Hi I'm Gerard Sharp. Welcome to the June 2011 issue of the monthly newsletter. This month the Tech Tip is on GC Liquid Vaporisation.

Dr Gerard Sharp...

Ardent Scientific has released a new GC eLearning module titled 'Fundamentals of GC'. Visit the site to launch some of the free content. The module is suitable for new users as well as anyone wanting to update their fundamental knowledge of GC. There's many interactive animations and voice overs explaining the content as you go along. Click here to view. Enjoy!


 

Tech Tip - GC Liquid Vaporisation

One of the least understood events that occurs in GC is the process whereby the liquid sample is converted into a gas in the GC inlet. A basic understanding of this is necessary to achieve good chromatography and to be able to troubleshoot inlet problems.

Basically when a liquid is converted into a gas in a GC inlet, the volume of the resultant gas can be 200 - 1200 times the volume of the liquid. The variables which determine this gas volume are the inlet temperature and pressure, the amount of liquid injected and the molar mass of the solvent. The gas volume can be calculated using the ideal gas equation. There is an inverse relationship between the molar mass of the solvent and the gas volume that results after the injection. This is one of the reasons why water is a difficult solvent in GC. Water expands four times more than methylene chloride because ot their relative molar masses (18 for water, 84 Daltons for methylene chloride).

So all a bit theoretical you say, well the practical implications of all this is the volume of the inlet liner has to be sufficient to contain the volume of the gas produced. The larger the better from this perspective. Thus large inner diameter liners are preferred especially when injecting large liquid volumes or injecting solvents with a low molar mass e.g. water, methanol, acetone etc. If there is a 'spill over' of the solvent gas then this can lead to contamination of parts of the injector which aren't meant to see the sample, such as the carrier gas inlet line and the septum purge line. The problem is magnified because these lines are cold because they are away from the heated injector zone and this leads to condensation of especially the semi volatile compounds. Contamination of the inlet line with sample will lead to sample carryover in subsequent chromatograms and chromatographically this will be seen as ghost peaks.The following is a guide for inlet liner diameter.

Inlet liner inner diameter:

  • Use a large diameter for large volume liquid injections ie. > 1μL
  • Use a large diameter for solvents with a small molar mass eg. water
  • Use a large diameter when doing split injections
  • Use a small diameter when doing splitless injection

eLearning

 

Chromatography eLearning is yet another way to enhance your chromatography knowledge. While not ever being able to replace a classroom course, eLearning does offer advantages on cost and rich interactive content.  

Also, unlike a classrom course you can learn at your own pace and return to the content time after time.

Ardent Scientific offers varying levels of membership for viewing eLearning content. The Single Membership starts at $300 / year and corporate memberships are also available. View here for full details.

 

Sent by Ardent Scientific
P.O. Box 549, Box Hill Vic 3128, Australia ph 0422 872 212
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