problems derivatizing oxalic acid with BSTFA
Posted: Mon Nov 22, 2010 10:13 pm
Hello all, I am having problems making calibration curves for oxalic acid (derivatized with BSTFA). I’ve done a lot of trouble-shooting and still I wind up with empty chromatograms (GCMS) on the key m/z values.
Here’s what I have tried so far:
1. Had oxalic acid along with 8 other diacids in a methanol mix that I ran at 5 concentration levels (the highest was 20 ppm). I blew the vials down to dryness under nitrogen, then added 100 uL hexane, 20 uL pyridine and 20 uL BSTFA all under nitrogen. Then I baked them at 60 degrees C for 35 minutes and ran them via GCMS within a week. I never saw oxalic acid or malonic acid in any of the chromatograms, though the other diacids were clearly present.
2. Tried putting these two diacids by themselves (i.e., the two of them together in a vial without any other diacids) and followed the same procedure as in (1). Still saw no oxalic acid or malonic acid in the chromatograms.
3. Tried making fresh solutions of oxalic acid and malonic acid in methanol to use and followed the same procedure as in (2) above; no help.
4. Tried everything as in (2), but left the vials in the oven for 45 minutes instead of 35 and ran them in the GCMS promptly. Still nothing.
5. Used solutions of oxalic acid and malonic acid (each alone) in acetonitrile (non-protic) in case residual methanol was eating my BSTFA, and followed procedure in (2). This finally worked for malonic acid, but still no luck with the oxalic acid!
I’ve done the stoichiometry and I have a large excess of BSTFA relative to oxalic acid, even at the highest level of 20 ppm. And I have been simultaneously derivatizing some cloud water samples with BSTFA, and oxalic acid shows up quite clearly on those; there is always a large peak at exactly the time I would predict based on the retention times of the other diacids, and its mass spectrum looks just like the one in the NIST library for oxalic acid. The cloud water vials get exactly the same treatment: blown down to dryness under nitrogen and the same BSTFA method. And of course the GCMS program (ramping rate, starting temp, solvent cut time, etc.) is the same for the cloud water and the calibration curves.
I’m stumped; can anyone think of any reason why the oxalic acid for my calibration curves is being so tricky, while oxalic acid in my samples is getting derivatized? Thanks in advance!
Here’s what I have tried so far:
1. Had oxalic acid along with 8 other diacids in a methanol mix that I ran at 5 concentration levels (the highest was 20 ppm). I blew the vials down to dryness under nitrogen, then added 100 uL hexane, 20 uL pyridine and 20 uL BSTFA all under nitrogen. Then I baked them at 60 degrees C for 35 minutes and ran them via GCMS within a week. I never saw oxalic acid or malonic acid in any of the chromatograms, though the other diacids were clearly present.
2. Tried putting these two diacids by themselves (i.e., the two of them together in a vial without any other diacids) and followed the same procedure as in (1). Still saw no oxalic acid or malonic acid in the chromatograms.
3. Tried making fresh solutions of oxalic acid and malonic acid in methanol to use and followed the same procedure as in (2) above; no help.
4. Tried everything as in (2), but left the vials in the oven for 45 minutes instead of 35 and ran them in the GCMS promptly. Still nothing.
5. Used solutions of oxalic acid and malonic acid (each alone) in acetonitrile (non-protic) in case residual methanol was eating my BSTFA, and followed procedure in (2). This finally worked for malonic acid, but still no luck with the oxalic acid!
I’ve done the stoichiometry and I have a large excess of BSTFA relative to oxalic acid, even at the highest level of 20 ppm. And I have been simultaneously derivatizing some cloud water samples with BSTFA, and oxalic acid shows up quite clearly on those; there is always a large peak at exactly the time I would predict based on the retention times of the other diacids, and its mass spectrum looks just like the one in the NIST library for oxalic acid. The cloud water vials get exactly the same treatment: blown down to dryness under nitrogen and the same BSTFA method. And of course the GCMS program (ramping rate, starting temp, solvent cut time, etc.) is the same for the cloud water and the calibration curves.
I’m stumped; can anyone think of any reason why the oxalic acid for my calibration curves is being so tricky, while oxalic acid in my samples is getting derivatized? Thanks in advance!