A micropipette is a standard laboratory instrument engineered for the highly accurate measurement and transfer of extremely small volumes of liquid, typically in the microliter range. This level of precision is indispensable in modern scientific disciplines, including biology, chemistry, and medicine. The tool allows scientists to reliably handle reagents and samples in processes like DNA sequencing, pharmaceutical formulation, and clinical diagnostic testing. Because experiments often depend on exact concentrations and volumes, the micropipette’s ability to ensure consistent, repeatable liquid transfers is fundamental to reliable research results.
The Internal Mechanism of Volume Control
The most common type of micropipette operates on the air displacement principle, which relies on moving a volume of air to draw up and dispense liquid. The instrument contains an internal piston that moves within an airtight chamber, separated from the liquid by an air cushion. When the user turns the volume adjustment dial, it precisely changes the mechanical stop point for the internal piston.
This adjustment dictates the distance the piston can travel, controlling the exact volume of air that is displaced. Pressing the plunger down causes the piston to move, expelling the pre-set volume of air from the disposable tip. When the user releases the plunger, the piston moves back up, creating a partial vacuum inside the tip. Atmospheric pressure then forces the liquid up into the tip to fill the void left by the displaced air, precisely matching the volume set on the dial.
Step-by-Step Operation: Aspiration and Dispensing
The proper use of a micropipette involves a sequence of steps centered around the plunger’s two distinct resistance points, known as the first and second stops. After a disposable tip is firmly attached to ensure an airtight seal, the user first depresses the plunger only to the first stop. This action sets the internal mechanism to displace the exact volume of air corresponding to the desired liquid volume.
With the plunger held at the first stop, the tip is immersed into the source liquid. The plunger is slowly and smoothly released back to its resting position, drawing the measured volume of liquid into the tip. The user then moves the pipette to the receiving vessel and dispenses the liquid by depressing the plunger again to the first stop.
To ensure all the liquid is transferred, the user continues to press the plunger past the first stop to the second stop. This second, harder stop is a “blowout” mechanism that expels any residual liquid remaining inside the tip. The tip is then removed from the receiving vessel while the plunger is held at the second stop, preventing any liquid from being re-aspirated.
Critical Factors for Measurement Accuracy
Achieving the highest possible accuracy requires attention to several technical details beyond the basic operation. Controlling the speed of the plunger is important, as releasing it too quickly during aspiration can cause air bubbles or liquid to splash up into the pipette body, leading to errors. The tip immersion depth should be just a few millimeters (2 to 3 mm) below the liquid surface to prevent excess liquid from clinging to the outside of the tip or aspirating air.
Maintaining a near-vertical angle, no more than 20 degrees from vertical, during aspiration and dispensing helps ensure consistent liquid column formation. Before aspirating the sample, a technique called pre-wetting is performed by aspirating and dispensing the liquid three to four times. This action coats the inner walls of the disposable tip, humidifying the air space and reducing volume variations caused by evaporation or liquid adhesion.
For long-term reliability, the instrument requires periodic calibration checks to verify that the volume dispensed matches the volume set, as mechanical components can drift over time.