Microbiology: Everything you need to know about micropipettes

In the microbiology lab, we all know how to use a micropipette, but do we really know how it works?

For years, we’ve just called it a “micropipette.” But after chatting with some experts, we learned its proper name is actually “air-cushion micropipette.”

To be fair, “air-cushion micropipette” is a bit of a mouthful, isn’t it?

Imagine the lab if every other second you heard, “Has anyone seen my air-cushion micropipette?” or “Oh no, my air-cushion micropipette is off for calibration again!”

What a hassle! No wonder we stick to the shorter version.

Still, the term “air-cushion” caught our attention. It made us curious—where does that come from, and how exactly does it work inside these devices?

And hey, if there’s a cushion, does that mean we can take a nap on it? 😉

What is the purpose of an air-cushion micropipette?

An air-cushion micropipette is a tool used to measure and transfer liquid volumes with high precision, often at the microliter scale (between 10 and 1000 µL)… hence the “micro” in micropipette.

It’s commonly used in microbiology labs but also in other “minor” fields like … chemistry (haha).

Other names for the air-cushion micropipette include:

• “single chanel micropipette” (when there is only 1 tip)
  
• “manual micropipette”
  
• “mechanical micropipette”

How does an air-cushion micropipette work?

The secret of the air-cushion micropipette lies in its internal mechanism. Let’s take a look at what’s inside!

Inside, you’ll find a piston, a chamber filled with air (the famous air cushion), and a tip where the cone is attached (don’t forget the “e” in cone!).

Here’s what happens inside the micropipette when you push the piston:

When you press the piston (1), a precise volume of air is expelled. However, as you can see, there’s always a small volume of air left inside the pipette and the tip—this is the famous air cushion.

When you release the piston (2), a specific volume of liquid is drawn in, equal to the volume of air expelled in step (1). The air cushion always remains between the liquid and the piston.

This air cushion also helps protect the micropipette from contamination between samples.

Why are air-cushion micropipettes so sensitive?

By looking at how they work and what’s inside, it’s easy to see why air-cushion micropipettes are so sensitive… even though they look very robust!

• A poorly fitted or incompatible tip: This can cause an air leak at the junction between the tip and the micropipette. The volume drawn won’t be accurate, and worse, liquid might leak onto the bench.
  
• Temperature variation: The air cushion inside the micropipette is what allows precise volume measurement. Depending on the temperature, the density of the air can change, which affects accuracy:
  
  • If the air is warm, it expands and takes up more space → less liquid is drawn.
    
  • If the air is cold, it contracts and takes up less space → more liquid is drawn.

That’s why you should use the micropipette at the same temperature where its calibration (metrology) was performed.

The temperature of the liquid you’re handling also matters. If possible, let the sample reach room temperature before pipetting.

This also explains why pipetting volatile or foamy liquids is tricky. In such cases, try using the reverse pipetting technique for better results.

Pipettes Tips

How can we talk about air-cushion micropipettes without mentioning their tips (also called cones or points)? The two are an inseparable pair!

Micropipette tips are affordable and sterile, making them single-use only.

There are two main types:

• Universal tips, which fit most micropipettes.
  
• Specific tips, designed to fit only one brand of micropipette.

Each type has its pros and cons… but we’ll get into that another time.

Every volume (or volume range) of a micropipette has its corresponding tips to ensure optimal performance.

Of course, there are different qualities of tips… but that’s a topic that deserves an entire article on its own.

Good news: we’re planning to write one soon! Stay tuned! 😉

What is the nominal volume of an air-cushion micropipette?

You’ll often hear about the nominal volume of micropipettes. This is the volume indicated on the top of the micropipette.

• For fixed-volume micropipettes (one specific volume), it’s straightforward: there’s only one volume.
  Example: For a P1000, the nominal volume is 1000 µL.
  
• For variable-volume micropipettes, the nominal volume is the largest volume the micropipette can handle.
  Example: For a P200 (range: 20 µL to 200 µL), the nominal volume is 200 µL.

Important note: The recommended working range for a variable-volume micropipette is usually between 10% and 100% of its nominal volume. However, the closer you get to 10%, the less accurate the measurements become.

That’s why most suppliers recommend not going below 35% of the nominal volume.

For example, with a P1000, you should ideally avoid pipetting less than 350 µL to maintain accuracy.

This is the moment we all have flashbacks to our lab classes, proudly pipetting 50 µL with a P1000! 😅

The Different Types of Air-Cushion Micropipettes

The air-cushion principle applies to several types of micropipettes. Here they are:

Fixed-volume single-channel micropipette

This is the most basic type of air-cushion micropipette. It can only pipette one specific volume (but it does it very well, with great precision).

These micropipettes are available for volumes such as 20 µL, 25 µL, 50 µL, 100 µL, 200 µL, 500 µL, 1000 µL, and 2000 µL.

Variable-volume single-channel micropipette

The variable-volume single-channel micropipette can pipette only one volume at a time, but this volume can be adjusted by the microbiologist within the micropipette’s working range (from 100% to 35% of the nominal volume).

Here are the most common variable-volume micropipettes:

Model	Volumes
P2	0,2 to 2µl
P10	1 to 10µl
P20	2 to 20µl
P100	10 to 100µl
P200	20 to 200µl
P5000	1 to 5ml
P10000	1 to 10ml

Electronic Micropipette

With an electronic micropipette, aspiration and dispensing are triggered electronically. The microbiologist doesn’t need to mechanically press the piston.

This greatly helps reduce musculoskeletal disorders (MSDs) in the hand and arm. Who hasn’t experienced shoulder pain after hours of pipetting?

Electronic pipettes can also perform multiple dispensing.

For example: You can aspirate 1 mL once and then dispense 10 times 100 µL. This eliminates a lot of repetitive movements, saving time and reducing the risk of MSDs.

As with variable-volume micropipettes, the dispensed volume can be adjusted within the pipette’s working range (from 100% to 35% of the nominal volume).

Multichannel Micropipette

Unlike single-channel micropipettes, multichannel micropipettes can aspirate and dispense multiple samples at the same time. There are two main types: 8-channel and 12-channel models.

They are primarily used for high-throughput applications, such as pipetting into 96-well microplates (and beyond).

Electronic Multichannel Micropipette

Just like the mechanical multichannel micropipette, this one allows you to dispense multiple samples simultaneously (8 or 12 channels).

However, in the electronic version, the aspiration and dispensing are triggered electronically, offering greater ease of use and precision.

Conclusion

Although air-cushion micropipettes may seem robust, it’s important to remember that they are precision instruments. They must be handled with great care.

You should also be mindful of factors that can affect accuracy, such as the tip, type of liquid, temperature, piston speed, and more.

Air-cushion micropipettes are best suited for aqueous solutions at or near room temperature. For other cases, such as handling viscous, volatile, or non-aqueous liquids, positive displacement pipettes are the better choice.