Emission interference

Emission interference was common in many early instruments which were accessories for UV/visible spectrophotometers, which operated in most instances on a d.c. system. The interference was caused by emission of the element at the same wavelength as that at which absorption was occurring. All modem instruments use a.c. systems which are of course blind to the continuous emission from the flame. However, if the intensity of the emission is high, the noise associated with the determination will increase, since the noise of a photomultiplier detector varies with the square root of the radiation falling upon it. 

This effect can be reduced by either increasing the source current or by closing down the slit, both methods resulting in an increase in the signal-to-noise ratio. 

This is by far the most frequently encountered interference in AAS. Basically, a chemical interference can be defined as anything that prevents or suppresses the formation of ground state atoms in the flame. A common example is the interference produced by aluminium, silicon and phosphorus in the determination of magnesium, calcium, strontium, barium and many other metals. This is due to the formation of aluminates, silicates and phosphates which, in many instances, are refractory in the analytical flame being used. 

In order to overcome this type of interference, two techniques may be emphasised, both of which release the element under investigation. The first relies upon the application of chemistry, in the knowledge that, in many instances, a compound may be added which will lead to the release of the element that we are interested in by the formation of a preferential complex. Thus, a chelate such as EDTA can be added to complex the cation thus preventing its association with an anion that could lead to the formation 

Secondly, virtually all chemical interferences may be overcome by using the high-temperature nitrous oxide—acetylene flame. 

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Improved LIF sensing discrimination power is required for sample matrices that contain multiple fluorophores with similar spectral emission properties or when background emission is problematic. Distinguishing among airborne bioagent hazards and common emissive interferants (albuminous, epithelium, and cellulous materials as well as aromatic hydrocarbons), is a prime example where higher selective detection is required. This can be achieved via the lifetime properties of each fluorophore, by an optode approach or both. 

Different optical arrangements are represented in different manufacturers equipment. Right-angle fluorescence measurement is one of the most common approaches, with emitted light passing through the emission interference filter to a photomultiplier tube. 

Analytical Techniques. The primary method used to determine the metallic element concentration in the tailings was IX Plasma Atomic Emission Spectrometry (IXP). It was used for the determination of both major and minor components. In the former case, the analysis is straightforward, but in the case of minor constituents, it was necessary to use matrix matching, i.e., to use standard solutions having the same concentration of the major component as the unknown, to compensate for the background emission interference of the other solutes. This requires the initial determination of the major components to define the appropriate doping levels. The... 

The structure of the flame itself consists of two major zones, the primary reaction zone and a secondary reaction or postheating zone. The primary reaction zone, the area of the flame just above the burner surface, is the region where combustion, atomization, and excitation occurs. Some typical flame combustion products formed in this region are CO, C02, H2, N2, and H20 molecules, as well as O, H, OH-, and C- radical species. The secondary reaction zone is a much cooler region where the flame gases mix with atmospheric components that may include impurities and emission interferants. Between these two zones lies a smaller, but important, intermediate region, where little reaction occurs. In this region of the flame, the fraction of the atoms in the ith excited state, a is controlled only by the prevalent temperature and can be represented by the Boltzmann distribution ... 

An interferent is a substance or factor which distorts the relationship in pure solution and low concentration between salt concentration and emission. Interferents can be classified as follows. 

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. 

Sodium silicate is somev at more difficult to analyze than many other materials because of the formation of the relatively long lived radionuclide Na whose emissions interfere with the detection of other elements. Nevertheless we were able to determine, in a sample of sodium silicate, that many heavy elements of toxicological concern were undetectable down to the ppm to ppb level in the undiluted silicate (13), An XRF spectrometer can be configured to perform sequential multi-elemental analyses. It is less sensitive to the elements of lower atomic number. Also, since the X-rays penetrate only to a depth of about 10 urn, the sample must be homogeneous. Solid samples must be presented to the X-ray beam with a flat surface. However, the relative ease of sample preparation and the ability to run glasses and solutions with only minor dilution make X-ray fluorescence a useful technique where analysis for a wide range of impurities is required,... 

The extended fine structure of the secondary electron spectrum is a result of secondary electron emission ftom an atom when the direct secondary electron emission interferes with the emission of secondary electrons scattered by the nearest neighbors of the ionized atom. In the previous section we have considered the amplitudes of these processes in the single-scattering approximation. 

Traditional analytical lines for B identification are 345.1 and 249. 8 nm of B II and B I, correspondingly. Nevertheless, if high level of Fe presents in the rock, those lines are difficult to detect because of strong Fe emission interference. The first one may be detected with high boron content, but at average and low levels the Fe emission interference is very strong (Fig. 8.5a, b). From the other side, B III emission is situated in the spectral range which is free from Fe emission (Fig. 8.5c). Thus this line was selected as the analytical one for this task. The representative... 

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