Prediction of aspirated volume of a liquid

A method of aspirating a liquid, comprising:

• providing an aspirating probe which comprises a probe tip and a piston pump, wherein the probe tip and piston pump are in fluid communication;
• measuring an initial gas pressure (PA/Dinitial) in the tip of the aspirating probe prior to the tip entering the liquid to be aspirated;
• moving the tip into the liquid, whereby a volume of gas is located between the top of the liquid and the piston of the piston pump;
• moving the piston of the piston pump a predetermined distance which corresponds to a selected volume of liquid to be aspirated, whereby a column of liquid is aspirated into the tip;
• measuring the gas pressure in the volume of gas when the piston stops moving and the column of liquid aspirated into the tip has equilibrated (PA/Dfinal);
• obtaining Volume/unit pressure, wherein Volume/unit pressure is the change of the gas volume for each change of unit pressure;
• determining the piston volume created by the movement of the piston;
• and after aspiration determining the volume of the column of liquid aspirated by the following formula:
• Vliquid aspirated=Vpiston volume−((Pinitial−Pfinal)*Volume/unit pressure),
• wherein
• Pinitial is initial gas pressure before the liquid enters the tip,
• Pfinal is final pressure of the volume of gas after the column of liquid has equilibrated.

A method of analyzing a sample for an analyte, comprising:

• providing a source of a sample;
• providing an aspirating probe which comprises a probe tip and a piston pump, wherein the probe tip and piston pump are in fluid communication;
• measuring an initial air pressure (PA/Dinitial) in the tip of the aspirating probe prior to the tip entering the liquid to be aspirated;
• moving the tip into the sample, whereby a volume of air is located between the top of the liquid and the piston of the piston pump;
• moving the piston of the piston pump a predetermined distance which corresponds to a selected volume of liquid to be aspirated, whereby a column of liquid is aspirated into the tip;
• measuring the air pressure in the volume of air when the piston stops moving and the column of sample aspirated into the tip has equilibrated (PA/Dfinal);
• obtaining Volume/ADC, wherein ADC is pressure as expressed in analog/digital counts and Volume/ADC is the change in the volume of the gas per unit ADC;
• determining the piston volume created by the movement of the piston;
• and
• after aspiration determining the volume of the column of sample aspirated by the following formula:
• Vliquid aspirated=Vpiston volume−((PA/Dinitial−PA/Dfinal)*Volume/ADC),
• wherein
• PA/Dinitial is initial air pressure before the sample enters the tip,
• PA/Dfinal is final pressure of the volume of air after the column of liquid has equilibrated
• dispensing the sample into a sample holder;
• optionally dispensing a reagent into the sample holder;
• performing a measurement on the sample;
• and
• determining a concentration of analyte in the sample based on the measurement on the sample and the determined volume of the column of sample.