Harmony in Gas Chromatography

Martin Perkins

12th May 2016

Analytes, Gas Chromatography, GC, GC-MS, Ray Perkins, Solute, Solvent,

Great gas chromatographers create elegant methods, in the same way that great composers create elegant music.

In GC and in music, elegance arises from the way that the various elements harmonise together to yield something that works as a whole.

For some, the GC, the raw power of modern GC-MS can stimulate an approach to method development that is akin to heavy metal music; thrilling, powerful and brutally effective but also raw and untidy.

In time, as their skill develops; most chromatographers begin to understand that GC is also about sound chemistry and some discover that there is beauty to be found in a great GC method.

The elements of an elegant GC method

  1. Your analytes and solvent share similar functionality
  2. The stationary phase of your column matches the functionality of both your analytes and your solvent

Methods that harmonise in this way are robust and tend to be free of the problems that other, less stylish approaches to gas chromatography suffer from.

In the case of solvent selection, like dissolves like, so solvent selection should be first and foremost about achieving high recovery of analytes. The closer you can get to 100% recovery into the solvent, in the finish, the better your data is likely to be and the more robust your method will prove.

If both solvent and solutes share a similar functionality, it is simple to choose a column stationary phase that is compatible with both and there will be no uncomfortable compromises to be made. It is highly likely that the best column for the job will have a functionality that matches your analyte/solvent combination.

If your analytes are highly soluble in your stationary phase, your separation will work brilliantly:

  • The analytes will be retained well
  • Small differences in solubility between analytes will result in good separations being achieved
  • The peak-shapes of your analytes will be Gaussian
  • Your solvent peak won’t tail much
  • You may be able to use a higher oven start temperature
  • Peak splitting will be absent
  • You will be able to use the shortest possible column
  • You will achieve the shortest possible run-time

In other words, your column will have excellent selectivity for your analytes.

It is important to realise that the benefits of selectivity are largely cost free; to achieve a similar degree of separation on a non-ideal phase will need more theoretical plates and increasing plate count comes with costs in the form of longer run times.

Another way of looking at this is to say that the right way to work, is for the functionality of your analytes to determine both your choice of solvent and your choice of column with the aim of all thee matching.

All too often method development is approached the other way around.

The column is fixed – often an HP-5 or equivalent (for no good reason), and you end-up using a solvent that has an imperfect degree of compatibility with analytes, column or both.

An awful lot of time is wasted solving problems that stem from this back-to-front approach, to GC method development.

As in music, so in gas chromatography – harmony is something to be aspired to.

Back-to-front is always a clumsy way to go about things!