NanoSight Pro – Size, Polysdispersity, and Span

Introduction

The diversity of particle characterization techniques, each with their own data outcomes and definitions for reporting particle size, results in confusion when employing multiple analytical tools in a laboratory. A question that commonly arises is “which size definition is the most suitable for a particular technique and which one is optimal for data comparison?” Often, the complexity of the measured system adds further difficulty to interpreting the data. 

NanoSight Pro NTA

One of the light scattering techniques that is often employed in the characterization of particle size is nanoparticle tracking analysis (NTA), which can be performed by Malvern Panalytical’s NanoSight Pro product. NanoSight Pro uses NTA to provide high resolution size and concentration data in the characterization of nano and biomaterials. NanoSight Pro delivers comprehensive insights through direct images and particle-by-particle analysis, offering an instant understanding of sample complexity and heterogeneity. The high-resolution size distribution provides detailed information, enabling users to interpret data effortlessly and extract valuable insights for informed decision-making. 

The data is presented to the user in diverse formats, including graphical representations, direct numerical values, indirect images, and video footage of particles. These various outputs obtained offer the possibility of a comprehensive data interpretation. 

Particle size definitions

Both the mode and mean values of particle size are often reported and published. The modal particle size corresponds to the most frequently occurring particle size. This represents the dominant peak in the size distribution, signifying the highest frequency of particles within the given range. The mean particle size represents the average size across the entire population/distribution. Additionally, people often consider the median particle size (also reported as D50), which signifies that 50% of the total particles are smaller than this size, or conversely, 50% are larger than this size.

D values

The particle size distribution is constructed from the combined characteristics of individual particles. It can be interpreted by examining values such as D10, D50, and D90. The D values represent the percentage of particles within the entire population that are equal to or smaller than a specific size. For example, D10 = 56 implies that 10% of particles in the entire population are 56nm or smaller, while D50 = 256 indicates that 50% of the total sample comprises particles 256nm or smaller.

Figure 1 illustrates the distinction between mode, mean and median particle size presented on the typical polydisperse NTA size distribution along with the D values: D10, D50, and D90.

The mean size often coincides with, or is very similar to, the modal size (Figure 2A) for a relatively monodisperse population. However, caution must be exercised when characterizing more polydisperse samples (commonly analysed by NTA, Figure 2B and 2C). Reporting a single size value may not sufficiently describe complex systems. Furthermore, reporting the mean size for heterogenous distributions may lead to a skewed interpretation of the sample. Reporting the modal size and size distribution range comprehensively describes unknown polydisperse samples. Furthermore, you can report all the modal sizes for visible peaks within unknown particle mixtures. This approach ensures a more thorough and nuanced understanding of the varied particle populations within the system.

Span – Polydispersity indicator

The three values—D10, D50,  D90 are often collectively used to calculate a single value known as 'span.' 

Instead of relying solely on a single point in the distribution as a specification, it is advisable to incorporate other size parameters to effectively characterize the width of the distribution. Including additional size parameters offers a more comprehensive understanding of the overall range and variability within the distribution. 

The span, calculated as (D90 – D10) / D50, is a widely used metric for quantifying the size distribution width. This calculation provides a numerical representation of the breadth of the distribution, offering valuable information about the overall spread of particle sizes. This information can be used to assess a sample polydispersity (Table 1).

This metric encapsulates the degree of consistency in particle sizes. D10, D50, D90, and the Span collectively provide a comprehensive overview of the particle size distribution, offering insights into the overall uniformity or diversity of particle sizes (Figure 3).

Summary

Together, the mode and mean values, the size distribution range, and span offer a comprehensive understanding of the particle size distribution, shedding light on both the most prevalent size and the central tendency within the sample.