**Conclusion**

In this application note we have sought to provide a direct link between a sample’s mean-free-path and the quality of DLS measurements which can be performed in one of several configurations of the LS Spectrometer. To do so we have determined the cross-correlation function magnitude (or intercept) as a function of sample mean-free-path. We have then related the intercept to DLS analysis quality in two ways: by calculating the repeatability of mean hydrodynamic radius measurements as a function of intercept; and calculating particle size distributions for two limiting cases of a very turbid and a dilute sample. We have shown that while measurement precision decreases as the cross-correlation intercept is reduced, accurate and reliable results can nonetheless be obtained for intercepts as low as ~0.05. Furthermore we have demonstrated the failure of autocorrelation measurements to produce an accurate particle size distribution for a turbid sample.

Our analysis was based on fixing measurement duration, scattered intensity, and correlation times included in the analysis. By doing so we have attempted to directly show the importance of the cross-correlation function magnitude on DLS measurements. However, we must emphasize that the overall quality of the correlation function will also depend on the aforementioned parameters [3].

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**References**

^{[1] I. D. Block and F. Scheffold, Modulated 3D cross-correlation light scattering: Improving turbid sample }

^{ characterization, Review of Scientific Instruments 81,123107 (2010).}

^{[2] Stephen W. Provencher, Inverse problems in polymer characterization: Direct analysis of polydispersity with }

^{ photon correlation spectroscopy, Macromolecular Chemistry and Physics 180 (1), 201-209 (1979). }

^{[3] Klaus Schätzel, Correlation techniques in dynamic light scattering, Applied Physics B, 42(4) (1987).}