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Re: Trying to Understand Library Match Factors, MH B.08
tom_barrettNov 14, 2018 5:21 PM (in response to trifecta)
Hi Trifecta,
On the surface the last example is in enigma but I think that I may know the answer. My theory is that the sample spectra actually has some very, very small ions that we don't see due to scaling issues. One or more of the ions align with the library spectrum and we get a non zero LMS. I don't have the data to test this but if you will expand the y-axis you can check for this.
Tom
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Re: Trying to Understand Library Match Factors, MH B.08
trifecta Nov 15, 2018 12:19 PM (in response to tom_barrett)Hello Tom,
Thank you for your suggestion. I did zoom the y-axis and found one mass peak, 57, at an abundance of 0.8. Picture attached. This compares to the base peak mass 74 in the unknown of 1000 abundance. I could not find a way to directly access NIST MSSearch from MH Quant. Therefore I created a spectrum manually in NIST MSSearch, with these 2 mass-abundance pairs. I also created a NIST library of one entry, the compound of interest, using the NIST spectrum. I searched the unknown spectrum against this single spectrum library. The forward match factor was 0 (zero) and the reverse match factor was 70. This seemed a bit generous to me, but 57 is the base peak and if it's the only mass considered, well OK.
Can Agilent provide a document that shows the equation for the Agilent Library Search Match Factor ?
Also, can Agilent provide a document that describes whether reverse of forward search is used, and if both are used what are the weighting factors ?
Thanks for your help.
Trifecta
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Re: Trying to Understand Library Match Factors, MH B.08
trifecta Nov 15, 2018 12:21 PM (in response to trifecta) -
Re: Trying to Understand Library Match Factors, MH B.08
tom_barrettNov 15, 2018 12:51 PM (in response to trifecta)
Hi Trifecta,
Good to know I was right about the tiny ions. I have contacted the R&D team to see what sort of documentation we have but can tell you a few things now. The weighting in MH is 70 forward & 30 reverse. In Unknowns Analysis B.09.00 this is exposed and user adjustable in the method.
Will post more when I know more.
Tom
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