Quadrupole GC-MS System for Pesticide Analysis in Tea

This article discusses how triple quadrupole GC-MS with solid phase micro extraction (SPME) technology can be used to perform fast and cost-efficient analysis of pesticides in complex tea leaf matrices, while reducing the use of solvents, thereby providing a more environmentally sensitive sample preparation method. Proof of concept is demonstrated for pesticide analysis in heavy matrix without the need for typical laboratory extractions, and the results justify further method development.

Experiment

The method outlined below analyzes a list of pesticides by tandem mass spectrometry, saving a significant amount of time and money for laboratories and complying with industry regulations.

For this experiment, SPME (Supelco) was used to screen and confirm pesticides on the Thermo Scientific TSQ Quantum XLS triple quadrupole GC-MS using multiple reaction monitoring (MRM). This method minimizes solvent consumption and extraction time.

Dry tea leaves were homogenized with a blender and 1 g added to 20 mL volatile organic analysis vials. The dry tea was spiked with standards for the development of a curve, and 3 mL of 1% MeOH in deionized water saturated with NaCl was added. The vial was then capped, swirled gently, and placed on the autosampler (see Figures 1a and 1b, p. 30).

The SPME technology was programmed to incubate and agitate the sample and expose the SPME fiber to the vial headspace. This eliminated the need for traditional extractions, improving laboratory productivity by enabling fast sample preparation. Due to the high levels of aroma, flavors, and polyphenols in tea, however, the detection system must be able to deal with the volatile matrix.

After a designated period of time to reach equilibration, the fiber was inserted into the inlet of the GC to begin pesticide analysis. A matrix blank was analyzed in full scan to show the interfering compounds (see Figure 2, p. 31). Using MRM, triple quadrupole GC-MS separated the interfering volatile compounds from those of interest and provided the sensitivity required to reach the required MRLs for all of the evaluated pesticides. In this example, pesticide levels were examined in relation to Japanese MRLs, some of the most stringent in the world.

Results and Discussion

Calibration curves were developed for the 28 compounds listed in Table 1 (above). For these compounds, the correlation coefficient of the calibration curves was r2>0.995. Two or more transitions were used for each compound, and confirming ions were found to be within limits set by the European Union. An additional 28 compounds that do not currently have MRLs for tea in Japan were analyzed in the standard mix. The additional compounds were used to show applicability of the technique in a heavy matrix. Initial calculations show that laboratories using this technique can expect cost savings of approximately $158,000 per year from accelerated sample preparation and, ultimately, increased pesticide analysis throughput.