The "derived count rate" is a calculated parameter in the Zetasizer Nano software, and is representative of the scattering intensity that would be measured at the APD in the absence of the laser attenuation filter. The APD detector in the Zetasizer Nano exhibits a linear response across a broad range of light intensity. There is an upper threshold however, beyond which the detector response becomes non-linear, eventually reaching saturation. In the dynamic light scattering (DLS) technique, time dependent fluctuations in the scattering intensity are used to calculate the distribution of diffusion coefficients of the particles in the dispersion. As such, it is important that the measurement be conducted under conditions where the detector exhibits optimal "sensitivity" to light intensity fluctuations. For samples exhibiting large scattering signals, it is often necessary to reduce the amount of light reaching the detector in order to keep the signal within the linear range of the APD.
In the Zetasizer Nano system, the intensity of the scattered light detected is optimized by attenuating the laser beam prior to entering the sample. This is done through use of a number of attenuators (light filters) located within the optical path between the laser and the sample. Filter (or attenuation) selection is automatically adjusted by the software to ensure that the scattering intensity is within the optimal range of the APD.
The unique optical configuration of the Zetasizer Nano system facilitates size and zeta potential measurements across a broad range of particle sizes and sample concentrations. As a consequence of these enhanced capabilities,researchers can now compare sample and charge properties under both dilute and concentrated conditions. It is here that the derived count rate becomes important, since a meaningful comparison can only be conducted if the attenuation effects of the light filters are normalized. As stated above, the derived count rate is the scattering intensity that would be measured at the APD in the absence of a laser light attenuation filter. Within the Zetasizer Nano there are 11 attenuation settings, with typical attenuation factors given in the table below. The derived count rate is the measured count rate divided by the attenuation factor, as shown in the expression below.
Derived Count Rate = (Measured Count Rate) / (Attenuation Factor)
The derived count rate, attenuator setting, and attenuation factor are not displayed in the Zetasizer software by default. However, all three, along with a multitude of other relevant parameters, can easily by displayed by configuring the user workspace.
