Product Survey: Hand-held Dispensers
Step by Step
by Harald Zähringer, Labtimes 06/2013
Classical micropipettes are perfect for pipetting different amounts of liquids into a limited number of reaction tubes. Dispensing equal amounts of liquids into many reaction vessels, however, calls for handheld-dispensers or stepper pipettes.
Repetitive or stepper pipettes are specialised micropipettes adapted for quick sequential dispension of liquids. Electronic steppers may even be an alternative to expensive liquid handling robots.
Though pipetting and liquid handling robots are commonplace in many labs, there is still a lot of manual pipetting left for PhD students, technicians and other lab workers. Setting up a pipetting robot makes sense for high throughput applications but starting a liquid handler to just fill a few microplates or prepare a serial dilution is not very economical. It’s a much better idea in these cases to pipette by hand, however, not necessarily with a classical micropipette but rather a hand-held dispenser alias repetitive or stepper pipette.
Hand-held dispensers are special micropipettes, optimised to accurately dispense repetitive amounts of liquids at a remarkable speed. They are available in four different formats: manual, manual with electronic display, electronic with manual tip removal and electronic with electronic tip rejection.
At first sight, manual dispensers look similar to micropipettes; however, they significantly differ from their cousins in certain features. The most obvious is a syringe kind of tip locked into the tip holder via a bayonet lock that replaces the typical micropipette tip. Hand-held dispensers work according to the positive displacement principle in contrast to micropipettes, which are based on the air-displacement concept.
Positive displacement tips used for hand-held dispensers are basically syringes, with a precision-moulded piston moving up or down the cylindrical barrel that comes in direct contact with the liquid. The liquid is drawn into the syringe with a constant aspiration force, which facilitates accurate aspiration and dispensing of viscous or volatile liquids. Positive displacement tips also come in handy to aspirate bigger volumes that, afterwards, can be dispensed in small repetitive steps. They are available in different sizes, ranging from 0.05 ml to 50 ml, with most manufacturers offering at least three volume sizes.
The liquids are drawn into the syringe by shifting a filling lever, usually positioned at the lower end of the hand grip. Once the syringe is filled, the liquid may be dispensed by pushing a second lever at the opposite end of the grip or a button on its top. Before dispensing, however, the number of steps must be selected by turning a stroke-setting wheel to the respective position. Depending on syringe capacity and dispensed volume, different numbers of steps are possible. Usually, a table printed on the instrument shows the possible combinations, e.g., 49 steps of 20 µl if a 1 ml syringe is used. After dispensing, the empty syringe is released by pushing an ejection key or lever and a new one may be locked in.
Filling and dispensing lever as well as stroke wheel and ejecting key are essential parts of every manual stepper pipette. How these components are arranged and positioned, however, in the hand grip is quite different for every model and certainly has an impact on ergonomics. Of course, all manufacturers claim that their steppers are ergonomic; even the square-edged models with long heavy levers, whose design seems to date back to the nineteen-fifties! These models may be okay for dispensing a few drops but your Lab Times editor won’t be too eager to perform extensive dispensing tasks with a stepper, whose grip is shaped like a rectangular stick!
Some manual stepper pipettes are equipped with an electronic display that indicates the selected tip size and after choosing the number of steps, displays the dispensing volume. Messing up dispensing volumes should be almost impossible with this feature.
The real dispensing fun, however, starts with electronic hand-held dispensers. No more clumsy filling and dispensing levers, no fiddling around with stroke wheels. Touching an operation button instead will do to start the desired dispensing programme automatically. Electronic hand-held dispensers are almost indistinguishable from electronic micropipettes. The only difference that immediately catches the eye is the syringes used as positive displacement tips. All other features, such as grip design, arrangement of keyboard and operation buttons as well as the positioning of the display are pretty much the same as in electronic micropipettes.
The operator may choose between several programmes, such as pipetting, diluting, multiple dispensing, multiple aspiration or sequential dispensing where aliquots of different volumes may be dispensed. Electronic dispensers use a slightly different tip system than mechanic instruments. The tips (syringes) of manual dispensers usually have a gradation of five and the smallest dispensed volume equals the tip content divided by 50. Most electronic steppers use a gradation of 20 and divide the tip content by 100. Hence, they offer more volume choices, provided that the locked-in tips are compatible with the stepper model and also allow dispensing of odd volumes.
Overlooking the many numbers and symbols flickering on the displays of some steppers is not always easy. Some display every single instrument setting, from operation mode, volume, selected tip to aspiration/dispensing direction to battery status, in and out speed and the number of dispensed aliquots.
Granted, electronic steppers are much more ergonomic than manual dispensers. Most instruments, however, are not completely electronic, since the tips still have to be removed manually: either by pushing a tip ejection lever or, even worse, by pulling out the tip from a locking mechanism. That’s not only inconvenient or time consuming, it also exposes the operator to a considerable risk if, for example, the tips were used to dispense infectious or radioactive liquids. Using an electronic dispenser with an electronic tip ejection mechanism eliminates this risk; it simultaneously increases the working speed and reduces the workload during extensive dispensing tasks.
Electronic dispensers are more costly than mechanical steppers but they are still much cheaper than liquid handlers. Hence, buying a couple of high quality, electronic dispensers instead of a liquid handling robot could be an option for many labs and is certainly worth thinking about.
First published in Labtimes 06/2013. We give no guarantee and assume no liability for article and PDF-download.
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