Simplified Method of Processing for Multi-Residue Analysis by GC-MS/MS

Topics Covered

Introduction

Tandem mass spectrometer coupled to gas chromatography, such as GC-MS/MS, operated in multiple reaction monitoring (MRM) mode, has become the method of choice for targeted screening of multi-residue analysis in complex food matrix samples. The popularity of this method is due to the high specificity of results and the capability to monitor many product ions from a large number of pesticides. Analytical laboratories following established regulations are required to measure multiple MRMs for each target pesticides, i.e. quantitation ion (one) and qualified ion(s). The requirement to run several hundreds or even thousands of MRMs in one method is therefore common. MRM method development is complicated and time consuming, requiring the set-up of an MRM acquisition table and results processing.

Experimental

Instruments

Bruker SCIONTM triple quadrupole (TQ) mass spectrometer coupled to a Bruker 451-GC and CP 8400 Autosampler (see Figure 1) [1].

GC-MS Conditions

• Column: BR-5ms 15m X 0.25 mm X 0.25 µm
• Oven: 70°C (1 min) to 290°C (4 min) at 12°C/min Injection: Splitless, 270°C, 2 µL
• EI source: 70 eV, 260°C, 80 µA
• Transfer line Temp: 280°C
• Q2: Ar (1.5 mTorr)
• MRMs with Collision Energy (CE) – see Table 1

Figure 1. SCION TQ MS with 451-GC and CP8400 Autosampler.

Table 1. MRMs with Collision Energy (CE).

Topics Covered

Samples

Standards of 49 pesticides were prepared in 1:1 hexane/acetone (1, 5, 20, 50 and 100 ppb). Matrix sample was prepared by QuEChERS of pumpkin extract followed by SPE cleanup and reconstituted in 1:1 hexane/acetone.

Results

The new MS Workstation software MSWS 8 for SCION TQ uses compound-based screening (CBS) with a factory installed compound library containing hundreds of pesticides. Each library entry includes MRM information stored under each pesticide name.

Set-Up of an MRM Method Can Be Greatly Simplified By:

• Selecting all 49 pesticides from the library to export to Acquisition Method (Figure 2).
• Obtaining retention times (RT) and their windows from a few initial test runs.
• Inputting the average peak width and required data points per peak (10 to 15 normally) to automatically compute and assign the scan (dwell) time for each MRM for a timed MRM approach (Figure 3).

With The Use of CBS Data Processing Is Also Simplified By:

• Automatic data creation from acquisition method.
• Obtaining ion ratios from a standard data file without manually defining the ratios.
• Linking processing method to acquisition method for automatic update of RT, ion ratios and other method parameters.

Analysis Was Performed By 10 Replicate Injections of 1 ppb Pesticides Spiked In Pumpkin Extract Matrix between Two Solvent Calibration Curves, Each from 1 to 100 ppb.

• The 1 ppb spikes showed excellent signal and reproducibility with RSD<12% (n=10), and mostly 3.5-8% (Figure 4).
• Ion ratios of qualifier/quantifier are consistent for 1 ppb matrix spikes with RSD<6%.
• Calibration curves before and after QuEChERs extract samples both showed good linearity (R2>0.997) and with similar slope values, demonstrating good robustness.

Figure 2. MRM method for 49 Pesticides built with CBS.

Figure 3. Timed MRM window with autosegmentation of 49 Pesticides.

Figure 4. MRM Chromatograms of 1 ppb pesticides in QuEChERS matrix (n=10).

Conclusion

• Compound-based scanning (CBS) greatly simplified MRM- based method development and data processing for multi- residue analysis.
• The SCION TQ MS demonstrated good sensitivity, reproducibility and robustness in matrix.
• Ongoing work aims to provide all RT times for the pesticides in the CBS library and the capability to predict RT based on each pesticide’s retention index.

Authors

Helen (Qingyu) Sun and Kefei Wang, Bruker Chemical & Applied Markets Division (CAM), Fremont, CA.

References

[1] A compound-based approach to simplify method development and data processing for multi-residue analysis by GC-MS/MS. Poster at ASMS 2012, WP 573.Acknowledgement: Entire software development team at Bruker CAM Fremont, CA, for making the CBS from concept to reality.

Acknowledgement

Entire software development team at Bruker CAM Fremont, CA, for making the CBS from concept to reality.

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