 Positive-displacement pipette

A positive-displacement pipette transfers liquid with a piston that contacts the sample directly, avoiding the intermediate air cushion of an air-displacement pipette. In the handheld designs described by the manufacturer, the capillary and piston form a disposable assembly. The mechanism can reduce effects associated with the expansion or contraction of trapped air.^[ref]

This distinction is useful when handling liquids whose viscosity, density or volatility differs substantially from water. The instrument still measures a volume: a selected $25\mu \mathrm{L}$ does not imply a fixed mass for every liquid. For an illustrative density of $1.20mg/\mu \mathrm{L}$, the corresponding mass is $30\mathrm{mg}$ by $m=\rho V$.

A gravimetric check therefore needs the appropriate density conversion. Direct displacement also does not make every delivery perfect: instrument condition, the fitted assembly and the test procedure remain relevant. Repeated measurements distinguish a systematic displacement of the mean from scatter in individual deliveries.^[ref]