Quantitative analysis atomic fluorescence

In addition to the spark emission methods, quantitative analysis directly on soHds can be accompHshed using x-ray fluorescence, or, after sample dissolution, accurate analyses can be made using plasma emission or atomic absorption spectroscopy (37). 

Both WDXRF and EDXRF lend themselves admirably to quantitative analysis, since there is a relationship between the wavelength or energy of a characteristic X-ray photon and the atomic number of the element from which the characteristic emission line occurs. The fluorescence intensity of a given element is proportional to the weight fraction. Emitted fluorescence radiation is partly absorbed by the matrix, depending on the total mass absorption coefficient ... 

Atomic Fluorescence Spectrometry. Principles of Quantitative Analysis... 

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. 

Thomas and Sniatecki [51] also performed an analysis of trace amounts of arsenic species in natural waters using hydride generation IPC-ICP-MS. Six arsenic species were determined with detection limits in the range 1.0-3.0 fig l-1 and total arsenic was determined using hydride generation by atomic fluorescence detection. It was found that the predominant species present in bottled mineral water samples was always As(V) with very low levels of As(III). The authors described how the system required . .. further work using special chromatographic software. .. to improve the quantitative measurement at a natural level. ... 

Fluorescence occurs when a molecule absorbs light at one wavelength and reemits light at a longer wavelength. An atom or molecule that fluoresces is termed a fluorophore. Fluorometry is defined as the measurement of the emitted fluorescence Hglit. Fluorometric analysis is a widely used method of quantitative analysis in the chemical and biological sciences it is accurate and very sensitive. 

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. 

Atomic Identification and Analysis. Atomic emission and absorption spectroscopy and X-ray fluorescence and absorption are used for elemental analyses. These methods vary in their sensitivity and quantitative applicability. A summary of the usually accepted virtues and limitations of these methods is given in Table III. 

Both solid and liquid samples can be analyzed by XRF as described earlier in the chapter. Very flat surfaces are required for quantitative analysis, as discussed subsequently. Liquids flow into flat surfaces, but cannot be run under vacuum. The best solvents are H2O, HNO3, hydrocarbons, and oxygenated carbon compounds, because these compounds contain only low atomic number elements. Solvents such as HCl, H2SO4, CS2, and CCLt are undesirable because they contain elements with higher atomic numbers they may reabsorb the fluorescence from lower-Z elements and will also give characteristic lines for Cl or S. This will preclude identification of these elements in the sample. Organic solvents must not dissolve or react with the film used to cover the sample. 

Quantitative analysis can be carried out by measuring the intensity of fluorescence at the wavelength characteristic of the element being determined. The method has wide application to most of the elements in the periodic table, both metals and nonmetals and many types of sample matrices. It is comparable in precision and accuracy to most atomic spectroscopic instrumental techniques. The sensitivity limits are of the order of... 

Atomic emission spectroscopy (AES) and atomic absorption spectroscopy (AAS) are In a manner similar to our discussion of molecular spectroscopy, where we compared UV absorption with UV excitation and subsequent fluorescence, these two determinative approaches are the principal ways to identify and quantitate trace concentration levels of metal contamination in the environment. As the need developed to quantitate increasing numbers of chemical elements in the Periodic Table, so too came advances in instrumentation that enabled this to be achieved at lower and lower IDLs AES and AAS techniques are both complementary and competitive. Atomic fluorescence spectroscopy (AFS) is a third approach to trace metal analysis. However, instrumentation for this has not as yet become widespread in environmental testing labs and it is unlikely that one would see atomic or what has become useful x-ray atomic fluorescence spectroscopy. Outside of a brief mention of the configuration for AFS, we will not cover it here. 

Various techniques can be used for quantitative analysis of chemical composition, including (i) optical atomic spectroscopy (atomic absorption, atomic emission, and atomic fluorescence), (ii) X-ray fluorescence spectroscopy, (iii) mass spectrometry, (iv) electrochemistry, and (v) nuclear and radioisotope analysis [41]. Among these, optical atomic spectroscopy, involving atomic absorption (AA) or atomic emission (AE), has been the most widely used for chemical analysis of ceramic powders. It can be used to determine the contents of both major and minor elements, as well as trace elements, because of its high precision and low detection limits. 

The quality of an XRF analysis depends on the counting statistics. Therefore, the reduction of the measuring time to achieve a very fast semi-quantitative analysis will be limited by the user s demand on the analytical quality. Concentration calculations in XRF analysis are based on fluorescence intensities from a layer at the surface of a sample, whose thickness may vary depending on the element and basic material, or from a layer of several centimeters down to a few layers of atoms. Thus, the limit of accuracy in Standardless XRF analysis also depends on the surface quality of the sample (Fig. 7). 

Recent developments, such as the windowless EDX detector, have allowed the light element range to be extended down to C. Automated WDX spectrometers with computer control of the operating parameters are now available, such as the Microspec WDX-2A system. This considerably simplifies WDX analysis. The ideal system, however, requires both an EDX and WDX system mounted on the microscope simultaneously. This would permit the rapid determination of the elements present with the EDX system, and a detailed analysis of these elements using the WDX spectrometer. Combined EDX/WDX systems have already been developed where components of the hardware are shared, such as a computer to perform corrections on the measured data for atomic number, absorption, and fluorescence effects. These corrections are necessary when performing quantitative analysis. 

Hydride generation techniques are superior to direct solution analysis in several ways. However, the attraction offered by enhanced detection limits is offset by the relatively few elements to which the technique can be applied, potential interferences, as well as limitations imposed on the sample preparation procedures in that strict adherence to valence states and chemical form must be maintained. Cold-vapor generation of mercury currently provides the most desirable means of quantitation of this element, although detection limits lower than AAS can be achieved when it is coupled to other means of detection (e.g., nondispersive atomic fluorescence or micro-wave induced plasma atomic emission spectrometry). 

See also Amperometry. Atomic Emission Spectrometry Flame Photometry. Chemiiuminescence Overview Liquid-Phase. Flow Injection Analysis Principles. Fluorescence Quantitative Analysis. Ion Exchange Ion Chromatography Instrumentation. Liquid Chromatography Overview. Ozone. Sampling Theory. Sulfur. Textiles Natural Synthetic. 

Emission of UV/VIS radiation Atomic fluorescence spectroscopy (AFS) Quantitative elemental analysis of ultratrace concentrations (sub-ppb)... 

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. 

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