Chromatography Forum

Hello

I have been doing a fair amount of reading on LC/MS lately, and I wanted to ask a question.

One of the references I looked at said the following: "the analytes that can be used with APCI or APPI are generally analytes which can be analyzed by gas chromatography".

This statement seemed strange to me. I do understand that both of these interfaces require that the analyte goes into the gas phase prior to being ionized, and therefore the analytes do need some volatility. But they only need to go into the gas phase transiently (as opposed to gas chromatography where they need to be in the gas phase for the whole analysis).

If this statement was really true then almost all LC/MS work done in the Pharmaceutical industry would have to be done with the electrospray interface. And I don't think that's correct.

So please share your thoughts experience on 'what range of analytes can be handled by APCI and APPI interfaces'.

Thank You

I would see the statement is true; it just does not cover what else works well with these techniques. The suitability will also depend on whether a quantitative measure is needed, verse just a qualitative ID.

I also read into it that the compounds that work well with APCI and APPI are less polar, non-ionic, etc.
The things you look for with GC separations as opposed to LC.

Adam
Compounds in GC-MS degrade in three areas of injector, column and ion source. The injector vaporization is similar in nature (flash thermal vaporization from a spray) to that of APCI and/or APPI. However, GC-MS with a PTV injector and high liner flow rate (split injections or splitless with high column flow rates) can be gentler to labile compounds since with the use of PTV injectors the degree of sample degradation is much lower than in the high temperature APCI/APPI vaporizer. Intra column degradation occurs particularly to labile compounds that elute at temperatures close or higher than those used at the injector (Iprodion and tetril as two examples). The GC-MS ion source is metallic by nature and thus a source of degradation and despite some vendors claims of inert it is active and for example cholesterol degrade to cholestane in part or mostly in all vendors GC-MS ion sources. However, if one uses a PTV injector, short column with high column flow rate and contact free fly-through ion source (as in GC-MS with Cold EI), the range of compounds amenable for GC-MS can be even greater than of LC-MS with APCI/APPI. This subject is discussed in more details at the Advanced GC-MS Blog Journal http://blog.avivanalytical.com/2013/08/ ... nable.html
You can also read our paper on this subject A. B. Fialkov, A. Gordin and A. Amirav, J. Chromatog. A. 991, 217-240 (2003).
In general, the main reasons to use LC-MS with APCI/APPI is that it is available in the same LC-MS with ESI that can ionize larger compounds and that the Pharma industry wants to know what eluted from an LC column at a given elution time.
Otherwise, GC-MS with Cold EI provides the same or even bigger range of compounds that can be analysed with the exception that very polar compounds such as free diacids and quaternary ammonium salts do not wet standard PDMS based GC columns hence have very little peak capacity while labile small compounds such as the TATP explosives or the Aldicarb pesticide are analyzed much better on GC-MS with Cold EI with short column and high column flow rate than on LC-MS with APCI since as above injector degradation can be much lower in PTV injectors.