A serological pipette is a precise laboratory instrument, typically a long, calibrated glass or plastic tube, used for the accurate measurement and transfer of liquid volumes ranging from less than one milliliter up to 50 milliliters. This tool is fundamental in scientific fields like cell culture and reagent preparation, where consistency in liquid handling is necessary for reliable experimental results. Mastering the proper technique is paramount to ensure both accuracy and sterility in the laboratory setting.
Preparing the Equipment
The first step toward accuracy involves selecting the correct size of pipette for the task at hand. Choosing one where the target volume is a significant portion of the total capacity helps minimize measurement error. The pipette’s basic markings, including the major and minor graduation lines, indicate the volume in milliliters.
A mechanical pipettor, often called a pipet-aid or pipette controller, must be securely attached to the top of the pipette. This device creates the controlled vacuum necessary for drawing up and dispensing liquid safely. The pipette is gently but firmly inserted into the pipettor’s gasket to ensure an airtight seal, which is critical for maintaining accurate control over the liquid column and preventing erratic aspiration or dripping.
For applications involving cell media or sterile reagents, a quick sterility check of the packaging is necessary before use. Disposable plastic pipettes come individually wrapped and sterilized. The wrapper should be checked for any tears or damage that could compromise sterility.
Mastering Aspiration and Volume Reading
With the equipment prepared, the next focus is on the technique for drawing up the liquid, known as aspiration. The pipette tip should be immersed only slightly into the source liquid, avoiding contact with the container’s bottom, which could introduce contaminants or restrict flow. Holding the pipette vertically, or at a slight angle, the user should engage the pipettor mechanism slowly to draw the liquid up past the desired volume mark.
This controlled, slow aspiration prevents the formation of air bubbles within the liquid column or splashing, which can compromise the true volume and introduce contamination. After drawing past the mark, the liquid level is carefully adjusted down to the precise volume by gently engaging the exhaust mechanism. Accuracy hinges on correctly reading the liquid’s curved surface, known as the meniscus.
To read the volume correctly, the user must align their eye level precisely with the meniscus. The measurement is always taken at the lowest point of the concave curve of the liquid, where the liquid meets the pipette wall. Reading from an angle above or below this point, a phenomenon called parallax error, will lead to an inaccurate volume measurement.
Understanding the pipette’s calibration type is necessary for accurate aspiration. Serological pipettes are typically marked as “To Deliver” (TD), meaning they are calibrated to dispense the specified volume, often leaving a small amount of liquid in the tip. Less common “To Contain” (TC) pipettes require all the liquid to be expelled to achieve the correct measure.
Accurate Dispensing and Post-Use Handling
The dispensing phase requires the same degree of control as aspiration to ensure the full, measured volume is transferred. The liquid should be released into the receiving vessel at a controlled rate to prevent splashing or aerosol formation. For optimal delivery, the pipette tip should be touched gently against the inner wall of the receiving vessel; this wall contact helps to wick the liquid out of the tip, ensuring complete delivery.
A significant consideration during dispensing is the “blow-out” requirement. Many serological pipettes are designed as “blow-out” types, indicated by two small, continuous etched rings near the top. For these marked pipettes, the last remaining drop of liquid in the tip must be forcibly expelled using the pipettor’s blow-out function to deliver the full calibrated volume. If the pipette lacks these double rings, it is a “self-draining” type where the liquid remaining in the tip is intended to stay and should not be blown out.
After the liquid has been transferred, the pipette must be removed carefully from the receiving vessel to prevent accidental contamination. The pipette should then be immediately removed from the mechanical pipettor without delay. This prevents residual liquid from wicking up into the device, which could damage internal components or cause cross-contamination.
Proper post-use handling is essential for safety and contamination control. Disposable pipettes must be placed immediately into the appropriate waste receptacle, such as a designated biohazard bag or sharps container. Reusable glass pipettes are placed directly into a cleaning solution or a designated soak container to await sterilization.