Emission line, intensity

In Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), a gaseous, solid (as fine particles), or liquid (as an aerosol) sample is directed into the center of a gaseous plasma. The sample is vaporized, atomized, and partially ionized in the plasma. Atoms and ions are excited and emit light at characteristic wavelengths in the ultraviolet or visible region of the spectrum. The emission line intensities are proportional to the concentration of each element in the sample. A grating spectrometer is used for either simultaneous or sequential multielement analysis. The concentration of each element is determined from measured intensities via calibration with standards. 

Previous experience in arc and spark emission spectroscopy has revealed numerous spectral overlap problems. Wavelength tables exist that tabulate spectral emission lines and relative intensities for the purpose of facilitating wavelength selection. Although the spectral interference information available from arc and spark spectroscopy is extremely useful, the information is not sufficient to avoid all ICP spectral interferences. ICP spectra differ from arc and spark emission spectra because the line intensities are not directly comparable. As of yet, there is no atlas of ICP emission line intensity data, that would facilitate line selection based upon element concentrations, intensity ratios and spectral band pass. This is indeed unfortunate because the ICP instrumentation is now capable of precise and easily duplicated intensity measurements. 

Chemiluminescence from the bO — X,0 band system of PI. Reassignment of vibrational level numbering and modified molecular constants Chemiluminescence from the bO —> X O system of Asl and the b0+ — Xi0, X2l systems of Sbl Transition probabilities for the 4-p-ns (n = 6—9) transitions of Arl from emission line intensity measurements. Lifetimes of the nsCijl) (n = 6—9) levels Stimulated emission from the B — X bands of Cdl and " Cdl... 

Figure 2.3 Perturbations and predissociations affect absorption and emission line intensities in quite different ways. Two pairs of absorption and emission spectra are shown. The first pair illustrates the disappearance of a weakly predissociated line in emission without any detectable intensity or lineshape alteration in absorption. The second pair shows that emission from upper levels with slow radiative decay rates can be selectively quenched by collision induced energy transfer. The opposite effect, selective collisional enhancement of emission from perturbed, longer-lived levels, is well known in CN B2 +—X2 +(u = 0,v") emission spectra (see Fig. 6.14 and Section 6.5.5). (a) the CO B1S+—X1S+(1,0) band in emission (top) and absorption (bottom). The last strong lines in emission are 11(16) and P(18). Emission from levels with J > 17 is weak because the predissociation rate is larger than the spontaneous emission rate. (Courtesy F. Launay and J. Y. Roncin.) (6) The CO A ll—X1 + (0,0) band in emission (bottom) and absorption (top). The a 3 + —X1 +(8,0) band lines appear in absorption because the A1 FI a 3 + spin-orbit interaction causes a small amount of A1 character to be admixed into the nominal a 3 + levels. These a —X lines are absent from the emission spectrum because collisional quenching and radiative decay into a3II compete more effectively with radiative decay into X1 + from the long-lived a 3 + state than from the short-lived A1 state. In addition, collisions and radiative decay into a3II cause the P(31) extra line (E) (arising from a perturbation by d3A v = 4) to be weakened in emission relative to the main line (M). (Courtesy F. Launay, A. Le Floch, and J. Rostas.)...

A related problem is ionization interference. If the analyte atoms are ionized in the flame, they cannot emit atomic emission wavelengths, and a reduction in atomic emission intensity will occur. For example, if potassium is ionized in the flame, it cannot emit at its atomic emission line at 766.5 nm and the sensitivity of the analysis will decrease. If a large amount of a more easily ionized element, such as cesium, is added to the solution, the cesium will ionize preferentially and suppress the ionization of potassium. The potassium ions will capture the electrons released by the cesium, reverting to neutral potassium atoms. The intensity of emission at 766.5 nm will increase for a given amount of potassium in the presence of an excess of cesium. The added cesium is called an ionization suppressant. Ionization interference is a problem with the easily ionized elements of groups 1 and 2. The use of ionization suppressants is recommended for the best sensitivity and accuracy when determining these elements. Of course, as ionization increases, ion emission line intensity increases. It may be possible to use an ionic emission line instead of an atomic emission line for measurements. 

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. 

There are always four stages in any scheme of x-ray emission analysis. The excitation of characteristic radiation from the specimen by bombardment with high-energy photons, electrons, protons, etc. the selection of a characteristic emission line from the element in question by means of a wavelength or energy-dispersive spectrometer the detection and integration of the characteristic photons to give a measure of characteristic emission line intensity and finally, the conversion of the characteristic emission line intensity to elemental concentration by use of a suitable calibration procedure. 

The quantitative aspects of LIBS even for laboratory applications may be considered its Achilles heel, first due to the complex nature of the laser-sample interaction processes, which depends upon both the laser characteristics and the sample material properties, and second due to the plasma-particle interaction processes, which are space and time dependent. All those parameters influence strongly on plasma conditions and, consequently, on emission lines intensities. Consequently it may sound correct, that LIBS is a mix of great potential and severe limitation, that one of the major problems that precludes its more quantitative use is a lack of reproducibility of spectra at a given wavelength on a shot to shot basis, and that the sensitivity remains modest, precision mediocre and matrix dependence strong (Hahn and Omenetto 2012). 

For field applications the laser-sample distance is constantly changing and the physical form of the rock is different (pebbles of different size from tenth of centimeters to tenth of microns). All those parameters influence strongly on plasma conditions and, consequently, on emission lines intensities. Laser-target distance corrections may be achieved by constantly measuring it with corresponding changes in the optical... 

The relative intensities of X-ray emission lines from targets varies for different elements. However, one can assume a ratio of KaJKa2 = 2 for the commonly used targets. The ratio of Ko2lKjii from these targets varies from 6 to 3.5. The intensities of Kj32 radiations amount to about 1 percent... 

The intensity, I, of an emission line is proportional to the number of atoms, N, populating the excited state... 

X 10 J/K), and T is the temperature in kelvin. From equation 10.35 we can see that excited states with lower energies have larger populations and, therefore, the most intense emission lines. Furthermore, emission intensity increases with temperature. 

Standardizing the Method Equation 10.34 shows that emission intensity is proportional to the population of the excited state, N, from which the emission line originates. If the emission source is in thermal equilibrium, then the excited state population is proportional to the total population of analyte atoms, N, through the Boltzmann distribution (equation 10.35). 

When possible, quantitative analyses are best conducted using external standards. Emission intensity, however, is affected significantly by many parameters, including the temperature of the excitation source and the efficiency of atomization. An increase in temperature of 10 K, for example, results in a 4% change in the fraction of Na atoms present in the 3p excited state. The method of internal standards can be used when variations in source parameters are difficult to control. In this case an internal standard is selected that has an emission line close to that of the analyte to compensate for changes in the temperature of the excitation source. In addition, the internal standard should be subject to the same chemical interferences to compensate for changes in atomization efficiency. To accurately compensate for these errors, the analyte and internal standard emission lines must be monitored simultaneously. The method of standard additions also can be used. 

Usually, the ultraviolet and visible regions of the spectrum are recorded. Many of the most intense emission lines lie between 200 nm and 400 nm. Some elements (the halogens, B, C, P, S, Se, As, Sn, N, and O) emit strong lines in the vacuum ultraviolet region (170-200 nm), requiring vacuum or purged spectrometers for optimum detection. 

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. 

In principle, therefore, the surface concentration of an element can be calculated from the intensity of a particular photoelectron emission, according to Eq. (2.6). In practice, the method of relative sensitivity factors is in common use. If spectra were recorded from reference samples of pure elements A and B on the same spectrometer and the corresponding line intensities are and respectively, Eq. (2.6) can be written as... 

The quantification algorithm most commonly used in dc GD-OES depth profiling is based on the concept of emission yield [4.184], Ri] , according to the observation that the emitted light per sputtered mass unit (i. e. emission yield) is an almost matrix-independent constant for each element, if the source is operated under constant excitation conditions. In this approach the observed line intensity, /ijt, is described by the concentration, Ci, of element, i, in the sample, j, and by the sputtering rate g, ... 

The potential of LA-based techniques for depth profiling of coated and multilayer samples have been exemplified in recent publications. The depth profiling of the zinc-coated steels by LIBS has been demonstrated [4.242]. An XeCl excimer laser with 28 ns pulse duration and variable pulse energy was used for ablation. The emission of the laser plume was monitored by use of a Czerny-Turner grating spectrometer with a CCD two-dimensional detector. The dependence of the intensities of the Zn and Fe lines on the number of laser shots applied to the same spot was measured and the depth profile of Zn coating was constructed by using the estimated ablation rate per laser shot. To obtain the true Zn-Fe profile the measured intensities of both analytes were normalized to the sum of the line intensities. The LIBS profile thus obtained correlated very well with the GD-OES profile of the same sample. Both profiles are shown in Fig. 4.40. The ablation rate of approximately 8 nm shot ... 

The depth profiling of the Cu-Ag-Si samples was performed with the laser beam focused on to the sample surface to the spot of approximately 30-40 pm. To obtain the best depth resolution the laser fluence was maintained near the 1 J cm level, close to the threshold of the LIBS detection scheme. The intensity profiles of Cu and Ag emission lines are shown in Fig. 4.43. The individual layers of Cu and Ag were defi-... 

The intensity /k, (2 a) of a spectral emission line, i. e. the radiative recombination of an electron of a species A from a higher energy level k to the lower level i, is characteristic of a sputtered element or molecule A and is calculated by use of the equation ... 

High pressure xenon lamps are also employed in some TLC scanners (e.g. the scanner of Schoeffel and that of Farrand). They produce higher intensity radiation than do hydrogen or tungsten lamps. The maximum intensity of the radiation emitted lies between k = 500 and 700 nm. In addition to the continuum there are also weak emission lines below k = 495 nm (Fig. 14). The intensity of the radiation drops appreciably below k = 300 nm and the emission zone, which is stable for higher wavelengths, begins to move [43]. 

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

However, the optical train illustrated in Figure 22B allows the determination of fluorescence quenching. The interfering effect described above now becomes the major effect and determines the result obtained. For this purpose the deuterium lamp is replaced by a mercury vapor lamp, whose short-wavelength emission line (2 = 254 nm) excites the luminescence indicator in the layer. Since the radiation intensity is now much greater than was the case for the deuterium lamp, the fluorescence emitted by the indicator is also much more intense and is, thus, readily measured. 

---