Atomic emission spectroscopy sensitivity

The classical wet-chemical quaUtative identification of chromium is accompHshed by the intense red-violet color that develops when aqueous Cr(VI) reacts with (5)-diphenylcarba2ide under acidic conditions (95). This test is sensitive to 0.003 ppm Cr, and the reagent is also useful for quantitative analysis of trace quantities of Cr (96). Instmmental quaUtative identification is possible using inductively coupled argon plasma—atomic emission spectroscopy... 

Bromo-2-pyridyla2o)-5-diethylamiQophenol (5-Br-PADAP) is a very sensitive reagent for certain metals and methods for cobalt have been developed (23). Nitroso-naphthol is an effective precipitant for cobalt(III) and is used in its gravimetric determination (24,25). Atomic absorption spectroscopy (26,27), x-ray fluorescence, polarography, and atomic emission spectroscopy are specific and sensitive methods for trace level cobalt analysis (see... 

Colorimetric procedures are often used to determine copper in trace amounts. Extraction of copper using diethyldithiocarbamate can be quite selective (60,62), but the method using dithhone is preferred because of its greater sensitivity and selectivity (50—52). Atomic absorption spectroscopy, atomic emission spectroscopy, x-ray fluorescence, and polargraphy are specific and sensitive methods for the deterrnination of trace level copper. 

The emerging analytical technique of laser-induced breakdown spectroscopy (LIBS) is a simple atomic emission spectroscopy technique that has the potential for real-time man-portable chemical analysis in the field. Because LIBS is simultaneously sensitive to all elements, a single laser shot can be used to record the broadband emission spectra, which provides a chemical fingerprint of a material. 

Measurements of the intensity and wavelength of radiation that is either absorbed or emitted provide the basis for sensitive methods of detection and quantitation. Absorption spectroscopy is most frequently used in the quantitation of molecules but is also an important technique in the quantitation of some atoms. Emission spectroscopy covers several techniques that involve the emission of radiation by either atoms or molecules but vary in the manner in which the emission is induced. Photometry is the measurement of the intensity of radiation and is probably the most commonly used technique in biochemistry. In order to use photometric instruments correctly and to be able to develop and modify spectroscopic techniques it is necessary to understand the principles of the interaction of radiation with matter. 

Nickel is normally present at very low levels in biological samples. To determine trace nickel levels in these samples accurately, sensitive and selective methods are required. Atomic absorption spectrometry (AAS) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES), with or without preconcentration or separation steps, are the most common methods. These methods have been adopted in standard procedures by EPA, NIOSH, lARC, and the International Union of Pure and Applied... 

Inductively Coupled Plasma Atomic Emission Spectroscopy An Atlas of Spectral Information, Winge, R.K., Fassel, V.A., Peterson, V.J. and Floyd, W.A., Elsevier, New York, 1985. A compilation of sensitive lines for use in ICPAES. 

Eluate from a chromatography column can be passed through a plasma to atomize and ionize its components and measure selected elements by atomic emission spectroscopy or mass spectrometry. An atomic emission detector directs eluate through a helium plasma in a microwave cavity. Every element of the periodic table produces characteristic emission that can be detected by a photodiode array polychromator (Figure 20-14). Sensitivity for sulfur can be 10 times better than the sensitivity of a flame photometric detector. 

Atomic emission spectroscopy can be employed, generally with an inductively coupled plasma for thermal excitation. The sample is introduced into the plasma as a mist of ultrafine droplets, and the monochromator and detector are set to measure the intensity of an atomic emission line characteristic of the element. This technique is powerful, general, sensitive, linear, and able to measure over 70 elements, and, as a result, is widely used. Response is typically linear over four orders of magnitude in concentration with relative standard deviations of 1 to 3%. In low-salt aqueous solutions, detection limits range from 10 to 1000 nanomolar without preconcentration. Significant problems with saline samples remain, but use of Babington nebulizers alleviates these problems somewhat. 

A number of analytical methods were developed for determination of elemental mercury. The methods are reviewed in Refs. [1-4]. They include traditional analytical techniques, such as atomic adsorption spectroscopy (AAS), atomic fluorescence spectroscopy (AFS), and atomic emission spectroscopy (AES). The AAS is based on measurements of optical adsorption at 253.7 or 184.9 nm. Typical value of the detection limit without pre-concentration step is over 1 pg/l. The AEF is much more sensitive and allows one to detect less than 0.1ng/l of mercury... 

Ultraviolet-visible (UV-Vis) spectrophotometric detectors are used to monitor chromatographic separations. However, this type of detection offers very little specificity. Element specific detectors are much more useful and important. Atomic absorption spectrometry (AAS), inductively coupled plasma-atomic emission spectroscopy (ICPAES) and inductively coupled plasma-mass spectrometry (ICP-MS) are often used in current studies. The highest sensitivity is achieved by graphite furnace-AAS and ICP-MS. The former is used off-line while the latter is coupled to the chromatographic column and is used on-line . 

The ion atmosphere of nucleic acids directly affects measured biochemical and biophysical properties. However, study of the ion atmosphere is difficult due to its diffuse and dynamic nature. Standard techniques available have significant limitations in sensitivity, specificity, and directness of the assays. Buffer exchange-atomic emission spectroscopy (BE-AES) was developed to overcome many of the limitations of previously available techniques. This technique can provide a complete accounting of all ions constituting the ionic atmosphere of a nucleic acid at thermodynamic equilibrium. Although initially developed for the study of the ion atmosphere of nucleic acids, BE-AES has also been applied to study site-bound ions in RNA and protein. 

To overcome the limitations of other techniques, buffer exchange-atomic emission spectroscopy (BE-AES) was created (Bai et al, 2007). Measurements of the ion atmosphere done with BE-AES provide a rigorous thermodynamic measure of the number of ions associated with a nucleic acid. BE-AES is sensitive to a wide number of elements and has proven to... 

Inductively coupled plasma atomic emission spectroscopy (ICP-AES) and x-ray fluorescence spectrometry (XRFS) are also used for elemental determination in environmental studies, although they are generally less sensitive than ICP-MS techniques. 

The determination of trace metal impurities in pharmaceuticals requires a more sensitive methodology. Flame atomic absorption and emission spectroscopy have been the major tools used for this purpose. Metal contaminants such as Pb, Sb, Bi, Ag, Ba, Ni, and Sr have been identified and quantitated by these methods (59,66-68). Specific analysis is necessary for the detection of the presence of palladium in semisynthetic penicillins, where it is used as a catalyst (57), and for silicon in streptomycin (69). Furnace atomic absorption may find a significant role in the determination of known impurities, due to higher sensitivity (Table 2). Atomic absorption is used to detect quantities of known toxic substances in the blood, such as lead (70-72). If the exact impurities are not known, qualitative as well as quantitative analysis is required, and a general multielemental method such as ICP spectrometry with a rapid-scanning monochromator may be utilized. Inductively coupled plasma atomic emission spectroscopy may also be used in the analysis of biological fluids in order to detect contamination by environmental metals such as mercury (73), and to test serum and tissues for the presence of aluminum, lead, cadmium, nickel, and other trace metals (74-77). 

The separation of yttrium from the lanthanides is performed by selective oxidation, reduction, fractionated crystallization, or precipitation, ion-exchange and liquid-liquid extraction. Methods for determination include arc spectrography, flame photometry and atomic absorption spectrometry with the nitrous oxide acetylene flame. The latter method improved the detection limits of yttrium in the air, rocks and other components of the natural environment (Deuber and Heim 1991 Welz and Sperling 1999).Other analytical methods useful for sensitive monitoring of trace amounts of yttrium are X-ray emission spectroscopy, mass spectrometry and neutron activation analysis (NAA) the latter method utilizes the large thermal neutron cross-section of yttrium. For high-sensitivity analysis of yttrium, inductively coupled plasma atomic emission spectroscopy (ICP-AES) is especially recommended for solid samples, and inductively coupled plasma mass spectroscopy (ICP-MS) for liquid samples (Reiman and Caritat 1998). 

Once in solution, the preferred method for measurement of boron is inductively coupled plasma atomic emission spectroscopy (ICP-AES) or inductively coupled plasma mass spectrometry (ICP-MS). The most widely used nonspectrophotometric method for analysis of boron is probably ICP-MS because it uses a small volume of sample, is fast, and can detect boron concentrations down to 0.15 pgL . When expensive ICP equipment is not available, colorimetric or spectrophotometric methods can be used. However, these methods are often subject to interference (e.g., nitrate, chloride, fluoride), and thus must be used with caution. Azomethine-H has been used to determine boron in environmental samples (Lopez et al. 1993), especially water samples. Another simple, sensitive spectrophotometric method uses Alizarin Red S (Garcia-Campana et al. 1992). 

Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and mass spectrometric (ICP-MS) techniques are modern, sophisticated, very sensitive, and generally quite expensive. These methods have been used successfully for the determination of metals in tobacco and tobacco smoke (20A90, 20A112). The success of these two methods can be affected by the metal-solvent matrix employed. In many cases preseparation procedures and other special handling techniques are required. Each method for analysis of metals in tobacco and tobacco smoke has its own advantages and challenges. 

Since only the electronic levels of the sample are probed with this technique, many parameters of interest to the material cannot be direcdy assessed. For example, morphology or structural information of the material cannot be determined through luminescence measurements. In addition, the specific nature and location of the observed defects described above cannot be determined without more sophisticated luminescence microscopy measurements. Also, the material must be inherently luminescent to provide any useful information from this technique. This luminescence may not be visible at ambient temperatures, however some sample cooling might be required to obtain usable data. With these limitations in mind, however, the solid-state luminescence technique has many benefits. Many of these advantages are also commonly profiled in molecular or atomic emission spectroscopy treatments. Luminescence is one of the most sensitive of analytical techniques, with a large linear concentration range and very low limits of detection. 

The hottest flames generally used in atomic absorption and emission spectroscopy rarely reach temperatures of 4000 K. It is apparent from the data in Table 10.1 that even at the highest temperature, the excited-state population is very small in comparison to the ground-state population. This is true even for the relatively easily excited alkali metals, which are readily determined by atomic emission spectroscopy. Elements such as zinc show poor sensitivity by atomic emission because an extremely small number of the atoms is thermally excited. 

Why, then, do the alkali metals exhibit good sensitivity by atomic emission spectroscopy The answer is that one measures the difference between a theoretically zero signal in the absence of the sample and a finite signal in the presence of the sample. Therefore, the small signal arising from the sample can be readily amplified and measured. The limit of detection is governed by the noise level of the photo-... 

The analytical application of atomic-absorption or atomic-emission spectroscopy generally involves obtaining the sample in an appropriate solution for measurement and calibrating the instrument properly. Commonly used methods for different materials are described below. Frequently, a releasing agent will have to be added, or a solvent extraction will be required to concentrate the element and increase the sensitivity. Standards should be treated in a similar manner. 

Inconspicuous instrumental, environmental, or chemical effects often cause a loss of instrument response. In atomic emission spectroscopy, for example, sensitivity is affected by such instrumental factors as flame temperature, aspiration rate, and slit width. In amperometric measurements, diffusion currents vary with temperature, and a significant loss in sensitivity may occur with a drop in sample temperature. In ion-selective electrode measurements, sensitivity may be affected by chemical effects, such as changes in ionic strength or pH. 

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