mysterious compound; can M+28 or M+29 make sense?

[Aurelien]

Hey guys,

I have been doing some lcms to identify a mysterious compound from a reaction mixture. The mass i found is 28-29 above the mass of the expected structure. Can this 28-29 difference be explained by an added ion (ie somthing like M+X+...) (runing conditions: gradiant of MECN (0.1%TFA) in H20 (0.1% TFA), 22 to 55% in 20 min)

Thanks

Aurelien

[sassman]

It isn't necessarily related to your compound in any way. Could be a completely different molecule. You haven't really given us enough information to differentiate. Is it present in a control sample, blank, etc.

A very useful little program called "Molecular Weight Calculator" is available here: http://www.alchemistmatt.com/. Check out the "Formula Finder" under "Tools". Having said all of that, here are a few possibilities for fragments:

28
CH2N
CO
C2H4

29
CHO
CH3N
C2H5

[Aurelien]

Thanks for this molecular mass calculator.
It is related to my molecule because it is a reaction intermediate that disappears slowly upon appearance of product.
It must be somehow related to my starting material.
So I had an idea of what it could be but the mass of this "idea molecule is" 29 mass unit below the experimental mass obtained by LCMS. So I was just wondering if i was not missing an obvious ion that could have added to the ion peak (eg M+Na or Na+K instead of M+H).
Ionisation method is electrospray positive ionisation.
Hope it's clearer now

Thanks

Aurelien

[jbshan]

It is just called as adduct ions (Na, K, H2SO4,H3PO4 etc) which usually appear in ESI

shan

[lmh]

As a first step, I'd strongly suggest working out whether it's 28 or 29. The reason is the nitrogen rule, which depending on odd or even mass can tell you whether your product/adduct could have the expected number of nitrogens, or an unexpected number.

In electrospray, assuming you don't have a very unusual analyte, you will not have a free-radical ion. All the common adduct ions have odd masses, so if your analyte has an even neutral mass, it should give odd-mass adduct ions (and vice versa); the exception is ammonium, which adds a mass of 18.

An adduct that is 28 units heavier than the expected neutral mass of the analyte molecule suggests to me that the analyte molecule isn't what you expected, and contains one nitrogen more or less than you expected (Nitrogen rule).

A mass difference of 29 still suggests to me that you should be thinking about whether your parent structure is correct. It's not a common adduct, but there are lots of explanations, including Sassman's list (but beware of the nitrogen rule when considering CH2N), and also slightly more esoteric formula-changes such as N2.

Good luck!