Micropipette calibration by differential pressure measurements

The pipetting of small volumes, typically ranging between a few and one thousand microliters (mu l), is widely used in many laboratories and essentially relies on air displacement to aspirate and dispense the liquid contained in a small plastic tip. While these pipetting devices are largely dependable, they require rigorous calibration at periodic intervals which can be conducted by gravimetry (weight measurement of water) or by spectrophotometry with the dilution of a dye from a known concentration. The aim of this work is to describe an entirely new and reliable method based on the measurement of the pressure increase in a reference volume caused by the pipette piston opening. Relying on the physical properties of the perfect gas, this method offers several advantages and can be conducted at any time without other high-precision and expensive devices. Thanks to advances in digital pressure sensors recently introduced into smartphones and digital watches, their sensitivity and precision allows the use of differential pressure measurements for reliable reading over a large volume range. This new method is adequate even for measurements of volumes below 1 mu l, a condition that is difficult to achieve using standard gravimetry and requires a consumable such as dye in the case of spectrophotometry. Moreover, a clear advantage of the method is to allow rapid verification of the pipetting condition with the very same tip that can be subsequently used for handling the liquid; a condition that cannot be satisfied by any method currently available. Easily adaptable to automated liquid handlers, or multiple channel pipetting devices, this method also offers the possibility of assessing the precision of the pipetting, with the very same conditions and pipette tip, immediately prior to any critical operation.