M + 18 ion in ESI... water?

[ZeroAir]

I am doing injecting a standard of the following molecule: (C22 fatty acid methyl ester)



I am using ESI in positive ion mode, my ionizing solution is 50:45:5 Chloroform:Methanol:Water, with .1% TFA. In the mass spec I see two peaks (with isotope trailers), one at 355 which corresponds to the M+H of the compound, and one at 372 which is M+H20, is this possible? Are there certain molecules that exhibit more susceptibility to water adducts, and if so what is this characteristic called?

Thanks,

Luke

[Jetjamnong]

Is this Docosanoic acid methyl ester (C23H46O2) M+H = 355, M+NH4 = 372, M+H3O (Water adduct) = 373
I think this is't posible to adduct with water.
Try to see that below link for cuase of adduct formation.
http://www.rci.rutgers.edu/~layla/AnalM ... s_LCMS.pdf

[ZeroAir]

Thanks, this is a useful table. I am still not sure what the result of the peak above the molecular ion peak is. I am preparing more samples (other chain lengths) of fatty acid methyl esters to see how they respond. Thanks for your help JJ!

Luke

[Alp]

ZeroAir

The only thing I can think of is
ammonium ion/NH4+ is 18 AMU.

If M= 354 (M+H is 355) then M+NH4+ would be 372. You state you are only using water/MeOH/Chloroform and TFA so where would the ammonium be coming from, (your system might be saturated with it).

It might also be a background contaminant. Have you run a blank so you might subtract it from your sample?

Alp

[ZeroAir]

Alp,

Yeah a few weeks ago I was running the system with some ammonia because I was in negative ion mode, I think that must be the source of this. Thank you both for your assistance!

Luke

[seegers04]

I would definitely agree with the ammonia adduct. Lipids are very fond of making them.

[ZeroAir]

Why do you say this? Is there a specific functional group that is responsible? It seems odd that the hydrophobic lipid tail would do this...