Obviously optical density, Heywood shape factor, refractive index are all necessary variables to determine shape by something like a Dynamic Light Scattering or Laser Diffraction based nano-sizer instrument. However, I always need to enter the physical density of the material in these interfaces as well, but I have not been able to think of a reason why this is needed.
We have recently had an incident where some samples were discovered to have had the wrong s.g. entered at measurement. The discrepancy was less than 1%, so my suspicion is that it will be irrelevant when compared to the threshold of instrument noise, operator error etc. However, I do not know how physical density / specific gravity are used mathematically in the calculations. Maybe it has a non-linear relationship. The instrument uses a Fraunhofer Diffraction model.
Roland Kerr It doesn't need to be known for the particle size distribution. The entering of the density (not the specific gravity) for the material enables the (calculated/derived) SSA (= 6/D[3,2]) to be changed from say m2/cm3 to m2/g. Your last comment ('The instrument uses a Fraunhofer Diffraction model') is incorrect - the correct Mie modeling should be applied. However, interestingly, and not relevant to this conversation, there is a link between density and refractive index in the Gladstone-Dale relationship. We'll be dealing with that in a newly branded webinar (original was in 2006) on March 17th. See: https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W200317LDBasics2.html
or more conveniently as: https://tinyurl.com/vykzfzn
Roland Kerr DLS doesn't need the optical properties for an intensity size distribution. It'll require these for a transformation from the intensity to volume distributions.
One reason you'd need to know the particle density is if you are doing powder analysis and for some reason to want to estimate aerodynamic size.
Or you want to convert a mass fraction to volume fraction.
John Francis Miller Hi John Yes, you're correct in that density is required for aerodynamic diameter. However, this parameter would probably be measured in an appropriate instrument and the question referred to laser diffraction.
You second answer needs expanding - for a single material, of constant density, then mass and volume distributions are identical. In a mixture of different densities, then density would be needed in the manner you state.
Alan F Rawle Laser diffraction is a useful surrogate for assessing particle size distributions of aerosols emitted from inhalers. I used to do it a lot because (a) it's a LOT cheaper than cascade impaction, (b) much quicker, (c) needs much less material (e.g., one dose).
During the development of novel powders for inhalation (typically porous particles via spray drying), the combination of cascade impaction and laser diffraction provided a fair estimate of the particle density. Subsequently, it permitted the use of laser diffraction as a rapid screening tool.
Re. the mass vs. volume fraction, I must say I was being a bit lacking in my answer. What I meant is that you may have a suspension manufactured on a mass per unit volume basis but need to know the volume per unit volume basis. An instrument as a laser diffractometer may record it simply for completeness or if an estimate of particle concentration is needed. I've never come across such a need, though.
John Francis Miller Thanks for adding to your earlier response. Yes, your comments are very valid with respect to aerosols/MDI's and cascade impactors. However, we find, because of the regulatory and pharmacopoeia requirements LD users are often asked to refer back to the 'real' answer provided by these (rather crude) techniques. The advantage of LD is so obvious here - the ability to characterize the front, middle, and trailing portion of any plume.
Your mass/volume comment is also relevant and has direct impact in a way that perhaps you didn't think of. The calculated volume concentration provided on our laser diffraction equipment (derived from Bouguer's Law 1729) is actually stated as volume/volume ([100*volume]/volume if I'm pedantic). So a knowledge of particle density is usually needed if one is doing a determination of the imaginary part of the refractive index as, for example, in:
June 22nd 2010 Determination of the optical properties of CeO2 and CaCO3 by the volume concentration experiment - the 3 S's, Beer-Lambert and Mie
https://www.brainshark.com/malvern/vu?pi=zF2zI6Y3fz5chz0&tx=preview
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