Chromatography Forum

How do calculate the RF value (Response Factor) of an unknown peak?

Generaly this is done by running a set of linearity samples of known amounts of the unknown peak, then caluclating a response factor from the slope.

Where this isn't possible because you don't have any of the unknown peak then sometimes you can use different detection methods such as LC-NMR or LC-ICP-MS (if you're unknown contains metal atoms for instance).

If you really don't know what your unknown peak is then it is almost impossible to caluclate an RF value...

Paul.

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Paul Hurley

Is it a good idea to assume that the RF value of an unknown is 1 in a calculation for impurities?

Paul, almost impossible?
lanetl, if you assume that, then why bother with an analysis?

Well, It might be possible to do some form of mass balance and work out how much of your known sample must be this unknown peak by weight, and then tie that to a linearity, but I'd still not trust it, just in case there's something there I can't see by that method....

Lanetl: If you mean assume a relative response factor of 1 against a known peak (such as the parent drug) then yes, until you know more about you're unkown then that's all you can do, but you should specify this when you quote your results, i.e quote '0.5% rRt 1.8 as Drug' or similar. This should work as long as you think you'r unknown is similar to your known (ie a degredant of the parent peak).

If you mean an absolute RF of one, then no, thats ridiculous, you might as well just write down numbers at random....

Paul.

Yes, Relative Response factor is what I mean. You have all been very helpful. I wish i new about this site when i started learning. Thankyou again.

Hi Lanetl,

Welcome to this message board. You have hit on a particularly difficult problem here (I have the same problem with one of our compounds).

Three options I can give here:

1) If you have a MS you might be able to get some more structural info of what the unknown is.
2) Do some prep LC / extra purification to see if you can get enough of the unknown for NMR / further MS for identification. If you can get it identified ask the sysnthesis lab for help...
3) Run with a detector that may give you constant RF's (e.g. Charged aerosol detector) regardless of compound structure - there has been a recent paper in Anal. Chem. that details doing this with 2 HPLC systems with gradient elution.

Also search "relative response factor" in this forum for possible further info.... good luck!