Flame atomic emission spectrometers

Flame atomic emission spectrometers have separate controls for maintaining gas pressures, selecting wavelengths and adjusting slit widths (optimal values depend on the wavelength examined figures are given in the instrument handbook). The power control allows the adjustment of (relative) radiation intensities (readouts). 

A flame atomic emission spectrometer or flame photometer incorporates a burner, monochromator, or filters, a detector and a method of introducing the sample solution into the flame. 

Flame atomic emission spectrometers have similar optical systems to those of UV-visible spectrometers, but the source of radiation is provided by the sample itself. A flame photometer is a simpler instrument emplopng narrow bandpass optical filters in place of a monochromator Fig. 1). The sample is prepared as a solution, which is drawn into a nebulizer by the effect of the flowing oxidant and fuel gases. The fine droplets produced pass into the flame where sample atoms are progressively excited. The emitted radiation passes through the monochromator or Alter and is detected by a photocell or photomultiplier tube. 

At present, however, the usual flame emission method is obtained by simply operating a flame atomic absorption spectrometer in the emission mode (see Fig. 21.3). 

A major breakthrough came in Australia when Alan Walsh1,2 realized that light sources were available for many elements which emitted atomic spectral lines at the same wavelengths as those at which absorption occurred. By selecting appropriate sources, the emission line widths could be even narrower than the absorption line widths (Figure 2). Thus the sensitivity problem was solved more or less at a stroke, and the modern flame atomic absorption spectrometer was bom. 

The association of a spectrometer with a liquid chromatograph is usually to aid in structure elucidation or the confirmation of substance identity. The association of an atomic absorption spectrometer with the liquid chromatograph, however, is usually to detect specific metal and semi-metallic compounds at high sensitivity. The AAS is highly element-specific, more so than the electrochemical detector however, a flame atomic absorption spectrometer is not as sensitive. If an atomic emission spectrometer or an atomic fluorescence spectrometer is employed, then multi-element detection is possible as already discussed. Such devices, used as a LC detector, are normally very expensive. It follows that most LC/AAS combinations involve the use of a flame atomic absorption spectrometer or an atomic spectrometer fitted with a graphite furnace. In addition in most applications, the spectrometer is set to monitor one element only, throughout the total chromatographic separation. 

Why are monochromators of a higher resolution found in ICP atomic emission spectrometers than in flame atomic absorption spectrometers ... 

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 human eye is a useful detector for qualitative analysis but not for quantitative analysis. Replacing the human eye with a spectrometer and photon detector such as a PMT or CCD permits more accurate identification of the elements present because the exact wavelengths emitted by the sample can be determined. In addition, the use of a photon detector permits quantitative analysis of the sample. The wavelength of the radiation indicates what element is present, and the radiation intensity indicates how much of the element is present. Flame atomic emission spectrometry is particularly useful for the determination of the elements in the first two groups of the periodic table, including sodium, potassium, lithium, calcium, magnesium, strontium, and barium. The determination of these elements is often called for in medicine, agriculture, and animal science. Remember that the term spectrometry is used for quantitative analysis by the measurement of radiation intensity. 

Warm up the flame photometer or flame atomic absorption spectrometer in emission mode, following manufacturer s directions. Using an air-acetylene burner, set the flow rates of air acetylene to (a) oxidizing flame, (b) stoichiometric flame, and (c) reducing flame, following manufacturer s directions. 

Spectral emission lines in flames are the same width as atomic absorption lines in flames, on the order of 0.01 nm. Why is the spectral bandpass in an atomic emission spectrometer much smaller than that in an AAS ... 

The majority of commercial atomic absorption spectrometers permit both flame atomic absorption and flame atomic emission measurements to be performed. Thus, flame AES is no longer considered as an independent instrumental technique, except for the determination of sodium and potassium (as well as calcium or lithium) in biological samples by flame photometers. 

Flame AAS may be used to determine the concentration of most metals by using an acetylene-based flame. An instrumental arrangement for a flame atomic absorption spectrometer is presented in Fig. 11. There are virtually no spectral interferences that affect AAS. Nonetheless, the main limit to analysis by AAS is due to physico-chemical interferences. The proliferation of ICP instruments has significantly reduced the overall application of flame AAS in most industrial applications. More than likely, the use of flame atomic absorption will dwindle to only a few specialized applications (similar to flame emission instruments). 

Although flame emission measurements can be made by using an atomic absorption spectrometer in the emission mode, the following account refers to the use of a simple flame photometer (the Coming Model 410 flame photometer). Before attempting to use the instrument read the instruction manual supplied by the manufacturers. 

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. 

Potassium at trace concentrations in aqueous samples can be measured by a flame photometer at a wavelength of 766.5 nm. Either a flame photometer or an atomic absorption spectrometer operating in flame emission mode can be used for such analysis. 

Optical-Emission Spectrometer. Similar to flame photometer (atomic-absoiption photometer) except that an electric spark rather than a flame is used to vaporize (atomize) unknown samples. 

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