A fast-response method for determination of atmospheric isoprene using quadrupole ion trap mass spectrometry

A fast-response detection system for measurement of atmospheric isoprene (2-methyl-1,3-butadiene) is described which involves a tubular silicone membrane sampling system interfaced to a quadrupole ion trap. This method can be adapted to determination of a variety of atmospherically important alkenes that have short atmospheric lifetimes and, thus, for which high-frequency measurements are desirable. In this paper, we describe the development of a novel multimembrane air-sampling system in conjunction with selective chemical ionization and tandem mass spectrometry for the rapid quantitative determination of isoprene and other alkenes. In this method, we use vinyl methyl ether as an alkene-selective CI reagent, with which isoprene undergoes a [2 + 4] cycloaddition reaction; followed by loss of methanol, to generate an adduct ion at m/z 94 [M + 58 - 32](.+), This adduct ion undergoes loss of a methyl radical upon collision-induced dissociation. Quantitation is based on measurement of this fragment ion (m/z = 79), In this method, isoprene and other volatile organic compounds are preconcentrated using a combination silicone membrane and cryogenic trap inlet system. A linear calibration curve was obtained for gas-phase isoprene concentrations between 0.5 and 10 ppb. The multimembrane sampling system was shown to be relatively insensitive to changes in sample humidity and to membrane temperature.