Patent ID: 8277756

Claim:
A method of identifying a blockage at a receiving opening of a pipetting needle in an analysis apparatus when taking liquid from a liquid-containing vessel and/or when delivering liquid into a liquid-containing vessel, said pipetting needle at or close to the receiving opening having an electrode (E 1 ) or being itself an electrode (E 1 ), comprising: (a) said pipetting needle being displaced such that the receiving opening is immersed in said liquid-containing vessel; (b) an amount of liquid being sucked up into said pipetting needle or delivered from said pipetting needle; (c) further displacing said pipetting needle such that the receiving opening comes out of the liquid in the liquid-containing vessel; (d) detecting during said displacement of the pipetting needle capacitances (K measurement ) between the electrode (E 1 ) and a counter-electrode (E 2 ) in dependence on the position of said pipetting needle; (e) comparing the detected capacitances (K measurement ) to reference values (K reference ) for the respective position of said pipetting needle; and (f) delivering information about the presence of a blockage at the receiving opening of the pipetting needle if a deviation between the detected capacitances (K measurement ) and the reference values (K reference ) at one position or a plurality of positions of the pipetting needle exceeds a threshold value; whereby said reference values (K reference ) of the capacitance are determined in dependence on the position of said pipetting needle by a procedure comprising: (a) the pipetting needle being displaced such that said receiving opening is immersed in said liquid in said liquid-containing vessel; (b) further displacing said pipetting needle such that said receiving opening is moved out of said liquid in said liquid-containing vessel without an amount of liquid being sucked up into said pipetting needle; (c) detecting during said displacement of said pipetting needle capacitances (KO) between the electrode (E 1 ) and a counter-electrode (E 2 ) in dependence on the position of the pipetting needle; and (d) correcting said detected capacitances (KO) in dependence on the position of the pipetting needle in relation to the change in the height of the surface of the liquid in the vessel, which change occurs in dependence on the liquid volume to be removed in the liquid removal operation, and the vessel dimensions, thereby obtaining said reference values (K reference ).