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5 Pages

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ENCH 213
Diane Beauchemin

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Flame atomic spectroscopy • oxidant acts as nebulizer gas as well • spray chamber contains obstructions (baffles, impact bead) to either block big droplets or break them small ones • flame not efficient at exciting ground-state atoms ◦ used mainly inAAS • A= Ebc, where ◦ b is path length, E is a constant, c is concentration • long thin flame: increase path length, therefore increasing absorption Flame atomizer • combustion of a fuel/oxidant mixture • temperature varied by changing fuel/oxidant ratio (i.e. their flow rates) • “rich” flame = relatively rich in fuel ◦ excess carbon reduces formation of MO, MOH • “lean” flame = excess oxidant ◦ hotter than “rich” flame • organic solvent in sample may act as additional oxidant or fuel ◦ depending on the solvent A rich flame in atomic absorption spectroscopy would a) decrease the concentration of metal oxides in the flames. b) be hotter than a lean flame. c) be preferred for refractory elements. Flame atomizer • flame chose according to: analyte and the nature of matrix most common Flame selection • air/acetylene ◦ lower T ◦ favors neutral atoms ◦ path length = 10 cm • N 2/acetylene ◦ higher T ◦ may favor excited atoms ◦ path length = 5 cm ◦ watch out for explosion! Possible problems • background emission from flame ◦ C2, CH and OH emission bands ◦ + CN with N2O ◦ may interfere with analyte signal • spectroscopic interference ◦ overlap (total or partial) of analyte signal with that due to other elements, molecules or flame ◦ background correction to subtract the flame’s contribution ◦ select other analyte wavelength, if interference from elements or molecules Other possible problems • vaporization or chemical interference ◦ formation of refractory compound (stable, hard to break into atoms) with SO 42-and PO 43- 2+ 3 Ca (aq)+ PO 4 - (aq)Ca (P3 ) 4 2 ◦ suppression of analyte signal ▪ ex: suppression of Mg signal byAl Antidotes to vaporization or chemical interference • increase residence time • look higher in the flame (equivalent of increasing residence time) • use a hotter flame (N2O...) 3+ 2+ • add a high concentration of a releasing agent (La or Sr ) to tie up the interferent ◦ Ca + PO 43-+ La → LaPO + Ca 4+ 2+ • use protective chelation to prevent the interfering anion from reacting with analyte ◦ Ca + EDTA+ PO 43-→ Ca(EDTA) + PO 43- ▪ EDTA: protective agent ▪ Ca(EDTA) is more stable in sltn than Ca (PO ) ,3howe4 2 easily decomposes in flame Yet other possible problems • increase in T required to break MO into M • increase in T may also favor excitation and/or ionization of M ◦ not desirable forAAS ◦ compromise required + - • ionization interference for easily ionized elements (Na, K): M → M + e ◦ add an ionization suppressor (100-1000 mg/L Cs to sample and standards) to get neutral Na and K atoms (forAAS) ▪ even more easily ionized than analyte An analyst notes that a 1-mg/L Na solution gives a flame atomic emission signal of 110, while the same solution containing also 20 mg/L K gives a reading of 125. Yet, a solution containing only 20 mg/L K exhibited no reading. Explain these observations. a) K, which is easier to ionize than Na, increases the concentration of electrons in the flame, which shifts the Na ionization equilibrium back towards Na atoms, leading to more Na atoms that can be excited. (ionization interferene) b) K increases the flame temperature, which improves the excitation of Na atoms c) K interferes with Na. Flame atomic absorption spectroscopy (FAAS) • Beer’s law is true for monochromatic radiation • absorbance:A=l
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