When performing electrophoretic light scattering (ELS) measurements, the sample is exposed to an externally applied electric field, and we observe the extra movement that the particles undergo as a result of the applied field with the changes in the phase difference between the un-scattered reference laser beam and the scattering from the particles. The electrophoretic mobility µ is the particle velocity v divided by the electric field strength E, where the field strength is the applied voltage over the distance between the electrodes [typically referenced in units of Volts/cm]. We can express this in an equation as:
where the voltage is the applied voltage at the electrodes and the distance is the spacing of the electrodes. Fortunately, these data are recorded with every Zetasizer .dts data file, so it is just a matter of finding the correct values to compute the field strength which was applied in a specific measurement record.
What is the distance between the electrodes?
The spacing between electrodes depends on the specific setup of the measurement cell. Different cuvette types have different electrode spacing. The disposable capillary cell has a spacing of 6.1cm whereas the universal dip cell has a spacing of 2.1 mm. The actual spacing for the cuvette used during a measurement is recorded under
- Record View Parameters – Measurement – SOP – SOP Measurement – Electrode Spacing
- Record View Parameters – Measurement – Zeta – Measured Voltage
and both can easily be displayed in the records view by editing the desired workspace.
How to display the field strength in a report?
This is a little more complicated and can be achieved with a custom calculation, utilizing the parameters:
- Measurement -> SOP -> Measurement -> ElectrodeSpacing and Measurement -> Zeta -> EffectiveVoltage from within Report Designer. I have added the field strength to the attached report named “Zeta Mono with Field Strength” to spare you the details.
You can download the zipped report Zeta-with-Field-Strength and save it to the folder C:\Users\login_name\Documents\Malvern Instruments\Zetasizer\Reports\en-US\Zeta and then enable it in a workspace to use with your software.
NB: you can also customize this report with the report designer tool. The screenshot below shows the field strength of a typical zeta transfer standard measurement, displayed in the customer report “Zeta Mono with Field Strength“.
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