Pipettes - A Complete Buying Guide

What is a Pipette?

The pipette, sometimes spelt as pipet, has been used in various forms since the late 19th-century primarily to reduce contamination in the transfer of liquid samples.

It wasn’t until the 1950s that the modern version of the scientific pipette was patented. Today, the majority of different pipette types work in a similar way, although the quantity of liquid and measurement accuracy varies between each type. Many modern pipettes work by creating a partial vacuum above the chamber where the liquid is held. This vacuum is then selectively released to draw and dispense liquid from the chamber.

Pipettes are now used by chemists, biologists, and medical experts all over the world. There are various types of pipettes, each offering different levels of accuracy and precision.

How to Use a Pipette

In its most basic form, you could compare the use of a pipette with a straw. It involves the creation of a pressure gradient and either pulling or pushing liquid. The transfer of liquid was initially achieved through the application of mouth pipetting. However, a range of tools has subsequently been developed, allowing for the effective suction and movement of fluids. Roller pumps and squeezy pumps are now attached to glass or plastic pipettes so that the liquid can be measured in increments. It is important that you know the scale to which liquid is measured in the pipette being used.

It is quite common to hold a pipette in a similar manner to a syringe. However, this method doesn’t allow for the desired level of comfort or flexibility. Instead, you should take hold of the pipette with the lower guard positioned directly above your index finger.

Different Types of Pipettes

Many pipettes are traditionally made from glass. However, there has been an increase in the manufacture and use of plastic pipettes, which are particularly well suited to applications where a defined quantity of liquid isn’t required. Modern pipettes feature either polyethylene (PE) or polypropylene (PP) tubing, with the appropriate choice depending on factors such as the physical properties of the liquid and the volume that is to be transferred.

Manual pipettes come complete with squeezable bulbs or pumps that draw the liquid into the tubing. Such devices are also found on semi-automatic or electronic pipettes. Bear in mind that you should try to choose a pipette with a maximum volume capacity that is closely matched to the quantity of liquid that you want to transfer.

This table provides further detail and examines some common uses of the main pipette varieties:

Graduated Pipettes

The liquid volume of a graduated pipette is marked in defined increments along the tubing. You can also be confident of precision when transferring liquid with these pipettes. There must be a source of vacuum for the effective operation of the graduated pipette. They may come complete with either plastic or glass tubing and will feature a tapered tip. A small pipette is typically advised for the accurate measurement of small quantities of liquid. However, larger pipettes may be preferred where the level of accuracy is less important.

There are three types of graduated pipettes. Type 1 and type 3 pipettes feature a nominal value at the bottom (zero at the top). Type 1 pipettes also allow for the partial delivery of liquid in a range of volumes. Type 3 pipettes allow exclusively for transfer of liquid at the nominal value. The nominal value can be found at the bottom of the type 2 pipette, with the option of delivering the solution for any volume.

Specialist Pipettes

In addition to the types of pipette described above, there are also a number of specialist pipettes available for particular applications. Some examples of specialist pipettes include:

• Ostwald-Folin Pipettes - These pipettes are particularly suitable for the measurement of viscous fluids, including whole blood. They are commonly found in medical laboratories
• Semi-Automatic Pipettes - These pipettes enable the electronic transfer of fluid, minimising the need for manual labour involved with repetitive pipetting
• Van Slyke Pipettes - These graduated pipettes have traditionally been used for medical applications. They feature serological pipettes for volumetric analysis
• Glass Micropipettes - These pipettes are used for physical interaction with microscopic samples as necessary in the processes of microinjection and patch clamping. They are commonly made from borosilicate, aluminosilicate or quartz, with each being suitable for specific applications
• Microfluidic Pipettes - These recently developed pipettes feature a localised flow zone at the tip and enable control of the nanoliter environment in front of the pipette

FAQs

How Accurate are Pipettes?

The accuracy of a particular pipette will depend largely on the type of pipette in question. The level of accuracy will be dependent on the degree to which the delivered volume matches with the volume specified on the pipette. The condition of equipment and use of the pipetting technique will also have some impact on accuracy.

In the majority of instances, discrepancies in accuracy will be relatively minor and insignificant. However, you should be aware that there will be a risk of major inaccuracies when performing pipetting tasks such as the measuring of dilution for sensitive techniques such as HPLC.

The process of measuring pipetting accuracy is quite straightforward. Simply pipette the same volume of water ten times into a tared balance, keeping a record of the weight in each instance. You should then proceed to measure the relative standard deviation in the dispensed masses through the calculation of the standard deviation which will be expressed as a percentage of the average. This process should be repeated for a range of volumes for a true idea of measuring accuracy.

You should be aware that there is a distinct difference between pipetting accuracy and precision. The latter is specific to the closeness of different measurements to each other instead of a standard value. The mean volume should be the same as the set volume, with no variation between different pipettings.

How Do You Measure a Pipette?

The measuring process will differ depending on the type of pipette and the amount of liquid that you are measuring. The graduated pipette is typically used for the measurement of liquid volumes up to 50ml. A graduated cylinder is commonly used for the measurement of larger liquid quantities. You may also use a micropipette for measuring small volumes.

It may be necessary to calculate the measurement of liquid volume with a basic graduated pipette. As an example, you might be required to draw 8ml of water into the pipette. You may then fill the pipette to the zero line and dispense the water until the 8ml line is reached. However, this will result in an additional 2ml of water being present in the pipette. If using a serological pipette, then you may insert a liquid solution up to the 2ml point and dispense to the tip without any waste. Reference may also be made to a reverse scale for measuring convenience.

How Do You Make a Pipette More Accurate?

The following methods may be used in order to increase the accuracy of a pipette:

• Check the pipette for any signs of damage prior to each use
• Clean the pipette following use
• Use a pipette holder for vertical storage and the prevention of corrosion
• Follow the appropriate pipetting technique
• Pre-wet the pipette tip, removing any liquid residue
• Use the reverse pipetting technique when working with viscous liquids or volatile solvents
• Use the largest volumes possible

How Do You Stop Air Bubbles in a Pipette?

There will be a risk of air and bubbles being drawn into the pipette or dispensed into the wells if particular care isn’t taken. The build-up of bubbles can have a bearing on the optical density values and results. The reverse pipetting technique is recommended for the avoidance of such issues, particularly in relation to the transfer of highly viscous liquids such as 20% glycerol and blood serum.

The following steps should be followed for effective reverse pipetting:

• Push the plunger slightly past the first stop
• Insert the pipette tip into the liquid and ensure the smooth transfer of liquid to the top
• Dispense the liquid to the first stop, leaving any residue in the tip
• Repeat the pipetting procedure or leave the residue in the tip for discarding as necessary