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Cutting down the cross-talk

Cutting down the cross-talk

27 Jun 2012 by Evoluted New Media

Regulatory demands for the use of pesticides are ever increasing and triple quad GC-MS seems to be the tool to meet them. However ‘cross-talk’ issues can cause analytical problems – but there is a way to beat them say Kefei Wang and Quingyu Sun

The application of pesticides to crops to improve yield has been industry practice for decades. However, widespread overuse as well as illegal use of pesticides can lead to significant risks to human health and the environment. Because of regulatory demands, pesticide control laboratories are under more and more pressure to maximise the range of pesticides detected in a single run and across a variety of complex matrices. Mass spectrometry techniques are now gaining more acceptance with food safety and environmental monitoring laboratories as researchers turn to MS studies to accurately monitor pesticide residues in foods.

[caption id="attachment_28605" align="alignleft" width="200" caption="Figure 1. Scan time vs. Peak Area for 1 pg OFN"][/caption]

The European Union is determined to tackle this problem, and the majority of EU countries have adopted legislation and regulations to control the use of pesticides, imposing strict Maximum Residue Limits (MRLs) for most substances¹. The US Food and Drug Administration (FDA), in conjunction with the Environmental Protection Agency (EPA), also regulate the level of pesticide chemicals in the food chain through a regulatory tolerance publication process². The FDA Food Safety Modernisation Act (“FSMA”) became law in January 2011 and represents the greatest expansion of the FDA’s food regulatory powers since the enactment in 1938 of the Federal Food, Drug, and Cosmetic Act³.

The average growth of sales of Gas Chromatography MS (GC-MS) systems has been up 3-7%, year on year for the past five years, with the fastest growing GC-MS technology segment being the GC-MS Triple Quadrupole (GC-MS TQ) 15-20%, year over year⁴. The triple quadrupole mass spectrometer is rapidly becoming the instrument of choice for compound analysis in complex matrices due to its selectivity in multiple reaction monitoring (MRM) mode and the capability of simultaneously monitoring multiple target compounds, even when their retention time windows are overlapped. As more target compounds are located in a single run, a triple quad method can have hundreds to thousands of MRMs with decreasing scan times (i.e. dwell time) for each MRM. The resulting short scan time can lead to an overall decrease in sensitivity.

[caption id="attachment_28606" align="alignright" width="200" caption="Figure 2. Zero cross-talk with 100pg OFN at 1ms scan time (note the different scales, i.e. MCps vs. Cps, is used in top and bottom panels to illustrate the zero cross-talk"][/caption]

Another potential problem in using short scan time for multiple MRMs is “cross-talk”. This describes a scenario in which there are two MRMs with the same m/z fragment ions from different precursor ions and with short scan time, the collision cell (Q₂) does not have enough time to clear the fragment ion from the first MRM before the second MRM fragmentation takes place. In these cases the product ion from the first MRM can appear in the second MRM chromatogram as a “ghost peak”. The cross-talk effect can be particularly problematic if the first MRM is of high intensity, as it can lead to more plausible false positives on the following MRM.

There are several design features which can be incorporated into a triple quad GC-MS which will assist with cross-talk and reductions of false positives, including a collision cell design with no apertures and no lenses and 180° orientation enabling two “noise-cancelling” design elements. One such example is the Bruker SCION TQ triple quadrupole mass spectrometer. Its design features also allow high sensitivity, with a scan rate of 14,000Da/s high-speed acquisition of 1ms to 100ms and with greater than 5% impact on the percentage relative standard deviation (%RSD), no peak height change and insignificant peak area loss.

Here we present several experiments to demonstrate the effect on cross-talk using a Bruker 451 GC (with CP 8400 Autosampler) coupled to a SCION TQ triple quadrupole mass spectrometer. For all tests, 1?l sample was injected using the splitless mode at 230°C. Bruker BR5-ms columns with 0.25mm ID and 0.25?m film thickness were used. A 30m column was utilised for the scan time experiment, and a 15m column for the no cross-talk tests. The samples and the MRM parameters are presented along with results. All the data were processed using the Bruker MS Workstation.

Six replicates of 1pg octafluoronaphthalene (OFN) were injected for monitoring 272>222 at 20V at six different scan times ranging from 100ms to 1ms.

Figure 1 summarises the results. As shown, peak heights remain visually the same and there is no significant loss (~13%) of OFN peak areas from scan time 100ms to 1ms. Additionally, %RSD (n=6) remains good at 5.4% with 1ms scan time.

[caption id="attachment_28607" align="alignleft" width="200" caption="Figure 3. No cross-talk effects for 1ng of Methoxchlor (227>212) and Vinclozolin (285>212) with 10ms scan time (note the different scales, i.e. MCps vs. Cps, are used in top and bottom panels to illustrate the zero cross-talk)."][/caption]

Experiment A: 100pg OFN was injected to monitor the following two MRM transitions: OFN: 272>222 @ 20V and Dummy: 500>222 at 20V; Scan time: 1ms. As shown in Figure 2, no cross-talk has been observed under the worst case scenario.

Experiment B: 1ng of a pesticide standard mix in acetone:hexane (50:50) solvent was injected. Two MRM transitions belonging to two pesticides Methoxchlor (227>212) and Vinclozolin (285>212) were monitored at a scan time of 10ms for each. Figure 3 shows the results, and again in this case, no cross-talk was observed on either chromatogram.

This article described studies evaluating the scan time effect on the signal intensity and the zero cross-talk effect, while using a SCION TQ mass spectrometer in the analysis of pesticides. Bruker’s SCION GC-MS TQ demonstrated negligible loss of peak intensity (height and area) under the extremely fast scan time of 1ms and zero cross-talk. The fast scanning sequence and the absence of active lens cleaning before the collision cell acquired the next data set brings zero cross-talk, eliminating false positive results providing greater confidence in pesticide scan results with short scan time.

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