REPORT FOR ANALYTICAL
CHEMISTS
Atomic Absorption Spectroscopy J . W . Robinson, Esso Research Laboratories, Baton Rouge, Louisiana
Introduction
The determination of trace metal impurities has always provided a challenge to the analytical chemist. With progress in industrial technology and in all the disciplines of science, this problem becomes increasingly important. Considerable technical effort has been spent on improving well known instrumental techniques—spectrography, polarography, ionophoresis—and improving chemical methods, e.g., by solvent extraction, organic reagents. As most of these processes are already well known, a major breakthrough in this field of analytical work will probably come Figure 2.
about only by the development of a new technique. Atomic absorption spectroscopy seems to be such a technique. As far back as 1955, A. Walsh and his coworkers pointed out that the phenomenon responsible for Fraunhoffer lines could be utilized for trace metal analysis. His experiments showed that only comparatively simple equipment was required for the detection and determination of several metals at the parts per million level of concentration. Since that time, further pioneering work on his part has established this as a scientific field of major importance, potentially capable of solving composition
equilibrium and kinetic problems in many branches of science. It seems possible that with the development of commercial equipment for this work, its value will be fully realized. Principle
The analytical application is based on the fact that metal atoms absorb strongly at discrete, characteristic wave lengths, which coincide with the emission spectra lines of the particular metal. A simple presentation of the process is shown in Figure 1 (page 19 A). The process for emitting spectra, as in flame photometry and emission spectrography, is the reverse
Schematic of equipment used for atomic absorption spectroscopy
^
Mu. V VOL. 32, NO. 8, JULY 1960
·
17 A
REPORT
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.
ANALYTICAL CHEMISTRY
Established 1 9 3 4 .
James W. Robinson was educated at the University of Birmingham, Eng land (BSc. Hons. 1949, Ph.D. 1952) where he studied under R. Belcher. He entered the British Civil Service where he worked, among other things, on the first transatlantic telephone cable. In 1954 he was General Sec retary of the International Symposium of Analytical Chemistry held at Bir mingham. In 1955 he came to Louisiana State University and worked with Prof. Philip W. West on the detection of nerve gases, helping to develop the West-Robinson reagent for phosphate. He joined Esso Research Laboratories, Baton Rouge in 1956 where he is cur rently engaged in analytical research. He became interested in atomic ab sorption spectroscopy when he was faced with the increasing demand for trace metal analysis as the petroleum industry becomes more and more con cerned with the production of chemi cals. Direct communication with A. Walsh of Australia further inspired this interest. Research work carried out at Baton Rouge confirmed many of the potentials of the phenomenon. of the absorption process, as shown above. This relation between emis sion and absorption points out areas in which the phenomena will be similar in behavior, and factors which will produce reverse effects on the two phenomena. For ex ample, any process which increases the abundance of atoms from a solution should improve both meth ods. By contrast, any method which increases the percentage of excited atoms present will do so a t
the expense of the number of unexcited atoms present. Equipment A schematic diagram of equipment used for this work is shown in Figure 2 (page 17 A ) . I n operation, the liquid sample is atomized in a s t a n d a r d or modified flame photometer burner. M a n y metals are reduced in the flame to the atomic state. T h e hollow cathode emits the spectrum of the metal used to m a k e the cathode. This beam of light traverses the flame and is focused on the entrance slit of a monochromator and readout system. T h e monochromator is set to read the intensity of the chosen spectral line. Light with this wave length is absorbed by the metal in the flame. T h e degree of absorption is a function of the concentration of the metal in the sample. I n practice, it has been necessary to chop the light source and modulate the detector. This eliminates the interference of emitted light from the flame which, of course, is subject to the interferences of conventional flame p h o t o m etry and would seriously complicate the measurement of true absorption.
Source Atomic absorption spectral lines are extremely narrow. If a continuum is used as a source—e.g., a hydrogen lamp—these absorption " b a n d s " are measurable only under conditions of high resolution not a t tainable in commercial equipment. F u r t h e r , the energy of a hydrogen lamp over this narrow wave band
Figure 1.
is very low. T h e q u a n t i t y of energy absorbed by the metal is therefore low with an inherent loss in sensitivity. These difficulties have been overcome by using a hollow cathode as the source. This emits a strong spectrum with emission lines of band width similar to the absorption lines. M e a s u r e m e n t of absorption at this high intensity is comparatively easy. A t t e m p t s have been made to emit several spectra simultaneously by using an alloy as the cathode. R e sults are satisfactory for a short time only. Because of the different rate of sputtering of the metals involved, one metal slowly covers the entire cathode. This causes a drifting signal from all the elements concerned. Advantages T h e p a r a m o u n t a d v a n t a g e held by atomic absorption over other procedures is its high degree of freedom from interference from its environment. L a b o r a t o r y work has shown t h a t only rarely is the accuracy of the procedure affected by the presence of other elements, probably because we are dealing with the physical properties of an atom. This eliminates m a n y chemical problems. Also, as it is very unusual for two metals to absorb at the same wave length, direct interference by other metals present is most unlikely. T h e method also is almost independent of the t e m p e r a ture of the atomizer. We can understand this better when it is realized t h a t we are usually dealing with atoms in the unexcited or ground state. In a
Simplified concept of atomic absorption spectroscopy
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A
REPORT FOR ANALYTICAL CHEMISTS
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ANALYTICAL CHEMISTRY
Figure 5.
Modified Perkin-Elmer Model 13
r flame, it is unusual for the number of excited atoms to exceed 1% of the total number of atoms. Any variable which affects this percent age, such as matrix, solvent, or flame temperature, will seriously change the intensity of the emission temperature. For example, if the number of excited atoms increases from 1.0% to 1.1 %, the emission in tensity should increase about 10%. However, in absorption we are deal ing with the other 99% of the atoms and the same variable will change the population of ground state atoms from 99% to 98.9%. This effect is minor and in practice can often be neglected. An equally im portant feature is the high degree of sensitivity of the procedure (Table I, page 24 A). Another attraction to this field is the simplicity of the equipment required. In many cases only a simple flame photometer with a modulated light source and detector is required. In some cases—e.g., analysis of alkali metals—light filters provide sufficient spectral resolution. However, it is better to use a simple prism or grating as a monochromator, particularly where transition and noble metals are in volved.
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It would be unusual indeed if any system, new or otherwise, could be devised which did not suffer from some built-in problems. Atomic ab sorption spectroscopy is no excep tion. Perhaps the most important problem is that when a flame is used to produce the atomic state, several elements are not detectable. These elements include aluminum, titanium, tungsten, molybdenum, silicon, and a few others. At pres ent, the most probable explanation appears to be that these elements form oxides in the flame. This for mation would prevent realization of the atomic state which is essential for detection. As described below, it appears likely that this problem can be overcome for metal samples. But an alternative method of atomization other than the flame seems to be essential for liquid samples. Attempts to break down these oxides with a high temperature
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A
REPORT
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flame, such as oxycyanogen, did not solve the problem. Other observed difficulties center around the efficiency of production of the atomic species in the flame. If aqueous solutions a r e used, t h e predominant anion affects the signal to a noticeable degree. Further, the formation of strongly bound salts in solution—e.g., CaPCU—has a pronounced effect on the response. Where possible, these effects should be avoided or corrected for by suitable standardization procedures. When organic solvents are used, the degree of absorption is enhanced manyf old. Again, it is believed t h a t this is caused b y a more efficient production of metal atoms from solution. I n this instance, further complications arise because the organic solvents themselves absorb in the flame. This problem can be overcome b y choosing an optical path through t h e flame above t h e zone where combustion of t h e solvent takes place.
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This testing procedure has found considerable use in several fields of endeavor. The agricultural chemist has used it for the determination of sodium, potassium, magnesium, zinc, a n d calcium. T h e medical profession h a s used it for determination of t h e same elements in blood serum, etc. Manufacturers of noble metals use t h e procedure for t h e rapid determination of platinum, gold, silver, and rhodium. The chemical a n d pharmaceutical industries are beginning t o use the method for t h e determination of trace metals in their products. This is particularly useful where alkali metal salts are concerned, because in other procedures mutual interference takes place and separation of the metals from each other is long and tedious. So far, these applications have used only liquid samples. Recently, however, Walsh h a s shown t h a t the metallurgical indust r y could find use for this procedure. His method is to drill a hole through the sample a n d insert it into t h e chamber which can be evacuated rapidly. Using a sputtering technique, the metal is atomized, then analyzed as described above. There
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Comparison with Other Procedures Trace metal content can be de termined by various other methods, such as wet chemical, polarographic, flame photometric, and emission s p e c t r o g r a p h s . T h e first two are unrelated and will not be discussed. However, in m a n y ways atomic absorption spectroscopy competes with t h e emission spectro graph and t h e flame photometer and a comparison of t h e three tech niques would seem t o be pertinent. The flame photometer and t h e spectrograph are emission tech niques. This means t h a t they de pend on the production of excited atoms for t h e generation of a meas urable signal. A n y variable which affects this product acts as a direct interference. I n flame photometry, we must control feed rate, viscosity, flame temperature, solvent, etc., in order to obtain reproducible re sults. W i t h t h e conventional oxyhydrogen flame, comparatively few metals emit line spectra. T h e m a jority emit band spectra which cover large p a r t s of t h e spectrum. This gives rise to serious interfer ence problems when more t h a n one metal is present, which is t h e usual case. Considerable progress h a s been made on this problem by J o h n A. D e a n of t h e University of Tennessee, who has successfully employed solvent extraction of the metal followed by flame photome try. At one stroke, this largely eliminates mutual interference effects, and simultaneously concen trates t h e metal and t a k e s advan tage of the enhancement effect of organic solvents. Valuable though this work is, it necessitates a cer tain amount of preparation work, which is always a detriment to a n y analytical procedure. A final problem, as y e t unre solved in flame photometry, is t h e determination of alkali metals. I t is ironical t h a t although t h e flame has been used most widely for determination of these metals, they are mutually interfering elements.
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32, NO. 8, JULY 1960 ·
23 A
REPORT FOR ANALYTICAL CHEMISTS This problem can be overcome to a limited degree by careful s t a n d a r d i zation. W o r k in high t e m p e r a t u r e flame photometry—e.g., the oxycyanogen flame pioneered by B . L. Vallée— indicates t h a t m a n y more elements exhibit line spectra in this flame. M a n y interference problems arc eliminated under these conditions— unfortunately not the alkali metal interference problem. Inasmuch as flames are less noisy and more controllable t h a n arcs, more precise results should be obtainable in this way. I t seems, therefore, t h a t with suitable equipment and burner design, the high t e m p e r a t u r e flame holds the promise of a very useful analytical tool. Spectrographs Technique W i t h the spectrograph, two types of electrical discharges are used— the spark and the arc. Broadly speaking, the spark is more reproducible and precise but less sensitive t h a n the arc. Although atomic absorption can be used by the metallurgist for the determination of major constituents in alloys, ores, etc., in other fields we are concerned primarily with metal a n a l ysis a t low concentration levels. For this work in spectrography, wc arc restricted m a i n l y to sensitive arclike conditions. Discussion is therefore restricted to analyses from ai'c spectra. T h e most important attribute of the arc is its universal application. Virtually all metals and metalloids exhibit spectra, as do some nonmetals—e.g., carbon and phosphorus. As an instrument for qualitative analysis, it is without equal from the point of view of both speed and certainty of identification. To take a d v a n t a g e of this versatility, good equipment and a high degree of skill and experience on the p a r t of the operator are necessary. W i t h q u a n t i t a t i v e analysis, however, the position is not nearly so rosy. Reasonable semiquantitative results can be obtained by comparison with synthetic standards, but this requires considerable knowledge of the sample before such s t a n d a r d s can be made. T h e problem of obtaining accurate q u a n t i t a t i v e results is much 24 A
·
ANALYTICAL CHEMISTRY
Table I.
Approximate Sensitivity limits (P.p.m.) Flame Photometry
SpectroOxycyanogen Element graphic" aqueous solvent* 0.8 Aluminum 10 Antimony 15 50 Arsenic 40 100 Barium 0.4 0.05 Beryllium 0.01 1.0 Bismuth 8 3 Boron 0.1 5 Cadmium 4 0.5 Calcium 0.4 10 Cerium 10 300 Cesium 15 10 Chromium 2 5 Cobalt 2 4 Copper 0.05 2 Gallium 4 2 Germanium 4 Gold 50 ND Hafnium 200 Indium 4 Iron 0.5 6 Lanthanum 5 Lead 10 15 Lithium 0.1 0.01 Magnesium 0.005 0.01 Manganese 0.08 0.2 Mercury 20 Molybdenum 2 300 Nickel 4 1.0 Osmium 250 Palladium 0.5 Phosphorus 6 ND Platinum 7 ND Potassium 4 1.0 Rubidium 30 6 Scandium 20 Selenium 70 Silicon 40 ND Silver 0.6 3 Sodium 1 0.005 Strontium 0.2 0.03 Tantalum 300 Tellurium 20 Thallium 40 1.7 Thorium 4 Tin 15 100 Titanium 0.4 20 Tungsten 10 6 Uranium 200 ND Vanadium 4 3 Yttrium 0.1 Zinc 2 ND Zirconium 2 200 ND = N o t detected Blank = N o information available
Oxyhydrogen* ND ND ND 1.0 ND
Atomic Absorption" ND 2.5 10 2.5
3 5 0.08
0.1 0.5
0.02 1.0 4.0 0.6 1.0
10 10 10 0.5
ND 1.0 50 2 14 0.2 0.6 0.1 200 10 1.6 100 ND ND 0.05 0.5 ND 1.5 0.001 0.02 ND 0.6 500 10
1 2.5 0.5 1.0 0.1 0.5 100 ND 1.0 2 10 0.1 2 ND 0.2 0.01 10.0 ND 10 100 ND
ND ND ND ND
ND 0.1
"h R. L. Mitchell, Macaulay Institute, Scotland, private communication. Studies at Esso Research Laboratories, Baton Rouge, La. " A. Walsh, C.S.I.R.O., Australia, private communication, and studies at Esso Research Laboratories. T a b l e I illustrates t h e limits of detection of t h e elements listed. I t does not indicate the limits of q u a n titative determination of these same elements. With spectrography, this figure is usually 10 t o 20 times as high as t h e concentration quoted, and not infrequently, 100 times these concentrations. W i t h flame p h o t o m e t r y , t h e q u a n t i t a t i v e limits of determination are of t h e same order as t h e figures quoted, p r o vided interferences are eliminated.
W i t h atomic absorption spectros-"" copy, the q u a n t i t a t i v e limits of determination are of the same order as listed. However, it can be a n ticipated t h a t these limits will be lowered from these preliminary values. Including instrumental features such as long flames, multipass systems where t h e light beam traverses t h e flame numerous times, and t h e use of integration of signal will reduce t h e sensitivity limits b y a significant factor.
REPORT FOR ANALYTICAL CHEMISTS Table II. Relative Interference Effects Flame Photometry Spectrographic Oxycyanogen Oxyhydrogen Interference Matrix High Appreciable Appreciable Other metals High Appreciable Often high Flame temp, or excitation High Appreciable Appreciable conditions
more difficult. This will be ap parent when it is appreciated what a tremendous effect the matrix, or base material, exerts on the inten sity of spectra exhibited by the trace metals present. For example, if two samples contain equal amounts of silicon, one being a steel sample and the other alumina, the silicon spectrum may be as much as 15 times as intense in one case as the other. It is not uncom mon to report semiquantitative re sults as " 0 . 1 % to within a factor of 10" i.e., the result is between 1 and 0.01 %. This illustrates the dif ficulties involved in obtaining quantitative results accurate to
PERIODIC GROUP
1
1
In atomic absorption spectros copy, however, there is a high degree of freedom from interference from the matrix and other elements present. This is probably because we are dealing with absorption by unexcited atoms. This is the state of about 99 % of the atoms present, and it is difficult to affect this ratio substantially. Also, because the atoms absorb at very selective wave lengths, mutual interference is rare. This permits reproducible results to be obtained with consid erably less preparation. These techniques are compared in the Tables I and I I . Elements detected are shown in Figure 6. It can be concluded from these comparisons that atomic absorp tion has much more freedom from interference than flame photom etry or emission spectrography. It would be expected, therefore, to provide more reliable and more accurate results. However, it is not as universal in its application as the arc, because certain elements can-
Atomic Absorption Minor Minor Minor
within 5% of the true value under any conditions. Of course, this problem can be overcome by reducing samples to a common matrix. This requires considerable experience and tech nique on the part of the spectrographer and points up the excel lence of work carried out in emis sion laboratories in this country and elsewhere, such as the Macauley Institute for Soil Research, Aberdeen, where the standard de viation is as low as 2%. For good precision concentration and spark techniques are used whenever pos sible. This necessitates consider able preparation.
2
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Figure 6. Elements detected by atomic absorption spectroscopy VOL. 32, NO. 8, JULY 1960 ·
25 A
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ANALYTICAL CHEMISTRY
not be detected by this method using the flame as an atomizer.
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REPORT
Commercially Available Equipment
Much work has already been done to establish atomic absorption as a reliable technique with fundamental advantages over emission techniques. However, the procedure is new and the only equipment available commercially is at best modified flame photometers or spectrophotometers. In Australia, Walsh has provided a prototype suitable for most applications. This equipment (Figure 3) embodies multielement hollow cathodes, source modulation, a burner designed for this work, and a simple monochromator and detector system which can be purchased quite cheaply. The main theme of his equipment is simplicity. It is to be hoped that future manufacturers of such instruments will maintain this simplicity and low cost, at least for simple routine applications. The only other source of equipment is provided by Hilger and Watts. This equipment (Figure 4), is a conventional Uvispek spectrophotometer to which has been added an attachment consisting of a burner and hollow cathode attachment. This equipment does not embody source modulation, and although it can still be used for many purposes, any emission signal from the flame is not eliminated from the final signal and would, of course, confound the result obtained. Such emission would be subject to all the interferences encountered in flame photometry. In this country, Perkin-Elmer has already shown interest in the method. To date, its equipment has been restricted to instruments suitable for research in assaying the value of the field. Figure 5 shows a modified Model 13 as used at Perkin-Elmer and at Esso Research Laboratories, Baton Rouge, La., for work in this field. This equipment embodies all the requirements for research on this subject—e.g., modulation, high resolution, single-beam, and doublebeam circuits and special light-
gathering lens between the hollow cathode and sample. It does not include a specially designed burner, and it is anticipated that the inclu sion of such a burner would in crease the sensitivity of the system as much as tenfold. It is hoped that in a comparatively short time an abundance of inexpensive and reliable instruments will be avail able to the analyst.
ARL VACUUM X-RAY QUANTOMETERS* Vacuum x-ray analysis of all elements for the Iron and Steel Industries in cluding low concentra tions of Phosphorus— Sulfur—Silicon. Rapid analysis of elements in High and Low Alloy Steels-Cast I r o n Sinters — Slags — Ores.
Conclusions
A new technique is now available to the analytical chemist for de termining trace metal concentra tion. At present, several metals cannot be determined by this method, but it is possible that alternate atomizing methods will overcome this shortcoming. The only equipment available for studying or utilizing this phenome non is a modification of existing spectrophotometric equipment. I t is hoped, however, that this un favorable situation will soon be remedied. A wealth of research should be forthcoming from studying and applying this phenomenon. Here is a tool which will give us directly a measure of the number of atoms in the excited and ground state under any set of experimental con ditions. This should find direct ap plication to spectrographic and wave mechanical problems. Other possible uses include iso tope analysis. This, of course, would require high resolution. Analysis of exhaust gases from jet engines should be permissible di rectly in the exhaust gases. In the medical science, a start has already been made in the use of this method for the analysis of body tissue. The attraction in this case is the small amount of sample required. In chemical and metallurgical work the applications are countless and do not need further emphasis. No doubt much work remains to be done on interference studies, equipment design, etc., but it does seem that here we have a new ana lytical tool which will find much application to composition studies pertinent to many areas of funda mental and applied research.
SPECIAL FEATURES: Fixed and scanning spectrometers to analyze all elements from magnesium through uranium. • Simultaneous analysis of these elements in about two minutes. • Foolproof flexibility for rapid analysis of powders, liquids, and solid samples. • Economical —utilizes a vacuum system, eliminating the expense and supply problems of helium. • Dual sample handling facilities permit sample interchange without breaking vacuum. • Let ARL evaluate your analytical control or research requirements. Write today for complete information. *τ Μ.
APPLIED RESEARCH LABORATORIES, INC. _ BRANCH OFFICES
subsidiary of BAUSCH & LOMB INCORPORATED P. O. BOX 1710, GLENDALE 5, CALIFORNIA ί
NEW YORK
· PITTSBURGH · DETROIT · CHICAGO LAUSANNE. Switzerland * LONDON. England
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HOUSTON
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Circle No. 133 on Readers' Service Card VOL. 32, NO. 8, JULY 1960
LOS ANGELES
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