Look at it from a different angle – Choosing the right scattering angle for a DLS measurement
Depending on the sample characteristics the choice of measurement angle can influence the outcome of a dynamic light scattering (DLS) measurement. Therefore, most high-end instruments offer different measurement angles, while more economic configurations are constrained to one angle.
Introduction
From the many parameters to choose before conducting a dynamic light scattering (DLS) measurement, one of the most important is the angle of detection. State of the art devices offer the selection of three angles: back angle (e.g. 175°), side angle (90°) and forward angle (e.g. 15°). While forward angle is used for special applications and typically in comparison with side- or backscattering results, the choice between side and back angle can be more complex. In this application report we discuss the different parameters, which have to be taken into consideration, in order to make a mindful and correct choice.
Historically, the first dynamic light scattering configuration was realized in side angle. Due to the lack of moving parts, it had the advantage, that it didn´t require fine mechanics. Since this alignment became the official recommendation of the first ISO Standard for Photon correlation spectroscopy (ISO 13321:1996), it quickly became also the standard configuration. However, the physical model behind dynamic light scattering can only address a single scattering event. This means, that the incident laser light must be scattered only once before the scattered photon reaches the detector without further influences. As in side angle the laser path inside the cuvette is relatively long, this was ensured by strongly diluting the samples, which was also clearly stated in the same ISO standard.
In some applications however, a dilution is not desired as it may change the characteristic of the sample, therefore novel DLS configurations were developed, which are able to determine the particle size in a larger concentration range.
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