In HPLC -MS analysis, observed mass and theoretical mass of phenolics such as P-coumaric acid did not match. Observed mass is less by 18 then theoretical value. what will be the possible reason??
how should i justify for further pathway prediction??
Hi Savita,
[M+H]+ - 18 indicates loss of water.
Look here:
https://www.researchgate.net/figure/ESI-MS-a-and-MS-MS-b-spectra-of-p-coumaric-acid_fig4_230572928
Regards
Markus
Dear Savita Dattatraya Girawale
I am quite confused when I read your question. How can you calculate the observed mass of p-coumaric acid?
Please pay attention to the difference between the nomimal mass, which we calculate based on its formula, and the mass-to-charge ratio of its ion peak measured by MS which is not its mass.
According to your question, I guess that ´´the observed mass´´ of p-coumaric acid you mentioned is actually its mass-to-charge ratio (m/z). Based on its m/z, you can elucidate how p-coumaric acid is ionized in the ion source and then you can use this ion as an indicator for quantitation.
I hope that my answer can clear your question.
Best regards,
Pham Quynh Khoa.
I guess that the "observed mass" is just m/z value of the base peak (ion). And because most (not every ion!) of the ions formed have a single charge, so the m/z value is taken as an equivalent to mass itself (but I am not an author of the question, so it is just my guess)...
I think that for the identification of phenolics (and derivatives) people mostly use a negative ion mode.
In fact, you usually should see p-coumaric acid at m/z ~163 (in negative mode).
@Jerzy Mierzwa Thanks for your reply and suggestions.
Actually I did MS analysis of P-coumaric acid and its derivatives in positive mode. As per your suggestion I will repeat same in negative mode, hopefully it will give me required m/z values.
Thanks.
The signal of the protonated molecule at m/z 165 might be small or even missing. See if you can find the sodiated molecule at m/z 187 (positive mode). It would confirm the molecular weigth of 164.
A cumaric acid ester could also explain the abscence of other adduct ions such as [M+H]+ and [M+Na]+. In that case you should find another group of adduct ions, which might be of relatively low intensity. I have seen this with monocaffeoyl quinic acid derivatives which give very weak signals (10 % rel. Int. or so) at m/z 355 [M+H]+ and m/z 377 [M+Na]+ and a huge base peak at m/z 163 (caffeoyl fragment).
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