A micropipette is a precision laboratory instrument designed to measure and transfer minute volumes of liquid, typically in the microliter (µL) range. This instrument is foundational in molecular biology, biochemistry, and clinical diagnostics, where even fractional volume errors can compromise experimental results. Since the micropipette relies on air displacement to draw up and dispense liquid, the operator’s technique directly influences the final volume accuracy and experimental reliability. Mastering the correct handling procedures is necessary for generating trustworthy data in any scientific setting.
Identifying Components and Setting Volume
The air-displacement micropipette is operated using several primary components, including the plunger, the volume adjustment wheel, the digital display, and the tip ejector. The plunger, located at the top, controls the internal piston that creates the vacuum necessary for aspiration and dispensing. The volume adjustment wheel allows the user to set the desired volume, which is read on the digital display, usually in microliters.
Before use, the correct volume must be set precisely within the instrument’s designated range, such as a P200 (20–200 µL) or P1000 (100–1000 µL) model. To adjust the setting, the volume wheel should be turned slowly, approaching the target volume gradually to prevent overshooting. Never turn the adjustment wheel past the upper or lower limits of the pipette’s range, as this can damage the internal piston mechanism. Once the volume is set, an appropriately sized disposable tip must be seated firmly onto the tip cone to ensure an airtight seal, which is necessary for accurate air displacement.
Aspirating the Liquid: First Stop Control
The process of drawing liquid into the tip, known as aspiration, begins with the controlled use of the plunger’s first stop. The first stop is the point of initial resistance, and pressing the plunger to this depth displaces a volume of air exactly equal to the volume set on the digital display. The user must depress the plunger smoothly to this exact point before immersing the tip into the source liquid.
The pipette tip should be immersed just a few millimeters below the liquid surface—typically 2 to 3 millimeters for small volumes—to prevent liquid from clinging to the outside of the tip. The pipette must be held vertically, or at an angle no greater than 20 degrees from vertical, to prevent the liquid from running up into the pipette shaft. This contamination can lead to inaccurate results. Once immersed, the plunger is released slowly and steadily to draw the liquid up into the tip. A rapid release can cause air bubbles or splashing, resulting in an incorrect volume. After the plunger is fully released, a brief pause allows the liquid column to stabilize before the tip is withdrawn.
Dispensing and Tip Ejection
Expelling the measured liquid requires a transition from the first stop to the second stop, which is sometimes called the “blow-out” stop. To dispense, the tip is placed against the inner wall of the receiving vessel, often at a slight angle of about 10 to 45 degrees. The plunger is then pressed smoothly to the first stop, which dispenses the set volume of liquid.
To ensure all residual liquid is expelled from the tip, the plunger is pressed down past the point of initial resistance to the second stop. This final blow-out pushes out the small amount of air needed to overcome the surface tension that causes the last drop to cling to the plastic tip. While holding the plunger fully depressed at the second stop, the tip is carefully withdrawn from the side of the container. Releasing the plunger while the tip is still submerged can cause the liquid to be sucked back into the tip, leading to an error in the final volume. The final step is to use the tip ejector button to safely discard the used tip into an appropriate waste receptacle without touching it.
Care, Storage, and Common Mistakes
Maintaining the accuracy of a micropipette over time depends on proper care and storage, as the instrument is sensitive to mechanical and environmental stresses. When not actively in use, the pipette should always be stored vertically on a dedicated stand. This prevents residual moisture or liquid from running into the air-displacement mechanism, which can cause corrosion and damage the internal piston. Storing the pipette horizontally can also affect its smooth operation by causing lubricant to pool.
A new, sterile tip should be used for every different solution to avoid cross-contamination of reagents. Pipetting liquids significantly hotter or colder than the ambient temperature can introduce inaccuracies because the temperature difference affects the density of the liquid and the air cushion volume. Liquids with high viscosity or volatility, such as ethanol or glycerol, may require a specialized technique like reverse pipetting. Micropipettes require periodic calibration and servicing, ideally at least once a year, to verify that the volume dispensed matches the volume displayed, ensuring continued experimental reliability.