9/13/10. Each spectral line is characteristic of an individual energy transition

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1 Sensitive and selective determination of (primarily) metals at low concentrations Each spectral line is characteristic of an individual energy transition 1

2 Atomic Line Widths Why do atomic spectra have finite width to them? Atomic Line Widths cont d Line broadening from the uncertainty effect Doppler broadening Line Spectra and Continuum Spectra 2

3 [M +,X - ] aq [M +,X - ] aq solution mist [M 0 emission ] gas ground state [M*] gas excited state [MX] solid excitation or absorption (via heat or light) vaporization [M 0 ] gas atomization [MX] gas atomization [X 0 ] gas [M + ] gas [X - ] gas Atomic spectroscopic methods are categorized based on the type of atomization Sample Introduction the Achilles Heel of Atomic Spectroscopy Flame Atomic Absorption Spectrometry Only a small percentage of the aqueous sample is atomized much of the sample goes to waste 3

4 Electrothermal or Graphite Furnace Atomizer Temperature profiles for a natural gas-air flame Atomization occurs in an electrically heated graphite tube The graphite tube is flushed with an inert gas (Ar) to prevent the formation of (non-absorbing) metal oxides Flame absorption profiles graphite tube Hydride Generation Atomic Absorption Spectrometry Atomization process Dry (remove water) Ash or Char (destroy organics) Atomize Small sample size Typically µl Enhanced sensitivity over flame atomization The entire sample is analyzed Much of the background matrix can be eliminated Reaction mechanism for As with NaBH 4 as reducing agent BH H 2 O + H + H 3 BO H + 2As H + 2AsH H + 2AsH 3 2As + 3H 2 4

5 Instrumentation for Atomic Absorption Spectroscopy Challenge: atomic absorption lines are narrow - too narrow for goodquality monochromators! Solve the problem by using line sources with bandwidths even narrower than the absorption line width Two Types Spectral when the absorption or emission spectra of an interfering species overlaps or lies close to that of the analyte Chemical species in the sample matrix interfere with the atomization of the analyte Enhances or decreases the volatility of the analyte 5

6 Example: Determination of Ba in the presence of Ca Both Ca and Ba atomize simultaneously Ca (g) + oxidant ----> CaOH (g) CaOH (g) exhibits broad band molecular absorption The observed absorbance is in error due to the non-atomic signal coming from CaOH (g) Monitor a wavelength nearby the atomic line of interest using a continuous light source to generate the nearby wavelength Abs Total Abs Back = Abs Atomic Solution: a. use a higher flame temperature b. use D 2 lamp correction c. use a different line (if overlapping spectral lines is the problem) Example: Determination of Calcium in the presence of phosphate Most chemical interferences result from a change in the atomization behavior of the analyte Usually the atomization signal is depressed The interference usually comes from analysis of an analyte in low concentration in a complex matrix (e.g. seawater) An equilibrium exists in aqueous solutions between calcium and calcium phosphate Ca 2+ + PO 4-3 <-----> CaPO 4-1 CaPO 4-1 is less volatile (i.e. more difficult to atomize) than Ca 2+ This equilibrium tells us that as [PO 4-3 ] increases, [CaPO 4-1 ] increases and [Ca 2+ ] decreases Net result, the absorbance of atomic calcium decreases as phosphate content in the sample increases Solutions: 1) use a hotter flame 2) use a releasing agent (cation that reacts preferentially with interfering ion) 3) use a protecting agent (EDTA, 8HQ) 4) ionization? Add an ion suppressor 5) Calibration using the method of standard additions 6

7 Analytical Figures of Merit and AAS Accuracy: Flame AAS 1-2% RSD ETA 5-10% RSD Scope: > 60 metals or metalloids Typically requires aqueous samples Inductively Coupled Plasma - ICP Much higher temperatures than flame AAS Higher number of atoms in the excited state Stronger signal! ICP Burner Head Temperature regions in a typical Ar ICP RF induction coil ~ 2 kw of power at 27 or 41 MHz See Figure 10-1 See Figure

8 Optical Diagram for a spectrometer with a CID 8