Methods of Measuring Molecular Shape, Weight and Size

Who wants to measure molecules?

The answer is a surprisingly broad range of people and companies. Molecular weight and size is most often used in identification of proteins, and in characterisation of polymers.

Molecular Weight is commonly measured in industries such as::

• Pharmaceutical industry
• Biotech research
• Medical analysis
• The food industry
• Petroleum and polymer industries

Measurement methods for molecules

Molecules are not simple shapes – unlike the fairly compliant, mathematically-uncomplicated atom, which is spherical, molecules are difficult to define as a simple shape. Hence, two measurements have become industry standards – the radius of gyration and the hydrodynamic radius – and analytical instruments are geared towards obtaining these.

Radius of gyration

Scientific instruments can be used to identify the centre of mass and dimensions of a molecule; this is its radius of gyration. It is measured directly using static light scattering; however this method has limitations at sizes lower than 10-15nm, and also for large molecules like polysaccharides. Viscometers are the tool which is most often used for determining radius of gyration, using the Flory-Fox equation.

Hydrodynamic radius

This is a behavioural property of a substance – a measurement of size based on how it moves, which means that it is much more useful in industrial or practical settings.

Size exclusion chromatography

Size exclusion chromatography is the method by which scientists determine the molecular size (not the weight!), of a particular substance.  Molecules are separated in columns packed with porous substances, which might include glass beads, polystyrene gels, silica gel, etc. Larger molecules elute more quickly through the columns, since the molecules cannot fit into as many spaces.    A concentration detector is placed at the bottom of the GPC columns to determine the amount of material of each size fraction. In traditional SEC/GPC systems, operators need to pass known standards through the columns before the sample.  By creating a calibration curve of size versus elution time, particle size of unknown samples can be calculated.

In more recent times, addition of other detection techniques such as Static Light Scattering and the Intrinsic Viscosity detector provide direct measurement of molecular weight and size so that traditional calibration techniques are not required.

Gel permeation chromatography

SEC is also commonly referred to by other names. When an aqueous solution is used to carry the sample through the column, the technique is often called “Gel Filtration Chromatography”, and the name “Gel Permeation Chromatography” is often used when solvents are used to carry the substance in question through the packed columns.

Get the right instrument

Using the right method to measure molecules is very important, just as using the right instrument for measuring molecules is essential too. ATA Scientific offers a range of scientific instruments, including the Malvern Omnisec – Advanced Multi-Detection SEC/GPC, that can assist you in undertaking your measurements, so contact us today to find the instrument you need.