Sero Pipette: How To Use It?

Sero pipettes are among the most commonly used liquid handling tools in laboratories, particularly in cell culture, microbiology, and routine solution preparation. While most laboratory personnel become familiar with their basic operation early on, issues such as volume inconsistency, poor reproducibility, or sample contamination are often caused not by reagents or instruments, but by improper pipetting techniques. Proper use of serological pipettes affects not only volumetric accuracy but also the reliability and reproducibility of experimental results.

What is a serological pipette?

A serological pipette is a disposable or reusable laboratory instrument used to accurately measure and transfer liquid volumes, typically ranging from 1 mL to 100 mL. It is commonly used together with a serological pipette controller or bulb and is especially suitable for handling relatively large volumes of liquids.

Serological pipettes are usually graduated along the entire length of the tube and are often blow-out pipettes, meaning that any remaining liquid in the tip is expelled to ensure accurate volume delivery. They are widely applied in cell culture, microbiology, clinical laboratories, and general laboratory procedures.

What is the structural design and working principle of a serological pipette?

The structural design of a serological pipette focuses on accurate volume measurement, stable liquid control, and minimal liquid retention. It consists of several key components and operates based on air pressure differential and gravity flow principles.

Tube Body
The pipette body is typically made from high-transparency materials, the main types include PS disposable serological pipettes and glass serological pipettes, allowing clear visualization of the liquid. The tube is printed with clear, continuous graduations for accurate volume reading.

Graduation System
Most serological pipettes feature full-length graduations, extending close to the tip. This design allows precise measurement across a wide volume range and supports linear volume control during aspiration and dispensing.

Upper End
The upper end connects to a pipette controller or bulb. By regulating airflow, a pressure difference is created, enabling liquid aspiration and dispensing.

Tapered Tip
The tapered tip helps concentrate liquid flow, reducing wall retention and improving transfer accuracy and reproducibility.

Blow-out Design
Most serological pipettes are blow-out type, meaning that after the liquid drains to the final graduation, an additional burst of air is applied to expel any remaining liquid, ensuring accurate volume delivery.

Working Principle
Serological pipettes operate based on the combined effects of air pressure differential and gravity. When air is withdrawn from the pipette using a controller, atmospheric pressure forces liquid into the tube. Releasing the pressure allows the liquid to flow out in a controlled manner, enabling accurate and repeatable liquid transfer.

How to choose the right serological pipette?

Selecting the right serological pipette requires consideration of volume range, application type, sterility requirements, graduation accuracy, and handling preference, ensuring accurate and reproducible experimental results.

Choose the appropriate volume capacity
Common serological pipette sizes include 1 mL, 2 mL, 5 ml serological pipette, 10 mL, 25 mL, 50 mL, 100 mL. It is recommended to select a pipette size close to the target transfer volume, as using oversized pipettes for small volumes may reduce accuracy.

Consider sterility requirements
For applications such as cell culture and microbiological work, sterile, individually wrapped serological pipettes are recommended. For routine liquid handling where sterility is not critical, non-sterile options may be more cost-effective.

Evaluate graduation clarity and accuracy
High-quality serological pipettes should feature clear, durable graduations that are easy to read. Accurate and consistent graduations help minimize volumetric errors during use.

Confirm blow-out design
Most serological pipettes are blow-out type, designed to deliver the full calibrated volume. Users should verify the pipette type and follow proper blow-out procedures when required.

Ensure compatibility with serological pipette controller
The pipette should be compatible with the available pipette controllers or bulbs to ensure secure fitting and smooth operation.

How to use a serological pipette?

**Before Use**

Before using the sero pipette, check the markings, accuracy level, and the position of the graduation marks.
Clean the pipette with chromic acid washing solution to remove any grease on the inner wall of the pipette.
Rinse the pipette with tap water to remove any remaining washing solution, then rinse with distilled water until clean.
After cleaning, the inner wall of the pipette should not have any water droplets clinging to it.
Before drawing up the solution, use filter paper to dry the inner and outer ends of the sero pipette.
Rinse the inner wall of the pipette 2 to 3 times with the solution you intend to transfer to ensure the concentration remains unchanged.

**Drawing Liquid**

While using the sero pipette, hold the area above the graduation mark with your right thumb and middle finger. Insert the pipette tip about 1 cm below the surface of the liquid to be measured. Avoid inserting it too deeply to prevent excess liquid from adhering to the outer wall of the pipette tip. Likewise, do not insert it too shallowly to avoid drawing in air as the liquid level drops.
Gently use your left hand to squeeze the rubber bulb, drawing the liquid into the pipette. Keep an eye on the rising liquid level. As the liquid level in the container drops, lower the pipette accordingly.
When the liquid level rises to about 1 cm above the graduation mark, quickly use your right index finger to seal the pipette’s opening and remove the pipette from the solution.

**Adjusting the Liquid Level**

Lift the pipette slightly away from the liquid surface, keeping the pipette tip in contact with the inner wall of the container holding the solution. Maintain the pipette in an upright position.
Gently relax your index finger (sometimes you can slightly rotate the pipette) to allow the liquid to slowly flow out from the bottom tip until the bottom of the meniscus aligns with the graduation mark.
Once aligned, immediately press your index finger tightly against the pipette’s opening. Remove any excess droplets by touching the pipette tip to the container wall, then remove the pipette and insert it into the receiving container.

**Releasing the Solution**

Place the sero pipette into the container that will receive the solution, ensuring the pipette tip touches the container wall.
Tilt the container slightly while keeping the pipette upright.
Relax your right index finger to allow the solution to flow freely down the container wall.
After the liquid stops flowing, wait for about 15 seconds before removing the pipette.
If the pipette is not marked with the word “blow,” do not blow out the remaining solution in the pipette tip, as the calibrated volume of the pipette does not include this residual liquid.

**Precautions**

Sero pipette (volumetric pipettes) should not be dried in an oven.
Pipettes should not be used to transfer solutions that are too hot or too cold.
Use the same pipette throughout a single experiment whenever possible.
After use, immediately rinse the pipette with tap water followed by distilled water, and place it on a pipette rack.
Sero pipettes are often used in conjunction with cell culture flasks, so it’s common practice to perform a relative volume calibration between the two before use.
To minimize measurement errors when using a sero pipette, always start from the top (zero) mark and release the required volume of liquid downwards, rather than drawing exactly the needed volume from the start.

Summary

Although serological pipettes are basic consumables, their proper use reflects a laboratory’s commitment to quality. Consistent technique forms the foundation of reliable scientific research.