science

Positive displacement pipette

Positive displacement pipette is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Positive displacement pipette rather than just read about it. In short: Positive displacement pipettes are a type of pipette that operates via piston-driven displacement. 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.

Key takeaways

Positive displacement pipette belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Positive displacement pipette to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Positive displacement pipette from memory before moving on to harder problems.

Reference excerpt

Positive displacement pipettes are a type of pipette that operates via piston-driven displacement. 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.

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. Several liquid handling companies suggest that positive displacement pipettes can be used to accurately pipette very viscous, volatile, hot or cold, or corrosive samples.

Viscous liquids Viscous liquids, such as glycerol, flow very slowly. Glycerol has high dynamic viscosity, and if a researcher aspirates a sample of glycerol too quickly with an air displacement pipette, It will draw up an air bubble. 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. Surfactants also produce this effect, but the remaining liquid film is thinner. In a positive displacement pipette, the aspiration strength remains constant, so the tip fills evenly. Also, the piston slides along the internal sides of the pipette tip and pushes the total volume out, so no liquid is left behind.

Volatile liquids Volatile liquids such as acetone, hexane, and methanol, evaporate continuously in air displacement pipettes. 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 imprecisely. As drops leak out, they can contaminate the bench, ultimately causing cross-contamination from sample to sample. These drops can also produce a health hazard. 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. Also, the capillary/piston (CP) tips used for positive displacement pipetting are disposable.

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. 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. Hot samples, such as mammalian cell cultures at 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. Positive displacement pipettes do not have an air cushion and are less affected by liquid temperature, yielding greater pipetting accuracy.

Corrosive and hazardous liquids Corrosive and radioactive liquids may damage the piston, seal, and tip holder in an air displacement pipette. 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.

Pipetting technique Positive displacement pipettes operate very similarly to air displacement pipettes. Steps for operating a positive displacement pipette

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

Worked examples

Example 1 — a first encounter with Positive displacement pipette

Start with the simplest possible case. Write down what Positive displacement pipette claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Positive displacement pipette before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Positive displacement pipette ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Positive displacement pipette

In research: Positive displacement pipette appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Positive displacement pipette in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Positive displacement pipette is common in secondary-school and first-year university syllabi. It links to neighbouring topics Laboratory equipment, Volumetric instruments, so understanding it makes those chapters shorter.
In everyday life: Look for Positive displacement pipette outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Positive displacement pipette in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Positive displacement pipette means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Positive displacement pipette out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Positive displacement pipette in simple terms?

Why does Positive displacement pipette matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Positive displacement pipette?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Positive displacement pipette.