Compare concentrations using LC/MS

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?




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 ${y = 1 - \frac {C\_t}{C\_0} = 1 - \frac {A\_t}{A\_0} }$.


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