I R SPECTROSCOPY

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ir spectroscopy by D.lohithasu M Pharm

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INFRARED SPECTROSCOPY : 

INFRARED SPECTROSCOPY

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CONTENT What is FT-IR Spectroscopy Why FTIR is used Why is it Preferred over Dispersive method of IR spectroscopy FT-IR basic theory Working of FT-IR Advantages of FT-IR Applications of IR

INTRODUCTION: 

INTRODUCTION Spectroscopy is the branch of science dealing with the study of interaction of electromagnetic radiation with matter. IR spectra is mainly used in structural elucidation to determine the functional groups.

IR REGION: 

IR REGION From application and instrumentation point of view infrared region is subdivided into But the mostly used region is 2.5 µ to 25 µ . REGION WAVE LENGTH m WAVE NUMBER cm -1 Near infrared 0.75-2.5 13300-4000 Mid infrared 2.5-50 4000-200 Far infrared 50-300 200-33.33

Types Of Vibrations: 

Types Of Vibrations Stretching Vibrations: based on bond axis

Types Of Vibrations: 

Types Of Vibrations Bending Vibrations: based on bond angle

principle: 

principle In any molecule ,it is known that atoms or groups of atoms are connected by bonds. These bonds are similar to springs and not rigid in nature. Different atoms which are at either end of the bond have different strengths due to different masses. As a result the bonds vibrate at different frequencies and is characteristic to every portion of the molecule. This frequency is called the NATURAL FREQUECY OF VIBRATION. When Applied infrared frequency = Natural frequency of vibration then absorption of IR radiation takes place and a peak is observed.

Hooke’s law: 

Hooke’s law Hooke's law of elasticity is an approximation which states that the extension of a spring is in direct proportion with the load added to it as long as this load does not exceed the elastic limit.

Hooke’s law: 

Hooke’s law K = force constant in dynes/cm m = atomic masses

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What is a FTIR Spectrometer? A spectrometer is an optical instrument used to measure properties of light over a specific portion of the electromagnetic spectrum. FTIR (Fourier Transform Infra Red) spectrometer obtains an infrared spectra by first collecting an interferogram of a sample signal using an interferometer, then performs a Fourier Transform on the interferogram to obtain the spectrum. An interferometer is an instrument that uses the technique of superimposing (interfering) two or more waves, to detect differences between them. The FTIR spectroscopy uses a Michelson interferometer .

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Why is FT-IR used? Traditional IR Spectrometers – monochromation – NaCl / KBr prisms. Diffraction gratings – Mid IR – adequate dispersion; high resolution – wider spectral region. Most advanced IR – Interferometry procedure – Interferometer.

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FOURIER TRANSFORM Fourier transform is a mathematical method, used to do the conversion from the Time-domain spectrum to the Frequency-domain spectrum. Being a transform, no information is created or lost in the process, so the original signal can be recovered from the Fourier transform. T ransformation of a signal is a continuous complex process ,thus the use of FT and of an algorithm simplifies the computerized calculations.

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Fourier Transformation F( w ) = f(t)e dt iw t Converts from units of time to units of frequency

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Principle of FT-IR Relies on Michelson Interferometer – Interferometric Infrared Spectroscopy Interferometer is used to change the frequency of the EMR from the source to a proportionately slower oscillating signal,slower oscillation enables the detector to respond accurately. Sample compound placed in the beam, before /after interferometer, it absorbs particular frequencies , so that their intensities are reduced in the Interferogram and the obtained Fourier Transform is the IR absorption spectrum of the sample.

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Sensitivity Wavelength accuracy- backlash in mechanical movements (movable mirrors and gratings) Speed FT-IR vs Dispersive IR spectrometer

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Schematic of Michelson Interferometer

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Interferometer He-Ne gas laser Fixed mirror Movable mirror Sample chamber Light source Detector Beam splitter FT Optical System Diagram

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FTIR Theory The heart of the FTIR is a Michelson interferometer. The mirror moves at a fixed rate. Its position is determined accurately by counting the interference fringes of a collocated Helium-Neon laser. The Michelson interferometer splits a beam of radiation into two paths having different lengths, and then recombines them. A detector measures the intensity variations of the exit beam as a function of path difference. A monochromatic source would show a simple sine wave of intensity at the detector due to constructive and destructive interference as the path length changes.

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Mirror travel Frequency, (cm -1 ) x = 0 x = 0 400 4000 Interferogram: Single beam spectrum of air: FT 100% H 2 O H 2 O CO 2 400

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Applications of FTIR Qualitative “ fingerprint ” check for identification of drugs Used for screening compounds and rapid identification of C=O groups Can be used to characterize samples in solid states (creams and tablets) Can detect different crystal isoforms ( polymorphs ) Water content measurement Analysis of solids, liquids, and gasses Can also be used on satellites to probe the space In remote sensing.

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References Spectrometric Identification of Organic Compounds by Robert M. Silverstein , Francis X. Webster Organic Spectroscopy by William Kemp Introduction to Spectroscopy by Pavia, Lampman , Kriz Instrumental Method of Analysis by Willard, Meritt , Dean, Settle, Wadworth Instrumental Methods of Chemical Analysis by Gurdeep R. Chatwal , Sham K. Anand www.pubmed.gov www.sciencedirect.com