3-(Bromoacetyl)Pyridine - a Highly Specific Derivatization Reagent for the Determination of Organophosphorus Acids by Liquid Chromatography Positive Electrospray Ionization Tandem Mass Spectrometry

Avi Weissberg, Analytical Chemistry, IIBR, Ness Ziona, Israel
Moran Madmon, Analytical Chemistry, Iibr, Ness Ziona, Israel
Shai Dagan, Analytical Chemistry, Iibr, Ness Ziona, Israel

Negative electrospray ionization (ESI-MS/MS) is the first choice for detection and identification of organophosphorus acids such as alkyl alkylphosphonic acids, dialkyl phosphates, dialkyl thiophosphates etc. However, in the presence of interfering background chemicals, their ESI mass spectra are dominated by chemical noise in the lower mass region. In addition, poor chromatographic retention, mainly of the small highly polar acids render identification of such acids difficult, mainly in complex matrices such as urine. Attachment of a pyridine moiety by derivatization allows to shift the parent ions to a higher mass region, separate the analyte from interferences as well as improve the chromatographic retention and sensitivity as positive ESI is usually more sensitive and stable than negative ESI. However, loss of analytical specificity in MS-MS spectra of these derivatives, due to charge retention on the reagent leaving group, may hamper their identification.

In this work, derivatization of several polar organophosphorus acids with commercially available, mostly pyridine-based reagents, possessing a highly proton-affinitive amine, was evaluated. A simple and generic derivatization procedure of such acids with the reagents, in presence of potassium carbonate and 18 crown 6 in acetonitrile, was developed. After derivatization, the samples were analyzed by LC-ESI-MS/MS and a fragmentation study was carried out. In all derivatizing reagents the resulting acid-derivatives were highly LC retainable and responsive in ESI-MS-MS operating in the positive-ion mode. However, in MS-MS experiments loss of specificity was observed for most of them. Interestingly, each derivatizing ragent exhibits a different MS-MS behavior towards the analytes. Some undergo considerable collision-induced dissociation exclusively at the amine tag portion while others undergo dissociation both, at the analyte portion and at the amine portion. From all tested reagents, only 3-(Bromoacetyl)pyridine derivatives demonstarted highly specific, providing rich informative MS-MS spectra exhibiting analyte-characteristic product ions during MS-MS.