Excitation interferences

The design of the DCP-OES allows the use of both aqueous and most non-aqueous solvents, providing standards and samples are prepared under similar conditions. It is more expensive to operate than AAS but cheaper than ICP-OES. The limitation of DCP-OES is the susceptibility to excitation interferences and increased signals from easily ionisable elements (EIEs). It has lower limits of detection and wider linear range for most elements but not as good as ICP-OES. 

Figure 3.18 Left panel The molecular LICS scheme for Na2. In this scheme a 2i photon. The two-photon process proceeds from an initial sate, assigned here as r = SJ = 37, via the v = 35,7 = 36,38 levels, belonging to the interacting A Eu/ TIu electronic states, acting as intermediate resonances. The a>i photon dresses the continuum with the (initially unpopulated) v = 93,7 = 36 and v = 93,7 = 38 levels of the A Eu/ Ilu electronic states. Right panel Experimental Na(3d) fluorescence (solid) and Na(3p) fluorescence (dashed) (both uncalibrated) for the Na2 — Na(3s)- -Na(3p)/Na(3d) LICS scenario of the left panel, as a function of the a>i frequency. The (o frequency is fixed at 17 474.12 cm . Taken from Ref [89].

Chemical interferences fall into two categories as in hydride—AAS (i) Interferences in the liquid phase that prohibit or limit the formation of the volatile hydride, and (ii) Interferences in the gas phase that diminish the analyte available for excitation. Interferences of the first type are caused mainly by the Group 8, 9, 10, and 11 transition elements. The degree and elimination of the interferences are the same as in AAS (Chapter 3, Section 6.2). 

Due to the high excitation power of the ICP, the spectrum obtained is very rich with lines caused by atoms and ions, which makes it possible to correct the interference due to the light scatter using a double beam technique. When using a plasma source for excitation, interference caused by the scatter may be corrected by a method based on self-absorption. In this technique the slope of the fluorescence graph is compared to that of the plasma emission graph. 

Similar to hollow cathodes, electrically heated graphite furnaces in a low-pressure environment can be used for sample volatilization. The released analyte can then be excited in a discharge between the furnace and a remote anode. This source is known as furnace atomic nonresonance emission spectrometry (FANES), and was introduced by Falk et al. [287]. Owing to the separation of volatilization and excitation, its absolute detection limits are in the picogram range. For real samples, however, volatilization and excitation interference may be considerable. 

Another type of such coupling is the configuration interaction (CI) between a true discrete excitation and a continuum excitation. This autoionization phenomenon is clearly within the TDLDA framework, A nice example can be found in copper where 3d -> ef, ep excitations interfere with the 3p -> 4s transition, The resulting 3d partial photoionization cross section is shown in Figure 8, In addition to the prominent Fano line shape, an overall diminultion (relative to the LDA) of the cross section is found due to intrashell 3d polarization. The interesting dip around 80 eV is again a Cl effect, but this time the 3d ef,ep excitations interfere with the continuum channels, 3p es,ed. 

Lew frequency Eddy current probing For frequencies below some 100 Hz the SQUID is coupled with a completely superconducting flux antenna. This antenna has to be within the cryogenic vessel. The Eddy current excitation is done in a conventional way. But care must be taken, that interference between the excitation field and the flux anteima and SQUID is... 

The -frmction excitation is not only the simplest case to consider it is the frmdamental building block, m the sense thatv the more complicated pulse sequences can be interpreted as superpositions of 5-frmctions, giving rise to superpositions of M avepackets which can in principle interfere. 

Figure Al.6,8 shows the experimental results of Scherer et al of excitation of I2 using pairs of phase locked pulses. By the use of heterodyne detection, those authors were able to measure just the mterference contribution to the total excited-state fluorescence (i.e. the difference in excited-state population from the two units of population which would be prepared if there were no interference). The basic qualitative dependence on time delay and phase is the same as that predicted by the hannonic model significant interference is observed only at multiples of the excited-state vibrational frequency, and the relative phase of the two pulses detennines whether that interference is constructive or destructive.

Quack M and Sutcliffe E 1983 Quantum interference in the IR-multiphoton excitation of small asymmetric-top molecules ozone Chem. Phys. Lett. 99 167-72... 

Due to the rather stringent requirements placed on the monochromator, a double or triple monocln-omator is typically employed. Because the vibrational frequencies are only several hundred to several thousand cm and the linewidths are only tens of cm it is necessary to use a monochromator with reasonably high resolution. In addition to linewidth issues, it is necessary to suppress the very intense Rayleigh scattering. If a high resolution spectrum is not needed, however, then it is possible to use narrow-band interference filters to block the excitation line, and a low resolution monocln-omator to collect the spectrum. In fact, this is the approach taken with Fourier transfonn Raman spectrometers. 

Problems arise if a light pulse of finite duration is used. Here, different frequencies of the wave packet are excited at different times as the laser pulse passes, and thus begin to move on the upper surface at different times, with resulting interference. In such situations, for example, simulations of femtochemistry experiments, a realistic simulation must include the light field explicitely [1]. 

Selectivity The selectivity of molecular fluorescence and phosphorescence is superior to that of absorption spectrophotometry for two reasons first, not every compound that absorbs radiation is fluorescent or phosphorescent, and, second, selectivity between an analyte and an interferant is possible if there is a difference in either their excitation or emission spectra. In molecular luminescence the total emission intensity is a linear sum of that from each fluorescent or phosphorescent species. The analysis of a sample containing n components, therefore, can be accomplished by measuring the total emission intensity at n wavelengths. 

Atomic emission is used for the analysis of the same types of samples that may be analyzed by atomic absorption. The development of a quantitative atomic emission method requires several considerations, including choosing a source for atomization and excitation, selecting a wavelength and slit width, preparing the sample for analysis, minimizing spectral and chemical interferences, and selecting a method of standardization. 

Choice of Atomization and Excitation Source Except for the alkali metals, detection limits when using an ICP are significantly better than those obtained with flame emission (Table 10.14). Plasmas also are subject to fewer spectral and chemical interferences. For these reasons a plasma emission source is usually the better choice. 

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. 

Sensitivity Sensitivity in flame atomic emission is strongly influenced by the temperature of the excitation source and the composition of the sample matrix. Normally, sensitivity is optimized by aspirating a standard solution and adjusting the flame s composition and the height from which emission is monitored until the emission intensity is maximized. Chemical interferences, when present, decrease the sensitivity of the analysis. With plasma emission, sensitivity is less influenced by the sample matrix. In some cases, for example, a plasma calibration curve prepared using standards in a matrix of distilled water can be used for samples with more complex matrices. 

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