Use and variations

Pipettes are commonly used in molecular biology, analytical chemistry as well as medical tests. Pipettes come in several designs for various purposes with differing levels of accuracy and precision, from single piece glass pipettes to more complex adjustable or electronic pipettes. Many pipette types work by creating a partial vacuum above the liquid-holding chamber and selectively releasing this vacuum to draw up and dispense.

Pipettes that dispense between 1 and 1000 μl are termed micropipettes, while macropipettes dispense a greater volume of liquid. Two types of micropipettes are generally used: air-displacement pipettes and positive-displacement pipettes. In particular, piston-driven air-displacement pipettes are micropipettes which dispense an adjustable volume of liquid from a disposable tip. The pipette body contains a plunger, which provides the suction to pull liquid into the tip when the piston is compressed and released. The maximum displacement of the plunger is set by a dial on the pipette body, allowing the delivery volume to be changed. Larger capacity tubular pipettes, such as volumetric or graduated pipettes, are used by temporarily attaching a pipetting dispenser. Pipetting syringes typically handle volumes in the 0.5mL to 25mL range, for aliquot transfer and incremental dispensing in titrations, with a positive displacement method of operation. No disposable tips or pipetting aids are needed with the pipetting syringe.

[edit] Piston-driven air displacement pipettes

Single-Channel Pipettes designed to handle 1-5ml and 100-1000µl with locking system

These pipettes operate by piston-driven air displacement. A vacuum is generated by the vertical travel of a metal or ceramic piston within an airtight sleeve. As the piston moves upward, driven by the depression of the plunger, a vacuum is created in the space left vacant by the piston. Air from the tip rises to fill the space left vacant, and the tip air is then replaced by the liquid, which is drawn up into the tip and thus available for transport and dispensing elsewhere. These pipettes are capable of being very precise and accurate, however, being air displacement, are subject to inaccuracies caused by the changing environment, particularly temperature and user technique. For these reasons this equipment must be carefully maintained and calibrated, and users must be trained to exercise correct and consistent technique. These micropipette were invented and patented 1960 by Dr. Hanns Schmitz (Marburg/ Germany). Afterwards, the co-founder of the biotechnology company Eppendorf, Dr. Heinrich Netheler, inherited the rights and initiated the global and general use of micropipettes in labs. In 1972, the adjustable micropipette was invented at the University of Wisconsin-Madison by several people , primarily inventor Warren Gilson and Henry Lardy, hence one of the bigger producers is the original company called Gilson Inc., as a result they are colloquially referred to as Gilsons [1] [2].

Several different type of air displacement pipettes exist:

  • adjustable or fixed
  • volume handled
  • Single-channel, multi-channel or repeater
  • conical tips or cylindrical tips
  • standard or locking
  • manual or electronic
  • manufacturer

[edit] Positive displacement pipette

These are similar to air displacement pipettes, but are less commonly used and are used to avoid contamination and for volatile or viscous substances at small volumes, such as DNA. The major difference is that the disposable tip is a microsyringe (plastic), composed of a plunger which directly displaces the liquid.

[edit] Pipetting syringe

Pipetting syringes combine the functions of volumetric (bulb) pipettes, graduated pipettes, and burettes into one hand-held device, being calibrated to ISO volumetric A grade standards. A glass or plastic pipette tube is provided with a thumb operated piston and PTFE seal which slides within the pipette in a positive displacement operation. This arrangement provides improvements in precision, handling safety, reliability, economy, and versatility with volumes between 0.5ml and 25ml. The pipetting syringe is designed to handle all laboratory fluids - aqueous, viscous and volatile, hydrocarbons, essential oils, and mixtures - accurately and incrementally. Developed by Blackwood-Eddy pty.ltd. (Australia) in the early 2000's, the pipetting syringe's simplicity and effectiveness have led to its widespread adoption by Australasian high schools and University chemistry departments, and for a wide variety of industrial applications.

[edit] Vacuum assisted pipette

Serological pipettes are used for large volumes and for sterile work, but require a pipette dispenser

Non-piston-driven vacuum assisted pipettes are hollow narrow cylinders which work like a straw and require the use of some kind of additional suction device. Originally pipettes were made of soda-lime glass, but currently many are made of borosilicate glass which is tougher and more chemically resistant. Disposable and single use pipettes are often made of polystyrene. All of these are commonly used in chemistry, mainly with aqueous solutions. There are two types. One type, the volumetric(or bulb)pipette, has generally a large bulge with a long narrow portion above with a single graduation mark as it is calibrated for a single volume. Typical volumes are 10, 25, and 50 mL. The second type, the graduated pipette, is straight-walled and as the name implies has graduation marks along most of its length.

The pipette is filled by dipping the tip in the fluid, then drawing up the liquid by using a pipette filler to create a partial vacuum above the fluid. The surface of the fluid inside the pipette is generally concave and this is called the fluid meniscus. The fluid level is read at the bottom of the meniscus and by aligning it with the graduation marks while holding the pipette at eye level. Liquid is dispensed by releasing the vacuum created by the pipette filler ; slow release for gradual dispensing and complete removal for fast dispensing. With safe fluids the finger may be used on the open end of the pipette to control the vacuum. While moving the pipette to the receiving vessel, care must be taken not to shake the pipette because the column of fluid may "bounce". Complete emptying of the pipette is accomplished by either blowing the pipette out with air from the filler (blow-out pipettes), or touching the tip against the side of the receiving vessel, according to pipette type.

[edit] Volumetric pipettes

Volumetric pipettes allow the user to measure a volume of solution extremely accurately and then add it to something else. They are commonly used to make laboratory solutions from a base stock as well as prepare solutions for titration. They are typically marked to indicate one single volume in a particular size pipette (as are volumetric flasks). Many different sizes are available.

[edit] Graduated pipettes

Graduated pipettes use a series of marked lines (as on a graduated cylinder) to indicate different calibrated volumes. These also come in a variety of sizes. These are used much like a burette, in that the volume is found by calculating the difference of the liquid level before and after liquid is dispensed. Historically, the accuracy of a graduated pipette was not as good as that of a volumetric pipette; however, with improved manufacturing methods, the accuracies listed by the manufacturer can equal volumetric pipets.[citation needed] Two types of graduated pipettes exist:

  • Mohr pipettes or drain-out pipettes have a 0ml mark before the start of the conical end, which is a dead volume.
  • serological or blow-out pipettes have no 0ml mark as that corresponds to an empty pipette.

Graduated pipettes have +/- tolerances that range from 0.6% to 0.4% of the nominal volume when measured at 20C.Graduated pipettes are manufactured according to ISO specifications for accuracy and the arrangement of the graduations. A grade pipettes are more accurate than B grade pipettes and Volumetric pipettes are the most accurate of all.

[edit] Pipette Dispensers

Various methods exist to handle the liquids inside a pipette. Before the advent of more sophisticated pipette dispensers, it was common practice to "mouth pipette" i.e. to aspirate fluid into the pipette by applying suction with one's mouth. Mouth pipetting is now considered unsafe due to the possibility of accidentally ingesting or inhaling toxic chemicals or pathogens. The main three types of pipette dispenser are the bulb filler, pipette pump and the electronic controller.

[edit] Pipette accessories

  • Pipette fillers are used to fill the pipette easily, avoiding the need for mouth pipetting.
  • Pipette dispensers are battery-operated and are designed to be used with disposable pipette tubes. These pipettes cannot be calibrated and their accuracy is determined by that of the printed graduations on the disposable tubes.
  • Light-guided pipetting systems are pipetting accessories which are computer based. They utilize flat screen LCD monitors or LED arrays to light up source and destination wells in microplates or vials for accurate well to well pipetting. Some of these systems use text to speech to alert the operator during plate or volume changes when pipetting lab protocols.
  • Pipette tips. The pipettors and injection molded plastic disposable tips form together a reliable pipetting system. It is recommended to use original manufacturers tips to guarantee the precision and accuracy of the pipettes. The precision-made pipettor tips provide excellent reproducibility and accuracy. Pipettor tips are available in autoclavable boxes, refills and bulk packaging. Non-sterile, pre-sterilized and filtered tips are usually available in single trays as RNase, DNase and endotoxin certified free.

[edit] Pasteur pipette

Dispensable pipettes.

Pasteur pipettes, also known as teat pipettes or droppers, are plastic or glass pipettes used to transfer small amounts of liquids, but are not graduated or calibrated for any particular volume. Transfer pipettes, also known as Beral pipettes, are similar to Pasteur pipettes. However, they are made from a single piece of plastic and their bulb can serve as the liquid-holding chamber.
A commercial variant of the pasteur pipette is the dispensable pipettes which are often made of plastic and intended to be used to administer medicine into the eye or ear of a patient (see image).

[edit] The smallest pipette

A zeptoliter pipette has been developed at Brookhaven National Laboratory. The pipette is made of a carbon shell, within which is an alloy of gold-germanium. The pipette was used to learn about how crystallization takes place.[1]

[edit] References

  1. ^ Aimee Cunningham (2007-04-18). "A New Low: Lilliputian pipette releases tiniest drops". 171,. Science News. pp. 244–245. http://www.sciencenews.org/articles/20070421/fob4.asp. 

[edit] External links

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