Why Are the PSD Results Different?

Size Representation: In SEM, the particle size is measured as a projection of the particle's 2D image, typically giving a diameter based on the area of each particle in the image. In LD, the particle size is calculated based on how light scatters, which assumes spherical symmetry and weighs particles according to their volume.

Weighting of Larger Particles: In LD, larger particles contribute disproportionately to the PSD because the measurement is based on volume. A few larger particles can shift the PSD significantly, making the overall distribution appear coarser. In contrast, SEM treats each particle equally, so a single large particle does not skew the distribution.

Sample Size and Representativeness: LD analyzes a large number of particles simultaneously, so the PSD is generally more representative of the bulk powder. SEM, on the other hand, is limited to a smaller sample size (the particles in the image field), so it may not capture rare large particles or small particles that are clumped.

Shape Sensitivity: SEM can accurately measure non-spherical particles, while LD assumes spherical particles, which can lead to inaccuracies in the LD results if the particle shape deviates significantly from a sphere.

To compare the two techniques:

Number-Based vs Volume-Based: You should convert the number-based distribution from SEM to a volume-based distribution to make it more comparable to the LD results. This can be done using the relationship between particle size and volume, but keep in mind that SEM results may still differ because LD over-represents larger particles.

Size Range Sensitivity: SEM is more sensitive to small particles because each particle is counted equally. LD, on the other hand, may underrepresent small particles since their contribution to the overall volume is small. For submicron powders, this could lead to significant differences in the results.

Advantages and Disadvantages of Each Technique

SEM PSD:

Advantages:

1. High precision for measuring individual particles.

2. Provides detailed information on particle shape and morphology.

3. Accurate for irregularly shaped and submicron particles.

Disadvantages:

1. Labor-intensive and time-consuming.

2. Small sample size, which may not be fully representative of the bulk material.

3. Difficult to handle agglomerated particles.

LD PSD:

Advantages:

1. Rapid analysis with a large number of particles.

2.Provides statistically representative data of the bulk powder.

3. Suitable for a wide range of particle sizes.

Disadvantages:

1. Assumes spherical particles, which can lead to inaccuracies with irregular shapes.

2. Overestimates the contribution of larger particles due to volume-based weighting.

3. Small particles might be underrepresented in the PSD.

Why Are They Not Perfectly Comparable?

The PSD from SEM and LD will not perfectly align because they represent the particle size distribution differently: SEM based on number and LD based on volume. The differences are due to the ways the techniques weigh particles, sample sizes, and assumptions about particle shape.

In your results and discussion section, you can explain that both methods are valuable but reflect different aspects of the particle size distribution:

SEM provides detailed insight into the size and morphology of individual particles, which is crucial for understanding how particle size and shape contribute to material properties.

LD offers a statistically robust overview of the particle size distribution, particularly useful for bulk materials, though it may overemphasize larger particles.

Thus, both techniques complement each other, and using them together provides a more comprehensive understanding of the particle size distribution of your submicron powders.

I hope this helps?

One comment: a volume or mass distribution is the same as a value ($) distribution. Consider 2 (spherical) particles of diameters 1 and 10 units respectively. On a number basis each particle has equivalent weighting and the average would then be 5.5 units. On a volume basis the 10-unit particle makes up 1000/1001 parts of the total system and thus 1000/1001 parts of the value. The smaller particle only makes up 1/1001 parts of the value of the system. So, if they're particles of gold, which would you prefer? Most materials are sold on the basis of weight or volume and not numbers of particles. A second comment is that 'sub-micron' powder is almost an oxymoron - such systems contain a fused collection of aggregates and looser agglomerates rather than single discrete independent particles. Also see the attached.

I like Gideon C. Irogbele 's answer. It covers many important areas. There are issues of representative sampling to consider, especially with SEM.

If you are going to compare the techniques, you have to make sure they are on the same basis. You should be able to transform one basis into another. And you should consider which basis is appropriate for your application. I used to analyze mineral particles in coal. In that case, the mass of the particle was important. It mattered that 90% of the mass was larger than 20 um even if 70% of the features were smaller than 20 um. If I was studying the effects of inhaling particles, I would probably be more interested in the number of particles of a certain size and their potential health effect even if they amounted to only 2% of the mass of sample.

You should also use the same size scale. You can probably transform that as well, but it would be nice to start with the same size scale. I generally prefer a geometric/logarithmic scale.