creek
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Registered: 27-9-2004
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Spectrometry
Hello everyone,
I am a non chemistry guy, but I have stumbled upon a problem about spectrometry. What I want to understand is where does the equation
Intensity = concentration * factor
comes about. What is the intensity conc. and the factor in the above.
Also if there are other elements involved say if we looking at steel, and we are trying to determine the actual concentration of zn in the steel
sample, and for arguments sake it contains traces of Ni and Cu along with Fe ofcourse. All these will show up in the spectrum and interfere in the
equation. How do we set up the equation in this case, and I believe they have to be set up as matrices, Can anyone explain or make reference so that I
can understand.
Many thanks,
P.S. I understand my question might not be as clear, I will try to clarify myself if you have difficulties.
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JohnWW
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Registered: 27-7-2004
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Assuming you are talking about a form of absorption spectrometry, like the UV/visible absorption spectrometry of solutions containing colored
molecules or ions, which owe their absorbance of light and hence color to absorbed light wavelengths being the same as those required to cause
transitions of electrons from the ground state to excited states:
The "intensity" is the total fraction of absorbance of light of a particular wavelength per unit thickness of the solution in the testing
cell (in most laboratory benchtop spectrometers, 1 cm), and the "factor" is the unit absorptivity per mole per unit thickness of the colored
substance. The "factor" depends on the quantum-mechanical circumstances of the electron transition, which determine the numbers of such
transitions occurring; with, in this type of spectrometry, charge-transfer transitions (as in high-valent transition metal compounds like MnO4-,
CrO4--, FeO4--, VO4---, in which there are no or few free d electrons, resulting in a much greater absorptivity of light of characteristic wavelengths
(due to much more numerous transitions per mole of substance) than unpaired d->d or f->f electron transitions (in low-valent transition metals
and rare-earth metals respectively) which are in many cases "forbidden" by quantum-mechanical rules. Atomic absorption spectrometry is
similar in principle except that the solution, which has to be of a metal or metalloid, is aspirated into a gas flame through which light of
characteristic wavelengths is passed and the absorbance measured photometrically.
However, if you are talking about some form of emission spectrometry, in particular arc emission spectrometry (which is the most convenient and rapid
method of analysis of steel alloys such as you mention, as it is non-destructive): the sample has to be electrically conductive and non-volatile, i.e.
a refractory metal alloy, so that it may be used directly as one (or both) of the electrodes for a DC electrical arc which emits light which is
analysed. The light emitted by the arc is of characteristic wavelengths which are emitted by atoms of the metals in the sample when the outer
electrons decay from excited states back to the ground state. In this case, the "intensity" is the total intensity of the wavelength of
interest (of which there may be several) emitted by a metal as photometrically measured, being directly proportional to the metal's concentration
in the sample, and a "factor" which is, again, dependent on the number of such transitions occurring per mole of the metal. The same applies
to flame emission spectroscopy, used for more volatile metals and their compounds.
For arc emission spectroscopy, which I have personally used, see for example:
http://www.iupac.org/publications/analytical_compendium/Cha1...
John W.
[Edited on 28-9-2004 by JohnWW]
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