Chromatography Forum

At least this is how I would look at it from an auditor's perspective - how can you prove that your system is precise at all of the various injection volumes? That would be the biggest question to answer if you were to do it your way - in my opinion.

The same way you prove that your dilutions are precise: by examining the linearity of the calibration plot. In any case this should be done routinely as part of PQ ("Performance Qualification", for the uninitiated).

What it boils down to is: can you prepare your calibrators more accurately / precisely than the autosampler can make its injections?

By the way, I prefer to keep injection volume constant to minimize possible matrix effects, but either approach can be validated.

Thank you, Tom!! My thoughts precisely. By varying the injection volume you run into a host of problems that you don't have to deal with using a fixed injection volume, including transfer time to the head of the column, refocusing, matrix effects (especially with MS!!), etc.

As James points out, in a regulatory environment (be it USEPA, ICH, AOAC or other) you are normally limited to a controlled injection, i.e.., a fixed volume injection.