Ch21-To-PDF

Ch21-To-PDF - Quantitative Chemical Analysis Quantitative...

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Quantitative Chemical Analysis Quantitative Chemical Analysis Chapter 21: Atomic Spectroscopy Atomic Spectroscopy
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2 Three basic experiments: Atomic emission Atomic Absorption Atomic fluorescence
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3 The basic atomic absorption spectrometer
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4 Flames
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5 Premix burner Distribution of drop sizes produced by a particular nebulizer ( Impact bead ) End view of flame: slot ~ 0.5 mm across
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6 The hollow cathode lamp – the source lamp Emission beam ~ 500 volt potential, Ne or Ar at 1 – 5 Torr (~ mm Hg) Ionizes gas to Ne + or Ar + Ions are accelerated toward cathode by potential Metal is sputtered to gas phase atoms by impact of ions Gas phase metal atoms are excited by collisions with high-energy electrons and emit atomic spectral frequencies
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7 The hollow cathode lamp Source lamp (hollow-cathode lamp) for steel
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8 Linewidth – three sources 1) “ Natural” linewidth from uncertainty broadening , δ E δ t h /4 π Lifetime of excited state of isolated gas-phase atom is ~ 10 9 s, so δ E ~ 5 × 10 26 J How many nm broad is this peak? Consider the 248.3 nm line of iron. This λ corresponds to E = 8.0 × 10 19 J (energy of the photon) Relative uncertainty δ E / E (5 × 10 26 J)/( 8.0 × 10 19 J) = 7 × 10 8 δ E / E = 7 × 10 8 = δλ / λ ∴ δλ ≅ (7 × 10 8 λ = (7 × 10 8 )(248 nm) = 10 5 nm . A very narrow spectral line!
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9 Linewidth – three sources 2) Doppler broadening Doppler linewidth = ( λ )(7 × 10 7 ) (T/M) 1/2 In the analyte flame, for iron at λ = 248.3 nm at T = 2,500 K, and M of Fe = 56, Linewidth ~ 0.001 2 nm 3) Pressure broadening Collisions shorten lifetimes of atomic excited states, and so makes the bands broader ~ same size as Doppler broadening
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10 Source (HC lamp) linewidth must be narrower than the analyte (atomic vapor in the flame) linewidth for Beer’s law to apply Overall, Monochromator bandwidth is ~100× greater than atomic lines. Can’t use a monochromator for a source in AA spectroscopy
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Ch21-To-PDF - Quantitative Chemical Analysis Quantitative...

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