Laser Diffraction for Particle Sizing – Sample Concentration/Dilution (1)

“We need to characterize particle size distribution quickly and statically. The estimated size range falls into that covered by laser diffraction. Now what?”

Now – sample preparation: Can we actually prepare the samples to be measured properly?

When it comes to sample prep, the #1 thing to be considered is sample concentration, or oftentimes, dilution. Here we will talk about wet analysis and what you would need to understand regarding sample concentrations.

1. Why does sample concentration matter?

The upper limit:

“Laser diffraction” is a “first-principle” technology with no need for calibrations. 

The angular-dependent profile of the scattered light is directly determined by the particle size and its optical properties. Therefore, the particle size is calculated from the light patterns detected by the instrument. But pay attention here. There is a key assumption – “single scattering event (SSE)”. In other words, the algorithm assumes that the incident light only hits one particle before it is detected. If the particle concentration is too high, then the incident light will interact with multiple particles before it finally gets to the detector. This detected light properties (angle, intensity) will be misunderstood by the instrument – it still thinks it only hits one particle

The lower limit:

There is also a lower limit of the sample concentration to make sure that every measurement we take it gets a sufficient signal-to-noise ratio.

2. How will it affect results?

Too Concentrated:

In this case, the result will appear smaller than it should. Generally speaking, particles diffract light away from the incident direction. When an incident light hits multiple particles, it would be further away from the incident direction, meaning a wider diffracted angle. Wider angle is correlated with smaller particle size.

Too Few Particles:

The noise level is defined as the small but random fluctuations in the data after the background signal has been subtracted. When there are very few particles in the system, it will not essentially differ from the background, which means, poor signal-to-noise ratio.

3. What is the right concentration range to be measured？

The shortest answer is Obscuration.

The obscuration value is located on the top-left corner in the software. The green bar varies with the sample concentration inside the system. It turns orange when its value exceeds the pre-set range, which will be excluded by the software.

The recommended obscuration ranges for wet analysis are as follows:

Particle Size	Obscuration Range
0.01 – 1 µm	1% – 5%
1 – 100 µm	5% – 10%
100 – 3500 µm	10% – 20%
Poly-dispersed samples	Choose based on largest

Here’s for dry analysis:

Particle Size	Obscuration Range
Cohesive, fine particles	0.5% – 5%
Easily dispersed, coarser particles	1% – 8%

Stay tuned for the next blog to continue the discussions 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? 

Further reading

• Wet or liquid dispersion method development for laser diffraction particle size distribution measurements
• Laser diffraction: my top Q&A
• Wet dispersion application note