25


Figure 2. Coagulant concentration vs. zeta potential IEP Determination


Most IEP studies investigate the change in zeta potential as a function of pH, but other chemistry changes such as surfactant concentration are also examined. Figure 3 shows an IEP study for the protein lysozyme, particle size approximately 3.5nm. The pH was altered using NaOH as the base and HCl as the acid. As the sample passed through the IEP the physical aggregation of the protein was visible to the eye as the aggregate size increased.


Figure 4. Zeta potential of an emulsion sample; 800 analyses with one cell Guidance from ISO Standards


International Organization for Standardization (ISO) standards are useful for chemists new to an analytical technique because they provide information on the theory, use, and verification process for a given technique. Working group 17 within ISO TC24/SC 4, particle characterisation, recently completed the first 2 of three documents on methods for zeta potential determination; Part 1: Introduction, and Part 2: Optical Methods. Part 1 provides excellent background and theoretical reviews. Part 2 provides expert information on how the measurements are made, suggests best practices when using optical techniques, and discusses potential sources of error. A summary of useful guidance procedures are described below.


Dilution: Measurements without dilution are preferred whenever possible. Dilution for zeta potential measurements can be troublesome because zeta potential is not a property of the particles alone but also on the chemical equilibrium between particle surface and dispersing medium. Any alteration of the surface chemistry therefore affects the zeta potential. The worst possible approach is to dilute with DI water, which is almost certain to alter the surface chemistry. The ISO standard suggests diluting the sample using the so-called equilibrium dilution procedure, which uses the same liquid as in the original system as a diluent. This is typically performed by extracting the supernatant from the sample after either sedimentation or centrifugation.


Verification: All analytical instruments should undergo a verification process on a regular basis, and at least one a year. There is only one standard reference material for zeta potential available at this time, the NIST SRM 1980. This is a powder hematite sample that is not easily dispersed and is not very popular. The ISO standard acknowledges this, so users typically test their system with the sample supplied by the vendor. The system meets the requirements defined by the ISO standard if the following criteria are met after making three measurements of the reference material.


Repeatability (coefficient of variation) must be less than 10% Accuracy of the mean value must be within 10% of the published value


Figure 3. Iso-electric point (IEP) determination of lysozyme


Older zeta potential cells using gold coated electrodes suffered damage when measuring proteins like lysozyme due to Joule heating at the electrode surface. This made IEP measurements as shown in Figure 1 almost impossible because the operator was not sure when the cell became too damaged for accurate measurements.


The newest generation of zeta potential cells such as those supplied with the HORIBA SZ-100 system includes electrodes coated with a porous carbon layer that minimises damage by protein ’baking‘ on the surface, greatly extending the lifetime of the cell. Carbon coated electrodes improve data quality and drastically reduce the cost of ownership since the cells can now last for hundreds of measurements, depending on the sample.


Figure 4 shows over 800 zeta potential measurements of an emulsion using a single cell. Note that the change seen around measurement number 100 is due to the preparation of a new sample once it was realised how long this experiment would take.


Reproducibility can also be tested and the system meets the ISO standard requirements if three separate aliquots are analysed and the coefficient of variation for the mean value is less that 15%. Note that in the standard the reference values are stated in units of electrophoretic mobility, but most customers typically use the calculated zeta potential values.


Conclusions


Zeta potential is a growing analytical technique used in many industries for dispersion stability studies and IEP determination. The new ISO 13099 standards are helpful documents for chemists new to the technique or experienced users looking for best practice and verification procedure recommendations.


Note: All data presented in this article were analysed on the HORIBA SZ-100 system. References


[1]. ISO 13099-1 Methods for Zeta Potential Determination - Part 1: Introduction. [2]. ISO 13099-1 Methods for Zeta Potential Determination - Part 2: Optical Methods


New Handbook of Accessories for SPM


JPK Instruments has released a new handbook of accessories for its family of SPM systems. JPK's family of SPM solutions have been available for over ten years. To keep these functioning and still useful and being able to add new capabilities has always figured prominently in the design process of the newest systems. The latest range of accessories is described in a new 20-page handbook available in both digital and print form. It provides users with unlimited possibilities for their NanoWizard®


, ForceRobot® and CellHesion® systems. Of the new items, four stand out for mention. The Advanced Force Spectroscopy


software module provides users with open and flexible software enabling advanced force measurements from single protein unfolding and DNA stretching experiments to the probing of cells and tissues. This links nicely with the ExperimentPlanner™ software module which allows the user ready-access to design and develop unique experimental protocols by providing full control over all system parameters.


The recently-announced QI™ Advanced software module for NanoWizard systems delivers quantitative mechanical properties from the most difficult-to-image samples. These include the soft, sticky and brittle samples found in applications of biological and polymeric materials. It is straightforward to learn this advanced performance technique and to obtain adhesion, stiffness and dissipation data in real-time while scanning. Operating in ambient conditions or in fluids, it may also deliver electrical conductivity or molecular recognition information (sample dependent). Electrical measurements require specific sample mounting and the newest imaging methods are even using advanced cover slips coated with materials such as ITO (indium tin oxide). A new CoverslipHolder™ with electrical sample connections is now available for electrical measurements such as conductive AFM or STM importantly on a coverslip for use in combination with high NA optics. Integrated optical systems have always been a feature of instrumentation from JPK and this accessory is no exception to this offering an insert that may be used for life science applications too.


Circle no. 593


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