I have tried simultaneously analyzing three chemicals, i.e. benzyl alcohol (BA), benzaldehyde(BAD), and benzoic acid using GC-FID while using TR-1, TR-5, or TR-wax columns. With all three columns, I have been able identify both BA and BAD but benzoic acid peak is not emerging in the chromatogram. I have done all possible variations with GC-FID but still no sucess. Sample is prepared in acetonitrile with concentration of ~50 ppm. Does anyone else also has experienced the same problem? And any suggestions to overcome this issue?
Hello Muhammad Saifullah
the explanation is simple, benzoic acid is very polar and will bind with the acid group to any active surface in your GC. And believe me there are a lot of active spots in each GC. You can easily get around this by derivatizing the carboxyl group (and thus the OH group of the alcohol). Either esterify or silylate, I tend to do the latter with hexamethydisilazane. Pipette the sample and the reagent into the injection vial, then let react at 70°C for 20 min, let cool, inject. I think then you will see the bezoic acid as silyl ester and the alcohol as well. Another advantage is that the peaktailing, typical for polar aromatics, is suppressed.
For the derivatized analytes I would then take the TR5 column, the polarity of the column should be enough.
Good luck and best regards
Joachim Horst
Typically aromatic organic acids are derivatized (TMS as an example or the FAME [fatty acid methyl ester] method for oils) which is found in the AOAC.
Thanks Bruce Neagle and Bruce Neagle . I would definitely consider trying the derivatization approach. While I'm preparing the derivatization agent, I'll also perform the same analysis using HPLC coupled with a UV detector. In this setup, I'll use a C18 column and a mobile phase consisting of ACN and water.
I've used HPLC for this analysis previously, but I've encountered an issue where the retention time of benzaldehyde varies significantly from one run to another, and eventually approaches to that of benzoic acid. Is it possible that this variation is due to the oxidation of benzaldehyde to benzoic acid?
It's worth noting that several studies have conducted a similar analysis using HPLC, but none have mentioned any difficulties in measuring benzaldehyde. In contrast, benzaldehyde consistently shows reproducible results when analyzed via GC.
I would greatly appreciate your feedback on this matter.
You can also use Methanol and water. Be sure to use a ionization pH buffer like pH 3 Phosphate buffer in your mobile phase to ensure you only have 1 ionized form.
Hi Muhammad, you can also use a solution of tetramethylammoniumhydroxide pentahydrate in methanol. Some drops to your sample solution and then inject it. The benzoic acid is esterified in the injector block and you will get the methyl ester.
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