THEORY, INSTRUMENTATION AND APPLICATIONS OF DISPERSIVE & FTIR Presented By Bhavana Vedantam, Dept. Of Pharmaceutical Analysis

Contents It Contains… Introduction to IR Spectroscopy Dispersive IR Spectroscopy FT - IR Spectroscopy Conclusion

INTRODUCTION Spectroscopy is the branch of science dealing with the study of interaction of electromagnetic radiation with matter. IR spectroscopy is Absorption spectroscopy in which molecular vibrations observed due to absorption of IR radiation. Infrared radiation was discovered in 1800 by William Herschel.

The range of EMR between the visible and microwaves region is called INFRARED region(14000-40 cm-1 ).

IR REGION From application and instrumentation point of view infrared region is subdivided into Region Wave length (m) Wave number(cm-1 ) Near infrared 0.75-2.5 14000-4000 Mid infrared 2.5-50 4000-400 Far infrared 50-300 400-40 Mid IR (4000-40 cm-1) Functional group region (4000-1400 cm-1) Finger Print/Single bond region (1400-40cm-1) Stretching vibrations occurs in F.G. region Bending vibrations occurs in F.P. region

General Purpose Of IR Radiation Infrared light is used in industrial, scientific, and medical applications. In Telescopes to detect  planets Finding heat leaks from houses

Contd... Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, and to detect overheating of electrical apparatus. Night-vision devices Remote temperature sensing, short-ranged wireless communication, spectroscopy, and weather forecasting.

Pharmaceutical Applications Qualitative determination of substances Structural elucidation by determining the functional groups. Detection of impurities Identification of geometrical isomers for both organic & inorganic samples Detection of presence of water in sample Quantitative determination of sample by using Beer’s-Lamberts law

PRINCIPLE In any molecule , atoms or groups of atoms are connected by bonds which are similar to springs and not rigid in nature. Bonded atoms having different strengths due to different masses. Absorption of IR energy will cause vibrational changes in molecule and a peak will be observed, when

Conditions to obtain IR spectrum Contd… Conditions to obtain IR spectrum Selection rule for IR spectroscopy: Dipole moment of bonds should change during vibration. When Applied infrared frequency = Natural frequency of vibration When covalent bonds are polar in nature When sample obeys these conditions, then it gets vibrated by absorbing radiation and gives IR spectrum.

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. K = force constant (in dynes/cm) m = atomic masses of atom 1 & 2 Used to calculate approximate position of band

TYPES OF VIBRATIONS Stretching Mode In plane bending vibrations Scissoring Rocking Out plane bending vibrations Symmetric Asymmetric Wagging Twisting

Instrumentation Radiation sources They must produce intense & steady radiation. Nernst Glower heated rare earth oxide rod (zirconium, yttria, thoria) (~1500 K) 1-50 µm (mid- to far-IR) Globar heated Silicon Carbide rod (~1500 K) W filament lamp 1100 K 0.78-2.5 µm (Near-IR) Hg arc lamp Hg plasma 50 - 300 µm (far-IR)

Monochromators Filtration of desired frequency of radiation Monochromators are 2 types Prismatic Monochromator Grating Monochromator Prismatic Monochromators: Composed of glass or quartz and coated by alkyl halides (NaCl) These are 2 types Mono pass Prismatic Monochromator: radiation will pass once through the prism Double pass Prismatic Monochromator: radiation will pass twice through the prism

Grating Monochromators Grating monochromators introduced in 1950’s High dispersion and resolution than prismatic monochromators These are 2 types Reflection Grating Monochromator are common than Transmittance Gratings Gratings are linear grooves or lines which are made up of Aluminium.

Detectors/Transducers Thermocouple Thermoelectric effect -dissimilar metal junction cheap, slow, insensitive Bolometer Ni, Pt resistance thermometer (thermistor) Highly sensitive <400 cm-1 Golay cell Metal cylinder with Xe gas Faster than others & having wide wavelength range Pyro electric Tri glycine sulfate piezoelectric material fast and sensitive (mid IR) Photoconductive (non-thermal) PbS, CdS, Pb Se light sensitive cells fast and sensitive (near IR)

TYPES OF IR INSTRUMENTS Non- Dispersive Filters used for wavelength selection & having sample specific Detector Dispersive Sequential scanning of each wave number takes place FTIR systems Widely applied and quite popular in the far-IR and mid-IR spectrometry.

Dispersive IR Instrument Dispersive IR instruments are introduced in 1940’s. Double-beam instruments are mostly used than Single beam instrument. In dispersive IR sequential scanning of wave numbers of light takes place.

Instrumentation In double beam spectrometer , beam separates into two and passes to sample & reference. Prismatic monochromators have been replaced with Grating monochromator. Dispersive IR failed due to monochromator containing narrow slits which limit the wave number of radiation.

(X) It containing all movable parts which causes mechanical slippage (X) Slow scan speed (X) Less resolution, accuracy and sensitivity (X) Only narrow frequency range can be studied (X) Involvement of stray light (X) Atmospheric absorptions by CO, water also takes place. To overcome all these problems FTIR has been developed

Fourier Transform IR Instrument FTIR collects all wavelengths simultaneously and scans at once. FTIR works based on Michelson Interferometer which having Beam splitter Fixed mirror Movable mirror

FTIR Instrumentation He-Ne gas laser Beam splitter Movable mirror Light source He-Ne gas laser Beam splitter Movable mirror Sample chamber Fixed mirror Detector Interferometer

When the beams are combined an interference pattern is created Combined beam reaches detector by passing through sample Obtained spectrum is referred as Interferogram This will be amplified and translated into IR spectrum by FTIR

Advantages Fast & sensitive All frequencies can be modulated at once Simple mechanical design with only one moving part No stray light is involved When using He-Ne laser as internal standard, no need of external calibration Availability of easy sampling accessories Air pollutants like CO, ethylene oxide etc. can be analysed

Conclusion FTIR having significant advantages over Dispersive IR due to its fast and accurate analysis.

References Instrumental Methods Of Chemical Analysis; By Gurudeep R. Chatwal, pg No. 2.29-2.82 Infra Red Spectroscopy: Fundamentals And Applications; By Barbara Stuart, pg No. 16-23 Introduction to Spectroscopy, 4th edition, By Pavia, Lampman, Kriz. Elementary organic chemistry, By Y.R.Sharma; 2007,Pg No.69-137