Direct determination of dialkyl phosphates by strong anion‐exchange liquid chromatography/atmospheric pressure chemical ionization mass spectrometry using a quadrupole ion trap instrument

The direct determination of dialkyl phosphates (DAPs) in water by strong anion‐exchange (SAX) liquid chromatography/atmospheric pressure chemical ionization (APCI) mass spectrometry was investigated. The SAX high‐performance liquid chromatography (HPLC) column was eluted with methanol/water gradients containing ammonium formate (AF) separating the DAPs which included six dimethyl‐ and diethyl‐substituted phosphates, thiophosphates, and dithiophosphates. The high buffer concentrations required for separation were compatible with −ve APCI, but in +ve APCI the DAPs were unstable giving anomalous ions such as [M+15] + and [M+29] + . These ions are believed to result from ion molecule reactions with CH 3 OH   2 +in the plasma. DAPs are very stable in −ve APCI being detected as abundant [M–H] − ions, even with 200 mM AF. At higher AF concentration formate clusters ([M+45] − and [M+91] − ) were seen. Fragmentation by collision‐activated dissociation (CAD) was more efficient for deprotonated ethyl‐substituted DAPs which lost ethylene followed by ethanol. APCI instrument detection limits were in the low ng/mL range and the response was highly linear. Isotope dilution quantitation using d 10 ‐diethyl dithiophosphate (DEDTP) as an internal standard produced an instrument detection limit of 2 ng DEDTP/mL and method detection limit (MDL) of 9.3 ng/mL with accuracy of 99% (spike concentration, 25 ng/mL). DAP mixtures required storage in cold, dry conditions and alcohol solvents should be avoided because of solvolysis reactions. Copyright © 2008 John Wiley & Sons, Ltd.