In: Biology
In a lab you are asked to pipette 130 microL using a pipette with a range of 20-200 microL. This pipette has recently been calibrated so is functioning correctly. You correctly move the dial of the pipette to 130 microL. On your first attempt to dispense 130 microL, you realise that the volume expelled was only 105 microL. On the second occasion it was 170 microL. Give explanations to what operator errors could have led to these outcomes?
Answer: The analytical results should be accurate and precise. For this, regular calibration of pipettes is necessary. Calibration should ensure that the pipette works properly. In the air-displacement mechanical action pipettes, operators are the major cause of errors. If the pipette is not pre-wetted before the initial delivery, some of the sample volumes may be lost due to evaporation at the tip. If the volatile sample liquid is aspirated and expelled at least three times before the actual delivery, the risk of errors can be reduced. The temperature of the pipette and the liquid sample being dispensed should be equal. Air displacement pipettes are affected by relative humidity, air pressure and the liquid sample vapour pressure. All these factors are temperature dependent. Sample volume variations can occur if the pipette is handled for too long time. Reverse mode of pipetting is another significant cause of the operator error in pipetting. The immersion depth is critical to have a note. Inappropriate pause timing during aspiration is also the cause of operator error in the pipette.