Atomic emission spectroscopy precision

When the problem has been defined and needed background information has been studied, it is time to consider which analytical methods will provide the data you need to solve the problem. In selecting techniques, you can refer back to the other chapters in this book. For example, if you want to measure the three heavy metals (Co, Fe, and Ni) that were suspect in the Bulging Drum Problem, you might immediately think of atomic absorption or inductively coupled plasma atomic emission spectroscopies and reread Chapter 8 of this book. How would you choose between them Which would be more accurate More precise Does your lab have both instruments Are they both in working order What if you have neither of them What sample preparation would be needed ... 

Chapters 7 and 8 describe two major techniques for the monitoring of trace elements in environmental samples atomic absorption (AA) and inductively coupled plasma-atomic emission spectroscopy (ICP). AA is most ideally suited for analyses where a limited number of trace metal concentrations are needed with high accuracy and precision. ICP has the advantage of multielement analysis with high speed. 

Atomic absorption (21,22,25-29) differs from atomic emission spectroscopy in that the quantitative measure of an element is made by observing the absorption of light passing through an atomized sample instead of the emission from the thermally excited atom (Fig. 6). The precision of measurement is greater than arc, spark, or flame emission and comparable to inductively coupled plasma spec-... 

Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) is used for multi-element determinations in blood and tissue samples. Detection in urine samples requires extraction of the metals with a polydithiocarbamate resin prior to digestion and analysis (NIOSH 1984a). Other satisfactory analytical methods include direct current plasma emission spectroscopy and determination by AAS, and inductively coupled argon plasma spectroscopy-mass spectrometry (ICP-MS) (Patterson et al. 1992 Shaw et al. 1982). Flow injection analysis (FIA) has been used to determine very low levels of zinc in muscle tissue. This method provides very high sensitivity, low detection limits (3 ng/mL), good precision, and high selectivity at trace levels (Fernandez et al. 1992b). 

The structure and energy of isolated atoms of chemical elements are well-known (Figure 3.13). The atomic emission spectroscopy allows one to determine the composition of electronic shells and the energy of electrons in an atom with a desired high precision. The structure of atoms has specifically been investigated by measurements of the frequency, intensity, and broadening of their spectral hnes. 

In the atomic spectroscopy experiment in Figure 20-1, a liquid sample is aspirated (sucked) through a plastic tube into a flame that is hot enough to break molecules apart into atoms. The concentration of an element in the flame is measured by absorption or emission of radiation. For atomic absorption spectroscopy, radiation of the correct frequency is passed through the flame (Figure 20-2) and the intensity of transmitted radiation is measured. For atomic emission spectroscopy, no lamp is required. Radiation is emitted by hot atoms whose electrons have been promoted to excited states in the flame. For both experiments in Figure 20-2, a monochromator selects the wavelength that will reach the detector. Analyte concentrations at the parts per million level are measured with a precision of 2%. To analyze major constituents, a sample must be diluted to reduce concentrations to the ppm level. Box 20-1 describes an application of atomic emission for space exploration. 

Analytical methods of atomic spectroscopy have been used in forestry and wood product research since their earliest development. Nowadays, almost all of the spectroscopic techniques available are employed in the analysis of metals and trace elements in diverse samples of industrial and environmental origin. The techniques that find most regular application include flame atomic absorption spectroscopy (F-AAS), graphite furnace atomic absorption spectroscopy (GF-AAS), inductively coupled plasma atomic emission spectroscopy (ICP-AES) and, occasionally, also direct current plasma atomic emission spectroscopy (DCP-AES). In many applications F-AAS is a sufficiently sensitive and precise technique however, in the analysis of some environmental samples for trace elements (forest soils, plant material and water) where concentrations may be very low (of the order of 100 ng mL" ) the greater sensitivity of GF-AAS and ICP/DCP-AES is required. In considering the applications of atomic spectroscopy to forestry and... 

In general, gas chromatography will undoubtedly continue to be the method of choice for characterization of light hydrocarbon materials. New and improved detection devices and techniques, such as chemiluminescence, atomic emission, and mass spectroscopy, will enhance selectivity, detection limits, and analytical productivity. Laboratory automation through autosampling, computer control, and data handling will provide improved precision and productivity, as well as simplified method operation. 

Since the mid-1960s, a variety of analytical chemistry techniques have been used to characterize obsidian sources and artifacts for provenance research (4, 32-36). The most common of these methods include optical emission spectroscopy (OES), atomic absorption spectroscopy (AAS), particle-induced X-ray emission spectroscopy (PIXE), inductively coupled plasma-mass spectrometry (ICP-MS), laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS), X-ray fluorescence spectroscopy (XRF), and neutron activation analysis (NAA). When selecting a method of analysis for obsidian, one must consider accuracy, precision, cost, promptness of results, existence of comparative data, and availability. Most of the above-mentioned techniques are capable of determining a number of elements, but some of the methods are more labor-intensive, more destructive, and less precise than others. The two methods with the longest and most successful histoty of success for obsidian provenance research are XRF and NAA. 

The method of standard additions is widely used in atomic spectroscopy (e.g. determination of Ca2+ ions in serum by atomic emission spectrophotometry) and, since several aliquots of sample are analysed to produce the calibration graph, should increase the accuracy and precision of the assay... 

The absorption, emission, photodetachment, and photoelectron spectroscopy experiments are capable of providing accurate and precise values for the electron affinities of atoms. The best precision is about 1 part per million, more than precise enough for chemical purposes. The state of the ion must be identified and some excited-state electron affinities of atoms have been reported. The photoelectron spectroscopy and photodetachment procedure can give the accurate and precise electron affinities of molecules and radicals when the state of the anion is assigned. 

More common methods for elemental analysis - to determine the elemental contents of a sample - include spectroscopy and spectrometry. Spectroscopy measures changes in atoms that cause a specific light photon to be either absorbed (absorption spectroscopy) or emitted (emission spectroscopy). This light has a precise wavelength or energy, characteristic of a specific element in the periodic table. The simplest (and oldest) form of elemental analysis was not spectroscopic, in fact, but colorimetric. This method was based on the reaction of a strongly colored chemical in a solution. The appearance of a specific color in the solution revealed the identity of the element of interest. If the color intensity is proportional to the amount of that element present, the method can also be used to estimate the amount of the element present. 

Argon plasma offers a number of advantages as a source for emission spectroscopy. Argon is an inert gas and will not react with the sample so chemical interference is greatly reduced. At plasma temperatures, atomization is complete and elemental spectra do not reflect molecular components. Detection limits are high for most elements. Accuracy and precision are excellent. In addition, ICP/OES requires less sample preparation and less sample amount than other techniques. 

Among the various types of atomic spectroscopy, only two, flame emission spectroscopy and atomic absorption spectroscopy, are widely used and accepted for quantitative pharmaceutical analysis. By far the majority of literature regarding pharmaceutical atomic spectroscopy is concerned with these two methods. However, the older method of arc emission spectroscopy is still a valuable tool for the qualitative detection of trace-metal impurities. The two most recently developed methods, furnace atomic absorption spectroscopy and inductively coupled plasma (ICP) emission spectroscopy, promise to become prominent in pharmaceutical analysis. The former is the most sensitive technique available to the analyst, while the latter offers simultaneous, multielemental analysis with the high sensitivity and precision of flame atomic absorption. 

The elements Na, K, Li, Mg, Ca, Al, and Zn are among the most common elements subjected to pharmaceutical analysis and coincidentally are also among the elements most readily determinable by flame emission. Although it is not as versatile as other methods, flame emission spectroscopy exhibits sensitivity greater than or approximately equal to that of flame absorption spectroscopy for the above elements (Table 2). Where precision of the analysis is critical, however, the analyst should consider the alternative of atomic absorption spectroscopy. 

Sodium and potassium levels are difficult to analyze by titrimetric or colorimetric techniques but are among the elements most easily determined by atomic spectroscopy (2,38) (Table 2). Their analysis is important for the control of infusion and dialysis solutions, which must be carefully monitored to maintain proper electrolyte balance. Flame emission spectroscopy is the simplest and least expensive technique for this purpose, although the precision of the measurement may be improved by employing atomic absorption spectroscopy. Both methods are approved by the U.S. (39), British (40), and European (41) Pharmacopeias and are commonly utilized. Sensitivity is of no concern, due to the high concentrations in these solutions furthermore, dilution of the sample is often necessary in order to reduce the metal concentrations to the range where linear instmmental response can be achieved. Fortunately, the analysis may be carried but without additional sample preparation because other components, such as dextrose, do not interfere. 

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