Malvern Mastersizer 2000, User Manual

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Chapter 2 Overview
Page 2-2 MAN 0384
2
How the Mastersizer 2000 works
Scientists have for centuries tried to predict the way particles scatter and absorb
light. There are many theories and models that the modern particle size analyst can
use.
One of the simplest theories used is the Fraunhofer model . This model can pre-
dict the scattering pattern that is created when a solid, opaque disc of a known size
is passed through a laser beam.
This model is satisfactory for some particles but it does not describe the scattering
exactly. Very few particles are disc shaped and most particles are transparent.
The accepted theory which accurately predicts the light scattering behaviour of all
materials under all conditions is known as the
Mie theory .
Mie theory was developed to predict the way light is scattered by spherical particles
and deals with the way light passes through, or is adsorbed by, the particle. This
theory is more accurate, but it does assume the user knows some specific informa-
tion about the particle, such as its refractive index and its absorption.
The key point about these theories is that if the size of the particle and other details
about its structure are known, the way it will scatter light can be predicted accu-
rately. Each size of particle will have its own characteristic scattering pattern, like a
fingerprint, that is unlike any other size of particle.
So how does the Mastersizer measure the size of particles? It works backward from
the above theories by using the optical bench to capture the actual scattering pattern
from a field of particles. Then, using the theories, it calculates the size of particles
that created that pattern.
There are three distinct procedures in measuring a sample on the Mastersizer:
1.
The sample is prepared and dispersed to the correct concentration and then
delivered to the optical bench. This is the purpose of the sample dispersion
units. Sample preparation is the most important stage of making a measure-
ment. Remember that, if the sample is poorly prepared (unrepresentative or
badly dispersed) then the basic data will be incorrect; no amount of analysis of
this data will give a correct answer.
2.
The capturing of the scattering pattern from the prepared sample - this is
known as the “measurement”. This is the function of the optical bench.
The detector array within the optical bench is made up of many individual
detectors. Each detector collects the light scattering from a particular range of
angles. A typical light scattering pattern is shown below.
MAN0384-1.0 Mastersizer 2000.book Page 2 Tuesday, March 20, 2007 8:56 AM