Lecture #6 Atomic Absorption Spectrometry Reading: Chapter 9, page 230 – 253; Problems: 9-1,3,5,6,8 • Sample atomization: flame, electrothermal and others. • Atomic absorption instrumentation: lamps and spectrophotometer. • Interferences in atomic absorption spectroscopy. • Application of atomic absorption spectroscopy. Flame atomization Because many complex processes occur during the atomization, it is the most critical step in flame spectroscopy. It is important to understand the characteristics of flames and the variables that affect these characteristics. Flame burner Flame burner Three functions of the flame: (1) sample holder, (2) desolvation source, (3) volatilization source. Two critical aspects of the flame: (1) Temperature --- determined by the fuel (next slide). (2) Burn velocity --- must be correlated with the gas flow rate. For determination of metals that form stable oxides, a flame containing an excess of fuel normally provides better sensitivity for atomic absorption. Flame structure Burn velocity (cm/s) 39-43 370-390 300-440 900-1400 158-266 1100-2480 285 Flame structure • Primary combustion zone: thermal equilibrium not reached, seldom used for spectroscopy. • Interzonal zone: rich in free atoms, most widely used for flame spectroscopy. • Secondary combustion zone: products from inner core are converted to stable oxides. Highest temperature is located in the flame about 1 cm above the primary combustion zone. Flame absorbance profiles for different atoms Mg: increase is due to the increased number of Mg atoms produced by the longer exposure to the heat of the flame; decrease is due to the oxidation of Mg within the secondary combustion region, where stable MgO is formed. Ag: not easy to be oxidized, a gradual increase in absorbance is observed from the base to the periphery of the flame. Cr: easily oxidized to stable oxides, a continuous decrease in absorbance is normally observed. Electrothermal atomization: graphite tube furnace Electrothermal atomization: graphite tube furnace • Appeared on market in early 1970’s • Provides enhanced sensitivity WHY? Short atomization time: milisecond to seconds at high temperature, 2000-3000 oC AND atoms are in optical path for 1 or more seconds This translates to smaller sample volumes: 0.5 - 10 µL and lower detection limit: 10-10 - 10-13 g. • Good precision (not best) 5 – 10% compared to 1% or better for flame atomization The external stream prevents the entrance of outside air and a consequent incineration of the tube; the internal stream excludes air, and serves to carry away vapors generated from the sample matrix. Glow discharge atomization (see figure 9-8 for details) Glow discharge atomization (see figure 9-8 for details) 1. Argon gas is ionized by a current between an anode supporting the nozzles and the sample which acts as the cathode. 2. Upon sputtering by the argon ions, free atoms of the sample are drawn by a vacuum to the axis of the cell where they absorb the radiation from the spectrometer source. Hydride atomization (see figure 9-9 for details) 1. 3BH4- + 3H+ + 4H3AsO3 3H3BO3 + 4AsH3 (gas phase) + 3H2O 2. Hydride gas is heated for atomization in a quartz tube. Cold-vapor atomization: mercury (Hg) determination Mercury is the only metallic element that has appreciable pressure at ambient temperature. 1. Mercury in the sample is converted to Hg2+ by reaction with H2SO4. 2. Conversion of Hg2+ to Hg metal by reduction with SnCl2 or SnSO4. 3. Hg vapor is transferred to a quartz tube for observation. Other Components of Atomic Absorption Instrumentation Radiation Source Sample Holder Wavelength Selector Detector Signal Processor Radiation sources: lamps Radiation sources: lamps 1. Hollow Cathode Lamp: • Most widely used lamp. • Cathode material is made of the element of interest. • An individual lamp is needed for each element --- atomic absorption spectroscopy is a one-element-at-a-time measurement. 2. Electrodeless Discharge Lamp: not commonly used. self-learning (page 239). Spectrophotometer: detection of radiation (see figure 9-13 for details) • Many spectrometers are capable Single beam of achieving a bandwidth on the design. order of 1 Å. • Most of the spectrometers use photomultiplier tubes (PMT). Double beam design Interferences in atomic absorption spectroscopy self-learning, page 241-246. • Spectral interferences and corrections: 1. the two-line correction method; 2. the continuum-source correction method; 3. background correction based on the Zeeman effect; 4. background correction based on source self-reversal; • Causes of chemical interferences: 1. formation of compounds of low volatility; 2. dissociation equilibria; 3. ionization equilibria; Application of atomic absorption spectroscopy Learning and practice from lab experiment #1. • Sample preparation; • Measurement with organic solvent; • Calibration curves; • Standard addition method; • Detection limit: with flame, 0.001 – 0.02 ppm; with graphite furnace, 2×10-6 - 1×10-5 ppm. • Accuracy: relative error for flame, 1% - 2%; for graphite furnace 5% - 10%.