I need to detect 0,1-1% of Benzoic Acid in Benzyl alcohol using a GC-FID, so I started making a standard with 1% acid in alcohol but I'm still not able to detect acid signal. Does anyone know temperatures that I should use for this kind of analysis?
Just google for 'benzoic acid gc' there is plenty of stuff. I had a quick look and i would choose for derivatizing like don in 'Determination of benzoic acid in soft drinks by gas chromatography with on-line pyrolytic methylation technique'
This may be a method better suited for HPLC and a detector capable of collecting uv-vis spectra.
If GC-FID is your only option here is a reference detailing method conditions for looking at benzyl alcohol, will help you come up with a good starting point. This is a GCMS method, but the inlet and oven parameters should still be applicable. http://scholarsresearchlibrary.com/DPL-vol3-iss2/DPL-2011-3-2-218-225.pdf
+ 1 for derivatization, if you are locked with GC. As experienced by Shubhankar, low detection is difficult and the peak shape of AcOH is at best, tailing...
first thing to think about before designing a GC method is the boiling point and polarity of your analytes. Benzoic acid has a b.p. of 250oC!! This temp is quite close to the limit of most caplillary columns. The peak will be likely to appear at the end of your chromatographic sequence when the tempertaure gradient is at its highest and column bleeding is quite likely. plus it is quite polar and can form hydrogen bonds with the stationary phase so will result in peak tailing. both factors will give you less sensitive and inaccurate results.
There is loads out there on organic acid derivatisation techniques. The above suggestions about converting benzoic acid to its methyl ester are good ones to begin with.... you could also think about TMS (trimethylsilyl) derivatisiation. Both derivatives are much less polar and have lower b.ps, which will improve your peak shape.
Next step would be to ensure your sample prep method is robust and repeatable!!
Agree completely. Another thing to consider with regard to the boiling point is if any sample condenses in a cold spot (carrier supply and septum purge and split vent line particularly) it will be impossible to remove without replacing that component. Start with a small injection volume and high split until you are sure you're not injecting too much.
Thank you very much for all your answers.
So, you think it would be better to use another kind of instrument for this analysis, isn't it? Actually we own an HPLC but I suppose we should buy a new column, so I'm not sure i would be able to change instrument, for the moment.
Talking about derivatization, it seems the best way for me, actually. A colleague suggested me I could try using silanization method used in Fatty Acid Oil analysis; what do you think about this solution?
Additional information: my sample contains actually: Benzoic Acid, Benzyl Alcohol but also Benzaldehyde, so I have to find a method in which this last one won't interfere.
Prepare sample benzoic acid in CH2CL2 or diethylether or Acetonitrile, into autosampler vial (total recovery vial is recommended) + 10 uL from this solution + 10 uL MSTFA or BSTFA, heat at 80 C for 5 min, cool, inject 1 uL. You shall get very good signals as O-TMS.
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