Temperatures flame atomic absorption spectroscopy

The hemicellulose and pectin, weighed at 100, 200, and 300 mg. Then each was dissolved into erlenmeyer containing 25 cm solution 30 mg/cm of cadmium in 0.1 N nitric acid and the pH adjusted to acid pH was performed at pH 2. The solution was stirred with a magnetic stirrer at room temperature for 2 h. Then, each was centrifuged at 10,000 rpm for 30 min. The supernatant was taken and diluted with 0.5 to 50 cm aquabidest. Then 0.25 cm of dilution was taken and diluted to 25 cm with aquabidest. The cadmium levels were measured [34]. The cadmium in the supernatant was estimated using flame atomic absorption spectroscopy [Hitachi Analyst 100) at 228.3 nm [23, 32, 34,51]. 

Instead of employing the high temperature of a flame to bring about the production of atoms from the sample, it is possible in some cases to make use of either (a) non-flame methods involving the use of electrically heated graphite tubes or rods, or (b) vapour techniques. Procedures (a) and (b) both find applications in atomic absorption spectroscopy and in atomic fluorescence spectroscopy. 

Essentially the same spectrometer as is used in atomic absorption spectroscopy can also be used to record atomic emission data, simply by omitting the hollow cathode lamp as the source of the radiation. The excited atoms in the flame will then radiate, rather than absorb, and the intensity of the emission is measured via the monochromator and the photomultiplier detector. At the temperature achieved in the flame, however, very few of the atoms are in the excited state ( 10% for Cs, 0.1% for Ca), so the sample atoms are not normally sufficiently excited to give adequate emission intensity, except for the alkali metals (which are often equally well determined by emission as by absorption). Nevertheless, it can be useful in cases where elements are required for which no lamp is available, although some elements exhibit virtually no emission characteristics at these temperatures. 

Atomic absorption spectroscopy is highly specific and there are very few cases of interference due to the similar emission lines from different elements. General interference effects, such as anionic and matrix effects, are very similar to those described under flame emission photometry and generally result in reduced absorbance values being recorded. Similarly, the use of high temperature flames may result in reduced absorbance values due to ionization effects. However, ionization of a test element can often be minimized by incorporating an excess of an ionizable metal, e.g. potassium or caesium, in both the standards and samples. This will suppress the ionization of the test element and in effect increase the number of test atoms in the flame. 

Fruchier rial. (1980), determined by X-ray fluorescence IXRF), except Aland Naby ncuiran activation analysis (NA At. Mg by flame atomic absorption tlidnum borate fusion (FAA), and B by plasma emission spectroscopy (sodium carbonate fusion) (PE5) Saether (1980), determined by XRF after low-temperature ashing (LTA) of raw oil shale samples In = 10). 

FAA FA FBC FC FEBEX FFFF FGD FP FSU FT FTIR FUETAP Flame atomic absorption Fly ash Fluidized bed combustion Filter cake Full-scale engineered barriers experiment (in crystalline host rock) Flow-field flow fractionation Flue gas desulphurization Fission products Former Soviet Union Fourier transforms Fourier transformed infrared spectroscopy Formed under elevated temperature and pressure... 

The inductively coupled plasma13 shown at the beginning of the chapter is twice as hot as a combustion flame (Figure 21-11). The high temperature, stability, and relatively inert Ar environment in the plasma eliminate much of the interference encountered with flames. Simultaneous multielement analysis, described in Section 21 1. is routine for inductively coupled plasma atomic emission spectroscopy, which has replaced flame atomic absorption. The plasma instrument costs more to purchase and operate than a flame instrument. 

How would emission intensity be affected by a 10 K rise in temperature In Figure 21-14, absorption arises from ground-state atoms, but emission arises from excited-state atoms. Emission intensity is proportional to the population of the excited state. Became the excited-state population changes by 4% when the temperature rises 10 K, emission intensity rises by 4%. It is critical in atomic emission spectroscopy that the flame be very stable or emission intensity will vary significantly. In atomic absorption spectroscopy, temperature variation is important but not as critical. 

The development of fast and accurate procedures for the determination of calcium in biological materials represents one of the important early achievements of atomic absorption spectroscopy. The diflBculties encountered with calcium in emission flame photometry are well known (Dll, L6, S6, SIO), but spectral interferences and extreme dependency on flame temperature, serious obstacles in emission, are either nonexistent or of lower importance in absorption. Chemical interferences, however. 

W21. Willis, J. B., Atomic absorption spectroscopy with high temperature flames. Appl. Opt. 7, 1295-1304 (1968). 

Amos M. D. and Willis J. B. (1966) Use of high-temperature pre-mixed flames in atomic absorption spectroscopy, Spectrochim Acta 22 1325— 1343. 

Atomic spectroscopy can be divided into several broad classes based on the nature of the means of exciting the sample. One of these classes is generally known as atomic emission spectroscopy, in which excitation is thermally induced by exposing the sample to very high electric fields. Another class is known as flame emission spectroscopy or flame photometry, in which excitation is thermally induced by exposing the sample to a high-temperature flame. These methods differ from atomic absorption spectroscopy, in which the absorption of light from a radiation source by the atom is observed rather than the emission from the electronically excited atom. 

Atomization of the sample is usually facilitated by the same flame aspiration technique that is used in flame emission spectrometry, and thus most flame atomic absorption spectrometers also have the capability to perform emission analysis. The previous discussion of flame chemistry with regard to emission spectroscopy applies to absorption spectroscopy as well. Flames present problems for the analysis of several elements due to the formation of refractory oxides within the flame, which lead to nonlinearity and low limits of detection. Such problems occur in the determination of calcium, aluminum, vanadium, molybdenum, and others. A high-temperature acetylene/nitrous oxide flame is useful in atomizing these elements. A few elements, such as phosphorous, boron, uranium, and zirconium, are quite refractory even at high temperatures and are best determined by nonflame techniques (Table 2). 

The Sampling Boat. The author has had some success with a different approach. Here, the sample is loaded into a narrow boat-shaped vessel, made out of tantalum or a similar material, and dried. The boat is then placed into the middle of a standard air-acetylene flame. For lead, selenium, cadmium, silver, and zinc, encouraging results have been obtained. The detection limit for lead, for example, is about one-fifteenth that of conventional atomic absorption spectroscopy. However, at least at the present stage of development, drawbacks exist here also. Interferences are multiplied, because of the low temperatures at which atoipiza-... 

Why is a high-temperature flame, for example, the nitrous oxide-acetylene flame, sometimes required in atomic-absorption spectroscopy ... 

The relative number of atoms in a particular energy state can be determined by use of the Boltzmann equation [refer to equation (2-23)]. Walsh has calculated these ratios for the lowest excited states of several typical elements and several flame temperatures. Table 9-2 indicates that the number of atoms in the ground state is much greater than the number in the lowest excited state at temperatures commonly used in atomic absorption spectroscopy. 

Early work in atomic absorption spectroscopy used the lower temperature flames thus the method was restricted to those elements that could be converted to atoms at lower temperatures. Since some metals, such as molybdenum, rhenium, and tin, are only partly converted to gaseous atoms in low temperature flames, higher temperature flames were developed with success. 

The high-temperature flames, however, did not solve all the problems of atomic absorption spectroscopy for many elements. If excess oxygen is present in a flame, a significant fraction of certain metal elements is converted to oxides. If the oxide is particularly stable, it is not redissociated into atoms. Thus the refractory oxides, such as MgO, CaO, AlO, MoO, and others, resist decomposition even in high temperature flames. To inhibit formation of these oxides, it is possible to use a fuel-rich flame to produce a reducing ... 

Amos and Willis suggested another combination of fuel and oxidant, acetylene and nitrous oxide, as another approach to the analysis of refractory elements. The combination of acetylene and nitrous oxide produces a high-temperature flame (2950°C) with little free oxygen to react with the metal elements. This flame is now very successfully used in atomic absorption spectroscopy and permits satisfactory atomic absorption analysis for many refractory elements. Use of nitrous oxide and acetylene requires a burner head that will withstand the temperatures produced by the flame. A common burner head for this combination of fuel and oxidant is 5 cm long and 0.05 cm wide. 

Other physical interferences are similar to those observed in flame emission and atomic absorption spectroscopy. They include effects due to viscosity and temperature of the sample solution. Any factor that can alter the rate of uptake of the sample solution requires control. The best method to use to control these effects is to prepare a blank with physical properties similar to those of the test sample. 

Early in the development of atomic absorption spectroscopy it was recognized that enhanced absorbances could be obtained if the solutions contained low-molecular-weight alcohols, esters, or ketones. The effect of organic solvents is largely attributable to increased nebulizer efficiency the lower surface tension of sueh solutions results in smaller drop sizes and a resulting increase in the amount of sample that reaches the flame. In addition, more rapid solvent evaporation may also contribute to the effect. Leaner fuel-oxidant ratios must be used with organic solvents to offset the presence of the added organic material. Unfortunately, however, the leaner mixture produces lower flame temperatures and an increased potential for chemical interferences. 

Atomic absorption spectroscopy instruments place a sample in a high temperature flame that yields atomic species and passes selected, element specific, illumination through the flame to detect what wavelengths of light the sample atoms absorb. Either acetylene or nitrous oxide fuels the analytical flame. This process again demands that solid samples be digested (dissolved in an acid or fused with a salt) and dissolved to form a solution that can be aspirated or sprayed into the instrument s flame, while protecting the sample material from contamination or adulteration. 

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