Multi-element spectra

Fig. 12-21. Multi-element spectra from a multi-element standard solution using Mo and W excitation, respectively. The peaks arise from an amount of 1 ng per element. The different peak heights reflect different sensitivities.

Radiochemical separations are necessary for many elements when only a Nal detector is available. Even with a Ge(Li) detector, a radiochemical separation increases the sensitivity and accuracy and permits the determination of some elements whose radioactivities are masked by stronger activities in the multi-element spectrum of a coal sample. For example, mercury, selenium, gallium, and zinc in most coals are below the limit of detection instrumentally even with the resolution of a Ge(Li) crystal (7), but can be determined after radiochemical separations as is described later. 

In ICP-MS a multi-element tuning solution is applied for the mass calibration of mass spectra. Figure 6.3 shows the mass spectrum for phosphorus determination at m/z = 31. In this case, the mass calibration was performed with the aid of a solution of a phos-phorus/sulphur mixture. From the known masses of the isotopes of the atomic ions, the masses of the polyatomic ions occurring were determined and identified in accordance with the isobaric polyatomic ions (15N160+ and 14N16OH+). Mass accuracy is the deviation of experimental determined mass of an atomic, polyatomic, cluster, molecular or fragment ion from the exact mass of species expected. Polyatomic, cluster and molecular ions were calculated from the atomic masses (see Appendix I), the mass accuracy is usually presented in ppm. 

However, studies of magpie feathers have, until now, concerned mostly a few elements such as lead, cadmium and mercury. The objective of the presented research was to evaluate how suitable the method is for a broader spectrum of elements found in many Polish areas which differ in type of industrialization and urbanization. Also the studies aimed at testing the severity of contamination in certain areas, both multi-elemental and regarding individual elements... 

Figure 9.6 shows a typical NAA spectrum obtained with a multichannel analyzer equipped with scintillation (upper curve) or semiconductor (lower curve) detectors. Each peak can be ascribed to a certain y-energy, which in most cases identifies the nuclide. A number of nuclides can be identified simultaneously with semiconductor detectors, but with Nal(TI) scintillation detectors the poor resolution limits simultaneous multi-element analysis. 

The spectrum of an element in the ICP discharge resembles most closely one obtained in the DC arc rather than in a spark or a flame, so that the ICP discharge provides a rich spectrum for qualitative and simultaneous multi-element quantitative analysis. Even molecular spectra such as that due to CN are minimized in the ICP discharge or are located in a separate region of the discharge. In addition, the... 

The principle of this process is shown schematically in Fig. 7 a. Mass spectrometry of the secondary ions provides a sensitive multi-element analysis of fresh surfaces. Figure 7 b shows the spectrum of a vanadium surface containing zirconium near the surface. With a current intensity of 5x I0 A a surface area of 0.1 cm was bombarded with 3 keV Ar" ions. Only 100 seconds were required for recording of the spectrum. In this time a monomolecular layer of vanadium oxide was analyzed which corresponds to a weight of about 10" g. Zirconium in the surface is identified by registration of the mass lines 90, 91, 92 and 94. These lines represent the naturally occuring isotopes of this group IV a element. In the secondary ion... 

Each spectrometer for sequential or simultaneous multi-element plasma AES measurements is equipped with a monochromator in order to (i) have an adequate wavelength selection, and (ii) collect as much light as possible from a selected spectrum area in the radiation source. A monochromator consists of a) an entrance slit, b) a collimator to produce a parallel beam of radiation, (c) a dispersing element (a prism or a grating), d) a focusing element reforming the specific dispersed narrow bands of radiation, and (e) one or more exit slits to isolate the desired spectral band or bands. 

Fig. 11.15 Energy-dispersive XRF spectrum of a multi-element standard, obtained in a TXRF spectrometer.

Depending on the X-ray source and the spectral modification devices, the LD are in the pg range for 2—3 kW X-ray tubes and in the fg range with excitation by means of synchrotron radiation. Figure 11.15 shows a typical TXRF spectrum the absolute detection limit values of typical TXRF instruments are shown in Fig. 11.10. Thus, TXRF permits to simultaneously determine trace elements in samples of small volume. Additional advantages are insensitivity to matrix effects, easy cahbration, fast analysis times and low cost. In practice, the method is in particular apphed for multi-element determinations in water samples of various nature and for the routine analysis of Si-wafer surfaces employed in the microelectronics industry. 

In the early stages of laser dififaction, the diffraction pattern was recorded by photographic plates (Stromgren 1975, Biichtemann et al. 1982). Today multi-element photodetectors of varying geometry are employed (Heuer and Leschonski 1985, Xu 2000, pp. 135). That means that a discrete power spectrum is measured which deviates significantly in shape from the continuous intensity spectmm (spatial intensity distribution). 

Let the number of individual units in multi-element models tend to infinity. For creep, an infinite number of Kelvin units gives an infinite number of retardation times this is called the spectrum of retardation times. The analagous development for stress relaxation leads to the spectrum of relaxation times. 

The superiority of the Ge(Li) detector, relative to the Nal(Tl) detector, for multi-element analysis is once more confirmed by the recent comparison of Nal(Tl) with Ge(Li) spectra of the same samples of atmospheric pollutants and forensic materials. Although often of lower efficiency than NaI(Tl) detectors, the higher resolution and higher P/C ratio of the Ge(Li) detector means that low-intensity peaks in a complex y-spectrum are more easily detected, more positively identified, and more precisely measured. 

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