Analysis of PCBs in Food and Biological Samples Using GC-Triple Quadrupole MS–MS

Dirk Krumwiede and Hans-Joachim Huebschmann, Thermo Fisher Scientific Inc., Bremen, Germany.

This application details a fast, reliable and highly selective trace-level screening method for the quantification of polychlorinated biphenyls (PCBs) in environmental, food and biological samples, using gas chromatography and a triple stage quadrupole mass spectrometer. The analytical strategy is analogous to the well-established US Environmental Protection Agency (USEPA) Method 1668A.

The international Stockholm Convention on persistent organic pollutants (POPs) recognizes polychlorinated biphenyls (PCBs) among 12 of the world's most dangerous chemicals, and PCB levels are reported to stay unaffected globally.¹ Monitoring PCB levels will continue for years, particularly for dangerous dioxin-like (dl) PCBs. In particular the 12 dl-PCBs — non-ortho- and mono-ortho-substituted PCBs — are the focus of food safety controls because of toxicity similar to 2,3,7,8-TCDD. The dl-PCBs also contribute significantly to the sample toxic equivalents (TEQ) value.

This application details the use of the Thermo Scientific TSQ Quantum GC triple quadrupole mass spectrometer for the quantification of PCBs and dl-PCBs in environmental, food and biological samples. The analytical strategy is analogous to US Environmental Protection Agency (USEPA) Method 1668A.² As a result of differences in analytical response, each PCB chlorination degree is measured against its own isotopically labelled internal standard. Internal standards are labelled with ¹³C on the biphenyl backbone, yielding 12 labels. Isotope dilution quantification is performed by spiking the ¹³C-labelled PCBs into each sample, enabling accurate identification of and correction for the concentration of the native (unlabelled) compounds.

Experimental Conditions

Sample analyses were performed using the Thermo Scientific TSQ Quantum GC GC–MS–MS system, equipped with a Thermo Scientific TRACE GC Ultra gas chromatograph. The TRACE GC Ultra was configured with split/splitless injector and sample introduction was performed using the Thermo Scientific TriPlus AS liquid autosampler. The capillary column was a Thermo Scientific TRACE TR-Dioxin 5MS column (5% phenyl film) of 30 m length, 0.25 mm inner diameter and 0.10 µm film thickness. Commercially available EPA 1668 standards (Wellington, Guelph, Ontario, Canada) were used. The treatment of samples, internal standards and overall analytical strategy complied with EPA Method 1668A. PCB samples were analysed in standard and then in extracted samples to evaluate matrix performance.³

MS–MS transitions from two PCB precursor ions were monitored and individual product ions for each chlorination degree were detected, providing five identification points for each PCB to comply with EU Commission Directive 96/23/EC.⁴ The TSQ Quantum GC was programmed in selected reaction monitoring (SRM) mode to use six retention time windows with overlapping masses for all 10 levels of chlorination. The high number of masses covered in each segment demonstrates the capacity of the TSQ Quantum GC for parallel multicomponent detection.

Results

With two independent transitions based on two different precursor ions, the TSQ Quantum GC method exceeds the high certainty required by the EU directives by providing five points of identification. The high speed of the TSQ Quantum GC analyser also generates a sufficient number of data points to effectively characterize chromatographic peaks, even when monitoring two chlorination degrees in each SRM window. This allows for reliable peak integration and quantification.

A number of challenging sample types were prepared to test the performance in matrix. The TSQ Quantum GC demonstrated excellent sensitivity, selectivity and robustness with these samples, as shown in Figure 1, which displays dl-PCBs in blood. PCB concentrations were measured at a range of different concentrations in calibration solutions and samples, down to 200 fg/µL. The selectivity of the TSQ Quantum GC virtually eliminates matrix interference, allowing for low detection limits, enhanced confidence in quantitative results and accurate identification of these compounds.

Figure 1

Conclusions

The Thermo Scientific TSQ Quantum GC facilitates the screening and quantification of PCBs at low levels in difficult matrix samples. The analytical set-up complies with USEPA Method 1668A, following an isotope dilution quantification protocol. The MS–MS measurement scheme using two precursor ions and SRM detection of individual product ions is a valuable solution for screening for PCBs in various complex matrices at the relevant levels. The TSQ Quantum GC offers superior and uniform selectivity for low-level PCB samples in complex matrices. For fast quality control of food samples, GC–MS–MS with the TSQ Quantum GC exceeds the current EU directives for a minimum of four identification points, in that the method described here offers five identification points. This method also provides a high productivity solution with increased sample throughput.

References

1. W.E. Turner et al., "Instrumental approaches for improving the detection limit for selected PCDD congeners in samples from the general U.S. population as background levels continue to decline." Proceedings of the Dioxin Conference, Oslo (2006).

2. Method 1668, "Revision A: Chlorinated Biphenyl Congeners in Water, Soil, Sediment, and Tissue by HRGC/HRMS," United States Environmental Protection Agency, Office of Water, EPA No. EPA-821-R-00-002 (December 1999).

3. D. Krumwiede and H-J. Huebschmann, "Analysis of PCBs in Food and Biological Samples Using GC Triple Quadrupole GC-MS/MS." Thermo Fisher Scientific Inc. Application Note AN10262. www.thermo.com.

4. EU Commission Directive 96/23/EC concerning the performance of analytical methods and the interpretation of results (12 August 2002). see http://eur-lex.europa.eu.

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