Chromatography Forum

I have two questions about this topic:
1. In GC MS, the internal standare often is used ( I think the IS is used to control the sample extraction and the injection sample to GC column), but in LC MS (or LC DAD), the IS rarely is used (if the IS is used, it is often isotope of sample, not a substance that is the same as GC MS). Why? If I want to use IS in LC/MS and I do not have isotope of sample, how can I do?
2. In ESI LC MS, I known about mechanism that is used to explain negative and positive mode, but in APCI and APPI LC MS, I don't know about the mechanism forming negative mode. Will someone help me?

1. Two things come to my mind right away I can comment on.
IS (internal standard) is often used in both GC and LCMS experiemnts. Most people will agree that the BEST IS to use is a deuterated or C13 isotope of the analyte.
When using a labelled isotope, the chemical properties are almost exactly the same between the analyte and the IS. This means that the problems with recovery of the analyte should show up for the IS as well and the IS can therefore be used to "correct for" loss of analyte through the sample prep procedure. As it elutes at the same time (always??) it will suffer the same matrix effect, if any, in the ionsource and will help rcorrect for this as well.
When using a compound that is different from the analyte, either a little bit or a lot different, you can reduce the effectiveness the IS has in correcting for analyte loss (recovery), matrix effects, ionization enhancement or supression and so forth.

2.
Phrase your question wisely and use google.
You can apply this to item 1. as well.