Fluorescence

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Fluorescence
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Topics

• Sensitivity (nM-pM)
• Contrast UV-vis measurements
• Derivatization
• Laser-Induced Fluorescence

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Atomic Absorption Spectroscopy

• Elemental analysis
• Dissolved metals
• Source
• Hallow Cathode Lamp
• Cathode of specific element
• Sample
• Acetylene-air flame, nebulizer
• Detector
• PMT

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Atomic absorption

• Gas-phase elemental atoms
• Narrow absorption lines (0.01 nm)
• Element specific source
• Flame conditions control sensitivity
• 1-100 ppm (mg/L)
• varies with element

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AA Spectrometer
PMT
Hollow cathode lamp
Flame
monochrometer
Coating of element
Air
nebulizer
Acetylene
sample
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Experiment

• Aspirate blank, set 100 T
• Aspirate sample
• Matrix effects are common
• Standard addition analysis

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Removal of Atomic Emission

• Place a chopper before the flame
• The signal from the source is modulated by the
  chopper
• Thus, a AC signal is produced on top of a DC
  signal that originates from emission in the flame

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Atomic Emission Spectroscopy

• Emission of light from exited species
• ICP source
• Multi-element analysis
• Fancy optics
• Greater sensitivity for most elements

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ICP source

• Ar tourch
• Very hot , 10000 K
• Abundance of e- inhibits ionization
• Lack of O2 inhibits oxide formation
• Excited atomic species predominate
• 5-20 L/min

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Sample Introduction

• Nebulizer
• Electrothermal vaporization
• Laser ablation
• In either case Ar gas in used to carry sample
  into the ICP

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Scanning Instruments

• Grating monochromator
• Slew-scan mode
• Slew-The detector and slit is quickly moved to a
  specific wavelength for a given element
• Slowly scan across the wavelength window near a
  line in very small steps-0.001 nm
• Not limited to a set of specific elements
• Echelle grating/prism
• Higher orders, 2-D plane, better dispersion
• Slew-scan mode

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Multi-channel Instruments

• Polychromators
• 60 photomultipliers in fixed positions
• Array-based
• Echelle and prism for 2-D dispersion plane
• Charge injection detector with 94672 different
  detector elements (8.7x6.6mm)
• A set of 39 elements makes up a read window for a
  given element
• CCD detectors

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CCD detectors

• Array of semiconductor detection elements
• 512x320 array 163840 individual detectors
• Charge of impinging photons are stored
• The charge accumulated is registered one by one
  by sending the charge to a preamplifier and
  readout, emptying the registrar
• Very sensitive

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Performance

• Sensitivity Ranges from 1-300 ng/mL
• Number of useful lines 1-24
• Calibration curves almost 3 orders of magnitude
• Internal standards
• Self-absorption at higher concentrations

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Other components of visible spectroscopy

• Photon Counting
• Each time a photon strikes the PMT, a pulse of
  electrons are produced
• Normally, an average current over time is
  integrated to produce the signal
• In photon counting each pulse is counted, and the
  total number of pulses in a time interval is
  summed
• Filters out noise
• Fluorescence and Raman Spec

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Fiber Optics

• Total Internal Reflections
• n1gtn2gtn3

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Lasers

• Qualities
• High intensities
• Narrow band width
• coherence
• Lasing medium
• Solid state, gas, dye
• Lasing mechanism
• Figure 7.5