When doing measurements in DLS (either size or zeta potential), I have the option to input an automatic number of measurements or to set it manually.
My question is: is there a drawback in setting a pre-defined number of measurements?
For example, I know that if the sample is stable it may take fewer runs for the measurements to be completed. However, is it possible that the sample may require more runs, BUT if the equipment is set to a pre-defined number, it will actually give me the wrong size?
Thank you in advance!
Thank you, Prof. M. R. Mozafari for your comment. For a typical "well-behaved" sample with sufficient scattering intensity there are two answers for the Zetasizer:
- The classic method was to take ~12-15 correlation functions of ~10sec each, eliminate those with higher scattering intensity, and average the remaining to come up with a measurement.
- The most recently developed 'adaptive correlation' method takes many (~15-50) short correlation functions of ~1sec each, then does statistical analysis on them to group correlation functions into a 'steady-state' and a 'transient' subset. The 'steady-state' average becomes the measurement.
For a "good" sample, both methods should deliver the same answer. The adaptive method is faster and can lead to a good result in a third of the time. Or as a hybrid approach more shorter runs can work, if you need to speed up the measurement slightly (for example 20 x 5sec correlations).
Hello Beatriz Dias Barbieri , yes there are differences!
The automatic mode takes into account the level of scattering intensity to determine a number of collect and average the signal from enough photons. See https://www.materials-talks.com/blog/2014/11/17/tips-tricks-for-nanoparticle-characterization/#runtime
Potential drawbacks:
- if you set the number too small, the result may be noisier and show a larger variation
- if you set the number too large, you may not see kinetic information that happens faster than the total runtime of the measurement.
If your sample is scattering significantly, is fairly stable and you just want to avoid the variability in the total duration, then it might be OK to use a similar total duration. The size quality report will alert you if insufficient signal was collected or if there may be additional issues with the correlation functions. So, for a stable samples, if you select a larger number of manual measurements to collect over a larger total duration than the automatic algorithm would have selected, there should be no problem. However, if you select a smaller total duration the result may be statistically less reliable than the automatic mode.
You may also like https://www.materials-talks.com/blog/2019/04/23/light-scattering-iso-standard-anyone-iso13321-iso22412/
Beatriz Dias Barbieri I concur with Ulf Nobbmann For initial experiments, automatic measurement is encouraged to gain an insight into the overall stability and distribution. Once you are pretty sure about the stability and expected variations, a manual measurement can be set up with a fair duration of scan.
Thank you very much, Ulf Nobbmann and Nabodita Sinha. That clarifies a lot!
Hi,
Dear Prof. Ulf Nobbmann
Thanks for very informative explanation. One question which then will follow is
What would be a right number to set for automatic measurement for non-viscous, aqueous sample of colloidal particles?
Thank you, Prof. M. R. Mozafari for your comment. For a typical "well-behaved" sample with sufficient scattering intensity there are two answers for the Zetasizer:
- The classic method was to take ~12-15 correlation functions of ~10sec each, eliminate those with higher scattering intensity, and average the remaining to come up with a measurement.
- The most recently developed 'adaptive correlation' method takes many (~15-50) short correlation functions of ~1sec each, then does statistical analysis on them to group correlation functions into a 'steady-state' and a 'transient' subset. The 'steady-state' average becomes the measurement.
For a "good" sample, both methods should deliver the same answer. The adaptive method is faster and can lead to a good result in a third of the time. Or as a hybrid approach more shorter runs can work, if you need to speed up the measurement slightly (for example 20 x 5sec correlations).
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