When to use HPLC or GC

[JenI]

Can some one help me out and give me the basics on why I have found many analysis for acids on hplc over gc? What are the problems with using GC to analyze acids? Thanks Jeni

[tom jupille]

The two techniques are, in a sense, complementary:

GC requires analytes to be at least somewhat volatile and thermally stable.

LC requires that they be at least somewhat soluble.

There are a lot more compounds in the latter category than the former!

[Bruce Hamilton]

Volatile organic acids tend to be very reactive ( unless derivatised ) and degrade/bind in the GC system as the reactions are accelerated by heat. Also, the shorter the chain, the smaller the FID response - formic is very, very low.

There are a few columns that can analyse some thermally-stable acids without derivatisation, but they are very sensitive to contamination. Samples with acids are often quite yucky.

The longer chain organic acids ( fatty acids ) are easily derivatised and give excellent GC response, so GC is the preferred technique because of the improved resolution.

As well as the points noted by Tom above, HPLC has the advantage of no derivatisation ( a range of different columns can separate acids ), and usually no degradation, and a range of detectors that easily see most acids, which makes samples easy to process.

Bruce Hamilton