At the end of a dispense a droplet can cling to the tip, held there by surface tension and adhesion. That droplet is volume you meant to deliver and did not, and at small volumes it can be a real fraction of the transfer. A touch-off, sometimes called a tip touch, is the small move that gets rid of it: the tip briefly touches the side of the well so the droplet wicks off against the surface rather than riding back up with the tip.
What a touch-off actually does
After the plunger has pushed out the volume, the head moves the tip to touch the well wall, usually at or just above the liquid surface, holds for an instant, and withdraws. The contact breaks the surface tension that was holding the droplet, and the liquid transfers to the well by adhesion. It is a mechanical finish to a dispense, not a change to how much the plunger moved, which is why it pairs with, rather than replaces, the air and pressure settings that clear the bulk of the liquid.
When it earns its place
Touch-off is most useful exactly where a free-falling drop is least reliable.
- Low volumes, where the droplet may never gain enough weight to fall on its own.
- Viscous liquids that string and cling rather than releasing cleanly.
- Dispensing into an empty well, where there is no pooled liquid to pull the drop out, as in a contact-dry dispense.
- Any transfer where a consistent, complete delivery matters more than keeping the tip clear of the well.
When to leave it off
The cost of a touch-off is contact. A tip that has touched one well has, in principle, touched that well's contents or its surface, so a touch-off works against the reuse of tips across many wells and against a sterile free dispense. If you are aliquoting one aspiration into a plate without changing tips, a touch-off would defeat the point. It also adds a small motion to every dispense, which costs time across a large run. Where a free dispense already releases the liquid cleanly, a touch-off buys nothing.
It works with blowout and dispense mode, not instead of them
Touch-off is one of three things finishing a dispense, and they are easy to confuse. Blowout expels a trailing air volume to push the last liquid out of the tip. The dispense mode decides whether the tip is above the well, in pooled liquid, or against a dry surface. Touch-off decides whether the tip physically wipes the last droplet against the wall. A well-built class for a difficult small-volume transfer often uses all three together: enough blowout to clear the tip, a contact dispense to place the liquid, and a touch-off to shed what remains.
A hanging droplet is delivered volume that never arrived. If your low-volume transfers read a little short and a little noisy, a touch-off is a cheap thing to try first.