Page:NIOSH Manual of Analytical Methods - 2520.pdf/4

 METHYL BROMIDE: METHOD 2520, Issue 2, dated 15 May 1996 - Page 4 of 5

bromine, independent of the source compound. Calibration can be done with brominated compounds that are liquid at room temperature, thereby eliminating the need for methyl bromide standards. Methylene chloride replaced carbon disulfide as desorption solvent. This revised Method 2520 (issued 5/15/96) for methyl bromide was evaluated at 7.8, 20.0, and 125.0 mg/ m³ (2, 5, and 32 ppm) [1]. Test atmospheres were generated by delivering methyl bromide gas from two diffusion tubes kept at −12 °C into an airstream flowing at 12 L/min. Humidity was added downstream when needed, and airflow passed through two mixing chambers before reaching a sampling manifold. The concentration was monitored with a total hydrocarbon analyzer and confirmed by calibrating against other brominated standards by GC-AED. Three compounds used as calibration standards (dibromomethane, 1-bromopropane, and 1-bromobutane) were chosen as closest in chemical structure to methyl bromide, although a compound-independent response was assumed [6]. A three-compound calibration was used during the development of the GC-AED analytical procedure. Since there was good agreement between the three compounds, the method was written with only one brominated standard, dibromomethane. When challenged with methyl bromide at a calculated concentration of 27 ppm, the capacity of the sampler (a 400-mg petroleum charcoal tube plus a 200-mg petroleum charcoal tube) at 40% RH and 20 °C was 1386.5 µg regardless of flow rate (10.5, 40, or 100 mL/min). However, at 100% RH and 39 °C, the average capacity fell to 298.6 µg. With a 9-g sodium sulfate drying tube in line, the capacity was increased to 651.8 µg (concentration averaged 33 ppm). Even with a drying tube in line, severe breakthrough occurred at the 10-L sample volume (50% was found on the back tube). Without the use of a drying tube, a 1-L sample volume is recommended, based on a 170.6 µg capacity (1.6 L) found at the 40 mL/min rate multiplied by the 0.67 caution factor.

Recovery (%)

Recovery fell below 70% for sample loadings less than 58 µg when carbon disulfide was used for desorption (Figure 1). This would not allow accurate sampling at 2 ppm, the exposure level most frequently encountered. Therefore, alternate desorption solvents were tested. Desorption with methylene chloride improved recovery at the 15-µg level to 76.7%. However, sample stability still fell below 70% after storage for six days at −10 °C regardless of sample level. This stability limitation remains a concern and rapid sample analysis is required.

Methylene chloride Carbon disulfide

Methyl bromide (µg)

Figure 1. Comparison of desorption solvents.

NIOSH Manual of Analytical Methods (NMAM), Fourth Edition