High-sensitivity elemental ionization for quantitative detection of halogenated compounds

The rising importance of organohalogens in environmental, pharmaceutical, and biological applications has drawn attention to analysis of these compounds in recent years. Elemental mass spectrometry (MS) is particularly advantageous in this regard because of its ability to quantify without compound-specific standards. However, low sensitivity of conventional elemental MS for halogens has hampered applications of this powerful method in organohalogen analyses. To this end, we have developed a high-sensitivity elemental ion source compatible with widely available atmospheric-sampling mass spectrometers. We utilize a helium-oxygen plasma for atomization followed by negative ion formation in plasma afterglow, a configuration termed as plasma-assisted reaction chemical ionization (PARCI). The effect of oxygen on in-plasma and afterglow reactions is investigated, leading to fundamental understanding of ion generation processes as well as optimized operating conditions. Coupled to a gas chromatograph, PARCI shows constant ionization efficiency for F, Cl, and Br regardless of the chemical structure of the compounds. Negative ionization in the afterglow improves halide ion formation efficiency and eliminates isobaric interferences, offering sub-picogram elemental detection for F, Cl, and Br using low-resolution MS. Notably, the detection limit for F is about one order of magnitude better than other elemental MS techniques. The high sensitivity and facile adoptability of PARCI pave the way for combined elemental-molecular characterization, a comprehensive analytical scheme for rapid identification and quantification of organohalogens.