Using patient urine as calibration standard

[chandra]

Hello to everyone

I don't have the pure compound of Isovalerylglycine. Few days back i came across a Isovaleric acidemia patient, collected his urine sample and after solvent phase extraction i run it into GCMS . I got the peak of Isovalerylglycine with 98% match with the NIST Library.I extracted the sample 10 more times and got the same result.I want to know is it ok to use this urine sample as calibration standard for Isovalerylglycine.

Thanks

Chandra

[Don_Hilton]

Just a mass spec match is not much proof that you have the right compund. Knoledge of the sample - that this compund would be expected in the sample helps. But, if you were reading a report in which a compund was described in the way you have described was used as a standard, what questions would you have. Reviewers of publications and procedures will have similar questions.

How sure are you that the compund is what you think it is? It it is a metabolite that has shown up in urine and is not found in other patients urine, could it be because the patient had something different for lunch?

If you are using this as a compund identity standard - your identification is no stronger than the confrimation of the standard material.

If you are going to use this as a quantitation standard, is it pure? There are a lot of compunds you can get out of urine that will never make it down the GC column, but the inclusion of these compunds will affect the recorded weight on the balance.

If results you are seeking are of value, are they worth getting a sample of isovalerylglycene of known (and certified) purity? If so - do it. If not, are the results you are seeking realy worth seeking?

(I used to teach some classes on the use of a mass spec system - and I had a favorite set of slides showing spectra of different compunds with similar spectra and close retention times.)

[chandra]

Thanks Don for your guidance ,could you provide me that slides.

[Consumer Products Guy]

http://www.tcichemicals.com/eshop/en/us ... ity/I0301/

[Don_Hilton]

I don't know if I have those slides still, but examples include
-- PAHs - in which the system ionizes and may not fragment at all. In many cases the compunds will elute close to each other coming off a chromatographic column.
-- Some gamma or delta lactones will fragment with loss of the side chain - resulting in spectra with little or no observable molecuar ion and futher fragments are from the ring system which has lost the side chain - vurtually identical. Similaritiy in retention time - close enough that they may be identifiable with retention index markers - but with care.
-- a number of terpenes have similar spectra to each other and will elute close to each other.

Those are off the top of my head.

How have you derivatized the dipeptide? And does the spectrum show a molecuar ion for the whole compund?