Pipettes

Pipettes are handheld instruments commonly used in academic, biotech, pharmaceutical and other labs to transfer precise amounts of liquid from one container to another. Pipettes range from very basic graduated glass tubes to sophisticated precision instruments capable of aspirating and dispensing liquid in increments as small as a fraction of a microliter. The most common pipette used in life science labs around the world is the single-channel air displacement pipette.

There are two basic pipette types, positive displacement and air pipettes. Modern pipettes work with a piston-stroke principle – Liquid is measured using a piston-cylinder system in the handle of the pipette. When the piston is depressed and then slowly released, the movement of the piston creates a vacuum in the disposable pipette tip and thus draws liquid up inside the tip; this process is termed aspiration. Since the piston is a long way from the liquid sample, the air cushion between the piston and liquid can have a number of effects on the accuracy of this process, depending on the liquid type. Pressing down on the plunger that had a tip full of liquid expels the liquid out from the tip; this is termed dispensing. Positive displacement pipettes work on a similar principle, however the tip of the positive displacement system contains its own piston and hence there is a minimal air cushion between the piston (that is part of the tip) and the liquid surface. Positive displacement pipettes are commonly used for samples of high viscosity (e.g. glycerol) or high vapor pressure (e.g. volatiles like alcohol), while air-displacement pipettes can be used for a large portion of laboratory applications.   

The LiteTouch System (LTS) is a design feature in Rainin pipette tips. LTS was invented by Rainin and provides an effort-free way to load pipette tips and reduces tip-ejection force by up to 84%. Tip ejection is the leading pipette-associated cause of repetitive strain injury (RSI). LTS also provides a greater channel-to-channel consistency in multichannel pipettes. The cylindrical pipette-shaft end and matching tip cup produce a reliable, consistent seal without excessive force. LTS tips feature a ring-shaped seal and flat, round shelf that provides a positive "stop." This stop lets the user know when the seal is made and makes it impossible to jam or pound the shaft into the tip – a common problem with conical-shaped tips that are typical with universal (UNV) pipettes.

Pipetting is usually a repetitive activity that, when loading and ejecting tips, can involve a high level of force. As with any repetitive activity, over time, the risk of developing musculoskeletal injury, such as carpal tunnel syndrome, tendinitis and other repetitive strain injuries (RSIs) increases. While good posture and stretching can provide relief, the key to preventing injury is reducing pipetting forces. The Rainin LiteTouch System (LTS) reduces tip ejection force by up to 85%. Read more about pipetting ergonomics 

Accuracy is the ability of a pipette to deliver the volume specified on the volume setting. In other words, the trueness of a measurement to the true value. Precision is the ability of a pipette to consistently deliver same-sized volumes of liquid over multiple aliquots. For example: a pipette can be precise with regard to dispensing the same volume time after time, yet inaccurate because it fails to dispense the correct volume. An accurate and precise pipette dispenses volumes at the intended volume with each dispense. 

The pipette, the disposable tip, and the operator form a 3-part system. Accuracy depends on the interplay of the elements in the system. More than with other lab instruments, the pipette operator has a high degree of influence on the accuracy of results. As such, it is important that operators are trained to use a pipette. There are some powerful, yet simple techniques that can greatly improve pipetting accuracy. The Rainin Pipetting Technique poster offers a good summary 

Pipette calibration refers to testing a pipette's precision and accuracy and resetting it to the manufacturer's specifications if it is out of range. The most common method of pipette calibration is the gravimetric method, which involves pipetting distilled water onto an analytical balance and weighing it to assess the accuracy of the measurement.