Instrumental Lecture 9

Instrumental Lecture 9 - Chapter 9 Atomic Absorption&...

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Unformatted text preview: Chapter 9: Atomic Absorption & Atomic Fluorescence Spectrometry Sample Atomization Atomic Absorption (AA) Atomic Fluorescence (AF) - Both AA and AF require a light source - Like Molecular Absorption & Fluorescence, in AA high intensity is NOT required, in AF high intensity results in greater sensitivity occurs in flame, plasma, etc. Molecular emission is generally unwanted & can cause interference AA AE AE Sample introduction for solutions: 1) Pneumatic nebulizers 2) Ultrasonic nebulizers 3) Electrothermal vaporizers 4) Hydride generation 3 BH4- + 3 H+ + 4 H3AsO3 3 H3BO3 + 4 AsH3 + 3 H2O 5) Cold vapor generation Hgo + Sn4+ Hg2+ + Sn2+ AA Slot Burner and Flame Sample introduction for solutions: 1) Pneumatic nebulizers 2) Ultrasonic nebulizers 3) Electrothermal vaporizers 4) Hydride generation 3 BH4- + 3 H+ + 4 H3AsO3 3 H3BO3 + 4 AsH3 + 3 H2O 5) Cold vapor generation Hgo + Sn4+ Hg2+ + Sn2+ There are many possible variations for the hydride generation apparatus Interferences in AA can be Spectral atomic spectral lines overlap or are too close to resolve these are rare & generally well known or characterized Matrix scattering of radiation during atomization (smoke), enhancement by matrix elements, structured background handled by background correction Chemical reactions that take place to alter the analyte (like Ca atoms reacting with PO4 to form a new species) change conditions Background Correction in AA Two-Line correction (not very common) Continuous source correction (very common) Zeeman background correction (common for graphite furnace instruments) Smith-Hieftje correction (relatively new `83) Normal or idealized situation for the absorption of source radiation in AA Problems arise if the sample absorption is interfered with by a matrix component - sloped baseline - overlapping molecular band Background correction is then needed Normally assume baseline is flat not structured. In the absence of peak would have flat baseline Peak height easily measured What if baseline is sloped? How is peak height measured? here & here Need measurement of baseline What if the baseline is really a mess? Use Background Correction The AA source (HCL or EDL) tells us the absorbance at the of interest Using another light source will allow us to determine the background absorbance Typically we are interested in points on either side of the peak line source for AA continuum source for background correction This shows (bottom ) that a continuum source is not suitable for AA. However, it will work for background correction. The atomic line absorbs minimally, but the background on either side of the line will be measured The Zeeman effect splits the absorption peak in a magnetic field & shifts absorption to higher & lower wavelength. The new absorption peaks interact differently with polarized light allowing analyte & background absorbance to be measured The Smith-Hieftje technique splits the HCL line Self-reversal or Self-absorption Another type of matrix interference not alleviated by background correction involves variable amounts of analyte ionization in flames or plasmas One other factor (other than temp.) that influences degree of ionization is the presence of another easily ionized species. Here K enhances the Sr AA signal by suppressing Sr ionization Atomic Fluorescence use an intense light source to excite AF of elements in a flame or plasma. Chapter 10: Emission Spectroscopy Using Plasmas, Arcs or Sparks Inductively Coupled Plasma (ICP) Direct Current Plasma (DCP) Arcs and Sparks Inductively Coupled Plasma (ICP) ICP Torch with sample introduction system (nebulizer and spray chamber) An alternate means of getting sample into ICP ICP Temps. The viewing area for each element is typically reported as mm above the load coil Direct Current Plasma (DCP) Torch Lab will involve use of DCP for multielement analysis Eagle Mount ...
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