MERCAPTANS, METHYL-, ETHYL-, and n-BUTYL-: METHOD 2542, Issue 1, dated 15 August 1994 - Page 3 of 4
CALIBRATION AND QUALITY CONTROL: 8. Calibrate daily with at least six working standards covering the range of the samples: a. Add known amounts of calibration stock solution to 1,2-dichloroethane in 10-mL volumetric flasks and dilute to the mark. b. Analyze together with samples and blanks (steps 10 and 11). c. Prepare calibration graphs: either quadratic curves, i.e., peak area vs. concentration of mercaptans, or linear curves, i.e., In(peak area) vs. In(concentration)² can be used for calibration. 9. Determine recovery (R) for each lot of glass fiber filters impregnated with mercuric acetate in the concentration range of interest. Prepare four filters at each of three concentrations plus three media blanks. a. Apparatus for preparing and sampling gas spikes is shown in Figure 1. The apparatus is composed of glass and Teflon. b. Sample at a rate of 0.2 L/min for 100 min. NOTE: Protect the samples from light. c. Extract the samples (steps 5 through 7) and analyze (steps 10 and 11). d. Prepare separate recovery graphs for each mercaptan (R vs. µg analyte). MEASUREMENT: 10. Set gas chromatograph to conditions given on page 2542-1. Set air and hydrogen flow rates on the flame photometric detector to manufacturer’s specification. Inject 1-µL sample aliquot via the split injection mode. Retention times: methyl mercaptan, 2.4 min; ethyl mercaptan, 3.2 min; n-butyl mercaptan 6.9 min. 11. Measure peak area. CALCULATIONS: 12. Determine the mass, µg, of methyl-, ethyl-, and n-butyl-mercaptans found in the samples ( ) and in the average media blanks ( ). 13. Calculate concentration, , of the mercaptans in the air volume sampled, (L): , mg/m³. EVALUATION OF METHOD: This method modifies OSHA Method 26, Methyl Mercaptan [2], and further develops and evaluates the procedure for ethyl- and n-butyl mercaptans, as well as methyl mercaptan [1]. The method was evaluated for recovery and extraction efficiency with gas spikes, 18 samples (6 at each level, 0.5, 1, and 2 × PEL) (Figure 1). Recoveries were 82.7% for methyl mercaptan, 89.3% for ethyl mercaptan, and 93.0% for n-butyl mercaptan. Test atmospheres of methyl- and ethyl mercaptans were generated with permeation tubes as the analyte source. Six samples at one level (1 × PEL) were collected and analyzed. Recoveries for generated samples, corrected for extraction efficiency, were 98.6 ± 8.9% for methyl mercaptan and 102 ± 7.8% for ethyl mercaptan. Samples of n-butyl mercaptan were not generated. Samples prepared by spiking impregnated filters with a solution of mixed mercaptans were found to be stable for at least three weeks at ambient temperatures when protected from light. The evaluation data are summarized in Table 2.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition
