Recently I recorded a mass spectra by using LCMS (Agilent 6530 Accurate-Mass Q-TOF LC/MS) using Acetonitrile as mobile phase in +ESI mode. Nature of compound is nitrogen containing heterocycle. The mass spectra shows the sodium and potassium adduct peaks. I fail to understand, exactly from where a sodium and potassium came in contact with sample, as I didn’t add these adduct in sample before running LCMS. Please explain.
Hi,
Sodium and potassium can come from a number of sources, mainly during sample preparation and analysis: residual salts can be present on the analytical column you're using, in the HPLC system (injector/tubing, etc.) that is being used or these can leach from glassware that you used to prepare your solvents in.
Do you also observe the protonated variant(s) of your molecule(s) of interest, or only the sodiated/potassiated ions? I'd expect a nitrogen-containing heterocycle to be protonated primarily.
Hope this helps.
Eef is right, many sources are possible. You must also consider that, although you (probably) use ultra pure Acetonitrile, standard solvents for LC-MS have purity of (just) 99.9%. If you check Acetonitrile's label, you will see that Na and K are among the substances that are indicated as traces in the Acetonitrile. In my personal experience, it is almost impossible to get rid of it.
By the way, if sample prep is always the same, their ratio is constant and they are not invalidating quantification of the pseudo-molecular ion.
Have a nice day
You can also try to clean up your system with water and ammonium acetate if it's necessary.
If the contamination it's stable and irrelevant you can analayze your sample if it isn't keep on washing with water.
Good work!
Hello Sameer,
I fully agree with Eef, I do not fully agree with Luca. Dedicated LC-MS solvents do have very low content of potassium and sodium (less than 50 ppb), which is sufficiently low to avoid the formation of many adducts.
Please find examples for specification of such LC-MS solvents at:
http://www.sigmaaldrich.com/Graphics/COfAInfo/SigmaSAPQM/SPEC/34/34967/34967-BULK________FLUKA____.pdf
http://www.sigmaaldrich.com/Graphics/COfAInfo/SigmaSAPQM/SPEC/14/14261/14261-BULK________FLUKA____.pdf
In general acetonitrile (ACN) is not so critical, because potassium and sodium salts do not solve well in ACN. But the potassium and sodium content is even more important for water. Usually water for LC-MS is supplied in bottles made of a special glass (borosilicate glass) that has a very low rate for leaching potassium and sodium into the water. Due to this it is extremely important that the waters stays in the original bottle until use, because otherwise you get potassium and sodium leaching from regular bottles into the water.
Please find further general information on LC-MS solvents at:
www.sigmaaldrich.com/lc-ms
www.sigmaaldrich.com/uhplc-ms
ACN and water could be great source of this adduct. Also glassware used for solvent containers contains sodium that leaches out into solvent. To some extent it can be controlled by adjusting some of your ESI voltages, potentially at the loss of your parent ion.
Sodium or potassium adduct can also be reduced by adding organic acids such as formic acid, acetic acid, which allows 'protonation' and hence it is possible to observe intense signals due to protonated adduct ions in the mass spectrum.
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