Pesticides in Imported Produce

The QuEChERS protocol uses less expensive and fewer solvents and provides a faster extraction method. Samples are homogenized via blending before centrifugation and extraction with a suitable reagent. The modified QuEChERS methodology developed by the U.S. FDA laboratory in Irvine presents an even simpler alternative to the conventional QuEChERS technique, allowing the extracted matrix to be diluted and injected directly into the GC/LC-MS to save further time.

Advances in both GC-MS and LC-MS have led the U.S. FDA to recommend them as the platform for QuEChERS screening. Modern LC-MS systems are generally considered to be more powerful and are able to separate a greater range of pesticide products. However, where routine detection of known volatile analytes is required, GC-MS systems are a suitable and lower priced alternative.

Despite the increased availability of sophisticated GC-MS and LC-MS systems, the functionality of many commercially available systems is still limited by technological aspects, struggling to deliver the levels of sensitivity and specificity required. Yet, triple quadrupole MRM overcomes many of these limitations and delivers the performance levels required for pesticide detection.

Technological Developments in GC-MS and LC-MS

MRM helps to maximize reliability in pesticide detection by fragmenting ionized analytes into multiple ions. When MRM is incorporated into GC-MS and LC-MS, there is a dramatic increase in signal to noise ratio, greater specificity, and better quantitative performance. Hardware advances in both GC-MS/MS and LC-MS/MS have refined the performance of triple quadrupole MS for pesticide detection to ensure high performance is maintained throughout high throughput analysis. In GC-MS/MS, an axial ion source reduces the contact of ions with hot surfaces and avoids the matrix build-up on the ion source. Higher signal to noise ratio is maintained, reducing the need for instrument cleaning and the resulting downtime, while ensuring high performance is maintained, crucial for a high-throughput laboratory.

Design advances in LC-MS/MS also deliver similar improvements in robustness and sensitivity by optimizing ion transfer. The systematic loss of sensitivity resulting from residue deposition is overcome by use of an open orifice rather than a capillary interface between the liquid chromatography and mass spectrometry elements. An Active Exhaust further reduces chemical noise and increases sensitivity and specificity of trace analyte detection by reducing gas recirculation within the ion source.

A further point of development has been extending LC-MS/MS to cover thermally labile pesticide species that commonly breakdown during liquid chromatography eluent “over-heating” prior to nebulization. This is achieved by the incorporation of a vacuum insulated probe within the ionization unit and around the liquid chromatography eluent to reduce heat transfer to the sample. Vacuum Insulated Probe Heated Electrospray (VIP-HESI) technology ensures high signal-to-noise ratios, superior robustness, and broadens the analysis range of liquid chromatography techniques.

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Table 1: MRM transitions of 30 pesticides by GC-MS/MS and LC-MS/MS systems.

Advantages of CBS

Developing a multiple reaction monitoring method can be a time consuming task as there can be hundreds of pesticides to identify in a single run. Traditionally a chromatographic run is divided into fixed segments and only the MRMs eluted in each segment are monitored. However, residues eluting near the joint of the two adjacent segments, MRMs must be set up in both segments to assure detection. The need for duplicated MRMs leads to slower duty cycles that must be carefully optimized to ensure sensitivity is not lost from short dwell times. Compound Based Scanning (CBS) streamlines method development for multi-residue analysis. Following a number of initial runs to locate the retention time window for each compound, the optimal scan time is automatically calculated by the software, which processes all the overlapped retention time windows. In this way, the duty cycle is optimized and fixed segments are no longer required, which is greatly advantageous in a high-throughput laboratory.