Can you give more information such as:

Instrument

What the solid is?

What the dispersant is?

Is it buffered?

What salt concentration?

Without a buffer, aqueous solutions will pick up CO2 from the atmosphere and eventually lead to a pH of about 5.5. If your formulation has a pH of, say, 6 - 8, it will be difficult to control. Depending on the nature of the surface, the change in pH may change the surface charge and, hence, zeta potential.

Regarding dispersant only, it depends. A true solution or pure liquid will give you meaningless results. If your dispersant contains micelles, you may detect those - this depends a lot on the instrument. If your dispersant contains any substances that scatter light strongly, this may be greater than from the particles.

For some instruments, the manufacturer suggests adding a low amount of electrolyte to ensure there is electrical conductance. It is likely that you have enough. The instrument will advise you if that is not the case.

Finally, if you are diluting your samples prior to measurement, it is possible that you are not waiting long enough for a new equilibrium to occur. Without knowing the details of your system, it's hard to suggest if this is the case or not.

Zeta potential (ZP) without a controlled and stated pH value is virtually meaningless. ZP is a holistic property of particles in a continuous phase. You are simply measuring noise-on-noise if there are no particles present. Typically speaking a powder measurement would be a laser diffraction experiment, not DLS (yes, I'm talking size measurement here as opposed to ZP). If you have the ability to take a DLS size measurement as well, the presence of sedimentation may be indicated. You may have (significant) sedimentation taking place, you may not have sufficient conductivity in the continuous phase.

As John indicates above, we need more information.

Thank you. I am just trying to determine whether the used Malvern Zetasizer Nano ZS is working properly, and I don't have any standards yet. I was just curious what the measured values on blank runs of HPLC grade water, DI, and tap water would actually indicate. Perhaps nothing.

I have been running platinized carbon supports in various solvents to determine the stability of the solids in an "ink". There just doesn't seem to be consistency in the measurements; one time the solution results in a - 30 mV zeta potential, other times it results in a -70 mV zeta potential. I know that the greater the magnitude of the zeta potential, the greater the stability. There is definitely sedimentation, though. When I perform DLS for particle size analysis using the same concentration and composition of solids and dispersant, I am able to get reproducible data.