Molecular flame emission

FIGURE 8-8 Molecular flame emission and flame absorption spectra for CaOH. Atomic emission wavelength of barium is also indicated. (Adapted from L. Capacho-Delgado and S. Sprague, Atomic Absorption Newsletter, 1965. 4. 363. Courlesy of Perkin-Elmer Corporation. Norwalk. CT. ... 

Common gas chromatographic detectors that are not element- or metal-specific, atomic absorption and atomic emission detectors that are element-specific, and mass spectrometric detectors have all been used with the hydride systems. Flame atomic absorption and emission spectrometers do not have sufficiently low detection limits to be useful for trace element work. Atomic fluorescence [37] and molecular flame emission [38-40] were used by a few investigators only. The most frequently employed detectors are based on microwave-induced plasma emission, helium glow discharges, and quartz tube atomizers with atomic absorption spectrometers. A review of such systems as applied to the determination of arsenic, associated with an extensive bibliography, is available in the literature [36]. In addition, a continuous hydride generation system was coupled to a direct-current plasma emission spectrometer for the determination of arsenite, arsenate, and total arsenic in water and tuna fish samples [41]. 

In principle, emission spectroscopy can be applied to both atoms and molecules. Molecular infrared emission, or blackbody radiation played an important role in the early development of quantum mechanics and has been used for the analysis of hot gases generated by flames and rocket exhausts. Although the availability of FT-IR instrumentation extended the application of IR emission spectroscopy to a wider array of samples, its applications remain limited. For this reason IR emission is not considered further in this text. Molecular UV/Vis emission spectroscopy is of little importance since the thermal energies needed for excitation generally result in the sample s decomposition. 

Minimizing Spectral Interferences The most important spectral interference is a continuous source of background emission from the flame or plasma and emission bands from molecular species. This background emission is particularly severe for flames in which the temperature is insufficient to break down refractory compounds, such as oxides and hydroxides. Background corrections for flame emission are made by scanning over the emission line and drawing a baseline (Figure 10.51). Because the temperature of a plasma is... 

The optical path for flame AA is arranged in this order light source, flame (sample container), monochromator, and detector. Compared to UV-VIS molecular spectrometry, the sample container and monochromator are switched. The reason for this is that the flame is, of necessity, positioned in an open area of the instrument surrounded by room light. Hence, the light from the room can leak to the detector and therefore must be eliminated. In addition, flame emissions must be eliminated. Placing the monochromator between the flame and the detector accomplishes both. However, flame emissions that are the... 

The best flame emission in the blue region of the visible spec-tmm (435-480 nanometers) is obtained from copper monochloride, CuCl. Flame emission from this molecular species yields a series of bands in the region from 428-452 nanometers, with additional peaks between 476-488 nanometers [1, 11]. ... 

The best flame emission in the red region of the visible spectrum is produced by molecular strontium monochloride, SrCl. This species - unstable at room temperature - is generated in the pyrotechnic flame by a reaction between strontium and chlorine atoms. Strontium dichloride, SrCl 2, would appear to be a logical precursor to SrCl, and it is readily available commercially, but it is much too hygroscopic to use in pyrotechnic mixtures. 

The photomultiplier detects both the thermal emission from the determinant and also any other atomic or molecular emission from either concomitant elements present in the sample or from the flame itself. Figure 8, for example, shows a typical section of a flame emission spectrum. While it is possible for some determinations by FES to work at a single fixed wavelength, as in flame AAS, it is advisable, at least initially, to scan the emission spectrum in the vicinity of the wavelength of interest to confirm the absence of spectral interferences. In any event, regular re-zeroing and aspiration of an appropriate standard to check for signal drift is essential. 

Both atomic and molecular emission and absoiption can be measured when a sample is atomized in a flame. A typical flame-emission spectrum was shown in Figure 24-19. Atomic emissions in this spectrum are made up of narrow lines, such as that for sodium at about 330 nm, potassium at approximately 404 nm, and calcium at 423 nm. Atomic spectra are thus called line spectra. Also present are emission bands that result from excitation of molecular species such as MgOH, MgO, CaOH, and OH. Here, vibrational transitions superimposed on electronic transitions produce... 

Molecular band emission can also cause a blank interference. This is particularly troublesome in flame spectrometry, where the lower temperature and reactive atmosphere are more likely to produce molecular species. As an example, a high concentration of Ca in a sample can produce band emission from CaOH, which can cause a blank interference if it occurs at the analyte wavelength. Usually, improving the resolution of the spectrometer will not reduce band emission, since tbe narrow analyte lines are superimposed on a broad molecular emission band. Flame or plasma background radiation is generally well compensated by measurements on a blank solution. 

In the early years of flame photometry, only relatively cool flames were used. We shall see below that only a small fraction of atoms of most elements is excited by flames and that the fraction excited increases as the temperature is increased. Consequently, relatively few elements have been determined routinely by flame emission spectrometry, especMly j ew of those that emit line spectra (several can exist in flames as molecular species, particularly as oxides, which emit molecular band spectra). Only the easily excited alkali metals sodium, potassium, and lithium are routinely deterniined by flame emission spectrometry in the clinical laboratory. However, with flames such as oxyacetylene and nitrous oxide-acetylene, over 60 elements can now be determined by flame emission spectrometry. This is in spite of the fact that a small fraction of excited atoms is available for emission. Good sensitivity is achieved because, as with fluorescence (Chapter 16), we are, in principle, measuring the difference between zero and a small but finite signal, and so the sensitivity is limited by the response and stability of the detector and the stability (noise level) of the flame aspiration system. 

Emission spectrometry using chemical flames (flame atomic emission spectrometry, FAES) as excitation sources is the earlier counterpart to flame atomic absorption spectrometry. In this context emission techniques involving arc/spark and direct or inductively coupled plasma for excitation are omitted and treated separately. Other terms used for this technique include optical emission, flame emission, flame photometry, atomic emission, and this technique could encompass molecular emission, graphite furnace atomic emission and molecular emission cavity analysis (MEGA). 

FIGURE 6-4 A portion of the flame emission speclrum for sodium, 800 ppm in naphtha isopropanol oxyhydrogen flame slit 0.02 mm. Note Chat the scale is expanded in the upper trace and flame conditions were changed to reveal greater detail for Na lines, but not for molecular bands. Note also that the lines at 589.00 and 589.59 nm are off scale in the upper trace. (Adapted from C. T. J. Alkemade and R. Herrmann, Fundamentdis oMna/yfrca/ Flams Specfroscopy, p. 229, New York Wiley. 1979, with permission.)... 

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. 

The basic instrumentation for atomic-fluorescence spectroscopy is shown in Figure 10.13. The source is placed at right angles to the monochromator so that its radiation (except for scattered radiation) does not enter the monochromator. The source is chopped to produce an AC signal and minimize flame-emission interference. As in molecular fluorescence (Chap. 9), the intensity of atomic fluorescence is directly proportional to the intensity of the light impinging on the sample from the source. 

The magnitude of the atomic absorption signal is directly related to the number of ground state atoms in the optical path of the spectrometer. Ground state atoms are produced from the sample material, usually by evaporation of solvent and vaporization of the solid particle followed by decomposition of the molecular species into neutral atoms. Normally these steps are carried out using an aspirator and flame. These are the same processes that are involved in flame emission spectroscopy as described in Chapter 9. When ground state atoms are produced, some excited state atoms also occur and, for easily ionizable elements, some ions and electrons are produced. 

FAES Nebulization Flame 2000-2800°C Emission measured by spectrophometer ppm-most ppb— alkaline earth elements Inexpensive Simple Sensitive for alkali and alkaline earth elements Spectral interferences Molecular species emission... 

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