Characterization of alkylphosphonic acid vapors using atmospheric flow tube-ion trap mass spectrometry

A key aspect of detecting hazardous compounds at ultra-trace levels for processing, compliance, and clean-up purposes involves developing methods that are not only sensitive, but also highly selective with minimal sampling effort. Atmospheric flow tube mass spectrometry (AFT-MS) using dielectric barrier discharge ionization has emerged as a technique that combines such features for vapor detection. AFT-MS is thus appealing for application to ambient screening for chemical warfare agents (CWAs) and their degradation products. Initial characterization of AFT-MS for CWA detection necessitates examining less harmful simulant species. A predominant hydrolysis product of most organophosphorus CWAs is methylphosphonic acid and most other hydrolysis products consist of some form of an alkylphosphonic acid.

An application of AFT-MS is presented wherein a homologous series of four alkylphosphonic acids (methyl-, ethyl-, propyl-, and t-butylphosphonic acid) were first qualitatively evaluated as anionic clusters with nitrate. These anionic adducts were subsequently quantified from non-equilibrium headspace vapor sampled over alkylphosphonic acid solutions in methanol.

The series of phosphonic acids demonstrated consistent relative ion abundances thought to be related at least in part to the relative vapor pressures depending on their alkyl chains. For quantitation, the resulting linear ranges were found to be 2 to 50 ppm_soln for methylphosphonic acid, 5 to 50 ppm_soln for ethylphosphonic acid, and 2 to 25 ppm_soln for propylphosphonic acid and t-butylphosphonic acid; quality controls of 15 ppm_soln were used to assess the quantitation accuracy.

Although measured over a limited dynamic range, the real-time analysis afforded by this method suggests the feasibility of using thermodynamically stable anionic adducts to monitor organophosphorus compounds via AFT-MS. In addition, this is proof-of-concept for the use of this ambient sensing technique to detect phosphonic acids. Furthermore, a discussion is included regarding gaps in clustering thermodynamics literature that would assist in uncovering physical or chemical explanations for observed trends.