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

Dispersive IR spectrophotometers


After this session students will be able to

       Identify the essential components of IR spectrophotometers

       Explain 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 components:

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

        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 properly, the instrument possesses virtually all the advantages that one might assign to an ideal analytical device.

IR spectrophotometer

1.         Radiation source

        Infrared radiation can be produced by electrically heating a source, often a Nernst filament 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.

2.       Detector

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 FT-IR.

Infrared Instrumentation History and development


        Scanning Instruments

          Dispersion Spectrometers (older technique)

         Multiplex Instruments

          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 frequencies.

          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 sample.

Disadvantages of Dispersion Infrared Instrumentation

          Slow Scanning process (time consuming)

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

       Limited energy throughput.

        optical dispersion process throws energy away

        The 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 scanning

        Wavelength reproducibility is not sufficient due to mechanical irrelevant response.

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

If “n” spectra are added  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


       The simplest IR spectrophotometers are Dispersive IR spectrophotometers

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

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


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