Principles of Spectroscopy PDF

Engineering Chemistry II MODULE 1: Principles of Spectroscopy It is a branch of science that deals with interaction of matter with light or electromagnetic radiation. OR Spectroscopy is a branch of science in which electromagnetic radiation of particular wavelength or range of wavelength is used f...

Engineering Chemistry II MODULE 1: Principles of Spectroscopy It is a branch of science that deals with interaction of matter with light or electromagnetic radiation. OR Spectroscopy is a branch of science in which electromagnetic radiation of particular wavelength or range of wavelength is used for qualitative and quantitative analysis of matter. Spectrum is a graph or plot of intensity of absorbed/ emitted radiation by sample verses frequency or Wavelength Spectrometer is an instrument design to measure the spectrum of a sample Spectroscopic techniques possess numerous merits over classical techniques as: Electromagnetic radiations are wave produced by motion of electrically charged particles (Photon).It consists of two components – Electric and Magnetic Initially electricity and magnetism were considered to be separate forces. Later in the year 1873, James Clerk Maxwell, the Scottish physicist, developed a combined theory known as electromagnetism. Engineering Chemistry II Electromagnetic field produce electromagnetic radiation also referred to as EM radiation. Electromagnetic radiation is a form of energy that is present all around us and takes various forms like microwaves, television waves, radio waves, gamma rays, X-rays, etc. Electromagnetic radiation can be defined as a form of energy that is produced by the movement of electrically charged particles travelling through a matter or vacuum or by oscillating magnetic and electric disturbance. The magnetic and the electric fields come at 90° to each other, and the combined waves move perpendicular to both electric and magnetic oscillating fields occurring the disturbance. Properties of Electromagnetic Radiation When electromagnetic radiation occurs, the electron radiations are released as photons. These are bundles of light energy or quantized harmonic waves which travel at the speed of light. Then based on the wavelength of the electromagnetic spectrum, the energy is grouped into different categories. These magnetic and electric waves travel perpendicular to each other and have some characteristics like wavelength, amplitude, and frequency. They can travel through empty space. Waves other than electromagnetic waves have to travel through some substance. For example, sound waves will need either a solid, liquid or gas to pass through. The speed of light which is 2.99792458 x 108 m/s is always constant. Engineering Chemistry II Waves and their Characteristics Electromagnetic radiation occurs when an atomic particle, like an electron, is accelerated by an electric field, causing it to accelerate. Electromagnetic waves and their characteristics is explained briefly in the points mentioned below. Wavelength Wavelength (λ) is the distance between successive crests of a wave, especially points in an electromagnetic wave or sound wave. It can be simply defined as the distance of one full cycle of the oscillation. If ‘λ’ is the wavelength, ‘c’ is the speed of light and ‘ν‘ is frequency. Then we can derive the relation given below. c=λν The shorter the wavelength, greater the frequency and greater the frequency, the higher the energy. Amplitude It is the distance from the middle of the wave to the maximum vertical displacement of the wave. Larger the amplitude, higher the energy and lower the amplitude, lower the energy. Amplitude tells us about the brightness or intensity of a wave compared to other waves. Frequency The number of cycles per second is defined as Frequency. It is defined as Hertz (Hz) or sec-1. If ‘E’ is the energy, ‘h’ is Planck’s constant which is equal to 6.62607 x 10-34 and ‘ν‘ is the frequency we can derive the relation given below. E = hν Thus, we can see that frequency is directly proportional to energy. Engineering Chemistry II Period Period is commonly characterised by the symbol ‘T’. It is the total time which a wave takes to travel 1 wavelength. Velocity In relation with electromagnetic radiation, the velocity is normally expressed as: Velocity = λν [where, ν = frequency] The wave velocity in vacuum for electromagnetic wave is = 186,282 miles/second or 2.99×108 m/s. More about Frequency Frequency is defined as the number of oscillations of a wave per unit time being measured in hertz(Hz). The frequency is directly proportional to the pitch. Humans can hear sounds with frequencies ranging between 20 – 20000 Hz. Sounds with frequencies over the human ears range are known as ultrasound and sounds with frequencies less than the audible range is known as infrasound. Engineering Chemistry II Electromagnetic Spectrum Accelerating charges produce electromagnetic waves. There are many levels in the structure of matter where moving (accelerating) charges exist. Some of the more obvious are electrons in an atom, freely-moving electrons in conducting metals, vibrating atoms in molecules, and charged particles in a nucleus. Thus, two factors result in the many different types of electromagnetic waves we observe—the source of the charge motions and the accelerations inherent in the motions. The many different types of EM waves are categorized according to their origins and their frequency/wavelength values. A typical organization of the Electromagnetic spectrum in simple terms is defined as the range of all types of electromagnetic radiation. The electromagnetic spectrum is a continuum of all electromagnetic waves arranged according to frequency and wavelength. The electromagnetic spectrum is a range of frequencies, wavelengths and photon energies covering frequencies from below 1 hertz to above 1025 Hz corresponding to wavelengths which are a few kilometres to a fraction of the size of an atomic nucleus in the spectrum of electromagnetic waves. Engineering Chemistry II Engineering Chemistry II Engineering Chemistry II Engineering Chemistry II Important Uses of Electromagnetic radiation: Engineering Chemistry II Electromagnetic Spectrum based on its origin: Emission Spectrum  Whenever electromagnetic radiation interacts with atoms and molecules of matter, the electrons in these atoms may absorb energy and jump to a higher energy state, losing their stability.  In order to regain their stability, they need to move from the higher energy state to the previous lower energy state.  To accomplish this job, these atoms and molecules emit radiation in various regions of the electromagnetic spectrum.  This spectrum of radiation emitted by electrons in the excited atoms or molecules is known as an emission spectrum. Absorption Spectrum Engineering Chemistry II  An absorption spectrum is like a photographic negative of an emission spectrum.  For observing the absorption spectrum, electromagnetic radiations are bombarded on a sample that absorbs radiation of certain wavelengths.  The wavelength of radiation absorbed by the matter contributes to the missing wavelength which leaves dark spaces in the bright continuous spectrum. Emission Spectra VS Absorption Spectra The main difference between emission and absorption spectra is that an emission spectrum has different coloured lines in the spectrum, whereas an absorption spectrum has dark-coloured lines in the spectrum. More differences between absorption and emission spectrum are as follows: Emission Spectra Absorption Spectra Produced when atoms release energy Produced when atoms absorb energy Comprise coloured lines in the Comprise dark lines or gaps in spectrum the spectrum Engineering Chemistry II Emission spectra can emit all the while the absorption spectrum colours in an electromagnetic can have a few colours missing spectrum, due to the redirection of absorbed photons. The type of photons emitted is helpful The wavelengths of light in figuring out the kind of elements the absorbed is helpful in figuring substance is made of as each element out the number of substances in radiates a different amount of energy the sample and has a unique emission level Engineering Chemistry II Engineering Chemistry II Engineering Chemistry II Engineering Chemistry II Engineering Chemistry II NOTE: Absorption spectroscopy and not Emission spectroscopy is used to study the spectra of organic compound Because emission of radiation from an organic compound requires very high temperature where organic compounds generally decompose. Continuous spectrum  It consists of unbroken luminous bands of all wavelengths containing all the colours from violet to red. These spectra depend only on the temperature of the source and is independent of the characteristic of the source.  Incandescent solids, liquids, Carbon arc, electric filament lamps etc, give continuous spectra.  when a ray of white light falls on a prism it experiences refraction twice.Once when it travels from the rarer medium (air) to a denser medium (glass) and again from the denser medium (glass) to a rarer medium (air).  Finally, we observe a band of colours, called spectrum, formed out of a ray of white light. If we observe this spectrum more closely, the colour having a smaller wavelength deviates the most and vice versa.  Thus, a spectrum of colours ranging from red to violet is observed where red having the longest wavelength suffers the least deviation.  This kind of spectrum is called a continuous spectrum as violet merges into blue, blue into green and so on. Line spectra  The emission spectrum of atoms in the gas phase, do not exhibit a continuous spread of wavelength from one colour to others.  Rather, the emitted light consists of a specific wavelength having dark spaces existing between them. Such kind of spectra is known as atomic spectra or line spectra.  Line spectrum(also called atomic spectrum) is produced when when atom of a particular element is irradiated with energy* ,causing excitation of outer shell electron(s) to higher state. Engineering Chemistry II  When these electron(s) return to their ground state , they emit their excitation energies corresponding to a particular frequency(and hence a particular wavelength, for the medium). This in spectroscopy shows as line.(characteristic of an element)  Line spectra are sharp lines of definite wavelengths.  It is the characteristic of the emitting substance. It is used to identify the gas.  Atoms in the gaseous state, i.e. free excited atoms emit line spectrum. The substance in atomic state such as sodium in sodium vapour lamp, mercury in mercury vapour lamp and gases in discharge tube give line spectra Band Spectrum  It consists of a number of bright bands with a sharp edge at one end but fading out at the other end.  Band spectra are obtained from molecules. It is the characteristic of the molecule.  If a Molecule(may be of same or different element) is irradiated, apart from the exitation- there will be additionally rotational or vibrational movements taking place(as is permitted by the degrees of freedom). This is what accounts to the several line spectrum combined as a Band Spectrum. We can conclude that when an atom absorbs the light the electron present in it get excited and jumps from lower energy level(stable) to higher energy level (unstable ) means jumps from ground state to excited state , so while returning back to ground state they emit light radiation and shows colour. So, when the electron returns back to ground state within the atom the coloured line is absorbed and each line is called to form line Spectra. Whereas when , large number of electrons return back to ground state and several coloured lines appears and they are closely spaced , means very close to each other form a band. Engineering Chemistry II Calcium or Barium salts in a bunsen flame and gases like carbon-di-oxide, ammonia and nitrogen in molecular state in the discharge tube give band spectra. When the bands are examined with high resolving power spectrometer, each band is found to be made of a large number of fine lines, very close to each other at the sharp edge but spaced out at the other end. Using band spectra the molecular structure of the substance can be studied. What is Atomic Spectroscopy? Atomic spectroscopy refers to the study of the electromagnetic radiation absorbed and emitted by atoms. Since chemical elements have unique spectra, we can use this technique to analyze the composition of elements in a sample. Electrons are in certain energy levels of an atom ,these energy levels are called as atomic orbitals. These energy levels are quantized rather than being continuous. The electrons in the atomic orbitals can move from one energy level to another by either absorbing or releasing the energy they have. However, the energy that the electron absorbs or emits should equal the energy difference between the two energy levels (between which the electron is going to move). Since each and every chemical element has a unique number of electrons at their ground state, an atom will absorb or release energy in a pattern unique to its elemental identity. Therefore, they will absorb /emit photons in a correspondingly unique pattern. Engineering Chemistry II Then we can determine the elemental composition of a sample by measuring the changes in light wavelength and light intensity. What is Molecular Spectroscopy? Molecular spectroscopy refers to the study of the electromagnetic radiation absorbed and emitted by molecules. The molecules in the sample can absorb some wavelengths that we pass through the sample and can move to a higher energy state from the existing lower energy state. The sample will absorb particular wavelengths but not all, depending on the chemical composition of the sample. Then, depending on the absorbed wavelengths and the intensity of absorption, we can determine the nature of the energetic transitions that a molecule is able to undergo, and therefore, gather the information about its structure. Types of Molecular Energies( Responsible for molecular spectra) Engineering Chemistry II Engineering Chemistry II Engineering Chemistry II Engineering Chemistry II What is the Difference Between Atomic Spectroscopy and Molecular Spectroscopy? Atomic and molecular spectroscopy are two techniques in which we use an electromagnetic radiation source in order to determine the composition of a sample. The key difference between atomic spectroscopy and molecular spectroscopy is that the atomic spectroscopy refers to the study of the electromagnetic radiation absorbed and emitted by atoms whereas the molecular spectroscopy refers to the study of the electromagnetic radiation absorbed and emitted by molecules. Therefore, atomic spectroscopy determines the type of atoms present in a given sample while molecular spectroscopy determines the structure of molecules present in a given sample. Engineering Chemistry II Classification of Spectroscopy: Chemicals can be analyzed both quantitatively and qualitatively through a number of different analytical methods, but one big area of analysis is by using spectroscopy. Spectroscopy studies the interaction between electromagnetic radiation and matter, with the interactions giving rise to electronic excitations, molecular vibrations or nuclear spin orientations.  Dr. Poonam Hemnani, SFIT Spectroscopy methods can be categorized depending on the types of radiation, interaction between the energy and the material, the type of material and the applications the technique is used for. NUMERICALS:  Dr. Poonam Hemnani, SFIT Numerical Problems 1.Calculate the energy of a photon of radiation with a frequency of 8.5 x 1014 Hz. 2. Calculate the energy of a photon of radiation with a wavelength of 6.4 x 10-7 m. 3. An FM radio station broadcasts at a frequency of 107.9 Hz. What is the wavelength of the radio signal? 4. Calculate wavelength and frequency and wavenumber of a light. whose period is 2×1010 second. 5. The color orange within the visible light spectrum has a wavelength of about 620nm. What is the frequency and energy of orange light? Flame photometer The principle of flame photometer is based on the measurement of the emitted light intensity when a metal is introduced into the flame. The wavelength of the colour gives information about the element and the colour intensity of the flame gives information about the amount of the element present in the sample. Flame photometry is one of the branches of atomic absorption spectroscopy. It is also known as flame emission spectroscopy. Currently, it has become a necessary tool in the field of analytical chemistry. Flame photometer can be used to determine the concentration of certain metal ions like sodium, potassium, lithium, calcium and cesium etc. In flame photometer spectra the metal ions are used in the form of atoms. The  Dr. Poonam Hemnani, SFIT International Union of Pure and Applied Chemistry (IUPAC) Committ ee on Spectroscopic Nomenclature has named this technique as flame atomic emission spectrometry ( FAES). Principle of Flame photometer The compounds of the alkali and alkaline earth metals (Group II) dissociate into atoms when introduced into the flame. Some of these atoms further get excited to even higher levels. But these atoms are not stable at higher levels.Hence, these atoms emit radiations when returning to the ground state. These radiations generally lie in the visible region of the spectrum. Each of the alkali and alkaline earth metals has a specific wavelength. Element Emitted wavelength Flame color Sodium 589 nm Yellow Potassium 766 nm Violet Barium 554 nm Lime green Calcium 622 nm Orange Lithium 670 nm Red The intensity of the emission is directly proportional to the number of atoms returning to the ground state. And the light emitted is in turn proportional to the concentration of the sample.  Dr. Poonam Hemnani, SFIT Parts of flame photometer A simple flame photometer consists of the following basic components: Source of flame: A Burner in the flame photometer is the source of flame. It can be maintained in at a constant temperature. The temperature of the flame is one of the critical factors in flame photometry. Fuel-Oxidant mixture Temperature (°C) Natural gas- Air 1700 Propane- Air 1800 Hydrogen- Air 2000 Hydrogen-Oxygen 2650 Acetylene- Air 2300 Acetylene-Oxyen 3200 Acetylene-Nitrous oxide 2700 Cyanogen-Oxygen 4800 Requirement from Flame It should have proper temperature Temperature should remain constant throughout the operation There should not be any fluctuation during burning Functions of Flame To convert the analyte of the liquid sample into vapour state To decompose the analyte into atoms and simple molecules To excite the formed atoms/free atoms/simple molecules to emit radiant energy  Dr. Poonam Hemnani, SFIT Nebulizer: Nebulizer is used to send homogeneous solution into the flame at a balanced rate. It breaks up the liquid into small droplets. Optical system: The optical system consists of convex mirror and convex lens. The convex mirror transmits the light emitted from the atoms. Convex mirror also helps to focus the emissions to the lens. The lens helps to focus the light on a point or slit. Simple colour filters: The reflections from the mirror pass through the slit and reach the filters. Filters will isolate the wavelength to be measured from that of irrelevant emissions. Photo-detector: The intensity of radiation emitted by the flame is measured by photo detector. Here the emitted radiation is converted to an electrical signal with the help of photo detector. These electrical signals are directly proportional to the intensity of light.  The solvent is first aspirated to obtain fine solid particles.  These molecules in the solid particles are moved towards the flame to produce  Dr. Poonam Hemnani, SFIT gaseous atoms and ions.  These ions absorb the energy from the flame get excited to high energy levels from the ground state.  But as these ions are unstable, they return to ground state. While returning they emit characteristic radiation.  The intensity of emitted light is proportional to the concentration of the element. The oxidants in flame photometer are mainly air, oxygen or nitrous oxide. The temperature of the flame depends on the ratio of fuel and oxidant.  Dr. Poonam Hemnani, SFIT The processes occurring during flame photometer analysis are summarized below: Desolvation: Desolvation involves drying a sample in a solution. The metal particles in the solvent are dehydrated by the flame and thus solvent is evaporated. Vaporization: The metal particles in the sample are also dehydrated. This also led to the evaporation of the solvent. Atomization: Atomization is the separation of all atoms in a chemica l substance. The metal ions in the sample are reduced to metal atoms by the flame. Excitation: The electrostatic force of attraction between the electrons and nucleus of the atom helps them to absorb a particular amount of energy. The atoms then jump to the higher energy state when excited. Emission: Since the higher energy state is unstable the atoms jump back to the ground state or low energy state to gain stability. This jumping of atoms emits radiation with characteristic wavelength. The radiation is me asured by the photo detector.  Dr. Poonam Hemnani, SFIT Applications of flame photometer 1. Flame photometer can be applied both for quantitative and qualitative analysis of elements. The radiations emitted by the flame photometer are characteristic to particular metal. Hence, with the help of Flame photometer we can detect the presence of any specific element in the given sample. 2. The presence of some group II elements is critical for soil health. We can determine the presence of various alkali and alkaline earth metals in soil sample by conducting flame test and then the soil can be supplied with specific fertilizer. 3. The concentrations of Na+ and K+ ions are very important in the human body for conducting various metabolic functions. Their concentrations can be determined by diluting and aspirating blood serum sample into the flame. 4. Soft drinks, fruit juices and alcoholic beverages can also be analyzed by using flame photometry to determine the concentrations of various metals and elements. 5. Used in determination of calcium and magnesium in cement. 6. Used in determination of lead in petrol. Advantages of flame photometer 1. The method of analysis is very simple and economical. 2. It is quick, convenient, selective and sensitive analysis. 3. It is both and qualitative and quantitative in nature. 4. Even very low concentrations (parts per million/p pm to parts per billion/ppb range) of metals in the sample can be determined. 5. This method compensates for any unexpected interfering material present in the sample solution. 6. This method can be used to estimate elements, which are rarely analyzed.  Dr. Poonam Hemnani, SFIT Disadvantages of flame photometer In spite of many advantages, this analysis technique has quite a few disadvantages: 1. The accurate concentration of the metal ion in the solution cannot be measured. 2. It cannot directly detect and determine the presence of in ert gases. 3. Though this technique measures the total metal content present in the sample, it does not provide the information about the molecular structure of the metal present in the sample. 4. Only liquid samples may be used. Also, sample preparation becomes lengthy in some cases. 5. Flame photometry cannot be used for the direct determination of each metal atom. A number of metal atoms cannot be analyzed by this method. The elements such as carbon, hydrogen and halides cannot be detected due to their non-radiating nature. Dr. Poonam Hemnani, SFIT