View source for Positive displacement pipette

'''Positive displacement pipettes''' are a type of [[micropipette|pipette]] that operates via [[piston]]-driven displacement.<ref name="pipette.com">{{cite web|title=Positive Displacement Pipette|url=https://www.pipette.com/positivedisplacementpipette|access-date=19 December 2017}}</ref><ref name="gilson 1">{{cite web|title=Selecting the Right Pipette|url=http://www.gilson.com/Resources/GGP_Selecting_the_right_pipette.pdf|access-date=19 December 2017}}</ref> 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.<ref name="gilson 2">{{cite web|title=The working principle of positive displacement pipettes|url=http://www.gilson.com/Resources/PDTechniques.pdf|access-date=19 December 2017}}</ref>
== 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.<ref name="gilson 2" /> Several liquid handling companies suggest that positive displacement pipettes can be used to accurately pipette very viscous, volatile, hot or cold, or corrosive samples.<ref name="John Morris">{{cite web|title=Positive-displacement Pipettes for Accurate Transfer of "Problem Liquids"|url=http://www.johnmorris.com.au/files/files/microman%20e%20mail/images_microm/AppNote_TLS001%5b1%5d.pdf%20-%20PDPs.pdf|access-date=19 December 2017}}</ref><ref name="Sigma">{{cite web|title=BRAND Positive Displacement Pipettes|url=https://www.sigmaaldrich.com/labware/labware-products.html?TablePage=17190915|access-date=19 December 2017}}</ref><ref name="Eppendorf">{{cite web|title=Precise and Repeatable Pipetting of Problem Liquids|url=https://www.eppendorf.com/MY-en/liquid-guide/|access-date=19 December 2017}}</ref><ref name="Ranin">{{cite web|title=Specialty Pipettes|url=https://www.shoprainin.com/Pipettes/Speciality-Pipettes/c/MTSLH001?q=%3Aname-asc%3AproductFamilyCategory%3APositive+Displacement|access-date=19 December 2017}}</ref>
===Viscous liquids===
[[Viscosity|Viscous liquids]], such as [[glycerol]], flow very slowly. Glycerol has high dynamic viscosity,<ref name="ACS">{{cite journal |last1= Segur |first1= J. B. |last2= Oberstar |first2= H. E. |doi= 10.1021/ie50501a040 |title= Viscosity of Glycerol and Its Aqueous Solutions |journal= Industrial & Engineering Chemistry |volume= 43 |issue= 9 |pages= 2117–2120 |year= 1951 }}</ref> and if a researcher aspirates a sample of glycerol too quickly with an air displacement pipette, It will draw up an air bubble.<ref name="pipette.com" /> When a researcher attempts to dispense the liquid, some of it will stick to the pipette tip wall, dispense very slowly and remain in the tip.<ref name="Thermo">{{cite web|title=Effect of Liquid Properties in Pipetting Liquid Handling Note – No. 1|url=https://tools.thermofisher.com/content/sfs/brochures/D00178.pdf|access-date=19 December 2017}}</ref> [[Surfactant]]s also produce this effect, but the remaining liquid film is thinner.<ref name="Microman">{{cite web|title=ACHIEVE PIPETTING PRECISION OF PROBLEM LIQUIDS WITH MICROMAN® E|url=http://www.gilson.com/Resources/Gilson_MICROMAN_E_White_Paper_Digital_11.21.14.pdf|access-date=16 January 2018}}</ref>
In a positive displacement pipette, the aspiration strength remains constant, so the tip fills evenly.<ref name="gilson 2" /> Also, the piston slides along the internal sides of the pipette tip and pushes the total volume out, so no liquid is left behind.<ref name="Microman" />
===Volatile liquids===
[[Volatility (chemistry)|Volatile liquids]] such as [[acetone]], [[hexane]], and [[methanol]], evaporate continuously in air displacement pipettes.<ref name="Thermo" /><ref name="Microman" /> Some volatile liquids expand so quickly that they expand the air column in the pipette, which causes leakage: The pipette will lose drops and dispense liquid [[Accuracy and precision|imprecisely]].<ref name="Lab Manager">{{cite web|title=How Positive Displacement Pipettes Work|url=http://www.labmanager.com/how-it-works/2017/06/how-positive-displacement-pipettes-work#.WjmK7xNSyF0|access-date=19 December 2017}}</ref><ref name="Thermo" /> As drops leak out, they can contaminate the bench, ultimately causing cross-contamination from sample to sample.<ref name="Lab Manager 2">{{cite web|title=Preventing Contamination in Pipetting|url=http://www.labmanager.com/laboratory-technology/2016/02/preventing-contamination-in-pipetting#.WjmL2hNSyF1|access-date=19 December 2017}}</ref> These drops can also produce a [[Biological hazard|health hazard]].<ref name="Lab Manager" />
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.<ref name="Lab Manager" /> Also, the capillary/piston (CP) tips used for positive displacement pipetting are disposable.
===Hot or cold liquids===
In an air displacement pipette, the [[Room temperature#Ambient versus room temperature|ambient temperature]] is correlated with the volume of the air cushion and affects the aspiration volume.<ref name="Artel">{{cite web|title=Extreme Pipetting II: Thermal Exploration|url=http://www.artel-usa.com/resource-library/in-the-lab-pipettes/|access-date=19 December 2017}}</ref><ref name="Nature">{{cite web|title=Securing accuracy and precision when pipetting hot and cold liquids with Microman®|url=https://www.nature.com/articles/nmeth1086|access-date=19 December 2017}}</ref> Cold liquids, such as a suspension of restriction enzymes, which are usually handled at 0°C, cause the air cushion to shrink and the pipette to aspirate more liquid than expected, making the pipette over-deliver.<ref name="Artel" /> Hot samples, such as [[mammal|mammalian]] [[cell culture]]s at [[human body temperature|body temperature]] or [[polymerase chain reaction]] solutions at 60°C or higher, will cause the air cushion to expand, causing the pipette to aspirate less liquid than expected and making the pipette under-deliver.<ref name="Artel" /><ref name="Microman" />
Positive displacement pipettes do not have an air cushion and are less affected by liquid temperature, yielding greater pipetting accuracy.<ref name="Artel" /><ref name="Nature" />
===Corrosive and hazardous liquids===
[[Corrosive substance|Corrosive]] and [[radioactive decay|radioactive]] liquids may damage the piston, seal, and tip holder in an air displacement pipette.<ref name="John Morris" /> 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.<ref name="pipette.com" />
== Pipetting technique ==
Positive displacement pipettes operate very similarly to [[air displacement pipette]]s.
Steps for operating a positive displacement pipette<ref>{{cite web|title=Pipetting Techniques|url=http://www.gilson.com/Resources/GGP_Pipetting_Techniques.pdf|access-date=19 December 2017}}</ref>
# Set the pipetting volume.
# 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.
== References ==
{{reflist}}
[[Category:Laboratory equipment]]
[[Category:Volumetric instruments]]