Optimization of ion trap parameters for the analysis of dilute samples in the presence of an interfering matrix: Analysis of polychlorinated biphenyls in transformer oil

Abstract

The analysis of dilute samples in the presence of an interfering matrix was improved by optimizing the operating parameters of an ion trap in a GC-ion trap system. For a mixture of nine polychlorinated biphenyls (PCBs) in 1% transformer oil, two strategies were successfully employed to increase signal-to-noise ( S/N) ratios. By limiting the mass range scanned to those m/z values that include most of the ion fragments due to PCBs and increasing the number of microscans per scan in this regime, S/N values were increased by a factor of 1.5. Further improvements in S/N by up to a factor of ten are achieved by raising the radio frequency (rf) storage level so that low molecular weight ions due to the transformer oil are not stored. An upper limit on how high the rf storage level can be adjusted is determined by the storage efficiency of the ion trap which decreases at high rf storage levels. Ion dissociation is also a limiting factor at high rf storage levels. Best detection limits for five of the nine PCBs studied were obtained when the rf storage level was tuned to m/z = 74, a regime where most of the transformer oil ions are no longer stored.