11
enable accurate, close-fitting class boundaries to be defined. As shown in Figure 2, these stencils can then be applied to the FID data for quantitative analysis, assisted by the excellent retention time correspondence for parallel detection that is now possible with flow modulators for GC×GC. Additionally, the ability of ChromSpace to open multiple data files in a single window (and indeed, multiple data file formats) makes creation and translation of the stencils even easier.
Advanced Applications – Scripting and Soft EI Detection
Figure 2. Classification of C11 -alkylbenzenes in a heavy alkylate using GC×GC with dual FID/TOF MS
detection. The excellent retention-time correlation (highlighted in the left-hand panel for the selected peak) allows stencils to be transferred easily from TOF MS to FID.
Filtering scripts are simple expressions (based on mass spectral or chromatographic properties) used to extract target compounds or classes from the acquired data. They can be selected from a default list of popular chemical classes in the software, or created easily by the analyst using preconfigured buttons in the expression builder. Scripts are valuable time-saving tools even in simple analyses, but they offer particular benefits for highly complex samples investigated using the TOF MS instrument in Tandem Ionisation mode, which simultaneously generates electron ionisation spectra at both high (70 eV) and low (10–16 eV) ionisation energies. These low-energy ‘soft ionisation’ spectra typically display reduced fragmentation and enhanced structurally-significant ions, which greatly enhance the use of filtering scripts.
An example of this approach is shown in Figure 3, which shows an expression builder in the software interface that has been set up to search for spectral characteristics of C12
-alkylnaphthalenes, and application of this script to the heavy alkylate sample. Filtering scripts provide increased selectivity compared to EICs, because they only show peaks that pass the qualifier expression, meaning a clean baseline. This is in contrast to EICs, which will show all peaks with that ion in common.
References
1. R.C. Striebich, L.M. Shafer, R.K. Adams, Z.J. West, M.J. DeWitt and S. Zabarnick, Hydrocarbon group-type analysis of petroleum-derived and synthetic fuels using two-dimensional gas chromatography, Energy Fuels 28 (2014), 5696−5706.
Figure 3. Top: Scripting expression used to classify the C12 -alkylnaphthalenes. Bottom: Use of this expression
to filter the 14 eV TOF MS data from the analysis of the heavy alkylate, and generate a stencil for this compound group that can be applied to the FID data.
2. F. Adam, F. Bertoncini, D. Thiébaut, S. Esnault, D. Espinat and M.C. Hennion, Towards comprehensive hydrocarbons analysis of middle distillates by LC-GCxGC, J. Chromatogr. Sci. 45 (2007), 643–649.
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