Postcolumn derivatization liquid chromatography/mass spectrometry for detection of chemical-weapons-related compounds

Abstract

Postcolumn derivatization for liquid chromatography/mass spectrometry (LC/MS) analysis was characterized for detection of some compounds related to chemical-weapons (CW) agents using an Atmospheric Pressure Chemical Ionization (APCI) source. The derivatizing reagents were added directly to the LC eluent flow, and the derivatization reactions occurred in the APCI source under typical operating conditions. The compound S-[2-(diisopropylamino)ethyl] methylphosphonothioic acid was methylated using the derivatizing reagent trimethylphenyl ammonium hydroxide (TMPAH). Methylphosphonic acid was doubly derivatized to form dimethyl methylphosphonate, although the signal for the derivatization product was very sensitive to the amount of TMPAH. Arsenic compounds related to the CW agent lewisite, including chlorovinyl arsonous acid and arsenic (III) oxide, were derivatized using 2-mercaptopyridine. The thiol group reacted readily with the arsenic (III) center and provided a significant improvement in sensitivity relative to the underivatized signal using APCI or electrospray ionization. Triethanolamine and ethyl diethanolamine were derivatized with benzoyl chloride, a commonly used LC derivatizing reagent for alcohols, to modify their mass spectra. Postcolumn derivatization using an APCI source gives an alternative for detecting some difficult-to-ionize compounds. It has the limitations that sensitivity was not always improved even though the major mass spectral peaks can be shifted; it is necessary to carefully select the reagent; and some reagents introduced strong interference peaks at specific masses in the spectrum and may suppress the ionization of some derivatized analyte ions. The reagent also produced contamination in the source, which had to be cleaned daily.