# 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;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 May 27, 2017 at 16:32
• Even in GC one has to calculate something akin to absorption factors which I don’t know the name of right now.
– Jan
Oct 25, 2017 at 9:33

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 ${y = 1 - \frac {C_t}{C_0} = 1 - \frac {A_t}{A_0} }$.

• 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? May 28, 2017 at 8:03
• 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). May 28, 2017 at 19:04
• 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. May 28, 2017 at 19:23
• 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. Jun 6, 2017 at 17:35