BASIC INSTRUMENTATION OF IR AND VIBRATION MODES

Name: Aamna KhurshidDepartment: Msc Chemistry

BASIC INSTRUMENTATION OF IRThe instrument used in IR spectroscopy is

called IR spectrometer. It determines wavelength and absorbance of a sample in infrared region of electromagnetic spectrum.

 Spectromete

r

Dispersion (IR)

Interferomet

ric type(FTIR)

DIFFERENCE BETWEEN DISPERSIVE AND FTIR

INSTRUMENTATIONThe main parts of IR spectrometer are

following:1- Radiation source2- Sample cell3- Monochromator4- Detector5- Recorder

1-RADIATION SOURCE

1- Incandescent lamp Used in near IR region. Fails in far IR region. Temperature can be raised to 1100’C. Nichrome wire is tightly wound and is heated by

passing current. Gives less energy then Nernst and Goblar

RADIATION SOURCE

2- Nerst Glower Composed of rare earth oxides e.g., zirconium

oxide , yttrium oxide. Platinum ends leads to passage of electricity. Temperature: 1000-1800’C

RADIATION SOURCE3- Gobbler Silicon carbide rod Electrically heated at 1,500K. Less intense then Nernst glower.

RADIATION SOURCE4- Mercury Arc: Used in Far IR. Enclosed in quartz . Mercury vapors are stored under high pressure and

excitation of mercury is done by electric current. Not suitable for continuous spectrum.

2-SAMPLE CELLFor gas sample: The spectrum of gas can be obtained by

permitting gas to expand into an evacuated cell, also called cuvette.

For liquid sample: IR solution cell consist of two windows of pressed salt sealed. Samples that are liquid at room temperature are analyzed in pure form or in solution form.

 For solid sample: solid samples are examined as thin paste

or mull. Mull is formed by grinding sample 2-5 milligram in presence of one or two drops of nujol oil (hydrocarbon oil). The resulting mull is examined as a film between salt plates. Sodium chloride or potassium bromide plates are commonly used.

3-MONOCHROMATOR Splits polychromatic light into

monochromatic light. select desired frequency and reject

others. Two types: 1) Prism 2) Grating

4-DETECTORS1- Thermocouple: temperature measuring device consist of two dissimilar conductors. detects shorter IR wavelengths.

DETECTORS2-Bolometer: Used to measure low power. constructed of metal stripes. highly sensitive. works on change in resistance. operates as resistance temperature detectors.

DETECTORS3-Pyroelectric Fast Sensitive for mid IR4-Photoconducting detectors: for near IR fast and sensitive

RECORDER5-Recorder:

The radiant energy received by detector is converted into measurable electrical signal and is amplified by amplifier.

The amplified signals are recorded and plotted.

It records transmittance of sample as well as function of wave number.

VIBRATION MODESThere are two main types of modes of molecular vibration: 1- Stretching: change in inter-atomic distance along bond

axis. Symmetric Asymmetric 2- Bending: change in angle between two bonds. Rocking Scissoring wagging Twisting