Basic Question
Posted: Mon Apr 03, 2017 3:29 am
by Chilly0543
I posted this before in the wrong section so I am moving it to this forum.
My question is why during lcmsms is your precursor ion not the molecular mass? For instance when researching methods for say gabapentin, all of the Q1 values are 172.1 when the MW of gabapentin is 171.2. Why is that? Thank you, I know this is a very basic question.
Re: Basic Question
Posted: Mon Apr 03, 2017 7:03 am
by Rndirk
You put a charge on your compound during the ionization process, typically electronspray ionization (ESI).
More often than not this is a protonation or deprotonation. So if your molecular weight of the compound is M, you measure [M+H]+ in positive mode, or [M-H]- in negative mode. The molecular weight of hydrogen is 1.
Re: Basic Question
Posted: Mon Apr 03, 2017 8:52 pm
by James_Ball
You put a charge on your compound during the ionization process, typically electronspray ionization (ESI).
More often than not this is a protonation or deprotonation. So if your molecular weight of the compound is M, you measure [M+H]+ in positive mode, or [M-H]- in negative mode. The molecular weight of hydrogen is 1.
Agreed, usually it is just the shift of one mass unit either up or down depending on polarity.
If you see a larger shift, it can be caused by adduct formation.
https://www.sepscience.com/Information/ ... ray-Part-1
This gives a little more information on the ionization and adduct formation.