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Pipets are used widely in biology, chemistry, and medical laboratories to transfer small, measured volumes of liquid in the region of 0.1 μL to 10 mL. Pipets can be manual or electronic, single or multichannel. They must be accurate, precise, easy to use, and suited to both the liquid and the application.
Mechanics of a pipet
Pipets consist of a main body and a changeable tip. A piston creates a vacuum to aspirate the liquid into the tip. As the vacuum is released, the liquid is dispensed. There are two types of pipet design: air displacement and positive displacement.
In air displacement pipets, a cushion of air remains between the piston and the liquid sample. Air displacement pipets are highly accurate and are suitable for most laboratory applications with aqueous solutions. The piston is a permanent part of the pipet; only the sample-holding tips are replaceable.
In positive displacement pipets, the sample makes direct contact with the piston. The piston forms part of the changeable pipet tip to avoid cross-contamination. Positive displacement pipets are suitable for use with high-viscosity liquids that are too heavy to be displaced by air. They are also appropriate for volatile, corrosive, or radioactive liquids. The absence of air permits rapid pipetting without cavitation.
Freedom EVO® Platforms from Tecan (Männedorf, Switzerland; www.tecan.com) for automated liquid handling include a choice of pipetting arms. The Air LiHa™ uses air displacement technology but can be combined with other arms, including the liquid displacement liquid handling arm (Liquid LiHa).
Different pipetting modes are suitable for different samples. Modes include forward, repetitive, and reverse pipetting. Reverse pipetting is only possible with air displacement pipets. Check which mode your chosen pipet is calibrated for.
Figure 1 – VIAFLO 96/384 electronic handheld pipet systems from INTEGRA include 96- and 384-channel pipetting heads.Single-channel pipets transfer a single aliquot of one sample at a time. The Transferpette® S Single Channel Pipette from BrandTech® Scientific (Essex, CT; www.brandtech.com) is a manual pipet with a traditional control layout. It is available in eight sizes in both digital adjustable- and fixed-volume versions.
Multichannel pipets transfer liquids from multiple samples at the same time. They are particularly useful for preparing microplates. The VIAFLO 384 (Figure 1) from INTEGRA Biosciences (Hudson, NH; www.integra-biosciences.com) is an electronic handheld pipet system that includes 96- and 384-channel pipetting heads. It is controlled by hand but the movements are supported by a servo-assisted steering mechanism, enabling the precise targeting of microplate wells.
Manual, or mechanical, pipets require the user to depress and release a plunger to aspirate and dispense the sample. They are basic, affordable, reliable, and likely to be suitable for teaching laboratories. The Proline® Mechanical Pipet from Sartorius Biohit Liquid Handling (Göttingen, Germany; www.sartorius.com) is relatively lightweight and has a volume range of 100–1000 μL.
Electronic pipets are the best choice for daily, repetitive liquid handling. They are usually still operated by the user’s thumb, but require less pressure. Electronic pipets often provide additional functionality. For example, the Thermo Scientific™ Finnpipette™ Novus from Thermo Fisher Scientific (Waltham, MA; www.thermoscientific.com) includes an automated counting function and the option to dilute and mix samples. The sequential aspirate mode enables serial aspirating of different volumes, while the sequential stepper mode allows serial dispensing of different volumes.
Pipet controllers such as the FASTPETTE™ PRO-Lightweight from Labnet International (Woodbridge, NJ; www.labnetinternational.com) offer the user a choice of different operating modes and speeds, depending on volume and liquid viscosity.
Accuracy and precision
Effective pipetting requires accuracy and precision. These two factors are independent of one another.
Accuracy measures how close the average delivery volume is to the expected volume. If you expect 15 μL, but over three uses actually deliver 14 μL, 15 μL, and 16 μL, then an average accuracy of 15 μL is achieved, making the instrument accurate but not precise.
Precision is the capability of delivering consistent samples. If you expect 15 μL but the pipet consistently delivers 14 μL, the instrument is precise but not accurate.
Only if the pipet consistently delivers 15 μL for a 15 μL expected volume can it be said to be accurate and precise. Accuracy and precision vary with volume. Manufacturers publish accuracy and precision charts for different volumes for each pipet model.
Pipetting technique and environmental conditions both have an effect on accuracy and precision. Relevant environmental conditions include temperature, liquid density, air pressure, and humidity. Basic technique variations include angle of pipet and immersion depth of the tip. The recommended immersion depth varies with sample volume. Check the manufacturer’s guidelines for best technique with your chosen pipet.
Calibration and industry standards
Pipets should be periodically recalibrated after maintenance to ensure continued operation within the manufacturer’s specification. Pipets can be calibrated in the laboratory. BioPette™ Plus Pipets from Labnet International include a calibration tool with each unit.
For applications requiring high accuracy and precision, such as medical and pharmaceutical work, specialized companies offer calibration services under controlled environmental conditions. ISO 8655 defines the measurement specifications, methodology, and conditions required for ISO-compliant measurements for manual and electronic piston pipets. According to Gilson® (Middleton, WI; www.gilson.com), on average the PIPETMAN® permissible errors are two times better than those recommended by ISO 8655.
Volume range
The volume range of a pipet is between 0.1 μL and 10 mL. Most pipets have variable-volume ranges; some are continuously adjustable, while others have discrete volume settings. Fixed-volume pipets will only aspirate and dispense a single volume, but usually have higher accuracy and precision than variable-volume instruments.
Many manufacturers offer a series of pipets covering different volume ranges, with different settings within that range. As noted above, the accuracy and precision vary according to volume. The BRAND HandyStep® S Repeating Pipette from BrandTech Scientific offers nearly 60 different volume settings from 2 to 5000 μL.
Figure 2 – PIPETMAN L from Gilson is available in eight single-channel models from 2 μL to 10 mL, covering the volume range of 0.2 μL to 10 mL. It features an adjustable tip ejector for left- and right-handed users to increase comfort and lower forces when ejecting tips.Microvolumes for valuable samples
Some applications, such as DNA sequencing, involve working with microvolumes. Two PIPETMAN L models from Gilson provide the required accuracy and precision for such work; Model P2L covers 0.2–2 μL and Model P10L 1–10 μL (Figure 2).
Calibra® Digital 822 Micropipets from Socorex (Ecublens, Switzerland; www.socorex.com) are single-channel pipets with instant volume adjustment. Six models cover a volume range from 0.2 to 1000 μL.
Choosing the right pipet tips
Pipet tips are designed to be disposable in order to prevent contamination. Pipet tips are made from plastic; different plastics are suitable for different chemicals. They come in a range of sizes and formats that are appropriate for different applications. Tips from Hamilton Company (Reno, NV; www.hamiltoncompany.com) come in a choice of formats including standard, nonfilter, extended length, hydrophobic filter barrier, nonsterile, presterilized, and macrovolume.
To achieve the ultimate precision and accuracy, use of the manufacturer’s tips is recommended, although many pipets accept most brands of universal tips. For example, tips from Scilogex (Berlin, CT; www.scilogex.com) are proven to be compatible with pipets from many leading manufacturers.
Check that the packaging of your tip is suitable and convenient for your application. Tips come loose packed, individually packed in sterilized packing, or ready mounted on racks for multivolume pipets. The EZ Rack® System from Denville Scientific (Metuchen, NJ; www.denvillescientific.com) guarantees a stable, secure fit into the durable autoclavable racks. The system’s lockdown design eliminates poorly seated refills and accidental flip-over.
Cleaning and sterilizing your pipet
There are three main types of contamination to avoid when using pipets: pipet-to-sample, sample-to-pipet, and sample-to-sample. Changing tips circumvents sample-to-sample contamination. Sample-to-pipet contamination can be avoided through good technique, particularly by keeping the pipet upright. To prevent pipet-to-sample contamination, it is important to carry out thorough pipet decontamination.
Steam sterilization, also known as autoclaving, is the most common means of sterilization. Depending on the materials used in manufacture, some pipet parts cannot be autoclaved.
Some parts can be autoclaved but may suffer discoloration. Check your manufacturer’s guidelines for sterilization.
More thorough cleaning methods include chemical cleaning, UV radiation, and beta or gamma radiation. Your choice of cleaning method will depend on the types of liquid you handle. Some plastics are not suitable for certain cleaning methods, so check that your chosen pipet can be cleaned to the standard you require.
Ergonomics
Pipetting is a repetitive activity that carries a risk of repetitive strain injury to the user. Most manufacturers now make ergonomics a priority in their pipet design. Ergonomic factors to consider are the user’s hand position and the forces required to press the plunger and to eject tips. Some pipets, such as Hamilton’s SoftGrip Pipets, can be adjusted for left-handed operation.
Figure 3 – Portable Pipet-Aid XL from Drummond.The electronic Biohit eLINE® pipet family from Sartorius is designed for user comfort. The range of pipetting modes reduces the need for several work stages and enables liquid dispensing twice as fast as a mechanical pipet. ClipTip™ technology from Thermo Fisher Scientific has been developed to provide a complete seal on every channel and minimize the forces required to attach and eject tips.
The new Portable Pipet-Aid™ XL from Drummond Scientific (Broomall, PA) (Figure 3) is designed to reduce shoulder and neck strain when pipetting under the hood. The ergonomic design includes an adjustable sliding hand rest and a removable stand that enables the unit to be put down without contaminating the pipet.
Pipet manufacturers
A list of pipet manufacturers is given in Table 1.
Table 1 – Manufacturers of pipets
Katriona Scoffin, B.Sc., is a freelance science writer; e-mail: [email protected].
Please see our Pipettes / Micropipettes section to find manufacturers that sell these products