Zetasizer: What is the zeta potential deviation?
The Malvern Zetasizer software report for zeta potential shows a parameter with the name Zeta deviation. You can see this parameter directly below the mean Zeta potential in the standard Zeta Potential(M) report.
The mean zeta potential results from the fast field reversal (FFR) phase analysis light scattering (PALS) technique.
The fast field reversal provides a single number from the analysis. This single number is the mean zeta potential. The slow field reversal [also just “SFR”] yields the zeta distribution, a distribution of different charge values from the sample.
When you obtain a complete zeta distribution, we can then use the bin values of the distribution to derive an overall standard deviation of the mean. The mean (from SFR) is by definition the same as the one from FFR.
This deviation therefore is only available if there is a distribution, and thus it is only available if a slow field reversal measurement was taken.
Three different analysis modes
The Zetasizer software has three different analysis modes for zeta potential measurements:
- The first and simplest one is monomodal analysis. This analysis only applies a rapidly alternating electric field (of approximately 20Hz). Another name for this is fast field reversal or FFR. It will only obtain a mean zeta potential.
- The second technique is called general purpose. This will apply a combination of fast field reversal (FFR) and slow field reversal (SFR). SFR applies a constant field for about 0.6secs in one direction, followed by the reversed field direction. There is a short rest rest period of 0.2 sec with no applied field in between. It will obtain both a mean zeta potential as well as a zeta potential distribution. We recommend to only use this general purpose mode for up to 150 mM salt. The reason for the limitation is that in high salt solutions Joule heating and other negative influences could damage the sample.
- The third and most intelligent analysis mode is the auto mode. With this setting, an algorithm in the software will automatically decide whether a general purpose [both FFR and SFR] or a mono modal [FFR only] analysis will be performed. The software decides based on the measured conductivity of the sample. If the sample conductivity is higher than five milli Siemens per centimeter (5 mS/cm) the software will apply only fast field reversal, in other words only a mean zeta potential well then appear.
The preferred default mode is the auto mode. For more details check the Technical note “Zeta potential – an introduction in 30 minutes“. (This technical note historically also appeared under the title “MRK654-01 Zeta An Introduction in 30 Minutes” in some older Malvern references.)
How can I get an idea of the quality of my Zeta result?
We would recommend that you look at the Zeta quality report. If there are no negative indications of issues with the phase data, then the result should be reliable.
Another useful suggestion is to look at repeated runs. When highlighting several records the software will show the mean and the standard deviation of the highlighted values. You can view these at the bottom of the records view tab. With this information it is straightforward to get a standard deviation and thus an error determination of the records. This is the best method to assess a standard deviation for monomodal analysis results.
What are typical values to expect for the deviation of zeta potential?
It gets increasingly difficult to measure zeta potential values close to charge neutrality. A good rule of thumb is to expect about 10% of the mean value as zeta deviation. However, when getting closer to zero, this deviation is likely to be about two millivolts (2mV) or larger. As guidance, a good expectation for a zeta potential deviation is 10% or 2 mV – whichever is larger. An alternate way to express the typically achievable zeta ζ deviations would be a σ of 10% relative but not less than 2 mV.
When the measurement performs only FFR = monomodal analysis then no zeta potential distribution is obtained (because that requires SFR). This can happen either because you specifically requested it in the experimental setup, or due to the conductivity of your sample being higher than the cutoff in the automatic mode. Since there is no zeta distribution, there is no standard deviation of the mean, and you can detect this as 0.00mV on the standard Zeta Potential (M) report tab in the software.
To get an idea of the ‘error in zeta’, do a set of repeated runs to look at the statistics (i.e. its mean and standard deviation) of the mean zeta potential. Should there be no statistics bar in the records view, add it by selecting the Records View tab, and then select in the main top menu View – Statistics Bar – and place check marks in front of Mean, Standard Deviation, and Relative Standard Deviation. The statistics for any highlighted records are then displayed in the three grey rows at the bottom of the active window.
When the zeta quality report indicates that “phase data are good”, the result of the FFR = monomodal analysis is reliable. So you can still use the mean zeta potential value. The distribution data come from the SFR part. When these distribution data are poor, the distribution is not reliable. This can for example happen when the sample is better suited for monomodal analysis. (Examples are also sample concentration too high or too low, or high conductivity). In many cases the Auto mode should handle this, but sometimes it may not.
An especially unusual condition is the message “Phase data poor, distribution data good”. This message basically indicates that the raw phase data are bad, but that whatever distribution was derived from it looks OK. Often this is most likely an artifact and does not lead to repeatable distribution data.
What are the Total Counts in my Zeta Potential Distribution?
The zeta distribution is an intensity based distribution. It represents how much light (or scattering intensity, or photon counts) contributed to the signal from the various apparent zeta potential bins (or brackets). This in turn means, that anything in the sample which scatters more light, will proportionately contribute more to the distribution. This intensity distribution is different from a number distribution, or a volume distribution. For size distributions this is a question we get frequently, see blog with a link to relevant technical note. However, for zeta potential this question is rare.
Most often, only the mean zeta potential is relevant, representing an overall (intensity average) mean value of zeta potential values. The mean zeta for the above distribution was -68.6mV± 6.5mV.
Hope the above eliminates some confusion about the zeta potential deviation in the Zetasizer software.
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If you have any questions, please email me at firstname.lastname@example.org. Thanks! While opinions expressed are generally those of the author, some parts may have been modified by our editorial team.