Pipette like a pro
The pipette is one of the most widely-used pieces of lab equipment. Should you find yourself working in a biochemistry, molecular biology, biotechnology or cell biology lab, the pipette will quickly become your new extended arm.
When used well, modern pipettes allow us to quickly, accurately, and safely transfer micro-volumes of liquids. But things weren’t always this simple.
Read on to learn about the history of the pipette we know and love, how to pipette with precision, and some techniques that use the pipette.
Then put your pipetting expertise into practice by helping a food scientist test the protein content of his genetically modified corn in our Pipetting simulation.
We should count ourselves lucky
What did scientists do before the invention of the sophisticated plastic pipettes?
As recently as the late 60s, mouth-pipetting was the widely accepted way of the lab. Mouth-pipetting involves drawing up a solution into a pipette by sucking on it like a straw and transferring it to another vessel. Often single-use glass pipettes had to be individually shaped by hand each time.
On top of being much less accurate, this came with safety consequences. As you can imagine, it wasn’t uncommon to accidentally suck up the solution (often including infectious, toxic or even radioactive substances), contaminate the mouth piece with your fingers, or inhale harmful vapors.
According to a 1966 paper by the U.S. Army Biological Laboratories, the first case of infection by mouth-pipetting was recorded in 1893, when a physician sucked up a mouthful of infectious bacterial culture. Poor guy! In 1915, a survey of 57 labs reported 47 workplace-related infections, and 40% of those were a direct result of mouth-pipetting.
Mouth-pipetting of blood plasma samples at Cambridge University England in 1943 / Imperial War Museum
You owe it to German inventor, Heinrich Schnitger, that you won’t have to fear the same fate as these scientists if you work in a modern lab. Frustrated by the lack of accuracy in the results of his chromatography experiments, Heinrich designed the first piston-driven prototype with a removable plastic tip, patented in 1957.
Affordable manual pipettes then became widely available in the 70s, radically changing the daily life of many scientists. In some labs now you can even find multi-channel automated pipetting robots!
An automated pipetting robot / Andrew Alliance
The improved precision of modern micropipette has the capacity to yield more meaningful experimental results. However, the precision of the pipette is also dependent on the user. So don’t take his invention for granted, and learn the proper technique to allow you to use the pipette to its full potential.
How to pipette with precision
1. Pick the correct pipette
Pipettes are designed to transfer volumes within a given range. You should therefore find the correct pipette for the volume you are aiming to transfer. Pipetting volumes that are too low can cause you to lose precision, and pipetting volumes that are too high can damage the pipette.
Tip: Often, there’s a clue in the name. Pipettes tend to be named “P”, followed by the maximum volume they can transfer in microlitres (often displayed on top of the plunger). For example, the “P200” should not be used for transferring volumes above 200 microlitres.
2. Adjust the volume
The window with three digits on the side of the pipette displays the volume. Turn the adjustable wheel of the plunger until the volume you’re aiming for is displayed on the readout. The image below shows how to interpret the readout on three different Labster pipettes, P20, P200 and P2000, but these can vary between different pipette models.
3. Add a sterile tip
Don’t touch the pipette tip. Simply open the box of pipette tips and press the pipette firmly onto a tip to pick it up.
4. Pick up the solution
The plunger on top of your pipette has three positions. Its default position is the rest position. If you try pushing your thumb gently on the plunger until you feel resistance, you’ll find the first stop. By then pressing harder, past the point of resistance, you’ll reach the second stop.
To collect a solution, you first need to hold the plunger at the first stop. Holding the pipette vertically, dip the pipette 3-4 millimeters into the liquid. By slowly and steadily releasing the plunger back to the rest position, you should see the solution enter the tip.
5. Dispense the solution
To dispense the liquid into a new vessel, hold the pipette tip against the side of the vessel at 30-40 degrees, and then slowly and steadily push the plunger from the rest position all the way through to the second stop. Watch closely to make sure no liquid is left in the tip.
6. Eject the tip
Eject the tip into the appropriate waste bin by pressing the eject button beside the plunger. Remember to take a new sterile tip every time you pipette a new solution to avoid contamination.
Tip: A good way to test your pipetting technique and calibration is to practice by pipetting water, weighing what you dispense. One microlitre of water weighs 0.001g. If you’re pipetting is on point, you should find that the volume you set your pipette to will correspond to the weight of the water. For example, when your pipette is set to 200 microlitres, the scale should read 0.2g.
So what can I use pipetting for?
Pipetting is a key part of countless lab techniques, but here are just two common techniques requiring good pipetting skills.
Imagine someone brought you a bacterial culture, containing millions of densely packed bacteria, and tasks you with counting how many bacteria are in there.
You could start by diluting the very concentrated sample to a much more dilute sample that is easier to work with, reducing how many bacteria you need to count. Taking into account the dilution factor, you could then work out how many bacteria you started with.
However, diluting very small volumes is very difficult to do accurately. So, instead, you can do a serial dilution. This isn’t when you add milk to your cereal in the morning. A serial dilution is a series of sequential dilutions, each reducing the bacterial concentration by a set amount. For example, you might carry out three 1 in 10 dilutions instead of one 1 in 1000 dilution.
The Bradford Assay
You probably know that some foods are high in protein, and that proteins are important for a balanced diet. But if someone handed you a glass of milk, and asked you to quantify how much protein it contained, would you have any idea where to start?
One way this can be done is by using something called a Bradford Assay.
The Bradford Assay involves using a dye, Coomassie Brilliant Blue, which binds to proteins producing a blue color. Firstly, you add the dye to solutions with varying, known protein concentrations and record how much light the solutions absorb. Absorption is a measure of ‘blueness’, which corresponds to the protein concentration of the solution. Then, add the dye to your unknown sample, and record its absorbance. By comparing the absorbance of our unknown sample to the known samples, the protein concentration of the unknown can be calculated.
The best way to get a good grasp on these techniques is to try them out yourself.
Put what you have learned into practice with the Labster Pipetting simulation. In the simulation, your challenge is to help a food scientist at LabsterFood Inc. who is hoping to reduce malnutrition. Can you help him find out if the genetically modified corn cob he has made contains enough lysine, an essential amino acid, to fulfill an adult’s recommended daily dose?
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