Chem. 133 – 4/7 Lecture

Announcements I Lab –Should be starting Set 2 Period 1 –Set 2 Period 2 Lab Reports due Today Pass Out TH Cheng Award Letters + Return Lab Reports + Last Week’s Assignment Quiz scores somewhat low Additional Problem –Made error on board so 4/4 pts unless done all right (then +1 bonus) –C = -logT/  b (not -  blogT) and  C = [(dC/dT) 2  T 2 ] 0.5 –  C = [(-1/  bTln10) 2  T 2 ] 0.5 =  T /  bTln10

Announcements II Exam 2 –Format similar to Exam 1 –Will cover: Ch. 13 (starting at sect. 4), 14, 17, 19, and 20 (covering overview, theory, and atomization means; parts on interferences will be on Exam 3) –Will Review Topics –Possible Help Session Wed. 12 – 1 PM in Sequoia 446 HW Set 2.3 Solutions Posted

Announcements III Today’s Lecture –Chapter 20: Atomic Spectroscopy Theory (Boltzmann Distribution) Atomization (flame, graphite furnace, and ICP) –Review for Exam 2 –Chapter 20: Atomic Spectroscopy (parts not on Exam 2 – if time) Optical Parts of Instruments + Interferences

Atomic Spectroscopy Theory For emission measurements, a key is to populate higher energy levels In most cases, this occurs through the thermal methods also responsible for atomization Fraction of excited energy levels populated is given by Boltzmann Distribution More emission at higher temperatures and for longer wavelengths (smaller  E) Na(g) o (3s) 4p E N = number atoms in ground (0) and excited (*) states g = degeneracy (# equivalent states) = 3 in above example k = Boltzmann constant

Atomic Spectroscopy Theory Example problem: Calcium absorbs light at 422 nm. Calculate the ratio of Ca atoms in the excited state to the ground state at 3200 K (temperature in N 2 O fueled flame). g*/g 0 = 3 (3 5p orbitals to 1 4s orbital).

Atomic Spectroscopy Atomization Flame Atomization –used for liquid samples –liquid pulled by action of nebulizer –nebulizer produces spray of sample liquid –droplets evaporate in spray chamber leaving particles –fuel added and ignited in flame –atomization of remaining particles and spray droplets occurs in flame –optical beam through region of best atomization sample in fuel (HCCH) oxidant (air or N 2 O) burner head spray chamber nebulizer light beam nebulizerair liquid

Atomic Spectroscopy Atomization Atomization in flames – Processes –nebulization of liquid: MgCl 2 (aq) → MgCl 2 (spray droplet) –evaporation of solvent: MgCl 2 (spray droplet) → MgCl 2 (s) –Volatilization in flame: MgCl 2 (s) → MgCl 2 (g) –Atomization (in hotter part of flame): MgCl 2 (g) → Mg(g) + Cl 2 (g) Target species for absorption measurement

Atomic Spectroscopy Atomization Complications/Losses –Ideally, every atom entering nebulizer ends up as gaseous atom –In practice, at best only a few % of atoms become atoms in flame –The nebulization process is not that efficient (much of water hits walls and goes out drain) –Poor volatilization also occurs with less volatile salts (e.g. many phosphates)

Atomic Spectroscopy Atomization Complications/Losses (continued) –Poor atomization also can occur due to secondary processes such as: Formation of oxides + hydroxides (e.g. 2Mg (g) + O 2 (g) → 2MgO (g)) Ionization (Na (g) + Cl (g) → Na + (g) + Cl - (g)) –If the atomization is affected by other compounds in sample matrix (e.g. the presence of phosphates), this is called a matrix effect (discussed more later

Atomic Spectroscopy Atomization Electrothermal Atomization –Atomization occurs in a graphite furnace –Process is different in that a small sample is placed in a graphite tube and atomization occurs rapidly but in a discontinuous manner –Electrothermal atomization is more efficient; atoms spend more time in the beam path, and less sample is required resulting in much greater sensitivity Concentration LODs are typically ~100 times lower (e.g. 100 ppt for EA vs. 10 ppb for flame) Mass LODs are even lower (100 pg/mL*0.01 mL = 1 pg for EA vs. 10 ng/mL*2 mL = 20 ng for flame)

Atomic Spectroscopy Atomization Electrothermal Atomization (Process) –Sample is placed through hole onto L ’ vov platform –Graphite tube is heated by resistive heating –This occurs in steps (dry, char, atomize, clean) Graphite Tube in Chamber (not shown) L’vov Platform Sample in T time drychar atomize Clean + cool down Ar in chamber flow stops and optical measurements made

Atomic Spectroscopy Atomization Inductively Coupled Plasma (ICP) –A plasma is induced by radio frequency currents in surrounding coil –Once a spark occurs in Ar gas, some electrons leave Ar producing Ar + + e - –The sample is introduced by nebulization in the Ar stream –The accelerations of Ar + and e - induce further production of ions and great heat production –Much higher temperatures are created (6000 K to K vs. flames) ICP Torch Quartz tube Argon + Sample RF Coil

Atomic Spectroscopy Atomization Advantages of ICP Atomization –Greater atomization efficiency than in flame AA (partly because better nebulizers are used than with flames due to higher total instrument cost and partly due to higher temperatures) –Fewer matrix effects because atomization is more complete at higher temperatures –High temperature atomization allows much greater emission flux + more ionization allowing coupling with emission spectrophotometers and mass spectrometers –Emission and MS allow faster multi-element analysis

Chapter 20 Questions 1.Why would it be difficult to use a broadband light source and monochromator to produce light used in AA spectrometers? 2.List three methods for atomizing elements. 3.List two processes that can decrease atomization efficiency in flame atomization. 4.What is an advantage in using electrothermal atomization in AAS? 5.Which atomization method tends to result in the most complete breakdown of elements to atoms in the gas phase? 6.Why is ICP better for emission measurements than flame?

Exam 2 Topics to Know A.Electrochemistry (Ch. 13 and 14) 1.Nernst Equation – know how to use to determine cell potentials, concentrations of unknowns, and equilibrium constants.* 2.Conversion between K,  G and E (as in Quiz 3)* 3.Know equipment needed for potentiometry measurements. 4.Know purpose of reference electrodes 5.Know types and uses of indicator electrodes 6.Understand how ion-selective electrodes work 7.Some failings of ion-selective electrodes under specific conditions * means requires quantitative knowledge

Exam 2 Topics to Know – cont. A.Chapter 17 (Spectroscopy – Theory) 1.Light defining parameters (be able to convert between, E,, and for light).* 2.Know processes of absorption and emission. 3.Know alternative methods of excitation and de- excitation. 4.Know regions of electromagnetic spectrum and related transitions. 5.Know basics of spectral interpretation. 6.Understand and be able to use Beer’s Law equations.* 7.Know sources of deviations to Beer ’ s Law + region of best precision

Exam 2 Topics to Know C.Chapter 19 (Spectrometers – cont.) 1.Spectrometer Design (know main components + designs for UV and fluoresence spectrometers) 2.Main discrete and broad band light sources 3.Main methods of wavelength discrimination (interference filters, monochromators, polychromators, Fourier methods, and through energy dispersive detectors) 4.How interference filters work* 5.Components and calculations in grating monochromators/polychromators* 6.Light Detectors (basic types and how they work)

Exam 2 Topics (cont.) C.Ch. 19 – cont. 7.Polychromators/Array detectors – how they work 8.How energy dispersive detectors work and what types of light measurements they are used for 9.How FTIR works + advantages and disadvantages of FTIR D.Chapter 20 (Atomic Spectroscopy) 1.Methods for Elemental Analysis (solid + liquid samples) 2.Basic theory of atomic transitions

Exam 2 Topics (cont.) D.Chapter 20 – cont. 3.Atomization processes in flame, graphite furnace, and ICP and sources of inefficiency in atomization 4.Effect of temp. on Boltzmann distribution and on emission intensity* 5.Advantages and disadvantages of various atomization methods

Exam 2: Equations Provided Nernst Equation E = E º – ( /n)logQ Monochromator angular and linear dispersion equations: Angular dispersion =  /  = n/dcos  Linear dispersion = D =  y/  = F  /  Boltzmann Distribution Equation

Atomic Spectroscopy Absorption Spectrometers The lamp is a hollow cathode lamp containing the element(s) of interest in cathode The lamp is operated under relatively cool conditions at lower pressures to reduce Doppler and pressure broadening of atomic emission lines A very narrow band of light emitted from hollow cathode lamps is needed so that absorption by atoms in flame mostly follows Beer ’ s law The monochromator serves as a coarse filter to remove other wavelength bands from light and light emitted from flames Lamp source Flame or graphite tube monochromatorLight detector

Atomic Spectroscopy Absorption Spectrometers A narrower emission spectrum from hollow cathode lamp (vs. flame absorption) results in better Beer’s law behavior wavelength Intensity or absorbance hollow cathode lamp emission Atomic absorption spectrum in flame Sources of broadening: 1.Inherent width (Heissenberg Uncertainty Principle):  E ~ J (see text) or  ~ nm 2.Doppler broadening (due to atom motion; depends on temperature) 3.Pressure broadening (shorter lifetimes at higher pressures gives broader peaks)

Atomic Spectroscopy Interference in Absorption Measurements Spectral Interference –Very few atom – atom interferences –Interference from flame (or graphite tube) emissions are reduced by modulating lamp no lamp: signal from flame vs. with lamp then with lamp: signal from lamp + flame – absorption by atoms –Interference from molecular species absorbing lamp photons (mostly at shorter wavelengths and light scattering in EA-AA) –This interference can be removed by periodically using a deuterium lamp (broad band light source) D 2 lamp signal = lamp intensity – molecular absorption – atomic absorption (very minor) A molec abs = -log[I(D 2 )/I o (D 2 )] (which can be subtracted from A Metal )

Atomic Spectroscopy Interference in Absorption Measurements Chemical Interference –Arises from compounds in sample matrix or atomization conditions that affects element atomization –Some examples of specific problems (mentioned previously) and solutions: Poor volatility due to PO 4 3- – add Ca because it binds strongly to PO 4 3- allowing analyte metal to volatilize better or use hotter flames Formation of metal oxides and hydroxides – use fuel rich flame Ionization of analyte atoms – add more readily ionizable metal (e.g Cs) –Another approach is to use a standard addition calibration procedure (this won ’ t improve atomization but it accounts for it so that results are reliable)

Atomic Spectroscopy Interference in Absorption Measurements Standard Addition –Used when sample matrix affects response to analytes –Commonly needed for AAS with complicated samples –Standard is added to sample (usually in multiple increments) –Needed if slope is affected by matrix –Concentration is determined by extrapolation (= |X-intercept|) Area Concentration Added Analyte Concentration standards in water