Chromatography Forum

I want to do impurity analyses with LC MS and I have a question about the quantity of impurities in a sample.

When we want to do impurity analyses, can we get % amount of the impurity?

I know that if i have the impurity alone, i can draw a calibration curve and then calculate the concentration impurity in our sample. But if we dont have the impurity alone ( for unknown impurities), what can we do? if we want to know the concentrations of impurities, is it logical to do ratio analyses from the areas under the peaks?

Thank you for your kind interest

sevinc

is it logical to do ratio analyses from the areas under the peaks?

It is not logical, but that is the way it is done (for lack of anything better).

But the ions are not ionized in same Da. so the percent concentration will not be exact?? or we can not see the impurities that are not ionized. so is it logical to make ionized them in their m+1 s, i am in, in their max. inonized values, and use them in percent concentration calculations?

Regards

I'll repeat what I said: it's not logical (for the reasons you mentioned). But it *is* done for lack of anything better.

Forgive me if I don't understand the question but what I am hearing is: Can't I just take the ratio of the product to the impurities? If that is the question, I would point out that there is a very high probability that the product has exceeded the linear range of the detector and that is why the ratio won't work.

Again, forgive me if I don't understand the question.

Best regards,

AICMM

I'll also repeat what Tom said!

Yes, it would be even less logical to use peak area ratios if the product peak has exceeded the reasonable range of a calibration curve, but the whole process is, in any case, illogical. But there is no alternative.

In typical LC-MS ionisation (e.g. electrospray) the ionisation efficiency (signal per mole) of different chemicals can differ wildly, even when the chemicals don't actually appear all that different. Ideally we'd like to know the percentage impurity expressed in some useful basis (percentage by mass, molar percent, etc.). Because we have no idea of the ionisation efficiency of the impurity (because we don't have it as a standard) we can't convert area percent to molar or mass percent (irrespective of what we know about the impurity's mass). The area percent is fairly meaningless, because a strongly-ionising impurity will give a much larger value than a weakly ionising impurity. Unfortunately, what else can we do? "No standard" means "no calibration".