Page 339 - Instrumentation Reference Book 3E

322 Chemical analysis: spectroscopy

Figure 16.3 Internal view of multiple reflections of variable long path cell. Courtesy Invensys.

with a selector switch to monitor key points in the These penetrations form a path whose length is
circuit, in particular the degree of obscuration dependent on the number of reflections. The
in the measuring cell. By choosing the appropri- energy is absorbed at the analytical wavelength
ate absorption bands the analyzer may be made proportionally to concentration just as in other
suitable for a wide range of gases or liquids. For types of cells. The crystal used is made of KRS (a
gases, it may be used for C02, CO, SO2, CH4, composite of thallium bromide and iodide).
CzHs, C2H4, C6H6, C2H2, NH3, N20, NO, Ordinary transmission cells have limited applic-
NO2, C0Cl2, and HzO, with ranges of &300ppm ability for high concentrations, viscous or aqu-
and 0-100 percent. eous streams. In many cases, the infrared beam is
It may also be used for measuring water in grossly attenuated or the sample cannot be
ketones, hydrocarbons, organic acids, alcohols, pumped through such cells. Multiple internal
glucols, and oils. The accuracy is *1 percent and reflection overcomes these problems.
the response time for 90 percent change is 3 s. The applications to which this instrument has
The instrument is marketed by Anatek Ltd, as been put include (a) for gases: the determination
the PSA 401 process stream analyzer. of phosgene in methane and plastic production;
Another instrument based on the same princi- methane and carbon dioxide in synthetic and
ple is the Miran I1 Infra Red process analyzer- natural gases in the range 1 ppm to 100 percent;
the chief difference being the sample cell used for (b) for liquids: water in acetone distillation, pet-
gas and liquid streams. These cells are either long roleum waste treatments, urea in fertilizer pro-
path gas cells or multiple internal reflection cells. duction and isocyanates in urethane and plastic
The gas cells which are normally manufactured in production in the range 50ppm to 50 percent;
stainless steel have a variable path length (see (c) for solids: the percentage weight of film coat-
Figure 16.3). Energy passes through the sample ings such as inks and polymers; and film thickness
gas and reflects one or more times off the mirrors for nylon and polythene (up to 0.025mm).
in the cell before striking the detector. The path In recent years, there has been much growth in
length can be adjusted between 0.75 and 20.25 the use of fiber optic probes as a means to non-
meters by suitable adjustment of the mirrors. intrusively introduce infrared and near infrared
These gas cells are used to analyze the presence wavelengths to the process. Use of these probes
of low concentrations of components in gases or negates the need for sample systems since only the
for those gases requiring a long path length to tip of the probe is in contact with the process,
enhance sensitivity at a weak analytical wave- typically through a retractable mechanism
length. directly mounted to the stream of interest.
In a multiple internal reflection (MIR) cell, the
infrared beam is directed along or around an optical 16.1.1.2 Dispersive infrared analysis
crystal through which the beam passes (Figure
16.4). As the beam is reflected on the sample The previous section was devoted to analysis
crystal interface, it slightly penetrates the liquid. using only one absorption frequency. However,
all organic compounds give rise to a spectrum in
the infrared in which there are many absorption
frequencies giving a complete fingerprint of that
compound. Dispersive infrared can be used,
among other things, to identify a substance, for
the determination of molecular structure for reac-
tion kinetic studies, and for studies of hydrogen
Figure 16.4 Principle of MIR sampling technique.
Courtesy Invensys. bonding.