I've been trying to get a VFA + Lactic Acid analysis up and running using a Carbowax glass column in a Perkin-Elmer AutoSystem XL. After
having PE Service come and check out the long idle GC giving it a clean bill of health and a fine test chromatogram I began the analysis ... the first standard looked fine after that either
nothing, next to nothing, or all peaks (still too small) negative. Also the baseline is, in my opinion, way too noisy. I've tried a number of things, cleaning the FID, new column, different column ... I've run out of ideas and I'm reluctant to write P-E a blank check with the instructions of diagnosis and fix it.
If anyone has anything insightful to say I'd sure like to hear it.
Alvin J. Boning, Jr
PO Box 110910
Animal Science Department, IFAS
University of Florida
Gainesville, Florida 32611-0910
By Wilhelm Leung on Wednesday, October 27, 1999 - 06:18 pm:
You should derivatise your analyte e.g. to form methyl ester and then determine with a PEG packed column or Carbowax capillary column. You can refer to the AOAC Official Methods of Analysis for details. Also do not try to inject your sample in aqueous medium.
By John Hinshaw on Wednesday, October 27, 1999 - 06:59 pm:
Check the polarization voltage and its connection to the FID. This is the red wire with black plastic terminator that connects to a pin at the base of the FID on the top of the instrument. Also make sure the electrometer cable (thick black one with silver coax terminator) is securely attached.
If that's not the problem, then look to the column, the dervitization, and the inlet system.
For examples of typical baseline problems, follow this link GC Troubleshooting Guide.
By Al Boning on Thursday, October 28, 1999 - 12:41 pm:
In a recent conversation I was told that what I have observed is likely do to contamination of the detector ceramics leading to polarization current leakage around the detector and loss of sensitivity and that this could also explain the negative peaks.
By David McCalley on Friday, October 29, 1999 - 05:58 am:
I hope you manage to sort out the detector electronics. I was interested in Wilhelm's remark about derivatisation and not injecting water. Although water is not the easiest solvent, people have been injecting aqueous solutions into packed porous polymer columns for years without too much difficulty, principally for ethanol determination. The problem with VFAs in silage is that the low MW acids such as acetic and propanoic acid have very unfavorable partition coefficients between organic solvents and water making extraction/derivatisation procedures difficult. Direct aqueous injections on to graphitised carbon black/Carbowax columns works quite well.
See Analyst 1987 112 1213.
You can even inject water on to capillaries if you are careful, and Grob has published some work on this. However, for silage analysis I would recommend the packed columns as more robust for these very dirty samples.
By David McCalley on Friday, October 29, 1999 - 06:04 am:
Sorry Al- I am guessing from where you work that you are doing silage analysis? However, my remarks would apply also to similar analyses of aqueous solutions for VFA and lactic acid.
By Al Boning on Tuesday, November 9, 1999 - 06:19 am:
Problem solved ... replacing all the ceramic insulators (which were quite dirty) in the detector took care of the noise ... replacing the leaking connector between the instrument and the column inlet took care of the sensitivity.
Thanks to all that responded.
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