Electrophoretic light scattering (ELS) or zeta potential instruments use first principles in their measurement protocol. They cannot therefore, be calibrated. However, they can be verified or validated that they are operating correctly, by measuring a suitable zeta potential standard.
The only zeta potential sample which is traceable to the National Institute of Standards and Technology (NIST) is Standard Reference Material (SRM) 1980. This sample is composed of 500 mg/L goethite (α−FeOOH) suspension, saturated with 100 μmol/g phosphate in a 5 x 10-2 mol/L sodium perchlorate electrolyte solution at pH = 2.5. When prepared according to the procedure supplied from NIST, the certified value and uncertainty for the positive electrophoretic mobility of SRM1980 is 2.53 ± 0.12 μmcm/Vs. This mobility should then give a zeta potential of 32 ± 1.5 mV, if the Smoluchowski model is used.
Most samples studied (e.g. biological systems) are negatively charged, and hence use of a negatively charged standard is preferable. In addition, negatively charged particles will have a minimal interaction with measurement cells, which tend to be
negatively charged. Currently there is no negative zeta potential standard available which is traceable to NIST. However the use of carboxylate modified polystyrene latex microspheres as zeta potential standards is common. These samples are prepared by grafting carboxylic acid polymers to the latex surface, producing a highly charged surface layer, which is quite resistant to trace contamination. Malvern Instruments produces such a zeta potential transfer standard. The standard is identified as DTS1235 and is a carboxylated polystyrene latex dispersed in a pH 9.2 buffer. The standard is supplied in ready for use 10 mL syringes, a supply of which is included with each new Zetasizer Nano zeta potential capable insrument. The specified zeta potential value for the DTS1235 zeta potential standard is -42 ± 4.2 mV.
