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ACETIC ANHYDRIDE
Method number: 102
Matrix: Air
Target concentration: OSHA PEL: ACGIH TLV:
5 ppm (20 mg/m^3 ) 5 ppm (20 mg/m^3 ) TWA 5 ppm (20 mg/m^3 ) ceiling
Procedure: Samples are collected open face on glass fiber filters coated with veratrylamine and di- n -octyl phthalate. Samples are extracted with 50/50 (v/v) 2-propanol/toluene and analyzed by GC using a nitrogen- phosphorus detector (NPD).
Recommended air volume and sampling rate: 7.5 L at 0.5 L/min ceiling 7.5 L at 0.05 L/min TWA
Reliable quantitation limit: 0.094 ppm (0.39 mg/m^3 )
Standard error of estimate at the target concentration: 6.4%
Special caution: Ketene and acetyl chloride produce the same derivative as acetic anhydride. Coated filters should be used within a month of preparation.
Status of method: Evaluated method. This method has been subjected to the established evaluation procedures of the Organic Methods Evaluation Branch.
Date: October 1993 Chemist: Yihlin Chan
Organic Methods Evaluation Branch OSHA Salt Lake Technical Center Salt Lake City, UT 84165-
- General Discussion
1.1 Background
1.1.1 History
In OSHA Method 82, acetic anhydride is collected on a glass fiber filter impregnated with 1-(2-pyridyl)piperazine, which reacts with the anhydride to form a derivative (Ref. 5.1). Attempts at using 1-(2-pyridyl)piperazine for the derivatization of maleic, phthalic, and trimellitic anhydrides failed, however, because the resulting derivatives of these anhydrides were found to be unstable. These anhydrides were derivatized with veratrylamine instead (Refs. 5.2-5.4). Di- n -octyl phthalate was added to the filter to improve collection efficiency. In an effort to have a common sampler for all four anhydrides, the use of veratrylamine as the derivatizing agent for acetic anhydride was evaluated. The equation for the derivatization reaction is shown below:
The acetic anhydride-veratrylamine derivative (AcVA) can be analyzed either by HPLC or GC; GC was selected for this evaluation. The method was evaluated at the then current OSHA PEL of 5 ppm (20 mg/m^3 ) ceiling. But after the evaluation work had been completed and the method was being written, the PEL was reverted back to 5 ppm (20 mg/m^3 ) TWA (Ref. 5.5). The validity of the sampling capacity at a slower flow rate was reevaluated in order to accommodate a longer sampling time.
1.1.2 Toxic effects (This section is for information only and should not be taken as the basis of OSHA policy.)
The following is quoted from the ACGIH Documentation of TLV:
Smyth et al reported an oral LD50 in rats as 1.78 g/kg. Henderson and Haggard mentioned eye, nose and throat irritation and suggested that bronchial and lung injury were likely to occur from inhalation of acetic [an]hydride vapor. Fairhall considered acetic anhydride a marked lachrymator and found systemic effects unlikely. McLaughlin discussed serious corneal injury from the liquid in industry. Smyth found rats inhaling 1000 ppm for four hours survived, but 2000 ppm was fatal. The liquid causes skin burns. No cumulative effects are known. The value of 5 ppm, as a ceiling limit, is recommended by analogy with acetic acid and to prevent undue irritation. (Ref. 5.6)
The OSHA PEL for acetic anhydride is 5 ppm (20 mg/m^3 ) TWA. (Ref. 5.5)
1.1.3 Workplace exposure
Exposure to acetic anhydride may occur in the following operations: manufacture of cellulose esters, fibers, plastics, lacquers, protective coating solution, photographic films, cigarette filters, magnetic tape, and thermoplastic molding compositions; manufacture of pharmaceuticals and pharmaceutical intermediates; use in organic synthesis as an acetylating agent, bleaching agent, and dehydrating agent; synthesis of perfume chemicals, explosives, and weed killers; use in acetylation of animal and vegetable oils; use as an acetylating and dehydrating agent in textile dyeing, chemical treatment of paper, and chemical analysis. (Ref. 5.7) Of these, by far the greatest single application for acetic anhydride is in the manufacture of cellulose esters. It is estimated that 95% of the total U.S. production is used for this purpose. (Ref. 5.8)
1.1.4 Physical properties and other descriptive information (Ref. 5.9)
1.2.4 Precision (analytical procedure)
The precision of the analytical procedure, measured as the pooled relative standard deviation over a concentration range equivalent to 0.5 to 2 times the target concentration, is 0.53%. (Section 4.5)
1.2.5 Precision (overall procedure)
The precision of the overall procedure at the 95% confidence level for the ambient temperature 15-day storage test (at the target concentration) is ±12.5% (Section 4.6). This includes an additional 5% for sampling error.
1.2.6 Recovery
The recovery of AcVA from samples used in a 15-day storage test remained above 96.1% when the samples were stored at ambient temperature. (Section 4.7)
1.2.7 Reproducibility
Six samples collected from a controlled test atmosphere, with a draft copy of this procedure, were submitted to an SLTC service branch for analysis. The samples were analyzed after 2 days of storage at 5EC. No individual sample result deviated from its theoretical value by more than the precision reported in Section 1.2.5. (Section 4.8)
- Sampling Procedure
2.1 Apparatus
2.1.1 Samples are collected using a personal sampling pump calibrated, with the sampling device attached, to ±5% of the recommended flow rate.
2.1.2 Samples are collected with a four- piece polystyrene cassette containing two coated glass fiber filters assem- bled as shown. Each treated filter is coated with 10 mg of veratrylamine (3,4-dimethoxybenzylamine) and 10 mg of di- n -octyl phthalate. Di- n -octyl phthalate is added to improve the collection efficiency.
2.1.3 The treated filters are prepared as follows: To make 40 coated filters, weigh 0.4 g of veratrylamine and 0.4 g of di- n -octyl phthalate in a scintillation vial. Add 16 mL of methanol and shake well. Place glass fiber filters on a clean glass plate. Apply 0.4 mL of the methanol solution to each filter. Air dry for 5 minutes. Place filters in a wide-mouth jar. Dry under vacuum at room temperature overnight. Store the coated filters in a refrigerator and use within a month. Filters stored longer than this period developed several small interfering peaks (believed to be decomposition products of veratrylamine) in the chromatogram.
2.2 Reagents
None required.
2.3 Technique
2.3.1 Prepare the sampler for open-face sampling by removing the top piece and the end plug from the bottom piece. Attach the sampler to the sampling pump with a piece of flexible tubing and place it in the worker's breathing zone with the open face of the cassette facing down.
2.3.2 Replace the top piece and the end plug after sampling. Wrap each sample with a Form OSHA-21 seal.
2.3.3 Submit at least one blank with each set of samples. Blanks should be handled in the same manner as samples, except no air is drawn through them.
2.3.4 Record sample air volume for each sample.
2.3.5 List any compounds that could be considered potential interferences.
2.4 Sampler capacity
Sampling capacity is determined by measuring how much air can be sampled before breakthrough of analyte through the sampler occurs. Breakthrough is considered to occur when the effluent from the sampler contains a concentration of analyte that is 5% of the upstream concentration (5% breakthrough). The sampler capacity was determined to be over 30 L at a sampling rate of 0.5 L/min with an acetic anhydride concentration of 40 mg/m^3 (2 times the target concentration). At a sampling rate of 0.05 L/min, 5% breakthrough point was not reached in 18 L. (Section 4.9)
2.5 Extraction efficiency
2.5.1 The average extraction efficiency for AcVA from the treated glass fiber filter over the range of 0.5 to 2.0 times the target concentration was 99.8%. (Section 4.10.1)
2.5.2 The extraction efficiency at 0.2, 0.1, and 0.05 times the target concentration was found to be 100.9%, 107.8%, and 108.0% respectively. (Section 4.10.1)
2.5.3 Extracted samples remain stable for at least 24 h. (Section 4.10.2)
2.6 Recommended air volume and sampling rate
2.6.1. For TWA samples the recommended air volume is 7.5 L at 0.05 L/min.
2.6.2. For short-term samples the recommended air volume is 7.5 L at 0.5 L/min.
2.7 Interferences (sampling)
2.7.1 Acetyl chloride and ketene react with veratrylamine to form AcVA, causing positive interference. But in most industrial applications they are rarely used together with acetic anhydride. Other compounds that react with veratrylamine, such as isocyanates, acyl halides, and other anhydrides, may interfere by consuming part of the derivatizing agent.
2.7.2 Suspected interferences should be reported to the laboratory with submitted samples.
2.8 Safety precautions (sampling)
2.8.1 The sampling equipment should be attached to the worker in such a manner that it will not interfere with work performance or safety.
2.8.2 All safety practices that apply to the work area being sampled should be followed.
3.4.1 Transfer the two filters to separate scintillation vials.
3.4.2 Add 5.0 mL of the extraction solvent to each vial.
3.4.3 Cap the vials and shake them on a mechanical shaker for 30 min.
3.5 Analysis
3.5.1 GC conditions
column: HP-1 (5-m, 0.53-mm i.d., 2.65-μm film) zone temp: column 150 EC to 270EC at 10EC/min injector 270 EC detector 300 EC gas flow: column (He) 1.83 mL/min hydrogen 3.84 mL/min auxiliary (N 2 ) 27.4 mL/min air 110.5 mL/min split vent 68 mL/min (split ratio 37:1) ret. times: AcVA 3.8 min benzalazine 4.5 min (ISTD)
Figure 3.5.1.1. Chromatogram of a standard at the target Figure 3.5.1.2. Chromatogram of an extracted sample at concentration. Key: 1 = AcVA, 2 = benzalazine (internal 0.75 times target concentration. Key: 1 = AcVA, 2 = standard). benzalazine, 3 = veratrylamine, 4 = impurities.
3.5.2 Peak areas are measured by an electronic integrator or other suitable means.
3.5.3 An internal standard (ISTD) calibration method is used. A calibration curve is prepared by plotting micrograms per milliliter versus ISTD-corrected response of standards. Bracket the samples with analytical standards.
Figure 3.5.3. Calibration curve constructed from the data in Table 4.5. Equation for the line is Y = 0.203X! 0.574.
3.6 Interferences (analytical)
3.6.1 Any compound that produces an NPD response and has a similar retention time as benzalazine or AcVA is a potential interference. If any potential interferences were reported, they should be considered before samples are extracted. Generally, chromatographic conditions can be altered to separate an interference from the analyte.
3.6.2 When necessary, the identity or purity of an analyte peak may be confirmed with additional analytical data (Section 4.11).
3.7 Calculations
The amount of acetic anhydride per sample is obtained from the appropriate calibration curve in terms of micrograms uncorrected for extraction efficiency. The back filter is analyzed primarily to determine the extent of breakthrough. If any analyte is found on the back filter, it is added to the amount on the front filter. This total amount is then corrected by subtracting the total amount (if any) found on the blank. The air concentration is calculated using the following formulae.
3.8 Safety precautions (analytical)
3.8.1 Adhere to the rules set down in your Chemical Hygiene Plan.
3.8.2 Avoid skin contact and inhalation of all chemicals.
3.8.3 Wear safety glasses and a lab coat at all times while in the lab area.
- Backup Data
4.1 Determination of detection limits
Detection limits (DL), in general, are defined as the amount (or concentration) of analyte that gives a response (YDL) that is significantly different (three standard deviations (SDBR)) from the background response (YBR).
The direct measurement of YBR and SDBR in chromatographic methods is typically inconvenient and difficult because YBR is usually extremely low. Estimates of these parameters can be made with data obtained from the analysis of a series of analytical standards or samples whose responses are in the vicinity of the background response. The regression curve obtained for a plot of instrument response versus concentration of analyte will usually be linear. Assuming SDBR and the precision of data about the curve are similar, the standard error of estimate (SEE) for the regression curve can be substituted for SDBR in the above equation. The following calculations derive a formula for DL:
Table 4. Detection Limit of the Analytical Procedure
concentration mass on column ISTD- (μg/mL) (pg) adjusted response
0 0 0. 0.120 3.2 0. 0.240 6.5 0. 0.361 9.7 0. 0.481 13.0 0. 0.601 16.2 0. 0.721 19.5 0. 0.842 22.7 0. 0.967 26.0 0.
Figure 4.2. Plot of data to determ ine the DLAP.
4.3 Detection limit of the overall procedure (DLOP)
The DLOP is measured as mass per sample and expressed as equivalent air concentrations, based on the recommended sampling parameters. Ten treated filters were spiked with amounts of AcVA equally spaced from 0 to 6.01 μg/sample. The latter amount, when spiked on a sampler, would produce a peak approximately 10 times the baseline noise for a sample blank. These spiked filters were analyzed with the recommended analytical parameters, and the data obtained used to calculate the required parameters (A and SEE) for the calculation of the DLOP. Values of 1.35 and 0. were obtained for A and SEE respectively. DLOP was calculated to be 0.88 μg/sample (0.028 ppm, 0.12 mg/m^3 ).
Table 4. Detection Limit of the Overall Procedure
mass per sample ISTD-adjusted (μg) response
0 0. 0.60 1. 1.20 2. 1.80 3. 2.40 3. 3.01 3. 3.61 5. 4.21 6. 4.81 7. 5.41 8. 6.01 9.04 (^) Figure 4.3. Plot of data used to determine the DLOP/RQL.
4.4 Reliable quantitation limit
The RQL is considered the lower limit for precise quantitative measurements. It is determined from the regression line data obtained for the calculation of the DLOP (Section 4.3), providing at least 75% of the analyte is recovered. The RQL is defined as the amount of analyte that gives a response (YRQL) such that
therefore
Figure 4.4. Chromatogram of the RQL.
The RQL was calculated to be 2.94 μg per sample (0.094 ppm, 0.39 mg/m^3 ). Recovery at this concentration is 103.0%.
4.5 Precision (analytical method)
The precision of the analytical procedure is defined as the pooled relative standard deviation (RSDp). Relative standard deviations were determined from six replicate injections of analytical standards at 0.5, 0.75, 1, 1.5, and 2 times the target concentration. After assuring that the RSDs satisfy the Cochran test for homogeneity at the 95% confidence level, RSDp was calculated to be 0.53%.
Table 4. Instrument Response to AcVA
× target concn 0.5× 0.75× 1× 1.5× 2× μg/mL 15.03 22.55 30.06 45.09 60.
ISTD-adjusted 14.541 22.342 30.142 45.009 59. response 14.546 22.188 30.109 44.842 60. 14.553 22.316 30.066 44.994 60. 14.657 22.431 30.149 44.909 60. 14.686 22.320 29.908 45.081 60. 14.679 22.673 29.850 45.163 60.
&X 14.610 22.378 30.037 45.000 60. SD 0.0705 0.1640 0.1274 0.1153 0. RSD (%) 0.483 0.733 0.424 0.256 0.
The Cochran test for homogeneity:
Since the g statistic does not exceed the critical value of 0.5065, the RSDs can be considered equal and they can be pooled (RSDP) to give an estimated RSD for the concentration range studied.
Table 4. Storage Test for AcVA
time (days) percent recovery percent recovery (ambient) (refrigerated) 0 102.2 103.4 93.2 102.2 103.4 93. 0 98.8 98.0 104.3 98.8 98.0 104. 3 95.0 91.8 101.6 111.0 96.7 98. 7 103.0 106.5 99.9 90.9 107.0 109. 10 97.8 99.2 97.9 111.4 104.1 111. 13 92.3 96.5 98.3 105.9 100.8 102. 15 94.2 93.0 97.6 96.5 98.1 94.
Figure 4.7.1. Ambient storage test for AcVA. Figure 4.7.2. Refrigerated storage test for AcVA.
4.8 Reproducibility
Six samples were prepared by collecting them from a controlled test atmosphere similar to that which was used in the collection of the storage samples. The samples were submitted to an SLTC service branch for analysis. The samples were analyzed after being stored for 2 days at 5EC. No sample result had a deviation greater than the precision of the overall procedure determined in Section 4.7, which is ±12.5%
Table 4. Reproducibility Data mg/m 3 expected mg/m 3 found percent found percent deviation 33.58 32.20 95.9 !4. 33.58 32.63 97.2 !2. 33.58 32.14 95.7 !4. 33.58 30.11 89.7 !10. 33.58 29.71 88.5 !11. 33.58 31.31 93.2 !6.
4.9 Sampler capacity
The sampler capacity was assessed by sampling from a dynamically generated test atmosphere of acetic anhydride at 2 times the target concentration and at 25EC and 80% RH. The test atmosphere of acetic anhydride was generated by pumping an ethyl acetate solution of acetic anhydride at a rate of 9.9 mg/min (11 mg/mL × 0.9 mL/min) through a TSI Model 3076 atomizer where it was dispersed with an air stream of 3.5 L/min. The aerosol passed through an electrostatic charge neutralizer and was mixed with a dilution air stream of 47 L/min. The test atmosphere was drawn through a sampler consisting of two filters in series (separated by a spacer ring) at 0.5 L/min. Three samplers were used in each of the two experiments. At 15-min intervals, the flow was stopped and the back filters were replaced with new ones. This was repeated six times. At the end of the experiment, all the back filters as well as the front filters were analyzed for AcVA. The downstream air concentration was obtained by dividing the amount found on the back filter by the air volume. The upstream concentration was obtained by dividing the sum of amounts found on the front as well as all the back filters by the total air volume. The breakthrough is defined as the downstream concentration divided by the upstream concentration. The average breakthrough for each sampling period versus the air volume was plotted in Figure 4.9.1. No clear indication of exceeding the 5% breakthrough limit was observed in 45 L.
Additionally, two experiments with four samplers were conducted with vapor generator system where acetic anhydride in ethyl acetate was simply evaporated directly into an air stream. The
Table 4.10. Extraction Efficiency × target conc 0.05× 0.1× 0.2× 0.5× 1.0× 2.0× (μg) 7.46 14.92 29.83 74.58 149.2 298. EE (%) 113.0 102.3 100.4 98.2 100.6 96. 87.4 98.1 98.7 95.4 102.5 98. 109.3 103.6 101.9 104.3 99.2 98. 106.5 106.0 102.5 98.0 101.0 101. 120.4 132.6 104.4 96.8 104.4 99. 111.5 104.0 97.3 101.6 101.7 97. &X 108.0 107.8 100.9 99.1 101.6 98.
4.10.2 Stability of extracted samples
The stability of extracted samples was investigated by reanalyzing the target concentration samples 24 h after initial analysis. After the original analysis was performed three vials were recapped with new septa while the remaining three retained their punctured septa. The samples were reanalyzed with fresh standards. The average percent change was !4.8% for samples that were resealed with new septa, and !2.0% for those that retained their punctured septa.
Table 4.10. Stability of extracted samples punctured septa replaced punctured septa retained initial EE after initial EE after EE one day difference EE one day difference (%) (%) (%) (%) 100.6 94.3 !6.3 101.0 98.9 !2. 102.5 98.2 !4.3 104.4 103.5 !0. 99.2 95.5 !3.7 101.7 98.8 !2. (averages) (averages) 100.8 96.0 !4.8 102.4 100.4 !2.
4.11 Qualitative analysis
The GC/MS of AcVA can easily be obtained by using GC conditions similar to those given in Section 3.5. A Perkin-Elmer Ion Trap Detector interfaced to a Hewlett-Packard Series II GC was used to obtain the mass spectra shown at right.
Figure 4.11. Mass spectrum of AcVA.
- References
5.1 Chan, Y. "OSHA Method No. 82; Acetic Anhydride", OSHA Salt Lake Technical Center, unpublished, Salt Lake City, UT 84165, April 1990.
5.2 Chan, Y. "OSHA Method No. 86; Maleic Anhydride", OSHA Salt Lake Technical Center, unpublished, Salt Lake City, UT 84165, December 1990.
5.3 Chan, Y. "OSHA Method No. 90; Phthalic Anhydride", OSHA Salt Lake Technical Center, unpublished, Salt Lake City, UT 84165, October 1991.
5.4 Chan, Y. "OSHA Method No. 98; Trimellitic Anhydride", OSHA Salt Lake Technical Center, unpublished, Salt Lake City, UT 84165, November 1992.
5.5 Code of Federal Regulations, Title 29, 1910.1000, Table Z-1, U.S. Office of the Federal Register National Archives and Records Administration, Washington, DC, 1993.
5.6 Documentation of the Threshold Limit Values and Biological Exposure Indices, 5th ed., American Conference of Governmental Industrial Hygienists Inc., Cincinnati, OH, 1986.
5.7 "NIOSH/OSHA Occupational Health Guidelines for Chemical Hazards", U.S. Department of Health and Human Services, Government Printing Office, DHHS(NIOSH) Publication No. 81-123.
5.8 Grayson, M., Ed., "Kirk-Othmer Encyclopedia of Chemical Technology", 3rd edition, Volume 1, p. 151, Interscience Publishers, New York, NY, 1984.
5.9 Sweet, D.V., Ed., "Registry of Toxic Effects of Chemical Substances", 1985-86 edition, U.S. Department of Health and Human Services, Government Printing Office, DHHS(NIOSH) Publication No. 87-114.
