Atomic absorption spectrometry analytical range

Boyle and Edmond [679] determined copper, nickel, and cadmium in 100 ml of seawater by coprecipitation with cobalt pyrrolidine dithiocarba-mate and graphite atomiser atomic absorption spectrometry. Concentration ranges likely to be encountered and estimated analytical precisions (lcr) are l-6nmol/kg ( 0.1) for copper, 3-12nmol/kg ( 0.3) for nickel, and 0.0-1.1 nmol/kg ( 0.1) for cadmium. 

Graphite-furnace atomic absorption spectrometry, although element-selective and highly sensitive, is currently unable to directly determine manganese at the lower end of their reported concentration ranges in open ocean waters. Techniques that have been successfully employed in recent environmental investigations have thus used a preliminary step to concentrate the analyte and separate it from the salt matrix prior to determination by atomic absorption spectrometry. 

Only arc/spark, plasma emission, plasma mass spectrometry and X-ray emission spectrometry are suitable techniques for qualitative analysis as in each case the relevant spectral ranges can be scanned and studied simply and quickly. Quantitative methods based on the emission of electromagnetic radiation rely on the direct proportionality between emitted intensity and the concentration of the analyte. The exact nature of the relation is complex and varies with the technique it will be discussed more fully in the appropriate sections. Quantitative measurements by atomic absorption spectrometry depend upon a relation which closely resembles the Beer-Lambert law relating to molecular absorption in solution (p. 357 etal.). 

Non-linear concentration/response relationships are as common in pesticide residue analysis as in analytical chemistry in general. Although linear approximations have traditionally been helpful the complexity of physical phenomena is a prime reason that the limits of usefulness of such an approximation are frequently exceeded. In fact, it should be regarded the rule rather than the exception that calibration problems cannot be handled satisfactorily by linear relationships particularly as the dynamic range of analytical methods is fully exploited. This is true of principles as diverse as atomic absorption spectrometry (U. X-ray fluorescence spectrometry ( ), radio-immunoassays (3), electron capture detection (4) and many more. 

A convenient method is the spectrometric determination of Li in aqueous solution by atomic absorption spectrometry (AAS), using an acetylene flame—the most common technique for this analyte. The instrument has an emission lamp containing Li, and one of the spectral lines of the emission spectrum is chosen, according to the concentration of the sample, as shown in Table 2. The solution is fed by a nebuhzer into the flame and the absorption caused by the Li atoms in the sample is recorded and converted to a concentration aided by a calibration standard. Possible interference can be expected from alkali metal atoms, for example, airborne trace impurities, that ionize in the flame. These effects are canceled by adding 2000 mg of K per hter of sample matrix. The method covers a wide range of concentrations, from trace analysis at about 20 xg L to brines at about 32 g L as summarized in Table 2. Organic samples have to be mineralized and the inorganic residue dissolved in water. The AAS method for determination of Li in biomedical applications has been reviewed . 

Flame atomic absorption spectrometry can be used to determine trace levels of analyte in a wide range of sample types, with the proviso that the sample is first brought into solution. The methods described in Section 1.6 are all applicable to FAAS. Chemical interferences and ionization suppression cause the greatest problems, and steps must be taken to reduce these (e.g. the analysis of sea-water, refractory geological samples or metals). The analysis of oils and organic solvents is relatively easy since these samples actually provide fuel for the flame however, build-up of carbon in the burner slot must be avoided. Most biological samples can be analysed with ease provided that an appropriate digestion method is used which avoids analyte losses. 

Different analytical techniques such as ICP-OES (optical emission spectrometry with inductively coupled plasma source), XRF (X-ray fluorescence analysis), AAS (atomic absorption spectrometry) with graphite furnace and flame GF-AAS and FAAS, NAA (neutron activation analysis) and others, are employed for the trace analysis of environmental samples. The main features of selected atomic spectrometric techniques (ICP-MS, ICP-OES and AAS) are summarized in Table 9.20.1 The detection ranges and LODs of selected analytical techniques for trace analysis on environmental samples are summarized in Figure 9.15.1... 

The most suitable techniques for the rapid, accurate determination of the elemental content of foods are based on analytical atomic spectrometry, for example, atomic absorption spectrometry (AAS), atomic emission spectrometry (AES), and mass spectrometry, the most popular modes of which are Game (F), electrothermal atomization (ET), and hydride generation (HG) AAS, inductively coupled plasma (ICP), microwave-induced plasma (MIP), direct current plasma (DCP) AES, and ICP-MS. Challenges in the determination of elements in food include a wide range of concentrations, ranging from ng/g to percent levels, in an almost endless combination of analytes with matrix speci be matrices. 

The newcomer to AAS could easily be led into believing that he has been misled when informed that this analytical technique is free from interferences. This impression unfortunately arises from early work in the technique when, of course, only a few applications had been studied. With the increase of interest, a wider range of applications was studied and consequently more problems were encountered. However, the interferences encountered in atomic absorption spectrometry are now extremely well documented and many which were reported early in the literature were found to be due to instrumental imperfections and have now virtually disappeared. All interferences can be overcome by the use of simple techniques. 

UV—Vis = spectrophotometry FAAS = flame atomic absorption spectrometry FP = flame photometry Analytical range or detection limit given in original units. 

The analyte is accumulated on the filter as a solid compound which is further dissolved and directed towards the detector. Matrix interferences are circumvented and sensitivity is improved, as demonstrated in the determination of copper in silicate rocks by flame atomic absorption spectrometry [295]. The analyte was precipitated by rubeanic acid, the precipitate was separated by continuous filtration and the depleted sample zone was directed towards waste. After precipitate collection, a potassium dichromate stream was allowed to pass through the filtering unit, thus dissolving the retained precipitate and transporting it towards the detector. The dynamic concentration range was expanded (0.3—200 -igL 1 Cu) by passing different sample volumes through the... 

The limit of detection of an analytical method must be directly correlated with the concentration of the analytes in the sample.329 Generally, the magnitude order for the limit of detection for different types of methods (e.g., anodic stripping voltammetry, potentiometry, atomic absorption spectrometry, UV/Vis, etc.) is known. Also, the approximate concentration range of the analyte in samples is known. For validation of the method for the analysis of an analyte from a specific sample, the limit of detection must be lower than the concentration of the analyte in the sample. 

A wide range of analytical techniques is necessary to provide an unambiguous identification of pigments in a sample. Elemental techniques are often used, such as scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), X-ray fluorescence (XRE) spectrometry, scanning electron microprobe analysis (EPMA), X-ray photoelectron spectroscopy (XPS), particle-induced X-ray emission (PDCE), neutron activation analysis (NAA), atomic absorption spectrometry (AAS), inductively coupled... 

This technique comprises a group of quantitative instrumental analytical methods based on the capacity of free atoms of both emitting and absorbing radiation at a specific wavelength. The radiation lies within the range for ultraviolet and visible light. A distinction is made between atomic emission spectrometry (AES), atomic absorption spectrometry (AAS), and atomic fluorescence spectrometry. The most commonly applied techniques are flame-AAS, graphite furnace-AAS, and ICP-AES. With ICP, excitation takes place in a plasma at a temperature of 7000 K. 

Atomic absorption spectrometry is commonly used to measure a wide range of elements as shown in Table 2. Such techniques as flame, graphite furnace, hydride generation, and cold vapor are employed. Measurements are made separately for each element of interest in turn to achieve a complete analysis these techniques are relatively slow to use. More sensitive, but also more expensive, multielement analytical techniques such as inductively coupled plasma-atomic emission spectrometry and inductively coupled plasma-mass spectrometry can be used if lower (pgl and below) detection limits are required. These detectors can also be coupled with separation systems if speciation data, e.g., Cr(III) and Cr(VI), are needed. 

A variety of techniques based on different physical principles have been used for trace element measurements. The most commonly used include neutron activation analysis (NAA) [1], atomic absorption spectrometry [2,3], and mass spectrometry [4-7]. The two distinct advantages primarily responsible for the selection of NAA in earlier studies appeared to be the option to determine several trace elements simultaneously and the elimination of complex chemical separation steps. The poor precision values obtained by NAA have recently necessitated prechemical separation, which introduces problems of analyte loss, contamination, and blank correction. However, the major drawbacks are the requirement of a nuclear reactor facility, the slow turnaround of the samples, and the relatively high cost of analysis. Nonetheless, NAA is a well-established, multielemental, nondestructive technique with detection limits for most elements in the 1-50 p.g/liter range. This topic is covered by Heydom in Chap. 13 of this book. 

A further check on the occurrence of systematic errors in a method is to compare the results with those obtained from a different method, if two unrelated methods are used to perform one analysis, and if they consistently yield results showing only random differences, it is a reasonable presumption that no significant systematic errors are present. For this approach to be valid, each step of the two experiments has to be independent. Thus in the case of serum chromium determinations, it would not be sufficient to replace the atomic-absorption spectrometry step by a colorimetric method or by plasma spectrometry. The systematic errors would only be revealed by altering the sampling methods also, e.g. by minimizing or eliminating the use of stainless-steel equipment. A further important point is that comparisons must be made over the whole of the concentration range for which an analytical procedure is... 

In many instrumental analysis methods the instrument response is proportional to the analyte concentration over substantial concentration ranges. The simplified calculations that result encourage analysts to take significant experimental precautions to achieve such linearity. Examples of such precautions include the control of the emission line width of a hollow-cathode lamp in atomic absorption spectrometry, and the size and positioning of the sample cell to minimize inner filter artefacts in molecular fluorescence spectrometry. However, many analytical methods (e.g. immunoassays and similar competitive binding assays) produce calibration plots that are intrinsically curved. Particularly common is the situation where the calibration plot is linear (or approximately so) at low analyte concentrations, but becomes curved at higher analyte levels. When curved calibration plots are obtained we still need answers to the questions listed in Section 5.2, but those questions will pose rather more formidable statistical problems than occur in linear calibration experiments. 

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