Autosampler injecting too much liquid

[gradyb]

Does anyone know why my Accela Autosampler would be injecting too much liquid? When I set it to inject 10 uL for example it injects more than 200 uL.

[uzman]

Actually , it does not inject 200ul , it injects 10ul , the remaining volume is to fill the volume between the needle and the injection valve.

[gradyb]

It usually doesn't fill that volume with sample though, that volume is filled with the syringe wash solvent.

[lmh]

The Accela uses a concentric two-plunger syringe, with the middle plunger (very narrow) being responsible for measuring, the larger plunger for taking big volumes of liquid for needle-washing. A single drive mechanism operates the whole system, and the only thing that decides whether it's moving the inner or outer plunger is how far up or down it is. If the autosampler is confused and thinks its syringe is somewhere different to where it is, it may think it's moving the middle plunger (to measure 10uL) but it's actually moving the outer plunger (for which the same movement will take more than 200uL).
To unconfuse it, home the syringe, or turn on and off again.
If, on flushing the syringe, the syringe plunger gradually empties the syringe less and less completely, then something is wrong. The usual culprit is a semi-blocked needle, meaning that the drive belt keeps slipping. Alternatively if the drive screw has become dirty, the mechanism will slip. Either unblock the needle, or clean the screw thread (clean cloth, trimmed end of a wooden dowel, etc., while turning the screw manually using the screw slot on the end of it; you'll need to remove the syringe drive assembly to do this)

[gradyb]

Thank you lmh, that was very helpful. Homing the syringe and restarting the autosampler didn't fix the problem.

Also, the needle wasn't completely blocked but like you said, it might have been partially blocked. I went ahead and flushed the needle and transfer tubing. I haven't had any problems since.