As close as possible (gives rise to accuracy).
For example, if your injection volume is 10 uL, you can use a FL of 10 uL or a partial loop of 20 uL (if not, then 50 uL (say)).
For fixed loop injectors, like a classic Rheodyne manual injector, the typical recommendation has been to inject no more than 1/2 the volume of the loop (e.g., 10 uL loop, inject <= 5uL) as explained below.
Liquid flowing in a tube takes the shape of a parabola like that shown in the picture (thanks Wikipedia !)
If you try to fill a 10 uL loop with exactly (and only)10 uL there is the potential for the liquid travelling in the middle of the tube to flow out. This will lead to inaccuracte as well as imprecise sample loads. So, to ensure that the loop "captures" the entire desired injection volume and that none spills out, 1/2 the loop volume is the typical recommendation.
The ACQUITY Sample Manager operating in Partial Loop mode adheres to this recommendation. For Partial Loop with Needle Overfill (PLUNO) up to 70% of the loop volume can be safely injected. These recommendations also apply to the
HClass & IClass SM FL models.
For the HClass and IClass FTN Sample Managers, these restrictions do not apply. Volumes up to and including the volume of the loop can be accurately & precisely injected.