Page 45 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: FYK/LSX Revised Pages P2: FWQ/FPW QC: FYD
Encyclopedia of Physical Science and Technology en001d42 April 28, 2001 15:9

Atomic Spectrometry 769

FIGURE 3 Schematic representation of the most frequently used grating-based monochromators (A, B, C) and
polychromators (D, E, F). A and D are known as Czerny–Turner design. Fastie-Ebert (B) and Littrow (C) have similar
designs in that both use a single reflective mirror. A Rowland circle polychromator is depicted in E, and F represents
an Echelle spectrograph.

slit is then collected by a collimating mirror and converted is the polychromator of choice due to its superb spectral
to parallel light rays that are directed toward a wavelength- resolution.
dispersion element (diffraction grating or prism). The
original light is now separated into discrete wavelengths,
B. Photon Detection
which are imaged by the mirror on the exit plane of the
monochromator. A slit is used to allow for propagation of a Historically, photographic films and plates have been used
unique wavelength out of the monochromator and into the at the exit plane of the monochromators to detect the spec-
detector. In most instances, a monochromator can be con- tral patterns from analytes. The main advantage of this ap-
verted to a polychromator by simply removing the exit slit proach is its inherent multi-wavelength detection ability.
and replacing the traditional detector by an area detector Because photographic films and plates are area detectors
(see next section). On the other hand, any polychromator with fantastic spatial resolution, the entire spectrum from
can be used as a monochromator if only a single output a sample may be recorded on a 30-cm plate, which ulti-
channel is used during an experiment. Although many mately results in simultaneous multi-elemental analysis.
wavelength-selection devices are available commercially, Unfortunately, the dynamic range of photographic emul-
the designs shown in Fig. 3 are among the most popular. sion is limited and the response function is typically non-
The Czerny–Turner monochromator is extremely popular linear. Furthermore, the overall process is extremely time
because of its compact design, while the Echelle system consumingandchemicallyintensive(needforadarkroom,