Compound identification in urine

[chandra]

Hello to everyone,

As we know that acylglycines are important marker of inborn errors of metabolism.We can identify and quantify them in urine using GCMS. When i run hammers of glycines to identify them sometime i absorbed that the three identifying ions of a particular acylglycines were present in a specific peak at a specific retention time but when i select and search the NIST library match , it gives some other compound not the acylglycines. For example the identifying ions for propionylglycine are 159,188,131.When i extract ion using these three ions i found all these ions present at RT 15.4 min but when i select this peak and search for the match it gives the match with hexane-1,3-dicarboxylic acid, this compound consist above identifying ions in very small abundance and also contain other ions like 303,187.147.73 etc whose abundance is much more higher.
My question is how do we know this peak is of which compound because ions of both compounds are present in this peak , if both the compounds are co eluting then how to find their ratio in the particular peak please help.

Chandra

[rb6banjo]

For proper identification of an analyte using the procedure you're describing, the ratios of the intensities of the ions is extremely important. Typically, I measure the mass spectrum for the analyte on my system and calculate the best average of the values for the ratios (all normalized to the biggest one). Then the rules are that for it to be my analyte:

1) the peak must elute at the correct retention time.
2) the normalized mass ratios have to be within 20% of what I measure for my known standard. The percentage is somewhat arbitrary and is determined by how much risk you're willing to take if you're wrong.

Let's say 159 is the base peak and 188 is generally 60% of that and 131 is 20% of 159. If the analyte elutes at the correct time and the ratio of 188/159 = 0.71 and 131/159 = 0.17 I'd say it's a match because 0.71/0.60 = 1.18 (only 18% off) and 0.17/.20 = 0.85 (only 15% off). However if the ratios turned out to be 188/159 = 0.45 and 131/159 = 0.30 I would say that it's not my target compound (0.45/0.60 = 0.75, 25% off and 0.30/0.20 = 1.5, 50% off). You can choose as many marker ions as you want but I wouldn't choose less than 3.

Good luck.

[Peter Apps]

In this case it might be interesting to see where hexane dicaboxylic acid elutes, and what its ion ratios are.

The results of MS library are tentative at best.

Peter