Which option correctly defines Positive Displacement pipetting?

Which option correctly defines Positive Displacement pipetting?

• A. Pistonless pipetting
• B. Moving the piston in a pipet tip or barrel like a syringe with a disposable piston
• C. Air displacement pipetting
• D. Capillary action

Answer: (B)

Positive displacement pipetting uses a piston that moves inside the tip or barrel and is in direct contact with the liquid, like a syringe with a disposable piston. When you move the piston, it displaces the liquid directly, with no air cushion between the piston and the sample. This makes the volume delivered closely match the piston movement and reduces errors caused by liquid viscosity, volatility, temperature, or foaming. The disposable piston also helps prevent cross-contamination. In contrast, air displacement pipetting relies on a pocket of air between the piston and liquid; the liquid’s movement depends on this compressible air and can be affected by vapor pressure, viscosity, and temperature, making it less reliable for certain samples. Capillary action, on the other hand, uses surface tension to draw liquid and isn’t a controlled displacement mechanism like positive displacement.

Positive displacement pipetting uses a piston that moves inside the tip or barrel and is in direct contact with the liquid, like a syringe with a disposable piston. When you move the piston, it displaces the liquid directly, with no air cushion between the piston and the sample. This makes the volume delivered closely match the piston movement and reduces errors caused by liquid viscosity, volatility, temperature, or foaming. The disposable piston also helps prevent cross-contamination.

In contrast, air displacement pipetting relies on a pocket of air between the piston and liquid; the liquid’s movement depends on this compressible air and can be affected by vapor pressure, viscosity, and temperature, making it less reliable for certain samples. Capillary action, on the other hand, uses surface tension to draw liquid and isn’t a controlled displacement mechanism like positive displacement.