Zeta consistency Top 5 signs – Zetasizer

by Ulf Nobbmann, Tuesday, 21st June 2022

Zeta consistency is one of the main signs to evaluate your zeta potential data. You may be looking at a set of records that are supposed to be the same – yet they are different. So now the analytic work starts: are the data really the same? The crux is that zeta is a complex system parameter, it changes with pH, ionic species, additives, preparation…) Here is my list of the top 5 zeta consistency checks for investigating Zetasizer zeta data.

female scientist with Zetasizer, inspecting a sample above the instrument, to check for data consistency and zeta consistency.

Conductivity: better than 10%

We can check the foundation for a good zeta consistency. Before a zeta measurement even starts, the Zetasizer determines the conductivity of the sample. In other words, we apply a small voltage (in V=Volt) to the cell and measure the current (in mA=milliAmpere). If a higher current flows then the electrical conductivity is higher. The Zetasizer records the specific conductivity in units of milliSiemens per centimeter or mS/cm. You can display this in the zeta workspace and look at the variation. Typically the relative standard deviation should at most be 10% for a series of ~5 repeats.

Zeta consistency: Is there a time trend?

If the results are trending in one direction, maybe there is a component in the buffer interacting with your particles? Or Joule heating is making the temperature drift (in which case you can put pauses in between measurements). Therefore, it is always a good idea to take a look at a few repeated measurements to compare records versus time.

Wall potential – the hidden sign

Usually, we don’t consider the wall potential. Because this parameter is often just like an unwanted side effect or artifact of slow field reversal. (In mixed mode M3-PALS with capillary cuvettes). The main interest remains the mean zeta. On the other hand, if you did acquire zeta distribution data you might as well look at the wall potential. In essence, the wall potential is like a (flow-induced) zeta potential of the cuvette wall material. Often this parameter mirrors the particle zeta, but if buffer or additives prefer the wall, the wall potential may be different. If the wall potential changes in a series of runs, that’s a good indication that something in the sample is adsorbing. Consequently, this can lead to a change in zeta, since the sample changes. And this then impacts the zeta consistency as well.

Size + Zeta + Size: the fall-back safety

While size measurements are truly non-invasive, the application of an electric field may -for very sensitive materials- cause complications. And the best method to confirm is to do DLS size runs both before and after the zeta potential measurements. In the Zetasizer Advance series, this is quite intuitive, and for the classic software use the SOP Player feature. In both cases you create a list of instructions to make size measurements before as well as after the zeta measurements. Here, you can repeat each a few times to also observe if the size is stable.

Zeta consistency – Reference beam

The reference beam is part of the laser beam signal that mixes with the scattered light. The software aims to perform measurements with a reference beam count rate of ~2500 kcps (± ~1500 kcps). The exact count rate is not so critical as long as it is much higher than the scattering from the sample. And the software usually figures this out. In some obscure cases, it may not be optimal. For example, if the reference beam is very low (<500 kcps) this may indicate a non-optimal sensitivity configuration.

Previously

Have any questions about zeta consistency? Please email me ulf.nobbmann@malvern.com – Thanks! Opinions are those of the author. Our editorial team modifies them occasionally.

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