Elizabeth S in another note asked "why isotopically labeled standards favoured for internal standard in LC/MS and why are they not always used?

Labeled standards are used for several reasons. They mimic the recovery of the analyte of interest, they usually coelute with the analyte and ionize in an almost identical manner correcting for matrix effects, they will have similar stability to that of the anlayte, etc.

They are not always used because they are either expensive or not commercially available.

Some people recommend no internal standard. See the following interesting discusion of the Pharma Discussion list archive..

http://www.boomer.org/pkin/PK04/PK2004313.html

Even a labeled standard is not perfect, it will cause matrix effects for the analyte, see the pdf file on Tandem Labs site (many other useful files there).

http://www.tandemlabs.com/documents/AAPS03Hairui.pdf

I found a very interesting discussion on the topic in the Pharma Discussion group..