What instrument are you using? I haven't heard the term "pulse measurement".

If you have an instrument that uses the phase analysis light scattering method, it should be able to measure highly viscous samples. That's one of the reasons I invented the technique!

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If you dilute the sample, you will likely change the zeta potential. What you would need to do is run samples at a range of viscosities and extrapolate back to your original viscosity. This will add additional problems - you need to know the dielectric constant and viscosity of the diluted sample and, depending on the instrument you are using, you will need the refractive index.

Please provide more details:

1. The instrument you are using

2. Sample information including the medium and the liquid you propose to use as a diluent.

John Francis Miller : I'm using a HORIBA nanoparticle analyzer, which allows measuring particles' size (up to 10 micrometers) and Zeta potential of sample. In the settings screen of Zeta potential mode, there is a choice named "pulse measurement", of which further settings include number of scans and delay time between each scan. As far as I know, this sort of apparatus has not applied the M3-PALS techniques for working with highly viscous solutions, but I still want to know whether the measurements could be ameliorated using this HORIBA device. I have diluted 5 times the sample with 10 mM KCl, and then the medium (which contains water, organic surfactants and stabilizers (tween 80,...)), but the results of both cases still varied too much, with high deviation values, and I want to make this narrower.

Duy Khang Nguyen Vu I assume you are using the Horiba SZ-100. As you state, this instrument does not employ the PALS method (note, M3-PALS is Malvern's proprietary - and unnecessarily complicated - implementation). The SZ-100 uses the more traditional laser Doppler method (laser Doppler electrophoresis - LDE). It's worth understanding the fundamental difference between the two and why LDE is unsuited for highly viscous samples whereas PALS can be used successfully.

The physical setup of the instrument is identical. The difference is the the way in which the scattered light signal is processed and (usually) the frequency of the applied electric field.

LDE performs a Fourier transform of the scattered light signal to yield a Doppler frequency distribution. The Doppler frequency is proportional to particle velocity. If the electric field is known, the electrophoretic mobility can be calculated. Typical Doppler frequencies required for meaningful measurement are a few 10Hz. This is readily achieved in low viscosity aqueous media but the velocity scales inversely with the viscosity. Samples in high viscosity media with the same zeta potential as particles in low viscosity media will yield much lower Doppler frequencies for a given applied electric field. e.g., the Doppler frequency may be less than 1Hz. This is typically less than the resolving power of the measurement and, in practical terms, not useful. This could be addressed by increasing the electric field but that would mean introducing a lot of heat, especially if significant electrolyte is present.

PALS looks at the phase of the scattered light signal instead of the frequency. Recall that a phase change of 360 degrees per second is equivalent to 1Hz. Resolution of phase changes of less than one degree are readily achieved. Phase analysis can quantify the motion of particles moving with velocities equivalent to those that would yield 1Hz via the Doppler method. Hence, PALS can measure electrophoretic mobilities two or three orders of magnitude less than LDE for the same electric field strength.

You mention diluting with 10mM KCl(aq). I know that the SZ-100 can be used with a variety of types of cell including the "disposable" type originally introduced by Malvern. What type of electrode are you using? The disposable type requires higher voltages to be used to get the same electric field strength compared to the more traditional "dip" type.

You also mention highly variable results. Have you performed a system check with a transfer standard - e.g., a simple aqueous dispersion of known zeta potential? Can you make measurements on your instrument at 10mM KCl(aq)? Is your optical concentration suitable?

John Francis Miller : Thanks for your answer. For this HORIBA device, I used the "disposable" electrode (carbon electrode) for Zeta potential measuring. The system has been checked by appropriate standards and no problems occurred, regarding Zeta potential. I usually measure with the sample in "as is" status (no dilutions or other modifications before measuring), in order to avoid unwanted effects on results' accuracy. In some cases, however, the sample is too viscous, which is not good for both the electrode cell and the results. That's the reason for my seeking the suitable measurement conditions.