Spectral Line Coincidence

Little can be done to avoid spectral overlap interferences occurring at one of the fixed or preset wavelengths. Often the only solution to this problem is 

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Spectral line coincidence. Direct line coincidence occurs when the monochromator of the spectrometer is not capable of separating the analyte line from the matrix line. For example, only few commercial spectrometers (echelle monochromators) are capable of routinely resolving the cadmium-arsenic line pair at 228.8 nm (Figure 131). Another similar example is separation of the zinc line at 213.856 nm from the nickel line at 213.858 nm. 

In 1868, within a decade of the development of the spectroscope, an orange-yeUow line was observed in the sun s chromosphere that did not exactiy coincide with the D-lines of sodium. This line was attributed to a new element which was named helium, from the Greek hellos, the sun. In 1891 an inert gas isolated from the mineral uranite showed unusual spectral lines. In 1895 a similar gas was found in cleveite, another uranium mineral. This prominent yellow spectral line was then identified as that of helium, which to that time had been thought to exist only on the sun. In 1905 it was found that natural gas from a well near Dexter, Kansas, contained nearly 2% helium (see Gas, natural). 

Spectral overlap of emission and absorption wavelengths Is a potential cause of Interference In atomic absorption spectrometry (57) Thus, (a) the emission line of Fe at 352.424 nm Is close to the resonance line of N1 at 352.454, (b) the emission line of Sb at 217.023 nm Is close to the resonance line of Pb at 216.999 nm, and (c) the emission line of As at 228.812 nm Is close to the resonance line of Cd at 228.802 (57). To date, these practically coincident spectral lines have not been reported to be of practical Importance as sources of analytical Interference In atomic absorption analyses of biological materials. 

Hieronymus Theodor Richter was bom at Dresden on November 21, 1824. He became a metallurgical chemist at the Freiberg School of Mines. When he placed some of the zinc blende in the loop of a platinum wire and heated it in the flame of a Bunsen burner, he observed a brilliant indigo line which did not coincide with either of the blue lines of cesium (20, 52). Because of this characteristic spectral line the new element was christened indium. The publication of this contribution under joint authorship was a mistake which Professor Reich afterward regretted, for Richter tried to make it appear that he Was the sole discoverer (2, 51, 52). 

The use of anticoincidence shielding significantly reduces the Compton continuum and allows the detection of weak spectral lines usually masked by interfering Compton radiation. Further improvement resulted from separate recording of the coincidence and anticoincidence thus, radionuclides that normally decay with a coincidence scheme can be recorded without loss of efficiency. 

If the isotopic shift of a spectral line in an atom or in a molecule is more Hi... the Doppler width, it is in principle possible to selectively excite a parti, id... isotopic species from isotopic mixtures by monochromatic light of w.u. length in coincidence with the absorption of the particular isotopic spe> < In a typical example, 2°2Hg atoms in natural Hg vapor containing 204. o 201, 200, 199, and 198 isotopes are preferentially excited by the 2b i V resonance line of 202Hg atoms. It has recently been demonstrated tlm 235U atoms are enriched in the photoionization processes of Mi. t... 

It often takes time for the implications of experimental data to be understood and to be acted upon. Fraunhofer s earlier observation that the solar D-lines coincided with the spectral lines of a sodium lamp eventually prompted further important experiments. In 1849, Jean Bernard Leon Foucault (1819-1868), a Parisian physicist, made an unexpected discovery. He passed sunlight through a vapor of sodium and he found that the solar D-lines were darker. His conclusion was that the sodium vapor presents us with a medium which emits the rays D on its own account, and which absorbs them when they come from another quarter. The consequences of Foucault s experiment, however, were expressed more cogently by Sir William Thomson (later Lord Kelvin). He drew the following explicit conclusion That the double line D, whether bright or dark, is due to the vapor of sodium. . . That Fraunhofer s double dark line D, of solar and stellar spectra, is due to the presence of vapor of sodium in atmospheres surrounding the Sun and those stars in whose spectra it has been observed. ... 

Spectral interferences are uncommon in AAS owing to the selectivity of the technique. However, some interferences may occur, e.g. the resonance line of Cu occurs at 324.754 nm and has a line coincidence from Eu at 324.753 nm. Unless the Eu is 1000 times in excess, however, it is unlikely to cause any problems for Cu determination. In addition to atomic spectral overlap, molecular band absorption can cause problems, e.g. calcium hydroxide has an absorption band on the Ba wavelength of 553.55 nm while Pb at 217.0nm has molecular absorption from NaCl. Molecular band absorption can be corrected for using background correction techniques (see p. 174). The operation of a flame atomic absorption spectrometer is described in Box 27.6. 

The function of the spectrometer is to accept as much light from the source as possible and to isolate the required spectral lines. This may be impossible where there is a continuous spectrum in the same region as the analytical line for example, the magnesium line of 286.2 nm coincides with a hydroxyl band. In direct reading instruments, electronic devices may be used to supplement the resolution of the spectrometer by modulating the intensity of the analytical signal. In absorption and fluorescence the light source is modulated in emission the spectral line is scanned (816) or the sample flow modulated (M23). 

We point out that similar analyses and results have been performed and obtained also by other authors [33, 35, 38 0]. The spectral lines at 86meV and 123 meV excitation energy in the theoretical spectrum correspond to excitation of the modes V6 and vi, respectively. The first spacing deviates from the harmonic frequency of mode V6 in Table 3 because of the JT effect, while the second coincides with that of mode vi because of the linear coupling scheme adopted. For higher excitation energies the lines represent an intricate mixture of the various modes because of the well-know nonseparability of modes in the multi-mode dynamical JT effect. Overall, the excitation of the various modes can be characterized as moderately weak. The total JT stabilization energy amounts to 930 cm and is dominated by the contribution of mode ve- The barrier to pseudorotation is of the order of 10 cm only, consistent with the fact that the theoretical spectrum of Fig. 3 is obtained within the LVC scheme (see Sect. 2.1 above). 

Spectral interferences of analyte lines with other atomic spectral lines are of minor importance as compared with atomic emission work. Indeed, it is unlikely that resonance lines emitted by the hollow cathode lamp coincide with an absorption line of another element present in the atom reservoir. However, it may be that several emission lines of the hollow cathode are within the spectral bandwidth or that flame emission of bands or a continuum occur. Both contribute to the non-absorbed radiation, by which the linear dynamic range decreases. Also, the nonelement specific absorption (see Section 4.6) is a spectral interference. 

Tables VIII, IX and X demonstrate that the accuracy of the SPD is generally similar to the accuracy of the PMT. In the case of some spectral lines the SPD results were less accurate than the PMT results. This is caused by the lower resolving power of the SPD causing it to be more subject to specific spectral interference. The adverse effects of spectral line interferences resulted in relatively large concentration errors for the low analyte concentration sample. The effect of these spectral interferences is twofold. First, there is the analyte signal intensity error caused by coincident spectral line interferences from unresolved matrix lines. Second, there is the background interpolation...

Soc., 31, 337,1881) has tried to answer the question, Is the number of harmonic relations in the spectral lines of iron greater than what a chance distribution would give Mallet (Phil. Trans., 171,1003, 1880) and R. J. Strutt (Phil. Mag., [6], 1,311,1901) have asked, Do the atomic weights of the elements approximate as closely to whole numbers as can reasonably be accounted for by an accidental coincidence In other words Are there common-sense grounds for believing the truth of Prout s law, that the atomic weights of the other elements are exact multiples of that of hydrogen ... 

Hydrogen is a model for all the atoms of the kingdom, but we need several more items of information before we can move on. First, we need to know that a spherical orbital of hydrogen is called an s-orbital. Although it is convenient to think of s as standing for spherical, that is only a coincidence, and its actual origin is deep in the history of spectroscopy, where it stands for sharp, a comment on the appearance of certain spectral lines. 

The mechanism of the anomalous Zeeman effect is exactly the same as that of the normal Zeeman effect, but it exhibits more than three components. For the anomalous Zeeman effect it is characteristic that the n -compon-ent also splits into several lines and thus no longer coincides exactly with the original resonance line. In this case the Landen factor, g will vary for various terms which is the reason for the splitting of the spectral lines into the several components (Figure 12). 

In some cases the interference caused by a background spectrum line which directly coincides with the desired analyte can be avoided by background correction. For example, the OH molecular line overlaps the A11 line at 308.2 nm. Because the OH signal is relatively constant, it can be subtracted out with the blank signal. However, this will add background noise that can interfere with the determination at low analyte concentrations. This type of problem is much greater when the spectral line originates from a concomitant line which directly overlaps the analyte line, like the coincidence of the Cu I line at 213.859 nm with the Zn I line at 213.856 nm, or that of the... 

A search of wavelength tables, arranged by wavelength, reveals a sensitive cadmium line at 3261.1 A thus, cadmium may be the element producing the observed spectral line. To verify this tentative identification, a table of spectral lines of cadmium should be examined and the spectrum should be inspected to determine if other cadmium lines are present. Three or four lines in the spectrum coinciding with the wavelengths and relative intensities of other cadmium lines is considered positive proof that cadmium is present in the sample. 

Basically, ICP-AES is a low-interference method. In individual cases, however, the spectral and non-spectral types of interference described below may occur. Of these, line coincidences and interference due to sample feeding are the most significant in practice. 

This interference arises as a result of overlapping of the spectral lines. Line coincidences become apparent only when a critical concentration ratio between the interfering and analyzed elements is reached. They are dependent on the spectral resolution of the spectrometer. The line coincidences which may occur in the analysis of wastewater have been established in a test. The results of this test are summarized in Table b. 

Although the relative intensities of spectral lines in the ICP differ from those observed in the DC arc and AC spark, the published tables are invaluable for the selection of analyte lines in ICP sources, and the identification of spectral interferences in the spectrometer bandwidths. However, spectral lines are emitted by ICP sources that are not emitted by DC arcs and sparks. In order to facilitate spectral line selection in ICP-AES, numerous spectral line atlases are now available which list the best analytical lines and the potential interferences due to coincidences from major and minor constituents. Simulated... 

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