Arcs, atomic spectroscopy

A At Harvard 1 took a course on quantitative analysis for which we had to do the gravimetric analysis of calcium in limestone. But the instructor told us that we were wasting our time any sensible person would use atomic spectroscopy. I asked what it was and he told me to read a small book written by Gerhard Herzberg, who would later win a Nobel Prize for spectroscopy. 1 did, and that summer at home I made my own carbon arc for taking atomic spectra of various compounds. 

In addition to the continuum sources just discussed, line sources are also important for use in the UV/visible region. Low-pressure mercury arc lamps are very common sources that are used in liquid chromatography detectors. The dominant line emitted by these sources is the 253.7-nm Hg line. Hollow-cathode lamps are also common line sources that are specifically used for atomic absorption spectroscopy, as discussed in Chapter 28. Lasers (see Feature 25-1) have also been used in molecular and atomic spectroscopy, both for single-wavelength and for scanning applications. Tunable dye lasers can be scanned over wavelength ranges of several hundred nanometers when more than one dye is used. 

In the early days of atomic spectroscopy, dc and ac arcs and high-voltage sparks were popular for use in excitation of atomic emission. Such sources have almost entirely been replaced by the ICP. 

Among the various types of atomic spectroscopy, only two, flame emission spectroscopy and atomic absorption spectroscopy, are widely used and accepted for quantitative pharmaceutical analysis. By far the majority of literature regarding pharmaceutical atomic spectroscopy is concerned with these two methods. However, the older method of arc emission spectroscopy is still a valuable tool for the qualitative detection of trace-metal impurities. The two most recently developed methods, furnace atomic absorption spectroscopy and inductively coupled plasma (ICP) emission spectroscopy, promise to become prominent in pharmaceutical analysis. The former is the most sensitive technique available to the analyst, while the latter offers simultaneous, multielemental analysis with the high sensitivity and precision of flame atomic absorption. 

Bulk chemical analysis X-ray fluorescence spectroscopy Atomic absorption spectroscopy Inductively coupled plasma emission spectroscopy Direct-current plasma emission spectroscopy Arc emission spectroscopy Gravimetry Combustion Kjeldahl Impurities... 

Many analytical applications of atomic spectroscopy produce their spectra by arc or spark excitation techniques and these methods form the basis for much of the present practice in the field. The historical development in this area is most difficult to document since almost from the start, after observations of the spectrum from the sun, the attempt was to utilize high-energy sources. This led immediately to arc and spark methods. The present-day applications of the arc or spark are improvements of the early work with attempts to better stabilize and control excitation conditions within the arc or spark in an effort to improve analytical data derived from the spectra. These techniques will be discussed in Chapter 5, which deals with accessory equipment for arc and spark spectrochemical analysis. 

Atomic fluorescence is the most recent development in analytical atomic spectroscopy thus it has not had time to be evaluated as well as other techniques. Further developments in this field with respect to optimizing sources and sample cells, together with improvements in instrumental parameters and development of readily available commercial instrumentation, should lead to this technique serving in the area of analytical spectral methods to supplement the already well-established arc and spark emission, flame emission, and atomic absorption spectroscopy. 

A plasma may be defined as a gas containing a relatively large number of ions and free electrons. To produce a plasma, an energy source is required and for analytical atomic spectroscopy three different excitation methods have been used. They are (1) a dc arc, (2) radiofrequency energy coupled through a microwave cavity, and (3) radiofrequency energy inductively coupled to the plasma. 

This textbook is an outgrowth of the author s experience in teaching a course, primarily to graduate students in chemistry, that included the subject matter presented in this book. The increasing use and importance of atomic spectroscopy as an analytical tool are quite evident to anyone involved in elemental analysis. A number of books are available that may be considered treatises in the various fields that use atomic spectra for analytical purposes. These include areas such as arc-spark emission spectroscopy, flame emission spectroscopy, and atomic absorption spectroscopy. Other books are available that can be catalogued as methods books. Most of these books serve well the purpose for which they were written but are not well adapted to serve as basic textbooks in their fields. 

This book is intended to fill the aforementioned gap and to present the basic principles and instrumentation involved in analytical atomic spectroscopy. To meet this objective, the book includes an elementary treatment of the origin of atomic spectra, the instrumentation and accessory equipment used in atomic spectroscopy, and the principles involved in arc-spark emission, flame emission, atomic absorption, and atomic fluorescence. 

Giinther, D., Jackson, S.E., Longerich, H.P. (1999) Laser ablation and arc/spark sohd sample introduction into inductively coupled plasma mass spectrometers. Spectrochimica Acta Part B Atomic Spectroscopy, 54,381-409. 

Atomic spectroscopy refers to a wide range of techniques that are applied to materials that have been reduced to atomic or ionic forms or species in an electric arc, flame, or plasma. When one speaks of atomic spectra, molecules and compounds that have been of discussion up to now are no longer an issue. AS is in fact a monumental extension and improvement upon the flame color tests that were mentioned at the beginning of this article. 

Aluminum is best detected quaUtatively by optical emission spectroscopy. SoHds can be vaporized direcdy in a d-c arc and solutions can be dried on a carbon electrode. Alternatively, aluminum can be detected by plasma emission spectroscopy using an inductively coupled argon plasma or a d-c plasma. Atomic absorption using an aluminum hoUow cathode lamp is also an unambiguous and sensitive quaUtative method for determining alurninum. 

Automatic Atomic Emission Spectroscopy, 2nd edn, Slickers, K., Briihlsche Universitatsdmckerei, Giessen, 1993. A very useful practical guide to arc and spark methods in the metallurgical industry. 

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