Agilent InfinityLab ELSD User Manual
11
1
Introduction to the Detector
System Overview
Theory
There are four main processes, by which the path of electromagnetic radiation or
light can change direction, when passing through a medium containing a
suspended particulate phase, see
Figure 2
ELSD Scattering Mechanisms
The importance of each of these processes depends on the radius of the particle
(r) compared to the wavelength (
λ
) of the incident light. Rayleigh scattering is
predominant when r/
λ
is < 5·10
-2
. When particle dimensions are greater than
λ
/20
they no longer behave as point sources, and Mie scattering becomes
predominant. Once particle size approaches the wavelength of incident light then
reflection and refraction begin to prevail.
The relative importance of refraction and reflection can be understood by
examining the effects of the incident light on a single spherical particle whose
equilateral axis lies in the same plane as the photodetector and light source. With
this configuration, refraction is of greater significance than reflection. Most
organic compounds have refractive indices between 1.3 and 1.5. Changes in the
refractive index within this range will not greatly affect the quantity of light
reaching the detector. This accounts for similarities in the sensitivity of the
instrument to various compounds.
Reflection
Refraction
Mie
Rayleigh
