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Introduction to Spectroscopy

  1. Introduction to Spectroscopy ER. FARUK BIN POYEN ASST. PROFESSOR DEPT. OF AEIE, UIT, BU
  2. Contents:  1. Introduction  2. Classification of Methods  3. Atomic Transitions – Theory  4. Absorption Spectroscopy  5. Emission Spectroscopy  6. Scattering Spectroscopy  7. Atomic Fluorescence Spectroscopy  8. Few Definitions and Terminologies  9. Differences between similar terms  10. What is Electromagnetic Radiation?  11. Photon as a Signal Source  12. Basic Components of Spectroscopic Instruments 2
  3. Introduction:  Spectroscopy is the study of the interaction of electromagnetic radiation in all its forms with matter. The interaction might give rise to electronic excitations, (e.g. UV), molecular vibrations (e.g. IR) or nuclear spin orientations (e.g. NMR). Thus Spectroscopy is the science of the interaction of energy, in the form of electromagnetic radiation (EMR), acoustic waves, or particle beams, with matter.  Analytical instruments exploit spectroscopy for both identification (qualitative analysis) and measurement (quantitative analysis) of atoms, ions and molecules.  Can be used alone or in combination with chromatography to achieve both separation and measurement of multicomponent samples.  The optical system that allows production and viewing of spectrum (VIBGYOR, and invisible to plain eyesight) is called spectroscope.  Spectrometry: The measurement of the intensity (I) of radiation, in the form of EMR or high energy particles (electrons, ions, etc.), with some form of electronic device. 3
  4.  Spectrometer: An instrument that can measure radiation intensity as a function of energy (E), frequency (ν), wavelength (λ), wavenumber (σ), etc. in order to obtain a spectrum  Spectrophotometer: A spectrometer that uses a photon detector to measure the ratio of the radiant power incident on (P0) and emergent power from (P) a sample of matter, as a function of photon wavelength (λ), frequency (ν), wavenumber (σ) or energy (E photon).  To see a molecule, the wavelength of light must be smaller than the molecule (1 to 15 angstrom units X – ray region). 4
  5. Basic Types:  Mass Spectrometry: Sample molecules are ionized by high energy electrons. The mass to charge ratio of these ions is measured very accurately by electrostatic acceleration and magnetic field perturbation, providing a precise molecular weight. Ion fragmentation patterns may be related to the structure of the molecular ion.  Ultraviolet-Visible (UV – Vis) Spectroscopy: Absorption of this relatively high-energy light causes electronic excitation. The easily accessible part of this region (wavelengths of 200 to 800 nm) shows absorption only if conjugated pi-electron systems are present.  Infrared (IR) Spectroscopy: Absorption of this lower energy radiation causes vibrational and rotational excitation of group of atoms within the molecule. As of their characteristic absorptions, identification of functional groups is easily accomplished.  Nuclear Magnetic Resonance (NMR) Spectroscopy: Absorption in the low-energy radio- frequency part of the spectrum causes excitation of nuclear spin states. NMR spectrometers are tuned to certain nuclei (e.g. 1H, 13C, 19F & 31P). For a given type of nucleus, high-resolution spectroscopy distinguishes and counts atoms in different locations in the molecule. 5 Basis: Each chemical element has its own characteristic spectrum.
  6. Classification:  Spectroscopic measurement is on the basis on either energy absorbed or emitted.  Methods are differentiated as either atomic (apply on atoms) or molecular (apply on molecules).  Absorption Spectroscopy: It uses the range of the EM spectra in which a substance absorbs. This includes atomic absorption spectroscopy and various molecular techniques, such as infra-red spectroscopy in that region and Nuclear Magnetic Resonance spectroscopy in the radio region.  Emission Spectroscopy: It uses the range of EM spectra in which a substance radiates (emits). The substance first must absorb energy. This energy can be from a variety of sources, which determines the name of the subsequent emission, like luminescence. Molecular luminescence techniques include Spectroflourimetry.  Scattering Spectroscopy: It measures the amount of light that a substance scatters at certain wavelengths, incident angles, and polarization angles. The scattering process is much faster than the absorption/emission process. One of the most useful applications of light scattering spectroscopy is Raman Spectroscopy. 6
  7. Classification:  Absorption Spectroscopy - Emission Spectroscopy - Scattering Spectroscopy 7
  8. Common Types:  Fluorescence spectroscopy  X-ray spectroscopy and crystallography  Flame spectroscopy Atomic emission spectroscopy Atomic absorption spectroscopy Atomic fluorescence spectroscopy  Plasma emission spectroscopy  Spark or arc emission spectroscopy  UV/VIS spectroscopy IR spectroscopy  Raman spectroscopy  NMR spectroscopy  Photo thermal spectroscopy  Thermal infra-red spectroscopy  Mass Spectroscopy 8
  9. Atomic Transitions - Theory  Atomic electron transition is a change of an electron from one energy level to another within an atom or artificial atom.  The probability that an atomic spectroscopic transition will occur is called the transition probability or transition strength.  This probability will determine the extent to which an atom will absorb light at a resonant frequency, and the intensity of the emission lines from an atomic excited state.  The spectral width of a spectroscopic transition depends on the widths of the initial and final states.  The width of the ground state is essentially a delta function and the width of an excited state depends on its lifetime. 9
  10. Terminologies: Colorimetry  This is the technique used to determine the concentration of a solution having color.  It measures the intensity of color and relates the intensity to the concentration of the sample.  In colorimetry, the color of the sample is compared with a color of a standard in which the color is known.  Colorimeter is the equipment used to measure the colored samples and gives the appropriate absorptions. 10
  11. Terminologies: Spectroscopy  It is the science of studying the interaction between matter and radiated energy.  To understand spectroscopy, one must first understand spectrum.  The visible light is a form of electromagnetic waves. There are other forms of EM waves such as X-Rays, Microwaves, Radio waves, Infrared and Ultraviolet rays.  The energy of these waves is dependent on the wavelength or the frequency of the wave.  High frequency waves have high amounts of energies, and low frequency waves have low amounts of energies.  The light waves are made up of small packets of waves or energy known as photons. For a monochromatic ray, the energy of a photon is fixed.  The electromagnetic spectrum is the plot of the intensity versus the frequency of the photons.  When a beam of waves having the whole range of wavelengths is passed through some liquid or gas, the bonds or electrons in these materials absorb certain photons from the beam. It is due to the quantum mechanical effect that only photons with certain energies get absorbed. This can be understood using the energy level diagrams of atoms and molecules. Spectroscopy is studying the incident spectrums, emitted spectrums and absorbed spectrums of materials. 11
  12. Terminologies: Spectrometry  It is the method used for the study of certain spectrums.  Ion-mobility spectrometry, mass spectrometry (MS), Rutherford backscattering spectrometry, and neutron triple axis spectrometry are the main forms of spectrometry.  In these cases, a spectrum does not necessarily mean a plot of intensity versus frequency.  For example, the spectrum for MS is the plot between intensity (number of incident particles) versus the mass of the particle.  Spectrometers are the instruments used in spectrometry. The operation of each type of instrument depends on the form of spectrometry used in the instrument.  Spectrophotometry is the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength.  For the visible region, the perfect white light contains all the wavelengths within the region. Assume, white light is sent through a solution absorbing photons with a wavelength of 570 nm. This means the red photons of the spectrum is now reduced. This will cause a blank or reduced intensity at the 570 nm mark of the plot of intensity versus wavelength. The intensity of the light passed, as a proportion to the light projected, can be plotted for some known concentrations, and the resultant intensity from the unknown sample can be used to determine the concentration of the solution. 12
  13. Terminologies: Photometry  The term ―photo‖ means light and the term ―metry‖ refers to measurement.  Photometry is the science of the measurement of light, in terms of its perceived brightness to the human eye. In photometry, the standard is the human eye.  The sensitivity of the human eye to different colors is different.  This has to be considered in photometry.  Therefore, amplification methods are used so that the effect from each color would be same as that of the eye.  Since the human eye is only sensitive to visible light, photometry only falls in that range. 13
  14. Terminologies: Spectrophotometry  Spectrometers have developed into electronically operated complex machines, but they share the same principle as the initial spectrometers made by Fraunhofer.  Modern spectrometers use a monochromatic light that passes through a liquid solution of the material and a photodetector detects the light.  The changes of the light compared to the source light allow the instrument to output a graph of the absorbed frequencies. This graph indicates the characteristic transitions in the sample material. These types of advanced spectrometers are also called spectrophotometers because it is a spectrometer and photometer combined into a single device. The process is known as the spectrophotometry.  Spectrophotometer is the instruments used in this technique. It has two main parts, the spectrometer, which produces the light with a selected color, and the photometer, which measures the intensity of light. There is a cuvette where we can place our liquid sample. The liquid sample will have a color, and it absorbs the complementary color of it when a light beam is passed through that. The color intensity of the sample is related to the concentration of the substance in the sample. Therefore, that concentration can be determined by the extent of absorption of light at the given wavelength. 14
  15. Terminologies: Refraction, Reflection  Refraction: Refraction is an object’s visual proportion being distorted or refined when passing from one state to another through an angle. Refraction is the bending of light as it passes from one substance to another. Here, the light ray passes from air to glass and back to air. The bending is caused by the differences in density between the two substances.  Reflection: A reflection is a result of light bouncing off an object and hitting another clear surface, giving out an object’s mirror-like image. Reflection occurs when light changes direction as a result of "bouncing off" a surface like a mirror 15
  16. Terminologies: Diffraction, Scattering  Diffraction: The process by which a beam of light or other system of waves is spread out as a result of passing through a narrow aperture or across an edge, typically accompanied by interference between the wave forms produced.  Diffraction involves a change in direction of waves as they pass through an opening or around a barrier in their path.  Scattering: In scattering, light is intercepted by an object and sent off in many directions; this movement may appear to be random and not following the law of reflection. Rayleigh scattering refers to the scattering of light by molecules in air, and is what causes the sky to have a blue color. As because this type of scattering is proportional to the 4th power of the frequency, blue light (which has the highest frequency of visible light) scatters the most.  Rayleigh scattering can be considered an elastic collision of a photon with an atom or molecule.  Another type, Raman scattering, is due to an inelastic collision with a molecule, and is used by chemists and physicists to measure the vibrational quantum state of molecules. 16
  17. 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 17
  18. Absorption Spectroscopy  Atomic-absorption (AA) spectroscopy uses the absorption of light to measure the concentration of gas-phase atoms.  Since samples are usually liquids or solids, the analyte atoms or ions must be vaporized in a flame or graphite furnace.  The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels.  The analyte concentration is determined from the amount of absorption.  Applying the Beer-Lambert law directly in AA spectroscopy is difficult due to variations in the atomization efficiency from the sample matrix, and non-uniformity of concentration and path length of analyte atoms (in graphite furnace AA).  Concentration measurements are usually determined from a working curve after calibrating the instrument with standards of known concentration. 18
  19. Absorption Spectroscopy  Light source The light source is usually a hollow-cathode lamp of the element that is being measured. Lasers are also used in research instruments. Since lasers are intense enough to excite atoms to higher energy levels, they allow AA and atomic fluorescence measurements in a single instrument. The disadvantage of these narrow-band light sources is that only one element is measurable at a time.  Atomizer AA spectroscopy requires that the analyte atoms be in the gas phase. Ions or atoms in a sample must undergo desolvation and vaporization in a high-temperature source such as a flame or graphite furnace. Flame AA can only analyze solutions, while graphite furnace AA can accept solutions, slurries, or solid samples. Flame AA uses a slot type burner to increase the path length, and therefore to increase the total absorbance (see Beer-Lambert law). Sample solutions are usually aspirated with the gas flow into a nebulizing/mixing chamber to form small droplets before entering the flame. 19
  20. Absorption Spectroscopy The graphite furnace has several advantages over a flame. It is a much more efficient atomizer than a flame and it can directly accept very small absolute quantities of sample. It also provides a reducing environment for easily oxidized elements. Samples are placed directly in the graphite furnace and the furnace is electrically heated in several steps to dry the sample, ash organic matter, and vaporize the analyte atoms.  Light separation and detection AA spectrometers use monochromators and detectors for UV and visible light. The main purpose of the monochromator is to isolate the absorption line from background light due to interferences. Simple dedicated AA instruments often replace the monochromator with a bandpass interference filter. Photomultiplier tubes are the most common detectors for AA spectroscopy. 20
  21. Absorption Spectroscopy  Change of position of energy bands. 21
  22. 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.  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. 22
  23. Emission Spectroscopy  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.  Atomic emission spectroscopy (AES or OES [optical emission spectroscopy]) uses quantitative measurement of the optical emission from excited atoms to determine analyte concentration.  Analyte atoms in solution are aspirated into the excitation region where they are dissolved, vaporized, and atomized by a flame, discharge, or plasma.  These high-temperature atomization sources provide sufficient energy to promote the atoms into high energy levels.  The atoms decay back to lower levels by emitting light. Since the transitions are between distinct atomic energy levels, the emission lines in the spectra are narrow. 23
  24. Emission Spectroscopy  The spectra of samples containing many elements can be very congested, and spectral separation of nearby atomic transitions requires a high-resolution spectrometer.  Since all atoms in a sample are excited simultaneously, they can be detected simultaneously using a polychromator with multiple detectors.  This ability to simultaneously measure multiple elements is a major advantage of AES compared to atomic-absorption (AA) spectroscopy. 24
  25. 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, incident angles and light polarization angles.  It differs from other spectroscopy types primarily because of speed.  The scattering process is much faster than absorption or emission. 25
  26. Atomic-Fluorescence Spectroscopy (AFS)  Atomic fluorescence is the optical emission from gas-phase atoms that have been excited to higher energy levels by absorption of electromagnetic radiation.  The main advantage of fluorescence detection compared to absorption measurements is the greater sensitivity achievable because the fluorescence signal has a very low background.  The resonant excitation provides selective excitation of the analyte to avoid interferences. AFS is useful to study the electronic structure of atoms and to make quantitative measurements.  Analytical applications include flames and plasmas diagnostics, and enhanced sensitivity in atomic analysis.  As because of the differences in the nature of the energy-level structure between atoms and molecules, discussion of laser-induced fluorescence (LIF) from molecules is found in a separate document. 26
  27. Atomic-Fluorescence Spectroscopy (AFS)  Analysis of solutions or solids requires that the analyte atoms be dissolved, vaporized, and atomized at a relatively low temperature in a heat pipe, flame, or graphite furnace.  A hollow-cathode lamp or laser provides the resonant excitation to promote the atoms to higher energy levels.  The atomic fluorescence is dispersed and detected by monochromators and photomultiplier tubes, similar to atomic-emission spectroscopy instrumentation. 27
  28. Difference: Colorimetry and Spectrophotometry  A colorimeter quantifies color by measuring three primary color components of light (red, green, blue), whereas spectrophotometer measures the precise color in the human-visible light wavelengths. .  Colorimetry uses fixed wavelengths, which are in the visible range only, but spectrophotometry can use wavelengths in a wider range (UV and IR also).  Colorimeter measures the absorbance of light, whereas the spectrophotometer measures the amount of light that passes through the sample. 28
  29. Difference: Photometry and Spectrophotometry  Spectrophotometry is applied to the whole electromagnetic spectrum, but photometry is only applicable to the visible light.  Photometry measures the total brightness as seen by the human eye, but spectrophotometry measures the intensity at each wavelength on the whole range of the electromagnetic spectrum for which the measurements are necessary. 29
  30. Difference: Spectrometry and Spectroscopy  Spectroscopy is the science of studying the interaction between matter and radiated energy while spectrometry is the method used to acquire a quantitative measurement of the spectrum.  Spectroscopy does not generate any results. It is the theoretical approach of science. Spectrometry is the practical application where the results are generated. 30
  31. Difference: Diffraction and Refraction  Diffraction is bending or spreading of waves around an obstacle, while refraction is bending of waves due to change of speed.  Both diffraction and refraction are wavelength dependent. Hence, both can split white light in to its component wavelengths.  Diffraction of light produces a fringe pattern, whereas refraction creates visual illusions but not fringe patterns.  Refraction can make objects appear closer than they really are, but diffraction can not do that. 31
  32. Difference: Absorption and Emission Spectra  When an atom or molecule excites, it absorbs a certain energy in the electromagnetic radiation; therefore, that wavelength will be absent in the recorded absorption spectrum.  When the species come back to the ground state from the excited state, the absorbed radiation is emitted, and it is recorded. This type of spectrum is called an emission spectrum.  In simple terms, absorption spectra records the wavelengths absorbed by the material, whereas emission spectra records wavelengths emitted by materials, which have been stimulated by energy before.  Compared to the continuous visible spectrum, both emission and absorption spectra are line spectra because they only contain certain wavelengths.  In an emission spectrum there’ll be only few colored bands in a dark back ground. But in an absorption spectrum there’ll be few dark bands within the continuous spectrum. The dark bands in the absorption spectrum and the colored bands in the emitted spectrum of the same element are similar. 32
  33. Difference: Spectrometer and Spectrophotometer  Spectroscopy is the study of methods of producing and analyzing spectra using spectrometers, spectroscopes, and spectrophotometers.  The basic spectrometer developed by Joseph von Fraunhofer is an optical device that can be used to measure the properties of light. It has a graduated scale that allows the wavelengths of the specific emission/absorption lines to be determined by measuring the angles.  Spectrophotometer is a development from the Spectrometer, where a spectrometer is combined with a photometer to read relative intensities in the spectrum, rather than the wavelengths of emission/absorption.  Spectrometers were only used in the visible region of the EM spectrum, but spectrophotometer can detect IR, visible, and UV ranges.  A spectrometer is an optical instrument used to measure properties of light over a specific portion of the electromagnetic spectrum. The variable measured is most often the light's intensity but could also, for instance, be the polarization state.  A spectrophotometer is a photometer (a device for measuring light intensity) that can measure intensity as a function of the color, or more specifically, the wavelength of light. 33
  34. Electromagnetic (EM) Radiation  EM radiation—light—is a form of energy whose behavior is described by the properties of both waves and particles. Some properties of EM radiation, such as its refraction when it passes from one medium to another, are explained best by describing light as a wave. Other properties, such as absorption and emission, are better described by treating light as a particle. Particle Properties of EM Radiation When matter absorbs EM radiation it undergoes a change in energy. The interaction between matter and EM 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, wavelength, and wavenumber by the following equalities 𝐸 = hν = hc / λ =hcν where h is Planck’s constant, which has a value of 6.626 × 10–34 J⋅s 34
  35. Photon as a Signal Source  There are several characteristic properties of EM radiation, including its energy, velocity, amplitude, frequency, phase angle, polarization, and direction of propagation.  A spectroscopic measurement is possible only if the photon’s interaction with the sample leads to a change in one or more of these characteristic properties.  We can divide spectroscopy into two broad classes of techniques.  In one class of techniques there is a transfer of energy between the photon and the sample.  In the second broad class of spectroscopic techniques, the electromagnetic radiation undergoes a change in amplitude, phase angle, polarization, or direction of propagation as a result of its refraction, reflection, scattering, diffraction, or dispersion by the sample. 35
  36. Photon as a Signal Source Table1 Examples of Spectroscopic Techniques Involving an Exchange of Energy Between a Photon and the Sample Type of Energy Transfer Region of Electromagnetic Spectrum Spectroscopic Techniquea Absorption γ-ray Mossbauer spectroscopy X-ray X-ray absorption spectroscopy UV/Vis UV/Vis spectroscopy atomic absorption spectroscopy IR infrared spectroscopy raman spectroscopy Microwave microwave spectroscopy Radio wave electron spin resonance spectroscopy nuclear magnetic resonance spectroscopy Emission (thermal excitation) UV/Vis atomic emission spectroscopy Photoluminescence X-ray X-ray fluorescence UV/Vis fluorescence spectroscopy phosphorescence spectroscopy atomic fluorescence spectroscopy Chemiluminescence UV/Vis chemiluminescence spectroscopy 36 Table 2 Examples of Spectroscopic Techniques That Do Not Involve an Exchange of Energy Between a Photon and the Sample Region of Electromagnetic Spectrum Type of Interaction Spectroscopic Technique X-ray diffraction X-ray diffraction UV/Vis refraction refractometry scattering nephelometry turbidimetry dispersion optical rotary dispersion
  37. Basic Components of Spectroscopic Instruments  Sources of Energy  Wavelength Selection  Detectors  Signal Processors 37
  38. Basic Components – Sources of Energy  All forms of spectroscopy require a source of energy.  In absorption and scattering spectroscopy this energy is supplied by photons.  Emission and photoluminescence spectroscopy use thermal, radiant (photon), or chemical energy to promote the analyte to a suitable excited state.  Chemical Sources of Energy: Exothermic reactions also may serve as a source of energy. In chemiluminescence the analyte is raised to a higher-energy state by means of a chemical reaction, emitting characteristic radiation when it returns to a lower-energy state. When the chemical reaction results from a biological or enzymatic reaction, the emission of radiation is called bioluminescence. Commercially available ―light sticks‖ and the flash of light from a firefly are examples of chemiluminescence and bioluminescence. 38
  39. Basic Components – Sources of Energy  Sources of Electromagnetic Radiation. A source of electromagnetic radiation must provide an output that is both intense and stable. Sources of electromagnetic radiation are classified as either continuum or line sources. A continuum source emits radiation over a broad range of wavelengths, with a relatively smooth variation in intensity. A line source, on the other hand, emits radiation at selected wavelengths.  Sources of Thermal Energy. The most common sources of thermal energy are flames and plasmas. Flames sources use the combustion of a fuel and an oxidant to achieve temperatures of 2000–3400 K. Plasmas, which are hot, ionized gases, provide temperatures of 6000–10 000 K. 39
  40. Basic Components – Sources of Energy  Common Sources of Electromagnetic Radiation 40 Source Wavelength Region Useful for... H2 and D2 lamp continuum source from 160–380 nm molecular absorption Tungsten lamp continuum source from 320–2400 nm molecular absorption Xe arc lamp continuum source from 200–1000 nm molecular fluorescence Nernst glower continuum source from 0.4–20 μm molecular absorption Globar continuum source from 1–40 μm molecular absorption Nichrome wire continuum source from 0.75–20 μm molecular absorption Hollow cathode lamp line source in UV/Visible atomic absorption Hg vapor lamp line source in UV/Visible molecular fluorescence Laser line source in UV/Visible/IR atomic and molecular absorption, fluorescence, and scattering
  41. Basic Components – Wavelength Selection  The ideal wavelength selector has a high throughput of radiation and a narrow effective bandwidth.  A high throughput is desirable because more photons pass through the wavelength selector, giving a stronger signal with less background noise.  A narrow effective bandwidth provides a higher resolution, with spectral features separated by more than twice the effective bandwidth being resolved.  These two features of a wavelength selector generally are in opposition.  Conditions favoring a higher throughput of radiation usually provide less resolution.  Decreasing the effective bandwidth improves resolution, but at the cost of a noisier signal.  For a qualitative analysis, resolution is usually more important than noise, and a smaller effective bandwidth is desirable. In a quantitative analysis less noise is usually desirable. 41
  42. Basic Components – Wavelength Selection  Wavelength Selection Using Filters:  The simplest method for isolating a narrow band of radiation is to use an absorption or interference filter.  Absorption filters work by selectively absorbing radiation from a narrow region of the electromagnetic spectrum.  Interference filters use constructive and destructive interference to isolate a narrow range of wavelengths.  A simple example of an absorption filter is a piece of colored glass. A purple filter, for example, removes the complementary color green from 500–560 nm.  Commercially available absorption filters provide effective bandwidths of 30–250 nm, although the throughput may be only 10% of the source’s emission intensity at the low end of this range. Interference filters are more expensive than absorption filters, but have narrower effective bandwidths, typically 10–20 nm, with maximum throughputs of at least 40%. 42
  43. Basic Components – Wavelength Selection  Wavelength Selection Using Monochromators:  A filter has one significant limitation—because a filter has a fixed nominal wavelength, if you need to make measurements at two different wavelengths, then you need to use two different filters.  A monochromator is an alternative method for selecting a narrow band of radiation that also allows us to continuously adjust the band’s nominal wavelength.  Radiation from the source enters the monochromator through an entrance slit.  The radiation is collected by a collimating mirror, which reflects a parallel beam of radiation to a diffraction grating.  The diffraction grating is an optically reflecting surface with a large number of parallel grooves.  The diffraction grating disperses the radiation and a second mirror focuses the radiation onto a planar surface containing an exit slit. In some monochromators a prism is used in place of the diffraction grating. 43
  44. Basic Components – Wavelength Selection  Schematic diagram of a monochromator that uses a diffraction grating to disperse the radiation. 44
  45. Basic Components – Wavelength Selection  Radiation exits the monochromator and passes to the detector.  A monochromator converts a polychromaticsource of radiation at the entrance slit to a monochromatic source of finite effective bandwidth at the exit slit.  The choice of which wavelength exits the monochromator is determined by rotating the diffraction grating. A narrower exit slit provides a smaller effective bandwidth and better resolution, but allows a smaller throughput of radiation.  Monochromators are classified as either fixed-wavelength or scanning.  In a fixed-wavelength monochromator we select the wavelength by manually rotating the grating. Normally a fixed-wavelength monochromator is used for a quantitative analysis where measurements are made at one or two wavelengths.  A scanning monochromator includes a drive mechanism that continuously rotates the grating, allowing successive wavelengths to exit from the monochromator. Scanning monochromators are used to acquire spectra, and, when operated in a fixed-wavelength mode, for a quantitative analysis 45
  46. Basic Components – Detectors  In initial days, human eyes served as detectors but they are only limited to visible spectra and vary from person to person.  In modern days, sensitive transducers convert the signal consisting of photons into an easily measurable electrical signal.  Ideally the detector’s signal, S, is a linear function of the electromagnetic radiation’s power, P, 𝑺 = 𝒌𝑷 + 𝑫 where k is the detector’s sensitivity, and D is the detector’s dark current, or the background current when we prevent the source’s radiation from reaching the detector.  There are two broad classes of spectroscopic transducers: thermal transducers and photon transducers. 46
  47. Basic Components – Detectors Transducer Class Wavelength Range Output Signal Phototube photon 200–1000 nm current Photomultiplier photon 110–1000 nm current Si photodiode photon 250–1100 nm current Photoconductor photon 750–6000 nm change in resistance Photovoltaic cell photon 400–5000 nm current or voltage Thermocouple thermal 0.8–40 μm voltage Thermistor thermal 0.8–40 μm change in resistance Pneumatic thermal 0.8–1000 μm membrane displacement Pyroelectric thermal 0.3–1000 μm current 47 Transducers for Spectroscopy
  48. Basic Components – Detectors 48  Photon Transducers:  Phototubes and photomultipliers contain a photosensitive surface that absorbs radiation in the ultraviolet, visible, or near IR, producing an electrical current proportional to the number of photons reaching the transducer  Other photon detectors use a semiconductor as the photosensitive surface. When the semiconductor absorbs photons, valence electrons move to the semiconductor’s conduction band, producing a measurable current.  One advantage of the Si photodiode is that it is easy to miniaturize.  Groups of photodiodes may be gathered together in a linear array containing from 64– 4096 individual photodiodes.  With a width of 25 μm per diode, for example, a linear array of 2048 photodiodes requires only 51.2 mm of linear space.  By placing a photodiode array along the monochromator’s focal plane, it is possible to monitor simultaneously an entire range of wavelengths.
  49. Basic Components – Detectors 49  Thermal Transducers:  Infrared photons do not have enough energy to produce a measurable current with a photon transducer.  A thermal transducer, therefore, is used for infrared spectroscopy. The absorption of infrared photons by a thermal transducer increases its temperature, changing one or more of its characteristic properties.  A pneumatic transducer, for example, is a small tube of xenon gas with an IR transparent window at one end and a flexible membrane at the other end.  Photons enter the tube and are absorbed by a blackened surface, increasing the temperature of the gas.  As the temperature inside the tube fluctuates, the gas expands and contracts and the flexible membrane moves in and out.  Monitoring the membrane’s displacement produces an electrical signal.
  50. Basic Components – Signal Processors  A transducer’s electrical signal is sent to a signal processor where it is displayed in a form that is more convenient for the analyst.  Examples of signal processors include analog or digital meters, recorders, and computers equipped with digital acquisition boards.  A signal processor is also used to calibrate the detector’s response, to amplify the transducer’s signal, to remove noise by filtering, or to mathematically transform the signal. 50
  51. References:    51