I have a question about HRMS (High Resolution Mass spectrometry). HRMS is a powerful technique that can discriminate between possible target compounds without the need of reference standards, unlike QqQ (triple quad systems). My question is simple why do HRMS don't need reference standards for analysis?

A short explanation would really help I cannot find the answer online or anything related to it.

Thank you, hopefully I can be helped with this part. I put a section of a book as an attachment maybe that will help.

Greets, Student

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    $\begingroup$ I smell a rat. I've never work with an instrument that didn't use some sort of standard for setting it up. I can believe that you don't need to run standards with each particular sample, but never run a standard? No way. $\endgroup$ – MaxW Apr 29 '20 at 15:44
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    $\begingroup$ I assume that the quoted text is about a GC-MS or LC-MS setup. With a low res MS you'd need retention time and MS to identify a compound. With a high res MS you just need LC or GC to give a clean separation then the MS alone could identify the compound. However the MS itself would definitely need standards to tune it. Say injecting a pure compound of some sort. $\endgroup$ – MaxW Apr 29 '20 at 15:55

The statement is not 100% correct. Ordinary mass spectrometry will not give you exact masses to 5 or 6 decimal places, say, m/z 415.14 for a singly charged molecular ion. Now there will be hundreds of compounds with this mass. On the other hand HRMS gives you a accurate and precise mass say 415.14509. This detailed number drastically reduces the number of matches in a spectral library. Not only that, the isotopic distribution pattern and their masses reduces the choice to one compound. Hence the claim that HRMS will not need a reference.

Like any claim, there are always exceptions. Structural isomers can be a problem. If we have isotopologues (isotopic isomers), even HRMS will fail. Say you have two analytes but both are acetonitriles. Say one is

$^{13}$CH$_3$CN, and the other analyte is CH$_3$$^{13}$CN, HRMS will give the identical molecular mass to five decimal places. One might look up their fragmentation pattern but MS is not that great at low masses (high noise). Here you would need another spectroscopic technique such as microwave rotational spectroscopy. These techniques are used an in emerging field called position specific isotopic analysis.

  • $\begingroup$ Thanks for the answer. I understand your part. But still I then want to know how do they use HRMS without reference standards? Short procedure, I can't still imagine that part. Is is due the fact they created a database with accurate masses and can then conclude the unknown compound or am I not right. Thank you very much $\endgroup$ – tommy11 Apr 29 '20 at 14:43
  • $\begingroup$ Yes, you are right. HRMS is used in analytical problems where the analytes are unknown. Therefore you have no clue which standard to buy? So HRMS would allow the users to match exactly with the library or database with very high confidence. $\endgroup$ – M. Farooq Apr 29 '20 at 14:58
  • $\begingroup$ Even if the compound is not listed in the library, one can deduce the exact chemical formula (not the structure) with HRMS. $\endgroup$ – M. Farooq Apr 29 '20 at 14:59

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