Chromatography Forum

What is difference between gas chromatograph and high performance liquid chromatograph?

This is a very general question and can only get a very general answer. At the simplest, the difference is given in the names. The names indicate the mobile phases. On a more practicle level, this requires ( or allows) different methods of sample introduction, management of partition coefficients, and detection of analytes.

The topic is large enough to write a book - like one on methods of analytical chromatogrpahy. There are many. And, before I go off and write another, I'll let you take a look in the library.

please elaborate your question so more help is given

what do you need this information for? what is your experience? and so on

Hi all,
I don't know what information Smith want to know. On my experience I have the similar question.
One of the clients gave me a sample and wanted to known certain organic components presented on it. I can see those on both GC and HPLC. I wonder which instrument I have to use. Due to the different extraction for each machine, I can't run both of them.
Please advise. Thank all.
Spectro.

To grossly oversimplify the differences between GC and HPLC:

1. In GC the mobile phase is a gas. In HPLC it's a liquid (blinding flash of the obvious!)

2. In order to be analyzed by GC, a compound must be volatile at some reasonable temperature (up to about 300 degrees C) and thermally stable at that temperature (translation: it has to vaporize before it decomposes). Only a minority of compounds fit that description (I'm guessing 10-20% ??).

3. GC is more efficient than HPLC, with a peak capacity that is around 3-5 times higher (for the more experienced, I'm assuming about 10,000 plates for a typical HPLC column and 100-250,000 plates for a good GC column; feel free to plug in your own numbers!).

1. For GC, analytes have to be volatile & thermally stable (except for pyrolisis GC, long story). For LC, they have to be soluble in mobile phase.

2. In GC, compounds are separated primarily by their volatility (again, long story); in LC, by polarity (for typical RP and NP columns; other variants are also possible)

3. They use different detectors:
GC: mostly FID, ECD, MS...
LC: DAD, RI, FLD, MS (but a bit different than the GC-one)...

4. as Tom Jupille said, GC is still far more efficient (it uses capillary columns, while LC still uses mostly packed ones)

5. For GC, there are far less paramers to vary (thus, it's a bit simpler, but somewhat more limited).

6. Expenses: GC uses some carrier gas, minute amounts of solvents, and a lot of power. LC uses a lot of solvents.

7. LC can be used not only for analysis, but also for isolation (OK, you need some specific modules...). Preparative GC is possible, but not used too often (due to lower capacity, I guess).

8. GC requires more sample purification (no non-volatile compounds should be present in the sample, as well as particles). For LC, it is usually enough just to filter the sample (you can always wash impurities that are stuck on column).