Intensity, of emission line

The intensities of emission lines of selected elements are recorded continuously by a computer during measurement. Using calibration curves obtained by means of calibration standards under identical discharge conditions, concentration depth profiles of the chosen elements in the analysed sample are determined from the recorded emission line intensity. 

P. W. J. M. Boumans. Line Coincidence Tables for Inductively Coupled Plasma Atomic Emission Spectrometry. Pergamon Press, Oxford, 1980, 1984. Lists of emission lines for analysis and potentially overlapping lines with relative intensities, using spectrometers with two different resolutions. 

Table 3. List of emission lines and their relative intensities for the most important mercury lamps ).

Minerals generally present difficult problems in chemical analysis, and these problems grow more serious when the elements being determined are as difficult to separate as are those named above. The time and effort that x-ray emission spectrography can save are therefore great, but there are obstacles to be surmounted. Among these are (1) Absorption and enhancement effects are often serious. (2) The element of interest may be present at low concentration in a matrix that is unknown and variable. (3) Satisfactory standards are not always easy to obtain. (4) Simple equipment sometimes does not resolve important analytical lines- completely. (5) Sample preparation and particle size often influence the intensities of analytical lines Class II deviations (7.8) can be particularly serious with minerals. 

The n — 1 multiplet rule states that nuclei i and j will exhibit only Ji] and Wj lines. The two missing lines are the high field line of the low field multiplet and the low field line of the high field multiplet. In addition the intensity of emission and absorption over both multiplets balances as would be expected. For example, a high field E/A multiplet would at zero-field exhibit E and A n — 1 multiplets for nuclei i and j respectively. 

The simplest fluorescence measurement is that of intensity of emission, and most on-line detectors are restricted to this capability. Fluorescence, however, has been used to measure a number of molecular properties. Shifts in the fluorescence spectrum may indicate changes in the hydrophobicity of the fluorophore environment. The lifetime of a fluorescent state is often related to the mobility of the fluorophore. If a polarized light source is used, the emitted light may retain some degree of polarization. If the molecular rotation is far faster than the lifetime of the excited state, all polarization will be lost. If rotation is slow, however, some polarization may be retained. The polarization can be related to the rate of macromolecular tumbling, which, in turn, is related to the molecular size. Time-resolved and polarized fluorescence detectors require special excitation systems and highly sensitive detection systems and have not been commonly adapted for on-line use. 

In either case, the information on the vibrational transition is contained in the energy difference between the excitation radiation and the inelastically scattered Raman photons. Consequently, the parameters of interest are the intensities of the lines and their position relative to the Rayleigh line, usually expressed in wavenumbers (cm 1). As the actually recorded emissions all are in the spectral range determined by the excitation radiation, Raman spectroscopy facilitates the acquisition of vibrational spectra through standard VIS and/or NIR spectroscopy. 

Electromagenetic Radiation. Atomic and Molecular Energy. The Absorption and Emission of Electromagnetic Radiation. The Complexity of Spectra and the Intensity of Spectral Lines. 

Table 8.7). Thus, intensity and concentration are directly proportional. However, the intensity of a spectral line is very sensitive to changes in flame temperature because such changes can have a pronounced effect on the small proportion of atoms occupying excited levels compared to those in the ground state (p. 274). Quantitative measurements are made by reference to a previously prepared calibration curve or by the method of standard addition. In either case, the conditions for measurement must be carefully optimized with reference to the choice of emission line, flame temperature, concentration range of samples and linearity of response. Relative precision is of the order of 1-4%. Flame emission measurements are susceptible to interferences from numerous sources which may enhance or depress line intensities. 

Self-absorption is a phenomenon whereby emitted radiation is reabsorbed as it passes outwards from the central region of the flame (cf. arc/spark spectrometry). It occurs because of interaction with ground state atoms of the analyte in the cooler outer fringes of the flame and results in attenuation of the intensity of emission. It is particularly noticeable for lines originating from the lowest excited level and increases with the concentration of the analyte solution (Figure 8.24). 

Organic solvents enhance emitted intensities mainly because of a higher resultant flame temperature (water has a cooling effect), a more rapid rate of feed into the flame because of the generally lower viscosity, and the formation of smaller droplets in the aerosol because of reduced surface tension. The resultant enhancement of spectral line intensity may be 3-to over 100-fold. Conversely, the presence of salts, acids and other dissolved species will depress the intensity of emission from the analyte and underlines the need for careful matching of samples and standards. 

A technique that utilizes a solid sample for light emission is spark emission spectroscopy. In this technique, a high voltage is used to excite a solid sample held in an electrode cup in such a way that when a spark is created with a nearby electrode, atomization, excitation, and emission occur and the emitted light is measured. Detection of what lines are emitted allows for qualitative analysis of the solid material. Detection of the intensity of the lines allows for quantitative analysis. 

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 LIBS technique may be extremely useful for sorting of fluorite ores. Figure 8.8 clearly demonstrates the opportunities of time-resolved LIBS in comparison with the steady-state method in the case of fluorite-carbonate ores. Fluorite and calcite both has Ca as a major element and its emission lines dominate in the steady-state spectra making sorting impossible. After a delay of several ps the intensity of Ca lines is strongly diminished and a F line with a longer decay becomes visible in the fluorite spectrum. 

For resonance lines, self-absorption broadening may be very important, because it is applied to the sum of all the factors described above. As the maximum absorption occurs at the centre of the line, proportionally more intensity is lost on self-absorption here than at the wings. Thus, as the concentration of atoms in the atom cell increases, not only the intensity of the line but also its profile changes (Fig. 4.2b) High levels of self-absorption can actually result in self-reversal, i.e. a minimum at the centre of the line. This can be very significant for emission lines in flames but is far less pronounced in sources such as the inductively coupled plasma, which is a major advantage of this source. 

The factors influencing the intensity distribution of emission lines are very difficult to determine quantitatively. Although a number of empirical or semi-empirical expressions have been proposed in the literature (7), their numerical evaluation would require knowledge of a number of poorly defined parameters of tlm particular lamp in question. Therefore all of these, types of calculations are based on certain reasonable but somewhat arbitrary assumptions. In Figure 3, the. intensity distribution is shown for the liyperfine emission line at 2.r)37-A., from a cooled electrodeless discharge containing pure 2ll2Hg, with the. intensity... 

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