Purge-and-Trap Gas Chromatography Method for Analysis of Methyl Chloride and Methyl Bromide in Seawater

Abstract

Purge-and-trap technique coupled with gas chromatography with electron capture detector (GC-ECD) is developed for analyzing the concentrations of methyl chloride (CH3Cl) and methyl bromide (CH3Br) in seawater. The optimum conditions were established as follows: purge time (12 min), purge gas flow rate (50 mL min−1), purge temperature (40°C), desiccant (Mg(ClO4)2), liquid nitrogen trap, boiling water desorption (2 min). The detection limits were 2.7 and 0.3 pM, the precision was < 2% and < 8%, and the recoveries were 89%–97% and 75%–85% for CH3Cl and CH3Br, respectively. Experiments for sample storage were carried out to test the reliability of the storage procedure used in this study, and the results showed that there was no significant change in CH3Cl and CH3Br concentrations in seawater within 24 h using this procedure. Merits of this method are that exact concentrations could be estimated and the analysis could be completed within 6 h in situ. This method was also used to measure in situ concentrations of CH3Cl and CH3Br in the surface seawater of the Jiaozhou Bay, and the results showed that the concentrations of CH3Cl and CH3Br ranged from 130 to 346 pM and 3.90 to 20.7 pM, respectively, which were in good agreement with those values reported in literature in other coastal waters. Our study shows that this method meets the demands of the measurement of in situ concentrations of CH3Cl and CH3Br in seawater.