Research Note: Calibration of micropipettes for pressure-probe studies

Abstract

In order to determine cell water-relation parameters with the pressure probe, the ratio of inner to outer pipette diameters (α = D_i/D_o) must be estimated. Four different methods for estimating a were compared. I: measurement of the volume of oil expelled from a pipette and the corresponding movement of an oil/water meniscus within the pipette; II: measurement of D_i and D_o on fractured pipettes; III: measurement of D_i and D_o on intact pipettes; IV: measurement of D_i and D_o on capillary tubing. Methods I and II gave mean values for α (± SE) that were in excellent agreement [0.587 ± 0.004 (n = 12), and 0.593 ± 0.003 (n = 14), respectively]. Method III gave variable results for D_o < 80µm; for D_o > 80 µm a mean value α = 0.624 ±0.007 (n = 4) was obtained. Method IV gave a mean value α = 0.544 ± 0.001 (n = 5). It is suggested that any method for calibrating micropipettes should be checked against an independent method (e.g. Method I) that allows the determination of α for D_o < 100 µm. In the present study, Method II gave the best combination of accuracy and convenience.