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Chem. 133 – 4/10 Lecture

Chem. 133 – 4/10 Lecture. Announcements I. Quiz Today + Turn in Additional Problem Exam 2 April 17 th Topics: Ch. 13, 14, 17, 19, 20 Will review topics next Tuesday Next Lab Report due dates – April 24 th and April 30 th. Announcements II. Today’s Lecture Atomic Spectroscopy

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Chem. 133 – 4/10 Lecture

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  1. Chem. 133 – 4/10 Lecture

  2. Announcements I • Quiz Today + Turn in Additional Problem • Exam 2 • April 17th • Topics: Ch. 13, 14, 17, 19, 20 • Will review topics next Tuesday • Next Lab Report due dates – April 24th and April 30th

  3. Announcements II • Today’s Lecture • Atomic Spectroscopy • Absorption Spectrometers • Emission Spectrometers • ICP-MS • NMR (if time) • Introduction • Theory

  4. Atomic SpectroscopyInterference 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) • D2 lamp signal = lamp intensity – molecular absorption – atomic absorption (very minor) • Amolec abs = -log[I(D2)/Io(D2)] (which can be subtracted from AMetal)

  5. Atomic SpectroscopyInterference 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 PO43-– add Ca because it binds strongly to PO43- 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)

  6. Atomic SpectroscopyInterference 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|) standards in water Area Analyte Concentration Concentration Added

  7. Atomic SpectroscopyEmission Spectrometers • In emission measurements, the plasma (or flame) is the light source • Flame sources are generally limited to a few elements for which AA spectrometers can be used • A monochromator or polychromator is the means of wavelength discrimination • Sensitive detectors are needed • ICP-AES is faster than AAS because switching monochromator settings can be done faster than switching lamp plus flame conditions Plasma (light source + sample) Monochromator or Polychromator Light detector or detector array Liquid sample, nebulizer, Ar source

  8. Atomic SpectroscopyEmission Spectrometers • Sequential vs. Simultaneous Instruments • Sequential Instruments use: • A standard monochromator • Select for elements by scanning the monochromator to specific wavelengths • Run at moderate rates (faster than AA but slower than simultaneous instruments). • Simultaneous Instruments use: • A 1D or 2D polychromator (Harris Color Plate 24/25) • 1D instruments typically use photomultiplier detectors behind multiple exit slits • 2D instrument shown in 4/1 lecture slide 13 • Selected elements (1D instruments) or all elements can be analyzed simultaneously resulting in faster analysis and less sample consumption.

  9. Atomic SpectroscopyInterference in Emission Measurements Emission Spectrum • Interferences • Atom – atom interferences more common than in atomic absorption because monochromators offer less selectivity than hollow cathode lamps • Interference from molecular emissions are reduced by scanning to the sides of the atomic peaks • Chemical interferences are less prevalent due to greater atomization efficiency Atomic peak background

  10. Atomic Mass Spectrometry • Most common arrangement consists of ICP torch placed to MS interface • The Ar+ ions (and electrons) collide with metals leading to ionization • The MS interface consists of skimmer cones to allow ions in, and to drop the pressure in stages, and ion optics • ICP-MS typically is the most sensitive elemental analysis method • Interference can arise from metals (e.g. 138Ba2+ vs. 69Ga+) or from ICP species (e.g. 40Ar+ and 40Ca+) • Use of secondary isotopic masses and collision cell reactions can reduce these interferences collision cell Plasma (atomizer + ion source) Mass spectrometer (e.g. quadrupole) Liquid sample, nebulizer, Ar source

  11. Atomic SpectroscopyComparison of Instruments

  12. Atomic SpectroscopySome Questions • Why is AES with a plasma normally more sensitive than AES with a flame? • List two ways in which a process in a flame can lead to reduced sensitivity and a way to deal with each process so its effect on the analysis is minimized. • Why can a simultaneous ICP-AES be more sensitive than an sequential ICP-AES if use for analysis of 12 metals? • If a sample matrix produces molecular emissions that interfere with atomic emissions, how would this be observed and how can this be accounted for? • What can cause interferences in ICP-MS?

  13. Nuclear Magnetic Resonance (NMR) SpectrometryMajor Uses • Identification of Pure Compounds (Qualitative Analysis) • Structural Determination (e.g. protein shape) • Quantitative Analysis • Characterization of Compounds in Mixtures • Imaging (MRI) – not covered

  14. NMR SpectrometryTheory • Spin • a magnetic property that sub atomic particles have (electrons, some nuclei) • some combinations do not result in observable spin (paired electrons have no observable spin; many nuclei have no observable spin) • Electron spin transitions occur at higher energies and are the basis of electron paramagnetic spectroscopy (EPR) • Nuclear spin given by Nuclear Spin Quantum Number (I)

  15. NMR SpectrometryTheory • Nuclear Spin (continued) • I = 0 nuclei → no spin (not useful in NMR) – e.g. 12C • I = ½ nuclei → most commonly used nuclei (1H, 13C, 19F, many others) • I > 1 nuclei → used occasionally, important for spin-spin coupling • number of different spin states (m) = 2I + 1 • examples: • 1H (I = ½), 2 states • 2H (I = 1), 3 states up state (m = +1/2) up state (m = 1) down state (m = -1/2) middle state (m = 0) down state (m = -1)

  16. NMR SpectrometryTheory • Effect of External Magnetic Field on Nuclei States • aligned nuclei (m = +1/2) have slightly lower energy (are more stable) than anti-aligned states (m = -1/2) • the greater the magnetic field (B0), the greater the energy difference between the states Applied Magnetic Field B0* “up” state – m = +1/2 “down” state – m = -1/2 path made by vector tips Note: arrows drawn at angles because spin vectors precess about B0 *Note: technically B0 is the magnetic field at the nucleus which is not quite the same as the applied magnetic field

  17. NMR SpectrometryTheory • Energy depends on nucleus, spin state (m), and magnetic field g (gamma) = magnetogyric ratio (constant for given nuclei) and h = Planck’s constant • Energy difference Energy ΔE B0

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