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Lecture

11GFAAS.pdf

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Department
Chemistry
Course
ENCH 213
Professor
Diane Beauchemin
Semester
Fall

Description
Graphite furnace atomic absorption spectrometry (AAS) or electrothermal atomization AAS (ETA-AAS) Graphite furnace or electrothermal atomizer • electrically heated graphite container (tube, cup...) into which the sample in injected (5-100 µL) • graphite tube with pyrolytic coating ◦ less loss by adsorption into porous graphite ◦ less refractory carbide formation by decreasing surface area Transversely heated graphite furnace • the graphite tube that is heated transversely maintains nearly constant temperature over its whole length ◦ reduces memory environment Graphite furnace or electrothermal atomization AAS • whole sample is in light path during 3-5 s (long residence time) ◦ higher sensitivity than with flame ◦ lower detection limit (0.001-0.2 µg/L vs 1-60 µg/L for FAAS) • but precision of 1-10% (vs 0.5- 1% with flame) ▪ have to pipet sample and as the tube is heated it changes shape ◦ poor reproducibility of manual injection (5-10%) ◦ better reproducibility with autosampler (~1%) Processes in graphite furnace depends onsize ofsample Argongas shuttlingout solvent The temperature of a graphite furnace is a) isothermal at approximately 2700 °C b) programmed to move at a constant rate from room temperature to approximately 2700 °C c) programmed to move in steps from room temperature to approximately 2700 °C Compared to a flame, a graphite furnace a) offers lower sensitivity in atomic absorption spectroscopy (has higher sensitivity) b) requires less sample c) operates at a higher temperature (temperature about the same) Optimization of GFAAS • GFAAS (or ETA-AAS)= empirical technique • temporal behavior and nature of vaporized species are highly dependent on sample matrix ◦ temperature program must be optimized for each type of sample ▪ change temperature for each step and look at absorbance (optimum step for drying, ashing (w/out ashing sample) ◦ matrix modifier can be added to increase matrix volatility L’Vov platform • often improves sensitivity ◦ heated with a delay compared to the tube Can also ◦ volatilization into a hotter environment pipette liquid sample ◦ more efficient atomization • GFAAS can be applied to solutions, liquids or solids (0.1-100 mg powder) Problems in GFAAS • addition of a matrix modifier is often required to prevent premature loss of analyte during drying or ashing steps ◦ example: Pb in chloride matrix → loss of PbCl (very 2 volatile) ▪ addition of NH NO 4eact3 w/ Cl and prevents Favours atomization of formation of PbCl 2 analyte • numerous possible matrix-related problems ◦ background correction mandatory for broad- band absorption (flame AA not always required, but b/c have 100% of matrix have to do it for GFAAS) ◦ method of standard additions for samples with complex matrix Both analyte and background absorption A matrix modifier such as NH NO 4r M3(NO ) is use3 2o a) prevent premature evaporation of the analyte b) prevent ionization of the analyte c) prevent formation of metal oxides Only background Alternative background correction approach absorption • Zeeman effect • when strong magnetic field is applied parallel to light path through furnace ◦ absorption line splits: take difference between matrix and both ◦ pulsing the magnetic field corrects for broad- band absorption Matrix(b/c broadband)
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