The complementary nature of X-ray fluorescence methods and optical emission methods in spectrochemical analysis

Abstract

THE advantages of spectrochemical methods of analysis over conventional chemical methods of analysis would appear so well known as to require no discussion. It may be worthwhile, however, to point out, for purposes of this paper, that spectrochemistry offers one basic advantage over wet chemistry, and that is high speed of analysis. This high speed is gained in several ways, one of the most important of which is t,he small amount of time for sample prcparrrt.ion that is required. The fact that a metal sample can bo cast from a spoon at the furnace, in a form just as suitable for spectrochemicnl analysis as for chcmicsl, puts the two systems on a par at that point. However, in one minute or less after pouring, such a sample can be surfacrtl and thus prepared for spectrochemical analysis, while to put it in solution chemically may require from several minutes to an hour, depending upon the particular alloy involved. The reason such a sample is ready for optical emission spectrochemical analysis rests, of course, on the fact that metals are electrical conductors. 13ecauso of this, a spark or arc-like clischnrgo can be passed directly to the sample, and usrd to volatilize the various elemental constituents, nntl in turn, excite their spectra. As soon ns R sample which is an electrical nonconductor must bc anelysed, a good deal of the speed of analysis attained by the short time for snmplc prcp~~rnt ion is immediately lost. It is at t.his point that S-ray fluorescence mhLttracti\-r, as a sample in any form, be it elcct~rira.lly conductive or not,, can be irradinterl by S rays an11 caused to in turn emit X radiations which arr characteristic of each of the elements prcs(lnt in the sample. Since this is the case, this would stem t,o immediately lead to t.he conclusion that. X-ray analysis is the preferred method, and that optical emission methods must trdcc sccon~l place. Actually, with the present statrsof t.he sciences inl-ol\-et], thercnrc large areasof analytical work where one excels over the other, nncl areas where both methods are rff(,cti\-c. T~II~, at this time it is far better to think of these two methods as being complementary, rnthcr than competitive. In thinking in terms of the analysis of all the elements in the periodic system, the basic differences in the cxcit&ion of optical spcctrn, as compared t,o S-ray spectra should be kept in mind. Optical emission analysis is concerned with the outermost electrons of the atom, while the S-ray spectrum is concerned with the innermost. This mn.kes optical emission phenomena reoccur for various elements throughout the periodic syst.cm as electron s1u~1Is are added, while it makes X-ray phenomena change in a regulnr me-nncr from ono end of the perioclic system to the other as the atomic nuclei increase in charge. Bec~se of t.his, in optical emission,phenomena, the elements in the same group or column of t.he periodic system behave similarly while in X ray the elements in the same series or row \vill br Inore similt%r. In considering the sensitivity of detection and measurement of one elrment, xvhich w-ill bo referred to as the analyte in the presence of R large qunntit.y of another element, which will be termed the matri.~, the physical and spectral chnrecteristics of both elements are of @most equal importance. t It is well known in optical emission analysis that though the probnbility of the electron trensit,ion determines the basic sensitivit.)of a spectrum line, the practical scnsitil-ity of analysis is dependent also upon the bnckgrotmd present. at the wnve1engt.h of the lincl being utilized. This background may consist of line, band, or continuous spectrum or any colnl)in~~~ion of these, but in general its characteristics will be dependent upon the m1ltrix element. ‘i’llus in determining the sensitivity of detection of a spectrum line, the matrix element as well ads the means of spectrum excitation must be considered. HARVEY’S t,nbles [l] provide this basic information for a .semiquantitMive method of optical emission rma.l+s emplnying the d.c. arc,