1.1. History

Previous methods involved oxidation of NO to nitrogen dioxide (NO₂) using a chromate compound and subsequent conversion of NO₂ to nitrite using triethanolamine-impregnated molecular sieve (TEA-IMS) sampling tubes. Common methods used a combination sampling tube and NO was determined colorimetrically (as NO₂^-) using a modified Griess-Saltzman reaction (8.1.-8.2.). This method, like most colorimetric procedures, can have significant interferences.

A differential pulse polarographic (DPP) method (8.3.) was later developed to improve analytical sensitivity and decrease the potential for interferences. The sensitivity of the DPP method was more than adequate for measuring workplace concentrations of NO; however, the nitrite ion is unstable in the pH range (pH 1-2) used during analysis (8.4.).

Method no. ID-190 uses the TEA-IMS sampling tube/chromate oxidizer approach. Samples are analyzed by IC.

1.2. Principle

A known volume of air is drawn through the sampling device which captures any nitrogen dioxide (NO₂) in the sampled air and also converts any NO to nitrite ion (NO₂^-). The sampling device consists of three glass tubes connected in series. The front and back tubes contain TEA-IMS, the middle or oxidizer tube contains an inert carrier impregnated with a chromate salt. The first TEA-IMS tube does not capture NO; this tube is only used to capture and convert to NO₂^- any NO₂ present in the sampled air. The middle tube oxidizes the sampled NO to NO₂. The back TEA-IMS tube then captures and converts this NO₂ to NO₂^-. Both TEA-IMS samples are desorbed using an aqueous triethanolamine (TEA) solution and analyzed as NO₂^- by IC. The front tube analytical results are reported as NO₂ and the back tube as NO.

The conversion mechanism of NO₂ gas to NO₂^- has been proposed by Gold (8.5.). The following is Gold's proposal for the reaction of equivalent amounts of NO₂ and TEA in an aqueous solution:

2NO2<=> N2O4 N2O4 + (HOCH2CH2)3N --> (HOCH2CH2)3NNO+NO3- (HOCH2CH2)3NNO+NO3- + H2O --> (HOCH2CH2)3NH+NO3- + HNO2 HNO2 --> H+ + NO2-

Nitrogen dioxide disproportionates to NO₂^- and nitrate (NO₃^-) in the presence of TEA and water. The NO₂^- formed from the above reaction can be analyzed via conventional analytical methods (8.1.-8.4., 8.6.-8.7.) including IC. Unfortunately NO₃^- is found in the commercial TEA-IMS sorbent as a significant contaminant. This contamination ruled out further research to also measure this NO₂-TEA disproportionation product by IC.

This reaction path requires a stoichiometric factor of 0.5 for the conversion of
gaseous NO₂ to NO₂^-. Experiments indicate the stoichiometric factor of 0.5 is seen only when NO₂ concentrations are greater than 10 ppm (8.5., 8.8.-8.9.). The conversion factor has been experimentally determined to average approximately 0.6 to 0.7 when concentrations are below 10 ppm (8.1.-8.3., 8.5.-8.9.). The deviation from ideal stoichiometry is believed to be due to competing reactions; however, evidence to support a competing mechanism has not been found (8.5.).

1.3. Advantages and Disadvantages

1.3.1. This method has adequate sensitivity for determining compliance with the OSHA Time Weighted Average (TWA) Permissible Exposure Limit (PEL) for workplace exposures to NO.

1.3.2. The sampling device can be used to simultaneously collect NO and NO₂; however, results for NO₂ may not reflect short-term exposures (see Section 5.2. for more details).

1.3.3. The analysis is simple, rapid, easily automated and is specific for NO₂^-.

1.3.4. After analytical sample preparation, NO exposures (as nitrite ion) can also be determined by colorimetric or polarographic analytical techniques (8.1.-8.3.).

1.3.5. A disadvantage is the potential interference from large amounts of soluble chloride salts present in commercial molecular sieve. Prior to TEA impregnation, the molecular sieve should be washed with deionized water (DI H₂O) to remove any soluble chloride salts.

1.3.6. Another disadvantage is the need for a concentration-dependent conversion factor when calculating results.

1.4. Physical properties (8.10., 8.11.)

Nitric oxide (CAS No. 10102-43-9), one of several oxides of nitrogen, is a colorless gas. A deep blue color is usually noted when NO is in the liquid state and a bluish-white color when solid. Other physical characteristics of NO are:

Formula weight	30.01
Specific gravity	1.27 at -150.2 °C (as liquid)
Melting point	-163.6 °C
Boiling point	-151.8 °C
Vapor density	1.04 (air = 1)
Solubility	4.6 mL NO in 100 mL H2O
Synonyms	nitrogen monoxide*,
	mononitrogen monoxide

*Nitrogen monoxide has also been used as a synonym for nitrous oxide (N₂O).

1.5. Some industrial sources for potential nitric oxide exposures are:

agricultural silos
arc or gas welding (esp. confined space operations)
electroplating plants
food and textile bleaching
jewelry manufacturing
metal nitrosyl carbonyl production
nitric acid production
nitrogen fertilizer production
nitro-explosive production
nitrosyl halide production
pickling plants

Nitrogen dioxide and nitric oxide usually exist together in industrial settings. Nitric oxide is reactive in air and produces NO₂ according to the following equations (8.10.):

2NO + O2 ----> 2NO2 d(NO2)/dt= K(O2)(NO)2

(K is a temperature dependent constant. At 20 °C, K = 14.8 X 10⁹)

An experimental approximation of the NO/NO₂ distribution found in various industrial operations is shown (8.10.):

Source	% NO2	% NO
Carbon arc	9	91
Oxyacetylene torch	8	92
Cellulose nitrate combustion	19	81
Diesel exhaust	35	65
Dynamite blast	52	48
Acid dipping	78	22

The potential for exposure to both NO₂ and NO should be considered because NO is easily oxidized to NO₂ and both oxides are likely to coexist in industrial settings.

1.6. Toxicology (8.11.-8.14.)

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Information listed within this section is a synopsis of current knowledge of the physiological effects of nitric oxide and is not intended to be used as a basis for OSHA policy.

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1.6.1. Nitric oxide is classified as a respiratory irritant. The main route of exposure is inhalation; however, physiological damage can also occur eyes or skin.

The term "silo-fillers' disease" has been used to describe exposure to nitric as well as other nitrogen oxides. The national population-at-risk for exposure to nitrogen oxides has been estimated by NIOSH to be approximately 950,000 employees (National Occupational Hazard Survey, 1972-74). When encountering either NO or NO₂ at high concentrations, both species will usually be present. Little scientific data is available regarding exposures to NO only. The majority of collected data concerns exposure to NO₂ since NO appears to be only one-fifth as toxic as NO₂ at low concentrations. Symptoms immediately following NO exposure are usually mild or not apparent. Severe symptoms may not appear up to 72 hours after exposure.

1.6.2. Mild exposures to NO can result in symptoms such as:

cough	shortness of breath
painful breathing	chest pains
increased breathing rate	weakness
methemoglobinemia

More severe exposures (>100 ppm) are characterized by pulmonary edema , cyanosis, pneumonia, severe methemoglobinemia, respiratory failure, and death.

1.6.3. The IDLH (Immediately Dangerous to Life or Health) concentration is 100 ppm NO. The LCLo (Lethal Concentration - Low) for inhalation by mice is 320 ppm.

1.6.4.Mechanism for toxicity:

Nitric oxide is slightly soluble in water and forms nitrous and nitric acid. This reaction occurs with lung tissue and produces respiratory irritation and edema. Alkali present in the lung tissue neutralizes the nitrous and nitric acids to nitrite and nitrate salts which are then absorbed into the bloodstream. The end result is the formation of nitroxy-hemoglobin complexes and methemoglobin in the circulatory system.

The formation of hemoglobin complexes is thought to contribute to the toxicity of NO but is not considered to be the sole source of the toxic reaction. The respiratory damage from nitrous and nitric acid appears to be more significant.

4.1. When other compounds are known or suspected to be present in the sampled air, such information should be transmitted to the laboratory with the sample.

4.2. Any compound that has the same retention time as nitrite, when using the operating conditions described, is an interference.

4.3. Interferences may be minimized by changing the eluent concentration, column characteristics, and/or pump flow rate.

4.4. If there is an unresolvable interference, alternate polarographic or colorimetric methods may be used (8.1.-8.3.).

4.5. Contaminant anions normally found in molecular sieve, such as NO₃^-, SO₄^2-, and PO₄^3-, do not interfere. Large amounts (greater than 4 to 5 µg/mL) of Cl^- can interfere.

5.1. Equipment

5.1.1. A three tube sampling device is commercially available (NO/NO₂ sampling tubes, Cat. No. 226-40-special order, water-washed, SKC, Eighty Four, PA) and can be used to simultaneously sample NO₂ and NO, or sample only NO₂. This device consists of three flame-sealed glass tubes:

1) Nitrogen dioxide is collected in the first tube which contains 400 mg TEA-IMS.
2) The second (oxidizer) tube converts NO to NO₂ and contains approximately 1 g of a chromate compound impregnated on an inert carrier.
3) The last 400 mg TEA-IMS packed tube collects the converted NO₂.

All molecular sieve used for tube packing must be washed with DI H₂O before impregnation with TEA. The dimensions of each TEA-IMS tube are 7-mm o.d., 5-mm i.d., and 70-mm long. A 3-mm portion of silylated glass wool is placed in the front and rear of each tube. The dimensions of the oxidizer tube are 7-mm o.d., 5-mm i.d., and 110-mm long .

When the three tubes are connected in series as shown below, NO₂ and NO can be collected simultaneously. The first TEA-IMS tube must be in place to prevent the collection of NO₂ by the second TEA-IMS tube.

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THREE-TUBE SAMPLING DEVICE

Text Version: The first tube in the Three-Tube Sampling Device is a nitrogen dioxide (NO2) sampling tube (TEA-IMS Tube). The second tube in the series is an oxidizer tube, and the third is another NO2 sampling tube that is identical to the first tube. The three tubes are connected with short lengths of plastic tubing (Tygon or equivalent). The three tubes should be connected as close to one another as possible. The sampling device is connected to the sampling pump with flexible plastic tubing. The set of three tubes that compose the sampling device is available from SKC, Inc. as catalog 226-40.

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5.1.2. Personal sampling pumps capable of sampling at a flow rate of approximately 0.025 L/min are used.

5.1.3. A stopwatch and bubble tube or meter are used to calibrate pumps. A sampling device is placed in-line during flow rate calibration.

5.1.4. Various lengths of Tygon tubing are used to connect the sampling tubes and pump together.

5.2. Sampling Procedure

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Note: If sampling for both NO₂ and NO is necessary, two separate pumps and sampling devices should be used. The differences in OSHA exposure limits [the NO₂ PEL is a 1 ppm Short-Term Exposure Limit (8.15.). Nitric oxide is a TWA PEL.] and flow rates dictates a need for a separate assessment of NO₂. Nitric oxide is collected at a 0.025 L/min pump flow rate. Nitrogen dioxide can be collected at this flow rate; however, a longer sampling time will be necessary to collect a detectable amount of NO₂ than for a short-term measurement. Concentrations of NO₂ may vary in the workplace during a longer sampling period.

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5.2.1. Calibrate the sampling pumps to a flow rate of 0.025 L/min.

5.2.2. Connect the sampling device to the pump. The different sampling schemes are listed:

a) Sampling for NO₂ only: - Use a single TEA-IMS tube (8.8.).
b) Sampling for both NO and NO₂: The three-tube device is used. The device must be assembled as shown above.
Label the first tube "NO₂".
Label the tube following the oxidizer section "NO".

5.2.3. Place the sampling tube or device in the breathing zone of the employee.

5.2.4. Collect the sample at the listed flow rates and sampling times:

a) For NO₂ only: 0.200 L/min for at least 15 min (8.8.) per sample.
b) For both NO and NO₂: 0.025 L/min for 4 h per sample (Note: The front ube of the three-tube device can be submitted for NO₂ analysis; however, analytical results may not represent short-term exposures).

5.2.5. The maximum recommended air volume is 6 L per NO sample.

Take enough samples for NO to cover the workshift.

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Note: One oxidizer tube per sample is sufficient for concentration ranges of NO usually encountered in industrial settings. A color change from orange to blue-green will be noticeable if the oxidizer is depleted.

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6.1. Precautions

6.1.1. Refer to instrument and standard operating procedure (SOP) (8.16.) manuals for proper operation.

6.1.2. Observe laboratory safety regulations and practices.

6.1.3. Sulfuric acid (H₂SO₄) can cause severe burns. Wear protective eyewear, gloves, and labcoat when using concentrated H₂SO₄.

6.2. Equipment

6.2.1. Ion chromatograph (Model 2010 or 4000, Dionex, Sunnyvale, CA) equipped with a conductivity detector.

6.2.2. Automatic sampler (Model AS-1, Dionex) and 0.5 mL sample vials.

6.2.3. Laboratory automation system: Ion chromatograph interfaced to a data reduction and control system (AutoIon 400 or 450, Dionex).

6.2.4. Micromembrane suppressor, anion (Model AMMS-1, Dionex).

6.2.5. Separator and guard columns, anion (Model HPIC-AS4A and AG4A, Dionex).

6.2.6. Disposable syringes (1 mL) and filters.

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Note: Some syringe pre-filters are not cation- or anion-free. Tests should be done with blank solutions first to determine suitability for the analyte being determined.

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6.2.7. Erlenmeyer flasks, 25-mL, or scintillation vials, 20-mL.

6.2.8. Miscellaneous volumetric glassware: Micropipettes, volumetric flasks, graduated cylinders, and beakers.

6.2.9. Analytical balance (0.01 mg).

6.3. Reagents - All chemicals should be at least reagent grade.

6.3.1. Deionized water (DI H₂O) with a specific conductance of less than 10 microsiemens.

6.3.2. Triethanolamine [(HOCH₂CH₂)₃N]
sodium carbonate (Na₂CO₃)
sodium bicarbonate (NaHCO₃)
sulfuric acid (H₂SO₄, concentrated 95 to 98%)
sodium nitrite (NaNO₂)

6.3.3. Liquid desorber (1.5% TEA):
Dissolve 15 g TEA in a 1-L volumetric flask which contains approximately 500 mL DI H₂O. Add 0.5 mL n-butanol and then dilute to volume with DI H₂O.

6.3.4. Eluent (2.0 mM Na₂CO₃/1.0 mM NaHCO₃):
Dissolve 0.848 g Na₂CO₃ and 0.336 g NaHCO₃ in 4.0 L DI H₂O.

6.3.5. Regeneration solution (0.02 N H₂SO₄):
Place 1.14 mL concentrated H₂SO₄ into a 2-L volumetric flask which contains about 500 mL DI H₂O. Dilute to volume with DI H₂O.

6.3.6. Nitrite stock standard (1,000 µg/mL):
Dissolve 1.5000 g NaNO₂ and dilute to the mark with DI H₂O in a 1-L volumetric flask. Prepare every three months.

6.3.7. Nitrite standard (100 µg/mL):
Dilute 10 mL of 1,000 µg/mL nitrite stock standard to 100 mL with liquid desorber. Prepare monthly.

6.3.8. Nitrite standard (10 µg/mL):
Dilute 10 mL of 100 µg/mL nitrite stock standard to 100 mL with liquid desorber. Prepare weekly.

6.3.9. Nitrite standard (1 µg/mL):
Dilute 10 mL of 10 µg/mL nitrite stock standard to 100 mL with liquid desorber. Prepare daily.

6.4. Working Standard Preparation

6.4.1. Nitrite working standards (10-mL final volumes) may be prepared in the ranges specified below:

Working Std	Standard	Aliquot
µg/mL	Solution µg/mL	mL
0.5	1	5
1	1	*
3	10	3
6	10	6
10	10	*
30	100	3
50	100	5

* Already prepared in Section 6.3.

6.4.2. Pipette appropriate aliquots of standard solutions (prepared in Section 6.3.) into 10-mL volumetric flasks and dilute to volume with liquid desorber.

6.4.3. Pipette a 0.5- to 0.6-mL portion of each standard solution into separate automatic sampler vials. Place a 0.5-mL filter cap into each vial. The large exposed filter portion of the cap should face the standard solution.

6.4.4. Prepare a reagent blank from the liquid desorber solution.

6.5. Sample Preparation

6.5.1. Identify which tube is the collected NO₂ sample and which is NO. Analyze these two tubes as separate samples.

6.5.2. Discard the oxidizer tube appropriately. This tube contains a chromate salt and may be considered a hazardous waste. Local regulations or restrictions should be consulted before disposal.

6.5.3. Clean the 25-mL Erlenmeyer flasks or scintillation vials by rinsing with DI H₂O.

6.5.4. Carefully remove the glass wool plugs from the sample tubes, making sure no sorbent is lost in the process. Transfer each TEA-IMS section to individually labeled 25-mL Erlenmeyer flasks or scintillation vials.

6.5.5. Add 10 mL of liquid desorber to each flask containing NO samples, shake vigorously for about 30 s. Allow the solution to stand for at least 1 h. (Note: Add 3 mL to NO₂ samples - see reference 8.8. for further details regarding NO₂ analysis and result calculations)

6.5.6. If the sample solutions contain suspended particulate, remove the particles using a pre-filter and syringe. Fill the 0.5-mL automatic sampler vials with sample solutions and push a filtercap into each vial. Label the vials.

6.5.7. Load the automatic sampler with labeled samples, standards and blanks.

6.6. Analytical Procedure

Set up the ion chromatography and analyze the samples and standards in accordance with the SOP (8.16.). Typical operating conditions for equipment mentioned in Section 6.2. are listed below.

Ion chromatograph
Eluent:	2.0 mM Na2CO3/1.0 mM NaHCO3
Column temperature:	ambient
Sample injection loop:	50 µL
Pump
Pump pressure:	approximately 1,000 psi
Flow rate:	2 mL/min
Chromatogram
Run time:	6 min
Average retention time:	approximately 2 min

7.1. Obtain hard copies of chromatograms from a printer. A typical chromatogram is shown in Figure 1.

7.2. Prepare a concentration-response curve by plotting the concentration of the standards in µg/mL (or µg/sample if the same volumes are used for samples and standards) versus peak areas or peak heights. Calculate sample concentrations from the curve and blank correct all samples.

7.3. The concentration of NO in each air sample is expressed in ppm and is calculated as:

ppm NO =

MV × µg/mL NO2¯ × solution volume × conversion × GF formula weight × air volume

Where:

MV (Molar Volume)	=	24.45 (25 °C and 760 mmHg)
µg/mL NO2	=	blank corrected sample result
Conversion [NO2 (gas)/NO2-]	=	varies with concentration*
GF (Gravimetric factor NO/NO2)	=	0.6522
Formula Weight (NO)	=	30.01

*The conversion of gaseous NO₂ to NO₂^- is concentration dependent. The final concentration of NO should be calculated using whichever example given below is appropriate:

Below 10 ppm NO From 0 to 10 ppm, the average ratio has been experimentally determined to be (8.1.-8.3., 8.5.-8.9.): 1 µg NO2 (gas) = 0.630 µg NO2- or conversely: 1 µg NO2- = 1.587 µg NO2 (gas) Simplifying the equation and calculating the ppm NO using a 10-mL sample volume gives:   ppm NO = µg/mL NO2¯ × 10 mL × 0.843 air volume, L Above 10 ppm NO Above 10 ppm NO, the expected stoichiometric factor of 0.5 mole of nitrite to 1 mole of nitrogen dioxide gas is seen (8.5., 8.8.-8.9.). Therefore, the following calculation should be used for sample results above 10 ppm and a 10-mL solution volume:   ppm NO = µg/mL NO2¯ × 10 mL × 1.063 air volume, L

7.4. Reporting Results

Report all results to the industrial hygienist as ppm nitric oxide.