8Biotage
®
ELSD-A120 User Manual
1 Introduction to the Detector
Detector Overview
Figure 1 Principles of operation
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.
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. The majority of organic
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