Method development for measuring trace levels of penicillins in aqueous environmental samples

A method based on liquid chromatography/mass spectrometry with an electrosp ray ion source and a single quadrupole instrument (LC/ES-MS) has been devel oped for determining trace levels of eight widely used penicillins in aqueo us environmental samples. Analyte extraction was performed from 4 L tap wat er, 2 L groundwater, 1 L river water, 0.2 L treated sewage and 0.1 L raw se wage, by using a Carbograph 4 cartridge. During removal of the solvent, pen icillins were purposely allowed to convert into their penicilloyl methyl es ters. This 'in situ' derivatization step resulted in a dramatic enhancement of the response of the ES-MS system for non-amphoteric penicillins. Analyt e recoveries were better than 80% irrespective of the type of aqueous sampl e, with the exception of amoxicillin (76%) and ampicillin (77%) in tap wate r. At the level of 50 ng/L of each analyte in ground water, the within-day precision was in the range 6-10%. Calibration curves were linear for inject ed amounts up to 800 ng, with R-2 in the range 0.9952-0.9995. When injectin g large equivalent volumes of the aqueous samples, the electrospray matrix effect altered in-source collision-induced dissociation (CID) spectra of th e analytes by severely weakening signals for fragment ions, as compared to spectra of reference standards. Remedies to obviate this anomalous unwelcom e effect are suggested. On the basis of a signal-to-noise ratio of 10, limi ts of quantification were estimated to range between 2 (cloxacillin) and 24 ng/L (amoxicillin) in river water. Copyright (C) 2001 John Wiley & Sons, L td.