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Pipettes and pipette tips

Pipettes and pipette tips


Pipettes come in a great variety of shapes sizes and functions. The following are just a few of those which are referenced in the suppliers media: 

Single channel pipettes
Variable volume pipettes 
Digital pipettes
Positive displacement pipettes 
Multichannel pipettes
Fixed volume pipettes
Glass pipettes
Pasteur pipettes
Air Displacement pipettes
Pipetting Aids or Pipette Controllers

To add more confusion the word pipette is often spelt "pipet" in the United States but is the same thing!

Pipettes are one of the most frequently found tools in a laboratory and have been used for over a century in various guises. Although today’s modern pipettes are light years away from the tubing and glassware used in mouth pipetting, they do all perform the same function; pipetting.

Pipetting is simply the aspiration and dispensing of fluids through use of a mechanical device or electro-mechanical device. Pipettes are available in many shapes and sizes, but the products that are generally referred to in a laboratory context are “Hand Held” which are designed for regular and repetitive use and to accommodate a variety of fluid volumes.

Laboratory samples have decreased in size as time has progressed, for various reasons, but mainly convenience and the ability to use a sample for more than one analysis. Combine this with advances in micro-fluid handling, smaller sample sizes (e.g. molecular biology), the increase in sample processing and there is a well defined set of volumes that most pipettes and pipetting devices accommodate. These are further consolidated by human pipetting tolerances and the ability to visualise small samples and manipulate hand held devices. Today’s pipettes cover the range 0.1ul to 10ml comfortably and with great accuracy and precision.

These hand-held pipetting devices can be sub-divided into various categories depending on their application, size and required functionality. All pipettes rely on a plunger/piston mechanism that moves up or down through a piston housing and aspirates fluid on the upstroke and then dispenses it on pipette plunger depression or down stroke. This is all performed through application of thumb pressure (generally) to the pipette thumb button. These pipettes can however be broadly categorised into air displacement and “positive displacement”. Air displacement pipettes are by far the most important and widespread and are so called because there is a small air gap between the fluid and piston and it is this volume of air displaced that relates to the volume of fluid being aspirated and dispensed. These pipettes use economical and disposable pipette tips, generally made of virgin polypropylene. These tips are generally sold loose in bags or boxes or in racks of 96, packed in boxes of 10 racks (960 tips) for microplate format pipetting.

Fixed volume pipettes (air displacement) are the original hand held invention but were followed some years later by the now very popular, “variable volume pipette”. The ability to change the volume provided a single tool that could replace many fixed volume units. Fixed volumes are still used however, especially when there is a possibility of accidental adjustment. The methods for changing the volume in variable volume pipettes vary but most rely on the ability to adjust the length of stroke of the piston.

Some variable volume pipettes later developed in adjustment and display design and became known as the digital pipette, due mainly to a click action and a volume display mechanism that was clearer ate fixed intervals. The earlier pipettes had a vernier type mechanism of volume adjustment and did not “click into place” and were considered less accurate in everyday use.

These advances were soon followed by extra “features and benefits” for example reduction of “hand warming effects” through low thermal conductivity materials and air gaps in the body construction.

Reduction in thumb pressures through use of different spring pressures and various other leverage systems or tip cones that would fit other manufacturers tips. Add to these features other elements such as colours, labels, and variations on almost any level and you have the extensive ranges of pipettes available today!

All these manufacturers have addressed the same issues over the years, to be able to compete in this narrow but volume market. Pipettes all serve the same function, but certain points were always a key consideration, two of the most important were adding a tip ejector, to avoid touching the pipette tip (and therefore sample material) and the second was a “blow out” function, to ensure that no fluid was retained within the tip and to ensure accuracy and precision of pipetting.

ELISA (Enzyme Linked Immuno Assay) and increased microplate use also led to the development of multichannel pipettes. These 8 or 12 channel devices allow fast aspiration or dispense across 8 or 12 channels of a microplate. 4 channel pipettes used to be manufactured but now are rarely seen. There are also pipettes available in 8 & 12 tip formats that have variable spacing for use in cell culture and wide spacing plates!

Positive displacement pipettes are used  for specific applications where the accurate dispensing of viscous, volatile or hazardous liquids is required. They also completely eliminate the risk of cross contamination between samples. However, due to the more costly capillary piston disposable tips the use of filter (barrier) tips on a standard air displacement pipette  has become a  more economical choice.

Positive displacement tips are effectively plastic syringes that are attached to a plunger connection mechanism in the pipette. The fluid comes into contact with the disposables plunger and the unit is discarded after use. The lack of elastic air space makes them much easier to use when dealing with difficult to handle liquids. Stepper Pipettes also function as positive displacement products with a large dynamic range and the advantage of multi-dispensing.

Pipettes have had limitations in high throughout use and are restricted to aspiration, dispense and mixing. Over the last 20 years there has been a move toward electronic pipettes, which are battery powered, rechargeable hand held pipettes driven by a DC or stepper motor with microprocessor control. These electronic pipettes provide extra functions such as dilutions, multiple dispensing (one aspiration and multiple dispenses), speed controls and more. They have also reduced the fatigue associated with pipetting over long periods by reducing thumb pressure.

Which pipette to choose? We would say, reliable, robust, feels good in the hand, accurate, precise and with good after sales service and selection of tips that fit!