Which automatic micropipettor operates like a hypodermic syringe for viscous samples with constant aspiration force?

• A. Air displacement micropipettor
• B. Positive displacement micropipettor
• C. Dispenser/Dilutor Pipet
• D. Manual micropipettor

Answer: (B)

The idea being tested is how pipettor design handles viscous samples and maintains a steady suction. A positive displacement micropipettor uses a disposable piston that directly contacts the sample inside the tip. When the piston moves, it draws the liquid in without relying on an air cushion. This direct contact means the suction and the dispensed volume are governed by the sample itself, giving a consistent aspiration force even for thick, sticky, or volatile liquids. It’s essentially like a syringe: the piston gets pushed or pulled, and the liquid moves with it, so viscosity doesn’t distort the volume.

In contrast, an air displacement micropipettor relies on an air column between the piston and the liquid. The air is compressible, so the pressure and volume depend on the air’s behavior, which becomes unreliable with viscous or foaming samples. That’s why they’re not ideal for such liquids or for maintaining a constant aspiration force.

Dispenser/dilutor pipets are designed more for bulk dispensing or diluting rather than precise, viscosity-resilient sampling, and manual micropipettors refer to how the instrument is operated rather than the mechanism that handles the liquid.