I use a glass cuvette for DLS analysis (and made the necessary specification in the protocol on the Zetasizer software to select the cuvette type/manufacturer). Ever since I started using this cuvette (it's wasn't new), I was getting huge standard deviations in size measurements (~20-30%) and very low count rate (~200 kcps). I even worked under various concentrations (1-10 mg/mL) to ensure this wasn't a concentration-dependent issue.
I randomly tried using a disposable polystyrene cuvette yesterday with the same protocol and concentration and found a much higher count rate (2000 kcps) and lower standard deviation (within 10%) for the same exact sample. I now know this isn't a sample or instrument problem but a cuvette issue.
Are disposable or polystyrene cuvettes better for DLS use? Or is there an issue with my cleaning protocol (see next paragraph)? If there is something such as a sub-visible stain or scratch, how can this be determined?
Protocol for cleaning polymer sample:
1.) Rinse 3 times with DI water and dispose.
2.) Rinse 3 times with ethanol.
3.) Wet a kim wipe and wet the outer surface of the cuvette.
4.) Air dry the cuvette using the laboratory air supply. Once dry, load sample.
Normally the glass or quartz cuvettes provide better quality and less flare from the surface - this is important where the particles are small, in low concentration, or have low optical contrast.. It sounds like your glass cuvette is etched or damaged in another way or is not the type supplied by Malvern specifically for the Zetasizer ZS. This is almst certainly confirmed by low (absolute) count rates. Take care because the attenuator will normalize the count rates so there's no danger of saturating the detector (among other things). Did you get error messages on the Quality Report or the Expert Advice?
If in any doubt then confirm the performance of all elements in the system by running a standard material. ISO and ASTM standards recommend a standard around 100 nm but others will be perfectly adequate.
I don't like Step 3 in your protocol. This has possibly put minute scratches on the outside of the cuvette.
Thanks for the fast response Alan. I did make sure that the cuvette is the same one supplied by Malvern. I never actually received an error from the run (just complaints about polydispersity for certain samples and high standard deviation).
Would BSA be a suitable standard? And maybe wetting lens paper with ethanol would be a better idea for next time? I read somewhere that would be best.
BSA will be a difficult sample to measure as it's small with poor optical contrast. I assume then you're working under a Class 100 hood? You will need to be very careful with the filtration (with a 0.02 um filter) and everything will need to be scrupulously clean. As the international standards recommend, use a 100 nm or similar (60/220 nm) latex material. This should give you the correct size and a low PDI - plus stability...
You should never need to clean the outside of the cell. If there are fingerprints on it the fingerprints can be removed with a little solvent and the source of the fingerprints must be banned from the lab. Normally this is blown with a dust blower to remove any material that may be on the outside.
Hi Rahul,
As a matter of interest, what measurement position was used for both sets of measurements? In a back scatter DLS measurement, the system will automatically choose a measurement position that seems optimal based on measurement quality. I have known that a slightly marked glass cuvette lead to some anomalies in this optimisation.
Hi Alexander. I had backscatter set so I think measurement position was automatically chosen. What would you recommend trying and how can measurement position be manually input?
Hello Rahul,
as Alex suspects, this may be due to an inconsistent measurement position. The positioning algorithm may have gone to the wall of the cuvette (and you can force this to the center for optically clear samples, in the advanced measurements settings). Alan alerted about the automatic attenuation, and in order to compare one can look at the derived count rate instead ( http://www.materials-talks.com/blog/2015/06/11/derived-count-rate-what-is-it/ ).
Please also make sure that the correct cuvette type is selected in the settings (i.e. ZEN0040 is different from DTS0012, and PCS8501 is different from ZEN2112) as the different cuvettes have different wall positions, which the software needs to know in order to optimize correctly.
The easiest might be to just send the data file: http://www.materials-talks.com/blog/2014/08/19/faq-how-can-i-submit-a-data-file-to-the-help-desk/
Finally, a good system check is to measure just toluene in a sample cuvette http://www.materials-talks.com/blog/2014/11/17/tips-tricks-for-nanoparticle-characterization/#toluene