Numerical investigation of band spreading generated by flow-through needle and fixed loop sample injectors

The present study reports on a computational fluid dynamics study of the band broadening occurring in injector systems frequently used in contemporary liquid chromatography instruments. The aim of this work is to determine band broadening originating purely from the injection volume in absence of any other possible contribution (e.g. band broadening due to the injection valve) and to unravel the mechanism behind it. Simulations of the dispersion process in flow through needle injectors were performed. In addition, fixed loop injectors were also simulated and comparison with flow through needle injectors was made. The results are also represented in a dimensionless form, allowing to generalize them to different experimental conditions. It was shown that two different injection regimes exist (the convection regime and the diffusion regime), both leading to narrow injection bands, while operating the injection needle in between the two regimes leads to broad injections bands. It was also found that the band broadening in the flow-through needle injector is strongly affected by the holding time between sample uptake and the actual injection. As a result, fixed, full loop injectors produced narrower injection bands compared to flow-through needle injectors operated with a realistic holding time.