tlahren wrote:
Doing a library search with SIM data will not result in a match that makes sense or is defensible. Unless you had 20 or so major ions in your SIM acquisition, in which case you are not benefiting by doing SIM at all (assuming you are using single quadrupole MS). SIM should only be used when you KNOW your chemical elutes at a specified RT and you are fairly certain you won't have any background contribution to your quant or qualifier ions in a given matrix or prep method. In Low Resolution MS, SIM is used to enhance the signal of selected ions only. If done properly, background subtraction should not be needed for adequate quantitation, and a library search will be meaningless because you will not have enough ions to reproduce a spectrum for search.
My suggestions is that if your goal is purely "qualitative" and you don't want to know how much chemical there is then switch to Full Scan and do a library search after subtracting out the time in front of the peak.
many thanks prof."tlahren for your replay ..
Yes i know the eluted chemicals ,and i know its RT .
my job about analysis of biological samples like urine, blood and viscera in forensic lab.i do scan mode for many cases ,but in other which give positive morphine or tetrahydrocannabinol in immuonoassy .i take these samples and make confirmation by elute in GC MASS AFTER HYDROLYSES..
my problem is about SIM mode ,as when i elute morphine with BSTFA .some cases give peak in exact RT .library read morphine TMS with match 90%.OR ABOVE
other give v,small peak whith match 65%.and i do subtract to these peak .give high match become 85%or Less ..my question .is these true (do abstraction in SIM mode 0.
or SUBTRACTION ONLY DONE ON PEAK SCAN MODE --
these is big problem we face in our lab as cases read 60% we consider it negative .when matching equal 85% or above we consider it positive ..
so by subtraction the sample become positive ..
and we deal with addicted person as we belong to ministry of justice---
..
sorry for these details and wast your time .but these is an important issue we face ,we need your help
many rhanks