Suppose I run a reaction and then pass the results through LC/MS to find the conversion. One idea could be to compare the peaks of the reactants to the peak of the product. But:

  • MS peak will depend on the levels of ionization of each of the compounds
  • UV spectra depends on the molar extinction coefficient of the compounds

So it looks like I can't compare the peaks. Is this right and if so what are the possible ways of finding the conversion?

  • 2
    $\begingroup$ You;re completely right, its one of the 'issues' with trying to monitor reactions by LCMS. If its a reaction you plan on running a lot, its not too much effort to measure the UV absorption and then calibrate the LC results, but failing that, GCMS or NMR are both viable options $\endgroup$
    – NotEvans.
    May 27, 2017 at 16:32
  • $\begingroup$ Even in GC one has to calculate something akin to absorption factors which I don’t know the name of right now. $\endgroup$
    – Jan
    Oct 25, 2017 at 9:33

1 Answer 1


You can indeed find the conversion (not the absolute concentrations), even without calibrating the system.

At time t = 0, take an exactly measured volume of reaction mixture (e.g. 1 mL) and dilute it in a volumetric flask with water and/or acetonitrile to an exact volume (e.g. 100 mL).

Inject an exact volume (e.g. 50 uL) of the diluted solution in the LCMS and note down the (UV-based) peak area of the limiting reagent. This is your ${A_0 = \epsilon \cdot C_0 \cdot k}$, where ${C_0}$ is the concentration of your limiting reagent at this time, and ${k}$ is a constant related to the sample size and dilutions.

At any time t, do exactly the same as above, and you get an area ${A_t = \epsilon \cdot C_t \cdot k}$. Under these conditions, ${k}$ is the same, and of course so is ${\epsilon}$.

As you know, the conversion is ${y = 1 - \frac {C_t}{C_0} = 1 - \frac {A_t}{A_0} }$.

  • $\begingroup$ Could you describe what A and ϵ are? And if this is a well-known equation, could you please name it and/or give the references? $\endgroup$ May 28, 2017 at 8:03
  • $\begingroup$ It's based on Beer-Lambert. ${A}$ is the the LCMS peak area from a UV detector, which is what I wrote, and is proportional to absorbance. ${\epsilon}$ is the molar extinction coefficient. Not sure why my answer got a-1 mark, considering that it answers the question (how to measure conversion by LCMS). $\endgroup$ May 28, 2017 at 19:04
  • $\begingroup$ Thanks! I don't think we'll be able to do the measurements at t=0 in our case, but still this clears some things up for me. $\endgroup$ May 28, 2017 at 19:23
  • $\begingroup$ In some special cases you need no t=0 measurement. E.g. if your reaction is 1st-order (or at least pseudo-1st order), plotting ${log(A_t)}$ vs t should give you a straight line, from which you can extrapolate ${log(A_0)}$. Ordinary reactions (zero order, 2nd order, etc) should all be amenable to such treatment. With different functions of course. $\endgroup$ Jun 6, 2017 at 17:35

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