In my full MS spectra, I'm having 2 to 5 more masses, around the mass of interest. For example for m/z 1448, I also see 1446; 1447, 1449; 1450. Is this related to the isotope abundance or to the quality of my experiments? And about, the isotope abundance, I read this in an abstract:"The isotope abundance of the [M+H]+ ... was also analyzed". Which practical information can I get from this analysis, while studying the fragmentation pattern of a compound or while investigating unknown compound? (I don't have experience with LC-MS).

Roy,
One of the simplest things to get from isotope abundances is an estimate of the number of carbons in the mass you are looking at. C-13 is about 1.1% of C-12 so for a fragment with ten carbons the M+1+H mass should be roughly abouot 11% the heigt of the M+H mass. Check any good mass spec book for good coverage of the subject.

Regards,
Mark

Here's a good site that will calculate a theoretical abundance for you, given a molecular formula, although it doesn't seem to work for larger molecules at the moment.

http://www2.sisweb.com/mstools/isotope.htm

At > ~ 91 carbons, your C-13 peak will exceed your C-12 (monoisotopic) peak. For a simple hydrocarbon, this corresponds to ~ 1280 m/z. Other elements such as Cl, Br, S, will also have their own contributions. The book "Interpretation of Mass Spectra" by McLafferty is a good one. It was written for EI fragmentation, not LC/MS, but all the isotope info still applies.

I thank u all for ur comments. I don't understand the following statement as given by MG: "At > ~ 91 carbons, your C-13 peak will exceed your C-12 (monoisotopic) peak". On the other hand I'm still concerned about the large number of masses around my mass of interest, making the spectrum even look a bit strange.

Roy, to clarify that statement: Suppose you have a compound that works by LC/MS, and you expect to see the ion at 1280 m/z. If the compound contains mostly C and H, and does not contain Cl, Br, or anything weird, you will also see a peak at 1281 m/z at approximately equal abundance. In mass spec lingo, this is sometimes called an "A+1" peak. If you add carbons from there, your "A+1" peak will exceed your "A" peak in abundance. This is due to the natural abundance of carbon-13. As you increase the number of carbons in a molecule, the number of molecules containing at least one C-13 in a given sample increases. At high masses, you'll also start to see some "A+2" contribution from molecules containing 2 carbon-13's.

Going back to what Mark posted on Jan.23, in a sample of compound containing 10 carbons, about 11% of the molecules will contain one C-13, so you'll see an "A+1" peak at 11% of the abundance of your "A" peak.

Thank u all for ur comments