What is the standard for dynamic light scattering?

How can we ensure compliance with recommended light scattering quality standards? Is there an international standard for light scattering? Recently an interesting question arrived at our help desk about the validity of data in the classic Zetasizer Nano software:

Hello Support! Is there a summary of Malvern Panalytical documents that covers ISO 22412:2017 – DLS? A customer is writing a paper about her DLS data and she would like to strengthen her argument with ISO data.

Since this is a relevant topic – even if you are not writing a paper – we decided to share the answer here. So, can we use guidance from the international standards organization (ISO) to “backup” the result from a DLS measurement?

What international standards are relevant for DLS?

There are two standards directly related to dynamic light scattering scattering

  • ISO 13321:1996 Particle Size Analysis — Photon Correlation Spectroscopy
  • ISO 22412:2017 Particle Size Analysis — Dynamic Light Scattering

the older one ISO13321 has been withdrawn and revised by ISO22412. That revision brought many of the terms and requirements of the older version up to date.

How to ensure DLS measurements fulfill ISO requirements

Let’s look at a number of ISO22412:2017 criteria and how you can ensure your measurements fulfill the requirements. Below are 9 tips for best data quality in dynamic light scattering results.

 

 

1. Sample Preparation: use filters for the dispersant

The standard recommends filters of 200nm pore size or smaller to filter dispersants used in the preparation of samples.

2. Avoid number fluctuations, at least 1000 particles in the scattering volume

Typically, this is not a problem, and the size quality report checks for indications in the correlation function. If we know concentration and size, then the Concentration utility calculator can predict the number of particles in the scattering volume. Just go to Tools – Calculators – Concentration Utilities, enter relevant parameters in “Concentration &Scattering” section and find the “Number of Particles in Probe Volume” in the “Results” section.

Number of particles in scattering volume for 80nm particles a 4E-6 volume fraction is about 1400 particles, for backscattering

3. Scattering intensity should be significantly greater than the dispersant alone

Typical samples will have derived count rates several times higher than the dispersant. In real situations this means a sample should probably have at least 100kcps count rate. Otherwise, the measurement might take an extended long time to complete.

4. Particles may have different size and poly dispersity at different angles

For very small particles (especially below 30nm) there is no effect at all. In other words, results are not changing with scattering angle. But especially when comparing with measurements from other systems, keep in mind that results at different angles can be different. (Also see previous blog post on How do DLS results differ with angle?)

5. Correlation function should have a good intercept

The intercept should be at least 80% of its maximum achievable value. Typically, the best achievable value is close to 1.0 so aim for intercepts of 0.8 or higher. The intercept is noted in many reports, for example the Intensity PSD (M) report, and can be observed at the Correlation (M) report directly.

6. The intensity auto correlation function shall be fitted until 1% of intercept value

By default, this is implemented in the Zetasizer software, and no user modification is required.

7. At least 20 points within the correlation function must be available for the cumulants fit

This is typically very easily satisfied and can be manually checked by displaying the Cumulants Fit (M) report in the software.

8. Avoid particle-particle interactions, collective diffusion for certain ranges

You can obtain the average interparticle distance from the concentration utilities. To access go to [Tools – Calculators – Concentration Utilities, enter relevant parameters in “Concentration &Scattering” section and find the “Average particle spacing (µm)” in the “Results” section].

Average particle spacing of 80nm particles at volume fraction of 4E-6 is about 8 microns

If the probing scale is much smaller than the interparticle spacing, self-diffusion is observed

  • λ/{2 · n · sin(Θ/2)} < average particle spacing ⇒ self-diffusion of single particles: good!
  • λ/{2 · n · sin(Θ/2)} > average particle spacing ⇒ collective diffusion of groups of particles: bad!

Here are typical scenarios in the Zetasizer Nano, in aqueous samples, for the length scales

  • “back scatter”, NIBS or 173°: λ/{2·n·sin(Θ/2)} = 633nm/2/1.33/sin(173°/2) ≈ 240nm
  • “side scatter” or 90° ≈  340nm
  • “forward scatter” or 13° ≈ 2.1µm

Thus, of all optical configurations, collective diffusion has the least influence in back scattering to a certain degree. For the above example with an average particle spacing of 8µm, we would observe self diffusion at back (173°), side (90°), and forward (13°) angle in the Zetasizer.

9. At least 1.2 M photons accumulated over duration of measurement

This was only part of ISO13321 yet is still a sensible recommendation. It leads to a statistical standard error of less than 1% ( =1/√N ). When selecting automatic mode in the classic Zetasizer software, this requirement should always be OK.  (Only exception when the size quality report flags “insufficient signal collected” as one of the messages.)

The above 9 tips for good DLS measurements should help on your way to perfect size results.

Previously

If you have any questions, please email me at ulf.nobbmann@malvernpanalytical.com. Thanks! While opinions expressed are generally those of the author, our editorial team may have modified some parts.