Hi, so I have been reading a bunch of research papers on how to do dynamic light scattering experiments in an non-ergodic medium.
1. Couple of them suggest to take the reading from a single ensemble over a long time, so that the probe particle can diffuse further.
2. Some suggest, taking the reading from different sub-ensembles and averaging the reading.
I am keen on carrying out the experiments taking the second approach route, where I want to study the Polystyrene particle in a dilute Poly-vinyl alcohol solution. The problem is, how do I set the measurement reading positions?
I am using Malvern Zetasizer ZS, where the measurement position can be set (0.0 to 6.5 mm). Any leads will be highly appreciated! Thank you!
Richa Ghosh Use the automatic settings until you have the familiarity with what’s going on. Invariably these will be the best for your measurements. The measurement position simply moves to the place within the cell where the scattering is optimum. If the scattering is weak then it will measure further into the cell to increase the scattering volume. If the measurement position is near the wall then the scattering is high and the scattering volume reduced. This also will help with multiple scattering. Read the manual or look any number of webinars on the Malvern Panalytical website.
Alan F Rawle the automatic measurement position stays fixed throughout the experiment being conducted. This defeats the purpose as for getting the reading of the ergodic medium, the position has to be varied.
Moreover, even if the experiment is repeated by keeping the automatic settings, more often than not the automatic measurement position remains the same.
Richa Ghosh Not sure what you mean. With DLS you can only acquire data at one position which you can vary but not during a single experiment. If you wish to change the position, then you can do it manually and that will move you further into the medium from the cell wall (or vice versa). Here you need to understand the photon counts you're getting (as the attenuator will automatically compensate to avoid either too low counts or detector saturation/damage). You'll also need to examine potential multiple scattering effects. Yes, for a repeated measurement on the same sample then the automatic position should stay the same - if it doesn't then it tells you that your sample is changing...
Alan F Rawle Thanks for the feedback. So while manually changing the measurement position within a certain range, as you have mentioned that I will have to be wary of the photon counts (to my understanding, the counting rate needs to be around 350-400, if I am not wrong). Is there anything else that I need to keep an eye out for apart from the count rate?
Also, what do you mean by examining potential multiple scattering effects? What variable do I need to keep an eye out for? I understand that the probe particle scatters light and so does the gel which I am studying. Thank you again for your response!
Richa Ghosh You need to look at the actual photon count (which you can find in the report parameters) rather than the optimized attenuated count (~ 200 kcounts). This will tell you the (real) amount of scattering. Usually, one is aiming for an attenuator position of 7 but good data can still be collected outside this point. If your attenuator position (in automatic settings) is 1 or 2 (high scattering: too concentrated; large particles; high relative refractive index/optical contrast) or 10 or 11 (too dilute; poor scattering; low relative refractive index; small particles) then I might worry. In terms of multiple scattering then the international standards (ISO and ASTM) advise a concentration ladder to examine the effect of varying this parameter. If you have multiple scattering, then the reported size will appear to decrease with increasing concentration. If you're running with a polystyrene latex standard, then it's easy to see this fall off. Remember that DLS isn't measuring a particle size; it's measuring a diffusion coefficient and thus you need a certain amount of time to collect the information from moving/diffusing particles. It's not a rigid system in space we're evaluating with DLS but a constantly changing one; hence the use of correlation to process such data. We're typically thinking of experiments of the order of 30 - 60 seconds typically. My thought is that a non-ergodic medium isn't going to remain stable enough over such a time frame to collect information, but I'd like you to prove me wrong...
Good luck with your research.
Alan F Rawle Just making sure that I got your advice right, by actual photon count you meant the reading off the count rate meter in Zetasizer Nano ZS or the derived count rate?
By the documentation it seems that 'derived count rate' is defined as the scattering intensity that would be measured at the APD in the absence of a laser light attenuation filter.
Count rater meter: enables the count rate signals to viewed before a measurement is run
Richa Ghosh From Ulf Nobbmann on Materials Talks:
'The avalanche photodiode APD inside the Zetasizer is a very sensitive light detector. If it gets too much light, it could be damaged. It also will become non-linear at very high count rates. Recommended count rates for DLS measurements are 100 – 500 kilo counts per second kpcs. In automatic mode, the software will automatically adjust the laser intensity to reach an appropriate scattering intensity from the sample'.
See: https://www.materials-talks.com/derived-count-rate-what-is-it/
Also see the attached.
Hello Richa Ghosh - typically in a non-ergodic DLS setup the sample might be in the center of a goniometer, and then slowly turn / move up and down while the data are being acquired. In the Zetasizer, the change of measurement position during a correlation function acquisition is not possible.
As you suggest, you could try to acquire correlation functions at different positions and then average them. I am not sure how many positions might be necessary to get a good space average.
The measurement position can be set in Measurement-Advanced-Measurement settings-Positioning method
An additional comment, the scattering volume in the ZS may already provide a good spatial average, depending on the scale of your expected inhomogeneities in your sample.
https://www.materials-talks.com/scattering-volume-in-dynamic-light-scattering-how-to-find-it/
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