Topics in..
. Chemical Instrumentation
Edited by 5.
Z. LEWIN.
N e w York Universify, N e w York 3, N. Y.
These articles. most of which are to be contributed Lu auesl oulhors, are intended lo s e r e lhe reahis of lhis JOURNAL by calling a i e h o n lo new devdoome~tlsin the theoru. desion. or availabilitu of chemical IaCoraIor~ instrumenlation, oi by present& us& insights and"e~planationsof topi; that are of practical importance to those who use, or leach the use of, modern instrumenlalion and inslrumalal lechnipes.
X. Emission Spectroscopy.
Part One
Stephen E. Wiberley and Herbert H. Richfol, Department of Chemistry Rensseber Polytechnic Institute, Troy, New York The field of emission spectroscopy is a very broad one and for adequate coverage would require an excessively long article. This review will attempt to emphasize a few of the highlights and recent develapments in this field. Flame photometry mill not he included since a previous article in this series dealt exclusively with this subject. G e n e r a l Principles Emission spectra can arise from excited atoms or molecules with atoms yielding line spectra and molecules band spectre. on the photographic plate. Under high resolution the band spectrum of a. simple molecule consists of many fine lines closely grouped together so the differences betweensome hand spectra. and line spectra are not ae pronounced as a casual inspection of the photographic plate would first indicate. The instrumentation for emission specbroscopy will he discussed in terms of line or atomic spectra because the major use liesin this area,rather than in the area of band spectra. However, the same instrumentation can also produce excellent band spectra as well.
An electron in the ground state may be excited to a higher energy level if suitable energy can be imparted to the electron by a flame, arc or spark. On returing from this higher energy level to a. lower level, radiation is emitted with a. characteristic wavelength such that tt = hc/h where Ah' ia the energy in ergs, h is Planck's constant, approximately 6.6 X lo-%' ergsee, c is the speed of light, approximately 3 X 10-'0 cm/sec, and A is the wa~elength in centimeters. I n emission spectroscopy the wavelengths are reported in angstrom units with the angstrom being originally delined as 10-lo m in terms of the standard meter bar. Because the meter bar was not an adequate standard for wavelength measurements, the angstrom was redelined a4 a unit of length equal to 1/ 6438.4696 of the wavelength of the red line of cadmium. This value was almost but not exactly 10-'Om. Mast recently ( I ) the meter has been delined as 1,650,763.73 wavelengths (in vacuum) of the orangered line of K+. With this new delinition the angstrom is again exactly lo-" m or em. Since only certain discrete energy transitions are dl.llow.ed, the radiation is quan-
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5. E. Wiberley i s Amociote Dean of the Sroduote School a t R.P.I. He war edumted ot W8llioms College I8.A. 19411 and 1.P.I. (MS.1948; Ph.D. 19501. He teacher molyticol rhemldry and ir ovthor of three extbaokr and oppmrimotely 5 0 papers in uch research are03 or spectroscopy and nstrumentol onolysis.
Herbert H. Richtol is Arrirtont Proferror of inalyticol Chemistry ot R.P.I. He war tducoted at St. Lawrence Univerlity 18.5. 9541, and New York Un~verrity (Ph.0. 9 6 1 ) . He teacher anolyticol chemistry and has relearch interests in the fields of pectrorcopy, luminescence and energy ronsfer processes.
tieed and the wavelen@,ht.hsof the emitted radiation are characteristic of the atom producing the radiation. By measuring these wavelengths or by comparison with spectra, of known elements the atom or element can be identified qualitatively. The intensity of a given emission line of an element is a. function of the probability of the electronic transition taking place and the number of atoms of the element concerned. By measuring the line intensity either photographically or with a phomultiplier tube, quantztatzue analysis can be carried out. :lgure 1.
Elements reodily detectable spectrographically.
(Continwd on page A9281 Volume 40, Number 12, December 1963
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Chemical instrumentation -
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Detection of the Elements Of the total of over 100 elements which bave been discovered, including those made artificially in trace amounts, some 87 have been investigated spectragraphically. However, only 72 elements can be readily detected and these elements are shorn in Figure 1. De Gramant studied the emission lines that persisted as the concentration of the element was decreased. In 1920 he published ( 8 ) n table of persistent lines also knaan as sensitive or principle lines (raies ultimes or letzten l i n k ) . I n this table he indicated the raies sensible8 and rales ultimes as the persistent lines in the visible and ultraviolet rezions of the
in this sense by spectrographers today. Using a DC open arc as a source of eacitation and B selected RC line in the range of 20004600 elements detectable in the 10-100 ppm limit include As, Cd, Co, Ga, Ni, Pb, Sb, Sn, Sr, TI, V, Zn, and Zr and > 1001000 limit include Hg, Se, Te, Th, and U (4). With the Stallwoad Jet, which cnntrols the direct current arc by surrounding a small diameter electrode vertically with a stream of a mixture of 70 parts argon to 30 parts oxygen, the sensitivity is improved by a factor of 2 or more for most elements. In addition, quantitative accuracy is improved (6). Although not shown in Figure 1 such nonmetsllio elements as C. S. Se. P. F. CI. Br. and I and the gaseous elements buch as H., O., and N2can be determined spectrogmphically. The eharacteriatic spectml lines of the non-metallic elements have excitation potentials two to four times greater than those of the metallic elements with the principal lines being below 2000 A 80 either a vacuum or an inert atmosphere must be used in the spectrographic arrangement.. Fluorine, chlorine, and bromine combined k t h an alkaline earth can be analyzed with a D C arc by measurement of the molecular bandhead. For example, calcium fluoride has a strong handhead a t 5291 A enabling measurement of fluorine in the 50 ppm range. Carbon can be analyzed ffom bandheads a t 3590, 3883, and 4216 A of eyanogen formed from the combination of nitrogen in the air with carbon in the arc. I n the study of detection limits the purity of the electrodes is an important factor. High purity graphite electrodes are sold with an analysis report containing an estimation of the impurities present. I Until recently moat graphite electrodes ' contained boron and vanadium as impurities as well as &J~
