Trace analysis: LC-MS/MS for low-level pesticide detection

To complicate matters, detection and quantitation workflows required to detect pesticides vary across sample matrices.

Pesticides are applied to crops to protect them from the negative effects of various pests.

When done inappropriately, an abundance of residues on foods and food products can have harmful health effects.

Multiple pesticide residue detection​

Gas chromatography (GC) and/or liquid chromatography (LC) coupled to mass spectrometry (MS) can enable detection and identification of multiple pesticide residues at the same time in a given matrix.

However, to determine the amount of each pesticide accurately, analytical methods are expected to achieve desired sensitivity so the low quantitation levels for each analyte are met.

Targeted methods can be based on triple quadrupole mass spectrometry while non-targeted screening is typically done by LC high resolution accurate mass (HRAM).

Liquid chromatography-tandem mass spectrometry (LC-MS/MS) has become the method of choice for multi-residue analysis and quantitation due to its high selectivity and sensitivity.

It allows for analysis of a small volume of sample to obtain levels of detection at or below maximum residue limits (MRL).

European Union (EU) regulation 396/2005 and Commission Directive 2006/125/EC set MRLs for pesticides in food products from plant and animal origin.

These guidelines have established levels of detection that represent an analytical challenge in reaching such low limits of quantitation but advances in quantitation technologies can meet these regulations.​

There are more than 1,600 substances identified and used as pesticides according to the British Crop Production Council (BCPC) Pesticides Manual.

These can be classified by function such as fungicides, herbicides, and insecticides or can be labelled by chemical classes including organophosphates, carbamates and organochlorines.

Single and multi-analyte methods​

Using targeted multi-analyte methods for an extensive list of pesticides combined with various single residue methods, a comprehensive analysis of hundreds of pesticide residues can be developed.

Single residue methods can be applied to groups of residues that are chemically similar and so would not separate well if analyzed in a group. It is a challenge to know when to use which method, depending on the sample and potential analyte mix.

When using complex sample matrices, homogenization and use of QuEChERS (Quick, Easy, Cheap, Effective, Rugged and Safe) methods for sample preparation and analyte extraction has proven to generate a clean enough sample for LC-MS/MS to detect all residues within the sample.

In addition to the sample clean-up process, LC-MS/MS delivers increased robustness and throughput, ensuring increased confidence in the method with better quality results, faster than before.

Reliable separation and detection of pesticide compounds can be achieved using the timed-selected reaction monitoring (SRM) mode, which reduces the number of SRM transitions monitored in parallel within each retention time (RT) window.

Good and robust performance at short dwell times helps in reaching the expected limits of quantitation for several pesticides across multiple matrices for each sample, every day.

• Dr Debadeep Bhattacharyya, senior manager of product marketing (Triple Quadrupole Mass Spectrometry), for all business in the applied market division at Thermo Fisher Scientific