Improvements in Robustness of Signal Response in Multiresidue Pesticide Analysis with Xevo TQ Absolute XR Mass Spectrometer

Typical LC-MS/MS pesticide multiresidue methods can contain more than 200 analytes acquired via multiple reaction monitoring (MRM) methods, with switching between positive and negative ionization modes. When analyzing challenging matrices with high m/z ions that can contaminate the quadrupoles of the mass spectrometer, charging can occur, resulting in a loss of signal. This charging can become more pronounced under fast acquisition rates, such as short dwell times and polarity switching. This can affect ion ratios and signal response leading to unplanned instrument downtime. The new StepWave™ XR ion guide within the Xevo™ TQ Absolute XR Mass Spectrometer showed improved robustness over the course of more than 14,000 injections for a single sample derived from a challenging matrix.

Benefits

• Xevo TQ Absolute XR Mass Spectrometer exhibits less quadrupole contamination— meaning no unscheduled downtime over 10 weeks of analysis
• Improved robustness of upto 6x longer for ion ratios, giving laboratories confidence in consistency
• Seamless batch review using waters_connect™ for Quantitation Software’s accelerated data validation workflows

Technological advancements have enabled testing laboratories to optimize workflows and reduce the analysis cost per sample. High-sensitivity mass spectrometry (MS) systems, featuring larger sampling cone orifices, now allow food testing labs to analyze crude extracts using a simple extract-and-dilute method—saving time, labor, and consumables. However, this approach can burden the MS system, as it often introduces large amounts of complex matrix ions, leading to increased chemical noise and the need for more frequent system maintenance.

Chemical noise is a common challenge in LC-MS/MS, particularly under electrospray ionization (ESI) conditions, where high solvent content can generate solvent-induced ions alongside matrix-derived ions from crude samples. This interference may stem from interactions between solvents and food components, leading to the formation of large molecular clusters such as protein complexes or oligosaccharide aggregates.

In targeted MRM mode, the impact of matrix loading may not be immediately evident during the chromatographic run, as many of these high-mass clusters exceed the mass range of a tandem quadrupole MS. However, they can still contaminate critical MS components—including the ion optics, quadrupoles, and detectors—resulting in unplanned maintenance and servicing requirements.

Fish feed is considered a challenging sample matrix, as it contains high levels of fatty acids, amino acids, phospholipids, carbohydrates, pigments, organic polymers (e.g. lignins), water insoluble content (e.g. cell walls), sugars, and vitamins. 

In this application note, an accelerated LC-MS/MS method for more than 200 pesticides in less than 6 minutes runtime was developed and a crude extract of fish feed was generated by QuEChERS extraction, without sample clean-up. These conditions were chosen to best challenge and demonstrate the StepWave XR performance, a novel slotted bandpass ion guide, within the Xevo TQ Absolute XR Tandem Quadrupole Mass Spectrometer. This study demonstrates maintained method performance with improved system robustness and thus gains in laboratory operation achievable, by acquiring more than 14,000 injections, without lengthy or unscheduled user interaction with the system.

Sample Preparation

Briefly, 2 g of fish feed (Figure 1) was weighed, rehydrated and extracted using DisQuE QuEChERS CEN pouch (p/n: 186006813) with 10 mL of acetonitrile. Following centrifugation, the sample extracts were collected, spiked with 204 pesticides, and diluted 1 in 10 with acetonitrile to give a final in vial concentration of 0.01 µg/mL. This solution was then aliquoted into 80 samples vials (4 plates of 20 vials) for the robustness study. 1 µL of sample was injected onto a reversed phase column using an LC method that had been accelerated from 20 minutes to 6 minutes on the new Xevo TQ Absolute XR Tandem Quadrupole Mass Spectrometer, acquiring data for 204 pesticides. To maximally challenge the analytical system, the waste divert valve was not used during this study.

Each analysis batch was divided into two sections. 45 injections of the batch acquired 204 pesticides, each with at least 2 transitions. These were used to monitor the ion ratio, calculated as identifier to quantifier ions. A pesticide standard in solvent was injected prior to analysis to obtain an average reference ion ratio value for each analyte. The remaining 5 injections of each batch were used to assess contamination on the quadrupole, referred to as the charging test. The ratio between 2 transitions of the same analyte should be ~1.

Charging may be most pronounced in heavy matrix and during fast MS acquisition rates, for example during polarity switching. Fluazinam was chosen as a representative analyte, with time windowed MRMs overlapping with ESI positive analytes.

Quadrupole Charging Test

Fluazinam was chosen to represent a negative ion compound being monitored in the same retention time window as a positive ion compound. Figure 2 shows the Xevo TQ Absolute XR Mass Spectrometer lasted more than 6 times longer than its predecessor.

Robustness Tests

Thirty solvent injections of the pesticide standard provided average values for each confirmation/quantitation ion ratio, which were used to set a ±30% tolerance for the ion ratio plot of the matrix injections. When fluazinam was measured on the Xevo TQ Absolute Mass Spectrometer (Figure 3), the ion ratio consistently exceeds this tolerance from 1,500 injections onwards, which is where the charge test plot (Figure 2) also began to show evidence of charging. On the Xevo TQ Absolute XR Mass Spectrometer (Figure 3), however, the ion ratio plot reached more than 14,000 injections without ever approaching the ±30% tolerance.

During polarity switching, negative ion compounds tend to be affected more readily by charging due to the generally lower ion current in negative ion mode. Examples in Figure 4 and Figure 5 of ion ratio comparisons of negative compounds on each instrument show the robustness of the Xevo TQ Absolute XR Mass Spectrometer. 

Table 1 shows a selection of compounds from the 204 analytes, selected to represent a wide cross section of parameters like retention time (RT), peak response, ion ratio, m/z, and negative ionization. Their calculated RSD% over 14,000 injections is displayed. Of the 204 analytes detected, 82% returned RSD% for ion ratio of less than 10%.

The StepWave XR ion guide was shown to prevent charging on the quadrupoles over a duration of 10 weeks in which >14,000 injections were made of a challenging matrix without prior sample cleanup. The StepWave XR ion guide is capable of effectively removing contaminant ions, preventing fouling of the quadrupoles. This results in improved system robustness- consistent ion ratios, and minimal intervention on the MS system.

This study can give testing laboratories confidence the Xevo TQ Absolute XR Mass Spectrometer will provide maximum uptime while delivering consistent performance.

1. Feed Tables webpage, available at https://www.feedtables.com/content/fish-meal-protein-65 (last accessed May 2025)