LASER light
When an atom becomes ‘excited and, while excited, is struck by a photon, the atom releases another identical photon having the same wavelength and direction as the incident photon. The generation of light photons in this way is called stimulated emission. The two photons, so produced, on collision with other excited atoms can then continue the process and produce two further photons. All the photons produced in this way are identical (wavelength, polarization etc.). The process is called light amplification by stimulated emission of radiation or LASER. In chromatography, LASER’S are used as light sources in light scattering detectors, thermal lens detectors and in LASER desorption ionization techniques for gas chromatography/mass spectrometry.
Author: RPW Scott
Book:Liquid Chromatography Detectors
Section:HPLC-Detectors Liquid-Light-Scattering
Liquid Light Scattering Detectors
Light
scattering detectors differ from evaporative light scattering detectors in that
they respond to the light scattered by a polymer or large molecular weight
substance present in the column eluent itself. The scattering is measured as it
passes through an appropriate sensor cell while illuminated by a high intensity
beam of light. The high intensity light source is achieved by the use of a laser
(light amplification by the stimulated emission of radiation) that also
generates the light at the appropriate wavelength for measurement. There are
two forms of the detector: the low angle laser light scattering (LALLS)
detector and the multiple angle laser light scattering (MALLS) detector.
Both devices are commonly used but the multiple angle laser light scattering
detector is more versatile as it provides molecular dimensions as well as the
HPLC-Detectors Liquid-Light-Scattering
Author: RPW Scott
Book:Liquid Chromatography Detectors
Section:HPLC-Detectors Refractive-Index-Based Thermal-Lens
index is negative and
consequently, the insertion of a liquid in the laser beam produces a concave
lens that results in beam divergence. Buffet and Momis (27) used the
thermal lens effect to develop a small volume detector, a diagram of which is
shown in figure 20.
Figure 20 The Layout of a
Thermal Lens Detector
The device
consists of a heating laser, the light from which is passed directly
through the sample via two lens and a half mirror. Another laser, the probe
laser, passes light in the opposite direction through one lens, through the
sample to the half mirror where the light is reflected onto a photocell. A
filter and a pinhole screen is placed between the mirror and the photo-cell to
remove the heating laser light. When an absorbing solute is eluted from the
column through the cell, a thermal lens is produced causing the probe light to
diverge, and the intensity of the light passing through the pinhole and on to
the photocell is reduced
HPLC-Detectors Refractive-Index-Based Thermal-Lens
Author: RPW Scott
Book:Liquid Chromatography Detectors
Section:HPLC-Detectors Liquid-Light-Scattering Low-Angle-Laser
and light scattering measurements; thus, by plotting
against
(c) a straight line will be produced with the intercept being .
The Low Angle Laser Light Scattering Detector
The optical
system of the low angle laser light scattering detector produced by LDC
Analytical of the Thermo Instruments Corporation is shown diagramatically in
figure 51. To conserve space, a folding prism is used that allows the device to
be contained to a reasonable size yet accommodate the length of the laser
generator. Light from the laser passes through a diverging lens, through a
chopper and then through the folding prism. On leaving the prism the beam
passes through some measuring attenuators and a calibrating attenuator shutter
and then through the cell. An annular mask is situated between the cell and the
relay lens and only allows light scattered in the cell at a low angle to pass
to the relay lens. Between the annular shutter and the relay lens is a safety
attenuator that ensures that
HPLC-Detectors Liquid-Light-Scattering Low-Angle-Laser
Author: RPW Scott
Book:Liquid Chromatography Detectors
Section:HPLC-Detectors Liquid-Light-Scattering Multiple-Angle-Laser
for their different responses.
The number of
different angles of measurement differs with different instruments, and some
measure the scattered light intensity at 16 different angles. In general, the
more data points taken at different angles, the more precise the results will
be. A diagram of a (MALLS) detector system which measures the light scattered
at three different angles is shown in figure 53.
Courtesy of Wyatt Technology Corporation
Figure 53 The Multiple Angle
Laser Light Scattering Detector (miniDawn®)
This device
(the miniDawn®), manufactured by Wyatt Technology Corporation, contains no
mirrors, prisms or moving parts and the light paths are direct and not "folded".
Light passes from the laser (wavelength 690 nm) directly through the sensor
cell. Light scattered from the center of the cell passes through three narrow
channels to three different photocells, set at 45o and 90o
and 135o to the incident
light. Thus scattered light
HPLC-Detectors Liquid-Light-Scattering Multiple-Angle-Laser
Author: RPW Scott
Book:Liquid Chromatography Detectors
Section:HPLC-Detectors Liquid-Light-Scattering Multiple-Angle-Laser
Figure 52
Determination of Molecular Weight from Low Angle Light Scattering Measurements
The detector
sensitivity appears to be similar to that of the refractive index detector and
with about the same linearity. However, the greatest advantage of this detector
is that it can provide molecular weight data for extremely large molecules.
The Multiple Angle Laser Light Scattering (MALLS) Detector
The multiple
angle laser light scattering detector differs from the low angle device, in that scattering measurements are made at a number
of different angles, none of which are close to the incident light. This
significantly reduces problems associated with light scattering from
particulate contaminants. Data taken at different angles to the incident light
allows the root-mean-square (rms) of the molecular radius to
be calculated in addition to the molecular weight of the substance. The
relationship that is used
HPLC-Detectors Liquid-Light-Scattering Multiple-Angle-Laser
Author: RPW Scott
Book:Liquid Chromatography Detectors
Section:HPLC-Detectors Liquid-Light-Scattering Low-Angle-Laser
Courtesy of LDC Analytical, Thermo Instruments
Corporation.
Figure
51 Optical Diagram of a Low Angle Laser Light Scattering Detector
Between the
forward detector lens and the rear detector lens is A filter holder and an
analyzer/polarizer. Finally the light is focused through a sensor aperture to
an opal diffuser that spreads the scattered light through a red filter
and onto the photo- multiplier.
The device is
frequently used with a refractive index detector in series to coincidentally
measure the refractive index of the eluent. This is necessary to calculate (K)
from the
HPLC-Detectors Liquid-Light-Scattering Low-Angle-Laser