Molecular weight

Looking at apparent molar mass

Molecular weight,
what is measured?

The measured quantity is the molecular weight, also referred to as molar mass, of macromolecules or proteins in dilute solutions. This characterization is only possible if the scattered intensities are expressed in absolute units.

Range of molecular weight accessible from SAXS measurement:

  • 1 to 300 kDa
Molecular weight
Figure 1. Scattering data acquired on lyzosyme in acqueous solution (top) and the corresponding Guinier fit of the low q data points (bottom).

In the absence of aggregation effects, a simple analysis allows one to estimate the mean size of the scatterers, supposedly similar in shape and size. In the low q range, the scattering of globular objects is well described by the Guinier law.


Typical samples for this measurement are:

  • Globular nano-objects in solution, in particular globular proteins

Methods & standards

Standards & methods used for molecular weight determination:

  • Guinier analysis
    Ref: A. Guinier, G. Fournet Small-Angle Scattering of X-rays. John Wiley, New York, 1955

XSACT analysis software implements the above method.

Examples of customer publications making use of molecular weight determination with SAXS:

SAXS Data Analysis

Why use SAXS for molecular weight determination ?

Advantages of SAXS for molecular weight determination:

  • SAXS is a non-destructive method, allowing to study samples in their bio-chemical environment.

  • SAXS does not require a reference protein for measurements.

  • Molecular weight measurements require absolute intensity measurement of scattering. With Xenocs SAXS equipment, this is possible without the need for intensity calibration.

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How To measure Apparent Molar Mass With SAXS

Macromolecules Or Protein Solution Samples. Methods, Standards, Advantages, Learn On SAXS Characterization