Analysis of Pesticide Residues in Eggs by Direct Sample Introduction/Gas Chromatography/Tandem Mass Spectrometry

Direct sample introduction (DSI) or "dirty sample injection" is a rapid, rugged, and inexpensive approach to large volume injection in gas chromatography (GC) for semivolatile analytes such as pesticides. DSI of complex samples such as eggs requires a very selective detection technique, such as tandem mass spectrometry (MS−MS), to determine the analytes among the many semivolatile matrix components that also appear. In DSI, the nonvolatile matrix components that normally would contaminate the GC system in traditional injection methods remain in a disposable microvial, which is removed after every injection. For example, 3 μg of nonvolatile residue typically remained in the microvial after an injection of egg extract using the DSI method. This analytical procedure involves the following: (i) weighing 10 g of egg in a centrifuge tube and adding 2 g of NaCl and 19.3 mL of acetonitrile (MeCN); (ii) blending for 1 min using a probe blender; (iii) centrifuging for 10 min; and (iv) analyzing 10 μL (5 mg of egg equivalent) of the extract using DSI/GC/MS−MS. No sample cleanup or solvent evaporation steps were required to achieve quantitative and confirmatory results with <10 ng/g detection limits for 25 of 43 tested pesticides from several chemical classes. The remaining pesticides gave higher detection limits due to poor fragmentation characteristics in electron impact ionization and/or degradation. Analysis of eggs incurred with chlorpyrifos-methyl showed a similar trend in the results as a more traditional approach. Keywords: Pesticide residue analysis; direct sample introduction; gas chromatography; tandem mass spectrometry; eggs