Latex Standards and the Measurement of Latex Standards by Photon Correlation Spectroscopy

Polymer latex spheres are very commonly used to verify the operation of particle size analyzers. This is because they are available as monosize dispersions of near perfect spheres. The sphere is the only three-dimensional shape whose size can be unambiguously described by a single figure and being monodisperse removes any uncertainty regarding the calculation of a mean size. Polymer latices have other benefits. They have a similar density to water, so particles less than 1 micron will remain in suspension during measurement. Dispersions can be stored at room temperature and have storage lifetimes of months or years.

Monodisperse Lattices for Calibration

A wide range of monodisperse polystyrene lattices are available from a variety of manufacturers, however not all are supplied with an individual calibration certificate. Duke scientific corporation is one manufacturer that supplies a calibration certificate with each sample, measured by transmission electron microscope, (TEM) which is traceable to NIST certified microspheres. The specification for the standards also includes a hydrodynamic diameter measured by photon correlation spectroscopy (PCS).

Standards for Photon Correlation Spectroscopy

Standards suitable for PCS are available from 20 nm to 900 nm. The easiest sizes to measure are in the range 60 nm to 300 nm. Particles larger than 60 nm are large enough to give very reproducible results with low power lasers at suitable dilutions for PCS. Particles larger than 300 nm start to show a marked variation in scattering intensity with angle, measuring standards smaller than this removes the requirement to consider the angle.

Certified Size

The result quoted on the Duke latex bottle is the certified TEM result. The PCS result is quoted in the specification sheet and is not a certified value. Table 1 lists a few comparisons of these two figures. For all Duke standards the size accuracy by PCS should be within the specified PCS range ±2% for samples prepared in a 1mM salt such as NaCl or KNO₃. This figure should account for uncertainties in sample preparation. Measurement precision should be ±1% or better. The peak width is expressed as the polydispersity. For polydispersities less than 0.2, this is equivalent to the variance of the distribution. A standard latex correctly dispersed should have a polydispersity less than 0.03.

Table 1. Quoted sizes from Duke Scientific Corp. Polystyrene latex standards specification sheets.

Comparison of Sizes Measured by TEM and PCS

It is easy to forget that different measurement techniques measure different properties of a particle and so can give different results. The question often arises, which is the correct result?

Electron Microscopy versus Photon Correlation Spectroscopy

For many people ‘seeing is believing’ so the electron microscope result is ‘correct’. In fact, samples prepared for electron microscope examination are often harshly treated, this treatment can distort soft materials such as polymer lattices and change or mask surface structures. It can make the size measurement of some types of materials like surfactant micelles impossible. PCS in contrast measures the Hydrodynamic diameter of dispersed particles in their native environment.

Factors that can Affect the Measured Size or a Polymer

Any surface structure such as a ‘hairy’ surface made up of polymer chains, or a change in the electrical double layer that affects the Brownian motion of the particle, will change the effective particle size. Increasing the surface structure or extending the double electric layer by using a very low salt dispersant, will reduce Brownian motion and increase the measured size.

For these reasons, the hydrodynamic size or PCS size of particles that are not smooth hard spheres, is usually larger than the TEM size.

Sample Preparation For Measurement By PCS

All latex standards are supplied at a concentration that is too high for PCS measurement, typically 2% w/v. The latex dispersion should be diluted with distilled, or preferably demineralised water filtered down to 0.2 microns.

Measurements in Salt Solutions

The final concentration should be such that the result is independent of the actual concentration, 0.002%w/v is a good guide, but the optimum concentration will be size dependent. A practical criterion, if a spectrophotometer is available, is that the optical density in a 1cm cell should be less than 0.04. If a size is required that more nearly matches the microscope result, a dilute simple salt such as NaCl or KCl should be used instead of pure demineralised water. This will compress the double electric layer and reduce the effective size. Table 2 shows a comparison of sizes in different salt concentrations.

Table 2. Comparison of measured sizes in various salt solutions.

The 105 nm latex was prepared at 0.001% w/v, the 220 nm latex at 0.0005% w/v.

All measurements were done at an angle of 90° and a temperature of 28°C with a 488 nm laser set at 30 mW. The analysis time was 300 s. The analysis was set to monomodal.

Troubleshooting

There are some simple settings to check if the result obtained is not as expected.

Temperature Equilibration and Calibration

If the sample temperature is input or read incorrectly the viscosity will be calculated incorrectly and the reported size will be wrong. For aqueous systems such a latex dispersion, at 20 °C a 1 degree error in the temperature will result in a 2.4% error in the viscosity used and therefore a 2.4% error in the calculated size.

Temperature equilibration can take several minutes if the sample temperature has to change by more than 2 or 3 degrees. Doing several measurements is a good check that the temperature is stable.

Viscosity

This is calculated from the temperature for aqueous systems. To ensure this calculation is being done, enter 0 (zero) for the viscosity on the measure document page.

Dispersant Refractive Index

The refractive index of the continuous phase, 1.330 for pure water.

Wavelength

The wavelength of the laser used in nanometers. 633 for a Helium Neon laser, generally 488 or 515 for an Argon ion laser.

Angle

For a variable angle system, check that the angle displayed on the measure document page is the same as the actual angle of the spectrometer.

Measurement Time

Reasonable results can be obtained in a few seconds, but a measurement time of over 100 seconds will ensure automatic dust rejection during data collection and therefore a more reproducible result. Setting the measurement time to auto will ensure sufficient quality data is collected for a reliable measurement of standard latices.

Summary

Conclusions

• Measure latex standards in a 10 mM salt solution, such as sodium chloride in dematerialized water, rather than in pure water.
• Dilute the samples on the day of measurement.
• Determine the correct sample concentration for each sample and set of measurement conditions.
• Set all measurement parameters to ‘Auto’.

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

For more information please visit Malvern Panalytical.