15
second dimension phase. This limits elution from the first dimension (typically non-polar) to that which is significantly less than the temperature required to elute hydrocarbons in the lubricant boiling range. Current means of addressing this issue involve a reduction in the second dimension polarity so that parity
of Tmax is found between the two phases. DB17 and 35 type high temperature phases have been successfully employed in our labs. However, this change reduces the orthogonality between the first and second dimensions and limits the accessible chromatographic space. Highly polar phases which have an equivalent maximum temperature to 5% phenyl 95% methyl polysiloxane phases would address this issue. Ionic liquids have the potential to achieve this aim and promise to make a significant impact on multi-dimensional GC.
A quick method of comparing column phases in the second dimension was devised. Samples were run on a range of columns and the difference between the test compound and the closest eluting hydrocarbon in the first dimension were compared. This is shown in Figure 5, where the arrow length describes retention time in the second dimension. This was normalised to a ratio of the modulation period and data is shown in Figure 6. The different selectivities of the phases can be observed by the changes in relative peak height across the columns studied. A number of trends can also be observed. Naphthalene appears to be well retained on all columns, IL 61 appears to be effective at retaining aromatic compounds; and retention tends to decrease with column polarity (N.B. Wax and IL61 are similar). It is this final observation which is most interesting and not what might be expected at first glance. The aim of increasing polarity in the second dimension was to improve column orthogonality and therefore improve separation between compounds. In general this is not what is shown in Figure 6, and leads to two observations. Firstly, the retention differences between non-polar and polar materials displayed on polar phases in one dimension is negated in two dimensions. This is
because as compound elutes from the first dimension it is already near or above the elution temperature for the second column and is poorly retained. Secondly, increasing the polarity differences between the two dimensions improves the potential chromatographic space but this is only useful if the analytes can access the entire space. If there is not the range of differently retained components in a mixture, increasing the orthogonality between the two dimensions will condense elution in the second dimension to a smaller region. It is better in this case to reduce the differences between columns to maximise the ‘useful’ chromatographic space.
In addition to these observations it is still the case that whilst significant improvements have been made in the maximum temperature of the polar ionic liquid phases, these commercially available columns are still 30-50 °C away from true parity with non-polar DB1 and 5 phases. Many reviews in the literature describe research phases [8] that can achieve these higher temperatures but these have yet to come to market for general use.
Conclusions and future work
The ability of ionic liquid coated GC columns to separate polar analytes of interest from the bulk hydrocarbon matrix in petroleum products has been investigated in both one and two dimensions. As polarity increases a number of effects were observed:
• A reduction in the loading of larger hydrocarbons.
• A reduction in the overall retention of components.
• An increase in the separation between non-polar and polar compounds.
In particular, fatty acid methyl esters and polar lubricant additives have been shown to elute in areas of the chromatogram less congested with hydrocarbon matrix. The increased temperature stability of the ionic liquid phases allows for lubricant products to be analysed in a single dimension. However, in two dimensions and in a single oven, analysis is still restricted by the maximum operating temperature of the polar phase.
[1]
L.Vidal, M-L. Riekkola, A. Canals, Anal. Chim. Acta. 715 (2012) 19-41.
[2] D.W. Armstrong, T. Payagala, L.M. Sidisky, LCGC North Am. 27 (2009) 596 598 600.
[3] F. Destaillats, M. Guitard, C. Cruz- Hernandez, J. Chromatogr. A. 1218 (2011) 9384-9389.
[4]J. V. Seeley, C. T. Bates, J. D. McCurry, S. K. Seeley, J. Chromatogr. A, In Press, Corrected Proof, Available online 30 July 2011.
[5]
Q.Gu, F. David, F. Lynen,
P.Vanormelingen, W. Vyverman, K. Rumpel, G. Xu, P. Sandra, J. Chromatogr. A.1218 (2011), 3056-3063.
[6] P. Delmonte, A-R. Fardin Kia, J. K.G. Kramer, M. M. Mossoba, L. Sidisky, J. I. Rader, J. Chromatogr. A. 1218 (2011) 545-554.
[7] C. Ragonese, P. Quinto Tranchida, D. Sciarrone, L. Mondello, J. Chromatogr. A. 1216 (2009) 8992-8997.
[8] C. F. Poole, S.K. Poole, J. Sep. Sci. 34 (2011) 888–900.
In two dimensions, the use of highly polar columns as the second dimension was found to be unsuitable for the samples chosen. Analytes interacting with the polar phase above their retention temperature were poorly retained. Poorly matched column sets did not make full use of the available modulation time.
Future work will concentrate on two specific areas. The first area of research will be reversing the column order so that the polar column is in the first dimension. This will reduce issues of poor retentivity at higher temperatures. The second area will investigate the effect of placing the columns in separate ovens, such as the Agilent LTM module, which allows for independent control of the two phases. This would overcome issues of maximum temperature and improve retention at high temperatures.
Acknowledgements
The author would like to thank Supelco for provision of the SLB-ionic liquid columns and Tom Lynch and the Investigational Analysis team for support.
When contacting companies directly from this issue of please tell them where you found their information.
Thank you
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