This is a very interesting question, but I am afraid it's quite impossible to answer. The real problem is this one: if you work on natural samples (soils or sediments), you cannot change the refractive index used from one sample to another one. If you do that, you will never be sure that you could compare each sample. Furthermore, if you find a refractive index which seems well adapted for small particles, what would happen for a mixed sample containing higher particles. We had this problem for example when working on riverine suspended particulate matter. Both sizes and mineralogy could change between high and low flow, but we had to kept the same methods for the analyses on Malvern. In my memory, we decided to keep the usual refractive index proposed by the company.

I am sorry, I have no experience.

Hello, I work with laser diffraction as well (Analysette 22), fortunately I can chose the analysis method (either Mie or Fraunhofer theory). Although, the software database icludes indexes, it is mostly for elements, not for soil itself. For example for the quartz it says RI = 1,54-1,5683 and the adsorption coefficient = 0. Because of the problems that you have desribed above I always use Franuhofer theory. But to help you in your effort, see this paper, maybe it will be helpful.

http://www.agriculturejournals.cz/publicFiles/00098.pdf

Good luck and please let me know when you will find out the suitable values :)

I'm so sorry Alexander, but i have no experience in this area.

It is interesting for my samples I have been finding that there is little effect of selecting a adsorption index of 0.01 or 1.0 on the weighted residual (the difference between the data and the fitted line). However, an adsorption index of 0.1 gave me a large weighted residual in comparison to 0.01 or 1.0. I decided to go with a adsorption index of 1.0 as it made the shape of the curve look a little less bimodal and in most cases got rid of a small weird peak out in the 1000 micron range.

@ Elena

Fraunhofer is thought to give very poor results for particles less than 10 microns and I think most people have switched to Mie because of this reason, but I am not sure what is better to use a less superior model like Fraunhofer or guess at the optical properties of the material. I will continue to use Mie theory in my work.

Here are two references that use similar settings as to what i have used.

Ryżak, M., and A. Bieganowski. “Methodological Aspects of Determining

Soil Particle-size Distribution Using the Laser Diffraction Method.”

Journal of Plant Nutrition and Soil Science 174, no. 4 (2011):

624–633.

Sperazza, M., J. N. Moore, and M. S. Hendrix. “High-resolution

Particle Size Analysis of Naturally Occurring Very Fine-grained

Sediment Through Laser Diffractometry.” Journal of Sedimentary

Research 74, no. 5 (2004): 736–743.

This is a very interesting question, but I am afraid it's quite impossible to answer. The real problem is this one: if you work on natural samples (soils or sediments), you cannot change the refractive index used from one sample to another one. If you do that, you will never be sure that you could compare each sample. Furthermore, if you find a refractive index which seems well adapted for small particles, what would happen for a mixed sample containing higher particles. We had this problem for example when working on riverine suspended particulate matter. Both sizes and mineralogy could change between high and low flow, but we had to kept the same methods for the analyses on Malvern. In my memory, we decided to keep the usual refractive index proposed by the company.

@ Alexander, thanks for those references. I will do some measurements using Mie and those settings and compare it with Fraunhofer. I am aware that the Fraunhofer theory is less precise for small particles, but according to the FRITSCH (distributing the product that we use) manual it is better to use Fraunhofer theory if you do not know the optical characteristics of the sample. Further, they suggest the transition between Fraunhoffer and Mie in the region of about 0,5 - 1 micrometers - particle size not significantly larger than wavelength of the light used..

I wonder if there is any easy method to estimate the optical properties of the sample....

I'm using also the Mastersize to analyse particle size. I found that the proportions of clay and small particles are always understimated. It doesn't matter for my study as I'm only interested in the sand and larger particles. However there is an article by Scott-Jackson and Walkington, 2005, in the Journal of Archaeological Science, 32:7. Where they do some experiments and tried to avoid this understimation of small particles.

@ Olivier

I think I agree with you that it is really impossible to answer. I will also, like you, stick with one setting as to enable a comparison between treatments/river basins/soils.

@ Elena

Let me know what you find out, I have also read that is little difference between Mie and Fraunhofer when looking at soils and sediments.

I don't think there is an easy way to estimate the optical properties. I have also read some work where the researchers looked at the mineralogy of the sample and looked up the optical properties and took the average vale. Although determining mineralogy is time consuming, and also some samples will still contain some organics which will add to the confusion.

@ Marta

Thanks for reference. And when you say it underestimates clay what is that in comparison to? I understand that the laser diffraction and sedimentation methodologies should not be compared. Some researcher have tried to build relationships between the two methods, but such relationships would be site specific. There is also a few people who suggest that trying to compared the two methods should not be even attempted (see the below reference).

Eshel, G., Levy, G.J., 2007. Comments on“ A Fast Method for Determining Soil Particle Size Distribution Using A Laser Instrument” By FJ Arriaga, B. Lowery, and DW Mays. Soil Sci. 171: 663-674 (2006). Soil Science 172, 413–415.

Thank you to everyone for participating in this discussion I have been finding it most helpful

A recent article by a Canadian researcher, definitely worth a read

Kowalenko, C.G., Babuin, D., 2013. Inherent factors limiting the use of laser diffraction for determining particle size distributions of soil and related samples. Geoderma 193-194, 22–28.

I am not sure if this method is better but I set up an old Turner Nepholometer to measure reflected light at 90 deg from the source. You can punch a hole in the Turner without hitting electronics so a test tube 20 cm X 2.5 cm diameter can move up and down past the light source. Take a sample and wash through 63um and then 32um screens to get wt of sand and one fraction of silt. And determine the wt of < 32um fractions. Add calgon and sconicate to break clay / then follow pipette procedure measuring turbidity at time zero followed by time and depth based on temperature of water etc. to get the size fractions you want. (18c: 10cm depth at 8 min = 0.016-0.008mm range; 10 at 32 min = 0.008 to 0.004mm; 10cm at 2h8min=0.004 to 0.002mm; 7cm at 5h58min=0.002 to 0.001mm; 7 cm at 24hours = Less than 0.001 mm) Determine reflected light (NTU units) that represents a size fraction (reading diference from before from after time). Reflected light is from surface area of a sphere 4 X Pi X radious squared. Determine Volume of that sphere is (4/3)X Pi X (radious X3). Determine relative (to the total weight of material) weight of particles in each time range that reprecents each size fraction. This works better than pipette method IMO because you don't need one liter containers, you don't disturb water when withdrawing samples and salts in the water are not needed in the calculations. Also; one uses such a small amount of total sediment you can really treat to break up the clay thats much harder to do with large samples.

Sorry for chiming in so late. I have been using laser particle sizers and coulter counters since 1996. Another good reference is Goossens (2008): Techniques to measure grain-size distributions of loamy sediments: a comparative study of ten instruments for wet analysis. Sedimentology 55, 65-96. With my own instrument, the Malvern Mastersizer 2000, I follow the recommendations of Sperazza et al. (2004). The key is to know as much as possible about the composition of the soil or sediment. Particle mineralogy can be determined via XRD and/or SEM-EDX analysis. If carbonate and organic matter are removed via pretreatment, I believe it is safe to assume that most soil and sediment residues are rich in quartz and clay minerals, although exceptions exist. The article by Kowalenko and Babuin (2013) mentioned above is interesting, but of limited use because the authors do not report the instrument settings. The authors cite articles from the 1990s when laser particle sizers were still single light source with Fraunhofer theory, which had its problems. The phenomenon they describe is well known to advanced users of laser particle sizers. The dual light source laser particle sizers of the 21st century are very useful for analyses of soils and sediments with broad size distributions, but which instrument one prefers depends on the character of the materials to be measured.

i am measuring particle size of MnHPO4 and Cu3(PO4)2 in 0.1 M PO4 buffer in the Mastersizer

does anyone know the refractory and adsorption indices for these two solids? If not any suggestion on what values I can use to give me reasonable results?