From skim milk to cosmetic products to semiconductors, the importance of understanding and characterizing nanometer-sized materials and their applications has never been more necessary. Among the various techniques and applications used to characterize materials on the nanometer scale, Dynamic Light Scattering (DLS) stands out as one of the more popular techniques. Serving many uses in many different industries, DLS is a technique that measures the intensity of light scattered from particles freely diffusing in Brownian motion and does so over the course of microseconds to calculate particle size using the Stokes-Einstein equation. As such, the data that is directly calculated and generated is data that is weighted based on the intensity of light scattered. In the Rayleigh approximation region, a theory that is applied to the measurement of particle sizing in the sub-micrometer region, particle scatter light is directly proportional to particle size.
More specifically, particles scatter light at a d6 proportionality, d referring to particle diameter, meaning that a singular 10 nm particle scatters as much light as 1 million 1 nm particles. Accordingly, a critical factor in a DLS measurement is the concentration of particles in a suspension, which is proportional to the signal that is received by the instrument’s detectors. For further information on the theory and applications of DLS, see PTL’s blog post on Dynamic Light Scattering as well as a blog post on issues experienced when conducting a measurement.
Method Optimization
Just as important as selecting the proper technique to adequately characterize materials on the nanometer scale is the methodology that is employed using that technique. Optimizing a method, which has many different aspects of focus, is the key to properly analyzing materials and obtaining truly representative and reliable results. At PTL, the creation and optimization of a method is broken into a series of stages. If a method is not sent in, a stage 1 method development will be done, where method parameters are established, which is then followed by a design of experiments testing plan that is implemented to evaluate these multiple factors and the impact of changes to those factors on the results. While it is entirely dependent on the sample type, typically, one of the most critical factors for investigation on DLS is linearity.
Linearity in Depth
Linearity can be described as the proportional relationship between the concentration of a sample and the reliability and reproducibility of the results; in layman’s terms, establishing the linear range of a sample would be to, through analyzing varying concentrations of a sample, establish a concentration range in which the results are reproducible and reliable within an expected range of variance of the method. For DLS, there are two ends to the linear concentration range with two different outcomes indicating deviation outside of that range. If a sample is too dilute, typically the signal-to-noise ratio is rather high, meaning that, in the absence of enough signal that would otherwise be contributed by sample particle light scatter, the instrument may be susceptible to using background signal/noise as actual results, which would typically skew the particle size on the upper/larger end. Conversely, if a sample is too concentrated, a phenomenon known as a multiple-particle scattering event may occur, where light reflected from one particle gets reflected by another particle due to crowding, which would typically skew the results on the lower/smaller end but may also skew the results on the higher/larger end as well in some instances.
Non-Invasive Backscatter
When NIBS is employed, DLS truly becomes a great investigative tool in addition to being an excellent quality control tool. The benefits of NIBS are two-fold. If particle size of a sample is not known, NIBS is a great tool in giving a rough estimate as to what side of the particle distribution the sample is on; for example, if the predominant population of a sample is on the larger end, it is expected that NIBS will pull back the measurement position to not swamp the detector with the intense light scatter from those larger particles, and, if a sample is on the smaller end of the distribution, NIBS will increase the measurement position to maximize light scatter received by the detector.
If a predicted particle size is known, NIBS saves vast amounts of time and energy that would be otherwise spent on adjusting laser attenuation and measurement position by using the best measurement position in a cuvette in order to optimize the amount of signal the instrument is receiving. Through the analysis of several aliquots at varying concentrations, which is what a linearity study would involve, the use of NIBS in conjunction with statistical analysis of the results makes an otherwise time-consuming and labor-intensive process quicker and more efficient. By saving time and effort aided by NIBS, method development work on DLS can then be streamlined, and more time and effort can be reserved for equally important aspects of developing a method: gauging robustness through randomizing analysis/sample preparation settings and, further along, gauging the precision of a method.
When it comes to establishing a quality method on DLS, linearity as a baseline is key! Contact us today to find out what DLS can do for you!
By Cesar Estrada, Particle Characterization Chemist III.
