When Should I Use a Positive Displacement Pipette?

Some liquids (viscous, foaming, high-vapor-pressure, and other “problematic” liquids) can not be transferred easily or accurately using traditional air-displacement pipettes.

 

The piston, a permanent part of the air-displacement pipette is moved within its cylindrical housing (the shaft), displacing a volume of air. This creates a vacuum in the pipette tip, causing liquid to move into the tip to displace it. There is always an air cushion between the piston and the liquid in the tip.

This procedure works perfectly well for the majority of day to day aqueous liquid transfers performed in most laboratories, so long as the pipette is well maintained. The piston regulates the volume of the air cushion, which, in turn determines the volume of liquid to be aspirated.

Difficult to Handle Liquids

When using an air-displacement pipette there is always the risk of creating aerosols in the tip that can contaminate the piston creating carry over and cross-contamination of other samples. This is undesirable, particularly for critical techniques such as forensic PCR. The introduction of filter tips has reduced this issue but still does not guarantee 100% contamination free pipetting.

Some liquids such as volatile, viscous or hazardous samples by their physical attributes, are just too difficult to pipette accurately, precisely and quickly with any confidence using a standard pipette.

The Positive Solution

Positive displacement pipetting has been introduced to overcome these problems This system uses a syringe-like tip so there is no compressible air cushion between the piston and the liquid in the pipette tip. The piston goes to the bottom of the tip and is in direct contact with the liquid. It directly expels the liquid from the tip, wiping the tip walls completely clean. This principle allows reproducible results, regardless of the pipetting speed, fluid viscosity or vapour pressure. The piston is disposable as part of the pipette tip and thus if tips are changed between each transfer the possibility of any cross contamination between samples is eliminated.

Eliminate Cross contamination

Disposable capillaries and pistons provide absolute protection against any trace of the previous sample. Thus the use of a positive-displacement pipette is highly recommended for critical PCR protocols.


 Viscous liquids

The small diameter of tips used with air-displacement pipettes demand that a constant aspiration strength must be generated in order to pipette viscous liquids such as highly concentrated protein solutions, oils, resins or fats . The air space between the sample and the piston behaves elastically. When expanding, this elasticity reduces the aspiration strength preventing proper pipetting. Sometimes, people get around this problem by cutting the end of the tip to increase its diameter but this will sacrifice a precise and accurate transfer.  When using positive-displacement pipettes there is no elastic air space to expand or contract, so the aspiration force remains constant, unaffected by the physical properties of the sample.

Aggressive liquids

Acids, bases and solvents are the worst enemies of any air-displacement pipette. During aspiration, aggressive vapours can enter the pipette body and attack the piston, seal or tip holder. This damage affects the accuracy of the pipette and may incur costly repairs. Thus laboratories working with aggressive liquids tend to prefer glass pipettes, even though they are not always easy to manipulate.

For frequent use of aggressive liquids, particularly in the domains of chemistry and biochemistry, positive-displacement pipettes are a good alternative to the potential hazards and inconvenience of glass pipettes. By design, MICROMAN® disposable capillaries and pistons shield all pipette parts against vapours allowing aggressive liquids to be pipetted simply and safely.

Hazardous samples

Radio-labelled compounds remain a very useful tool for highlighting metabolic pathways or for processing autodiagrams. These hazardous samples must be handled with extreme caution. Under these conditions, it is of utmost importance to protect pipette shafts from aerosol contamination. If contamination should occur, the pipette must be disassembled and cleaned thoroughly with special solvents. The use of MICROMAN® allows the operator to focus on other security parameters unrelated to pipetting.

Positive displacement pipetting can also prevent leakage when handling high vapour pressure liquids such as solvents, alcohols, ethers and chloroform. It prevents foaming when transferring surfactants and detergents. Blood and serum can be handled easily and with less risk of infection to the technician.

RAININ Pos-D- the Ergonomic Solution for Difficult to Handle Liquids

 

Rainin Pos-D™ positive displacement pipette guarantees ease, speed, accuracy, precision and safety. It is an excellent pipetting tool for applications in biotechnology, clinical, food and industrial laboratories.

Rainin Pos-D uses disposable capillary piston tips to ensure sample security. The pistonmoves within the plastic capillary, making direct contact with the liquid. The positive wiping action of the piston against the capillary wall ensures complete dispensing without droplets.

With no elastic air space to expand or contract, the aspiration force remains constant, unaffected by the physical properties of the sample. This means even very viscous or high density liquids such as 50% glycerol solutions, detergents, oil, syrup or even toothpaste can be easily pipetted with accuracy and precision. The risk of pipette damage from aggressive solvents is eliminated as the sample or its vapours can not come in contact with internal parts. Similarly there is no risk of contaminating the pipette when working with hazardous materials such as radioactive compounds. Eliminating cross contamination is also a major advantage for applications such as PCR. forensics and diagnostics.

The pre-assembled capillary tips are easy to use and also fit Gilson Microman®. For your convenience they are ALL supplied pre-sterilised and ready racked at prices you would normally pay for loose, non-sterile, unassembled capillary pistons.

RAININ Pos-D Pipettes

RAININ Capillary & Piston Tips

 

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