Chromatography Forum

Respected Forum,

I just would like to know what are the major causes of ion suppression in LC-MS? Coulb the anticoagulant used for plasma samples be the reason for the ion suppression?? How do you get around this ion suppression??

I would be greatful if all the experets throw some light on this very basic question!!

Thanks,

Keep Smiling!

Aniket

Ion suppression originates in the charged droplets formed by the electrospray interface. There is only so much charge that a droplet can hold, and all the species are competing for that charge. So if a large concentration of another species co-elutes with one of your drug metabolites, the competition will depress ionization of the target molecule. Another mechanism is if the interfering species forms a strong ion pair that neutralizes the ion of interest; TFA suppresses in this way. The usual anticoagulants are EDTA, citrate and heparin, and they should elute near the void for most RP-HPLC conditions.

The general approach to get around the problem of ion suppression is a significant reduction in the amount of interferences. The most efficient way to do this is solid-phase extraction, best with a SPE method that is somewhat orthogonal to your analytical method. Common tools are mixed-mode ion-exchangers. You can find well optimized methods and tools at the Waters webside (look for the Oasis applications booklet).

There is actually a very rational behind this. Ion-exchange and reversed-phase separations are orthogonal. If you cut a small slice of the elution pattern from an ion-exchanger, your interferences in reversed-phase are drastically reduced, since this slice is now spread over the entire RP elution window.

In addition to the concentration of the interfering compound, it's proton affinity and hydrophobicity will also determine the extend of ion-suppression if any.

For the moment, adequate chromatography or SPE can eliminate the problem, in the future, there is a possibility of eliminating the problem through multiple electrospray emitters (and probably somewhat lower flow rates)...