A pipette is a laboratory tool designed to measure and transfer small, precise volumes of liquid. It functions by creating a partial vacuum to draw up liquid and then releasing it to dispense the measured volume. This instrument is central to achieving accuracy and precision when handling liquids across various fields, including scientific research, medical diagnostics, and even some culinary or crafting applications, where consistent, exact liquid amounts are fundamental for reliable results.
Understanding Different Pipettes
Different types of pipettes serve distinct purposes based on their design and level of precision. Transfer pipettes, also known as Pasteur pipettes, are simple, disposable plastic tools with a bulb and a stem, commonly used for non-critical transfers and rough volume measurements. Graduated pipettes, made of glass or plastic, feature volume markings, allowing for the measurement of varying liquid volumes. These offer more precision than transfer pipettes and typically require an external bulb or pump for operation.
Micropipettes are highly precise instruments, often mechanical or electronic, engineered for handling very small volumes, typically in the microliter range. Found primarily in professional laboratory settings, they are indispensable for tasks demanding exceptional accuracy, such as molecular biology or pharmaceutical testing. These instruments work by displacing air to draw liquid into a disposable tip, ensuring minimal cross-contamination. The choice of pipette depends on the required accuracy and the volume of liquid being handled.
Step-by-Step Pipette Usage
Before beginning, ensure the pipette is clean and dry, and have both the liquid source and the destination container ready. For graduated pipettes, attach a rubber bulb or pump securely to the top.
To draw liquid, gently squeeze the bulb (on transfer pipettes) or use the suction device to create a vacuum. Submerge the pipette tip into the liquid and slowly release the pressure, allowing the liquid to rise above the desired mark. Drawing liquid slowly prevents air bubbles, which affect accuracy. For graduated pipettes, carefully release pressure to allow the liquid level, or meniscus, to descend to the precise graduation mark. Position your eye at the same level as the meniscus to ensure an accurate reading, typically at the bottom of the curve for most liquids.
Once the desired volume is aspirated, move the pipette to the receiving container. Dispense the liquid by gently depressing the bulb or plunger. For graduated pipettes, dispensing the liquid against the side wall of the container prevents splashing and ensures complete transfer. Some pipettes are designed “to deliver” (TD), meaning a small amount of liquid may remain in the tip after dispensing; this is normal and accounted for in their calibration.
Avoiding Common Mistakes
Several common errors can compromise pipetting accuracy. Air bubbles within the pipette tip indicate improper liquid aspiration, leading to inaccurate volume transfer. These can be avoided by aspirating slowly and ensuring the tip remains consistently submerged. Incorrect volume readings often occur when the meniscus is not viewed at eye level. Always align your gaze with the bottom of the meniscus to prevent parallax errors.
Cross-contamination is a significant concern. Using a fresh, clean pipette or thoroughly cleaning reusable pipettes between samples is essential to maintain sample purity. Splashing or foaming during dispensing can be minimized by touching the pipette tip against the inner wall of the receiving vessel. Working too quickly or using improperly fitting tips can also lead to inaccuracies and potential loss of liquid volume.
Pipette Care and Safety
Proper care extends the lifespan and maintains pipette accuracy. Reusable pipettes should be rinsed with distilled water immediately after use to prevent residue buildup, followed by proper drying. Storing pipettes upright in a dedicated stand or rack prevents damage and keeps them clean. Material compatibility is a consideration; pipettes are often designed for specific liquids, and using them with incompatible substances can cause damage or inaccurate results.
When handling potentially hazardous liquids, safety precautions are crucial. This includes wearing appropriate personal protective equipment, such as gloves and eye protection, and working in a well-ventilated area. Mouth pipetting is strictly prohibited due to the risk of ingesting hazardous chemicals or infectious materials. Always use mechanical pipetting aids to draw liquids. Disposable pipettes and tips should be disposed of in appropriate waste containers following laboratory guidelines.