Atomic Absorption and Emission Theory PDF

ATOMIC ABSORPTION AND EMISSION THEORY By Nancy Tyrer and Ela Kogut Edits by Katie Rankin Readings: Fundamentals of Analytical Chemistry, Chapter 28 Chemical Analysis, Chapter 13 Lab Manual, Experiments 4 and 5  Atomic Absorption/Emission Spectroscopy 2  Atomic abs...

ATOMIC ABSORPTION AND EMISSION THEORY By Nancy Tyrer and Ela Kogut Edits by Katie Rankin Readings: Fundamentals of Analytical Chemistry, Chapter 28 Chemical Analysis, Chapter 13 Lab Manual, Experiments 4 and 5  Atomic Absorption/Emission Spectroscopy 2  Atomic absorption spectroscopy (AAS) quantifies the absorption of light by ground state metal atoms in the gaseous state → Mo(g)  Atomic emission spectroscopy (AES) quantifies the emission of light by excited state metal atoms in the gaseous state → M*(g)  Atomic Absorption/Emission Spectroscopy 1. Analyte type:  Atoms of metals (e.g. Cu, Fe)  Ions of metals (e.g. Cu2+, Fe2+) Elements detectable by atomic absorption are highlighted in pink in this periodic table.  Can perform qualitative and quantitative (ppm/ppb) analysis of >70 elements by AAS → AAS one of the most widely used techniques for metal analysis 3 Atomic Emission Spectroscopy: Elements Elements in white cannot be analyzed by AES. 4  Atomic Absorption/Emission Spectroscopy 2. Type of radiation:  UV: 180-380 nm  Visible: 380-780 nm Interaction of UV- visible radiation with metals (atoms) results in change of electron distribution of valence electrons.  Atomic Absorption/Emission Spectroscopy 6 3. Process Measured:  Absorption (AAS), Emission (AES) or Fluorescence (AFS) 4. Type of Transition:  Electronic: Valence electrons promoted to higher energy levels, followed by decay to intermediate or ground state level (emission) 5. Qualitative Information:  Unique spectrum of analyte → fingerprint of element  Plot of IR vs. λ (nm) Atomic Spectrum: Molecular Spectrum: Line Spectrum Continuous Broad Band  Atomic Absorption/Emission Spectroscopy 7 6. Quantitative Information:  Plot of instrumental response vs. concentration of analyte  AAS:  A = abc (path length = width of burner slot for flame)  Abs = mC + b 1.000 0.800 Absorbance 0.600 y = 0.197x + 0.0165 0.400 0.200 0.000 0.00 1.00 2.00 3.00 4.00 5.00 6.00 Concentration of Ca2+ (ppm)  AES: IE = m C + b  Atomic Absorption/Emission Spectroscopy 8 What happens to the analyte species when its aspirated to the flame? Four Major Experimental Steps: 1. Conversion of solvated analyte ions to free gaseous atoms: Thermal energy in the form of a flame, Mn+(aq) Mo(gas) ∆ plasma or furnace is required to convert metal ions to gaseous atoms. 2. Excitation of atoms to higher electronic energy states: + 𝐸𝐸 ℎ𝑐𝑐 Mo(gas) M*(gas) Mo(gas) + or Δ λ AAS AES ℎ𝑐𝑐 AAS: 𝐸𝐸 = λ from hollow cathode lamp AES: 𝐸𝐸 = thermal energy from flame or plasma (ICP) → Studied in Instrumental 2  Atomic Absorption/Emission Spectroscopy 9 3. Measurement of absorption or emission of radiant energy at λmax (resonance λ): Absorption (AAS): Promotion of valence electrons to higher energy orbitals E1 E1 Absorption of EMR of energy (hc/λ) is equal to the energy difference between two electronic states. E0 E0 ℎ𝑐𝑐 Δ𝐸𝐸𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 = 𝐸𝐸 ∗ (𝐸𝐸1 ) − 𝐸𝐸0 = λ  Atomic Absorption/Emission Spectroscopy 10 Emission (AES): Decay of excited valence electrons back to ground state E1 E1 Loses energy (emits light) E0 E0 ℎ𝑐𝑐 Δ𝐸𝐸𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 = 𝐸𝐸 ∗ (𝐸𝐸 1) − 𝐸𝐸0 = λ  Atomic Absorption/Emission Spectroscopy 11 4. Extraction of qualitative or quantitative data Qualitative: Quantitative: Line spectrum Absorption Absorbance is proportional to concentration Abs = mC + b Emission Emission intensity is proportional to concentration IE = mC + b  Atomic Emission Spectra 12  Emission line represents transition from higher E level to lower E level  Each element has its own unique emission line spectrum → fingerprint  Atomic Emission Spectra  Each emission spectrum represents all possible transitions which could occur between energy levels of an atom  Only a certain # of limited transitions (λ’s) are absorbed or emitted due to quantization of energy  The more intense the line (resonance λ) the more likely transition will occur  Each transition has a different probability of occurring (based on nuclear charge, shape of orbital and distance from the nucleus) 13  Atomic Emission: Flame Spectroscopy Lithium 14 Observation Caused by... Persistent golden-yellow flame Sodium Cesium Violet (lilac) flame Potassium, cesium carmine-red flame Lithium Brick-red flame Calcium Crimson flame Strontium Yellowish-green flame barium, molybdenum Sodium Green flame Borates, copper, thallium Blue flame (wire slowly corroded) Lead, arsenic, antimony, bismuth, copper Qualitative method  Bohr Model of an Atom 15 In 1913, Niels Bohr formulated 3 rules regarding atoms: 1. Electrons can only be in discrete orbitals. 2. A photon can be emitted or absorbed by an atom only when an electron transitions from one orbital to another. 3. The photon energy equals the energy difference between the two orbitals. The photon energy is:  E = hf = hc/λ (recall that f = c/λ)  E is the energy difference between the two orbitals  Bohr Model: Electronic Transitions 16  An electronic transition occurs when an electron moves between two orbitals.  When absorption of a photon occurs, an electron is promoted to a higher energy level (e.g. from n = 1 to n = 2).  Emission of a photon occurs when an electron transitions from a higher to lower energy level (e.g. from n = 2 to n = 1). n=2 n=1  Electronic Energy Level Diagram Atoms 17  Instead of drawing circular orbitals in which the energy is higher for each successive orbital, we can draw an energy level diagram as shown on the right below.  The energy levels of an atom are represented by horizontal lines.  Energy increases as you move upward in the diagram.  Atomic Orbitals 18  Quantum mechanics (mathematic models) are used to predict the probability of where electrons are likely to be found near the nucleus within an atom.  Orbitals → 3-D map of where the e- is likely to be found within a defined space close to the nucleus (with a given degree of probability).  s, p, d, f orbitals  Only 2 e-s of opposite spin can be located in an orbital  Electron Configuration of Metals 19 Electronic configuration of atoms of an element is unique:  Electrons exist in discrete energy levels around the nucleus of an atom in atomic orbitals → s, p, d, f  Ground state configuration is the lowest energy state  Most stable state (shown in Periodic Table of Elements) Filling order of atomic orbitals Ground State Electron Configuration Ground state electron configuration for elements (only valence shell is shown) 20  Atomic Spectroscopy: Ground State 21 Ground state: Most stable arrangement of the nucleus in an atom of an element → minimum energy  Electron configuration of an element as found in the periodic table  Examples:  Na → [Ne]3s1  Ca → [Ar]4s2  For excitation to occur the valance e- MUST be supplied with a quantized amount of energy equal to the energy difference between the ground and excited state.  Atomic Spectroscopy: Excited State 22 Excited state: When an atom encounters energy (radiation or heat), it absorbs the additional energy and becomes excited → overall energy increase  Examples for Na:  [Ne]3s03p1  [Ne]3s03p04s1  [Ne]3s03p04s03d1  Examples for Ca:  [Ar]4s14p1  [Ar]4s1 4p1 3d1  [Ar]4s1, 3d1  Absorption of Radiation by Li (AAS) 23 Practice Problem. A lithium atom can absorb light of three wavelengths: 675 nm, 570 nm and 415 nm. a) State the ground state electron configuration for Li. b) State 3 possible excited state electron configurations corresponding to an absorption of energy at each wavelength. c) Draw a line spectrum for the electronic transitions, and match each excited state configuration from part b) to the corresponding line.  Ground State Energy Level Diagram for Li 24  Electronic Transitions for Li 25  Absorption Line Spectrum for Lithium 26 Emission line spectrum:  Atomic Absorption Spectrometer vs Atomic Emission Spectrometer 27  Atomic absorption spectrometers have 4 principal components: 1. A radiation source (hollow cathode lamp) 2. An atom cell → converts analyte metal species from aqueous ion to gaseous atom Mn+(aq) → Mn+(s) → Mn+(l) → Mn+(g) + e- → Mo(g) + hc/λ → M*(g) 3. A monochromator (λ selector) 4. A detector and readout device  LOD → mid to low ppb  LDR → 3-5 orders of magnitude  Atomic emission spectrometers have components 2, 3 and 4, but have NO radiation source  Atomic Absorption Spectroscopy vs Atomic Emission Spectroscopy Aspirate aqueous solution into flame or plasma AES, IE measured AAS, Abs measured AFS, IF measured 28  Instrumentation for Atomic Absorption Spectroscopy/ Single Beam Spectrometer: One path for light to travel 29  AAS vs AES AAS AES (flame) Process Measured Absorption by ground state Emission by excited state gaseous metal atoms gaseous metal atoms Chemical Equation Mo(g) + hc/λ → M*(g) M*(g) → Mo(g) + hc/λ Atomization Source Thermal energy (Δ) / Thermal energy (Δ) / reducing (Mn+(aq) → Mo (g)) reducing species of the flame species of the flame Excitation of Atoms Hollow Cathode Lamp Flame Quantitative A = mC + b IE = mC + b Analysis Detection Limits ppb (depends on the analyte) ppb (depends on the analyte) Sensitivity Higher sensitivity than AES Because population of excited state atoms in the flame is ~ 2%, AES has a lower sensitivity than AAS for most metals Other advantages of AAS: rapid, inexpensive, widely used, good accuracy, high selectivity Atomic Absorption Spectrometer Double Beam Spectrometer: Two paths for light to travel 31  References 32  Tyrer, N.; Kogut, E. CHEM25415 Lecture on Atomic Absorption/Emission Theory. Presented at Sheridan College, Brampton, ON, Fall 2012.  Tyrer, N. CHEM25415 Instrumental Analysis 1 Laboratory Manual; Sheridan College: Brampton, ON; Experiments 4 and 5.  Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry, 9th ed.; Brooks/Cole: California, 2014; Chapter 28.  Rouessac, F.; Rouessac, A. Chemical Analysis, 2nd ed.; John Wiley & Sons: New Jersey, 2007; Chapter 13.