The pipetting method is directed to mounting and ejecting a disposable pipette tip. The pipetting system has one or more tip mounting shafts with an upper locking section and a lower sealing section. The upper locking section has outwardly extending lobes spaced around the mounting shaft and located above a stop member. The lower sealing section on the tip mounting shaft is located below the stop. As the mounting shaft is inserted into the collar of the disposable pipette, the collar distorts out of round and engages the lobes on the upper locking section of the tip mounting shaft. Contemporaneously, the lower sealing section of the mounting shaft seals against the barrel of the disposable pipette tip.