A resource for chromatographers
For most HPLC analyses a priming injection is required prior to the analysis. This injection will usually result in a slightly different chromatogram to subsequent injections of the same solution.
The reason for this is not fully understood but is probably due to the presence of (at least) two types of active sites on the column for interaction with the analyte molecules. One of these sites equilibrates much more slowly than the other and is gradually saturated with the analyte in the first few injections thus stabilising the response for further injections.
For this reason it is common practice to perform one or more ‘test injections’ before starting the analysis injection sequence. The SST solution or the calibration standard is commonly used for the test injection. The test injections may be programmed into the injection sequence if the run will be unattended, e.g. it is started at the end of a working day.
This blog post is an excerpt from 'An Introduction to HPLC for Pharmaceutical Analysis' by Oona McPolin, available to purchase through the MTS website.