Multiresidue method for analysis of pesticides in liquid whole milk
Bennett, D.A.; Chung, A.C.; Lee, S.M.
Journal - Association of Official Analytical Chemists 80(5): 1065-1077
1997
ISSN/ISBN: 0004-5756 PMID: 9325583 Document Number: 471207
A multiresidue method to analyze liquid whole milk for 59 compounds was developed. The method involves a single extraction and cleanup strategy for many classes of compounds-organophosphorus compounds (OPs), organochlorine compounds (OCs), N-methylcarbamates (MCs), etc. Initial extraction is performed with ethanol-ethyl acetate as solvent and sodium sulfate as drying agent. A portion of the extract is concentrated to an oily consistency, and target analytes are partitioned into acetonitrile. Further cleanup is achieved by sequential solid-phase extractions - octadecyl (C-18)-bonded silica cartridges followed by aminopropyl (NH-2)-bonded silica cartridges. The solvent is exchanged to acetone for analysis by gas chromatography (GC) with electrolytic conductivity, flame photometric (FPD) or mass spectrometric (MS) detection and to methanol for analysis by liquid chromatography with postcolumn derivatization. Typical limits of detection (LODs; peak-to-peak signal- to-noise ratio gtoreq 3) were 0.3 ppb for OPs, 0.9 ppb for OCs and MCs, and 9 ppb for mass-selective detection. From the level of quantitation (LOG = 3.33 times LOD) to 10 times LOG, linear instrument response was observed for all detectors except the FPD which required a second-order calibration curve. Average recoveries for spikes at the LOG ranged from 69 to 127%, with standard deviations of about 10%. Similar accuracy and precision were observed for fortifications at 5 times LOG and 10 times LOG. The method was used to analyze 20 milk samples from various liquid milk processing plants. Incurred residues were confirmed by high-resolution GC/MS and GC/MS/MS.