A method for identification of some alkyl methylphosphonic acids in aqueous samples by liquid chromatography coupled to thermospray tandem mass spectrometry is described. The compounds are separated by reversedphase liquid chromatography and identified by their daughter spectra after collision-induced dissociation of the quasi-molecular ions produced by the thermospray interface. The combination of liquid chromatographic separation of the acids with detection of the compoounds by tandem m a s spectrometry gives enough information to identify the compounds. A twenty-to-fifty-times improvement in the signal-to-noise ratios, compared to normal thermospray detection, has been observed in these experiments.
An international treaty which bans production, stockpiling and use of chemical weapons has now been opened for signature and is expected to come into force within a short time. Chemical analysis of different sample materials for the presence of chemical warfare agents will play an important role in verification of such a treaty. Many of the chemical warfare agents are unstable, so that additional information can be obtained from determination of their degradation products. When any allegation of violation of the Chemical Weapons Convention is investigated, the unambiguous identification of such degradation products would give crucial information, especially if some time has passed since the alleged violation had taken place. Some of the nerve agents, a well known group of chemical warfare agents, are rapidly decomposed to 0-dkyl-substituted methylphosphonic acids and further to the methylphosphonic acid itself in matrices containing water. These methylphosphonic acids are polar, non-volatile compounds which are easily extracted from various matrices with water. The most common analytical regime for determination of these compounds has been gas chromatographic (GC) analysis after derivatization. Methylation with diazomethane'*2 or trimethylphenylammonium hydroxid3 and formation of trimethylsilyl ethers," rert-butyldimethylsilyl ethers' or pentafluorobenzyl esters6 of the acids have been the most popular techniques. A more direct approach would be liquid chromatographic (LC) analysis, but the lack of suitable detectors has prevented extensive use of this technique. Micro-LC with flame photometric' or phosphorus-sensitive"" GC detectors has, however, been used with success by Kientz er al., but has not yet been commercialized. The recent development of liquid chromatography/mass spectrometry (LC/MS) systems has made sensitive determination possible, and both thermospray," nebulization-assisted electrospray'2 and simple electrospray". l4 LC/MS interfaces have been used to identify and quantify alkyl methylphosphonic acids.
Both thermospray and electrospray are soft ionization techniques which give abundant quasi-molecular (e.g. W+H]' or p+NH,]') ions. The unambiguous identification of chemical warfare agents or their degradation products by mass spectrometric analysis is, however, dependent on the existence of certain specific fragmentation patterns in the spectrum. This fragmentation cannot be achieved by thermospray alone. Collision-induced dissocia-