. Polarographic (DPP)]

0.5 × PEL** -- ppm Found --			1 × PEL** -- ppm Found --			2 × PEL** -- ppm Found --
IC	DPP	RR	IC	DPP	RR	IC	DPP	RR

3.41 3.43 3.57 3.40 3.46 3.39	3.17 3.19 3.27 3.19 3.33 2.82	1.076 1.075 1.092 1.066 1.039 ***	5.93 6.21 6.21 5.95 5.88 6.15	5.55 6.03 5.91 5.61 5.58 6.00	1.068 1.030 1.051 1.061 1.054 1.025	10.11 10.20 10.73 10.33 10.11 11.96	10.11 10.39 10.23 10.09 10.03 10.49	1.000 0.982 1.049 1.024 1.008 1.140

n Mean Std Dev CV		5 1.070 0.020 0.018			6 1.048 0.017 0.016			6 1.034 0.057 0.055

* SKC tubes, lot no. 374, were used Transitional PEL of 5 ppm NO₂ was used * Excluded from statistical analysis as an outlier RR = Relative ratio, IC Found (ppm)/DPP Found (ppm)

Linear Regression Comparison (also see Figure 2)
	Correlation coefficient (r)	= 0.9938
	Slope (b)	= 1.0194
	Intercept (a)	= 0.1587
	Std dev of slope (Sb)	= 0.0295

Table 7
Qualitative Detection Limit - Nitrogen Dioxide
Rank Sum Test
For n(s) = n(b) = 10

------------------------- NO₂ -- (as nitrite) ---------------------------

Rank

0.08 µg/mL
Peak Area

0.16 µg/mL
Peak Area

0.32 µg/mL
Peak Area

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

0.50
0.57
0.70
0.72
0.83
1.03
1.05
1.13
1.13
1.16
1.86
1.89
1.89
1.89
1.93
1.99
2.10
2.10
2.16
2.18

RBL
RBL
RBL
RBL
RBL
RBL
RBL
RBL
RBL
RBL
STD
STD
STD
STD
STD
STD
STD
STD
STD
STD

0.50
0.57
0.70
0.72
0.83
1.03
1.05
1.13
1.13
1.16
2.14
2.29
2.41
2.46
2.58
2.77
2.79
2.83
2.90
2.93

RBL
RBL
RBL
RBL
RBL
RBL
RBL
RBL
RBL
RBL
STD
STD
STD
STD
STD
STD
STD
STD
STD
STD

0.50
0.57
0.70
0.72
0.83
1.03
1.05
1.13
1.13
1.16
4.02
4.15
4.33
4.49
4.61
4.64
4.67
4.78
4.81
4.96

RBL
RBL
RBL
RBL
RBL
RBL
RBL
RBL
RBL
RBL
STD
STD
STD
STD
STD
STD
STD
STD
STD
STD

Rb =

C =

55

99.9%

Qualitative detection limit for nitrogen dioxide = 0.08 µg/mL or 0.24 µg (3-mL sample volume). This corresponds to a concentration of 0.07 ppm NO₂ for a 3-L air volume.

Note:	(1) RBL	= Reagent Blank
	(2) STD	= Standard
	(3) Peak Area	= measured peak area/100,000

Table 7 (Cont.)
Quantitative Detection Limit - Nitrogen Dioxide (as NO₂¯)

Sample No.	Blank Peak Area	0.08 µg/mL Peak Area	0.16 µg/mL Peak Area	0.32 µg/mL Peak Area

1 2 3 4 5 6 7 8 9 10	0.50 0.57 0.70 0.72 0.83 1.03 1.05 1.13 1.13 1.16	1.86 1.89 1.89 1.89 1.93 1.99 2.10 2.10 2.16 2.18	2.14 2.29 2.41 2.46 2.58 2.77 2.79 2.83 2.90 2.93	4.02 4.15 4.33 4.49 4.61 4.64 4.67 4.78 4.81 4.98

n Mean Std Dev CV	10 0.88 0.25 0.282	10 2.00 0.12 0.062	10 2.61 0.27 0.105	10 4.55 0.30 0.067

Peak Area = measured peak area/100,000 The quantitative detection limit is calculated using the equation:

C_ld = k(sd)/m		C_ld = 10(0.248)/10.83 = 0.23 µg/mL

Where:
C_ld =	the smallest reliable detectable concentration an analytical instrument can determine at a given confidence level
k =	10, thus giving confidence that any detectable signal will be greater than or equal to an average blank reading plus ten times the standard deviation (area reading > Bl_ave + 10sd)
sd =	standard deviation of blank readings
m =	analytical sensitivity or slope as calculated by linear regression Quantitative detection limit = 0.23 µg/mL (as nitrite) or 0.69 µg (3-mL sample volume). This corresponds to 0.19 ppm NO₂ for a 3-L air volume.

Table 8
Nitrogen Dioxide Conversion factor

NO₂ ppm	Samples	Std Dev	CV	Mean*

12.89 25.20 49.79 97.90 192.57	7 7 6 6 7	0.038 0.037 0.022 0.020 0.025	0.074 0.070 0.043 0.044 0.068	0.519 0.533 0.517 0.450 0.368

* Average conversion factor. This was calculated from sample results and assumed a 100% recovery.

Table 9
Sampling and Analysis (Supelco Tubes)*
Nitrogen Dioxide

Test Level**	Found µg	Air Vol (L)	Found ppm	Taken ppm	Statistics

0.5 × PEL	5.80 6.25 7.71 8.52 5.06 8.71	2.19 1.90 1.71 2.24 2.40 2.56	1.41 1.75 2.40 2.02 1.12 1.81	2.62 2.62 2.62 2.62 2.62 2.62

				n Mean Std Dev CV	6 66.9% 17.0 0.26

1 × PEL	15.91 12.73 11.45 15.82 16.27 18.94	2.19 1.90 1.71 2.24 2.40 2.56	3.86 3.56 3.56 3.75 3.60 3.93	5.08 5.08 5.08 5.08 5.08 5.08

				n Mean Std Dev CV	6 73.0% 3.2% 0.043

2 × PEL	30.11 22.77 22.48 28.63 31.14 34.04	2.19 1.90 1.71 2.24 2.40 2.56	7.31 6.35 6.99 6.79 6.90 7.07	9.66 9.66 9.66 9.66 9.66 9.66

				n Mean Std Dev CV	6 71.4% 3.3% 0.047

* Supelco tubes, Lot No. 582-99, were used ** Transitional PEL of 5 ppm NO₂ was used

CV(pooled) = 0.15 Ave. Recovery = 70.4% Overall Error = ±59.6%

A block diagram of the major components of the dynamic generation system is shown below. The system consists of four essential elements, a flow, temperature and humidity control system, a nitrogen dioxide vapor generating system, a mixing chamber and an active sampling manifold.

Figure 1

Linear Regression Comparison
Ion Chromatographic vs. Polarographic Analysis of Nitrogen Dioxide

Dotted Line = Ideal Agreement Between Methods
Solid Line = Found Agreement Between Methods

Figure 2

Detection Limit

Figure 3