Picture perfect: model captures programmed temperature gas chromatography panorama

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  • Published: Sep 14, 2016
  • Author: Ryan De Vooght-Johnson
  • Channels: Laboratory Informatics / Chemometrics & Informatics
thumbnail image: Picture perfect: model captures programmed temperature gas chromatography panorama

Incomplete panorama

Ever since the 1960s, scientists have attempted to capture the mathematical panorama, and here researchers from Hawaii and China attempt to add yet more detail to the picture. Image: wikipedia

The decade that saw President Kennedy aim for the moon and four chaps from Liverpool say Hello, Goodbye to the States was also monumental for gas chromatography. Before then, gas chromatographists would typically run isothermal columns and would spend hours toiling in their laboratories, achieving only moderate resolution of their analytes at best.

Meanwhile, scientists elsewhere in small corners of California and Canada had other plans. In two seminal papers published back-to-back in 1960, the solid foundations of programmed temperature gas chromatography (PTGC) were laid. Rather than maintaining the column oven at the same temperature for the entire separation, the landmark manuscripts detailed how ramping up the temperature could both save time and increase the resolution of low-volatile analytes.

Fast forward to the Twenty-First Century, mathematics can be used to calculate at what temperature a column should be when certain variables are punched in. But this calculation does not account for all the jigsaw pieces, or as Wu and colleagues put it in their article in the Journal of Separation Science: ‘…a panoramic view of the true retention behaviour of a wide range of solutes was not well pictured.’

Overly simplistic

Firstly, this model assumes that analytes focus at the column head—a phenomenon not confirmed with empirical evidence. Secondly, Wu argues that it is overly simplistic and does not take into account the influence of the starting temperature and heating rate on retention times. Accordingly, Wu and colleagues at the University of Hawaii at Manoa and collaborators based in China teamed up to add the missing jigsaw pieces.

Wu took n-alkanes, 1-alkenes, 1-alkyl alcohols, alkyl benzenes and fatty acid methyl esters, and then resolved these over numerous stationary phases of various polarity—two non-polar, one moderately polar, and one polar. These model analytes of 8 to 20 carbons were then eluted from the experimental columns by linearly ramping up the temperature—starting at between 40 and 240 °C—at rates of increase ranging from 1 to 30 °C per minute.

Model fit

The phenomena of single- and dual-retention of analytes at the column head was studied by injecting analytes twice in short succession and resolving these over the same temperature ramp. Wu and his team then plugged the reams of data generated into linear, polynomial, power, logarithmic, and exponential models. Which fitting would be the best simulation?

What was made clear was that one model does not fit all. Whilst the isothermal model best explained the influence starting temperature has on the retention of analytes, the cubic equation model best fitted the heating rates. Secondly, chromatograms clearly showed the single retention of like analytes loaded over two injections.

The models established are ‘pictorial, simple and practical to visualize true retention behaviours of solutes,’ claim the authors. ‘Reproducibility of retention data such as retention indices in PTGC can be significantly improved, when it is operated at a single-retention behaviour region.’

Related Links

J. Sep. Sci., 2016, 39, 2785–2795. Wu et al. Phenomenon of dual- and single-retention behaviours of solutes and its validation by computational simulation in linear programmed temperature gas chromatography.

Article by Ryan De Vooght-Johnson

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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