Analysis of the degradation compounds of chemical warfare agents using liquid chromatography/mass spectrometry.

Abstract

The analysis of the degradation products of chemical warfare (CW) agents has been a challenge to analysts. The low volatility of these compounds makes them unsuitable for direct gas chromatography analysis without prior derivatization. Lack of a chromophore causes difficulties with classic detection methods after liquid chromatography separation. With the recent development of various interfaces that allow for the introduction of a liquid solvent stream into the mass spectrometer, the task of directly analyzing these compounds has become easier. For this report, we examined three different liquid chromatography/mass spectrometry (LC/MS) interfaces for their suitability for the analysis of CW degradation compounds. The interface types examined were particle beam electron impact ionization (PBI), electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). Several alkylphosphonates and thiodiglycol analogs that are produced from the degradation of organophosphorus nerve agents and sulfur mustard, respectively, were analyzed using each of the three techniques. Electron impact ionization following gas chromatography or particle beam introduction typically generates very reproducible, library-searchable mass spectra. Most of the CW breakdown compounds examined using the PBI interface did not produce a molecular ion. Despite the lack of a molecular ion, the mass spectra of the various compounds contained enough different structural information from fragment ions for the positive identification of each. The mass spectra generated using ESI are generally limited to protonated molecular ions with little or no fragmentation. For positive identification and confirmation, tandem mass spectrometry techniques quite often must be used. Many of the compounds in this study were characterized by prominent sodiated adducts along with the protonated molecular ion. Methylphosphonic acid produced protonated dimers, trimers, etc. Although the various adduct ions can be used for additional confirmation of the molecular weight of a compound, the adducts also can result in suppression of ionization of the compound and thus reduce sensitivity. Another 'soft' ionization technique that results in abundant protonated molecular ions is APCI. The mass spectra of the breakdown compounds produced using APCI were characterized generally by either a prominent protonated molecular ion or a dehydrated form of it. In addition, a number of structurally significant fragment ions were observed and their relative abundances could be adjusted by altering the APCI conditions. The data presented here indicate that each of the three techniques can be used successfully for direct liquid introduction and analysis of the non-volatile compounds produced from the degradation of CW agents. The mass spectra produced using each technique are quite different and could be utilized as additional confirmation of compound identity.