Ir Instrumentation

1. IR- Instrumentation

2. INSTRUMENTATION:
In a single beam instrument , the light transmitted through one sample is
measured at a time.
First, the solvent blank is placed in the cuvette and the instrument is
adjusted to read zero transmittance with the light shutter closed, i.e., no
light passes to the detector.
Then the shutter is opened and the instrument is adjusted to read 100%
transmittance.
The cuvette is removed and replaced by an identical one containing the
sample. The absorbance is then read.
The sequence of steps is maintained for every measurement.
It means that both source output and detector sensitivity must remain
constant, i.e., voltage supply to the source and detector should be stable.
Such a procedure compensates also for reflection, scattering or absorption of
light by the cuvette and the solvent.

3. Infrared Instruments
Schematic of a dispersive (double beam) IR spectrometer
• Sample compartments before the monochromator (opposite for UV-vis) to
diminish stray/scattered light problems. Possible because IR radiation does not
tend to photodecompose compounds, unlike the UV/Vis

4. RADIATION SOURCES:
A good spectrometric source should have a stable, high intensity output that covers a
wide range of wavelengths. No single source is suitable for all of the spectral regions.
Sources are divided into thermal and electric discharge sources.
Thermal radiation is the result of high temperature.
• Deuterium discharge lamp:This source uses an electrical discharge to dissociate
deuterium molecules into atoms. The process is accompanied by the emission of
continuous UV radiation in the range of about 160 to 380 nm.
• Tungsten filament lamp:The most common source of visible radiation is the
ordinary tungsten filament lamp. It consists of a thin coil of tungsten wire sealed
in an evacuated glass bulb. Electrical energy passing through the filament is
converted to heat causing it to glow ‘white hot’.
• Mercury is used under high pressure in Hg discharge tubes. It is not suitable for
continuous spectral studies because of the presence of sharp lines or bands
superimposed on a continuous background.
• Xenon discharge lamp operates with a low voltage DC source similar to that of the
H lamp but at xenon pressures in the range of 10 to 30 atm. The intensity in the
near UV is actually much greater than that of the H lamp, but even greater
intensity in the visible region may pose potential stray radiation problems.

5. MONOCHROMATORS:
The purpose of a monochromator is to disperse the radiation
according to the wavelength. Prisms and gratings are used extensively
for this purpose. The most popular materials for making prisms are
glass, quartz and fused silica. Of these, glass has the highest resolving
power. It disperses light strongly over the visible region of the
spectrum. However, it is not transparent to radiation with wavelength
between 350 and 200 nm, because glass absorbs strongly and,
therefore, cannot be used over this wavelength range.

6. CUVETTES:
• Virtually all UV spectra can be recorded in the solution phase and the
samples are placed in cells or cuvettes. Cells may be made of glass,
plastic or quartz. Quartz is transparent in all (200-700nm) ranges and
is normally used in UV region. Plastic and glass are only suitable for
visible spectra.

7. DETECTORS:
The four common types of detectors are:
1. Barrier layer cell or photovoltaic cell
2. Phototube or photocell or photo-emissive tubes
3. Photomultiplier tubes(PMT)
4. Semiconductor devices:
a. Light dependent resistor(LDR) and
b. Linear photodiode array(LPDA)