Positive displacement pipette
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Positive displacement pipettes are a type of pipette that operates via piston-driven displacement.[1][2] Unlike an air displacement pipette, which dispenses liquid using an air cushion in the pipette tip, the piston in a positive displacement pipette makes direct contact with the sample, allowing the aspiration force to remain constant.[3]
Applications
Since the piston makes direct contact with the sample, the aspiration force in a positive displacement pipette is unaffected by the sample's physical properties.[3] Several companies suggest that positive displacement pipettes can be used to accurately pipette very viscous, volatile, hot or cold, or corrosive samples.[4][5][6][7]
Viscous Liquids
Viscous liquids flow very slowly. If a researcher pulls the tip out of sample too quickly, the pipette will also draw up air bubbles.[1] When a researcher dispenses the liquid, some liquid will stick to the walls of the pipette tip and dispense very slowly and remain in the tip.[8] In a positive displacement pipette, the aspiration strength remains constant, so the tip fills evenly.[3] Also, the piston pushes the total volume out of the tip, so no liquid is left behind.
Volatile Liquids
Volatile liquids evaporate continuously in air displacement pipettes.[8] Some liquids expand so quickly that they expand the air column in the pipette, which causes leakage: The pipette will lose drops and dispense liquid imprecisely.[9][8] As drops leak out, they can contaminate of the bench, ultimately causing cross-contamination from sample to sample.[10] These drops can also produce a health hazard.[9]
Because there is no air cushion in a positive displacement pipette, liquids do not evaporate or leak. Drops will not fall from the tip, and vapors will not contaminate the internal parts of the pipette.[9]
Hot or Cold Liquids
In an air displacement pipette, the ambient temperature is correlated with the volume of the air cushion and affects the aspiration volume.[11][12] Hot liquids will cause the air cushion to expand, causing the pipette to aspirate less liquid than expected, making the pipette under-deliver.[11] Cold liquids cause the air cushion to shrink, causing the pipette to aspirate more liquid than expected, making the pipette over-deliver. Positive displacement pipettes do not have an air cushion and are less affected by liquid temperature, yielding greater pipetting accuracy.[11][12]
Corrosive and Hazardous Liquids
Corrosive and radioactive liquids may damage the piston, seal, and tip holder in an air displacement pipette.[4] Positive displacement pipettes use a disposable capillary/piston (CP) tip, so the pipette is not affected by corrosive samples over its lifetime. Since there is no contact between the sample and the pipette, there is little risk of contamination.[1]
Pipetting Technique
Positive displacement pipettes operate very similarly to air displacement pipette|air displacement pipettes]].
- Steps for operating a positive displacement pipette[13]
- Attach a CP tip onto the pipette.
- Hold the pipette vertically and press the plunger to the first stop.
- Put the CP tip into the sample and slowly release it, moving the button to the home position.
- Press the plunger to the first stop again to dispense the sample.
- Press the plunger to the second stop to eject the CP tip.
CP tips are disposable.[14]
References
- ^ a b c "Positive Displacement Pipette". Retrieved 19 December 2017.
- ^ "Selecting the Right Pipette" (PDF). Retrieved 19 December 2017.
- ^ a b c "The working principle of positive displacement pipettes" (PDF). Retrieved 19 December 2017.
- ^ a b "Positive-displacement Pipettes for Accurate Transfer of "Problem Liquids"" (PDF). Retrieved 19 December 2017.
{{cite web}}: line feed character in|title=at position 44 (help) - ^ "BRAND Positive Displacement Pipettes". Retrieved 19 December 2017.
- ^ "Precise and Repeatable Pipetting of Problem Liquids". Retrieved 19 December 2017.
- ^ "Specialty Pipettes". Retrieved 19 December 2017.
- ^ a b c "Effect of Liquid Properties in Pipetting Liquid Handling Note – No. 1" (PDF). Retrieved 19 December 2017.
- ^ a b c "How Positive Displacement Pipettes Work". Retrieved 19 December 2017.
- ^ "Preventing Contamination in Pipetting".
{{cite web}}:|access-date=requires|url=(help); Missing or empty|url=(help); Text "http://www.labmanager.com/laboratory-technology/2016/02/preventing-contamination-in-pipetting#.WjmL2hNSyF1" ignored (help) - ^ a b c "Extreme Pipetting II: Thermal Exploration".
{{cite web}}:|access-date=requires|url=(help); Missing or empty|url=(help); Text "http://www.artel-usa.com/resource-library/in-the-lab-pipettes/" ignored (help) - ^ a b "Securing accuracy and precision when pipetting hot and cold liquids with Microman®".
{{cite web}}:|access-date=requires|url=(help); Missing or empty|url=(help); Text "https://www.nature.com/articles/nmeth1086" ignored (help) - ^ "Pipetting Techniques".
{{cite web}}:|access-date=requires|url=(help); Missing or empty|url=(help); Text "http://www.gilson.com/Resources/GGP_Pipetting_Techniques.pdf" ignored (help) - ^ "Guide to Pipetting".
{{cite web}}:|access-date=requires|url=(help); Missing or empty|url=(help); Text "http://lifeserv.bgu.ac.il/wb/zarivach/media/protocols/GuideToPipetting.pdf" ignored (help)