i like to know which is a better method of introducing a water sample containing acetone into a GC.
by direct injection or headspace?
i got acetone results that are very much greater for the headspace method compared to direct injection.
By Bill Jenko on Sunday, October 29, 2000 - 08:49 pm:
You can probably purge and trap it too, although I've never tried that. Seems like overkill to P&T it, unless you are looking for a very small acetone concentration.
By Ron on Tuesday, October 31, 2000 - 06:07 am:
Water has a very large solvent expansion volume, so a 1 uL injection will probably result in backflash, loss of response, and low precision. Direct injection is probably the best technique for moderate to high levels of acetone, as the recovery in both static headspace and purge and trap is poor due to the solubility. Use an injection port temperature of 150C or cooler, the largest volume liner available for your system, and a small injection volume, preferably less than 0.25 uL. You might also want to try the full evaporation technique for headspace.
By jason on Monday, November 6, 2000 - 11:56 pm:
hi bill & ron:
thanks for replying.
i'm using GC-9A By shimadzu for the acetone analysis. the concentration of acetone in the water concentrate is about 1000ppm.
detector used: fid
column temperature:50 degree celsius
detector temperature:110 degree celsius
column: Nukol capillary column(0.53 micron)
vol. of sample injected: 5uL
Friends commented that it is wrong to inject a water sample into a GC as it will spoil the capillary column.
l like to hear your comments on the above operating parameters and opinion.
By jclark on Tuesday, November 7, 2000 - 05:18 am:
Injection of water is acceptable on many bonded-phase capillary columns these days and the Nukol is one. However, I would turn the detector temperature up considerably. You should also be able to achieve more than adequate sensitivity with 1 ul injections. I presume these are on-column injections.
You could also assay this on a methyl silicone phase for even better peakshape than the PEG-type phase will give you. Methyl silicone phases are even more rugged and impervious to water although you might have problems with the huge semi-invisible water peak interfering with your acetone peak. Wetting the non-polar phase with your polar injection solvent and analyte might be a problem.
By Ron on Tuesday, November 7, 2000 - 06:05 am:
I agree with the above recommendations to raise the detector temperature and lower the injection volume. Direct injection of water is ok on most modern capillary columns. If you have any questions about the suitablity of a column for direct injection of aqueous samples call your column manufacturers technical support department. They will be able to tell you about any potential problems.
By David McCalley on Tuesday, November 7, 2000 - 06:40 am:
Ron is right. He suggests using less than 1ul of water whereas you appear to be using 5ul, although you do not tell us whether you are splitting the sample or not.
How complex are your samples-is it just acetone in water or are there lots of other components? If the former, you could perform the analysis on a packed column such as a porous polymer material. Since these do not contain any coated liquid phase, you can inject aqueous solutions on to them for ever more with no damage. Many GC manufacturers will sell you a packed column which can be installed in place of the capillary glass liner-if you don't have a specialist packed column GC. You can then do direct on-column aqueous injections. However, you will need a pressure gauge on the carrier gas line which is sufficient to allow operation at packed column flow rates.
By jason on Saturday, November 18, 2000 - 06:03 am:
thanks to all who have provided constructive advice.
it seem that both techniques do give similar acetone results.perhaps it is due to error during the first time of analysis.
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