How application and workflow affect pipette selection

By choosing the right pipetting system and eliminating risk and variability from pipetting procedures, users can achieve enhanced accuracy, reduced costs and better overall productivity.

Evaluating application workflow is critical to selecting a pipette that will achieve accuracy. This paper explores helpful pipetting recommendations that can help users make educated choices in liquid handling instruments for greater ease of use, accuracy and instrument life, while reducing error risk and costs associated with failed experiments, rework and even repetitive stress injuries (RSIs).

Correctly assessing and reducing risks throughout a liquid handling workflow is critical to pipette selection and ongoing pipetting accuracy. Pipette selection can benefit from a full planning process which analyses all workflow aspects to determine liquid handling requirements for maximum efficiency and accurate data generation. These workflow aspects determine the pipette type required to achieve accurate results and enhance productivity.

Perhaps the most critical aspect to consider for any pipetting system is maximum tolerated data variability. This ensures a chosen apparatus can deliver required accuracy. Knowledge of sample type, liquid-handling volume/throughput and sample/assay specificity is also required to select handling formats that will provide the greatest accuracy and productivity for the process.

Nature of the sample

Most pipetting is straightforward. Aqueous samples at standard lab temperature and pressure can be pipetted accurately using simple air displacement. However, difficult samples such as viscous, dense, volatile, hot or cold liquids may be pipetted more accurately using positive displacement, which uses a piston and seal instead of an elastic air cushion to move liquid.

Helpful positive-displacement pipette attributes include less user training required for accuracy. Disadvantages include higher costs for disposable capillaries and pistons, and greater ergonomic forces required to plunge when pipetting manually. Fewer multichannel options are available as well, which can slow processes at higher throughput volumes.

Variability

Because some processes are more sensitive than others and some variability is expected, users must identify how much variability is tolerable to still produce solid data based on an individual process. Experiments dependent on a standard curve or generated through serial standards dilution can be severely affected by any amount of sub-optimal pipetting no matter how small, making features and expected accuracy of pipettes under consideration particularly critical to these efforts.

Pipetting volume

Workflows that require a large amount of liquid handling early, such as preparing buffers or plating cells, must often be balanced against smaller volume handling requirements later in the process. Needs for speed and precision must be carefully considered to manage both ends of this processing spectrum.

Consistently working at the outer limits on either the high or low end of a pipette’s volume-handling capability can compromise accuracy. If the number of samples being processed is high enough, significant efficiencies and even cost savings over time may be gained by moving assays to a plate format and shifting to multichannel pipettes.

Multichannel pipettes are an important option for fast, high-throughput applications such as 96-well plate ELISA work, or PCR and qPCR. Good multichannels such as 8- and 12-channel models should provide fast, secure, simultaneous tip-loading. They should not require excessive force to mount tips evenly, and tips should eject easily. A full pipetting platform, such as a manual 96-well pipettor, may also be helpful.

Adjustable spacer multichannel pipettes can also be useful for transferring multiple samples between different-sized tubes and plates with ease. This can cut format change time by up to 85% because as many as eight samples can be moved at the same time.

Manual or electronic?

Complex or repeated pipetting may benefit from electronic pipettes, since electronic pipettes can be programmed for specific protocols. They have also been shown to produce more consistent data than manual pipettes because microprocessors eliminate human error and variability - particularly noticeable in applications where pipetting errors can be compounded such as serial dilution.

Furthermore, electronic pipettes tend to be more ergonomic than manual pipettes. Ergonomic enhancement is usually achieved because the electronic mechanism reduces operator force required to plunge. It also reduces repetitive motion through the operator’s ability to program activities such as multidispensing, automatic mixing and serial dilution.

Cost may be a concern with electronic pipettes, however. Manual pipettes tend to be more economical, though they produce greater variability. However, particularly for highly sensitive or expensive pipetting applications, the cost of investing in an electronic pipette may be offset by a reduction in error and rework in a short span of time.

Choosing the right tip

The design, quality and fit of the pipette should be considered when choosing the right tip.

Design refers to properties that affect performance in different applications. For example, a wide orifice tip will behave differently from a low retention one. Standard tip design for regular, all-purpose pipetting includes thin clear walls to see the liquid being dispensed, a fine point for accurate dispensing and a good seal. Many specialty application tips also exist (Figure 1).

Quality refers to how a tip is manufactured, including whether it is free of defects that can cause costly sample loss and contaminants that disrupt experiments. This is a critical concern because some polypropylene labware consumables have been found to leach plasticisers which interfere with certain enzymatic reactions or cause false 260/280 nm spectrometer readings.

Fit refers to how well the tip matches the pipette. In almost every case, it is best to use a manufacturer’s recommended tip to gain the best fit, considering the tip-pipette a ‘comprehensive system’ rather than trying to use another brand of standard or universal tips.

Application-specific requirements should also be considered. For example, genomics application best practices include use of filter tips to minimise the effect of human or microbial DNA, RNase, DNase, Pyrogen or ATP on the sample or pipette. Filters can also block aerosols from the shaft, reducing contamination of later samples as well. They help protect the piston from microbial contamination, corrosives and salt deposits as well.

Conclusion

Pipetting is a critical action in many labs, and data accuracy/reproducibility generated by a liquid-handling workflow can be significantly improved by understanding application aspects such as the nature of the sample, volume, throughput and workflow-related risks. Tolerated variability is also a critical consideration. Selection of the right liquid handling instruments and options flows naturally from this analysis.

Taking time to analyse pipetting workflows can benefit users by providing greater processing ease and enhancing productivity.