Product Survey: Manual Micropipette

Not Dead Yet
by Harald Zähringer, Labtimes 01/2017

Different pipetting styles may lead to intra- and inter-individual imprecision of manual pipetting.

Manual pipettes are under pressure by faster and more ergonomic electric pipettes and liquid handlers. But they may spend a future life in robots, utilising manual pipettes instead of pipetting heads for liquid dispensing.

Though some experts, such as pipette calibration specialist and head of pipette calibration company ­Calibrate-It, Michel Bryce, have already proclaimed the end of manual pipetting, manual micropipettes are still the most prevalent pipetting tools in life science laboratories – in spite of liquid handlers and electronic pipettes, which are not only faster but also reduce the risk of muscle and strain injuries.

Bullet-proof construction

A major argument, besides the lower price, still speaking in favour of manual pipettes, is their simple and easy use. Lab technicians and researchers are trained on manual pipettes from day one in the lab – operating a manual pipette has almost become second nature to them. The rugged and almost bullet-proofed construction is another advantage of the manual pipette. The mechanics that drive plunger and piston inside the shaft is very robust and may be easily disassembled for cleaning. Even autoclaving at 121°C, to prevent contaminations, doesn’t bother manual pipettes – in contrast to most electronic pipettes that quit their work after autoclaving.

When calibrated properly, manual pipettes are very precise instruments, matching the accuracy and precision of electronic pipettes and liquid handlers. Let’s face it: the errors in manual pipetting are largely produced by the operator – not by the pipette. A group led by Giuseppe Lippi from the University Hospital of Verona, Italy, recently analysed the intra- and inter-individual imprecision of manual pipetting (Clin Chem Lab Med: Doi 10.1515).

Individual pipetting techniques

They randomly chose twenty laboratory operators and let them dispense 1 ml, 100 µl or 10 µl water for ten consecutive times with three recently calibrated, certified pipettes into a plastic container. The candidates applied their own pipetting techniques without any further advice from Lippi’s group. The team weighed the dispensed water after each pipetting step, calculated the intra- and inter-individual imprecision and expressed it as Coefficient of Variation (CV%).

It’s no surprise, that all operators chose forward pipetting, which is recommended for aqueous solutions and buffers (backward pipetting is used only on rare occasions to dispense, for example, viscous liquids). It is also not astonishing that the pipetting error is inversely correlated to the dispensed volume: the mean intra-individual pipetting imprecision was 5.7% for pipetting 10 µl, 0.8% for pipetting 100 µl and 0.2% for pipetting 1 ml. The corresponding CVs of inter-individual pipetting imprecision were 8.1%, 1.1% and 0.4%, respectively.

Error not related to lab experience

But it is kind of a surprise, that the imprecision is not correlated to the years of experience in the laboratory. Amongst the test candidates were ten laboratory technicians, five medical doctors and five biologists or chemists with an average lab experience of 24 (± 11) years. However, the recorded CV values, reflecting the individual pipetting performance, lay pretty close together. Obviously, lab experience is not a prerequisite for accurate pipetting. Or put the other way around: lab routine alone does not prevent significant pipetting errors.

The Italian group ruled out experimental variations by applying standardised procedures based on identical pipettes, tips, environmental conditions, etc. Hence, the only explanation for the variations is small differences during execution of consecutive pipetting runs: e.g., the pressure exerted on the plunger may be a bit higher or lower; the pipette may be inserted by the operator at different angles into the liquid or the aspiration velocity may vary. Inconsistent pipetting technique is another plausible source for errors: e.g., touching the container walls with the pipette tip irregularly, during liquid aspiration and/or dispensing, will inevitably lead to variations of the pipetted volume.

Practice or automation

Researchers have two possibilities to reduce these manual pipetting errors: they may either practice a proper and consistent pipetting technique or integrate their manual pipette into small robotic systems, which utilise standard manual pipettes instead of special pipetting heads for liquid dispensing.

While most of the established automation companies focus on complex liquid handling robots or workstations with integrated pipetting heads, start-ups such as US-based Opentrons or the Swiss company Andrew Alliance, concentrate on very lean robots, equipped with simple manual pipettes instead of complex and expensive pipetting heads.

Only essential parts

Opentrons’ standard pipetting robot OT-One, for example, is stripped down to the very essential parts of a robot system: it basically consists of a robot arm integrated into a basic cubic frame, made of square aluminium poles. The arm moves along the x, y and z-axes above a platform that houses up to 15 microplates. A standard single or eight-channel manual pipette is attached to the arm via a simple pipette holder made of plastic that is tightly fixed to the arm with screws.

An acrylic actuator mounted above the pipette has taken over the thumb’s job and presses down the plunger and tip-ejection buttons to aspire or dispense liquids. Sure, you have to calibrate OT-One’s labware and deck as well as the pipetting process yourself, with a downloadable app that also controls the robot and allows the running of different pipetting protocols. But it all looks way easier than installing and programming a full-blown liquid handler.

Pipetting companion robot

Andrew Alliance’s pipetting robot, Andrew, also employs common manual pipettes to automatically aspire and dispense liquids. Andrew, however, is built like a typical companion robot: one arm is holding different manual pipettes while the other is moving around similar to a human arm. It grabs an appropriate pipette from the pipette holder arm, adjusts the necessary volume, aspires an aliquot of liquid from a container placed on Andrew’s deck, swings to a new tube and finally dispenses the liquid into it.

Since Andrew is a skinny lightweight, weighing only ten kilos with a footprint of 29 x 25 centimetres, it may be installed directly on the bench to work side-by-side with human “pipetters”. And he will certainly prove to his human competitors that manual pipettes are still very precise and handy devices that are not dead yet.

First published in Labtimes 01/2017. We give no guarantee and assume no liability for article and PDF-download.

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