Analytical Instrumentation 9
is obtained offers good separation of major groups and even resolves multiple positional isomers within each carbon substituent group (Figures 3-5). Comparison of the sample mass spectrum at the retention time for dibenzothiophene to a reference spectrum reveals a clear match and confirms compound identification (Figure 6). As shown in this example, good use of the separation space is made by optimising parameters for both columns. Often, in order to optimise separations on the second column, the first dimension separation is intentionally slowed in order to produce broader peaks that will accommodate 3-5 modulations per peak. With careful attention to flows, temperatures, and modulator settings, good column efficiencies can be obtained in both dimensions. By employing GCxGC and optimising use of the separation space, peak capacity is significantly increased. This approach, in combination with the use of a powerful MS detector, allows speciation of sulphur compounds in diesel and provides a level of sample characterisation that cannot be obtained using one-dimensional GC.
Analysis of sulphurs in diesel provides a good demonstration of the power of GCxGC for complex matrices. Increased peak capacity allows identification of relevant constituents that cannot be differentiated by GC alone. As this technology is increasingly adopted by the petroleum and petrochemical industries, better materials characterisation and process optimisation can be achieved.
Figure 5: GCxGC-TOFMS extracted ion contour plot of the dibenzothiophene elution area for a diesel sample. Plotting m/z ions 184, 198, 212, 226, and 240 enhances visualisation of the separation of the dibenzothiophenes (including the alkyl substituted isomers).
Figure 3: GCxGC-TOFMS chromatogram (contour plot) of diesel analysed with orthogonal Rxi® 1ms and Rtx®
-FLD columns showing structured separation of aliphatics and aromatics.
-
Figure 6: Mass spectrum of dibenzothiophene for comprehensive two-dimensional GC of diesel. The sample mass spectrum (top) is an excellent match with the NIST library mass spectrum (bottom).
References [1] W. Rathbun, J. Chromatogr. Sci., 45 (2007) 636.
[2] F. Adam, D. Thiebaut, F. Bertoncini, M. Courtiade, M.-C. Hennion, J. Chromatogr. A, 1217 (2010) 1386.
[3] T. Dutriez, M. Courtiade, D. Thiebaut, H. Dulot, F. Bertoncinia, J. Vial, M.C. Hennion, J. Chromatogr. A, 1216 (2009) 2905.
Figure 4: GCxGC-TOFMS contour plot (TIC) of the dibenzothiophene elution region for a diesel sample. The dibenzothiophenes are more retained by the Rtx®
-FLD column, which means they are higher on the y-axis of the contour plot than most other components. Experimental
A Pegasus 4D GCxGC-TOFMS (LECO, St. Joseph, MI,USA) was used with electron ionisation at 70 eV and an MS source temperature of 250°C. The acquired mass range was 45-450 u at 200 spectra/sec. The primary GC column consisted of 2 Rxi®
0.25 µm) press-fitted together. The secondary column, a 1.5 m x 0.25 mm x 0.10 µm Rtx®
-1ms columns (each 60 m x 0.25 mm x -FLD
column (an experimental phase column), was connected by a press-fit to the end of the primary column. The secondary column was installed in its own oven and terminated in the source of the TOFMS. This column combination was operated with helium carrier at a corrected constant flow of 2.0 mL/min. One microliter 50:1 split injections of a diesel fuel composite were performed into a Sky™ 4.0 mm Precision®
inlet liner with wool at 275°C. The primary oven program was: 40°C
(hold 1 min.) to 320°C at 1.3°C /min. (hold 1 min.) The secondary oven program was a positive 5°C offset from the primary oven. For GCxGC a quad-jet, dual-stage thermal modulator was used with a temperature offset of 20°C and a modulation time of 3 sec. The hot pulse time was 1 sec. For one-dimensional GC runs, the modulation time was set to zero, and the MS data collection rate was 2 spectra/sec. LECO ChromaTOF software was used for GCxGC-TOFMS data processing and display.
Restek Corporation
Restek is a leading developer and manufacturer of chromatography columns, standards, and accessories. We provide analysts around the world with the innovative tools they need to monitor the quality of air, water, soil, food, pharmaceutical, chemical, and petroleum products.
www.restek.com [4] A. Kohl, J. Cochran, D.M. Cropek, J. Chromatogr. A, 1217 (2010) 550.
[5] F.L. Dorman, J.J. Whiting, J.W. Cochran, J. Gardea-Torresdey, Anal. Chem., 82 (2010) 4775.
[6] F.L. Dorman, E.B. Overton, J.J. Whiting, J.W. Cochran, J. Gardea-Torresdey, Anal. Chem., 80 (2008) 4487.
August / September 2011 •
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