Flame emission background radiation

Background emission by the flame (Figure 8.23) includes contributions from molecular species and continuum radiation from incandescent particles and depends upon the combination of fuel and support gases used. The sample solvent and matrix will further augment background radiation. 

In principle, atomic fluorescence is a simpler and more versatile technique than atomic absorption, but suffers from a susceptibility to quenching effects and to background noise arising from the scattering of radiation by particles in the flame. The latter is particularly serious for refractory materials and in high-temperature flames. Detection limits for some elements are lower than by atomic absorption or flame emission measurements, e.g. elements with resonance lines around 200 nm or below, such as As, Se,... 

An inductively coupled argon plasma eliminates many common interferences. The plasma is twice as hot as a conventional flame, and the residence time of analyte in the flame is about twice as long. Therefore, atomization is more complete and signal is enhanced. Formation of analyte oxides and hydroxides is negligible. The plasma is remarkably free of background radiation 15-35 mm above the load coil where sample emission is observed. 

Part of the continuum background radiation in the flame spectrum shown in Figure 24-19 is probably thermal emission from incandescent particles in the flame. Note that this background decreases rapidly as the wavelength approaches the ultraviolet region of the spectrum. 

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 atomic absorption, the ratio of signals is measured with and without absorption. In flame emission, however, the signal intensity is measured directly superimposed on a flame background. For these reasons, the exit-slit adjustment, radiation-detector quality, and so forth are generally less critical in absorption methods than in emission methods. 

Spectral interferences occur whenever any radiation overlaps that of the analyte element. The interfering radiation may be an emission line of another element, radical, or molecule, unresolved band spectra, or general background radiation from the flame, solvent, or analytical sample. If the spectral interference does not coincide or overlap the analyte element, spectral interference may still occur if the resolving power and spectral band pass of the monochromator permit the undesired radiation to reach the photoreceptor. 

To realize these important advantages, it is necessary that the output of the source be free of contaminating lines from other elements in addition, the atomizer should emit no significant background radiation. In some instances with electrothermal atomizers, background radiation is minimal, but certainly, it is not with typical flames. To overcome this problem, filters, located between the source and detector, have often been used to remove most of the background radiation. Alternatively, solar-blind photomultipliers, which respond only to radiation of wavelengths shorter than 320 nm. have been applied. For these devices to be used effectively, analyte emission must be below 320 nm. , ... 

The advantages of this technique over emission flame photometry lie in the fact that the interferences due to physical inter-element effects, background radiation and scattered light are absent. Unfortunately chemical interferences still exist so that the effect of phosphate on the absorption of the calcium line is the same as its effect on the emission. 

The procedure is strictly analogous to that used for absorbance measurements in UV and visible molecular spectrometry (p. 355). To avoid interference from emission by excited atoms in the flame and from random background emission by the flame, the output of the lamp is modulated, usually at 50 Hz, and the detection system tuned to the same frequency. Alternatively, a mechanical chopper which physically interrupts the radiation beam, can be used to simulate modulation of the lamp output. 

In addition to the emission due to the test element, radiation is also emitted by the flame itself. This background emission, together with turbulence in the flame, results in fluctuations of the signal and prevents the use of very sensitive detectors. The problem may be appreciably reduced by the introduction into the sample of a constant amount of a reference element and the use of a dual-channel flame photometer, which is capable of recording both the test and reference readings simultaneously. The ratio of the intensity of emission of the test element to that of the reference element should be unaffected by flame fluctuations and a calibration line using this ratio for different concentrations of the test element is the basis of the quantitative method. Lithium salts are frequently used as the reference element in the analysis of biological samples. 

The emitting species for sulfur compounds is excited S2. The lambda maximum for emission of excited S2 is approximately 394 nm. The emitter for phosphorus compounds in the flame is excited HPO with a lambda maximum equal to doublet 510-526 nm. In order to detect one or the other family of compounds selectively as it elutes from the GC column, the suitable band-pass filter should be placed between the flame and the photomultiplier tube to isolate the appropriate emission band. In addition, a thermal infrared filter is mounted between the flame and the photomultiplier tube to isolate only the visible and UV radiation emitted by the flame. Without this filter, the large amounts of infrared radiation emitted by the combustion reaction of the flame would heat up the photomultiplier tube, thus increasing its background signal. 

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