Measuring zeta potential - electrophoretic light scattering

This is a recording of a webinar I gave on How To Measure Zeta Potential More Confidently.

I show you how you can maximize the confidence in your measurements - and their interpretation - needed for robust decision making.

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TRANSCRIPT

So let's look at electrophoretic light scattering. If we shine a laser beam through a sample of particles, each particle will scatter the light and create a Doppler shift dependent on the velocity of that particle. It's like a police radar speed trap. And if you have millions of particles then you have millions of Doppler shifts contributing to the intensity of the scattered light. This varies with time in a similar way to a sound signal. You can use a spectrum analyzer to see the frequency components of the sound and you can do the same thing to see a spectrum -- or distribution -- of the Doppler shift frequencies of the scattered light. If you know the geometry of the equipment you can convert the frequency distribution to a velocity distribution. If you know the electric field, you can get an electrophoretic mobility distribution and then calculate a zeta potential distribution.
 
This has been available since the 1970s. Here we see a zeta potential distribution.
 
Another method is phase analysis light scattering (PALS) that I invented in the late 1980s. It uses a different way of processing the same signal. Why would you want to do this? Well, the laser Doppler method has a limitation as far as how much electrolyte you can have present in your sample. As you increase the electrolyte concentration, you start to see adverse effects like heating or bubble formation at the surfaces of the electrodes and other unwanted behavior that makes the measurement impossible using the laser Doppler method. Phase analysis gets around some of the limitations but sacrifices the ability to show you the distribution. Instead you get a single averaged value. Above 10 millimolar ionic strength, commercial instruments switch to using PALS simply because they cannot do the laser Doppler measurements. But PALS measurements are only valid if the sample is truly monomodal with respect to zeta potential. You have to assess whether that's an appropriate assumption or not for your particular purpose. I've developed a next generation electrophoretic light scattering system that does allow you to get the zeta potential distribution at these higher electrolyte concentrations. But there's another problem that arises with using high salt concentration and it also relates to the electrodes.

Видео Measuring zeta potential - electrophoretic light scattering канала John Miller