Spark-source optical emission spectrometry

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors are used in every technique except X-ray fluorescence where proportional counting or scintillation devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by optical emission spectrometry, but is now rarely used. 

Atomic emission spectrometry (AES) is also called optical emission spectrometry (OES). It is the oldest atomic spectrometric multielement method which originally involved the use of flame, electric arc or spark excitation. Recently there has been considerable innovation in new sources plasma sources and discharges under reduced pressure. Littlejohn et al. (1991) have reviewed recent advances in the field of atomic emission spectrometry, including fundamental processes and instrumentation. 

Furthermore, it is desired that atomization and excitation occur in an inert chemical environment to minimize possible interferences. Different flame, spark, and arc somces have been used as the excitation sources since the beginning of the twentieth century however, none of these approximates the fiiU fist of conditions fisted above. It was not until mid-1960s when the analytically useful plasma sources were developed, subsfantially improving fhe capabilities of OES. The first commercially available inductively coupled plasma optical emission spectrometry (ICP-OES) was introduced in 1974 and since then the revival of OES can be noted. 

Preferred methods in trace determination of the elements include atomic absorption spectrometry (AAS), optical emission spectrometry (OES) with any of a wide variety of excitation sources [e.g., sparks, arcs, high-frequency or microwave plasmas (inductively coupled plasma, ICP microwave induced plasma, MIP capacitively coupled micro-wave plasma, CMP), glow discharges (GD). hollow cathodes, or laser vaporization (laser ablation)], as well as mass spectrometry (again in combination with the various excitation sources listed), together with several types of X-ray fluorescence (XRF) analysis [51]. 

Several other methods have been used to determine the trace elements in the mineral matter of coal, as well as in whole coal and coal-derived materials. These methods include spark-source mass spectrometry, neutron activation analysis, optical emission spectroscopy, and atomic absorption spectroscopy. 

C. R.J. Conzemius, Analysis of rare earth matrices by spark source mass spectrometry 377 37D. E.L. DeKalb and V.A. Eassel, Optical atomic emission and absorption methods 405 37E. A.P. D Silva and V.A. Eassel, X-ray excited optical luminescence of the rare earths 441 37E. E.W.V. Boynton, Neutron activation analysis 457... 

Tt may be safe to say that the interest of environmental scientists in airborne metals closely parallels our ability to measure these components. Before the advent of atomic absorption spectroscopy, the metal content of environmental samples was analyzed predominantly by wet or classical chemical methods and by optical emission spectroscopy in the larger analytical laboratories. Since the introduction of atomic absorption techniques in the late 1950s and the increased application of x-ray fluorescence analysis, airborne metals have been more easily and more accurately characterized at trace levels than previously possible by the older techniques. These analytical methods along with other modem techniques such as spark source mass spectrometry and activation analysis... 

Emission spectrometry using chemical flames (flame atomic emission spectrometry, FAES) as excitation sources is the earlier counterpart to flame atomic absorption spectrometry. In this context emission techniques involving arc/spark and direct or inductively coupled plasma for excitation are omitted and treated separately. Other terms used for this technique include optical emission, flame emission, flame photometry, atomic emission, and this technique could encompass molecular emission, graphite furnace atomic emission and molecular emission cavity analysis (MEGA). 

This chapter deals with optical atomic, emission spectrometry (AES). Generally, the atomizers listed in Table 8-1 not only convert the component of samples to atoms or elementary ions but, in the process, excite a fraction of these species to higher electronic stales.. 4, the excited species rapidly relax back to lower states, ultraviolet and visible line spectra arise that are useful for qualitative ant quantitative elemental analysis. Plasma sources have become, the most important and most widely used sources for AES. These devices, including the popular inductively coupled plasma source, are discussedfirst in this chapter. Then, emission spectroscopy based on electric arc and electric spark atomization and excitation is described. Historically, arc and spark sources were quite important in emission spectrometry, and they still have important applications for the determination of some metallic elements. Finally several miscellaneous atomic emission source.s, including jlanies, glow discharges, and lasers are presented. 

For ion detection, several approaches are possible. Classical spark source mass spectrometry has developed the use of photographic plates. Very hard emulsions with low gelatin content are required. Emulsion calibration is similar to that described in optical emission spectrography, but usually varying exposure time are applied instead of using optical step filters. Provided an automated microdensitometer is used, mass spectrography is still a useful tool for survey analysis of solids down to the sub-pg/g level. 

The analytical chemistry of rare earths has been reviewed by Banks and Klingman (1961), Loriers (1964), Ryabchikov (1959), and Ryabchikov and Ryabukhin (1964). Fassel (1961) reviewed the analytical spectroscopy of rare earth elements. In volume 4 of this Handbook chapters can be found on the chemical spectrophotometric and polarographic methods (O Laughlin 1979), spark source mass spectrometry (Conzemius 1979, Taylor 1979), optical atomic emission and absorption (DeKalb and Fassel 1979), X-ray excited optical luminescence (D Silva and Fassel 1979), neutron activation (Boynton 1979), mass spectrometric stable isotope dilution analysis (Schuhmaim and Philpotts 1979), and shift reagents and NMR (Reuben and Elgavish 1979). 

Slavin (1971). The book by Slavin (Emission spectrochemical analysis) is an excellent book by the senior member of an atomic spectrometry family on emission spectrochemistry covering important advances since the 1950 s in theory and optical, electronic, measurement and computer technologies. The underlying technique is principally arc/spark emission spectrography/ spectrometry with mention of new excitation sources such as the flame, high-frequency discharge and plasma jet coming into play at that time. 

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