Validation of Nitrous Oxide Using SKC Passive Sampler SKC Inc. 863 Valley View Road Eighty Four, PA 15330

Transcription

1 Validation of Using SKC Passive Sampler SKC Inc. 863 Valley View Road Eighty Four, PA 15330

2 VALIDATION OF A DIFFUSIVE SAMPLER (SKC ) for NITROUS OXIDE Cynthia Kuhlman Linda Coyne SKC, Inc. 863 Valley View Road Eighty Four, PA ABSTRACT A partial validation was performed on the ability of a diffusive sampler (Cat. No , SKC Inc., Eighty Four, PA, U.S.A.) to accurately sample nitrous oxide in workplace air. Validation was conducted as outlined in a modified NIOSH protocol. A desorption efficiency (DE) study was performed using samplers spiked with nitrous oxide at 0.1, 0.25, 0.5, 1, and 2 times the PEL (50 ppm) for an 8-hour period. The desorption efficiency was determined to be 100% with a relative standard deviation (RSD) of 6.2%. The uptake rate was determined for samplers exposed to nitrous oxide levels ranging from to 890 ppm-hr and at 25 0 C and 20 to 80% relative humidity (RH). The mean uptake rate of these experiments was 0.81 ml/min and was based on the combined results of 66 samplers exposed and analyzed within 24 hours. A study of reverse diffusion showed that reverse diffusion does not take place. A storage study indicated that exposed samples could be stored for up to three weeks in a freezer and for one week at ambient temperatures. Samples were transferred from the passive sampler to a thermal desorption tube, desorbed, and analyzed using an electron capture detector (ECD). INTRODUCTION Nitrous oxide or N 2 O (CAS# ) is also known as dinitrogen monoxide or laughing gas. It is a colorless gas with a sweetish odor and taste and is very stable and inert chemically at room temperatures. Nitrous oxide is not classifiable as a human carcinogen; however, it is a narcotic at high concentrations. Although nitrous oxide is non-flammable it can support combustion and can form an explosive mixture with air. Nitrous oxide is used as an anesthetic in dentistry and surgery, as a propellant gas in food aerosols such as whipped cream, and as an inert gas to displace bacteria-inducing oxygen when filling packages of chips and other similar snack foods. Nitrous oxide is also used for leak detection and in motor racing as an oxidizer to increase the power output of engines. At high concentrations, nitrous oxide is an asphyxiant. Exposures to lower levels affect the central nervous system, the cardiovascular system, the liver, the hematopoietic system, and the reproductive system. Page 1 of 14

3 Epidemiological studies have shown increased risks of spontaneous abortion, premature delivery, and involuntary infertility among occupationally exposed populations. ACGIH used a guideline of 50 ppm based on an 8-hour TWA, NIOSH recommends 25 ppm as a time-weighted average, and OSHA does not currently regulate nitrous oxide. (1) Techniques available for sampling nitrous oxide include pumping ambient air through a spectrophotometer sample cell or filling a sample bag with a personal sampling pump at a calculated rate over the duration of the sampling period. The bag sample should be analyzed within two hours after completion of sampling to minimize compound loss. (2) These methods are highly accurate and reproducible. However, they are expensive, cumbersome, and analysis requires special equipment. The SKC Diffusive Samplers are inexpensive, simple to operate, unobtrusive, the sample can be stored up to three weeks in a freezer, and analysis is preformed with standard analytical equipment. OSHA has tested diffusive samplers for various methods and for monitoring exposures in the work environment. (3,4,5) Diffusive samplers have been evaluated recently for sampling lower levels commonly found in indoor air. (6,7) Detection of these lower levels is obtained easily when the diffusive sample is combined with thermal desorption. The validation of the diffusive sampler is important because the sampling can be affected by many environmental conditions such as temperature, humidity, chemical hazard concentrations, time of exposure, wind velocity, atmospheric pressure, sampler orientation, interfering compounds, and reverse diffusion of the compound of interest from the surface of the sorbent. (8,9) ASTM International (American Society for Testing and Materials) and ANSI (American National Standard Institute) have published guidelines for the validation of diffusive samplers. (10,11,12) ANSI has published a Standard Guide that describes testing to determine the effects of the factors listed above (13). Some of these are implemented in the SKC validation program. The validation of the SKC Diffusive Sampler for nitrous oxide was based on these guidelines and the factors tested qualify this study as a partial validation of the Diffusive Sampler for nitrous oxide. The disadvantages of diffusive samplers are that they cannot function in a static environment or at low wind levels. However, a study in the U.K. has shown typical workplace air velocities to be around a mean of 30 to 40 cm sec -1 (14), which is sufficient for the operation of diffusive samplers. Page 2 of 14

4 EXPERIMENTAL Reagents and Apparatus Nitrous oxide (Scott Specialty Gases [certified master grade], Plumsteadsville, PA,U.S.A.) was separated on a 30-meter, 0.32-mm ID CARBOPLT capillary column (Agilent Technologies, Wilmington, DE, U.S.A.). Pure nitrous oxide gas was used to make various concentration levels for a standard five-point curve. The standards were made daily and desorbed with the samples by thermal desorption and analyzed with an ECD. The Thermal Desorber was a Perkin Elmer Turbo Matrix and the GC was an Agilent 5890 with an electron capture detector. See Appendix A for GC and thermal desorber conditions. The diffusive sampler used was the SKC Diffusive Sampler containing 736 mg of washed Molecular Sieve 5A 60/80 (Alltech, State College, PA, U.S.A.). Standard atmospheres around the TLV (50 ppm, 25 o C, 80% RH) were generated using an atmospheric chamber. The atmosphere was calibrated using thermal desorption tubes packed with Molecular Sieve 5A 60/80. The analytical limit of detection for nitrous oxide was 2 ppm-hr per sampler. PROCEDURES The analytical recovery (desorption efficiency) was determined by using four samplers at each exposure level to be tested. The loading was based on equivalent 8-hour exposures to 0.1, 0.25, 0.5, 1, and 2 times the inhouse limit value (50 ppm). A known concentration of nitrous oxide was injected directly into the sampler for each expected exposure level based on the estimated sampling rate. The samplers were capped and stored at room temperature overnight to equilibrate, and were analyzed the following day. The amount recovered on analysis was determined as a percentage of the initial loading. The calculated uptake rate of the samplers for nitrous oxide was verified for concentrations in the range of to 890 ppm-hr at 25 0 C and relative humidity levels from 20 to 80%. Four samplers were exposed simultaneously to the atmosphere for each time period. Sampling times ranged from 15 minutes to 8 hours. After exposure, the molecular sieve was transferred from the samplers to a thermal desorption tube and analyzed. A set of 40 samplers was spiked with 41 µg of nitrous oxide for the storage study. After the samples equilibrated for two hours, 16 of the samples were run for Day-0 of the storage study. The remaining 24 samples were transferred to vials and divided into two sets. One set was stored at room temperature and the other set was stored at -8 0 C. Four samples from each set were analyzed weekly for three weeks. Page 3 of 14

5 A study of possible reverse diffusion in non-constant atmospheres was performed by exposing eight samplers to177.5 ppm-hr nitrous oxide at 25 0 C and 60% relative humidity for four hours. The samples were removed from the chamber and capped while the nitrous oxide was purged from the chamber. Then four of the samplers were uncapped and placed back into the chamber for another four hours. All eight samplers were analyzed the same day. Throughout the study, thermal desorption tubes packed with molecular sieve were used to determine the true concentration of the atmosphere. Samples were taken at 20 ml/min with a Pocket Pump (Cat. No MH, SKC Inc., Eighty Four, PA, U.S.A.). RESULTS and DISCUSSION The desorption efficiency results for nitrous oxide on the diffusive samplers are provided in Table 1. The mean recovery for nitrous oxide on the SKC Diffusive Samplers was 100% (6.2% RSD). The nitrous oxide results were consistent even at the low levels of concentration (Table 1). The results of the reverse diffusion study are shown in Table 2 and indicate that reverse diffusion of nitrous oxide does not take place on SKC Diffusive Samplers. Tables 3 through 5 show the uptake rate at different concentrations, temperatures, and relative humidities. The mean sampling rate of nitrous oxide at to 890 ppm-hr was determined to be 0.81 ml/min with an RSD of 12% and was based on the combined results of 76 samplers exposed and analyzed within 24 hours. The sampling rate is not affected by changes in concentration, temperature, or relative humidity. The storage stability results show that there was less than 10% change between the initial week and the third week period when the samplers were stored in the freezer. The samples can store for one week at room temperature. All storage stability results are shown in Table 6. QA RESULTS All analyses were run with a five-point calibration curve and check standards. The correlation coefficient for the calibration curves was 0.95 or better and all the check standards were within 10% of the original value. Thermal desorption tubes used to calculate the ppm concentration of the atmosphere showed that the appropriate nitrous oxide atmosphere was achieved. Page 4 of 14

6 CONCLUSION The SKC Diffusive Samplers have been partially validated for sampling nitrous oxide in occupational hygiene situations according to a modified NIOSH protocol. Nitrous oxide can be sampled accurately with the SKC Diffusive Samplers at an experimentally determined uptake rate of 0.81 ml/min (RSD ± 12%). The data obtained through this study shows that the diffusive samplers can be used at concentrations up to 890 ppm-hr and at various humidities. A desorption efficiency correction of 100% should be applied to Diffusive Samplers. The SKC Diffusive Samplers show at least a 91% recovery when stored up to three weeks in the freezer (-8 0 C). Page 5 of 14

7 REFERENCES 1) Occupational Safety and Health Guideline for, OSHA, 2) Burroughs, G.E. and Woebkenberg, M.L., Method 6600:, NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, NIOSH, Aug ) Yost, C. and Harper, M., Validation of Parachlorobenzotrifluoride, Benzotrifluoride, and Monochlorotoluene on Diffusive Samplers, Applied Occup. and Environ. Hyg., Vol. 15, No. 1, Jan. 2000, pp ) Hendricks, W., Method 1004: 2-Butanone (MEK) Hexone (MIBK), OSHA Analytical Methods Manual, Sept ) Harper, M. and Fiore, A., Determination of Methyl tert-butyl Ether in Air Using a Diffusive Sampler, Applied Occup. and Environ. Hyg., Vol. 10, No. 7, July 1995, pp ) Hendricks, W., The Marines Project: A Laboratory Study of Diffusive Sampling/Thermal Desorption/Mass Spectrometry Techniques for Monitoring Personnel Exposure to Toxic Industrial Chemicals, OSHA Salt Lake Technical Center, Salt Lake City, UT, Apr ) Hendricks, W., Performance of SKC Ultra Passive Samplers Containing Carboxen 1016, Carbotrap Z, or Chromosorb 106 when Challenged with a Mixture Containing Twenty of OSHA SLTC s Top Solvent Analytes, OSHA Salt Lake Technical Center, Salt Lake City, UT, Feb ) Harper, M. and Purnell, C.J., Diffusive Sampling A Review, Am. Ind. Hyg. Assoc. Journal, Vol. 48, Issue 3, Mar. 1987, pp ) Guild, L. V., Dietrich, D. F., and Moore, G., Assessment of Sampling Accuracy in Diffusive Samplers, Am. Ind. Hyg. Assoc. Journal, Vol. 52, Issue 5, May 1991, pp ) Cassinelli, M. E., Hull, R. D., Crable, J. V., and Teass, A. W. Protocol for the Evaluation of Passive Monitors, In Diffusive Sampling: An Alternative Approach to Workplace Air Monitoring, A. Berlin, R. H. Brown, and K. J. Saunders, eds., Royal Society of Chemistry, London, 1987, pp ) Harper, M. and Guild, L.V., Experience in the Use of the NIOSH Diffusive Sampler Evaluation Protocol, Amer. Ind. Hyg Assoc. Journal, Vol. 57, Issue 12, Dec. 1996, pp ) ASTM Standard D : Standard Practice for Evaluating the Performance of Diffusive Samplers, ASTM Annual Book of Standards, Section 11, Vol , ASTM International, West Conshohocken, PA, ) ANSI/ISEA Standard : American National Standard for Air Sampling Devices- Diffusive Type for Gases and Vapors in Working Environments, ISEA, Arlington, VA, 1998 (standard reaffirmed in 2003) 14) Baldwin, P. E. J. and Maynard, A. D., A Survey of Wind Speeds in Indoor Workplaces, Annals of Occupational Hygiene, Vol. 42, No. 5, May 1998, pp Page 6 of 14

8 Table 1. Analytical Recovery on SKC Diffusive Samplers PEL Spike (µg) Recovered (µg) Recovery (%) Mean: 113% Std. Dev.: RSD: 6.2% Page 7 of 14

9 Table 2. Reverse Diffusion ppm-hr on SKC Diffusive Samplers (25 0 C, 60% RH) 4-hr Exposure ppm-hr 4-hr Exposure ppm-hr and 4-hr Exposure to 0 ppm-hr µg µg µg µg µg µg µg µg Mean: µg Mean: µg Std. Dev.: 1.11 Std. Dev.: 0.67 RSD: 7.2% RSD: 4.4% Page 8 of 14

10 Table 3. Sampling Rate and Capacity on SKC Diffusive Samplers (25 0 C, 60% RH) ppm-hr Uptake (µg) Sampling Rate (ml/min) Mean: ml/min Std. Dev.: RSD: 11.2% Page 9 of 14

11 Table 4. Sampling Rate and Capacity on SKC Diffusive Samplers (25 0 C, 80% RH) ppm-hr Uptake (µg) Sampling Rate (ml/min) Mean: ml/min Std. Dev.: RSD: 13.7% Page 10 of 14

12 Table 5. Sampling Rate and Capacity on SKC Diffusive Samplers (25 0 C, 20% RH) ppm-hr Uptake (µg) Sampling Rate (ml/min) Mean: ml/min Std. Dev.: RSD: 11.4% Page 11 of 14

13 Table 6. Storage Study on SKC Diffusive Samplers 40-µg Spike Recovery (%) Storage Period Freezer (-8 C) Ambient Week Week Week Week Page 12 of 14

14 Appendix A GC/ATD Conditions GC Conditions: Agilent 5890 Column: Oven: Agilent CARBOPLT, 30 m, 0.32-mm ID, 1.5-µm film C isothermal, hold 10 minutes Range: 4 Attn: 0 Detector: ECD, C Injector: None Transfer line connects directly to the GC column. Thermal Desorber: Perkin Elmer Turbo Matrix ATD Temp: Valve: C Transfer: C Tube: C Rate: 40 0 C/s Trap: HIGH C, LOW C Timing: Purge: 1 minute Desorb: 10 minutes Trap hold: 10 minutes Option: Injection/tube 1 Inlet split: disabled Outlet split: enabled Operating mode: 2-Stg Desorb PNU: Outlet split: 100 ccm Desorb: 100 ccm Column: 16 psi Page 13 of 14