How to Configure the Optimum Delay Volume

For fast gradients over 0.5 min, the delay volume of the system can be easily reduced without changing the physical configuration of the system. The change is achieved by changing the behavior of the multisampler. The delay volume of the autosampler is due to the flow path from the injection valve through the metering device, needle, needle seat, and connecting capillaries back to the injection valve (see Schematic of injection steps in the Multisampler (Single needle)). For a 1290/1260 Infinity III Multisampler, the delay volume equates approximately to 78 µL (G7167B) or 265 µL (G7167A). To make an injection, the valve switches from mainpass (main path) to bypass so that the metering device can draw the sample into the needle capillary. The injection is made when the valve switches back to mainpass (main path) and the sample is flushed onto the column. The valve remains in this position during analysis so that the autosampler is continually flushed and hence the gradient has to flow through this delay volume to reach the column. This can be eliminated by switching the injection valve from mainpass (main path) to bypass after the injection has been made and the injected sample has been flushed onto the column. In practice, this can be done a few seconds after injection by selecting the Automatic Delay Volume Reduction (ADVR) function in the autosampler 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. For instance, a 1 µL injection under standard conditions effectively reduces the system delay volume by approximately 50 µL or 240 µL, depending on the installed Multisampler.

When using ADVR, it should be noted that the gradient has already started at the pump at the instant of injection. The question should be asked whether the gradient has already reached the autosampler, in which case a small step in the gradient results. This happens when the delay volume is less than the flush-out volume and is not necessarily a problem but may be a factor to be considered in a method transfer. With a flush-out factor of 5 and an injection volume of 10 µL, the autosampler allows 50 µL to pass through before switching to bypass which, with a delay volume of 50 µL, means that the gradient just reached the injection valve. Smaller injection volumes have no effect, but for larger injection volumes this introduces a small step in the gradient. The flow rate in use also has an impact on the decision to use ADVR or not. At a0.2 mL/min the delay time saved is 21 seconds, while at 1.0 mL/min it is 4 seconds.

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 install the 40 µL Analytical Head and the 20 µL Loop. To get the best results, it is also recommended to order the Low Dispersion Heat Exchanger and the micro flow cell for UV. This reduces the delay volume by 60 µL or 250 µL.

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