A pipet is a laboratory tool designed to precisely measure and transfer liquid volumes. These instruments are fundamental in various scientific, medical, and industrial settings, enabling accurate and reliable results. Pipets have evolved from simple glass tubes into sophisticated devices that ensure precise liquid handling.
Types of Pipets and Their Uses
Numerous types of pipets exist, each suited for specific applications. Volumetric pipets deliver a single, fixed volume with high accuracy, often used for preparing solutions. Serological pipets allow for measuring variable volumes, suitable for tasks like transferring cell suspensions. Transfer pipets are disposable plastic devices for moving small, unmeasured amounts of liquid, reducing cross-contamination risks. This article will primarily focus on the micropipette. Micropipettes are widely used across many disciplines due to their ability to handle extremely small liquid volumes with exceptional precision, making them indispensable in fields such as molecular biology, biotechnology, and clinical diagnostics.
Understanding the Micropipette
A micropipette is an air-displacement instrument that operates by creating a vacuum to aspirate and dispense precise liquid volumes, typically in the microliter range. It features several key components. The plunger, located at the top, controls both aspiration and dispensing of liquid, and it also functions as a volume adjustment mechanism on variable volume models. A volume adjustment knob allows users to set the desired liquid volume, which is then displayed in a visible window. Below the plunger, a tip ejector button provides a mechanism for safely removing disposable tips without manual contact, preventing contamination. The main body of the micropipette, often referred to as the barrel, houses the internal piston and creates the airtight space necessary for air displacement. At the bottom, the tip cone is where the disposable plastic tip is securely attached, forming a seal for accurate liquid transfer.
Mastering Micropipette Technique
Effective micropipetting begins with selecting the appropriate micropipette for the desired volume, ensuring it falls within the instrument’s specified range to prevent damage and maintain accuracy. Set the volume by rotating the adjustment knob until the correct reading appears in the display window. Attach a fresh, sterile disposable tip firmly to the tip cone, ensuring a snug, leak-proof seal.
To aspirate liquid, smoothly depress the plunger to the first stop, a point of resistance indicating the set volume. While holding the plunger at this stop, immerse the tip just below the liquid surface, typically 2-6 mm, to avoid drawing air. Slowly release the plunger, allowing the liquid to be drawn into the tip, and wait a second or two for full aspiration. Hold the micropipette vertically throughout aspiration to minimize air bubbles and ensure consistent volume.
For dispensing, place the tip against the inner wall of the receiving vessel at a slight angle. Press the plunger smoothly to the first stop, expelling most liquid. To ensure complete expulsion, press the plunger further to the second stop, also known as the “blow-out” stop. Without releasing the plunger, withdraw the tip from the vessel. Finally, eject the used tip into a waste receptacle.
Consistent speed and pressure during plunger operation contribute to improved accuracy. Additionally, pre-wetting the tip by aspirating and dispensing the liquid once is beneficial, especially with volumes greater than 10 µL.
Ensuring Pipet Accuracy
Maintaining pipet accuracy involves consistent care and monitoring. Regular calibration checks are essential to verify that the pipet is dispensing the correct volumes. The gravimetric method is a common standard for calibration, involving weighing dispensed distilled water and converting its mass to volume, considering environmental factors. This process helps identify any drift from manufacturer specifications. Proper cleaning and decontamination procedures are also important to prevent sample contamination and ensure instrument longevity. This includes regularly cleaning the exterior and performing more in-depth decontamination as needed. Correct storage practices are equally important; pipets should be stored upright on designated stands to prevent liquid from entering internal mechanisms and to protect them from physical damage. Adhering to these practices helps maintain the pipet’s functionality and the reliability of experimental results.
Troubleshooting Common Pipetting Issues
Users may encounter common issues that can affect pipetting accuracy and efficiency. Air bubbles in the tip after aspiration often indicate improper immersion depth or too rapid plunger release; correct this by ensuring the tip is just below the surface and releasing the plunger slowly. Inaccurate volume delivery might stem from worn seals, damaged tips, or an improperly calibrated pipet, necessitating tip replacement, professional servicing, or recalibration. Liquid retention, where droplets remain in the tip after dispensing, can occur with viscous liquids or if the tip is not fully blown out; pre-wetting the tip or using the second stop can mitigate this. Cross-contamination results from reusing tips, touching non-sterile surfaces, or improper tip ejection. Using a new, sterile tip for each sample and ensuring the tip does not come into contact with anything other than the source and destination liquids are crucial preventative measures. Addressing these issues systematically helps maintain the integrity of experiments and the reliability of data.