Method and apparatus for pipetting liquids

In an improved method for expressing multiple aliquots of liquid from a pipette, the method including the following steps:

• Step A:
• submerging the tip of the pipette into a source of the liquid;
• then
• Step B:
• loading the pipette with an initial volume of liquid by means of a piston pump driven by a step motor, the piston pump being pneumatically connected to the pipette, the initial volume of liquid including both an aliquot and a retained volume;
• then
• Step C:
• removing the tip of the pipette from the source of the liquid;
• then
• Step D:
• expressing a first aliquot of liquid from the pipette by means of the piston pump;
• then
• Step E:
• backsipping the liquid into the pipette by means of a first backsip stroke of the piston pump;
• then
• Step F:
• expressing a subsequent aliquot of liquid from the pipette by means of the piston pump;
• and then
• Step G:
• backsipping the liquid into the pipette by means of a subsequent backsip stroke;
• the improvement comprising the following additional steps:
• Step A':
• prior to said Step A, aspirating a blow out volume of air into the pipette;
• Step C':
• after said Step C and prior to said Step D, executing a preliminary backsip of the liquid into the pipette by means of a preliminary backsip stroke of the piston pump;
• in said Step E, the first backsip stroke having a magnitude for maintaining the backsipping within an allowable range which substantially reduces the risks of both unintendedly dripping liquid from the pipette and of bubbling air within the pipette;
• and
• Step E(1):
• prior to said Step E, determining a step count required by the step motor for performing the first backsip stroke;
• in said Step E, backsipping the liquid into the pipette by driving the step motor according to the step count of said Step E(1);
• in said step G, the subsequent backsip stroke having a magnitude for maintaining the backsipping within an allowable range which substantially reduces the risks of both unintendedly dripping liquid from the pipette and of bubbling air within the pipette;
• Step G(1):
• prior to said Step G, determining a step count required by the step motor for performing the subsequent backsip stroke;
• in said Step G, backsipping the liquid into the pipette by driving the step motor according to the step count of said Step G(1);
• the magnitude of the first backsip stroke being greater than the magnitude of the subsequent backsip stroke.