Atomic Spectroscopies 1 slide per page

Atomic Spectroscopies 1 slide per page - CHEM 2404 Forensic...

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CHEM 2404 Forensic Chemistry Lecture 5
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Electronic ground state Electronic excited state 0.5 1.0 1.5 2.0 2.5 Energy (Arbitrary Units) Distance (Arbitrary Units) Vibrational (bond stretches) energy levels of excited electronic state Vibrational (bond stretches) energy levels of ground electronic state Peak Width of Electronic Spectra of Molecules Electronic transition from ground vibrational and electronic state to one of several closely spaced vibrational energy levels in the excited electronic state Gives broad absorptions in electronic spectra Energy/Wavelength
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Atomic Spectroscopies (emission, absorption) Foregoing discussion general. Here, spectroscopy of atoms . Atoms have no rotational or vibrational motion. Spectra are due only to electronic transitions. Spectra are a series of narrow , often widely separated lines (rather than broad bands in molecular spectroscopy) Li atomic emission spectrum 400 nm 700 nm
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Atomic Spectroscopies (emission, absorption) Advantages high sensitivity (ppm levels of analyte routine, often ppb possible) rapid analysis simple analyses Disadvantages not as accurate as many wet chemical methods (precision ~ 1-2%) no evidence of speciation (molecular or chemical form) of analyte equipment expensive
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Flame Emission (Flame Photometry) & Atomic Absorption Spectrometry CaCl 2 ( aq ) aspirate 2 ( g ) dissociation Ca ( g ) + 2 Cl ( g ) flame excitation h ν emission Ca * lamp excitation Flame photometry AAS reaction with flame gases CaO CaOH ionization Ca + Similar techniques
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A sample is aspirated into a flame and atomized at the temperatures of the flame . The atoms then either: (a) absorb energy from the flame to reach an excited atomic electronic state. Decay from this excited state results in a spectrum of lines which is measured and recorded as flame emission spectrometry (flame photometry) (b) absorb energy from an appropriate light source to reach the same excited electronic state. The loss of radiation from the light source is recorded as the atomic absorption spectrum or Excited states Ground state Absorption Emission
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i.e., flame photometry and atomic absorption spectroscopy are very similar techniques: flame emission has no light source - atomization and excitation both done by flame. Emission spectrum measured. Measurement is from excited state atoms (decaying to ground state). atomic absorption spectroscopy has a light source - atomization by flame, excitation by light source. Absorption spectrum measured. Measurement is from ground state atoms (being promoted to an excited state). Difference between techniques is relative populations of atoms in ground and excited states .
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Used to determine concentrations of Na, K, Li, Ca, Ba, Cs, Rb and Sr Relatively inexpensive instrumentation Uses photoelectric detector and filters, not monochromator Detects presence of radiation, & is not wavelength specific Flame Photometer
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Interference filters used if: limited number of known wavelengths, energy throughput more critical than wavelength resolution cost is more important than flexibility.
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Atomic Spectroscopies 1 slide per page - CHEM 2404 Forensic...

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