An ion trap mass spectrometric technique using negative ion chemical ionization has been developed for the quantitative determination of the molecular halogen species Br-2, Cl-2, and BrCl. The technique utilizes NO2- as a chemical ionization reagent in an electron-transfer reaction to form the corresponding molecular anions of the halogen species, lending excellent selectivity to the measurement. Reaction rate experiments performed in the ion trap yield a rate constant for Br-2 + NO2- --> Br-2(-) + NO2 of (1.4 +/- 0.6) x 10(-9) cm(3) molecule(-1) s(-1), determined relative to published data for Cl-2 + NO2- --> Cl-2(-) + NO2. This paper describes a mass spectrometer pinhole inlet design and cryogenic preconcentration system for detection of the molecular halogens at atmospherically relevant concentrations. linear calibration curves were obtained for Cl-2 and Br-2 over 3 orders of magnitude and indicate limits of detection of 50 and 8 pmol for 3.8- and 5.1-L samples, respectively, corresponding to 220 and 50 parts per trillion (mole/mole). Quantitation is based on the total signal at m/z values of 70, 72, and 74 for Cl-2 and 158, 160, and 162 for Br-2. The effects of water vapor on the cryogenic preconcentration step are quatitatively assessed.
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