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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: Werde on July 30, 2019, 05:10:27 AM

Title: MS general question
Post by: Werde on July 30, 2019, 05:10:27 AM

Need a clarification. I'm a complete novice at MS and I will be doing some GC-MS (QQQ) experiments using PIS followed by MRM. Is MRM done on a GC-MS/MS, i.e. is the instrument on which MRM is performed necessarily a tandem MS? If I understood correctly, there is a second fragmentation of selected ions in MRM, so it has to be done on MS/MS... or not?

Title: Re: MS general question
Post by: DrCMS on July 30, 2019, 11:31:53 AM

Yes MRM needs an MS/MS.

Title: Re: MS general question
Post by: Werde on July 30, 2019, 05:39:31 PM

Thanks for answering. I have another brief question: what is the purpose of MRM and why is it used?

Title: Re: MS general question
Post by: Corribus on July 30, 2019, 06:38:19 PM

https://en.wikipedia.org/wiki/Selected_reaction_monitoring

Title: Re: MS general question
Post by: Werde on July 31, 2019, 10:30:26 AM

Thanks for the link, but that one tells mostly about proteomics. What would be the aid of MRM when screening small molecules (with the structure known to the analyst), i.e. what info can we get with MRM that cannot be obtained by e.g. PIS or full scan? Is the purpose of MRM to get higher sensitivity, lower the LOD, LOQ, find the better conditions for fragmentation of analytes...?

Sorry if I'm annoying with too many questions.

Title: Re: MS general question
Post by: MOTOBALL on August 02, 2019, 05:28:49 PM

MRM is typically used to (1) detect and identify one or more compounds in an incompletely-resolved mixture, or (2) detect a low- level analyte whose MS/MS characteristics are known.
Typically used in conjunction with a soft ionization process ( ie low energy) that produces very little fragmentation compared to electron impact; think chemical ionization (CI) for GC-MS or solids Probe and electrospray or atmospheric pressure CI for LC-MS.

Regards.
[ What is PIS ?]

Title: Re: MS general question
Post by: Werde on August 12, 2019, 05:47:20 AM

Quote from: MOTOBALL on August 02, 2019, 05:28:49 PM

MRM is typically used to (1) detect and identify one or more compounds in an incompletely-resolved mixture, or (2) detect a low- level analyte whose MS/MS characteristics are known.
Typically used in conjunction with a soft ionization process ( ie low energy) that produces very little fragmentation compared to electron impact; think chemical ionization (CI) for GC-MS or solids Probe and electrospray or atmospheric pressure CI for LC-MS.

Regards.
[ What is PIS ?]

Thanks for replying. PIS is both product ion scan and precursor ion scan, depending on which one is used.

Last part of your comment is interesting, since we are using MRM with electron impact mode...!?

Title: Re: MS general question
Post by: MOTOBALL on August 13, 2019, 08:06:09 PM

Your product ion = my daughter ion; your precursor ion = my parent ion.

Since you will be doing EI, perhaps your molecule gives a strong M+. or [M - H2O]+. etc.
Can you disclose the particular compound?
Also, the ions you intend to fragment and then monitor.

Regards,
Motoball