while measuring the zeta potential of sample, results are not consistent showing a high standard deviation for each measurement. For HPLC water it is showing a negative value of 2 mV with standard deviation of 1 mV.
Similar problem of standard deviation is also occurring in emulsions (oil in water). sometimes instrument is not able to provide any result in case of emulsions, could this be a problem of hardware?
So, I want to check whether my machine has laser related problem or calibration related problem or something else?
Hi Rohit,
Firstly, zeta potential is a property of a material in a continuous phase, so attempting to measure the zeta potential of just water is akin to measuring noise, as there are no particles to scatter the light.
In terms of your emulsion measurements, was any reason given by the instrument as to why a result could not be generated? Emulsions often show turbidity, and may cause some challenges for zeta potential measurements by electropheretic light scattering as light needs to be able to transmit through the sample without being multiply scattered.
There are a number of ways in which zeta potential measurements can be affected, so to check the performance of your system, I would recommend measuring a transfer standard, which will be available from most suppliers of zeta potential measurements and typically comes in a prefilled syringe. If this is measured within specification, then your problems are likely sample/sample loading related.
With regards to monitoring the laser, typically a laser intensity or mean count rate may be recorded for each measurement. You could check this for consistency, but this will be sample dependent. Also, ELS is a hetrodyne measurement and uses a reference laser beam which may also typically be monitored and is sample independent, so you could check for any changes in this.
Rohit Kumar Saw
Adding to Alex's excellent answer, the first thing to do is measure the (NIST-traceable) secondary/transfer standard supplied with the instrument. That will remove the instrument from the 3M's (machine, method, material) that are part of an assessment.
Second, the instrument measures mobility and not zeta potential. There is no calibration steps, simply verification with an appropriate material as above.
As stated, there needs to be particles in suspension in order that the ZP can be inferred from the mobility
Third, in order to induce mobility, then one needs conductivity in the continuous phase, typically of the order of 0.001M KCl or higher
Fourth, dilution of your emulsion in DI water may be problematical in inducing instability. Dilute in the mother liquor. If you do not know what this is then step outside the laboratory and ask.
Fifth, be in contact with your local distributor - MalvernPanaltyical-Aimil.
Sixth, the disposable cuvettes are exactly that. Do not try to reuse them.
Seventh, I recommend the answer by Sean Parlia in: https://www.researchgate.net/post/How_to_dilute_emulsion_samples_when_measuring_zeta-potential
Eighth, I recommend the answer by Alan F Rawle in: https://www.researchgate.net/post/Relationship_between_zeta_potential_and_Emulsion_stability
DLS/ELS instruments don't require calibration. They are absolute methods. All you can do is measure a standard of some sort to confirm the instrument is operating within the manufacturer's specifications.
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