This might be a question we ask before purchasing a new instrument. We might also be wondering this when trying out a new application. The spec sheet for your instrument will tell you the answer but what does that lower/upper size specification mean?

…it depends

In almost all cases, this is probably the more accurate, albeit vague answer. The lower size limit for most techniques will depend on sensitivity. If we are talking about a light scattering instrument, this will depend on the size and optical properties of the material. The quality of the data you can expect at the lower extreme sizes will likely also depend on the optical quality of your measurement. Describing the quality of your instrument’s optics could be its own series of blogs and something I touched on previously here.

What about large size limits? Generally sensitivity isn’t going to be an issue any more so what else could go wrong?

Some assumptions…

There are some conditions we should meet to reliably convert light scattering information to particle size. Here I will use the Zetasizer and Dynamic Light Scattering (DLS) as an example.

In a DLS measurement, we detect light that has been scattered only once by each particle in the sample. The average scattering intensity is consistent over time. By analyzing this light we can calculate a diffusion coefficient for the particles in the sample. We can then convert this into particle size. If any of these statements are not true, then the size measurement we receive is going to be inaccurate.

What does this have to do with measuring large particles? Well it turns out that as particle size increases, multiple scattering is more likely. An option is to dilute the sample. Here we need to be careful. Our average intensity will fluctuate due to low numbers of particles if we dilute too much.

It also turns out that larger particles might not only be diffusing but may be moving due to sedimentation or convection, so-called secondary particle motion. Luckily our low-volume disposable capillary cell can help with this. Read more about it here and here.

Defining a spec

When we come to ask “what is the largest particle size I can measure on my instrument?”, we are really asking at “what particle size can I achieve all the conditions I need for a reliable measurement?”

Again the answer here is “it depends” but this is something our R&D department has spent quite some time deliberating over.

The Zetasizer has an upper size specification of 10 μm. This is based upon polystyrene latex. To achieve this using the Zetasizer Nano range of instruments, we would need to adjust the density and/or the viscosity of the dispersant. This was done to control the secondary particle motion mentioned earlier. The Zetasizer Advance range can use the Low Volume Disposable Sizing Cell (LVDSC) to measure large particles. The geometry of the cell naturally controls secondary particle motion meaning 10 μm latex can now be accurately measured in water with no need to modify the density or viscosity.

Until recently this was demonstrable only by looking at the Z average particle size result. A new analysis mode allows distributions to be reported for an extended size range. This option is described in a recent technical note. This also goes into more detail into sample quality effects and sample optimization for large-sized particles.

Figure-1-TN210309-Zetasizer-Advance-Extended-Size-Range.jpg
Intensity weighted particle size distribution for a 10 µm polystyrene latex using the General-Purpose Extended distribution analysis model.

As an aside, some definitions I have seen used for specifications include the end point of a graph, the tail of a distribution, or a theoretical limit. This means we can’t make any sense of the specification with regards to understanding data quality for different applications.

Data I can trust?

At this point I am sure you are wondering how can you use DLS results that report large particle sizes? If “it depends” so much, how do we know a given result is valid? The Data Quality Guidance feature of ZS Xplorer is here to help, with indicators of any sample quality issues. This is particularly useful to identify any breaks in the assumptions we discussed previously.

Be specification smart…

If you have found yourself looking at specifications for different instruments or technology types, ask yourself what the number means when size range is quoted, and what this means for your application, and if possible, look at how these values have been demonstrated. To help you match the right instrument to your application, we have also produced an interactive tool to help you find which Zetasizer is the best fit.

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