introduction to spectroscopy

introduction to spectroscopy

1. Lecture # 01 Dr. Abdul Rehman Assistant Professor Department of Microbiology and Biotechnology Abasyn University, Peshawar Introduction to Spectroscopy Dr. Abdul Rehman 1

2. What is Spectroscopy..? Spectroscopy is the study of the interaction between matter and electromagnetic radiation as a function of wavelength ( λ ). Or Spectroscopy is the use of the absorption, emission or scanning of electromagnetic radiation by matter in order to qualitatively or quantitatively study of matter. The matter can be atoms, molecules or solids. In fact, historically, spectroscopy referred to the use of visible light dispersed accordingly to its wavelength e.g. by a prism. Later the concept was expanded greatly to include any interaction with radiative energy as a function of its wavelength or frequency. Dr. Abdul Rehman 2

3. Cont… Spectroscopic data is often represented by an emission spectrum, a plot of the response of interest as a function of wavelength or frequency. Spectral measurement devices are referred to as spectrometers , spectrophotometers , spectrographs or spectral analyzers . Dr. Abdul Rehman 3

4. History of Spectroscopy.. The history of spectroscopy began in the 17th century. Advances in optics, specifically prisms, enabled systematic observations of the solar spectrum. Isaac Newton first applied the word spectrum to describe the rainbow of colors that combine to form white light. During the early 1800s, Joseph von Fraunhofer made experimental advances with dispersive spectrometers that enabled spectroscopy to become a more precise and quantitative scientific technique. Since then, spectroscopy has played and continues to play a significant role in chemistry, physics and astronomy. Dr. Abdul Rehman 4

5. Electromagnetic Radiation.. Electromagnetic radiation—light—is a form of energy whose behavior is described by the properties of both waves and particles. Some properties of electromagnetic radiation, such as its refraction when it passes from one medium to another are explained best in Fig. 1 by describing light as a wave. Other properties, such as absorption and emission, are better described by treating light as a particle. The exact nature of electromagnetic radiation remains unclear, as it has since the development of quantum mechanics in the first quarter of the 20 th century. Nevertheless , the dual models of wave and particle behavior provide a useful description for electromagnetic radiation . Dr. Abdul Rehman 5

6. Fig. 1 Dual Nature of Electromagnetic Radiation Dr. Abdul Rehman 6

7. Wave Properties of Electromagnetic Radiation.. Electromagnetic radiation consists of oscillating electric and magnetic fields that propagate through space along a linear path and with a constant velocity. In a vacuum electromagnetic radiation travels at the speed of light , c , which is 2.997 92 × 10e8 m/s. When electromagnetic radiation moves through a medium other than a vacuum its velocity, v , is less than the speed of light in a vacuum. The difference between v and c is .sufficiently small (< 0.1 %) The oscillations in the electric and magnetic fields are perpendicular to each other, and to the direction of the wave’s propagation. Dr. Abdul Rehman 7

8. Particle Properties of Electromagnetic Radiation.. When matter absorbs electromagnetic radiation it undergoes a change in energy . The interaction between matter and electromagnetic radiation is easiest to understand if we assume that radiation consists of a beam of energetic particles called photons. When a photon is absorbed by a sample it is “destroyed,” and its energy acquired by the sample . The energy of a photon , in joules, is related to its frequency by the following equation. E = hv where h is Planck’s constant, which has a value of 6.626 × 10–34 J . s. Dr. Abdul Rehman 8

9. What is Wavelength..? Forms of electromagnetic radiation like radio waves, light waves or infrared (heat) waves make characteristic patterns as they travel through space. Each wave has a certain shape and length. The distance between peaks (high points) is called wavelength. Dr. Abdul Rehman 9

10. Techniques in Spectroscopy.. 1) Reflection of Light: Reflection is a process whereby incoming EM radiations is reflected after interaction with the surface of an object. 2) Scattering of Light: Scattering is a random process whereby EM radiation is absorbed and immediately re-emitted by particles or molecules. 3) Absorption of Light: Absorption is a process whereby EM radiation is absorbed by particles or molecules and converted to another form of energy. Dr. Abdul Rehman 10

11. Cont… 4) Emission Spectrum: The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of EM radiation emitted due to an atoms making a transition from a high energy state to a lower energy state. The energy of the emitted photon is equal to the energy difference between the two states. 5) Spectrum: The data that is obtained from spectroscopy is called a spectrum. A spectrum is a plot of the intensity of energy detected versus the wavelength of the energy. Dr. Abdul Rehman 11

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13. Types of Spectroscopy.. ABSORPTION SPECTROSCOPY Absorption spectroscopy uses the range of electromagnetic spectra into which a substance can be absorbed. “ Absorption” is the phenomenon that occurs when a transition from a lower level to a higher level takes place with transfer of energy from the radiation field to the atom or molecule. When atoms or molecules absorb light, the incoming energy excites a structure (in energy quanta) to a higher energy level. The type of excitation depends on the light wavelength. Electrons are promoted to higher orbits by ultraviolet or visible light. Vibrations are excited by infrared light and microwaves excite the rotations. An absorption spectrum is a way to represent the absorption of light as a function of wavelength. The spectrum of an atom or molecule depends on its energy-level structure, and absorption spectra are useful for identifying compounds. Dr. Abdul Rehman 13

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15. EMISSION SPECTROSCOPY “ Emission” occurs during transition from a higher level to a lower level if energy is transferred to the radiation field. When no radiation is emitted the phenomenon is called “nonradiative decay.” This type of spectroscopy relies on the range of electromagnetic spectra in which a particular substance radiates. The substance first absorbs energy and then radiates (that is, emits) this energy as light. The excitement energy that is absorbed first can come from a number of different sources, including collision (from high temperatures or other means), chemical reactions or light. Dr. Abdul Rehman 15

16. Cont… It also should be noted that atoms or molecules once excited to high-energy levels then can decay to lower levels by emitting radiation. This is called emission or luminescence. When atoms are excited by a high-temperature energy source this light emission commonly is called atomic or optical emission, and for atoms excited with light, it is called atomic fluorescence. Dr. Abdul Rehman 16

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18. SCATTERING SPECTROSCOPY “ Scattering” refers to light that is changed in direction (called redirection) from its interaction with matter. It may or may not occur with energy transfer. This spectroscopy form measures certain physical properties by determining the amount of light that a particular substance scatters at different wavelengths. It differs from other spectroscopy types primarily because of speed. The scattering process is much faster than absorption or emission. Dr. Abdul Rehman 18

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20. Spectrophotometry.. Spectrophotometry is the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. Spectrophotometry uses photometers that can measure a light beam's intensity as a function of its wavelength known as spectrophotometers. Important features of spectrophotometers are spectral bandwidth, the percentage of sample-transmission, the logarithmic range of sample-absorption, and sometimes a percentage of reflectance measurement . A spectrophotometer is commonly used for the measurement of transmittance or reflectance of solutions, transparent or opaque solids, such as polished glass, or gases. Dr. Abdul Rehman 20

21. Types of Spectrophotometr .. There are two major classes of devices: single beam and double beam. A double beam spectrophotometer compares the light intensity between two light paths, one path containing a reference sample and the other the test sample . A single-beam spectrophotometer measures the relative light intensity of the beam before and after a test sample is inserted. Although comparison measurements from double-beam instruments are easier and more stable, while single-beam instruments can have a larger dynamic range and are optically simpler and more compact. Dr. Abdul Rehman 21

22. Double Beam Spectrophotometer Dr. Abdul Rehman 22

23. Single Beam Spectrophotometer Dr. Abdul Rehman 23

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25. UV-visible spectrophotometry.. Ultraviolet–visible spectroscopy or ultraviolet-visible spectrophotometry (UV-Vis or UV/Vis) refers to absorption spectroscopy in the ultraviolet-visible spectral region. This means it uses light in the visible and adjacent (near-UV and near-infrared [NIR]) ranges. The absorption or reflectance in the visible range directly affects the perceived color of the chemicals involved. Dr. Abdul Rehman 25

26. Applications of UV/Visible Spectrophotometry.. UV/Vis spectroscopy is routinely used in analytical chemistry for the quantitative determination of different analytes, such as Transition Metal Ions Highly Conjugated Organic Compounds Biological Macromolecules. Organic compounds, also absorb light in the UV or visible regions of the electromagnetic spectrum. The solvents for these determinations are often water for water-soluble compounds, or ethanol for organic-soluble compounds. Dr. Abdul Rehman 26

27. IR spectrophotometry.. Infrared spectroscopy (IR spectroscopy or Vibrational Spectroscopy) is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic techniques, it can be used to identify and study chemicals. For a given sample which may be solid, liquid, or gaseous, the method or technique of infrared spectroscopy uses an instrument called an infrared spectrometer (or spectrophotometer) to produce an infrared spectrum. Dr. Abdul Rehman 27

28. Applications of IR Spectrophotometry.. Infrared spectroscopy is a simple and reliable technique widely used in both organic and inorganic chemistry, in research and industry. It is used in quality control, dynamic measurement, and monitoring applications such as the long-term unattended measurement of CO2 concentrations in greenhouses and growth chambers by infrared gas analyzers . It is also used in forensic analysis in both criminal and civil cases, for example in identifying polymer degradation. It can be used in determining the blood alcohol content of a suspected drunk driver . IR-spectroscopy has been successfully used in analysis and identification of pigments in paintings and other art objects such as illuminated manuscripts. Dr. Abdul Rehman 28

29. THANK YOU  Dr. Abdul Rehman 29