Viscous liquids are the most common reason a liquid handling method underperforms. Glycerol solutions, DMSO stocks, concentrated detergents, and PEG buffers all resist flowing at the speed a default water class expects, so the tip lifts before the liquid has finished moving. The result is short volumes that are also imprecise, because the error changes with tip wetting and temperature.
Slow down the flow rate first
The single highest-impact change is reducing aspiration and dispense flow rates. A viscous liquid needs time to enter and leave the tip. Cutting the flow rate to a fraction of the water default gives the meniscus time to keep up with the plunger, which is what actually determines the volume drawn.
Add settling and post-aspiration delays
Even at a slow flow rate, the liquid column keeps moving after the plunger stops. A post-aspiration delay of a second or more, with the tip still submerged, lets the volume equalize before the tip lifts. Add a matching delay after dispense so the liquid has time to leave the tip rather than string out behind it.
Use over-aspiration to counter retention
Viscous liquids coat the inside of the tip and stay there. Over-aspiration draws a small excess so the intended volume is still delivered after some is retained. Pair it with a blowout to clear the last drop, and confirm gravimetrically that you are not now over-delivering.
Mind the tip and the temperature
- Wider-bore or filtered tips reduce the pressure needed to move a thick liquid.
- Viscosity drops sharply with temperature, so a class validated at 20 C can be off in a cold room.
- Pre-wetting the tip stabilizes the first dispense, which is otherwise the least accurate.
Always verify a viscous-liquid class by weighing dispensed volumes. Visual inspection hides the small, systematic short-fills that these liquids produce.