AAS AES Compare
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Transcript of AAS AES Compare
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Principles of the atomic spectrometriesI. Atomic emission spectrometry
Atomic Spectrometry
(emission)
Instrumentation.1. A sample-introduction/atomization system
• Sample sprayer (nebulizer)
• Flame: desolvates, vaporizes and atomizes the fine sample to free atoms
2. The monochromator: 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-intensity-to-electrical signal transducer: photomultiplier tube (PMT)
4. An electronic data-reduction system: 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 – 0.008 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 “sees” the excited – state population of analyteatoms, not sees the ground-state atoms.
•To produce the desired signal, hot flame gases must thermally (collisionally) excite a significant fraction of the free atoms produced by dissociation.
1. Flame AES
2. Inductively coupled plasma AED
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2. Atomic absorption spectrometry
Transmittance: T = I/I0
Absorbance: A = log (I0/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 of the absorption lines is about twiceas wide as the emission profiles of the same element
2) The AAS “sees” both excited-state atomic populations and the ground-state atomic populations
3) The absorbance response directly proportional to the concentration of the analyte in the sample’
4) Element-selective, not as sensitive to atomizer temperature variations as that of AES
Absorption of radiation
ASS
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Sensitivity: is a convention for defining the slope of the absorbance versus concentration calibration for each element.
0044.0
. ysensitivit
absmeasured
Stdofconc =⋅⋅⋅
absmeasured
stdofconcysensitivit
⋅×⋅⋅= 0044.0
1. In terms of the concentration of the element in µg/mL required to produce 1% absorption
2. In term of absorption units,the µg of element per mL which will give an absorbance of 0.0044
The sensitivity is expressed:
Atomization
vaporization
http://lecturer.eng.chula.ac.th/fchvpv/matcharac/Lecture_6a.html
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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.
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AAS
http://lecturer.eng.chula.ac.th/fchvpv/matcharac/Lecture_6a.html
Atomic spectrometry
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Atomic spectrometry
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AAS and AES
1. Widely used for element trace analysis
2. Can give quantitative information
• By using calibration curve
• Created from standard sample with known concentration of analyte
3. Generally, require “digestion” of sample into solution
4. Limitations
• Limited ability to distinguish oxidation states and chemical environments of the analyteelements
• Insensititive to nonmetallic elements
Comparison of AAS and AES
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Atomic fluorescence
3. Flame atomic fluorescence spectrometry
1. A AFS Incorporates aspects of both atomic absorption and atomic emission.
2. The emission resulting from the decay of the atoms excited by the source light is measured. The intensity of this “fluorescence” increases with increasing atom concentration.
3. The source lamp for atomic fluorescence is out of line so that the detector sees only the fluorescence in the flame and not the light from the lamp itself.
4. An extra light beam to excite anlyte atoms radiactively. The absorption of the light from the source create a higher population of excited-state atoms in the atomizer.
5. The absolute sizes of the atomic emission signals detected are larger than those seen in the AES performed with the same conditions.
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HOMO and LUMO are acronyms for highest occupied molecular orbitaland lowest unoccupied molecular 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.
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. The energy difference between the HOMO and LUMO level is regarded as band gap energy.
When the molecule forms a dimer or an aggregate, the proximity of the orbitals of the different molecules induce a splitting of the HOMO and LUMO energy levels. This splitting produces vibrational sublevels which each have their own energy, slightly different from one another. 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
http://lecturer.eng.chula.ac.th/fchvpv/matcharac/Lecture_6a.html
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Electron transition promotes e- from HOMO to LUMO
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Broad UV/Vis absorption peak
http://lecturer.eng.chula.ac.th/fchvpv/matcharac/Lecture_6a.html
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Infrared spectrometry (IR)
http://lecturer.eng.chula.ac.th/fchvpv/matcharac/Lecture_6b.html
http://lecturer.eng.chula.ac.th/fchvpv/matcharac/Lecture_6b.html
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http://lecturer.eng.chula.ac.th/fchvpv/matcharac/Lecture_6b.html