theoretical question about ion enhancement

[Ruth]

Dear forum members,

I have a theoretical question. It sounds silly, but I hope you can help me out.

When using LC-MS matrix effects are a severe problem. There can be ion supression or enhancement. I perfectly understand that co-eluting substances supress the ionization of the analyte, causing a lower signal. But what is the mechanism of enhancement? How can there be more signal than amount of analyte?

Thanks!

[Gaetan Glauser]

These are only suggestions but:

- if it helps the escape of the ion from the droplet (ion evaporation)

- by charge transfer

any comment or other suggestions?

[Ruth]

Thanks for your answer... However, I stil don't understand how these mechanisms encounter for ion enhancement. If ion evaporation is helped, how can there be more ionized analytes than the real number of analytes? Same question for charge transfer...

[Gaetan Glauser]

OK I can see your problem. You just have to realize that the signal you obtain with a pure standard does not correspond to "the real number of analytes". It depends on the efficiency of the electrospray process. If your analyte of interest co-elute with another analyte which has a lower proton affinity, charge transfer can occur and you might see a signal enhancement for your analyte and a suppression for the co-eluting ion, compared with pure standards. Is that clearer?

[Ruth]

Ok, I think I understand. Is it than correct to say that when using pure standard the efficiency of ESI is not maximal, while it is when there is co-elution, because of charge transfer?

Thanks very much for helping me on this!

[Peter Apps]

Hi Ruth

LC-MS is not my field but I am reasonably sure that the same principles apply as in GC-MS and in GC-FID. In both cases the fraction of analyte molecules that are ionised and detected is tiny - closer to ppm than %, so there is plenty of opportunity for molecules that would otherwise not have been ionised and detected to be ionised by supplementary mechanisms. Only if every analyte molecule entering the source was ionised under normal conditions would ion enhancement result in detecting more analyte than was present, which is clearly impossible, as you recognise. If you can find a way of inceasing ionisation efficiency by a couple of orders of magnitude you will get very rich very quickly.

Peter

[Gaetan Glauser]

As Peter said, the efficiency is always far from maximal. In LC-MS interfaces, the ionisation process is even worse than in GC-MS (typically 50-100 times).

[Ruth]

Thanks! I completely understand it now!