tor have to convert intcilsity ratios to percentage composition b y referring to tables, or utilize direct-reading charts, or make calculations for
interelement effects. For these mathematical chores the computer 1s ideally suited I n fact they v,ould seem so siirple that a digital computer may appcar to be wasted in that RII analogLC type might appear adcquate to handlr this part of the routine. Vpon examining the problem in more detail, in thc light of what is known up to this time, it becomes apparent that in many cases, a digital computer could be justified even for this part of the operation. Take th case of a modern aluminum alloy piani, for instance. Sueti a plant ma:; easily produce 200-300 different types of alloys, c:wh type implying a different range of concentrations for the various alloying elemcnts. With ten t o twenty elements ir, each alloy, sevwal thousand analytical curves are ~nvolvedrelating the concentration of an element to an intensity ratio of a line of the element to a line of the internal stnndexd, which in this case is aluminum Because each curve usually requires three constants a t a minimum, representing background, slope and curvature, this would reyuirci the storage of many thousands of constants, just to handle this relatively simple part of the operation. Besides this, it would b e necessary for the operator to signal the computer as t o the proper alloy type being analyzed, which might be quite difficult in the early stages of wrap melt down, for instance. However, before getting involved into all of the pos,ihie difficulties, some of the details involved in converting line intensities to element concwtrations shoiild be reviewd Al! spectrochemist:, working in the field of optical emission learn, as
rGRATlNG
Figure 1. Optical ernkcton direct reader equipped with a digital computer
almost their first lesson, the need for an internal standard so that the fluctuations inherent in c.lectrica1 sources can be cancelled out by ratioing the intensity of the line of the element to be determined to the intensity of a line of the internal standard. This, in the first place, requires the choice of one element as an internal standard, which may not be obvious in a high alloy system, and in the second place, it forces this element to be treated differently from all other dement:: in the system as its intcrisity must bc considered as a constant irrespective of how it may vary ab the c.oiic.cntri,tion of the internal -t:tndarc! elcment variep “hi> facior alonc makes many analyticd tx::ve? of the alloying c1cme1lc; appear to vary from one allo:, Iype t o another, j lien in truth, chs. only thing which has chinged i i t l i c roncentration of the intei tini sta~11q:ird d e nwnt and heiicc the iritensity or the ~~, a controlling lint. F f o w e ~ with digital compute: thc ~nternalstsn-
dard concept may be xrappeci and. the major constituent (tan be trcated on equal terms with any other element. This also resolves another problem which is inherent in the “mutual standard’’ method proposed by Coulliette ( 2 ) . He statrd that in optical emission spectroscopy the ratio of concentrations of two element,s is some iunction of t i e rnt,io of the intensitiez of bpectram lines of t h e eleinents. Al~iiougtiia thc ASTM publiciation ‘ . ; ? l e t h i s for Eniission Spcatrn~hemicnl Analysib, 1957‘’ (3)L: ’ showed tliht, t,his was true only under very special circiimstanct.s, niosL workers have choscn t’o ignore thc.se limitations and have taken the reIxtior,ship as n iaw of spectrochemistry, with anornaious results in s o m ~cases. H o w docs the romputer allow the iriLi:r:\:il st:tndar.d c;mci.pt t o br hrvached‘.’ By opcrathg the system in absolutc intensity tei-ms the interna: standard element, aluminum in c !r c,xaaplc, may be determined wit,h respect to concentralion VQ:.
39, NO. 6, k A Y 1967
REPORT FOR ANALYTICAL CHEMISTS
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RELATIVE NUMBER O F ATOMS IN DISCHARGE
SUPPRESSION
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ABSOLUTE LINE INTENSITY
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ALUMINUM CONCENTRATION VI RELATIVE LINE INTENSITY
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.90 .92 .94 .96 .98 1.00 1.10 1.20 A I LINE INTENSITY
Figure 2. air)
Model for an optical emission system (aluminum alloy sparked in
along with all the other elements. I n other words, the conversion to intensity ratios, such as Icu/Idlfor the determination of copper in aluminum, is a step which destroys information a t the outset and may be avoided when a digital computer becomes part of the system. ,Many models of the action of the electrical discharge on the surface of a sample of a complex alloy sysLem may be proposed and tried, when a digital computer is available and absolute intensity data have been collected for a wide variety of standards covering all alloy types t o be analyzed. Besides tlw information availablc in the absolute intensities, the fixed relationship that the sum of the concentrations of all elements equals 100% may be used to good advantage. .llso the iterative power of the com~ i u t e ris a powerful aid, because r w n y nonlinear relntionships can be combined and made to converge to a fixed value, auch as 100% composition. The relation>liips between line intensity and concentration in one s:ich model are shown in Figure 2, which conccrns the determination of the mcjor constituent aluminum in alloy systems which involve Cu, Mg, Si, and Zn separately or in combination, with totitl alloying rlements often amounting to ten or fifteen percent. T h e suppressing effect of 3lg and Zn and the enANALYTICAL CHEMISTRY
hancing effect of Si on the total number of atoms present per stroke of the spark discharge is taken into account by utilizing the left hand scale and curve involving numbers of atoms, while the right hand scale and curve are utilized to convert normalized intensity values to concentration of aluminum. The advantage of finding a model which approaches reality is that the ensuing computations can be made relatively simple by utilizing a minimum of mathematical constants, because the gross effects involving such things as changes in the duration of each spark stroke or changes in the spark plasma temperature due to alloying elements are taken into account only once, and not repeatedly, in the interaction t e r n of every element’s effect on every other element’s determi n at i on. Preliminary studies of the aluminum alloy problem show that such a simplification can be achieved, as direct interactions by collisions of the second kind in the discharge between unlike atoms seem to play only a small part in the interelement effects observed, which seem more related to gross physical changes ir? the alloys such as heat conductivi ty and heat of evaporation. In support of this view, a ,itudy with a high resolution spectronietw just completed a t our laboratory shows that there are no line
wavelength shifts or line shape changes which can be attributed to interelement effects in the discharge itself. The only effects are changes of intensities of the various spectrum lines involved and many of these vary together in a proportional manner, indicating a change in the number of atoms involved in each spark stroke rather than element to element interaction in the discharge. What is the effect of all of this upon programming computers for complex alloy analysis? Up to this time the assumption has been that every element affected the analysis of every other element. This assumption has been forced on programmers by the internal standard concept because every change in the intensity of the aluminum reference line was of a necessity mirrored in the constituent line to aluminum line intensity ratio. Since much of this change was due to a pure dilution effect, this meant that every analytical curve of every element had to be corrected theoretically for the presence of every other element, whether it had an appreciable effect on the discharge or not. It is no wonder that programmers threw up their hands with regard to a universal set of equations and went back to the type of standard approach in which almost every alloy type has its own constants for the determination of each element. This eliminates the question of interelement effects by circumvention-by making the standard and the unknown as nearly equal as possible. Actually what happened was that some of the leaders in the aluminum industry changed from a discharge in air to one in pure nitrogen, as this tended to diminish the interelement effects and allowed their programmers to cope with the situation. In principle, however, it did not solve the basic problem generated by using intensity ratios. The goal of this new proposed way of treating data is to make computations relatively simple. One or, at most, a few factors are calculated from the intensities of a l l the element lines. These factors arc used to multiply the intensities of all of the element lines before they are applied to the analytical curves. Each analytical curve is
REPORT FOR AlNALYTlCAL CHEMISTS
Figure 3. X-ray fluorescence direct reader equipped with a digital computer
hopefully a universal one, with only three constants each, a t least for the aluminum alloy system. Thus the many thousands of constants may be reduced to well under one hundred. What is even more important is that the whole system may well be calibrated with far fewer standards in very much less time, and the results could be more accurate. Here is a case where a very thcrough analysis of a complex problem codd lead to a great simplification made possible by the iterative power of the computer. The net result could be a great improvement in the quality of analysis. Turning now t o x-ray fluorescence instruments, Figure 3 shows a typical multichannel system. This shows the high voltage x-ray tube supply, the x-ray tube which irradiates the sample, the polychromator-with a crystal spectrometer for each wavelength of each element of interest, an integrating or quantum counting interface console for receiving and storing the integ r a t d intensitiee of all of the lines simultaneously, and finally a digital computer-again the new relatively untried element labeled X in the system. I n operating s x h systems practice has been to run many of these as absolute devices without use of the internal standard principle which has so dominated optical emission analysis. I n some cases an external standard is utilized to provide overall control independent ANALYTICAL CHEMISTRY
of the characteristics of the sample being analyzed. With a computer a t the output of such a system, more sophisticated control and correction can be instituted to provide improved accuracy of analysis. Though x-ray fluorescence analysis has not been burdened with an “internal standard” complex, it has had to cope with some serious interelement effects caused by absorption and fluorescence phenomena. However, by taking the approach that the concentration of every element affects the intensities of the xray lines of every other element, some computer programs have been written which have provided a rather high degree of accuracy of analysis even in complex analytical systems.
“0°
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.9 6
Again, because of the power of the computer, some secondary effects can be taken into account by summing the analyses of all elements and comparing this sum to 100%. One of these variables in x-ray fluorescence is sample surface finish. It can be demonstrated very simply that any departure from a perfectly plane surface, even on a micro scale, will reduce the intensity of all x-ray fluorescence lines. However, the percentage reduction in intensity will be different for each line of each element depending upon each wavelength and the absorption of the sample for each of these radiations. Figure 4 shows this effect diagrammatically for the case of silicon in aluminum. As samples become rougher and rougher, the sum of all analyses will become less and less. By developing a roughness factor which applies to the sample as a whole and which is related to total analysis, individual corrections can be applied to each element to eliminate this extra absorption and provide results comparable to a polished sample. To do this one goes back to Sherman’s ( 4 ) approach in which the intensity of each x-ray line is developed from a physical model involving the absorptions, due to all elements in the sample, of the incoming beam with its wavelength distribution, and of the outgoing characteristic radiation. By changing the input and output angles, as compared to those which would be experienced for a plane
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Figure 4.
5.
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20.
Surface roughness
REPORT FOR ANALYTICAL CHEMISTS
surface, a roughness effect factor INCIDENT NORMAL TO SAMPLE can be developed for each element. R ADlAflON By iteration and summing and reIO peated correction, based upon the 1.47% A difference between the sum of all 8 WL a, elements and 100%, a result of high 6 accuracy can be developed, which RELATIVE takes into account changes in a I NTEN§ tTY sample finish as well as all interele4 1.282 A ment effects. Here again the power of the digital computer can be utilized to improve the quality of the analysis. 2 Besides using data, which are now being collected by direct reading systems, to better advantage by means of the computer, could additional data be collected and utilized S P E C T R O ~ETER to free such a system of many of its VIEWING DIRECTION constraints? Present spectrochemical systems are based upon the use of standards, which are as nearly Figure 5. Surface roughness effects on x-ray fluorescence measured by scat. identical in every respect to the un- tered radiation from sample known samples being analyzed as it is possible to get. Besides this grooves, a maximum intensity is they are based upon the implicit as- surface was properly conditioned a t obtained, while when the grooves sumption that all other variables the time the analysis was started. are turned a t right angles a miniexcept composition are invariant Background measurements could be mum is received. These intensity from calibration to analysis. I n made next to each spectrum line by values, as compared to that obsome cases this invariance is diffi- displacing the spectrum with regard tained from an ideal surface, a flat cult to maintain and this often to the secondary slits for part of the polished one, may be used to charmakes for the differences expe- sparking cycle. acterize the “roughness1’of the surrienced between duplicate analyses. I n the x-ray fluorescence analyface and provide a series of correcI n optical emission analysis for sis, the scattered radiation from the tion factors to be applied to the ininstance, the soundness and surface sample, which exists throughout the tensities of all fluorescence lines. finish of tbe casting a t the point of spectrum, carries much information Scattered radiation intensity impact of the spark can influence which can be utilized to good admeasurements have been used to the line intensities recorded. Like- vantage. Take, for instance, the determine the water contents of wise the conditions of the counter roughness factor discussed preslurries, and to automatically corelectrode and its spacing from the viously. If an x-ray line generated rect for changes in absorption of the sample determine the intensities by the target of the x-ray tube is on solids in automatic process control obtained. With all of the spectrum the short wavelength side of the systems utilizing x-ray fluoreslines usually available for each of principal absorption edge of the cence. the elements being analyzed, i t sample, then measurements of the Certainly many other measurewould seem that certain combina- intensity of this line should be able ments could be devised using infortions of lines could be used to her- to provide a very sensitive and acmation present in the spectrum ald the fact that nonstandard con- curate measurement of roughness, which is now being ignored in the ditions are present and that either as the line intensity will be atteninterests of simplicity. With a digthc analysis should be discarded, or, uated by the “hill and dale” effect ital computer as part of the system, lwtter yet, corrected for the lack with its increased absorption. Figmore and more of this peripheral of invariance. The ratio of intensi- ure 5 clearly shows the effect of information can be measured and ties of spark to arc lines of an ele- sample roughness on the intensity ment, for instance, might well indi- of scattered radiation. I n this case used to improve the quality of spectrochemical analysis. This type of cate if the gap spacing is correct, a steel sample was ground in one distudy would be very appropriate while the ratio of intensities of a rection with an 80 grit belt. Inhigh level line to a ground state line coming radiation normal to the surfor some of our large university or national laboratories interested in could indicate the amount of self- face was scattered by the surface improving spectrochemical methabsorption taking place and, hence, layers and absorbed more or less ods. indicate if proper spark craters are depending upon the direction from being formed. The ratio of intensi- which it was viewed, which in this With instruments more and more ties of lines in the prespark period case was 35” from the surface. As automated as the full capabilities of digital computers are recognized, to lines in the integration period the spectrometer receives radiation analyses should continue to immight indicate whether the sample scattered in the direction of the
t t t t t
REPORT FOU ANALYTICAL CHEMISTS
prove until once again the perfection and representativeness of the sample will limit the accuracy of the analysis. Can cybernation help in these areas also? Several papers given a t the 1967 Pittsburgh Conference show what can be expected when electron microprobe analysis is applied to samples used for both optical emission and x-ray fluorescence analysis. Withsuchequipmentitisrelatively easy to study the distribution of the elements present in a sample and to decide if i t is standard. Some of this work has been started at the Bureau of Standards ( 5 ) to determine which standards are sufficiently homogeneous for microprobe analysis. This should be extended to determine which standaids provide a set from the standpoint of being similar in the distribution of its components, no matter what that distribution may be. The use of a digital computer on
the output of a microprobe makes studies of this sort feasible for the first time as phases may be classified as to composition and size and frequency of occurrence, to get a true distribution function for each constituent. The type of subtle distinction that this can provide is shown in the comparison of two AlSi samples, at the 12% level of silicon, one of which has been modified by the addition of a few thousandths of a percent of sodium to act as a nucleating agent. Figure 6 is a photomicrograph and an electron heam scanning display showing concentration of silicon in the eutectic phase. Figure 7 shows the distribution of silicon within that phase providing regions of high silicon concentration in the unmodified sample, as compared to the modified sample which is much more homogeneous. This makes for low values in optical emission analysis, and high values in x-ray flnores-
-’-
H 5OP
Ibl
cencc for the unmodified samples as compared to the modified samples. It is not suggested t h a t every sample he inspected for a standard distribution of its components, hut such studies can be invaluable in setting up sampling procedures or in determining the effect of nucleating additions when samples are being cast. Finally with a computer hcing available full time as part of the system, how should it be employed when not in analytical use? First i t can be programmed to continuously cheek the sensitivity of the instrument and make computational corrections to keep this at a standard level. In optical emission a standard lamp can he viewed whenever a sample is not being analyzed and this can be used for the dual purpose of both fatiguing and calibrating the multiplier phototubes. Without this lamp, a dark current check of all the multipliers can be made and any changes noted and corrected. I n x-ray fluorescence, a standard epoxy briquette can be kept in the instrument when no unknown sample is being analyzed and the instrument can be kept in perfect calibration at all times by analyzing this standard repeatedly and averaging and updating the calibration until the unknown is introduced.
Figure 6. PhOtomictugraph (-) and silicon distribution elec . ( b ) [11.80% silicon in aluminum, aluminum p i n s . i
Figure 7. Frequency distribution of silicon in the eutectic of AI-Si alloy
condition of tde instrument by noting how the sensitivity of the instrument has varied with time and thereby pinpoint short term instability as compared to a long term continuous drift, for instance. This information can be the basis for the computer to sound the alarm in the event that one of the channels is becoming too erratic in its operation to provide reliable results. With time, more and more monitar functions will be delegated to the computer until i t can in essence look after the entire system itself. By providing automatic sample collection and preparation equipment, controlled hy the computer, cybernation will certainly some day take over spectrochemistry.
REPORT FOR ANALYTICAL CHEMSTS -
SAMPLE CAROUSEL
.
F m i 8 X-ray dffractior+fluorescence-optical emission system equipped with a digital Gmputer However long before that happeos, the computer will be used to bring various techniques together to bear on providing the ultimate in accurate analyses for an industry -for instance, by a combination system. A typical one might combine an optical emission speetrometer, a n x-ray fluorescence spectrometer, and an x-ray diffraction unit to provide a weighted analysis by means of the computer, which would be far superior to that obtainable from a single instrument in terms of aecuraey and rcliabilitg. Figure 8 shows such a system in diagrammatic form. Finally, what a b u t the speetrochemist? Michael, in his paper “Cybernation, the Silent Conquest” (I), implies that once we get OUI systems all worked out and running there won’t be anything left for us to do and we will proeced to go to pot with boredom and mischief making. Fortunately before that happens we have a tremendous job in spectrochemistry just to find out how to use these twin t o o l s automation and the digital computer-to the fullest extent. Finally after we work i t out for home consumption in the United States, we can take i t to the rest of the world to raise other areas to our standards of complexity and frustration. I n any event the vista at this time is exciting, stimulating, and still ever-cxpanding.
(1)
Liid.re c i i Donald N. Michael, “A Report to
the Center for the Study of Democratic Institutions,” (1962). ( 2 ) J. H. Coullirtie, Iso. EXG.CBM. And. Ed., 15,732 (1-3). (3) M. F. Hasler and J. W. Kemp, A S T M “Methods for Emision Speo tmehemiarl hnalysis, 1957.” pp. m. (4) Jacob Sherman, Spectmclun.. Acta, 7.m(195.5). (5) H. Yakowitr, D.
L. Victh, K. F. J. Heinneh. and R. E. Miehaeh, NaU. Bur. SLd. (US.) .%fisc. PdZ. %lo (1965).
