This application note will examine the effect of sample vial cap type on analyte concentration once the cap septum is punctured. The study used two separate LC instruments which have two different needle assemblies: a Waters ACQUITY UPLC with a fixed loop injector and a Waters ACQUITY UPLC H-Class with a flow through needle injector (FTN). The Reversed Phase QC Reference Material (QCRM) was used in this study as it contains both volatile and nonvolatile compounds and it is made in an aqueous solution of a volatile solvent, typical of many analytical samples.
An important property of a vial cap is the ability to maintain the vial analyte concentration after multiple injections over time. This ability is influenced by different factors such as the volatility of the analyte/diluent, the number of injections, the elapsed time and the injector/cap septum used. The results of this study can provide guidance in this area. However, it is always best to change vial caps as soon as feasible after piercing. For critical analyses, analysts are advised to do analyte concentration/time course measurements on their systems to confirm a sufficient vial cap resealibility for their needs.