Acrylonitrile sampling methods

Under 1910.1045AppD the primary sampling media recommended are activated charcoal tubes (two-section tubes) analyzed by gas chromatography after desorption with methanol. See Appendix D to 1910.1045: Sampling and Analytical Methods for Acrylonitrile for the detailed method and 1910.1045 for the standard requirements.

• Appendix D describes glass charcoal tubes with a 100 mg adsorbing section and a 50 mg backup section, separated by urethane foam, with glass wool in front.
• The method also notes alternatives (porous polymer tubes and passive dosimeters) and several validated variations of the charcoal-tube/GC approach.

Under 1910.1045AppD you should sample at 0.20 liter per minute or less and use a maximum sample size of 20 liters. See the Appendix D to 1910.1045 for these recommendations and 1910.1045 for the regulatory context.

• The flow rate must be known to within ±5 percent accuracy (pump calibration).
• If you suspect very high concentrations, use a smaller sampling volume to avoid overloading the tube.

Under 1910.1045AppD charcoal tubes must be placed vertically during sampling, with the smaller backup section closest to the pump, and air should not pass through any hose or tubing before entering the tube. See Appendix D to 1910.1045.

• Break the tube ends immediately before sampling to create an opening at least half the internal diameter (about 2 mm).
• Use the supplied plastic caps immediately after sampling (rubber caps are prohibited).

Under 1910.1045AppD the recommended charcoal tube is a 7 cm glass tube (6 mm O.D., 4 mm I.D.) with two sections of 20/40 mesh activated charcoal (100 mg front, 50 mg backup), urethane foam separators, and a glass wool plug in front; the pressure drop across the tube must be less than 1 inch of mercury at 1 liter per minute. See Appendix D to 1910.1045.

• The tubes described use charcoal from coconut shells fired at 600 °C prior to packing.
• Ensuring low pressure drop helps maintain accurate flow rates and sample volumes.

Under 1910.1045AppD breakthrough is indicated when the amount found on the backup section exceeds about 25 percent of the front section, and you should consider that sample loss may have occurred and adjust sampling (larger tubes, lower flow, or shorter sampling times). See Appendix D to 1910.1045 and 1910.1045.

• Employers are advised in the appendix to try larger-capacity charcoal tubes and/or reduce flow rates or sampling durations when breakthrough is suspected.

Under 1910.1045AppD acrylonitrile should be desorbed from the charcoal with chromatographic-quality methanol, using 1.0 mL of methanol and a 30-minute desorption with occasional agitation. See Appendix D to 1910.1045.

• After adding the solvent, cap the vial to minimize volatilization; if using an automatic injector, seal the vials promptly.
• Determine desorption efficiency for the charcoal lot being used (see appendix section on desorption efficiency).

Under 1910.1045AppD you must determine desorption efficiency for the specific compound and the same batch (lot) of charcoal used for sampling because efficiency can vary between laboratories and charcoal lots; the appendix gives a procedure using equivalent charcoal in sealed tubes spiked and analyzed to measure percent recovery. See Appendix D to 1910.1045.

• Desorption efficiency affects how much analyte is recovered into the solvent; it must be known at least once for each batch of charcoal used.
• The appendix describes preparing a sealed tube with the same charcoal and determining recovery under the lab's conditions.

Under 1910.1045AppD typical GC operating conditions include a flame ionization detector with nitrogen carrier at 50 ml/min, hydrogen to detector 65 ml/min, air to detector 500 ml/min, injector at 235 °C, detector manifold at 255 °C, and column temperature of 155 °C on a 4-ft × 1/4-in stainless column packed with Poropak Q. See Appendix D to 1910.1045.

• The appendix also recommends an electronic integrator for peak area measurement and provides injection technique guidance to minimize syringe blowback and evaporation.

Under 1910.1045AppD the recommended injection technique is the solvent-flush method using a 10-microliter syringe (flush with solvent, draw 3 µL solvent, then a 5 µL sample pocketed by air), make duplicate injections with no more than 3% area difference, and measure peak area with an electronic integrator. See Appendix D to 1910.1045.

• Pull the plunger back 1.2 µL before injection to reduce evaporation from the needle tip and ensure injected volume occupies ~4.9–5.0 µL of syringe barrel.
• An automatic sampler may be used if it matches the solvent-flush reproducibility.

Under 1910.1045AppD each personal sampling pump must be calibrated with a representative charcoal tube in the sampling line so the measured flow matches the in-line pressure conditions, and the flow determination should be within ±5 percent at the recommended flow rate. See Appendix D to 1910.1045 and 1910.1045.

• Calibrating with a tube in-line minimizes volume errors caused by differences in pressure drop across different tubes.

Under 1910.1045AppD capped charcoal tubes must be packed tightly and padded to prevent breakage, and for every batch of 10 samples you must submit one handled blank (same lot, same handling but no air drawn) labeled as a blank; do not use rubber caps and do not ship bulk material in the same container as tubes. See Appendix D to 1910.1045.

• Use the supplied plastic caps immediately after sampling.
• Submit a separate bulk sample of the source material in a glass container with a Teflon-lined cap, shipped separately from the tubes.

Under 1910.1045AppD high humidity can severely reduce breakthrough volume (condensation prevents efficient trapping), so you should consider alternative methods or modify sampling by using larger-capacity tubes, lower flow rates, shorter sampling times, or use other validated methods (porous polymer tubes or passive dosimeters) if humidity is a problem. See Appendix D to 1910.1045.

• The appendix warns that when condensation actually occurs in a tube, organic vapors will not be trapped effectively.
• Employers must verify method performance under their workplace conditions.

Under 1910.1045AppD you should record the temperature and pressure of the atmosphere being sampled (or the elevation if pressure readings are not available) in addition to the sample volume and flow rate accuracy. See Appendix D to 1910.1045.

• These environmental parameters are important because the validated ranges for the method were determined at specific temperature and pressure conditions and may affect concentration calculations.

Under 1910.1045AppD the method was validated over 17.5–70.0 mg/m3 at 22 °C and 760 mm Hg using a 20-liter sample, with a probable useful range of about 4.5–135 mg/m3 under the 20-liter condition, and the overall coefficient of variation (CVT) in the 17.5–70.0 mg/m3 range was 0.073. See Appendix D to 1910.1045.

• The appendix also notes that on average the method measured about 6% lower than the “true” concentrations in limited lab experiments, and it instructs not to apply a recovery correction to final results.

Under 1910.1045AppD employers should not apply a recovery correction to the final acrylonitrile results even though limited experiments showed an average 6% low result; the appendix explains that the difference may reflect random variation rather than bias. See Appendix D to 1910.1045.

• The appendix emphasizes that no recovery correction should be applied as a matter of procedure (see section about precision/accuracy).

Under 1910.1045AppD if interfering compounds are suspected you must provide that information with the sample to the laboratory and, if necessary, change chromatographic separation conditions (column, temperature, etc.) because compounds with the same retention time at the operating conditions are interferences. See Appendix D to 1910.1045.

• Retention time on a single column cannot be considered proof of identity; the lab should be instructed to alter conditions to resolve suspected interferences.

Under 1910.1045AppD an automatic sample injector can be used if it provides reproducibility at least as good as the solvent-flush method (duplicate injections with no more than 3% difference in area). See Appendix D to 1910.1045.

• The appendix requires duplicate injections and specifies the acceptable reproducibility to validate an automatic injector for use.

Under 1910.1045AppD to prepare a charcoal tube you score and open the tube, discard the glass wool, transfer the charcoal from the front (larger) section to one vial and the backup section to a separate vial, add 1.0 mL methanol, and desorb for 30 minutes with occasional agitation. See Appendix D to 1910.1045.

• The two sections are analyzed separately (front and backup).
• Use stoppered sample containers (glass stoppers or Teflon-lined caps) to minimize volatilization.

Under 1910.1045AppD you should use larger-capacity charcoal tubes, lower flow rates, or shorter sampling times if you suspect sample breakthrough or that the atmosphere contains a large amount of acrylonitrile. See Appendix D to 1910.1045.

• The appendix recommends larger tubes (similar to those used for vinyl chloride) and adjusting sampling parameters to minimize breakthrough problems.

Under 1910.1045AppD for every batch of 10 samples you must submit one blank tube from the same lot that was handled the same way except that no air was drawn through it; label it as a blank and submit it with the batch. See Appendix D to 1910.1045.

• The blank helps identify contamination introduced during handling, shipping, or analysis.

Under 1910.1045AppD an employer must assure themselves of the method's accuracy and precision under the unique conditions in their workplace and consider alternate validated methods or adjustments when environmental factors (e.g., high humidity) or interferences are present. See Appendix D to 1910.1045 and the general rule in 1910.1045.

• Validate desorption efficiency for the specific charcoal lot used.
• If standard charcoal-tube methods are inadequate, consult other validated methods available in the rulemaking record or from NIOSH/OSHA laboratories.

Under 1910.1045AppD rubber caps must not be used because they can interact with samples and allow loss or contamination; the appendix requires the supplied plastic caps and recommends using stoppered sample containers with glass stoppers or Teflon-lined caps for analysis vials. See Appendix D to 1910.1045.

• Use the supplied plastic caps immediately after sampling and capped vials for desorbed samples to prevent volatilization.

Prepare spiked tubes by injecting a known hexane solution of acrylonitrile into the activated charcoal and sealing the tube as described in the method. The open end is capped with Parafilm (or when using an automatic injector the injector vials with Teflon-faced septa may be used), a known amount of a hexane solution containing 0.239 g/ml acrylonitrile is injected into the charcoal with a microliter syringe, and the tube is capped again with Parafilm so the amount injected equals that present in a 20-liter air sample at the selected level. See the procedures in 1910.1045 App D for the exact steps and concentrations.

You should prepare six tubes at each of three levels: 0.5X, 1X, and 2X of the standard, plus a parallel blank tube. The method specifies preparing six tubes at each level (0.5X, 1X, and 2X based on the TLV) and allowing them to equilibrate (at least overnight) and also preparing a blank treated the same way except no analyte is added. See 1910.1045 App D.

Desorption efficiency equals the average milligrams recovered from the tube divided by the milligrams added to the tube. The method defines D.E. as (average weight in mg recovered from the tube) ÷ (weight in mg added to the tube) and instructs you to use this ratio in later corrections. See the formula and explanation in 1910.1045 App D.

Use the desorption efficiency curve to read the appropriate D.E. for the amount found on the front section, then divide the total weight by that D.E. to obtain the corrected mg/sample. Specifically, after subtracting blanks and summing front and backup weights, read the desorption efficiency from the plotted curve (desorption efficiency versus weight found) and divide the total measured mg/sample by that D.E. as described in 1910.1045 App D, section 10.4.

Prepare standards expressed as mg per 1.0 ml of methanol because samples are desorbed in 1.0 ml of methanol and analyzed under the same GC conditions. The density of acrylonitrile can be used to convert mg to microliters so you can measure volumes with a microliter syringe; prepare a series of standards across the range of interest and analyze them at the same time and GC settings as your samples. See the calibration and standards guidance in 1910.1045 App D, section 9.

Standards must be run with the samples because the method has no internal standard and instrument response can vary day-to-day or within the same day. The method explicitly notes that, since no internal standard is used, analyzing standard solutions at the same time as the samples minimizes the effect of day-to-day and same-day variations in detector response. See the note in 1910.1045 App D, section 9.

Break the ends to provide an opening at least one-half the internal diameter, keep the charcoal tube vertical, place the backup section nearest the pump, and do not pass sampled air through hoses or tubing before the charcoal tube. The method requires breaking the tube ends to ≥½ the internal diameter, placing the smaller (backup) section toward the sampling pump, positioning the tube vertically to minimize channeling, and sampling air directly into the tube (no tubing ahead of the tube). See 1910.1045 App D, section 5.3.

If the amount found on the backup section exceeds 25% of that found on the front section, the tube may be overloaded and sample loss is possible. The method warns that when the backup section value exceeds 25% of the front section value, breakthrough or sample loss should be suspected. See 1910.1045 App D, section 2.3.

Desorb each section by adding 1.0 ml of methanol, cap immediately, and sonicate for 30 minutes before analysis. The procedure calls for pipetting 1.0 ml methanol into each vial, recapping the vial as soon as solvent is added, and desorbing for 30 minutes in an ultrasonic bath before GC injection. See 1910.1045 App D, section 5.4.2.

Cap the sampled charcoal tubes with the supplied plastic caps immediately after sampling and do not use rubber caps. The method explicitly states to use the supplied plastic caps and warns against using rubber caps after sampling. See 1910.1045 App D, section 5.3.7.

Break the tube, remove and analyze the front (adsorbing) and backup sections separately by transferring each section's charcoal to its own vial and desorbing them individually. The method directs scoring and breaking the tube, discarding the glass wool, transferring the charcoal from the first (larger) section to a 2-ml vial and the backup to a separate capped vial, and desorbing/analyzing each section independently. See 1910.1045 App D, sections 5.4.1–5.4.2 and 10.2–10.3.

Subtract the blank (front and backup) values from the corresponding sample sections, add the corrected front and backup weights to get the total weight, then divide that total by the desorption efficiency (read from the D.E. curve) to obtain the corrected mg/sample. The method gives the steps: mg = mg sample − mg blank for each section, add front and backup corrected weights, then correct for desorption efficiency by dividing the total by the D.E. from the desorption efficiency curve (see 1910.1045 App D, sections 10.2–10.4).

Convert mg/sample to mg/m3 by dividing the corrected milligrams by the sample air volume in cubic meters, and convert mg/m3 to ppm using the provided formula that accounts for molecular weight, temperature, and pressure. The method gives: concentration (mg/cu m) = corrected mg/sample ÷ sample volume (m3), and ppm = (mg/m3) × 24.45 / M.W. × 760 / P × (T + 273) / 298, where P is sample pressure (mm Hg), T is sample temperature (°C), M.W. is molecular weight, and 24.45 is the molar volume at 25 °C and 760 mm Hg. See 1910.1045 App D, sections 10.5–10.6.

Calibrate each personal sampling pump with a representative charcoal tube in the sampling line and ensure the flow is known to within ±5 percent at the recommended flow rate. The method requires calibrating the pump with a representative tube in line and calls for a flow accuracy of at least ±5 percent. See 1910.1045 App D, sections 3.1 and 5.2.

The pressure drop across the tube must be less than one inch of mercury when measured at a flow rate of 1 liter per minute. The method specifies this maximum pressure drop limit and the reference flow rate for that measurement. See 1910.1045 App D, section 3.2.

A sample size of 20 liters is recommended, sampled at approximately 0.2 liters per minute, with the flow rate known within ±5 percent. The method recommends a 20-liter sample collected at roughly 0.2 L/min and stresses flow accuracy of at least ±5 percent. See 1910.1045 App D, section 5.3.5.

Record the temperature and pressure of the sampled atmosphere, submit at least one blank tube with each set of samples, and package samples to minimize breakage. The method instructs recording sample temperature and pressure, including at least one handling blank (no air drawn through it) with each sample set, and taking shipping/packing precautions to avoid breakage. See 1910.1045 App D, sections 5.3.6–5.3.9.

The NIOSH low-level modification differs in desorption solvent, GC column and operating conditions, injection volume, and recommended sampling rate compared with OSHA's methanol-desorption procedure. Specifically, NIOSH recommends desorption with 1% acetone in carbon disulfide (CS2) rather than methanol, specifies a 20% SP-1000 column on Supelcoport with different temperature and gas flows, uses a 2 µl injection, and recommends a 100 ml/min sampling rate; OSHA's laboratory modification uses methanol desorption and different GC settings. See the NIOSH and OSHA method differences described in 1910.1045 App D.