In many cases, it originates from laboratory glassware. Various alkali salts are used in the production of glass and these can be released from the glass by aqueous and/or acidic solvents. Furthermore, the sodium contamination can originate from the analyst...without wearing gloves, it is possible to transfer traces of salt just by touching the glassware, but this could be just enough for metal adduct ion formation.
In addition to [M+Na]+, [M+K]+ and [M+NH4]+ are also rather common.
EDIT: Rui Pinto, unfortunately I don't have a bibliographic reference for it, but I have noticed it happens quite often while recording HRMS (High Resolution Mass Spectrometry) spectra. It hasn't really bothered me much as the exact mass found has been in agreement with the calculated molar mass (that is, after subtracting the Na/K/NH4/etc. from the found total mass).
regards,
JR
Not a paper, but a nice piece on the contamination with alkali metal ions:
http://www.sepscience.com/Information/Archive/MS-Solutions/233-/MS-Solutions-3-Dealing-with-Metal-Adduct-Ions-in-Electrospray-Part-1
Alternatively, Na and K can come from the water used as solvent in the HPLC. Even distilled and deionized water has traces of Na and K ions. Those are not in any way considered contaminants of your sample, and I wouldn't worry about it. With highly acidic peptides I've sometimes even seen [M+3Na-2H]+ or [M+4Na-3H]+
In addition, if you are worried about the purity of your compound the "addition" of a small about on LiCl, for example, will give you and M+Li.
I totally agree with Dr. Vazquez's answer. I've also seen these quite often. On certain systems, we could get rid of the +Na mass by switching the solution bottles to plastic one. Obviously, Sodium acetate cannot be used as a buffer...
Thanks Pierre-André. Those [M+3Na-2H]+ ions were quite a surprise the first time!; at first I thought that somehow it had polymerised because I saw a whole mass ladder that ended up being the peptide with 1, 2, 3, 4, and 5 Na! :-)
Thanks to all for the interesting discussion. Usually a get MS spectra of my compounds using a direct insertion probe (solid crystalline sample), which gives very nice spectra with very visible M+. Now, we´re using a LCMS-ESI (sample dissolved on MeOH or MeCN), and we are obtaining these M+-adducts (more often M-Na+ as the base peak, but sometimes it appers M-MeCN+ as well)
Yes, HPLC-MS (ESI) is the way we have always worked, and that's why we see these things. You can get all sort of weird adducts (also with TFA or acetate), there are a few resources on the internet if you need extra information. Once you know they're there, it's not a big deal. Referees will not (should not) complain about that.
I agree with the answer given by Jan-Erik Raitanen. Na+ can come from the glasswares in the lab. Additionally, Na+ can also be present in H2O (water). So, in LC-ESI-MS experiments involving both organic and aqueous (H2O-based) solvents, there are higher chances of Na+ forming adducts with molecules, thereby showing peaks in the mass spectra that correspond to [M + Na+]+. Likewise, there can be K+ in water, which would be used for LC-ESI-MS experiments and hence, there can be peaks due to potassium adduct ions, viz., [M+K+]+.
However, not all molecules would have the ability to bind to Na+ and K+. Depending on the availability of the functional groups in a particular molecule, different molecules might have different abilities or capacities to bind to Na+ and K+. From my experience, I have seen that 'peptides' have higher propensity to bind to Na+ and K+, in particular, hydrophobic (non-polar) peptides have higher propensity to form Na+ and K+ adducts than hydrophilic (polar) peptides.
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