Improving analytical confidence in the determination of PCBs in complex matrices by a sequential GC-MS/MS approach

Abstract

A method is described for achieving increased confidence in the selective determination of PCBs using capillary gas-liquid chromatography with tandem mass spectrometry (GC-MS/MS). It is well known that quantitation of PCBs by MS is susceptible to a false positive interference that arises from the co-elution of a higher PCB homolog with a lower PCB homolog (i.e., M-Cl+ from a higher homolog is M+ for a lower homolog). Because the elution order of the PCB congeners is not exactly proportional to increasing Cl content, frequent switching from MS/MS windows for specific homologs must take place. This approach can yield significant errors when matrix-induced retention time shifts occur. We therefore explored an alternative approach that involves repetitive analyses of a single extract. We developed and optimized a method that requires three injections, with homolog classes sequentially monitored as: 1-4-7-10, 2-5-8, and 3-6-9, respectively. The sequential design of the method entails the use of separate, broad MS/MS windows for each homolog class, thereby minimizing adverse matrix effects on retention variability. However, a consequent tripling of overall analysis time is incurred for each sample. The homolog classes are determined with high confidence (99%) that overlapping higher homolog fragments do not interfere with the quantitation of lower homologs. The method was demonstrated for extracts in small samples (∼750–1000 mg) from seven different freshwater biota species (n = 20) to illustrate a wide range of matrix-induced shifts. Application of the method resulted in more accurate quantitation, correcting an average 5.3% relative error (false positive bias) in observed concentration.