Development of a chemical ionization ion trap mass spectrometric method for trace level determination of molecular halogens

An ion trap mass spectrometric technique using negative ion chemical ionization has been developed for the quantitative determination of the molecular halogen species Br₂, Cl₂, 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^- → Br₂^- + NO₂ of (1.4 ± 0.6) × 10^-9 cm³ molecule^-1 s^-1, determined relative to published data for Cl₂ + NO₂^- → Cl₂^- + 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 Cl₂ and Br₂ 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 Cl₂ and 158, 160, and 162 for Br₂. The effects of water vapor on the cryogenic preconcentration step are quatitatively assessed.