Techniques for quantifying gaseous HOCl using atmospheric pressure ionization mass spectrometry

Abstract

HOCl is speculated to be an important intermediate in mid-latitude marine boundary layer chemistry and at high latitudes in spring-time when surface-level O₃ depletion occurs. However, techniques do not currently exist to measure HOCl in the troposphere. We demonstrate that atmospheric pressure ionization mass spectrometry (API-MS) is a highly sensitive and selective technique which has promise for HOCl measurements both in laboratory systems and in field studies. While HOCl can be measured as the (HOCl·O₂)^- adduct with air as the chemical ionization (CI) reagent gas, the intensity of this adduct is sensitive to the presence of acids and to the amount of water vapor, complicating its use for quantitative measurements. However, the addition of bromoform vapor to the corona discharge region forms bromide ions that attach to HOCl and allow its detection via the (HOCl·Br)^- adduct. The ion–molecule chemistry associated with the use of air or bromoform as the CI reagent gas is discussed, with particular emphasis on the effects of relative humidity and gas phase acid concentrations. HOCl is quantified by measuring the amount of Cl₂ formed by its heterogeneous reaction with condensed-phase HCl/H₂O. Detection limits are ∽3 parts per billion (ppb) using air as the CI reagent gas and ∽0.9 ppb using bromoform. The atmospheric implications of this work are discussed.