Background emission sources

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... 

Natural silver halogenides, which are secondary ores for silver production, are characterized by intensive luminescence under powerful laser excitation (Fig. 4.67). The main source of the backgroimd liuninescence is the sorption of organic matter. The relatively long decay time of their liuninescence may enable us to lower the short-lived background emission by several orders of magnitude. 

The emissivity is calculated from the measured emission by ratioing the measurement from a blackbody source at the same temperature as the sample l48). Since there is a background emission from instrumental surfaces, four measurements are often made 149,150), to remove the background emission. The measured intensity, S(v, T) at any temperature has several components... 

Fundamental requirements for an atomic absorption experiment are shown in Figure 21-2. Principal differences between atomic and ordinary molecular spectroscopy lie in the light source (or lack of a light source in atomic emission), the sample container (the flame, furnace, or plasma), and the need to subtract background emission. 

Flame AFS combines features of both AAS and FES. The excitation of atoms is by the absorption of light. When individual element spectral line sources are used, the spectral selectivity should be as high as that in AAS, although scatter may be more of a problem in AFS. Quantification is by comparison of the intensity of fluorescence emitted by samples with that emitted by standards of known concentration. At low determinant concentrations, it is necessary to discriminate between small fluorescence emission signals and the background light levels associated with thermally excited emission from the flame. Therefore in AFS, as in FES, it is desirable to have low flame background emission. This is discussed further in Chapter 2, where instrumental aspects of flame spectrometric techniques are discussed. 

If the flame background emission intensity is reduced considerably by use of an inert gas-sheathed (separated) flame, then an interference filter may be used rather than a monochromator, to give a non-dispersive atomic fluorescence spectrometer as illustrated in Figure 14.36-38 Noise levels are often further reduced by employing a solar blind photomultiplier as a detector of fluorescence emission at UV wavelengths. Such detectors do not respond to visible light. The excitation source is generally placed at 90° to the monochromator or detector. Surface-silvered or quartz mirrors and lenses are often used to increase the amount of fluorescence emission seen by the detector. 

Cutter and Church (13) have determined selenium species in western Atlantic precipitation so that the emission sources using this sulfur analogue, which is enriched in fossil fuels (primarily coal), can be more exactly identified. The results show a correlation of both total Se and the Se IV Se VI ratio with increasing protons and excess sulfur in precipitation from Lewes, Delaware, and on Bermuda. Their hypothesis is that, although some reduced forms (1 nM/kg) may come from background oceanic emissions, most oxidized Se is a reflection of fossil-fuel emissions from North America. 

Source-receptor analysis was performed to allocate the air chemistry parameters to the individual emission sources. To estimate the impact of a source group on a certain pollutant, several simulations have to be accomplished. To minimize the associated uncertainties (non-linearity of chemical processes), the source group was suppressed. Due to the non-linear chemical processes, background concentrations and advection a non-linear fraction has to be introduced (DG-ENV 2001). The source-receptor analysis is an important tool for abatement and emission reduction strategies. 

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