Nebulizers atomic emission spectroscopy

For inductively coupled plasma atomic emission spectroscopy (ICP-AES) the sample is normally in solution but may be a fine particulate solid or even a gas. If it is a solution, this is nebulized, resulting in a fine spray or aerosol, in flowing argon gas. The aerosol is introduced into a plasma torch, illustrated in Figure 3.21. 

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. 

McKinnon P. W., Giess K. C. and Knight T. V. (1981) A clog-free nebulizer for use in inductively coupled plasma-atomic emission spectroscopy, in Barnes R. M. (ed) Developments in atomic plasma spectrochemical analysis. Heyden, London, 287-301. 

D. T. Gjerde, D. Wiederin, F.G. Smith, and B.M. Mattson, Metal speciation by means of microbore columns with direct-injection nebulization by inductively coupled plasma atomic emission spectroscopy, / Chromatogr., 73, 1993. 

K. E. LaFreniere, G. W. Rice, and V. A. Fassel, Flow Injection Analysis with Inductively Coupled Plasma-Atomic Emission Spectroscopy Critical Comparison of Conventional Pneumatic, Ultrasonic and Direct Injection Nebulization. Spectrochim. Acta Pt. B—At. Spec., 40 (1985) 1495. 

Typical biological fluids include blood and blood serum, blood plasma, urine and saliva. Measurement of calcium in serum was the first analysis to which the technique of AAS was applied and is an obvious example of how FAAS is useful for biomedical analysis. Other specimens e.g. dialysis fluids, intestinal contents, total parenteral nutrition solutions, may be analysed on rare occasions. Elements present at a sufficiently high concentration are lithium and gold when used to treat depression and rheumatoid arthritis respectively, and calcium, magnesium, iron, copper and zinc. Sodium and potassium can be determined by FAAS but are more usually measured by flame atomic emission spectroscopy or with ion selective electrodes. Other elements are present in fluids at too low a concentration to be measured by conventional FAAS with pneumatic nebulization. With other fluids, e.g. seminal plasma, cerebrospinal fluid, analysis may just be possible for a very few elements. 

In ICP-AES analysis, the liquid sample (i.e., solution) is nebulized into an inductively coupled plasma it has sufficient energy to break chemical bonds, liberate elements, and transform them into a gaseous atomic state for atomic emission spectroscopy. When this happens, a number of the elemental atoms will be excited and emit radiation. The wavelength of this radiation is characteristic of the element that emits it, and the intensity of radiation is proportional to the concentration of that element within the solution. The ICP-AES is used for both qualitative element identification and quantitative chemical composition determination. 

Direct nebulization of an aqueous or organic phase containing extracted analytes has been widely used in flame atomic absorption spectroscopy [69-72], inductively coupled plasma atomic emission spectrometry [73-76], microwave induced plasma atomic emission spectrometry [77-80] and atomic fluorescence spectrometry [81], as well as to interface a separation step to a spectrometric detection [82-85]. 

Despite the enormous benefits of the fledgling field of spectroscopy for chemistry, the underlying physical processes were completely unknown a century ago. It was believed that the characteristic frequencies of elements were caused by (nebulously defined) vibrations of the atoms, but even a remotely satisfactory quantitative theory proved to be elusive. In 1885, the Swiss mathematician Balmer noted that wavelengths in the visible region of the hydrogen atom emission spectmm could be fitted by the empirical equation... 

The history of flame spectroscopy is difficult to separate from the other types of emission spectroscopy, but it does have some unique features. The analytical use of flame spectra has a long history since the time of Bunsen and Kirchhoff. De Gramont made a major contribution in 1923 by introducing the very hot oxygen-acetylene flame. Lvmdegardh constructed the first flame photometer in 1928 with an atomizer (now called a nebulizer), a pressure control unit, and a photocell for detection. Ne-bulization was used as early as 1879, by Gouy when... 

In a flame atomizer, a solution of the sample is nebulized by a flow of gaseous oxidant, mixed with a gaseous fuel, and carried into a flame where atomization occurs. As shown in Figure 9-1, a complex set of interconnected processes then occur in the flame. The first is desolvation, in which the solvent evaporates to produce a finely divided solid molecular aerosol. The aerosol is then volatilized to form gaseous molecules. Dissociation of most of these molecules produces an atomic gas. Some of the atoms in the gas ionize to form cations and electrons. Other molecules and atoms are produced in the flame as a result of interactions of the fuel with the oxidant and with the various species in the sample. As indicated in Figure 9-1, a fraction of the molecules, atoms, and ions are also excited by the heat of the flame to yield atomic, ionic, and molecular emission spectra. With so many complex processes occurring, it is not surprising that atomization is the most critical step in flame spectroscopy and the one that limits the precision of such methods. Because of the critical nature of the atomization step, it is important to understand the characteristics of flames and the variables that affect these characteristics. 

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