Dispersive IR spectrophotometers – Instrumental Methods of Analysis B. Pharma 7th Semester

Dispersive IR spectrophotometers


After this session
students will be able to

the essential components of IR spectrophotometers

the construction and working of dispersive IR spectrophotometers

Infrared Spectrum

IR radiation is passed through a sample. Some of the
infrared radiation is absorbed, the rest is transmitted.
The resulting spectrum represents the molecular absorption and transmission,
creating a molecular fingerprint of the sample. No two unique
molecular structures produce the same infrared spectrum. This makes infrared
spectroscopy useful for several types of analysis.


Ideal Spectrometer system

An ideal Spectrophotometric instrument has only five

A source of illumination on the sample (a
light source which provides the electromagnetic energy required for the

A signal sorter (a dispersion or interference
device for resolving the frequencies from each other).

A sampling component (a method of
channeling the light to the sample and from the sample to the instrument).

A signal detector (a transducer which
converts light to a voltage or current).

A computer (to control data acquisition
and analysis).

If each of these components performs its task
, the instrument possesses virtually all the advantages that one
might assign to an ideal analytical device

IR spectrophotometer

1.         Radiation source

radiation can be produced by electrically heating a source, often a Nernst
or a Globar to 1000-1800 °C.

a.       Nernst
filament is fabricated from oxides of zirconium, thorium and cerium.

b.      The
Globar is a small rod of silicon carbide.


a. Thermal Detectors : measures IR energy by means of
its thermal effect, the heating effect of IR radiation produces
an electrical signal that can be measured, thermal noise is
always a problem.

b. Pyroelectric detectors : pyroelectric substances
are sandwiched between two electrodes, when IR radiation reaches the detector ,
temperature changes producing current that is proportional to the
rate of change of temperature, they exhibit fast responses so suitable for

Infrared Instrumentation History
and development


Scanning Instruments

Dispersion Spectrometers (older technique)


Fourier Transform Infrared (FT-IR) Spectrometers
(Modern technique)

Nearly all IR spectrometers, nowadays, are of
the FT type.


Dispersive IR spectrometers

Infrared Dispersion Scanning Instrumentation

Scanning instrument uses a frequency separation
device (grating) to resolve the IR radiation into individual

An exit slit isolates a specific
frequency for passage to the detector.

The IR spectrum is obtained by moving (scanning)
the grating over a given wavenumber region after passing through the

Disadvantages of Dispersion Infrared Instrumentation

Slow Scanning process (time consuming)

nature of spectral acquisition (Measure one frequency at a time-scanning takes
about 5 min)

Limited energy throughput.

dispersion process throws energy away

exist and entrance slits allow throughput of only a small fraction of the total
IR energy (<< 50%)

Difficult to increase the S/N by multiple

reproducibility is not sufficient due to mechanical irrelevant response.

      • Noise
        is random, it may be positive or negative.

If “n” spectra are
à   S/N
increases in proportion of                      

                This means that to improve S/N
by a factor of 2, you have to add and average 4 spectra (scans). Improvement of
S/N by factor of 10 needs averaging 100 spectra.          

All measurements, especially those we carry out with
instruments, generate Noise.

Detectors of all sorts generate electrical noise


simplest IR spectrophotometers are Dispersive IR spectrophotometers

essential components of IR spectrophotometers are radiation source, sample compartment,
dispersive devise and detector

scanning process and limited energy output are the disadvantages of dispersive
IR spectrophotometers


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