In my last blog, we covered:

  1. Why does sample concentration matter?
  2. How will it affect results?
  3. What is the right concentration range to be measured?

Here, we will continue the discussion on:

  • 4. How to prepare and control the sample to be in the right range?
  • 5. Why is the recommended obscuration range size-dependent?
  • 6. How do I optimize the obscuration for my own sample?
  • 7. Are there any concerns with the dilution of particles? 
  • 8. Are there alternatives to avoid sample dilution? 

4. How to control the actual sample concentration measured in the system?

The operation itself is truly simple: Take some samples from your batch and drop them into the system, while watching the obscuration bar until it reaches the right range. Even if you over-shoot, it can be easy to dilute it back. If you have not seen one, please do not hesitate to schedule a demo or virtual demo with us. The system may hold from 5.6ml to more than 600ml dispersant, depending on which accessory features you use. This dilution power is enough in many cases, but in some other scenarios, you may need to pre-dilute your samples and add the diluted solution to the system.

5. Why is the recommended obscuration range size-dependent?

You must have noticed that the recommended obscuration ranges are different for different particle sizes. Here is why: one particle of 1000µm in diameter can break into 109 particles of 1µm. It means, with the same volume of material scattering light, we will have significantly more fine particles than coarse ones. That means, there could be multiple scattering in a solution of fine particles, whereas for coarse particles, we still do not have enough to represent the bulk profile. Besides size, the refractive index can also affect the optimized obscuration range for your sample.

6. How do I optimize the obscuration for my own sample?

When developing a method, titration is always worth the effort to optimize controllable parameters, including obscuration. A titration over obscuration means to run the same test with everything but obscuration remains the same. Then we plot the measured size over obscuration to find a plateau where size is independent of obscuration.

7. Are there any concerns with the dilution of particles?  

In wet analysis, there is always this tricky question: How will dilution influence particle size?

First, the rule of thumb is to use the same dispersant as in the original sample. This minimizes the influence of dilution. Subsequentially, the stirring and sonication provides a mechanical shear on the particles. However, it only takes seconds for a particle to travel to the cell where it gets a scan before it circulates out of the system. Therefore, in most cases, it is safe to ignore the potential changes in particle size during this short amount of time.

8. Are there alternatives to avoid sample dilution?  

It is feasible to have a mathematical program to correct for multiple scattering. Actually, our Spraytec has a built-in algorithm to correct for multiple scattering. It is made for aerosol analysis and covers the range of 0.1 – 2000 microns.

In some other cases, thick topical creams have particles that are difficult to be diluted or dispersed. Microscopy can be useful with a simple spread of cream on the coverslip, except it can be time-consuming, labor-intensive, and less statistical. Morphologi 4-ID is well embraced by a lot of customers due to its automatic image-capturing and analysis features. It has a frequency of 10,000 particles per minute, comparing with the 10,000 scans per second by Mastersizer. The capabilities of actually seeing the particles, particle shape analysis together with Raman Spectroscopy make it the most suitable choice in a lot of cases.

Further reading