Chemistry 311: Instrumentation Analysis Topic 2: Atomic Spectroscopy. Chemistry 311: Instrumentation Analysis Topic 2: Atomic Spectroscopy
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1 Topic 1: Atomic Spectroscopy Text: Chapter 12,13 & 14 Rouessac (~2 weeks) 1.0 Review basic concepts in Spectroscopy 2.0 Atomic Absorption and Graphite Furnace Instruments 3.0 Inductively Coupled Plasmas 4.0 X-ray Fluorescence Winter 2009 Page 1 Electromagnetic Radiation : Wave-Particle Duality Light is a form of Electromagnetic Radiation EM Radiation described by a wave-particle duality mode EM Radiation behaves as a particle with no mass traveling as a sinusoidal wave at the speed of light. EM radiation, as the name suggests, has both Magnetic and Electric field components. These components are at right angles (orthogonal) to each other. Does not travel in a single plane as depicted on paper but rather as a 3D wave. ie., as an extended slinky Winter 2009 Page 2 1
2 Wavelength - λ space taken up by one cycle units of distance normally stated as Å or m or cm, 1 Å = 1 x m = 1 x 10-8 cm Key terms: Amplitude, Period {time required} and Frequency Frequency - ν: # of cycles per sec passing through a given point (units s-1) Frequency determined by source invariant Wavenumber, ν, the reciprocal of the wavelength: ν = 1/λ Power, P, the energy of the beam that reaches a given area per second. The flux of energy per unit time. Power is proportional to Amplitude, A2 Intensity, I, is the power per unit solid angle. Winter 2009 Page 3 Velocity or Speed of Light (c) In vacuum, c is independent of wavelength and is at a maximum value of x 10 8 m/s = constant c In air, velocity of light (v) is only ~0.03% less than c speed of light sometimes reported as 3.00 x 10 8 m/s. Valid in air or vacuum. General expression: c = 3.00 x 10 8 m/s = l ν Consequence of constant ν and c = f(medium)? Index of refraction (η) relates velocity of light in vacuum to medium Index of Refraction : η = c/v Winter 2009 Page 4 2
3 How is Electromagnetic Radiation related to Atomic and Molecular energy? Winter 2009 Page 5 Planck s Quantum Theory: A Quantum is the smallest quantity of energy that can be emitted (or absorbed) in the form of EM. E = h ν where: h is Planck s constant, which is h = 6.63 x J s Energy is always emitted (or absorbed) in whole number multiples of h ν The Photoelectric Effect: Albert Einstein proposed that light is composed of particles, called photons. Each photon has an energy content of; E = h ν Energy above a specific threshold can dislodge electrons from a metal surface. This phenomenon can be used to construct a photon detector. Winter 2009 Page 6 3
4 Winter 2009 Page 7 Energy Storage in Atoms and Molecules: Atoms and molecules have can store energy only in very specific ways. In other words, they have very specific and defined energy states. Since the energy of a photon is defined by it s frequency, only photons with very specific energy can be absorbed by a given atom or molecule. Subsequently, if an atom or molecule is in an excited state and then returns to a less energetic state, photons with specific frequencies will be emitted. E tot = E elec + E vib + E rot + E trans Winter 2009 Page 8 4
5 Winter 2009 Page 9 Basic Instruments and Components Various instruments are used to study absorption, emission, luminescence, etc. of Electromagnetic Radiation (EMR) as a function of wavelength (or frequency): Spectrometer: Measures the intensity of radiation emitted by the sample Spectrophotometer: an instrument with a monochromator to select λ Spectrograph: an instrument with a photographic plate as a detector Photometer: an instrument with a filter to select wavelength range Colorimeter: photometer using the human eye as the detector (visible λ) Fluorometer: an instrument that measures fluorescence. Spectrofluorometer: a Fluorometer with a monochromator Winter 2009 Page 10 5
6 These basic instruments are used to measure 6 phenomena; Absorption Fluorescence Phosphorescence Scattering Emission Chemiluminescence The optical and electronic principle employed in these instruments is basically the same for all the regions of the EMR, however there are some differences in the specific components used in various regions. Winter 2009 Page 11 Basic Instruments and Components Any spectroscopic instrument has five major components: 1. Stable source of radiant energy 2. Transparent container for holding the sample 3. Device that isolates a restricted region for measurement 4. Radiation detector 5. Signal processor or readout Winter 2009 Page 12 6
7 Line Sources for Atomic Absorption: Hollow cathode lamps: Analytical Problem: The smallest Bandwidth that can be obtained by a continuous source is very large compared to atomic absorption lines of nm to nm. Only a fraction of the source beam can be absorbed by an atomic sample large background signal occurs. Analytical Solution: Use atomic emission lines as the source. Winter 2009 Page 13 Line Sources: Electrodeless Discharge lamps: RF or Microwave radiation used to excite metal or salt of material of interest. Intensities of these lamps are one to two orders of magnitude larger than Hollow cathode lamps but performance is not as reliable. Winter 2009 Page 14 7
8 Wavelength Selectors: λ Band and Bandwidth: A narrow, continuous group of λ s. Effective Bandwidth is an indication of the quality of the λ selector. Winter 2009 Page 15 Winter 2009 Page 16 8
9 Wavelength Selectors: Types Filters Interference Filters Interference Wedges Absorption filters Monochromators Prisms Gratings Echellette Grating Concave Holographic Winter 2009 Page 17 Wavelength Selectors: Filters Interference Filters: Limit transference to narrow band. Wavelength passed depends on thickness of dielectric (t) and refractive index (η). UV to IR region. Bandwidth 1.5% to 0.15% of λ. 80% transmission to < 10% trans. Winter 2009 Page 18 9
10 Absorption Filters: Generally less expensive. Normally used in visible region for inexpensive devices. Colored glass or dye suspended in gelatin. Narrow bandwidth low transmission ~10%. Cut-off filters also can be combined. Winter 2009 Page 19 Absorption Filters: Cut-off filters also can be combined. Winter 2009 Page 20 10
11 Winter 2009 Page 21 Winter 2009 Page 22 11
12 Gratings Dispersion achieved through constructive interference Broad face narrow face For constructive interference: nλ = (CB + BD) = d (sin i + sin r) d = distances between blazes; i = incident angle; r = reflected angle Note: n means that a # order lines exist ie. 1st 900 nm, 2nd 450 nm, 3rd 300 nm Winter 2009 Page 23 Winter 2009 Page 24 12
13 Resolving Power of Monochromators: The ability to separate two (images) λ. Resolution of typical benchtop UV/Visible spectrometers 103 to 104. Winter 2009 Page 25 The Electromagnetic Radiation Transducers (Detectors): A radiation transducer is a device that converts the radiation into a quantifiable value. Early transducers were the human eye and photographic plates. Most modern transducers convert signals to an electrical signal. A generic relationship between the radiant power of the radiation and the signal is given by; S = kp + kd Note: kd is also called the dark current. Background current in absence of source Three major categories: Thermal detectors Sense the change in temperature. Examples: Thermoelectric detector; Bolometer; Pyroelectric Photon detectors Respond to incident photon arrival rates rather than to photon energies. Examples: Phototube; Photomultiplier tube (PMT) Multichannel detectors photographic emulsions, arrays of thermal detectors, etc. Winter 2009 Page 26 13
14 Important requirements for detectors: High sensitivity with a low noise level : Short response time. Long term stability to ensure quantitative response. Produces an electronic signal easily amplified. Performance Characteristics of Common Detectors Photon Detectors Phototube The photons strike photoemissive surface of the cathode and transfer energy to loosely bound surface electrons. The electrons escape from the surface and are collected at the anode causing current to flow. Photomultiplier tube (PMT) As in phototubes, an electron is emitted from a photoemissive surface. Ejected electron is accelerated by an electric field; strikes another electron active surface, causing additional emitted electrons. Page 27 Performance Characteristics of Common Detectors Semi-conductor based Devices: Photovoltaic or Barrier-Layer Cells: A simple rugged device containing a thin layer of semiconductor such as selenium coated with silver or gold. Photons are ejected from semiconductor and can flow to silver or gold collector electrode. Range: 350 to 750 nm Advantages: Low cost and rugged, no power required. Disadvantage: Lack of sensitivity at low light levels, difficult to amplify signal, can fatigue with continuous illumination. Silicon Diode Transducers: A silicon chip containing a reverse-biased pn junction. Sensitivity between vacuum phototube and photomultiplier. Range: 190 to 1100 nm Advantages: Low cost and can be miniaturized. Winter 2009 Page 28 14
15 Multichannel Photon detectors: Arrays of detectors usually contained on a semiconductor chip. Three types in common usage: 1) photodiode arrays (PDAs); 2) charge-injection devices (CIDs) and 3) charge-couple devices (CCDs). 2 and 3 are Charge-Transfer Devices (CTDs) Photodiode arrays (PDAs): Series of Silicon Diode Transducers on a single integrated circuit. 64 to 4096 diodes are possible, 1024 most common. Disadvantage: Not as sensitive as photomultiplier. Winter 2009 Page 29 Quantitative Aspects of Spectrochemical Measurements: As seen in the discussion of detectors, most spectroscopic detectors produce a signal that is proportional to the radiant power P of the EM. S = kp + kd The dark current, kd, is usually small and some instruments are equipped with electrical circuits which can reduce this to zero. For emission, luminescence and scattering the radiant power of the EM released is directly proportional to the concentration, ie., S = k C For atomic or molecular absorption, the magnitude that the EM beam is attenuated by the sample is proportional to the concentration. In order to measure the degree of attenuation, two measurements are required, one measuring the incident radiation, P 0, and the other the transmission, P. Winter 2009 Page 30 15
16 Key terms and relations: Winter 2009 Page 31 Beer s Law: Absorbance is directly proportional to the path length, b, the concentration, c, and a proportionality constant called the absorptivity, a, or in other words; A = abc Note: magnitude and units of a dependent on units of b & c When concentration is in Molarity and cell length is in cm, then a is equal to the molar absorptivity, ε, with units L mol -1 cm -1 A = εbc Winter 2009 Page 32 16
17 Operation of a Simple Spectrometer & Relation to Defined 1) Select appropriate wavelength and slit settings (if appropriate) 2) Nullify dark current, kd, by adjusting readout with detector isolated. In other words set to 0% transmission (this is often preset). 3) With solvent blank correct for absorbance of the solvent, scattering, reflectance, etc. by setting Transmission to 100% or zero the absorbance. 4) With sample in read transmission directly. (Note: Absorbance is a log scale) Winter 2009 Page 33 Atomic Absorption (AA) In Atomic Absorption, Sample Solution is spayed into a flame where it is vaporized and converted to atomic atoms. Atomic Atoms can absorb narrow bandwidths of light generated from a Hollow Cathode Tube. Application: Trace analysis of Metals (ppm ppb level) Instrument Cost: ~ $30K - $100K with accessories Winter 2009 Page 34 17
18 Flame Atomization Techniques: Many common steps to the production of gaseous atomic population Winter 2009 Page 35 Atomic Absorption/Emission and Energy Level Diagrams (Note: Na and Mg+ are isoelectronic) Winter 2009 Page 36 18
19 Atomic Absorption/Emission and Energy Level Diagrams Splitting of p orbitals shown in previous overhead is typical. If electron spin magnetic field and orbital motion magnetic field are the same, the energy of the state is higher than if they are different. Analogous to bringing 2 north magnetic fields together. Although, Na and Mg + are isoelectronic, in Mg + the negative electron is being moved further away from a 12 + nucleus rather than a 11 + nucleus, so higher energy is involved. For both Na and Mg+ cases there is 1 electron outside of the closed [Ne] shell. In Figure 8.2 (next page), the energy diagram for atomic Mg is presented. In this case there is two electrons outside of the closed [Ne] shell. If the magnetic spin of these electrons is the same, they are said to be in a triplet state (spins unpaired), if they are opposed, they are in a singlet state (spins paired). If spins are paired, the effects of magnetic electron and magnetic orbital spin splitting cancel and so the p-splitting effect is cancelled. However, in triplet states, all the p, d and f states are split. Therefore, many transitions are possible Winter 2009 Page 37 Winter 2009 Page 38 19
20 9/30/09 Winter 2009 Page 39 Atomic Energy Level Diagrams As depicted by the darker lines in the previous slides; Some transitions are more probable than others. The nature of the allowed and favored transitions is governed by a complex set of selection rules, in which some types of transitions are forbidden. For example, singlet - singlet transitions are much more probable than singlet triplet transitions. As a result the emission or absorption lines of these transitions are much more intense. As indicated by the previous diagrams, the complexity of the Energy Level diagrams increase with the number of electrons outside the closed shell. For a simple one electron alkali metal such as Lithium there is only 30 observed spectra, for magnesium there is 173, for Iron there is As we will discuss later, the number of observed emission lines is much lower in lower energy atomizers such as flames. Winter 2009 Page 40 20
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