The measurement of zeta potential in the NANOTRAC WAVE II particle size analyzer takes advantage of the same Frequency Power Spectrum methodology used for measuring nanoparticle size distributions. The same stable optics sample interface means no adjustments are required. The backscatter and laser amplified detection signals are collected as in the size measurement, and the rapid sequencing of applied electric fields prevents electroosmosis. The optical probe surface is coated to provide electrical contact with the sample. Two probes are used, one to determine the polarity of the particle charge at the slipping plane and one to measure the mobility of the particles in an electric field. Polarity is measured in a pulsed electric field, while mobility is measured in a high frequency sine wave electric field excitation. The zeta cell has two detection probes, on opposite sides, to detect polarity and mobility.

From the linear frequency power spectrum distribution (PSD), the Loading Index (LI), which is proportional to particle concentration, can be calculated. Loading Index values provide a single number for total scattering that can be used to determine particle mobility in microns / sec / volt / cm and particle polarity as + / -, positive or negative.

Measuring mobility and zeta potential begins by measuring the PSD and determining the LI with the excitation off. Then the PSD is measured with the high frequency sine wave on and a ratio is taken. Polarity is determined by measuring the LI before and after pulsed DC excitation. A ratio of LI after the excitation divided by LI before excitation of less than one is a positive polarity (concentration decreasing) and a ratio greater than one is negative (concentration increasing) for a positively charged probe surface.