Atomic fluorescence spectral analyses

In flame emission spectroscopy, light emission is caused by a thermal effect and not by a photon, as it is in atomic fluorescence. Flame emission, which is used solely for quantification, is distinguished from atomic emission, used for qualitative and quantitative analyses. This latter, more general term is reserved for a spectral method of analysis that uses high temperature thermal sources and a higher performance optical arrangement. 

The soil contains elements of biogenic as well as toxic character. The toxicity is usually manifested at relatively higher concentrations. The natural, safe concentrations of certain elements in the soil and plants are given in Table 7.5. Instrumental analytical methods are usually used for their determination, such as emission spectral analysis, atomic absorption spectrometry, photometry. X-ray fluorescence analysis, and polarography. 

Atomic fluorescence has many superior features for trace elemental analysis (spectral simplicity, wide dynamic range, and simultaneous multi-element analysis). However, major practical problems of this technique are connected... 

The principles of the laser-excited atomic fluorescence (LEAF) technique are very simple. A liquid or solid sample is atomized in an appropriate device. The atomic vapor is illuminated by laser radiation tuned to a strong resonance transition of an analyte atom. The excited analyte atoms spontaneously radiate fluorescence photons and a recording. system registers the intensity of fluorescence (or total number of fluorescent photons). The extremely high spectral brightness of lasers makes it possible to saturate a resonance transition of an analyte atom. Therefore, the maximum fluorescence intensity of the free analyte atoms can be achieved while the effect of intensity fluctuations of the excitation source are minimized. Both factors provide the main advantage of LEAF— extremely high sensitivity. The best absolute detection limits achieved in direct analysis by LEAF... 

It seems strange that in the middle of the 18 century it was possible to build a system based on chemical analysis. For a chemist in the 21 century, used to spectral analysis. X-ray fluorescence analysis and atomic absorption spectrophotometry, it is difficult to understand how, at that time, it was possible to say much about the composition of a specific mineral, but in Sweden it was possible. The method was blowpipe analysis, described in Chapter 10 Blowpipe and Spectroscope. 

Atomic fluorescence is an extremely sensitive technique for determination of elements in samples. We should reiterate that in atomic fluorescence an external Ught source is used to excite the analyte atoms. An ideal light source for AFS must be much more intense than ahoUow cathode lamp to achieve improvements in sensitivity. As a result, pulsed hollow cathode lamps and lasers are frequently used in AFS measurements. Excitation with alight source such as a hollow cathode lamp, which only emits radiation specific for the element of interest, makes AFS virtually completely free from spectral interferences. In addition, AFS is like AES in that a multi-element analysis can be achieved by putting several light sources around the atom cell, as discussed below. 

Since the X-ray spectral lines come from the inner electrons of the atoms, die lines are not related to the chemical properties of the elements or to the compounds in which they may reside. Because the characteristics of die X-ray spectra are associated with energies released through transitions of electrons within the inner shells of the atom, the spectra are simple. Most practical X-ray fluorescence analysis involves the detection of radiation release through electron transitions from outer shells to the K shell (K spectra), outer shells to die L shell (L spectra) and, in very few cases, from outer shells to the M shell (M spectra). 

Radiochemical methods of analysis are considerably more sensitive than other chemical methods. Most spectral methods can quantitate at the parts-per-mil-lion (ppm) level, whereas atomic absorption and some HPLC methods with UV, fluorescence, and electrochemical methods can quantitate at the parts-per-billion (ppb) levels. By controlling the specific activity levels, it is possible to attain quantitation levels lower than ppb levels of elements by radiochemical analyses. Radiochemical analysis, inmost cases, can be done without separation of the analyte. Radionuclides are identified based on the characteristic decay and the energy of the particles as described in detection procedures presented above. Radiochemical methods of analysis include tracer methods, activation analysis, and radioimmunoassay techniques. 

The relationship between the weight concentration of the element to be analysed and the intensity measured from one of its characteristic spectral lines is a complex one. For trace analysis several mathematical models have been developed to correlate fluorescence to the atomic concentration. A series of corrections must be introduced to account for inter-element interactions, preferential excitation, self-absorption and the fluorescence yield (the heavier atoms relax by internal conversion without photon emission). All of these factors require the reference samples to be practically the same structure and atomic composition than the sample under investigation, for all of the elements present. It is mostly because of these reasons that quantitative analysis by X-ray fluorescence is difficult to obtain. When operating upon a solid sample, a perfectly clean surface is important, preferably polished, since the analysis concerns the composition immediately close to the surface. 

This method is well adapted to quantitative analysis, where automatic identification of the spectral lines can be made by very sophisticated visual displays. Principal corrections (called ZAF) relate to atomic number (Z), nature of the isotope (A) and fluorescence (F). In semi-quantitative analysis, the software can give an approximate composition of the sample without the need for reference standards. 

The analysis of the solvatochromic effects on molecular absorption and emission (fluorescence and phosphorescence) spectra is further complicated by the variation of time scales for the solvent relaxation after the spectral excitation of the solute molecule. The spectral transition is a very fast process that takes place within approximately 10 s. Thus, during this short period of time the atomic nuclei do not practically move. The excited state reached by the respective vertical transition is often called the Franck-Condon state (Figure 11.1.4). 

Interferences are physical or chemical processes that cause the signal from the analyte in the sample to be higher or lower than the signal from an equivalent standard. Interferences can therefore cause positive or negative errors in quantitative analysis. There are two major classes of interferences in AAS, spectral interferences and nonspectral interferences. Nonspectral interferences are those that affect the formation of analyte free atoms. Nonspectral interferences include chemical interference, ionization interference, and solvent effects (or matrix interference). Spectral interferences cause the amount of light absorbed to be erroneously high due to absorption by a species other than the analyte atom. While all techniques suffer from interferences to some extent, AAS is much less prone to spectral interferences and nonspectral interferences than atomic anission spectrometry and X-ray fluorescence (XRF), the other major optical atomic spectroscopic techniques. 

Apart from high power of detection, maximum analytical accuracy is very important. This relates to the freedom from interference. Whereas interference from influences of the sample constituents on sample introduction or volatilization and excitation in the radiation source differ widely from one source to another, most sources emit complex spectra and the risks of spectral interference in AES are much more severe than in absorption or fluorescence. Therefore, it is advisable to use high-resolution spectrometers. This is especially the case when trace determinations are performed in matrices emitting complex spectra. Knowledge of the atomic spectra is also very important so as to be able to select interference-free analysis lines for a given element in a well-defined matrix at a... 

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