A micropipette is a common laboratory instrument designed to accurately measure and transfer very small volumes of liquid, typically in the microliter (µL) range. These instruments are essential for precision in various scientific fields, including molecular biology, chemistry, and clinical diagnostics. Micropipettes allow scientists to handle minute amounts of reagents and samples with high reliability, which is crucial for experiments where minor volume errors can significantly impact results. Their widespread application highlights their importance in modern scientific research and development.
Getting Started with a Micropipette
Before beginning any liquid transfer, it is important to understand the components of an adjustable volume micropipette. These instruments typically feature a plunger button, a volume adjustment knob, a digital volume display, a tip ejector button, and a tip cone or holder. The plunger controls liquid aspiration and dispensing, while the volume adjustment knob allows for precise setting of the desired volume, which is visible on the digital display.
Setting the desired volume accurately is a fundamental step. To adjust the volume, hold the micropipette upright and gently turn the volume adjustment knob, typically clockwise to decrease the volume and counterclockwise to increase it. It is generally recommended to dial down to the target volume to minimize mechanical backlash and ensure accuracy.
Once the volume is set, selecting the correct disposable pipette tip is equally important. Each micropipette has a specific volume range, and using a tip that matches this range ensures optimal performance and prevents leaks or aspiration errors. Firmly press the pipette shaft onto the tip to create a secure seal, avoiding excessive force that could damage the tip or the pipette itself.
Step-by-Step Pipetting Technique
The core of effective micropipetting lies in mastering the sequential steps for liquid transfer. Begin by depressing the plunger button smoothly to the first stop, which is a point of initial resistance. This action prepares the air displacement system for aspiration. While holding the plunger at the first stop, immerse the pipette tip into the liquid. The immersion depth is important; for small volumes, typically 2-3 mm, and for larger volumes, up to 5-6 mm is adequate, as immersing too deeply can cause liquid to cling to the outside of the tip.
Next, slowly release the plunger button to aspirate the liquid into the tip. A controlled, consistent release prevents air bubbles and ensures the correct volume is drawn. After aspirating, move the pipette to the target vessel.
To dispense the liquid, place the tip against the inner wall of the receiving vessel at an angle, typically between 10-45 degrees. Smoothly press the plunger to the first stop to dispense most of the liquid. Then, to ensure complete expulsion of any remaining liquid, press the plunger all the way down to the second stop, also known as the “blow-out” stop. Finally, while still holding the plunger down, slide the tip up the vessel wall to remove it, and then press the tip ejector button to discard the used tip into an appropriate waste receptacle.
Ensuring Accuracy and Precision
Achieving accurate and precise pipetting results depends on adhering to several best practices. Pre-wetting the pipette tip is a fundamental technique, involving aspirating and dispensing the liquid to be transferred at least once, and often three times, before the actual transfer. This process increases humidity within the tip, minimizing evaporation and ensuring more accurate volume delivery, especially for volatile liquids. Maintaining a consistent pipetting angle is also crucial; aspirating should typically occur with the pipette held vertically, while dispensing often benefits from a slight angle against the vessel wall to prevent bubbles and ensure complete liquid transfer.
Smooth and consistent plunger movement during both aspiration and dispensing is another key factor. Rapid or jerky movements can lead to air bubbles or incomplete liquid transfer, compromising accuracy. Avoiding air bubbles within the tip is paramount, as their presence indicates an inaccurate volume aspiration.
Factors such as liquid viscosity and temperature significantly influence pipetting accuracy. Viscous liquids require slower aspiration and dispensing speeds to ensure full volume transfer and prevent a film from remaining in the tip. Temperature differences between the pipette, liquid, and ambient air can cause thermal expansion or contraction, affecting the actual volume delivered. To prevent cross-contamination between samples, always use a new, sterile pipette tip for each different liquid or sample.
Proper Care and Storage
Maintaining micropipettes through proper care and storage is important for their longevity and continued accuracy. Routine external cleaning involves wiping the pipette with standard laboratory cleaning agents, such as 70% ethanol, to remove surface contaminants. For internal cleaning or contamination, disassembling specific parts and cleaning them according to the manufacturer’s instructions is necessary. This may involve using cotton swabs with cleaning solutions or even autoclaving for sterilization, depending on the pipette and the type of contamination.
Proper storage methods also contribute to the instrument’s lifespan and performance. Micropipettes should be stored upright on a stand when not in use to prevent liquid from entering the internal mechanism and to protect the tip cone from damage. Avoiding extreme temperatures and humidity during storage is also advised, as environmental conditions can affect calibration and material integrity.
Regular calibration checks are a fundamental aspect of proper maintenance. These checks ensure that the pipette continues to dispense the correct volume within specified tolerances, which is essential for reliable experimental results and compliance with laboratory standards. Regular calibration helps identify and correct any deviations, preventing costly errors and ensuring the integrity of scientific data.