In this process, when the sample has reached the column, the injection valve is switched back to bypass, and the next injection cycle starts but waits with switching to mainpass (main path) until the actual run is finished. You gain the sample preparation time when using this process.
Switching the valve into the bypass position reduces the system delay volume. The mobile phase is directed to the column without passing sample loop, needle, and needle seat capillary. This can help to have faster cycle times especially if low flow rates have to be used like it is mandatory in narrow bore and micro bore HPLC.
NOTE
Having the valve in bypass position can increase the carryover in the system.
The injection cycle times also depend on the injection volume. In identically standard condition, injecting 100 µL instead of 1 µL, increase the injection time by approximately 8 s. In this case and if the viscosity of the sample allows it, the draw and eject speed of the injection system has to be increased.
NOTE
For the last injection of the sequence with overlapped injections, it has to be considered that for this run the injection valve is not switched as for the previous runs and consequently the injector delay volume is not bypassed. This means that the retention times are prolonged for the last run. Especially at low flow rates this can lead to retention time changes which are too big for the actual calibration table. To overcome this, it is recommended to add an extra “blank” injection as last injection to the sequence.
base-id: 3714917131
id: 3714917131