Patent ID: 8480972

Claim:
A shaft for a sampling pipette, the shaft comprising: a first cylindrical portion configured to extend from a lower end of a pipette, wherein a first cross section of the first cylindrical portion is perpendicular to a longitudinal axis of the pipette and has a first diameter, the first cylindrical portion including an upper end opposite the lower end; a first annular shoulder including a first transition end adjacent the upper end of the first cylindrical portion; a second cylindrical portion, wherein a second cross section of the second cylindrical portion is perpendicular to the longitudinal axis of the pipette and has a second diameter; and a first end opposite the first transition end, the second cylindrical portion extending from the first transition end to the first end; and a second annular shoulder including a second transition end adjacent the first end of the first annular shoulder; a third cylindrical portion, wherein a third cross section of the third cylindrical portion is perpendicular to the longitudinal axis of the pipette and has a third diameter; and a second end opposite the second transition end, the third cylindrical portion extending from the second transition end to the second end; wherein the second diameter is greater than the first diameter; wherein the third diameter is greater than the second diameter; wherein the second diameter and the third diameter are selected so that a pipette tip contacts at least a portion of the second cylindrical portion to provide a seal and at least a portion of the third cylindrical portion to provide a progressive stop when the pipette tip is mounted on the shaft; and further, wherein a difference between the third diameter and the second diameter is in the range of approximately 0.2 millimeters and approximately 2 millimeters.