What is DLS Dynamic Light Scattering?
Dynamic Light Scattering measures sizes, often leading to terms such as 90° DLS, 173° NIBS, and backscatter DLS.
The Zetasizer Lab (NANO S90) device uses 90° dynamic light scattering, while Zetasizer Pro (NANO ZS) employs non-invasive back scatter (NIBS) at 173° optical configuration.
“Can both systems yield identical results?”
The answer to this question is that results are sometimes consistent and sometimes not.
The answer varies based on the sample being tested and the distribution observed. In principle, the intensity distribution can vary, but the volume or mass distribution should be the same (certain conditions may exist where this is not the case due to Mie scattering).
This can be a tricky concept to understand, so I will do my best to explain it here.
First, aside from having a larger scattering angle, the non-invasive back scatter (NIBS) system accommodates a much wider range of concentrations than the 90° system, meaning more concentrated samples can be measured with the NIBS system while yielding results consistent with the 90° system.
Since DLS dynamic light scattering is inherently a low-resolution technique, its ability to resolve particles of different sizes in a mixture is limited. The ability to distinguish between different size modes in a sample depends on several factors, including the relative size of the different populations present in the sample, the relative intensity of scattering by the different populations present in the sample, the polydispersity of the individual size distributions, the quality of sample preparation, and the quality of the data.
Measurements were conducted on mixtures of latex standards at diluted concentrations at 90° and 173°, using equipment capable of both DLS90 and NIBS.
Experiment
All latex standards used in the measurements were obtained from Duke Scientific in Palo Alto, California, and were traceable to NIST in Gaithersburg, Maryland. These are standards of 60nm and 220nm. All standards were supplied at 1% w/v concentration.
Results
Duke Scientific 60nm latex standards were prepared by taking 100μl of the sample and making it up to 40 ml with 10mM NaCl, providing a stock concentration of 0.0025% w/v. The stock 220nm latex concentration was 0.00025% w/v (10μl of sample was made up to 40 ml with 10mM NaCl). These two stock suspensions were mixed in various ratios as indicated in Table 1, and measured at 173° and 90°.
The intensity and volume size distributions obtained for different volume ratio mixtures at the two detection angles 90° (red plot) and 173° (blue plot) were superimposed for comparison. It can be observed that the intensity size distribution obtained for a particular volume ratio mixture is significantly different at each angle. This is expected because larger 220nm particles scatter more light when the sample is measured at the forward 90° angle compared to the backward 173° angle.
Figure 1: Intensity and volume size distribution of 1.1:1 (60:220nm latex standards) volume mixtures as measured at 90° (red) and 173° (blue).
Graphs of more diverse volume mixtures can be found in the following application note.
Download materials (English)
Conclusion
While dynamic light scattering is a low-resolution technique, the results presented in this application note demonstrate the ability to monitor changes in the relative concentration of each size population. Measurements performed on diluted latex standards at two different detection angles indicated that different size distributions in terms of intensity are anticipated for particles exhibiting angle dependence in scattering. However, using Mie theory to convert intensity data to volume allows for obtaining angle-independent distributions within a relative concentration range.
Read more materials
Differences between NIBS (Non-Invasive Back Scatter) and Backscatter
Reproducibility of Simple Backscatter and 90° Dynamic Light Scattering
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