In atomic emission spectroscopy

Figure 1.2 shows the basic instrumentation necessary for each technique. At this stage, we shall define the component where the atoms are produced and viewed as the atom cell. Much of what follows will explain what we mean by this term. In atomic emission spectroscopy, the atoms are excited in the atom cell also, but for atomic absorption and atomic fluorescence spectroscopy, an external light source is used to excite the ground-state atoms. In atomic absorption spectroscopy, the source is viewed directly and the attenuation of radiation measured. In atomic fluorescence spectroscopy, the source is not viewed directly, but the re-emittance of radiation is measured. 

If the linear dispersion of a spectrograph used in atomic emission spectroscopy is 2 mm for a difference in wavelength of 1 nm and the size of the exit aperture is 20 pm, calculate the spectral interval of the emissions which reach the detector. 

How would emission intensity be affected by a 10 K rise in temperature In Figure 21-14, absorption arises from ground-state atoms, but emission arises from excited-state atoms. Emission intensity is proportional to the population of the excited state. Became the excited-state population changes by 4% when the temperature rises 10 K, emission intensity rises by 4%. It is critical in atomic emission spectroscopy that the flame be very stable or emission intensity will vary significantly. In atomic absorption spectroscopy, temperature variation is important but not as critical. 

The ionization energy of Ar is 15.8 electron volts (eV), which is higher than those of all elements except He, Ne, and F. In an Ar plasma, analyte elements can be ionized by collisions with Ar+, excited Ar atoms, or energetic electrons. In atomic emission spectroscopy, we usually observe the more abundant neutral atoms, M. However, the plasma can be directed into a mass spectrometer (Chapter 22), which separates and measures ions according to their mass-to-charge ratio.17 For the most accurate measurements of isotope ratios, the mass spectrometer has one detector for each desired isotope.18... 

In atomic emission spectroscopy, the radiation source is the sample itself. The energy for excitation of analyte atoms is supplied by a plasma, a flame, an oven, or an electric arc or spark. The signal is the measured intensity of the source at the wavelength of interest. In atomic absorption spectroscopy, the radiation source is usually a line source such as a hollow cathode lamp, and the signal is the absorbance. The latter is calculated from the radiant power of the source and the resulting power after the radiation has passed through the atomized sample. 

In atomic emission spectroscopy, the analytical signal is produced by the relatively small number of excited atoms or ions, whereas in atomic absorption the signal results from absorption by the much larger number of unexcited species. Any small change in flame conditions dramatically influences the number of excited species, whereas such changes have a much smaller effect on the number of unexcited species. 

In atomic emission spectroscopy, the electrons of an element are excited by heating or by an electric discharge. The frequencies of emitted photons are then determined as the electrons release energy. 

Figure 11.15 Schematic diagram of an inductively coupled plasma located within its torch, as employed in atomic emission spectroscopy. From Dean, J. R., Atomic Absorption and Plasma Spectroscopy, ACOL Series, 2nd Edn, Wiley, Chichester, UK, 1997. Reproduced with permission of the University of Greenwich.

There are different techniques in atomic emission spectroscopy that are based upon the types of excitation and detection used. Under this heading arc and spark excitation and photographic and multiphotometric detection will be discussed. Flame photometry although by principle belonging to this group will be discussed together with atomic absorption spectrometry. 

Inconspicuous instrumental, environmental, or chemical effects often cause a loss of instrument response. In atomic emission spectroscopy, for example, sensitivity is affected by such instrumental factors as flame temperature, aspiration rate, and slit width. In amperometric measurements, diffusion currents vary with temperature, and a significant loss in sensitivity may occur with a drop in sample temperature. In ion-selective electrode measurements, sensitivity may be affected by chemical effects, such as changes in ionic strength or pH. 

Classical excitation sources in atomic emission spectroscopy do not meet these requirements. Flame, arc, and spark all suffer from poor stability, low reproducibility, and substantial matrix effects. However, the modem plasma excitation sources, especially ICPs, come very close to the specification of an ideal AES source. 

Spectral interference has been well studied and are probably best understood in atomic emission spectroscopy. The usual remedy to alleviate a spectral interference is to either increase the spectral resolution of the spectrometer (which often is not possible with a given type of instrument) or to select an alternative emission line. Three types of spectral interference can be discriminated 1. Direct wavelength coincidence with another emission line, 2. partial overlap of the hne under study with an interfering line in close proximity, 3. a linear or non-linear increase or decrease in background continuum (see Fig. 12.33). 

Because many elements have several strong emission Hnes, AES can be regarded as a multivariate technique per se. Traditionally, for quantitative analysis in atomic emission spectroscopy, a single strong spectral line is chosen, based upon the criteria of Hne sensitivity and freedom of spectral interferences. Many univariate attempts have been made to compensate spectral interferences by standard addition, matrix matching, or interelement correction factors. However, all univariate methods suffer from serious limitations in a complex and Hne-rich matrix. 

In atomic emission spectroscopy flames, sparks, and MIPs will have their niche for dedicated apphcations, however the ICP stays the most versatile plasma for multi-element determination. The advances in instrumentation and the analytical methodology make quantitative analysis with ICP-AES rather straightforward once the matrix is understood and background correction and spectral overlap correction protocols are implemented. Modern spectrometer software automatically provides aids to overcome spectral and chemical interference as well as multivariate calibration methods. In this way, ICP-AES has matured in robustness and automation to the point where high throughput analysis can be performed on a routine basis. 

Because of the unique characteristics of their emitted energies, lasers have been used for sample vaporization and excitation sources in atomic emission spectroscopy. They also have been used as sources for atomic absorption and atomic fluorescence analysis. Their application in these areas will no doubt increase as lasers become cheaper and more readily available. 

In addition to these induced effects, even undisturbed excited states will not live forever. The general deactivation is a radiationless process. Relatively few molecules exhibit spontaneous emission, called luminescence in the visible, or emission. This deactivation process of the excited state is a statistical effect and does not directly correlate with an act of excitation. Except induced absorption, plasma coupling, hot flames, or sparks can yield a relatively high population in the excited state which will depopulate by emission. This emission is used in analytics, especially in atomic emission spectroscopy. Since atoms in the gases are not influenced by the surrounding and their energies are not smeared by vibrational interactions, they will exhibit sharp characteristic lines for different metals. The advantages are discussed in more detail in Chap. 6 of this book. 

McCord P, Yau S-L, Bard AJ (1992) Chemiluminescence of anodized and etched sihcon evidence for a luminescent siloxene-like layer on porous silicon. Science 257 68-69 Mikulec FV, Kirtland JD, Sailor MJ (2002) Explosive nanocrystalline porous silicon and its use in atomic emission spectroscopy. Adv Mater 14 38-41 Parimi VS, Tadigadapa SA, Yetter RA (2012) Control of nanoenergetics through organized micro-stmctures. J Micromech Microeng 22 055011(1-6)... 

In recent years, the laser has become very useful in atomic emission spectroscopy. We con-sidcr here two laser-based techniques laser microprobe spectroscopy and laser-induced breakdown spectroscopy (LIBS). 

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