The Hybrid Multisampler is a type of sampler that can be operated in two injection modes: Flow-through and Feed Injection.
In flow-through injection mode, the Hybrid Multisampler has a delay volume of approximately 265 µL . This delay volume is due to the main path flow path. The mobile phase flows through the Injection Valve, the Metering Device, Sample Loop, and Needle, before being injected onto the column. The Injection Valve switches from main pass to bypass position, so that the Metering Device can draw the sample into the Needle Capillary and Sample Loop. To inject this sample in flow-through mode, the injection valve switches back to main pass and the sample is flushed onto the column. The injection valve is kept in this position during analysis, so that the sampler is continually flushed and hence the gradient has to flow through this delay volume to reach the column. This behavior can be eliminated by switching the injection valve from mainpass (main path) to bypass after the injected sample has been flushed onto the column. In practice, this switch can be done a few seconds after injection by selecting the Automatic Delay Volume Reduction (ADVR) function in the sampler setup menu. The flush-out factor (typically five times injection volume) ensures that enough time is allowed to flush the sample out of the injector before switching to bypass.
The delay volume can be completely eliminated using the Feed Injection mode. During Feed Injection, the mobile phase remains in bypass and the sample is directly mixed into the mobile phase in the injection valve.
When using ADVR, it should be noted that the gradient has already started at the pump in the instant of injection. The question should be asked whether the gradient has already reached the sampler, in which case a small step in gradient results. This behavior happens when the delay volume is less than the flush-out volume and is not necessarily a problem, but a factor to be considered in a method transfer. This gradient step happens when using larger injection volumes with ADVR. The flow rate in use also has an impact on the decision to use ADVR or not. At a 0.2 mL/min the delay time saved is 21 s, while at 1.0 mL/min it is 4 s.
The ADVR function is unlikely to be suitable for applications involving compounds that are known to cause carryover problems. The best way to reduce the delay volume is to inject the sample in Feed Injection mode. To get the best results, it is also recommended to order the Low Dispersion Heat Exchanger and the micro flow cell for UV.
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