Ultraviolet spectroscopy spectra , elements

Noble gases are intrinsically difficult to detect by spectroscopy. For example, solar photospheric spectra, which form the basis for solar abundance values of most elements, do not contain lines from noble gases (except for He, but this line cannot be used for abundance determinations). Yet, ultraviolet spectroscopy is the only or the major source of information on noble gas abundances in the atmospheres of Mercury and comets. In the Extreme Ultraviolet (EUV), photon energies exceed bond energies of molecules and the first ionization potential of all elements except F, He, and Ne, so that only these elements are visible in this part of the spectrum (Krasnopolsky et al. 1997). Other techniques can be used to determine the abundance of He where this element is a major constituent. Studies of solar oscillations (helioseismology) allow a precise determination of the He abundance in the solar interior, and the interferometer on the Galileo probe yielded a precise value for the refractive index and hence the He abundance in the upper atmosphere of Jupiter (see respective sections of this chapter). 

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. 

Elements that are readily observed in the visible spectrum of H II regions include O, N, Ne, S, and Ar. With the exception of O, all of these elements may have important ionization states that emit only in the ultraviolet or infrared (for example, Ne+, N+2, S+3). If we add UV spectroscopy we can study C and Si. Figure 1 shows an HST spectrum of one H II region in the SMC, showing the rich variety of forbidden emission lines and H, He recombination lines in the UV and optical spectrum. Other abundant heavy elements (such as Fe or Mg) may be observed in photoionized nebulae. One must always keep in mind that many elements in the ISM are strongly depleted onto grains,... 

Historically important in the development of modern atomic theory was the recognition that although polyatomic molecules show more or less broad bands of absorption and emission in the visible and ultraviolet regions of the spectrum, the characteristic light absorption or emission by individual atoms occurs at fairly narrow lines of the spectrum, which correspond to sharply defined wavelengths. The line spectrum of each element is so uniquely characteristic of that element that atomic spectroscopy can be used for precise elementary analysis of many types of chemically complex materials. 

Emission spectroscopy provides an ideal method for qualitative analysis, since each atomic species has its own unique line spectrum. Spectral lines have two characteristics useful for qualitative analysis (1) their wavelengths and (2) their intensities. It is the pattern of wavelength distribution that is primarily used for qualitative analysis, although the relative intensity distribution also can be helpful to verify spectral lines to identify an element. About 70 elements are easily identified by spectral methods. Those that are more difficult to identify include the gases and a few nonmetals, primarily because sensitive lines lie in the short ultraviolet portion of the spectrum that is difficult to observe. 

Spectroscopy n. The study of electromagnetic waves that are absorbed or emitted by substances when excited by the arc, a spark. X-rays, or a magnetic field. Each element emits fight of characteristic wavelengths, by which minute quantities can be detected and estimated. An example of a spectrophoto-mer for the ultraviolet and visible spectrum is the Perkin-Efiner Lambda 25/35/45 UV/ VIS (Courtesy of Perkin-Efiner Inc.). 

In the MCR framework, there are few cases in which the quantitative analysis is based on the acquisition of a single spectrum per sample, as is the case for classical first-order multivariate calibration methods, such as partial least squares (PLS), seen in other chapters of this book. There are some instances in which quantitation of compounds in a sample by MCR can be based on a single spectrum, that is, a row of the D matrix and the related row of the C matrix. Sometimes, this is feasible when the compounds to be determined provide a very high signal compared with the rest of the substances in the food sample, for example colouring additives in drinks determined by ultraviolet—visible (UV-vis) spectroscopy [26,27]. Recently, these examples have increased due to the incorporation of a new cmistraint in MCR, the so-caUed correlation constraint [27,46,47], which introduces an internal calihratimi step in the calculation of the elements of the concentradmi profiles in the matrix C related to the analytes to be quantified. This calibration step helps to obtain real concentration values and to separate in a more efficient way the information of the analytes to be quantified from that of the interferences. 

Optical emission spectroscopy includes the observation of flame-, arc-, and spark-induced emission phenomena in the ultraviolet, visible, and near infrared regions of the electromagnetic spectrum [38]. Qualitative and quantitative information can be gained from the intensity of the characteristic emission wavelengths. Analysis of lead in environmental samples (e.g., soils, rocks, and minerals) may be performed reproducibly down to the 5 ppm level. Emission spectroscopy is best used for the multi-elemental analysis of samples, because of the high cost of equipment. Usually, single element analyses are not performed on a emission spectrograph. 

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