Chemical Ionization of Large Linear Alkanes and Small Oxidized Volatile Organic Compounds by the Reactions with Atomic Gold Cations.

Abstract

Although the use of proton-transfer-reaction mass spectrometry (PTR-MS) in real-time measurements of atmospheric volatile organic compounds (VOCs) has expanded rapidly in recent years, PTR-MS has several serious limitations related to H3O+ reagent ion chemistry, such as considerable fragmentation for large alkanes, low sensitivity to formaldehyde, and inability to separate isomeric aldehydes and ketones. One of the key means to address these limitations is to develop other appropriate reagent ions. In this paper, the reactivity of Au+ toward large n-alkanes (C7-C10), formaldehyde, acetaldehyde, propanal, and acetone under high-pressure buffer gas has been explored by mass spectrometry and theoretical calculations. For large n-alkanes, extensive fragmentation was avoided with observations of hydride abstraction products. Formaldehyde could react effectively with the Au+ ion by splitting off H2 or CO. Propanal and acetone behaved with different reaction channels and could be easily distinguished. The hydride transfer for propanal and methyl anion transfer for acetone were observed. These results show that the use of Au+ reagent ion chemistry may settle some problems of VOCs detection by H3O+. In addition, reactions between Au+ and the tested VOCs were found to take place at the gas collision rate and the detection limit of Au+ was estimated to be as low as several parts per trillion by volume. Thus, the gold cation Au+ can serve as a useful reagent ion to identify and quantify trace amounts of linear alkanes and several small oxidized VOCs in the atmosphere.