A Complete Pipette Guide

Pipettes Types

There are many different types of pipettes, each with their own distinct features and most suitable applications. Some pipette types are ideal for more general use, whereas others are highly specialised for specific uses or industries.

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

Volumetric Pipettes

Volume pipettes allow for the measurement of a solution with a particularly high level of accuracy. Typically featuring a large bulb complete with a narrow long portion and single graduation mark, they are calibrated for a specific volume. Their highly accurate measurements are usually present in ml (millilitres).

Graduated Pipettes

The volume of a graduated pipette is marked in defined increments along the tubing. You can also be confident of precision when transferring liquid with this type. 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. However, larger versions may be preferred where the level of accuracy is less important.

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

Specialist Pipettes

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

• Ostwald-Folin - These are particularly suitable for the measurement of viscous fluids, including whole blood. They are commonly found in medical laboratories

• Semi-Automatic - These enable the electronic transfer of fluid, minimising the need for manual labour involved with repetitive pipetting

• Van Slyke - These graduated pipettes have traditionally been used for medical applications. They feature serological pipettes for volumetric analysis

• Microfluidic Pipette - These recently developed models feature a localised flow zone at the tip and enable control of the nanoliter environment in front of the instrument

How Accurate is a Pipette?

The accuracy of a pipette will depend largely on the type in question. The level of accuracy will be dependent on the degree to which the delivered volume matches with the volume specified on the instrument. The condition of equipment and use of 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 wider volume range for a true idea of measuring accuracy.

In terms of the different types, volumetric pipette accuracy is generally considered to be the best. They typically provide highly accurate measurements with only a tiny margin of error. Graduated pipette accuracy is usually also high but if accuracy is essential, a volumetric model will almost always be best.

You should be aware that there is a distinct difference between an accurate pipette and a precise one. 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 pipetting.

How Do You Make a Pipette More Accurate?

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

• Check for any signs of damage prior to each use

• Clean following use

• Use a dedicated holder for vertical storage and the prevention of corrosion

• Follow the appropriate pipetting technique

• Pre-wet the tip of the pipette, removing any residue

• Use the reverse pipetting technique when working with viscous liquids or volatile solvents

• Use the largest volumes possible

Pipette Sizes and Measurement

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

A small pipette is often preferred over a large pipette to maintain greater precision - quantity permitting. This is why micropipettes are ideal for precise measurements of small volumes.

It may be necessary to calculate the measurement of volume with a basic graduated pipette. As an example, you might be required to draw 8ml of water into the instrument. You may then fill it 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. If using a serological pipette, then you may insert a 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.

What is a Pipette Used for?

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

Materials

Many of these instruments 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 pipette plastic is either polyethylene (PE) or polypropylene (PP), with the appropriate choice depending on factors such as the physical properties of the liquid and the volume that is to be transferred.

FAQs

How Do You Stop Air Bubbles in a Pipette?

There will be a risk of air and bubbles being drawn in or dispensed into the wells if 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 but not as far as the second 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

Where Can I Buy a Pipette?

RS Components has a wide range on offer. This includes a variety of different types, sizes, materials and capacities, providing greater flexibility when it comes to choosing the best product for your requirements.

Click through below to buy online or browse our wider range of lab equipment.