Figure 14.6 shows a schematic of the optical system used in the CPL-300. The
light source for CPL measurements can be a laser, a xenon or mercury xenon lamp,
or a light-emitting diode (LED). In the CPL-300, a high-brightness xenon light
source is installed by default and the excitation wavelength can be varied depending
on the sample. The light is first passed through a monochromator that uses a prism or
a diffraction grating as the dispersive element. This light is then depolarized by a
quartz depolarizer and irradiates the sample. Since CPL signals are generally very
weak, the instrument must be capable of high-sensitivity measurements that are free
from artifacts. The CPL-300 uses a double-prism monochromator in order to achieve
low stray light levels, avoid higher-order light due to a diffraction grating and light
polarization due to Woods anomalies, and minimize the distortion of the CPL
spectrum. Circularly polarized fluorescence emitted from the sample should be
collected by the photo-detector placed in the angle of 90
or 180
toward the
extinction direction. The light from the sample first passes through a PEM and
polarizer, and is then passed through a monochromator and detected by a detector.
In the 90
arrangement, although the detected light is less likely to be affected by
excited scattered light, in the case of highly viscous or solid samples, there is a
possibility that the CPL spectrum will be distorted by fluorescence anisotropy. To
avoid this, the CPL-300 employs the 180
arrangement. A highly sensitive
photomultiplier tube is used for the photo-detector.
14.4 Calibration of CPL Instrument
The most important aspect of CPL measurements is the accuracy of the spectrum. It
is therefore important to have a reliable calibration method. In ultraviolet and visible
spectrophotometers, fluorescence spectrophotometers, and circular dichroism
Light source
Mirror
Prism
Lens
Depolarization
plate
Sample
PEM
Polarizer
Detector
Fig. 14.6 Optical system for CPL-300
314
S. Suzuki
light source for CPL measurements can be a laser, a xenon or mercury xenon lamp,
or a light-emitting diode (LED). In the CPL-300, a high-brightness xenon light
source is installed by default and the excitation wavelength can be varied depending
on the sample. The light is first passed through a monochromator that uses a prism or
a diffraction grating as the dispersive element. This light is then depolarized by a
quartz depolarizer and irradiates the sample. Since CPL signals are generally very
weak, the instrument must be capable of high-sensitivity measurements that are free
from artifacts. The CPL-300 uses a double-prism monochromator in order to achieve
low stray light levels, avoid higher-order light due to a diffraction grating and light
polarization due to Woods anomalies, and minimize the distortion of the CPL
spectrum. Circularly polarized fluorescence emitted from the sample should be
collected by the photo-detector placed in the angle of 90
or 180
toward the
extinction direction. The light from the sample first passes through a PEM and
polarizer, and is then passed through a monochromator and detected by a detector.
In the 90
arrangement, although the detected light is less likely to be affected by
excited scattered light, in the case of highly viscous or solid samples, there is a
possibility that the CPL spectrum will be distorted by fluorescence anisotropy. To
avoid this, the CPL-300 employs the 180
arrangement. A highly sensitive
photomultiplier tube is used for the photo-detector.
14.4 Calibration of CPL Instrument
The most important aspect of CPL measurements is the accuracy of the spectrum. It
is therefore important to have a reliable calibration method. In ultraviolet and visible
spectrophotometers, fluorescence spectrophotometers, and circular dichroism
Light source
Mirror
Prism
Lens
Depolarization
plate
Sample
PEM
Polarizer
Detector
Fig. 14.6 Optical system for CPL-300
314
S. Suzuki