Monroe L. Weber-Shirk S chool of Civil and Environmental Engineering Atomic-Absorption (AA) Spectroscopy 

Overview ä Excitation and Decay: Interplay of electron state and photons ä Emission ä Absorption ä Instrument Components ä Double beam configuration ä Light source ä Production of free atoms ä Graphite furnace ä Flame ä Detector ä Excitation and Decay: Interplay of electron state and photons ä Emission ä Absorption ä Instrument Components ä Double beam configuration ä Light source ä Production of free atoms ä Graphite furnace ä Flame ä Detector

Excitation and Decay: Interplay of electron state and photons ä Emission – decay from high energy state ä Absorption – excitation to high energy state ä Two fundamentally different ways of quantifying the concentration of an element! ä Emission – flame provides energy to excite atoms ä Absorption – Gas phase atoms absorb photons from light source ä Narrow band! (unlike molecular absorption) ä Emission – decay from high energy state ä Absorption – excitation to high energy state ä Two fundamentally different ways of quantifying the concentration of an element! ä Emission – flame provides energy to excite atoms ä Absorption – Gas phase atoms absorb photons from light source ä Narrow band! (unlike molecular absorption)

Instrument Schematic Flame Lamp Chopper Reference Beam Monochromator Detector Electronics Beam Recombiner t V V ref V sample

Light Source: Hollow Cathode Lamp ä The electric potential ionizes rare gas atoms and accelerates them into the cathode where they sputter metal atoms into the gas phase ä Collisions with gas atoms or electrons excite the metal atoms ä On decay the metal atoms emit light ä The electric potential ionizes rare gas atoms and accelerates them into the cathode where they sputter metal atoms into the gas phase ä Collisions with gas atoms or electrons excite the metal atoms ä On decay the metal atoms emit light anode cathode + - window light Cup made of metal of interest Power Supply

Atomization ä Atomic absorption and emission requires gas- phase atoms ä Liquid and solid samples must be vaporized ä Flame ä Graphite furnace ä The gas-phase atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels. ä What about the inevitable subsequent emission of photons on decay? ä Atomic absorption and emission requires gas- phase atoms ä Liquid and solid samples must be vaporized ä Flame ä Graphite furnace ä The gas-phase atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels. ä What about the inevitable subsequent emission of photons on decay?

Detector ä Photo multiplier tube ä Extremely sensitive ä Can detect single photons ä Produce a small current that is proportional to the number of photons ä Op-amps produce a voltage that is then digitized for analysis. ä Photo multiplier tube ä Extremely sensitive ä Can detect single photons ä Produce a small current that is proportional to the number of photons ä Op-amps produce a voltage that is then digitized for analysis.

Flame ä Different temperatures required for different elements ä Air-Acetylene flame ä Preferred flame for 35 elements ä Temperature of 2300 C ä Nitrous Oxide-Acetylene flame ä Temperature of 2900 C ä Optimal temperature produces free gas phase atoms but doesn’t ________ them ä Different temperatures required for different elements ä Air-Acetylene flame ä Preferred flame for 35 elements ä Temperature of 2300 C ä Nitrous Oxide-Acetylene flame ä Temperature of 2900 C ä Optimal temperature produces free gas phase atoms but doesn’t ________ them ionize

Calibration ä ________ Law ä Non linearity (according to Perkin Elmer) ä Stray light ä Non homogeneities of temperature and space in the absorbing cell ä Line broadening ä Absorption at nearby lines ä What else could be causing non linearity? ä ________ Law ä Non linearity (according to Perkin Elmer) ä Stray light ä Non homogeneities of temperature and space in the absorbing cell ä Line broadening ä Absorption at nearby lines ä What else could be causing non linearity? Beer’s Correct for this by subtracting V dark What is the physics? emission

Getting a Concentration ä Flame ä Measures absorbance proportional to the ______________ of the sample ä Analysis in the mg/L range (ppm) ä Graphite Furnace ä Measures an absorbance peak with an area proportional to _______ of the sample  Analysis in the  g/L range (ppb) ä Flame ä Measures absorbance proportional to the ______________ of the sample ä Analysis in the mg/L range (ppm) ä Graphite Furnace ä Measures an absorbance peak with an area proportional to _______ of the sample  Analysis in the  g/L range (ppb) concentration mass