Analysis Methods Atomic Spectroscopy

Hoenig, M., de Kersabiec, A.-M. Sample preparation steps for analysis by atomic spectroscopy methods present status. Spectrochim. Acta Part B 51, 1297-1307 (1996)... 

Figure 3.17 Graphical illustration of measurement uncertainty for individual sources with an analytical method for metal analysis using atomic spectroscopy...

Analysis of Surface Elemental Composition. A very important class of surface analysis methods derives from the desire to understand what elements reside at the surface or in the near-surface region of a material. The most common techniques used for deterrnination of elemental composition are the electron spectroscopies in which electrons or x-rays are used to stimulate either electron or x-ray emission from the atoms in the surface (or near-surface region) of the sample. These electrons or x-rays are emitted with energies characteristic of the energy levels of the atoms from which they came, and therefore, contain elemental information about the surface. Only the most important electron spectroscopies will be discussed here, although an array of techniques based on either the excitation of surfaces with or the collection of electrons from the surface have been developed for the elucidation of specific information about surfaces and interfaces. 

Instrumental Quantitative Analysis. Methods such as x-ray spectroscopy, oaes, and naa do not necessarily require pretreatment of samples to soluble forms. Only reUable and verified standards are needed. Other instmmental methods that can be used to determine a wide range of chromium concentrations are atomic absorption spectroscopy (aas), flame photometry, icap-aes, and direct current plasma—atomic emission spectroscopy (dcp-aes). These methods caimot distinguish the oxidation states of chromium, and speciation at trace levels usually requires a previous wet-chemical separation. However, the instmmental methods are preferred over (3)-diphenylcarbazide for trace chromium concentrations, because of the difficulty of oxidizing very small quantities of Cr(III). 

The problems involved in quantitative analysis using NMR spectroscopy, have been discussed by several authors and it is evident that it still causes a lot of problems as especially pointed out by Hays55 in his excellent review on the subject. Thus in liquid state NMR spectroscopy the quantitative estimation of atoms and groups involves the use of normal analytical method. In the case of solid state NMR spectroscopy, however, the application of the cross-polarization technique results in signal enhancements and allows repetition rates faster than those allowed by the carbon C-13 Tl. Therefore, the distortion of relative spectral intensities must always be considered a possibility, and hence quantitative spectra will not always be obtained. 

In this chapter we have limited ourselves to the most common techniques in catalyst characterization. Of course, there are several other methods available, such as nuclear magnetic resonance (NMR), which is very useful in the study of zeolites, electron spin resonance (ESR) and Raman spectroscopy, which may be of interest for certain oxide catalysts. Also, all of the more generic tools from analytical chemistry, such as elemental analysis, UV-vis spectroscopy, atomic absorption, calorimetry, thermogravimetry, etc. are often used on a routine basis. 

Young, S. M. M. and M. Pollard (2000), Atomic spectroscopy and spectrometry, in Ciliberto, E. and G. Spoto (eds.), Modern Analytical Methods in Art and Archaeology, Chemical Analysis Series, Vol. 155, Wiley, New York, pp. 21-54. 

Determination of trace metals in seawater represents one of the most challenging tasks in chemical analysis because the parts per billion (ppb) or sub-ppb levels of analyte are very susceptible to matrix interference from alkali or alkaline-earth metals and their associated counterions. For instance, the alkali metals tend to affect the atomisation and the ionisation equilibrium process in atomic spectroscopy, and the associated counterions such as the chloride ions might be preferentially adsorbed onto the electrode surface to give some undesirable electrochemical side reactions in voltammetric analysis. Thus, most current methods for seawater analysis employ some kind of analyte preconcentration along with matrix rejection techniques. These preconcentration techniques include coprecipitation, solvent extraction, column adsorption, electrodeposition, and Donnan dialysis. 

All reagents and solvents that are used to prepare the sample for analysis should be ultrapure to prevent contamination of the sample with impurities. Plastic ware should be avoided since these materials may contain ultratrace elements that can be leached into the analyte solutions. Chemically cleaned glassware is recommended for all sample preparation procedures. Liquid samples can be analyzed directly or after dilution when the concentrations are too high. Remember, all analytical errors are multiplied by dilution factors therefore, using atomic spectroscopy to determine high concentrations of elements may be less accurate than classical gravimetric methods. 

On the basis of the preceding discussion, it should be obvious that ultratrace elemental analysis can be performed without any major problems by atomic spectroscopy. A major disadvantage with elemental analysis is that it does not provide information on element speciation. Speciation has major significance since it can define whether the element can become bioavailable. For example, complexed iron will be metabolized more readily than unbound iron and the measure of total iron in the sample will not discriminate between the available and nonavailable forms. There are many other similar examples and analytical procedures that must be developed which will enable elemental speciation to be performed. Liquid chromatographic procedures (either ion-exchange, ion-pair, liquid-solid, or liquid-liquid chromatography) are the best methods to speciate samples since they can separate solutes on the basis of a number of parameters. Chromatographic separation can be used as part of the sample preparation step and the column effluent can be monitored with atomic spectroscopy. This mode of operation combines the excellent separation characteristics with the element selectivity of atomic spectroscopy. AAS with a flame as the atom reservoir or AES with an inductively coupled plasma have been used successfully to speciate various ultratrace elements. 

Atomic spectroscopy is an excellent method of analysis for trace or ultratrace levels of many elements in the periodic table. The major disadvantage of all atomic spectroscopic methods is that they provide no information on the oxidation state of the element or its speciation. This disadvantage can be redressed by the use of selective reagents coupled... 

Analysis for atoms means that atomic spectroscopy is limited to the elements. In fact, the keyword for atomic spectroscopy is metals. The vast majority of methods involving atomic spectroscopy are methods for determining metals. 

This series describes selected advances in the area of atomic spectroscopy. It is primarily intended for the reader who has a background in atomic spectroscopy suitable to the novice and expert. Although a widely used and accepted method for metal and nonmetal analysis in a variety of complex samples, Advances in Atomic Spectroscopy covers a wide range of materials. Each chapter will completely cover an area of atomic spectroscopy where rapid development has occurred. 

Inductively Coupled and Microwave Induced Plasma Sources for Mass Spectrometry 4 Industrial Analysis with Vibrational Spectroscopy 5 Ionization Methods in Organic Mass Spectrometry 6 Quantitative Millimetre Wavelength Spectrometry 7 Glow Discharge Optical Emission Spectroscopy A Practical Guide 8 Chemometrics in Analytical Spectroscopy, 2nd Edition 9 Raman Spectroscopy in Archaeology and Art History 10 Basic Chemometric Techniques in Atomic Spectroscopy... 

Analyte is measured at parts per million ( xg/g) to parts per trillion (pg/g) levels. To analyze major constituents, the sample must be diluted to reduce concentrations to the parts per million level. As we saw in the analysis of teeth, trace constituents can be measured directly without preconcentration. The precision of atomic spectroscopy, typically 1-2%, is not as good as that of some wet chemical methods. The equipment is expensive, but widely available. Unknowns, standards, and blanks can be loaded into an autosampler, which is a turntable that automatically rotates each sample into position for analysis. The instrument runs for many hours without human intervention. 

Investigation of atomic spectra yields atomic energy levels. An important chemical application of atomic spectroscopy is in elemental analysis. Atomic absorption spectroscopy and emission spectroscopy are used for rapid, accurate quantitative analysis of most metals and some nonmetals, and have replaced the older, wet methods of analysis in many applications. One compares the intensity of a spectral line of the element being analyzed with a standard line of known intensity. In atomic absorption spectroscopy, a flame is used to vaporize the sample in emission spectroscopy, one passes a powerful electric discharge through the sample or uses a flame to produce the spectrum. Atomic spectroscopy is used clinically in the determination of Ca, Mg, K, Na, and Pb in blood samples. For details, see Robinson. 

Several other methods have been used to determine the trace elements in the mineral matter of coal, as well as in whole coal and coal-derived materials. These methods include spark-source mass spectrometry, neutron activation analysis, optical emission spectroscopy, and atomic absorption spectroscopy. 

The application of analytical methods to speciation measurements in complicated systems has remained rather limited, despite the considerable technological progress during the past 25 years. The characterisation methods (e.g. spectroscopy, nuclear magnetic resonance) are often limited to the study of isolated compounds at relatively high concentrations. They, therefore, necessitate the prior employment of sophisticated separation and pre-concentration methods which introduce severe risks of perturbation. The trace analysis methods are often insensitive to the chemical form of the elements measured (e.g. atomic absorption, neutron activation). Those which possess sufficient element specificity (e.g. electron spin resonance, fluorescence, voltammetry) still require significant development before their full potential can be realised. 

The pure element standards can be also used as chemical composition standards. The certified properties are in this case the contents of all metallic traces at ultra trace level. BAM offers, e.g. BAM B Primary Cul with statements (certified values) of the content of 65 trace metal elements. This reference material is suitable for matrix matching in metal analysis, e.g. where using methods of atomic spectroscopy. 

The analytical chemist will choose the appropriate analytical technique (e.g., chromatography, spectroscopy, or titration) to satisfy the technical objective based upon his or her expertise and past experiences with similar analytical problems. Often, however, the analyte itself dictates the kind of analysis method to be used. For example, a residual volatile solvent would most probably be analyzed by gas chromatography (GC), while a residual catalyst, such as palladium, would best be analyzed by atomic absorption or emission spectroscopy. 

Atomic spectrometry is based on the generation of free atoms which can absorb or emit radiation due to defined transitions of the valence electrons of the outer shell of the atom. Comprehensive and critical reviews of atomic spectroscopy and its uses appear in Journal of Analytical Atomic Spectrometry. Advances in AAS and fluorescence spectrometry have been reviewed by Hill et al. (1991). Branch etal. (1991) has updated the use of AS for the analysis of clinical and biological materials, foods and beverages and discussed methods for individual elements. Cresser et al. (1991, 1992) have reviewed environmental analysis, including those for soils and plants and have included summary tables of methods. 

The use of photoresists to cover most of the metal surface and isolate individual pits has made possible detailed analysis of these entities, varying in radius from around 0.1 to 5 pm. The methods of examination include Auger spectroscopy, scanning electron microscopy, X-ray dispersive analysis, and atomic force microscopy (Ke and Alkyre, 1995). 

The fact that excited atoms give off specific colors and not a rainbow of colors suggested to Niels Bohr, a Danish physicist, that electrons are permitted in only certain locations within the atom. These locations are called energy levels. Each element behaves in its own unique way when excited by heat or electricity and produces a very specific pattern of lines of color called the atomic spectrum of that element (Figure 8.5). This unique chemical fingerprint is the foundation of atomic spectroscopy, a method of analysis used by forensic and medical laboratories to identify elements... 

The basic information in the study of sorption processes is the quantity of substances on the interfaces. In order to measure the sorbed quantity accurately, very sensitive analytical methods have to be applied because the typical amount of particles (atoms, ions, and molecules) on the interfaces is about I0-5 mol/m2. In the case of monolayer sorption, the sorbed quantity is within this range. As the sorbed quantity is defined as the difference between quantities of a given substance in the solution and/or in the solid before and after sorption processes (surface excess concentration, Chapter 1, Section 1.3.1), all methods suitable for the analysis of solid and liquid phases can be applied here, too. These methods have been discussed in Sections 4.1 and 4.2. In addition, radioisotopic tracer method can also be applied for the accurate measurement of the sorbed quantities. On the basis of the radiation properties of the available isotopes, gamma and beta spectroscopy can be used as an analytical method. Alpha spectroscopy may also be used, if needed however, it necessitates more complicated techniques and sample preparation due to the significant absorption of alpha radiation. The sensitivity of radioisotopic labeling depends on the half-life of the isotopes. With isotopes having medium half-time (days-years), 10 14-10-10 mol can be measured easily. 

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