WATERS CORPORATION 2424, Operator'S Manual, Page: 18 / 196

1-4 2424 ELS Detector Optics Principles
species before the detection step. This adds to the background noise and
decreases the analyte signal response, which causes low sensitivity (slope of
the calibration plot) and high limits of detection (LOD). The evaporated HPLC
solvents are condensed and captured in the recommended solvent trap and
exhaust routing. Nevertheless, small amounts of residual can persist, and
these should be exhausted into a fume hood to prevent their escape into the
laboratory.
Detection
The analyte particles enter the detection region where a light source impinges
on the particles. The light is thus scattered and focused onto a photomultiplier
tube (PMT) where its intensity is measured.
The size (diameter) of the analyte particles determines how the light is
scattered. The detector measures the intensity of the scattered light at 60°
relative to the excitation beam to minimize polarization effects and stray
light. Particles of different sizes exhibit different angular distributions of the
scattered light, and particles whose sizes and shapes vary have different
light-scattering cross sections. In general, larger particles scatter more light,
yielding more intense signals and peak responses.
A photomultiplier tube (PMT) converts the scattered light signal to a voltage
that can be recorded and analyzed. The stronger the scattering, the more
intense the final signal on the ELS detection chromatogram. The scattered
light is a rough measure of the mass of material represented by a
chromatographic peak. To some degree, this “mass” response can be
compound-independent. However, many factors can also affect the mass
response, particularly the density of the analyte in a small dried particle. For
example, a popped kernel of corn has a lower density than the unpopped
kernel from which it originated. Yet, because it is larger, in most cases it
would scatter more light. You should also remember that the output of an ELS
detector has no direct relation to the molecular weight of an analyte.
Types of light scattering
The three possible regimes of light scattering are
• Rayleigh
• Mie
• refraction-reflection