Particle Size Analysis of Polystyrene Latex Standard Using Dynamic Light Scattering

By AZoNano

Table of Contents

Overview
Introduction
Manufacturer Characterization of PSL
Sample Preparation
Materials and Methods
Results and Discussion
Conclusion
About Horiba

Overview

Dynamic light scattering offers quick, precise and repeatable nanoparticle size information. Applications include latexes, metal and oxide powders, drug delivery vehicles, and a large number of other materials. The HORIBA SZ-100 is a dynamic light scattering instrument that is perfect for these nanoparticle samples. In this application note, standard materials are characterized with the SZ-100 in order to prove the precision of the technique.

Introduction

Dynamic light scattering (DLS) is a preferred method for studying particles in the nano size range. Characteristics of the technique include:

  • Rapid measurements, typically taking just a few minutes
  • High repeatability
  • Coefficient of variation on the z-average size is higher than 5% for a large number of samples
  • High accuracy and is able to discern shifts in the z-average size of only a few percent

Polystyrene latex (PSL) beads are well-known size standards for particle analysis as they are well characterized, readily available, and relatively economical. Thus, they are often used for instrument qualification. This note shows the ability of the HORIBA SZ-100 to characterize these samples.

Figure 1. SZ-100 Nanoparticle Size Analyzer.

Manufacturer Characterization of PSL

Submicron latex beads are normally characterized using a range of techniques that include electron microscopy, dynamic light scattering and disc centrifugation or sedimentation. As these materials are spherical and have a very narrow size distribution, the results from a large number of characterization techniques are quite similar, often differing by less than 10%. This is significant since such similar results are not observed for many practical samples.

Non-spherical particles and broad size distributions seem to have a range of sizes when measured using different particle sizing techniques. By combining these differences it is possible to give the analyst a total picture of the sample. An essential consideration when using PSL standards to evaluate an operator, instrument, or laboratory is the method used to study particle size of the reference material. For instance, the certified mean size obtained for a nominal 100 nm PSL standard is 102 nm +/- 3 nm. However, the hydrodynamic size measured by DLS and also presented on the certificate of analysis is 95 to106 nm. The difference arises since the certified size is determined using a range of techniques while the hydrodynamic size is obtained only by dynamic light scattering. In order to precisely determine results from measuring a standard, it is important to read the manufacturer certificate of analysis and use the exact values, that is, DLS results for evaluation.

Sample Preparation

PSL suspensions are normally electrostatically stabilized. The use of charged surfactants, ensures that the particles do not begin to flocculate. This considerably increases the material shelf life for the end user. However, the Stokes-Einstein relation used to convert dynamic light scattering data to particle size rests on the assumption of free diffusion. In other words, if the particles interact strongly, DLS does not provide the correct size. Therefore, PSL latex concentrates are diluted with 10 mM 1:1 electrolyte such as NaCl.

Materials and Methods

Nominal 100 nm (part number #3100A, lot #36489) and 20 nm (part number #3020A, lot #35820) polystyrene latex spheres were purchased from Thermo Scientific. Concentrated PSL from the manufacturer was diluted with filtered aqueous 10 mM NaCl. Dynamic light scattering data was collected and analyzed with an SZ-100 nanoparticle size analyzer. Measurements were repeated six times in order to determine the coefficient of variation or the standard deviation of the six measurements over the mean of the six measurements.

Table 1. Measurement results for nominal 100 nm PSL

Mean determined z-average size (nm) CoV
Hydrodynamic size on manufacturer certificate 95-106 N/A
15 ppm 105.8 0.7%
100 ppm 105.8 1.5%

Figure 2. PSL dispersed on PSA300 slide.

The nanoparticles discussed in this note cannot be consistently imaged like the larger particles shown in this image. Therefore, DLS is an important method for their characterization.

Results and Discussion

The z-average diameter range stated by the manufacturer and the values obtained with the SZ-100 are listed in tables 1 and 2. The agreement between the two is excellent.

Table 2. Measurement results for nominal 20 nm PSL

Mean determined z-average size (nm) CoV
Hydrodynamic size on manufacturer certificate 20-22 N/A
15 ppm 22 6.4%
100 ppm 21 1.5%

Conclusion

The results of these measurements show that the SZ-100 can be validated with PSL latex standards.

About Horiba

HORIBA Scientific is the new global team created to better meet customers’ present and future needs by integrating the scientific market expertise and resources of HORIBA. HORIBA Scientific offerings encompass elemental analysis, fluorescence, forensics, GDS, ICP, particle characterization, Raman, spectral ellipsometry, sulfur-in-oil, water quality, and XRF. Prominent absorbed brands include Jobin Yvon, Glen Spectra, IBH, SPEX, Instruments S.A, ISA, Dilor, Sofie, SLM, and Beta Scientific. By combining the strengths of the research, development, applications, sales, service and support organizations of all, HORIBA Scientific offers researchers the best products and solutions while expanding our superior service and support with a truly global network.

This information has been sourced, reviewed and adapted from materials provided by Horiba.

For more information on this source, please visit Horiba.

Date Added: Oct 27, 2011 | Updated: Jan 16, 2014
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