Quantitative analysis atomic fluorescence spectrometry

Atomic Fluorescence Spectrometry. Principles of Quantitative Analysis... 

Chemical analysis of the metal can serve various purposes. For the determination of the metal-alloy composition, a variety of techniques has been used. In the past, wet-chemical analysis was often employed, but the significant size of the sample needed was a primary drawback. Nondestmctive, energy-dispersive x-ray fluorescence spectrometry is often used when no high precision is needed. However, this technique only allows a surface analysis, and significant surface phenomena such as preferential enrichments and depletions, which often occur in objects having a burial history, can cause serious errors. For more precise quantitative analyses samples have to be removed from below the surface to be analyzed by means of atomic absorption (82), spectrographic techniques (78,83), etc. 

The selection of the method of analysis is a vital step in the solution of an analytical problem. A choice cannot be made until the overall problem is defined, and where possible a decision should be taken by the client and the analyst in consultation. Inevitably, in the method selected, a compromise has to be reached between the sensitivity, precision and accuracy desired of the results and the costs involved. For example. X-ray fluorescence spectrometry may provide rapid but rather imprecise quantitative results in a trace element problem. Atomic absorption spectrophotometry, on the other hand, will supply more precise data, but at the expense of more time consuming chemical manipulations. 

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. 

Spectrophotometric techniques have been the basis of many coal analysis methods. One of the most widely used techniques for analysis of trace elements is atomic absorption spectrometry, in which the standards and samples are aspirated into a flame. A hollow cathode lamp provides a source of radiation that is characteristic of the element of interest and the absorption of characteristic energy by the atoms of a particular element. X-ray fluorescence is also employed as a quantitative technique for trace element determination and depends on election of orbital electrons from atoms of the element when the sample is irradiated by an x-ray source. 

One of the very first quantitative applications of x-ray fluorescence spectrometry involved the analysis of copper-based alloys for trace metals. IWenty years later the rapid development in the use of speciality alloys for, among others, the aircraft industry, required the availability of fast, accurate multielement instrumental methods. In the early 1950s two methods seemed to hold promise— x-ray fluorescence and ultraviolet emission (UVE). At that time, x-ray fluorescence was a technique limited to a wavelength range of about 0.5 to 8.0 A, in other words, all elements down to atomic number 14(Si). Even though it was unable to measure the lower atomic numbers, especially the important element carbon, it was able to provide data for S and R This, along with (at that time) a perceived minimum problem of interelement interferences, made x-ray fluorescence an ideal choice for the nonferrous industry. However, the UVE technique was the method of choice for most ferrous industry-based problems. This situation was to persist into the 1960s until the classic work of Shiraiwa and Fujino [14] provided the means for accurate... 

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. 

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. 

See also Atomic Spectroscopy, Historical Perspective Environmental and Agricultural Applications of Atomic Spectroscopy Inductively Coupled Plasma Mass Spectrometry, Methods Quantitative Analysis X-Ray Fluorescence Spectroscopy, Applications. 

X-ray fluorescence (XRF) spectroscopy is useful for qualitative elemental analysis of paint samples. It does not require dissolution of the sample and can be applied to dry films. When an energy-dispersive instrument is employed, XRF provides rapid information on the presence of elements of atomic number higher than or equal to 12 (e.g., above magnesium). However, from a quantitative point of view, the sensitivity, accuracy, and reproducibility of XRF measurements is lower than that of flame, electrothermal, or plasma atomic spectrometry. 

---