Xevo TQ-GC Tips & Tricks: How to extend MS data collection and reference mass spectral libraries

By December 17, 2018


The advantages of RADAR for simultaneous MRM and Full Scan MS acquisition on the Xevo TQ-GC

Busy, high-throughput food and environmental testing laboratories want to obtain the maximum amount of information from their sample analysis but need to avoid costly re-injection of extracts using different methods. Any methods utilizing advanced multiplexing of acquisition modes must remain sensitive and selective enough to detect compounds of interest at low levels.

Modern mass spectrometers now feature fast electronic components that combine with efficient ion transfer, enabling parallel acquisition modes to be used without a significant drop in performance.

  • Technology introduced in 2008 with the Xevo TQ MS called RADAR, which is available on all Waters Xevo instruments, allows the acquisition of full-scan MS and multiple reaction monitoring (MRM) scans in parallel.
  • The Auto-Dwell function provided by MassLynx Software ensures that the dwell and scan times are automatically set appropriately so that sufficient data points are acquired across the peak for good quantification.

For the Xevo TQ-GC mass spec, RADAR has the added benefit that electrospray ionization (EI) mass spectra acquired are library-searchable using the NIST/EPA/NIH Mass Spectral Library. Let’s take a look at how this is performed.

Xevo TQ-GC System

Xevo TQ-GC System

 

Setting up a method

Method set-up is simple; a RADAR function can be added via the MS method editor (Figure 1). As peak widths and scan time (for RADAR function) are added, MassLynx’s Auto-Dwell functionality ensures that dwell times are set to ensure adequate points per peak are obtained. MassLynx will also warn the user if scan speeds or mass range falls outside of the calibrated range.

Figure 1. MassLynx’s MS method editor showing RADAR scan.

 

Figure 2. Acquired data showing MS Scan Total Ion Chromatogram (TIC) alongside MRM TIC.

 

Figure 2 shows the acquired data, an MS full scan TIC with the associated MRM TIC. The acquired data can be processed in the same manner as any other data. The extracted spectrum (Figure 3) can be submitted to a library search, such as NIST MS search. Spectra search results are showing in Figure 4.

Figure 3. Spectrum extracted from chromatographic peak at RT = 15.30.

 

Figure 4. NIST MS search showing submitted spectra, match factor = 835.

Does RADAR acquisition impact data quality?

In addition to the advantages of how RADAR acquires MS data, it is also important to consider its impact on the quality of that data. Any impact on the sensitivity or quantitative performance of the system would detract from the usefulness of this acquisition method.

To test this, a suite of pesticides in leek (QuEChERS extraction) were analyzed using MRM, and separately, MRM and RADAR. The sample was analyzed using a Restek Rxi-5 sil-MS, 30 m, 0.25 mm x 0.25 µm column and a GC oven program (90°C, 1 min, 8.5 °C/min to 330 and hold 5 mins). RADAR mass range was set to m/z 50 > 650 and a scan time of 0.1 sec was used (6000 Da/sec).

Figure 5 shows a single MRM chromatogram trace for chlorpyriphos; with a MRM and RADAR acquisition (top) and MRM only acquisition (bottom). The data shows excellent peak shape in both cases and only a minimal loss of peak area when RADAR is included.

Table 1 shows a selection of pesticides showing peak areas with and without RADAR mode and the difference as a percentage. The data shows that, across a range of different compounds, the mean peak areas are less than 10%, which demonstrates that adding the useful RADAR function does not have a significant impact on the signal.

 

Figure 5. Single MRM trace for Chlorpyriphos with a RADAR acquisition included (top) and without RADAR acquisition (bottom).

 

Table 1. Selected pesticides showing peak areas with and without RADAR mode and the difference as a percentage.

 

Case study: Pesticides in raisins

To test the method in a real-world scenario, a crude raisin extract was injected onto the Xevo TQ-GC System using a Restek RXi-5 SiL MS 30 m, 0.25 mm x 0.25 µm column and a standard pesticides oven program (91°C for 1 min, 8.5°C/min to 300°C, hold 5 mins).

When we visually inspect the chromatogram, it is immediately evident there is a large pair of peaks (RT 17.70 and 17.77) with a significant intensity. On further investigation, these peaks coelute at the same retention time as captan.

A full-scan spectra can be extracted (Figure 8) and the spectra submitted to the NIST 17 mass spectral library (Figure 9). A library search indicates 13-Tetradece-11-yn-1-ol, which is potentially a coextracted matrix compound. This would tend to indicate shortcomings in the sample extraction methodology that may lead to frequent GC maintenance or problems with low-level residue quantification.

Figure 6. TIC showing MRM transitions alongside RADAR trace.

 

Figure 7. Reconstructed chromatogram showing the three transitions for captan and the TIC from the RADAR function.

 

Figure 8. Extracted spectrum from peak at RT 17.70.

 

Figure 9. Library search results from NIST/EPA/NIH mass spectral library for the peak at RT 17.7 mins.

 

The raisin extract was submitted for cleanup using a dispersive solid-phase extraction (SPE) technique (PSA) to determine whether cleanup will remove the coextractive. The new extract was analyzed using the same analytical method. The MRM and RADAR chromatograms are shown in Figure 10.

Here, the 13-tetradece-11-yn-1-ol peaks were absent from this chromatogram showing the benefits of cleanup. So we see that the RADAR tool, when used to acquire EI mass spectra, in combination with referencing the NIST Mass Spectral Library, provides great visibility of these issues without having to re-inject sample extracts.

Figure 10. Total Ion Chromatogram showing MRM transitions alongside RADAR trace for the QuEChERS with PSA sample.

 

A retrospective review of legacy data

Many laboratories will archive data for quality control or legislative purposes. As new information emerges on new possible contamination with novel or legacy compounds, it can be useful to carry out a retrospective review of previously acquired data.

Data acquired in a targeted MRM mode only will not reveal any new information – but an untargeted acquisition mode, such as MS scan, can be used to further investigate the composition of the sample extract. Since spectra generated are MS full scan, a search can be made for characteristic ions of the new compounds of interest and the identity of the peaks found can be confirmed by matching the MS spectrum to that in the NIST library.

When we consider that it doesn’t have a significant detrimental effect on the MRM data, and that RADAR acquisition mode provides added-value data essentially for free, this value gives enhanced insight into sample composition without a corresponding detriment to the data quality.

Conclusion

RADAR is a simple but powerful acquisition mode that can aid method development, track coextracted interference compounds, and allow re-processing of previously acquired data. RADAR is easy to set-up, does not significantly impact the quantitative data, and provides added value to an analysis.

 

Additional information

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