Flame: desolvates, vaporizes and atomizes the fine sample to free atoms
2. The The monoch monochro roma mato torr: to isolates a wavelength of light (characteristic of a particular quantized transition); to be scanned over the whole working range; to resolve close lines; to reduce the probability of spectral interferences. 3. A light-intensi light-intensity-t ty-to-elec o-electrical trical signal signal transduce transducerr : photomultiplier tube (PMT) 4. An electro electronic nic data-r data-reduct eduction ion syste system m : converts the electrical signal to an analytical response proportional to the concentration of the analyte.
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The intrinsic width of the absorption/emission lines of the elements : 0.002 0.00 2 – 0.0 0.008 08 nm Working range of the spectrometers: about 600 nm ~ 105 resolution elements are potential available, ~ 100 elements of the periodic table can be analyzed by using AAS or AES In AES, •The instrument instrument “sees” “sees” the excited excited – state population population of analyte analyte atoms, not sees the ground-state atoms. •To produce the desired signal, hot flame gases must thermally (collisionally) excite a significant fraction of fraction of the free atoms produced by dissociation. 1. Fl Flam ame e AES AES 2. Induct Inductively ively couple coupled d plasm plasma a AED
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2. Atomic absorption spectrometry
Transmittance: T = I/I 0 Absorbance: A = log (I 0/I) Beer’s Law:
A = abc
a: absorption coefficient b: length of the light path intercepted by the absorption cell c: concentration of the absorbing species in the absorption cell
The absorbance is directly proportional to the concentration of the absorbing species (element) for a given set of instrumental conditions
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Instrumentation
1) The bandwidth bandwidth of of the absorpt absorption ion lines lines is about about twice twice as wide as the emission profiles of the same element 2) The The AAS AAS “s “sees ees” ” both both excited-state excited-state atomic populations and the ground-state atomic populations 3) The absorbance absorbance respons response e directly directly proportional proportional to the the concentration of concentration of the analyte analyte in the sample’ sample’ 4) Element-sel Element-selectiv ective, e, not as sensitive sensitive to to atomizer atomizer temperature variations as that of AES
Absorption of radiation
ASS
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The sensitivity is expressed: 1. In te term rms s of of the the concentration of the element in element in g/mL requ required ired to produce 1% absorption 2. In te terrm of absorption units, the g of element per mL whic which h will give an absorbance of 0.0044
Sensitivity: is a convention for defining the slope of the absorbance versus concentration calibration for each element. conc. ⋅ of ⋅ Std measured ⋅ abs
The fuel/oxidant/sample droplet mixtures are burned in long, narrow slot burners to maximize the length of the atomization zone with the light path of the spectrometer.
1. A AFS Incorporat Incorporates es aspects aspects of both atomic atomic absorption absorption and atomic atomic emission. emission. 2. The emission emission resulting resulting from from the decay of the atoms excited excited by the source source light light is measured. The intensity of this “fluorescence” increases with increasing increasing atom concentration. 3. The source source lamp for atomic atomic fluoresce fluorescence nce is out of line so so that the detector detector sees sees only the fluorescence in the flame and not the light from the lamp i tself. 4. An extra extra light beam beam to excite excite anlyte anlyte atoms radiactiv radiactively. ely. The The absorption absorption of the the light from the source create a higher population of excited-state atoms in the atomizer. 5. The absolute absolute sizes of the atomic atomic emission emission signals signals detected detected are larger than than those seen in the AES performed with the same conditions.
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HOMO and LUMO are acronyms for highest occupied molecular orbital and lowest unoccupied molecular orbital, orbital , respectively. The difference of the energies of the HOMO and LUMO, termed the band gap can sometimes serve as a measure of the excitability of the molecule: the smaller the energy,, the more easily it will be excited. energy The HOMO level is to organic semiconductors what the valence band is to inorganic semiconductors. The same analogy exists between the LUMO level and the conduction band. band. The energy difference between the HOMO and LUMO level is regarded as band gap energy. When the molecule forms a dimer or an aggregate aggregate,, the proximity of the orbitals of the different molecules induce induce a splitting of the HOMO HOMO and LUMO energy levels. This splitting produces vibrational sublevels which each have their own energy, slightly different from one another. There There are as many vibrational sublevels as there are molecules that interact together. When there are enough molecules influencing each other (e.g. in an aggregate), there are so many sublevels that we no longer perceive their discrete nature: they form a continuum. We no longer consider energy levels, but energy bands
