Ions flame emission

Oxygen and nitrogen also are deterrnined by conductivity or chromatographic techniques following a hot vacuum extraction or inert-gas fusion of hafnium with a noble metal (25,26). Nitrogen also may be deterrnined by the Kjeldahl technique (19). Phosphoms is determined by phosphine evolution and flame-emission detection. Chloride is determined indirecdy by atomic absorption or x-ray spectroscopy, or at higher levels by a selective-ion electrode. Fluoride can be determined similarly (27,28). Uranium and U-235 have been determined by inductively coupled plasma mass spectroscopy (29). 

With flame emission spectroscopy, there is greater likelihood of spectral interferences when the line emission of the element to be determined and those due to interfering substances are of similar wavelength, than with atomic absorption spectroscopy. Obviously some of such interferences may be eliminated by improved resolution of the instrument, e.g. by use of a prism rather than a filter, but in certain cases it may be necessary to select other, non-interfering, lines for the determination. In some cases it may even be necessary to separate the element to be determined from interfering elements by a separation process such as ion exchange or solvent extraction (see Chapters 6, 7). 

Occasionally, separation, e.g. by solvent extraction or by an ion exchange process, may be necessary to remove an interfering element such separations are most frequently necessary when dealing with flame emission spectroscopy. 

Finally, and apart from the importance of micelles in the solubilization of chemical species, mention should also be made of their intervention in the displacement of equilibria and in the modification of kinetics of reactions, as well as in the alteration of physicochemical parameters of certain ions and molecules that affect electrochemical measurements, processes of visible-ultraviolet radiation, fluorescence and phosphorescence emission, flame emission, and plasma spectroscopy, or in processes of extraction, thin-layer chromatography, or high-performance liquid chromatography [2-4, 29-33],... 

Atomic absorption spectrometry is one of the most widely used techniques for the determination of metals at trace levels in solution. Its popularity as compared with that of flame emission is due to its relative freedom from interferences by inter-element effects and its relative insensitivity to variations in flame temperature. Only for the routine determination of alkali and alkaline earth metals, is flame photometry usually preferred. Over sixty elements can be determined in almost any matrix by atomic absorption. Examples include heavy metals in body fluids, polluted waters, foodstuffs, soft drinks and beer, the analysis of metallurgical and geochemical samples and the determination of many metals in soils, crude oils, petroleum products and plastics. Detection limits generally lie in the range 100-0.1 ppb (Table 8.4) but these can be improved by chemical pre-concentration procedures involving solvent extraction or ion exchange. 

Recalling Figure 9.4, we know that thermal energy sources, such as a flame, atomize metal ions. But we also know that that these atoms experience resonance between the excited state and ground state such that the emissions that occur when the atoms drop from the excited state back to the ground state can be measured. While there are several techniques that measure such emissions, including flame emissions... 

The technique of flame emission spectroscopy is used to determine the concentration of Ba, K, and Na ions by measuring the intensity of emission at a specific wavelength by the atomic vapour of the element generated from calcium acetate i.e., by introducing its solution into a flame. 

Johnson KE, Yerhoff FW, Robinson J, et al. 1983. Determination of barium at ng ml 1 levels by flame emission spectrometry after ion-exchange separation from 1000-fold amounts of calcium. 

A 250 mL sample of each solution from the polyethylene bottle was filtered through a Millipore filter (0.45 urn pore size). The concentrations of chloride, nitrate and sulfate ions in the filtrate were determined by ion chromatography using a YEW IC 100 of Yokogawa Hokushin Electric Co. Ltd. The concentrations of sodium and potassium were determined by flame emission spectrometry and concentrations of calcium and magnesium by atomic absorption spectrometry using a Hitachi 170-50 Atomic Absorption Spectrophotometer. An aliquot of each filtrate was used for the determination of Sr by ICP emission spectrometry after adding nitric acid (0.1 N), detailed analytical conditions of which are reported elsewhere (3). 

Figure 27-1 Predicted Influence of water content on sodium measurements for a lOOmmol/L NaCi solution by direct ion-selective electrode (tSE versus flame emission photometry or indirect ISE). Hatched areas represent nonaqueous volumes, which could consist of lipids, proteins, or even a slurry of latex or sand particles. (From Apple FS, Koch DD, Graves S, Ladenson JH. Relationship between d/rect-potent/ometric and flame-photometric measurement of sodium in blood. Clin Chem 1982 28 1931-5.)...

The concentration of lithium in serum, plasma, urine, or other body fluids has been determined by flame emission photometry, atomic absorption spectrometry, or electro-chemically using an ion-selective electrode. Serum analysis, the most useful specimen for lithium monitoring, is most commonly quantified by automated spectrophotometric assay. 

If the measured Na concentration in plasma is decreased, but measured plasma osmolahty, glucose, and urea are normal, the only explanation is pseudohyponatremia caused by the electrolyte exclusion effect see Chapter 27). This occurs when Na" is measured by either flame emission spectrophotometry or by an indirect ion-selective electrode in patients with severe hyperlipidemia or in states of hyperproteinemia (e.g., paraproteinemia of multiple myeloma). 

For routine flame-emission determinations of alkali metals and alkaline earth elements, simple filter photometers often suffice. A low-temperature flame is employed to prevent excitation of most other metals. As a consequence, the spectra are simple, and interference filters can be used to isolate the desired emission lines. Flame emission was once widely used in the clinical laboratory for the determination of sodium and potassium. These methods have largely been replaced by methods using ion-selective electrodes (see Section 2 ID). 

Reference 9 gives a review of applications of atomic absorption spectrophotometry to biological samples. Tiiis technique is widely used for metal analysis in biological fluids and tissues, in environmental samples such as air and water, and in occupational health and safety areas. Routine applications of flame emission spectrometry to biological samples are generally limited to the alkali and alkaline earth metals. Ion-selective electrode measurements (Chapter 13) have largely replaced the flame emission measurements in the clinical chemistry laboratory. 

With atomic fluorescence, the fluorescence signal is added to a constant small background signal which permits improvement of the fluorescence signal by electronic amplification until the system becomes noise-limited. This advantage is also inherent to flame emission however, with atomic absorption a difference measurement is made such that, as the concentration of the ion decreases, the signals of the blank and sample approach each other. A theoretical comparison of the sensitivities of the different flame methods has been presented by Winefordner (9). 

Atomic fluorescence flame spectrometry is receiving increased attention as a potential tool for the trace analysis of inorganic ions. Studies to date have indicated that limits of detection comparable or superior to those currently obtainable with atomic absorption or flame emission methods are frequently possible for elements whose emission lines are in the ultraviolet. The use of a continuum source, such as the high-pressure xenon arc, has been successful, although the limits of detection obtainable are not usually as low as those obtained with intense line sources. However, the xenon source can be used for the analysis of several elements either individually or by scanning a portion of the spectruin. Only chemical interferences are of concern they appear to be qualitatively similar for both atomic absorption and atomic fluorescence. With the current development of better sources and investigations into devices other than flames for sample introduction, further improvements in atomic fluorescence spectroscopy are to be expected. 

More than sixty elements can be determined by atomic-absorption or flame-emission spectroscopy, many at or below about 1 ppm [4]. Only metals and metalloids can be determined by usual flame methods, because the resonance lines for nonmetals occur in the vacuum-ultraviolet region however, a number of indirect methods for determining nonmetals have been described. For example, chloride can be determined by precipitating it with silver ion and then measuring either the excess or the reacted silver. Phosphorus (525.9 nm) and sulfur (383.7 nm) species (e.g., Sj) exhibit sharp molecular-band emission in the argon-hydrogen flame. 

Analytical schemes concerned with the determination of blood ions and gases can be divided into two categories analyses done in vivo and those done in vitro. By far the most common method of determining blood ions in vitro involves atomic spectroscopy. Atomic absorption and flame emission have both been used although the latter is the most popular. In the clinical lab nearly all of the remaining determinations (both in vivo and in vitro) are performed with ion-selective (for ions, NH3 and CO2) or amperometric electrodes (O2 and H2). Two important characteristics of ion-selective electrodes, sensitivity and selectivity, should be mentioned. The applicability of a specific electrode in any particular situation can be determined by considering, on one hand, the ionic constituents of the solution to be measured and, on the other hand, the sensitivity and specificity of the electrode in question. Proper consideration of these points will allow an investigator to determine the accuracy and validity of the measurement. 

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