Pipettors

According to the way of operation, manual and electronic pipettors can be distinguished. In manual pipettors, the piston is moved by a thumb using an operation knob. Accuracy and prcision of pipetting depend on expertise of the operator. In case of electronic pipettors, the piston is moved by a small electric motor. different speeds of aspiring and expressing of the liquid can be selected according to properties of the pipetted solution.

According to the principle of operation, pipettors can be divided into two big groups:
 * „Air displacement“ pipettors
 * Principle of so called air cushion is employed in this type of pipettes. A certain volume of air is betveen the piston and measured liquid. The volume of solution and of the air aspired or expressed with the piston may differ. It depends on density and viscosity of the liquid, wettability of the surface of the tip with pipetted liquid, temperature, athmospheric pressure and other influences. Thus, every pipettor must be calibrated on a regular base.


 * Air displacement pipettors can be designed as a single channel (for pipetting of a single volume at a time) or as a multi channel (most frequently with eight or twelve channels). Multi-channel pipettors are intended for simultaneous pipetting of the same volume of liquid to several wells of a microtitration plate. Every channel has its own piston; therefore, it is not necessary to use all 8 or 12 channels (i.e. less tips can be attached to the pipette).


 * Micropipettors are constructed for one fixed volume or it is possible to set the volume. Volumes can be set discretely (changing of plug-in modules) or continuously in a certain range (e.g. 10–100 &mu;L) using an adjustable screw or knob.At the lower end of a micrpipette there is a removable tip. At the other end, there is the knob operating the piston inserted into the cylinder of the pipettor.


 * „Positive displacement“ pipettors
 * Liquid is aspired into the tip without any air cushion - the piston is in contact with measured liquid. The liquid is then delivered at once or in several steps (in case of so called "steppers"). This type of pipettors is suitable for very viscous or voilatile liquids and for repetitive pipetting.