Multi-dispensing, or aliquoting, draws enough liquid in one aspiration to fill several wells, then dispenses it in portions. It saves time and tips, but it asks more of the class, because a single aspiration has to deliver many accurate drops in a row. Done carelessly it trades accuracy for speed. Done well it gives you both.
Decide whether to multi-dispense at all
Not every transfer should be an aliquot. If precision is critical, if the liquid is volatile and needs anti-droplet control, or if the target must be exact into an empty well, a single aspiration and dispense is often the safer choice. Reach for multi-dispensing when throughput matters and the per-well tolerance leaves room for it.
Discard the first and last aliquots
The first and last portions from a single aspiration are the least accurate. The first carries the effects of the initial draw, and the last scrapes the bottom of what the tip holds. Aspirate a little extra and throw away a lead and a trailing volume, delivering only the consistent middle. It costs a bit of liquid to gain real precision.
Correct against the stepping-down volume
As the tip empties, each aliquot is dispensed from a smaller remaining volume, so the correction cannot be a single number. Build the curve against the actual volume in the tip at each step. To deliver six 50 microliter aliquots, correct for 300 microliters at the first dispense, 250 at the next, and so on down, rather than correcting once at 50.
A few habits that help
- Pre-wet the tip on the first aspiration so the early aliquots are not outliers.
- Limit the number of aliquots per cycle, commonly to half a plate or one full plate, for easier and more consistent control.
- Use a slightly higher stop-back flow to break off cleanly between aliquots.
- Dispense-on-the-fly can add speed, but it is harder to optimize, so add it last.
The middle aliquots are the good ones. Aspirate extra, throw away the ends, and correct for a volume that keeps stepping down.