Constant Current: How to stop it?

How to stop high conductivity zeta potential from being a constant pain

So you have some physiological samples like a protein in PBS but you are concerned about its stability. Or maybe you have a seawater sample you are studying how to treat. Measuring zeta potential will help you understand these. But with the high salt content in the sample, you’re thinking this is going to be a big challenge. Previously, we introduced the patented diffusion barrier technique to help with these types of measurements. This method extends the time before the sample starts aggregating due to the measurement frying your sample.

High Conductivity Samples: A Polarizing Measurement

The newest improvement to measuring this sort of sample is the constant current mode available on the Zetasizer Advance series. A typical zeta potential measurement would use constant voltage. Here the same voltage is applied throughout the measurement but the current adjust to react to changes in your sample’s conductivity. These changes are due to effects like heating of your sample. When measuring a low conductivity sample, such as a polymer in water, this works fine but samples with higher conductivities cause the electrodes to become polarized. This is not a good thing!

If the electrodes are polarized, the current decreases. This means that in your samples, the particles move less than they should for the voltage being applied. This can make your measurement results less accurate. When measuring your high conductivity sample on a Zetasizer Advance instrument this is not a problem. The instrument can detect the higher conductivity and automatically switches to using constant current. In this mode, your Zetasizer will aim to use a voltage and current appropriate for your sample with constant voltage. However, the current stays the same throughout the measurement and your Zetasizer adjusts the voltage instead. This means the particles move at the speed they should do due to their charge, not because of a changing current.

Constant Voltage vs Constant Current

Let’s looks at some data. Here we can see changing the current to be constant improves the measurements for a polystyrene sample in the plot below. We measured the blue data points using only constant voltage whereas we measured the red data points using constant current. We can see the constant voltage trends towards zero as the electrodes polarize whereas the constant current data stays stable.

Whether it is looking into the treatment of seawater, studying the stability of protein physiological conditions, or identifying the isoelectric point of a slurry using constant current with the Zetasizer Advance instruments you can be confident that the zeta potential you are measuring really is that of the sample.

Further reading

• MADLS on the Zetasizer Advance: More than just a multi-angle measurement
• Starting the next chapter – moving from Zetasizer Nano to Zetasizer Advance
• Zetasizer Advance in bullet points
• Discover Zetasizer Advance – Meet the New Line-up of Dynamic Light Scattering Instruments from the Pioneers of DLS Technology – Q&A
• Peak number order in DLS – which is peak one?