Laboratories involved in pesticide residue analysis in foods are often challenged to analyse hundreds of different pesticides in hundreds of different sample types of varying matrix complexity. Unfortunately, after decades of development, there is still no ‘magic bullet’ method free of compromises.
The trend today is towards the use of the QuEChERS (Quick, easy, cheap effective, rugged and safe) approach1 or in other words faster, simpler, more generic extraction methods, using low sample amounts and low solvent volumes, but with ‘just enough’ clean-up. While this approach has revolutionised pesticide residues analysis by facilitating higher sample throughput, increased method scope and lower cost per sample, there still exists a compromise. Is the dispersive solid-phase extraction (d-SPE) clean-up ‘just enough’ or not enough, resulting in an increased additional and hidden cost?
The use of minimal clean-up can result in extracts with high concentrations of matrix co-extractives which inevitably lead to faster contamination of the analytical system, requiring unscheduled maintenance and increased downtime leading to increased costs. The ever-increasing sensitivity of mass spectrometers enabling higher fold dilution of the extracts, and hence lower concentrations matrix co-extractives, can to some extent, and for certain sample types, lessen the rate of contamination. Still, more complex samples, such as dry commodities, and more susceptible separation techniques, such as gas chromatography, will benefit from implementation of a more effective clean-up step before injection.
Solid Phase Extraction in a cartridge format is more effective than d-SPE because of the more intimate contact of the sorbent(s) with the sample. But why is there is a reluctance to use cartridge SPE? Most likely because of the time required for manual operation and time taken to determine a satisfactory balance between removal of co-extractive without loss of analytes. So, could automation be the answer?
Automated Online µSPE Clean-up
The use of online clean-up using SPE in a mini cartridge format was originally reported by Morris and Schriner2. Thermo Fisher Scientific has since collaborated with CTC Analytics to establish an automated online SPE system using miniaturized cartridges for GC-MS and LC-MS analysis. The fully integrated systems are built around the Thermo Scientific™ TriPlus™ RSH™ multi-purpose robotic autosampler, which supports fully automated workflows for GC-MS/MS and LC-MS/MS. The analyst simply transfers an aliquot of the un-cleaned supernatant fraction from the initial QuEChERS extract into a sample vial placed in a rack. The robotic sampler automatically conditions the μSPE cartridge (positioned in an adjacent rack), loads and elutes the sample then injects the cleaned-up extract into the chromatographic system for the analysis. The configuration and operation of the system are explained in detail in Application Note 65684 titled, Multi-pesticide residues analyses of QuEChERS extracts using an autom.... Since the clean-up steps automatically take place during ongoing analysis of the previous sample the instrument cycle times are not increased.
Results Using µSPE
The cartridges contain an optimised blend of sorbents for maximum removal of matrix components (pigments, lipids, etc.), but minimum losses of analytes with applicability to a wide range of sample types. Recovery of 195 LC-amenable pesticide compounds (parent pesticides, metabolites and isomers) spiked into grape, rice, and tea matrices were mostly within 70-120% with associated RSD <20% and LOQs of 0.01 mg/kg for 80% of the compounds in tea and 94% of the compounds in grape and rice samples. All obtained using the low cost, entry-level Thermo Scientific™ TSQ Fortis™ triple quadruple mass spectrometer. Even better results were also obtained for GC-amenable pesticides using the Thermo Scientific™ TSQ™ 9000 triple quadrupole GC-MS/MS system. At 0.01 mg/kg the recovery (n=6) was between 70-120% and the associated %RSD <20% for 205 of 209 pesticides in rice and 203 of 209 pesticides in wheat. A recent study also compared the online miniaturized SPE clean-up with manual d-SPE for the GC-MS/MS multi-residue analysis of pesticides in a range of spice matrixes, highlighting higher recovery and overall improvement in workflow efficiency3.
What are the benefits of automated online µSPE?
Lower % RSDs and higher % recoveries compared with manual d-SPE, leading to more reliable quantitation of targets analytes.
More efficient removal of matrix co-extractives for higher selectivity and sensitivity.
Improved robustness as demonstrated by %RSDs lower than 10% during the LC-MS/MS analysis of 200 consecutive injections of extracts of tea.
Extended automation to minimize the risk of human error.
Unattended operation and time savings compared to manual d-SPE.
Improved productivity with a decrease in system maintenance and increased instrument uptime.
Enabling excellent results even with entry-level triple quadrupole MS systems.
Unified clean-up operations for improved efficiency in routine testing laboratories.
1 Michelangelo Anastassiades Steven J. Lehotay, Darinka Štajnbaher, Frank Schenck, (2003) J. AOAC International Vol. 86, No. 2, pp.412-431
2Morris, B.D. and Schriner R.B. Development of an Automated Column Solid-Phase Extraction Cleanup of QuEChERS Extracts, using a Zirconia-based Sorbent, for Pesticide Residue Analyses by LC-MS/MS. J. Agric. Food Chem. 2015, 63, 5107-5119