Chapter 16 & 17

Introduction and Applications of

Infrared Spectrometry

DEFINITION OF INFRARED SPECTROMETRY...

The absorption of light as it passes through a medium varies linearly with the distance the light travels and with concentration of the absorbing medium.

THEORY OF INFRARED ABSORPTION SPECTROMETRY...

• Like in all spectometries, IR excites the analyte to be studied and gets a measurement of the changes. These changes can be related to the type of analyte being observed. The source of this excitation is of course infrared radiation. The Infrared spectral regions are as follows:

Region Wavelength Range, um Wavenumber Range,cm-1 Frequency Range, Hz

Near 0.78-2.5 12800-4000 3.8x1014-1.2x1014

Middle 2.5-50 4000-200 1.2x1014-6.0x1012

Far 50-1000 200-10 6.0x1012-3.0x1011

Most used 2.5-15 4000-610 1.2x1014-2.0x1013

Sources...• .The Nernst Glower – Rare earth oxides in a cylinder

shape. When electricity is run through it, it results in temperatures between 1200 K and 2200 K.

• .The Globar Source – Silicon carbide rod, has positive coefficient of resistance.

• .Incandescent Wire Source – Tightly wound spiral of nichrome.

• .The Mercury Arc – For far-infrared region of the spectrum

• .The Tungsten Filament Lamp – good for near-infrared region

• .The Carbon Dioxide Laser Source – Good for quantitative work

Transducers...• Thermal Transducers – Response based upon

heating effect of radiation are employed for detection of all but the shortest infrared wavelengths.

• Thermocouples – A potential develops between two plates of metal, a low-impedance device.

• Bolometers – Resistance thermometer made from strips of metal.

• Pyroelectric Transducers – A crystalline wafer of pyroelectric material that polarizes when an electric field is applied.

• Photoconducting Transducers – Thin film of a semiconductor material that absorbs radiation.

INFRARED INSTRUMENTS...

Three main types• Dispersive grating spectrophotometers,

qualitative.• Multiplex instruments, like Fourier

transform, for both quantitative and qualitative work.

• Nondispersive spectrophotometers, quantitative.

Applications of IR...

• The applications of infrared spectrometry can be split into three main areas:

• .Near-infrared – 4,000 to 14,000 cm-1

• .Mid-infrared – 670 to 4,000 cm-1

• .Far- infrared – less than 670 cm-1

Types Of Samples

• Gas in an evacuated cylinder

• Solutions

• Dissolved in solvents

• Liquid in cell

• Solids

• Pellets

• Mulls

Mid-Infrared Absorption Spectrometry...

Mid-infrared absorption and reflection spectrometry are major tools for determining the structure of organic and biochemical species.

- dPx/Px = dS/S

Qualitative Analysis...

The mid-infrared region is further broken down into the group frequency region(1,280 to 5,000 cm-1) and the fingerprint region (670 to 1,280 cm-1).

Quantitative Applications...

Quantitative infrared absorption methods differ somewhat from UV/VIS molecular spectroscopic methods because of the greater complexity of the spectra, the narrowness of the absorption bands, and the instrumental limitations of infrared instruments. Quantitative data obtained with dispersive infrared instruments are generally poorer in quality to data obtained with UV/VIS spectrophotometers.

Internal Working of a Spectrometer...

Figure: Illustration of the primary and secondary beams from a sample in a IR spectrometer.

Illustration of Electronic Transitions...

Binding Energy of Different Elements...

References...

• http://www.acs.org

• http://www.cas.org

• http://www.chemcenter/org

• http://www.sciencemag.org

• http://www.kerouac.pharm.uky.edu/asrg/wave/wavehp.html