Molecular weight

Looking at apparent molar mass

What is measured?

The measured quantity is the molecular weight of macromolecules or protein solution samples.

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.

SAXS Data Analysis

Advantages of SAXS

Why use 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.


All these measurements are possible directly in your lab.


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Molecular weight

Molecular Weight – How To measure Apparent Molar Mass With SAXS

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