Techniques for quantifying ambient HOCl using Atmospheric Pressure-Ionization Mass Spectrometry

It is important to be able to measure ambient HOCl concentrations at atmospheric pressure for a better understanding of phenomena such as spring-time surface-level O₃ depletion in the Arctic and the role of halogens in mid-latitude marine boundary layer chemistry. Atmospheric-pressure-ionization mass spectrometry (API-MS) is a highly-sensitive technique for quantitative measurement of other halogen compounds such as Cl₂. We report techniques for quantifying HOCl with API-MS (Perkin-Elmer Sciex, Model 300) in the negative ion mode. Gaseous HOCl samples were formed by bubbling air through an HOCl solution and into a Teflon reaction chamber where the HOCl was diluted with additional air. Air and bromoform ion sources were used interchangeably for chemical ionization of the HOCl samples. The HOCl*O₂^- (m/e=84, 86) adduct observed with air ionization and the HOCl*Br^- (m/e 131, 133, 135) adduct observed with bromoform ionization were used to monitor HOCl. Because the HOCl sample preparation method does not allow precise determination of the HOCl concentration directly, HOCl was titrated with an HCl/H₂O solution at 243 K to form Cl₂:

HOCl + HCl -> Cl₂ + H₂O

Molecular chlorine signals (m/e 70, 72, 74) were readily calibrated, and thus HOCl concentrations were determined. These techniques have been employed to detect HOCl concentrations below 100 ppb in the laboratory. The effects of relative humidity and acid concentration on the detection of HOCl and implications for the measurement of HOCl in the atmosphere will be discussed.