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Fluorescence

Fluorescence. Topics. Sensitivity (nM-pM) Contrast UV-vis measurements Derivatization Laser-Induced Fluorescence. Atomic Absorption Spectroscopy. Elemental analysis Dissolved metals Source Hallow Cathode Lamp Cathode of specific element Sample Acetylene-air flame, nebulizer Detector

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Fluorescence

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  1. Fluorescence

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

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

  4. 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

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

  6. Experiment • Aspirate blank, set 100 % T • Aspirate sample • Matrix effects are common • Standard addition analysis

  7. 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

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

  9. 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

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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. Fiber Optics • Total Internal Reflections • n1>n2>n3

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

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