Evaluation of Charcoal Tube and SUMMA Canister Recoveries for Methyl Bromide Air Sampling
Abstract
The performance of two sampling methods for ambient methyl bromide concentrations, charcoal tubes and SUMMA canisters, was determined in a laboratory using simulated field sampling procedures. A gas mixing and handling system was set up to generate controlled flows of air with known amounts of methyl bromide and moisture content. Samples were taken from the air flow inside the system using regular field sampling equipment with typical operating parameters. Initial tests with the charcoal tubes showed breakthrough at very high relative humidity (RH > 90%) and recoveries near zero for extremely dry air (RH < 10%). Most of the data were limited to a humidity range of 20% to 80% and concentrations between 20 ppb and 2000 ppb. Within this range, no major effects of either humidity or concentration were found. The average recovery was 49% ± 7% (s.d.) for the charcoal tubes and 78% ± 12% (s.d.) for the SUMMA canisters.
Also reported are field data about the relative performance of collocated charcoal tubes and SUMMA canisters. A linear regression of the log-transformed concentrations of the two methods indicated that at the 200 ppb level the charcoal tube results were 71% of the SUMMA canister data. This was not significantly different from the same regression done on the laboratory data, where the charcoal tubes yielded 63% of the SUMMA canisters. Adjusting the relative performance of 71% in the field by the laboratory measured recovery of the SUMMA canisters of 78%, gives a net recovery of 55% for the charcoal tubes under field conditions. This net recovery of the field samples is consistent with the laboratory measured recovery of 49%.