pipette
The pipette ( fr. Diminutive of pipe "pipe"; also suction tube, siphon or pipette ) is a laboratory device for dosing of liquids . The classic shape is a glass tube that is narrowed at the tip and either open at the other end or closed by a thick-walled rubber balloon (e.g. a peleus ball or other pipetting aid ). Often, especially in the medical field, plastic disposable items are also used. The micropipettes are technically more complex .
Pipette types
If a volume scale is attached to the pipette , it is called a graduated pipette. These are available for full or partial courses. A pipette that has only one or two marks for a defined volume is called a volumetric pipette. Graduated pipettes are suitable for different amounts of liquid and are therefore more flexible, but also less precise than volumetric pipettes. Both types of pipette are calibrated for liquids with a temperature of 20 ° C and for discharge (Ex.); If necessary, flow times are indicated on the pipettes.
A Pasteur pipette is made of soda-lime glass (ISO 7712) with a volume of 2 ml and a diameter of 7 mm, which is tapered at the lower end. It has a long, fine tip of 45 or 120 mm in length, can be fitted with a rubber suction cup at the end of the suction tube (length 25 mm) and is also suitable for cotton stoppers due to the narrowing . It is used to dose small amounts of liquid drop by drop. It has no mark to show the exact volume.
There are also Pasteur pipettes for single use made of PE-LD (low density polyethylene). They offer very good reproducibility of the number of drops per milliliter and are therefore ideal for distributing aliquot amounts of liquid. This Pasteur pipette can be deep-frozen when filled or, if necessary, transformed into a closed vessel by means of a heat seal. The integrated suction bellows can be squeezed very easily, so the fingers remain fatigue-free even with frequent pipetting. It is stable against gas or gamma-ray sterilization processes.
Scaled serological disposable pipettes made of plastic , usually with a volume of 5 or 10 ml, are nowadays used for frequent and simpler dosing work with less accuracy requirements and replace the glass graduated pipettes. They are also individually packaged and available with certified sterility, which simplifies quality assurance in the laboratory.
Some medications (eye, ear, nose drops) are offered in pre-dosed doses in sealed plastic containers. These containers are also pipettes; see picture in the gallery below.
In the case of a graduated pipette with a suction piston , suction and emptying are carried out by means of a glass piston (piston pipetting aid) with a retaining spring that is ground in at the top.
Most glass pipettes are calibrated in such a way that a drop of the liquid should remain in the pipette tip after being wiped off three times on a vessel wall ( "calibrated for discharge" , for this the word "Ex" is scratched, etched or lacquered on the pipette). Such pipettes are not blown empty. Pipettes calibrated in this way can only be used to fill in a certain volume and not to aspirate a certain volume (because more than the volume is aspirated and a remainder then remains in the tip). In addition, before draining or adjusting the volume, the outside of the pipette tip is wiped so that no additional drops of liquid are carried over and the pipetted liquid is also not sucked out by capillary action.
The meniscus of the graduated pipette shows 20.0 ml
Pasteur pipettes
Medicine pipettes
Pipetting aid (pipetting ball made of natural rubber for volumetric and graduated pipettes)
Disposable plastic pipettes
Micropipettes
A more modern mechanical variant is the microliter or piston-stroke pipette (KHP), or micropipette for short, developed by Heinrich Schnitger in 1957 . This pipette doses small volumes from 0.1 µl to 5000 µl more precisely than conventional glass pipettes. Piston-stroke pipettes work according to the displacement principle: a movable piston displaces the column of air below it when it is pressed down or pulls the column of air upwards with it in its upward movement and thereby also the liquid to be pipetted into the attached pipette tip. This pipette tip is a disposable plastic item, only it comes into contact with the liquid.
Positive displacement piston pipettes use special pipette tips with a built-in piston which comes into direct contact with the medium and thus eliminates the air cushion. This enables highly viscous solutions to be pipetted and the accidental transfer of substances in the form of aerosols via the air cushion is excluded.
As with graduated and volumetric pipettes, there are also piston-operated pipettes with a fixed volume and piston-operated pipettes with variable volumes. The desired number of microlitres can be set here using an interlocking. They are increasingly replacing pipettes with a fixed volume. Piston-stroke pipettes have the advantage of replaceable plastic pipette tips, which makes them suitable for working with frequently changing substances or substance concentrations. Electronically controlled piston-operated pipettes are particularly suitable for repetitive work as they take the strain off the thumb and can sometimes be used like a hand dispenser .
Multichannel pipettes
A special design of the piston-operated pipettes are multi-channel pipettes , which have 8 or 12 simultaneously operated channels. The channels are arranged in a grid of microtiter plates (8 rows by 12 columns) so that they can be processed quickly.
As with the micropipettes with a channel, the liquid only comes into contact with the pipette tips.
Hand dispenser
Hand dispensers, which are also known under the names Multipette, Stepper, Tacker and Repeater , are related to the piston-operated pipettes . Instead of the pipette tip, these have a direct displacement reservoir (disposable plastic article) similar to a syringe , which allows a larger volume of a substance to be taken up, which can then be dispensed repeatedly in small steps of the same volume. This in turn allows quick work with microtiter plates or many parallel reaction batches (e.g. in the polymerase chain reaction (PCR)).
Bottle dispenser
Bottle dispensers or tilting dispensers are glass devices that are placed on laboratory chemical bottles . By tilting the bottle, a calibrated reservoir is filled and this is emptied via a beak.
Wine lifter
A wine lifter is a glass tube in the manner of a volumetric pipette with an extension to a balloon. It is used to take liquid samples from barrels and glass balloons. To do this, it is dipped into a (wine) barrel at the upper bunghole. Closing the upper opening with a finger prevents the wine from flowing back or out. The removed wine is then emptied into a tasting glass by lifting your finger .
Accuracy classes
The commercial sizes of the volumetric pipettes are listed in the table below. The classification is made in two accuracy classes. In class A pipettes, the drain time for the liquid is extended by narrowing the drain tips to a capillary tip, so that the liquid continues to run on the glass wall during the drain . The error limits of these pipettes correspond to the regulations of the German calibration regulations. Class B pipettes have significantly shorter expiry times, but also twice as large error limits as those of class A. After the expiry of the class A and B pipettes, there are no waiting times.
With improved manufacturing methods, it has become possible to manufacture pipettes that are within the calibration error limits (class A) and yet have short expiry times. Class AS was introduced for such pipettes. However, they require a waiting period of 15 seconds after the expiry. In practice, class AS pipettes have become very popular and have almost replaced those of class A with longer drain times.
| Nominal volume [ml] |
Class a | Class AS | class B | |||
|---|---|---|---|---|---|---|
| Max. Error [%] |
Expiry time [s] |
Max. Error [%] |
Expiry time [s] |
Max. Error [%] |
Expiry time [s] |
|
| 0.5 | ± 1 | 10-20 | ± 1 | 4-8 | ± 2 | 4-20 |
| 1 | ± 0.7 | 10-20 | ± 0.7 | 5-9 | ± 1.5 | 5-20 |
| 2 | ± 0.5 | 10-25 | ± 0.5 | 5-9 | ± 1 | 5-20 |
| 2.5 | ± 0.4 | 7-11 | ||||
| 3 | ± 0.33 | 7-11 | ||||
| 4th | ± 0.25 | 7-11 | ||||
| 5 | ± 0.3 | 15-30 | ± 0.3 | 7-11 | ± 0.6 | 7-30 |
| 6th | ± 0.25 | 8-12 | ||||
| 7th | ± 0.21 | 8-12 | ||||
| 8th | ± 0.19 | 8-12 | ||||
| 9 | ± 0.17 | 8-12 | ||||
| 10 | ± 0.2 | 15-40 | ± 0.2 | 8-12 | ± 0.4 | 8-40 |
| 15th | ± 0.13 | 9-13 | ||||
| 20th | ± 0.15 | 25-50 | ± 0.15 | 9-13 | ± 0.3 | 9-50 |
| 25th | ± 0.13 | 25-50 | ± 0.13 | 10-15 | ± 0.32 | 10-50 |
| 30th | ± 0.12 | 13-18 | ||||
| 40 | ± 0.09 | 13-18 | ||||
| 50 | ± 0.1 | 30-60 | ± 0.1 | 13-18 | ± 0.2 | 13-60 |
| 100 | ± 0.08 | 40-60 | ± 0.08 | 25-30 | ± 0.16 | 25-60 |
Graduated pipettes are also available in three accuracy classes: Class A, Class B and Class AS. For fast-draining class AS pipettes, a waiting time of 15 seconds must be observed after the expiry.
use
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In the past, picking up liquids by sucking in with the mouth was only used for liquids whose vapor inhalation , accidental sucking into the oral cavity and swallowing could not lead to health damage Aspiration with non- graduated and non- calibrated glass tubes was referred to as pipetting).
The German Social Accident Insurance now generally forbids sucking in pipettes by mouth in Germany .
Since aspiration with the mouth can pose a significant health risk, a pipetting aid must be used. With this the liquid is sucked in after the pipette tip is immersed. When transporting the liquid, the upper end of an open pipette must be kept closed with the pipetting aid (previously with the index finger or thumb). The liquid is dispensed depending on the type of pipetting aid. In any case, it is important to wait 20 seconds after the leakage so that the remaining liquid can run in. Pipettes are usually not blown out with the breath. The pipette tip should then be briefly wiped off the edge of the vessel, as the pipettes are adjusted to the outlet ( Ex ).
If a part of the entire scale range is to be used with a graduated pipette, a distinction is made between forward and backward pipetting . If the substance is valuable or if you want to save yourself unnecessary work, you use the forward method. Here, the desired amount of substance is measured when it is sucked into the pipette and the pipette is completely emptied at the target location. Since more substance remains in the pipette tip than in another area of the shaft with the same volume, viscous liquids are often pipetted backwards . Here the liquid is sucked in up to the top mark of the pipette, the dosage is based on the scale when it is discharged. The rest of the liquid is then returned to the storage vessel, which creates the risk of contamination. Since the scales are used in opposite directions in both cases, there are pipettes with both types of scales.
The use of a pipette allows good control of the amount of liquid; in general, dropwise dosing is possible. In addition to burettes, for example, volumetric pipettes are among the most precise volumetric instruments in the laboratory.
In the case of viscous liquids, the pipettes, if they have not been calibrated for this liquid, are usually rinsed through with the correct solvent and using a squeeze bottle to deliver the desired amount.
The most common mistakes when using pipettes are for example:
- Do not hold the meniscus at eye level and this will result in an unclean reading.
- The pipette tip is not held against the vessel wall or is not held long enough.
- When attaching a pipetting aid, full pipettes are held by the glass bubble and break more easily.
- The pipette has an unclean, greasy inner wall, which means that liquids do not run off completely and the pipette is not completely emptied according to the calibration.
A tool with a similar principle, the wine lifter , is used by winemakers to suck up and measure wine from a wine barrel (should not be confused with a lifter ).
Further meaning
In digital image processing , the pipette is a tool with which a color value can be selected by determining (“taking”) it from an existing image; to do this, the mouse pointer takes on the shape of a pipette, the tip of which is clicked on a pixel . The existing image is not changed by this, the color is not literally "taken" but the color value is copied; Only in the following work steps does the selected color serve as the working color. Depending on the program and setting, it is also possible to determine an average value for the surrounding pixels (for example in Adobe Photoshop ).
See also
Web links
- Lecture on the accuracy and measurement uncertainty of piston-operated pipettes (PDF file; 197 kB)
- Influence of altitude on the volume result of a piston-operated pipette with an air cushion (PDF file; 128 kB)
Individual evidence
- ^ Walter Wittenberger: Chemische Laboratoriumstechnik , Springer-Verlag, Vienna, New York, 7th edition, 1973, pp. 87-88, ISBN 3-211-81116-8 .
- ↑ Marco Körner: Laboratory device of the month: The pipette. In: chemreporter.de. September 7, 2016, accessed May 15, 2018 .
- ↑ Peter A. Czeschinski: Protection against infection. Springer-Verlag, 2013, ISBN 978-3-322-83445-4 , p. 14 ( limited preview in Google book search).
- ↑ Max Gundel: Textbook of Microbiology and Immunobiology. Springer-Verlag, 2013, ISBN 978-3-662-25434-9 , p. 195 ( limited preview in Google book search).
- ^ Alfred Pingoud: Working methods of biochemistry. Walter de Gruyter, 1997, ISBN 978-3-11-016513-5 , p. 18 ( limited preview in the Google book search).
- ↑ Safety in the chemical university internship, An introduction for students, DGUV Information 213-026, PDF file .
- ^ Stefan Prasse: Digital visualization of chemical laboratory techniques. diplom.de, 1998, ISBN 978-3-8324-1078-0 , p. 35 ( limited preview in the Google book search).