Chem. 133 � 3/17 Lecture

Document Sample
Chem. 133 � 3/17 Lecture Powered By Docstoc
					Chem. 133 – 3/17 Lecture
                 Announcements I
• Quiz Today
• Turn in Term Project Proposal
• NMR Reading
   – From Skoog et al., 6th Edition
   – pp. 498 – 528
   – Will provide copies in class folder in main office for
   – Posted New NMR HW problems (3-7, 9, 17, 27, 31-34, 36,
     37) – see updated HW assignment
                 Atomic Spectroscopy
                 Absorption Spectrometers

              Flame or
              graphite tube

Lamp source

                                 monochromator            Light detector

•   The lamp is a hollow cathode lamp containing the element(s) of interest in
•   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 are needed
    so that for 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
            Atomic Spectroscopy
            Absorption Spectrometers
                                                                  hollow cathode
• A narrower emission                                             lamp emission
  spectrum from hollow
  cathode lamp (vs. flame

                               Intensity or absorbance
  absorption) results in                                              absorption
                                                                      spectra in flame
  better Beer’s law behavior

               Atomic Spectroscopy
     Interference in Absorption Measurements
• Spectral Interference
   – Very few atom – atom interferences
   – Interference from flame emissions are reduced by modulating
      •   no lamp: signal from flame
      •   then with lamp: signal from lamp + flame – absorption by atoms
      •   difference = I
      •   blank difference = Io
   – Interference from molecular species absorbing lamp photons
     occurs at shorter wavelengths (or 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)
      • I = difference (see above) – D2 lamp signal
      • Other methods (Zeeman method)
              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
       • Poor nebulization (e.g. viscous liquid)
       • Poor volatility due to PO43- – add Ca because it binds strongly to
         PO43- allowing analyte metal to volatilize better) or use hotter
       • Formation of metal oxides and hydroxides – use fuel rich flame
       • Ionization of analyte atoms – add more readily ionizable metal (e.g
   – 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
                                                           standards in water

• Standard Addition                            Area
   – Used when sample matrix affects
     response to analytes
   – Commonly needed for AAS with       Analyte
     complicated samples                Concentration

   – Standard is added to sample
     (usually in multiple increments)
   – Needed if slope is affected by                     Added
   – Concentration is determined by
                                            A  mX  b  0
     extrapolation (= |X-intercept|)
                                           X  b/ m
                   Atomic Spectroscopy
                   Emission Spectrometers
• In emission measurements, the plasma (or flame) is the light source
• A monochromator or polychromator is the means of wavelength
• Sensitive detectors are needed
• ICP-AES is faster than AAS because switching monochromator
  settings can be done faster than switching lamp plus flame
• In ICP-MS, a mass spectrometer replaces the monochromator
Plasma (light
source + sample)

               Monochromator or                     Light detector or
               Polychromator                        detector array

                   Liquid sample, nebulizer, Ar source
          Atomic Spectroscopy
            Emission Spectrometers
• Sequential vs. Simultaneous Instruments
• Sequential Instruments use a standard
  monochromator and switch through the
  elements by scanning the monochromator.
  These instruments are relatively slow (although
  much faster than AA instruments).
• Simultaneous Instruments use a 1D or 2D
  polychromator. 1D instruments typically use
  photomultiplier detectors behind multiple exit
  slits. Selected elements (1D instruments) or all
  elements can be analyzed simultaneously
  resulting in faster analysis and less sample
           Atomic Spectroscopy
     Interference in Emission Measurements
• Interferences                              Emission
  – Atom – atom interferences more
    common than in atomic absorption
    because monochromators offer less
    selectivity than hollow cathode lamps   Atomic
  – Interference from molecular flame       peak

    emissions are reduced by scanning to
    the sides of the atomic peaks
  – Chemical interferences are less
    prevalent due to greater atomization
  – In mass spectrometry, interference
    can occur due to two metals having
    the same mass (isobaric interference)      background
         Atomic Spectroscopy
       Comparison of Instruments
Instrument            Cost         Speed     Sensitivity

  Flame-AA        Low (~$10-15K)    Slow       Moderate
                                             (~0.01 ppm)
    GF-AA            Moderate      Slowest    Very Good
Sequential ICP-      Moderate      Medium     Moderate
 Simultaneous          High         Fast        Good
   ICP-MS            Highest        Fast      Excellent
         Atomic Spectroscopy
               Some Questions
1. Why is AES with a plasma normally more
   sensitive than with a flame?
2. 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.
3. What is the purpose of the dry, char,
   atomize and clean steps in graphite
   furnace heating?
          Atomic Spectroscopy
              Some More Questions
4. What specific type of spectral interference can
   be removed by periodically using a deuterium
   light in AAS?
5. What type of interference is greater in AAS? In
6. Rate the following spectrometers (from lowest
   to highest) in terms of speed: flame-AAS, GF-
   AAS, Simultaneous ICP-AES, Sequential ICP-
7. Sketch a block diagram of an ICP-AES,
   showing the 5 major spectrometer components
Nuclear Magnetic Resonance (NMR)
               Major Uses

• Identification of Pure Compounds
  (Qualitative Analysis)
• Structural Determination (e.g. protein
• Quantitative Analysis
• Characterization of Compounds in Mixtures
• Imaging (MRI) – not covered
             NMR Spectrometry
• 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
  – Nuclear spin given by Nuclear Spin Quantum
    Number (I)
             NMR Spectrometry
• Nuclear Spin (continued)
  – I = 0 nuclei → no spin (not useful in NMR)
  – 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:                          up state (m = +1/2)
     • 1H (I = ½), 2 states
                                                (m = 1)
                                       up statedown state (m = -1/2)
     • 2H (I = 1), 3 states
                                       middle state (m = 0)
                                       down state (m = -1)
                NMR Spectrometry
• Effect of External
  Magnetic Field on Nuclei
  States                            Applied Magnetic
                                    Field B0*
   – aligned nuclei (m = +1/2)
     have slightly lower energy
     (are more stable) than anti-                      “up” state – m = +1/2
     aligned states (m = -1/2)
   – the greater the magnetic                          “down” state – m = -1/2
     field (B0), the greater the
     energy difference between                              path made by vector tips
     the states
                       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
                  NMR Spectrometry
• Energy depends on
  nucleus, spin state (m),
  and magnetic field                           Energy
     E        B0
g (gamma) = magnetogyric
  ratio (constant for given                              ΔE
  nuclei) and h = Planck’s
• Energy difference
                                            gh          B0
   E  E (m  1 / 2)  E (m  1 / 2)       B0
                   NMR Spectrometry
                                                   B0 scanned at fixed 
• Transitions between              signal

  the ground and                            1H

  excited state can
  occur through                              19F

  absorption of light
                                                           13C (small because
                                                           most C is 12C)

E  h  
                 B0 or v 
• Lowest Resolution                                  B0
  Spectroscopy                     B0 is traditionally used for x-axis
  CH3CF2OH                         because older instruments involved
                                   changing B0 (most newer instrument
                                   don’t). A frequency plot at constant
                                   field would be reversed (1H at highest

Shared By: