internal standard variation

[jiangds06]

I saw 10% CV of internal standards response in seven different calibration standards. Sample preparation is ACN precipitation. According to your experience, is this because of different addition of IS to different calibration standards or just variation of LC-MS in different injections?

[tom jupille]

How did the analytes behave? And, more important, how did the ratio if IS to analyte response behave?

If the ratio was more reproducible than the areas, then it's probably an instrument issue or a sample prep issue (after addition of the IS). Correcting for those variations is the reason for using internal standards in first place.

If the ratio was equal or less reproducible than the areas, then variation in the amount of IS is the most likely problem.

[jiangds06]

yes, this is very good method to distinguish between the two.

How did the analytes behave? And, more important, how did the ratio if IS to analyte response behave?

If the ratio was more reproducible than the areas, then it's probably an instrument issue or a sample prep issue (after addition of the IS). Correcting for those variations is the reason for using internal standards in first place.

If the ratio was equal or less reproducible than the areas, then variation in the amount of IS is the most likely problem.