Influences on volume in piston-operated air-displacement pipettes
Air-displacement pipettes are easy-to-use instruments for small-volume liquid transfer, but they are prone to volume errors resulting from handling, ambient conditions and liquid properties. Analytical thermodynamic approaches were used to describe and quantify influencing factors affecting volume. Results were compared with measurements. Effects related to temperature difference between liquid and instrument by far prevail over the dependence on common temperature level of liquid, instrument and environment. An equation based on Fick's laws was developed to calculate volatilization effects in the captive air volume explaining the dependence on ambient moisture, pipetting pace, waiting period and unconditioned starting. Furthermore, dependence on barometric pressure, angle of inclination, liquid density, immersion depth and surface energy in tips was calculated and/or measured. The review allows a comprehensive insight into the size of volume errors in calibration and use. These effects are typically much higher than those resulting from gravimetric measurement.