Recent Advances in Atomic Absorption Spectroscopy J. W. ROBINSON Esso Research Laborafories, Esso Standard, Division of Humble Oil & Refining Co., Bafon Rouge, La.
b The phenomenon of atomic absorption is discussed; controllable variables and features of the equipment used for this work are pointed out. The relative limits of detection of various elements in oxyhydrogen and oxycyanogen flames are listed, for both aqueous and organic solutions. Lower sensitivity at various wave lengths of the same metal indicates that the analytical range is greater than originally suspected.
E
work in the field of atomic absorption spectroscopy promised that this might be a method for determining trace metals which would be remarkably free from interference by other metals and matrix effects. In 1955 (9) Walsh and his coworkers pointed out that the phenomenon of atomic absorption, which is responsible for Fraunhofer lines, might find wide application as a rapid and sensitive means of analysis. Since that time he has done considerable work to prove his point and with comparatively simple equipment has been able to determine numerous metals a t the few parts per million level. The object of our study has been to determine how widespread the application of this phenomenon is and which variables need close control to get reproducible analytical results. The principle of the process is based on the ability of atoms to absorb radiation a t certain well-defined characteristic wave lengths. The wave mechanics of this process and certain other fundamental characteristics have been discussed ( 1 , 6). ARLY
EQUIPMENT
The instrument operates as follows: The sample, which must be in the form of a liquid, is aspirated into a flame photometer burner and reduced to the atomic state. A hollow cathode emits the spectrum of the metal being examined and is lined up to traverse the flame and, therefore, the atoms produced from the sample. It then impinges on the grating or prism of a spectrophotometer. The relevant *ave length is directed to a detector and readout system. Light Source. The metal atoms absorb over a very narrow wave band. ConAequently, continuous light sources are unsatisfactory.
For metals such as alkali metals and mercury, vapor lamps are satisfactory. For other metals a hollow cathode can be used as the light source. This instrument emits the spectrum of the metal concerned with very narrow emission lines, comparable in width to the wave band which is absorbed by the metal atoms. A great proportion of the source emission line is therefore capable of being absorbed by the sample. This brings about the happy circumstance that in many cases we can examine a particular emission line and measure its absorbance against the background of the hollow cathode, which is virtually zero. Under these conditions, a favorable signal-to-background ratio exists and high sensitivity is possible. Although hollow cathodes have been used by spectrographers over a number of years for the production of good emission spectra, there is comparatively little information in the literature on the best way to makc these instruments. The relation between emission intensity and the type of gas used as the filler gas and the pressure under which this gas is used is described by Crosswhite, Dieke, and Legagneur (a). This paper clearly indicates conditions for making a n iron hollow cathode to give the optimum intensity emission signal. Besides a high signal strength, it is also important that the hollow cathode have a long life and emit a steady signal. For this reason, argon is sometimes used as the filler gas in preference to neon. Although the signal is lower, the lifetime is longer, because the gas cleans up more slowly. In practice, the three most common factors which limit the life of a hollow cathode are: LEAKAQE OF A I R . This can be recognized by the loss of emission signal from the metal, and by the typical purple color of electrical discharge in air. Oxygen in the air quickly forms metal oxides on the cathode and prevents the functioning of the instrument. The cause is usually a crack in the glass, or a faulty seal. Repair is usually impossible, although some extra life can be obtained by reversing the polarity of the leads and passing a high current through the getter. This may mop up the oxygen present and provide a temporary reprieve for the source unit. After cleanup, the leads are again reversed and the instrument is operated
in the normal fashion, a short time bcing allowed for it to stabilize out. No shift in wave length should occur. Loss OF FILLERGAS. One of the inherent problems of the hollow cathode is that the filler gas is continually being slowly absorbed or trapped by the metal cloud and by the glass surface. This cleanup of the gas results in a slowly falling signal-i.e., a drift-and as pointed out by Walsh (Q), the filler gas which provides the most intense emission spectra does not necessarily have the highest resistance to this problem. Loss OF METALFROM THE HOLLOW CATHODE. Also related to the problem of steady signal and long life is the shape of the hollow cathode itself. When in use, the hollow cathode is continually sputtering and throwing up small quantities of metal atoms from ita surface. It is these metal atoms which emit the spectra required. However, this results in a cloud of atoms which slowly diffuse throughout the hollow cathode and finally alight on the inside glass wall of the tube. For satkfactory operation, steps must be taken to cut this diffusion to a minimum and to prevent this cloud from diffusing to the signal outlet window of the hollow cathode. One method of reducing this diffusion k to use cavity electrodes. White (IO) describes the production and the desirable characteristics of these electrodes. In this system the hollow cathode is shaped as a sphere with a small hole in one side. I n principle, the metals inside the hollow sphere glow and emit the spectrum. The atomic cloud diffuses to other parts of the sphere and redeposits, Only a very small quantity actually diffuses out of the small hole and into the outer part of the hollow cathode. It is claimed that this results in an increased life of the hollow cathode and promotes a stable signal. Experiences in these laboratories have tended to bear this out. Based on these suggestions, the design of the hollow cathode which we have used is shown in Figure 1. The cathode is tubular rather than spherical. The tube is closed in a t one end and has a restricted opening a t the other end. The advantage of this shape is that there is a greater number of emitting atoms in the optical light path. Further, the ability to trap the diffusion metal should not be lost when a closed VOL. 33, NO. 8, JULY 1961
1067
Table
Element Figure 1.
Design of hollow cathode
AI
Ta tubc-shaped cathode is uscd rathcr than a sphere. On thc basis of this reasoning, it is hoped that the life of the tubeshaped cathode would be equal to the cavity electrode but that a n equal signal would be produced by a smaller current. Usually, if the instrument is run at aa low a current aa possible, the lifetime of the hollow cathode is extended because of the decrcascd loss of filler gas and diffusion of metal Burner. The design of IL satisfactory burner piovidm on(> of the most scrious challrngcs in t h r field of atomic absorption sppctroscopy as in the firld of flamr photometry. The burnrr has two prinripal functions to perform: It must intiodurr the saniplc into the flamc, and I t must redure the metal to atomic state. Control of both these stcps seriously affects the degrce of absorption or emission for a given sample. For u stcady signal, a constant feed rate and a constant rombustion prittern in the flame are essential. The prcsent work was carried out using the Beckman flame spcctrophotomctcr burner. Although this is satisfactory for obtaining data on useful absorption lines and relative limits of sensitivity of the detcction of various metals, considcrable gain in sensitivity can be obtaincd by using a burner which produces a flame with a long path length, sucah as a fishtail flamc. With this flnnic, the source travcrsrs an incrcascd number of atoms capable of contributing to the absorption signal Monochromator and Detector. The monorhromator uscd for this work WAS a T3nusrh & Lomh grating, 1800 lines pi^ mm. Thc dctrctor was a 11'28 multiplicr hototubc. This equipmcnt was instn led in a modified Perkin-Elmrr Modrl 13 spcctrophotomrtcr. l'hr powrr source for the hollow cathode was a Kcpco Model 1520n.
P
EXPERIMENTAL DATA
For cnch clcmrnt it was felt drsirable to obtain certain rrlevant information : choice of absorption line, thr relevant limits of detection, calibration data, and frccdom from intrrfcrcnce from othcr elcmcnts. To complcte this program for all the metallic dements in the periodic tablc will necessitate a large amount of experimental work. At present, only a limited number of hollow cathodes are available commercially. This nccrssitntcs fabricating hollow 1068
ANALYTICAL CHEMISTRY
V
Ni
Zn CO
Mo
cu
W
Cd Ne
Hg Ag
Absorbing Line, A.
I.
Solvent" Aqueoua Organic ND Aqueous ND Organic ND ND 3183 Aqueous Organic ND 4379 Aqueous ND Organic ND 3414 Aqueous 1 0 Organic 0.4 3524 Aqueous 10.0 Organic 3.0 2138 Aqueous 0 3 Organic 0.08 3453 Aqueous 10 Organic 8 3405 Aqueous 100 Organic 100 3529 Aqueous Organic Aqueous ND Organic ND 3274 Aqueous 0.5 Organic 0.2 3247 Aqueous 0.5 Organic 0.1 Aqueous ND Organic ND 22% Aqueous 1 Organic 0.1 5890 Aqueous 1. Organic 0.1 1 .o 5895 AGeous Organic 0.05 3302 Asueous 30 Oiganic 3 5Wfold excesb of Li and/or K did not interfere 2530 Aqueous 100 Organic 100 3281 Aqueous 0.6 Organic 0.3 3383 Aqueous 0.5 0.5 Organic
cathodes for a number of metals, many of which are difficult to handle because they are available only in powder or in chunks. It is anticipated that as progress is made in the manufacture of hollow cathodes, this problem will be solved for the analytical chemist. Choice of Absorption Line. T o ascertain which wave lengths can absorb, the emission sprctrum of a hollow cathode was scanned. The sample was aspirated into thc burner already positioncd in the optical path of the sourrc and the dctector. The spectrum was thrn rescanned. Any lines which are absorbed are detectrd, since their intensity decreases after the introduction of the atomic sample in the light path. In all cases the samples aspirated into the flame contain metals to the extent of 0.1% in aqueous or nonaqueous solution. Results are shown in Table I. Limits of Detection. The limits of detertion were ascertained by expos-
Absorption Data
Limits of Detection, P.P.& Oxyh drogen Oxyc anogen &ne &me ND ND
N 1)
N I1
N I) 300 300
0.5 0.1 0.5 0.2 0.5 0.06 20 1
2 1
1 0.2 0.2 0.1 ND ND 0.3 0.1 0.5 0.2 0.6 0.2 3. 0.5
0.2 0.2 0.3 0.3
ing tlie photomultiplier detector to thp mrtal absorbed, as determined in the previous euperiment. T h e instrument was then set for maximum sensitivity---i .e., to maximum zero suppression and maximum gain on t h e amplifier. The intensity of the pnrticular line was regulated by varying the inlet slit on the monochromator, such that lowas constant in all cases. Solutions of the elementa being examined were then aspirated into the flame and the degree of absorption was meaaured. The concentration of the solution was progressively decreased until the absorbed signal could not be differentiated from the noise level of the recorded signal. To be consistent, this was defined as "8 of the noise level of the signal of the particular absorption line. Calibration Curves. Calibration curves ran bc obtained in the normal way by applying the standard nbsorption formrila
on Various Elements
Element
Abmrbing Line, A.
Mg
2853
&%Solventa Aqueous Organic
Ph
2170
AqllCO~S
2833
Organic
Aqueous
Organic
Limits of Detection. P.P.M. Oxyhydrogen Oxycyanogen flame flame 0.1 0.01 50
. . ...
VARIABLES
50 1.5 1.5
900 p.p.m. of Sn, Na, Bi, Cu, Zn, B. Fe, Ni, Si did not int,erfere with signal from 10 p.p,m Pb
Sn Fe
3719
Aqueous Organic Aqueous
ND ND
NI) ND
4.0
900 p.p.m. Cu, Mn, Zn W, Ti, Co, Na, Cr, AI, V, Ni, do not interfere with signal of 10 p.p.m. Fe
Pt
2852
TI
2659 2929 3064 2768
Cr
3776 5350 3.570 3605 4254 4275 4290
Aqueous Organic Aqueous Organic Aqueous Aqueoue Aqueous Aqueous 3rganic Aqueous Aqueous Aqueous Organic Aqueous Organic Aqueous Organic Aqueous Organic Aqueous Organic
ND ND 10 10 10 40 300 0.6
ND
ND 0.02 0.02
0.2 200 0.5 0.2 1.0 0.2 1 .o 1.5 2.0 0.5 2.5 0.8
Included alcohol, acetone, benzene, and n-heptane. Different absorption lines of the 8ame element have widely different sensitivities, indicating that the procedure may be used for the analysis of more concentrated solutions. (I
I,
= lae-Kcl
It is often more convenient to take advantage of the Ringbohm relationship log C = R‘ ( 1 0 - I ! ) In this case the log (concentration) is plotted against the quantity of light absorbed (Io - Il). This is particularly useful if the zero is suppressed below the recording limits of the instrument. If the position of zero is reproducible, analytical results can be calculated without making absolute measurements of Io and Il. Freedom from Interference. Frecdom from interfrrrnre by other mrtals was studied by making u p solutions which contained the subject element and the interferencr rlemrnts. The subject metal was prcscnt in sufficient quantity to give a reliable and reproducible reading on the instrument. The quantity of the “interfering” ele-
limits of sensitivity. I t is felt that with suitable optics and instriimrntntion, these could be reduced signifirnntly. However, these data should indicate relative limits of detection of these metals and the pertinent, wave length.
ment present was adjusted to any desirable level. As a preliminary study, these solutions were made up such that the interfering element was present in concentrations approximately 100 times as great as the element being examined. If no interference was found a t this concentration, no further study was made. Interference has been encountered in some instances. A well known example is the depressing effect of aluminum on magnesium. This interference is further modified by the anion present. It is probably caused by chemical interference-Le., the magnesium forms a compound and does not reach the atomic state. RESULTS OBTAINED
Results obtained for elements examined hitherto, listed in Table I, are not intended to indicate absolute
The rnhancrmrnt of various rhromium lines in Table I is not equal in all cases. This indicates that the process of enhancement cannot be explained by one simple phenomenon, such as R population increase, but is probably a result of increased population, change in temperature, and change in combustion pattern. The quantity of light absorbed by a given metal is directly proportional to the number of unexcited atoms of that metal in the light path. The production of these atoms from the solution is subjert to any variable which affects the efficiency of this process. During this work it was necessary to control a number of variables in order to obtain reproducible quantitative results : feed rate, solvent, flame position, burner design, the predominant anion, and slit widths. The effect of the first five of these can be traced to their effect on the efficiency of production of atoms from the solution. Solvent. Generally, organic solvents enhance the degree of absorption by a given concentration of metal ions. A possible explanation for this icr that the solvent controls the efficiency of the flame in producing metal atoms from solution (8). Also, in aspirating burners, the viscosity and density of the solvent affect the sample feed rate, which is itself a variable. Sample Feed Rate. The sample feed rate dirrrtly affects the number of atoms introdiiccd into the atomizrr prr unit timr. I t s effect on atomic absorption is vrrv similar to its effect in flame photometry, b u t to a lesser degrer. With aqueous solutions, when the feed rate is increased, the absorption inrreascs up to a maximum. Further increase in feed rate lrads to inefficient production of atoms from the residues remaining after each droplet has been evaporated. The effect is not so pronounred with absorption as with emission flame photometry, presumably because the extra excitation step required in emission methods is unnecessary for absorption (9, 4 ) . With organic solvents, evaporation is accompanied by combustion, and this step is much easier to take than with aqueous solvents. This results in a more efficient production of atoms in the flame. The effect of the organic solvent is illustrated in Table I; in many cmes the limit of sensitivity of detection of metals in organic solvents is increased over that obtained from aqueous solvents. VOL. 33, NO. 8, JULY 1961
1069
Flame Position. When a droplet of solution is introduced into a flame, a series of steps takes plrtce. First, i t must evaporate, leaving a residue of solid material, including the metal. This residue must then decompose, liberating metal atoms, and finally these atoms may recombine Kith other constituents of the flame forming such things as oxides, peroxides, hydroxidw, and perhaps cven carbonyls. Since the lifetime of free atoms is usually short under such circumstances, it would be anticipated that concentration of metal atoms is dependent on flame position. Usually i t is comparatively low a t its point of introduction a t the base of the flame and high in the middle. The signal in the top of the flame depends on the readiness of the metal to form oxides. If oxides do not form easily, the signal remains high; otherwise it decreases. Figure 2 shows the relative absorption for a constant concentration of silver in aqueous and in organic solution. The data were obtained by moving the flame in a vertical plane and measuring the absorption signal a t various flame positions. Instrumental conditions and feed rate were kept constant throughout. The organic and the aqueous solvents g v e two different curves, showing that the combustion pattern for these two types of solvent is somewhat different. Thus the solvent has two effects on the rate of production of atoms in the solution: The density and viscosity affect the rate of feed, and its chemical composition affects its rate of breakdown and liberation of atoms in the flame. Burner Design. The design of a suitable burner for this work constitutes a considerable challenge to workers in the field of flame photometry. Under ideal conditions it would be preferable for the solvent to have no effect on the number of atoms produced by the burner and for the feed rate to be independent of the viscosity of the samples. These things can come about only when a fixed feed rate is possible, and when the effect of the solvent is small compared to the effect of the plasma of the flame. Experimental work has shown that as the concentration of metal increases, the noise associated with the signal also increases. Although the mean signal remains stable over a period of time, the observed signal on a continuous basis incorporates a certain amount of signal noise. A possible explanation for this high noise level a t high concentration may lie in the process of production of atoms from the solution. It is possible that in high concentrations evaporation of droplets results in variation in the size of the residue. On further heating, this residue may explode into the flame and produce a local, high concentration of metal 1070
0
ANALYTICAL CHEMISTRY
6“r 5”
t
I
f
I
10 30 ABSORPTION
50
70
SIGNAL (10-1, )
Figure 2. Relation between absorption and height of flame.
A
Aqueous solution Organic solution
atoms. This could cause considerable noise, since the source is not exposed to ti continuous steady number of unexcited atoms. It is felt that this problem could be alleviated by proper burner design which would produce very small particles during the atomization stage. This, in turn, would reduce the size of the residue and therefore maintain the noise level from this source at a low level. EQUIPMENT DESIGN
Equipment used in this field, hereto has mostly been modified flaqe photometers and a hollow cathode attachment. Although this is satisfactory for obtaining information on the best absorption lines and relative limits of sensitivity between various metals for routine appliation, certain features should be incorporated in atomic absorption equipment that would not be incorporated in equipment for flame photometry. Modulation. Most metals emit strongly a t the wave lengths a t which they absorb strongly. This means that the signal registered by the detector is a confounded signal of absorption and emission. The emission signal is subject t o all the errors normally met in flame photometryi.e., interelement interferences, and background interferences-and for this reason would be a direct interference in atomic absorption. In 1955 Walsh (9) overcame this problem by modulating the source and tuning the detector to the same frequency. Under these circumstances the detector sees only the interrupted signal from the source but not the direct signal from the flame which is interposed. Subsequent work in these laboratories and elsewhere has shown that this is a satisfactory method for eliminating this problem. However, this is not always a necessary step. If the metal does not emit a t the wave length a t which it absorbs, modulation is not entirely necessary. This is usually the case where the absorption wavelength is lesa than 2800 A.-e.g., the zinc line a t 2138 A. However, a t wave
lengths greater than 2800 A,, strong emission sometimes takes place and for results to be accurate and reliable, modulation is essential. This is illustrated by the effect of modulation on reproducibility and interference in the determination of,sodium (6, r). Under these circumstances modulation is not absolutely necessary for certain elements. However, if a general-purpose instrument is to be used in this field, modulation is a very desirable feature. Two methods of modulation are possible: One uses a mechanical chopper system and tuned detector and the second uses either the alternating current from the mains or rectified alternating current from the same source. In each case the detector is tuned to the frequency of the hollow cathode signal. Double-Beam System. The work carried out in this study was performed on an instrumpnt using a single light beam. This requires that the hollow cathode be powered by a very stable power source, t o produce a steady signal. I n a double-beam system, however, certain advantages can be claimed from the fact that small variations in the source signal can be automatically compensated for by the instrument. To obtain information on the relative value of these two systems, a series of experiments was carried out using silver as the subject metal. The work was carried out on R Perkin-Elmer Model 13 spectrophotometer, which can be used as either a single-beam or a double-beam instrument. This has the advantage that many components of the instrument are common to both systems. This elimimtes some sources of apparent but not real differences. The results showed that there was no significant advantage in sensitivity using the double-beam over the single-beam system, although it is conceivable that with increased zero suppression the sensitivity in the case of the doublebeam system could be greater than in the single-beam system. Although the single-beam system was satisfactory in this case, a very reliable power source was necessary to keep the signal from the hollow cathode constant. The cost of this power source could be a significant part of any equipment obtainable commercially for this type of work. For the double-beam system, however, variation in the hollow cathode is compensated for automatically by the instrument and this enables a cheaper power source to be used for exciting the hollow cathode. One big advantage of the single-beam system lies in the fact that the correct wave length can be easily located for each element. Usually it is necessary to scan the approximate wave length range of the
absorption band to identify the lime from the spectrum of the hollow cathode. With a double-beam instrument, this procedure is very difficult. It would seem desirable, therefore, that commercial equipment be capable of being used as a single- or a double-beam instrumen t. UNDETECTED METALS
One of the most serious problems encountered in this method of analysis is the fact that a number of met& cannot be detected by this process. These metals form refractory oxides and it is conceivable that in flame they are never reduced to the atomic state. Use of Oxycyanogen Flame. I n an effort to overcome this problem an oxycyanogcn flame was used instead of an oxyliydrogen flame. T h e possible advnntage of this system was t h a t the oxycyanogen flame reaches a temprrat,ure of 4500" C., which is ronsidrrably above that of the oxyhydrogen flame, which reaches a temperature of 3000" C. It waR hoped that at this elevated temperature the oxides formed by metals which are insensitive to this process would be broken down and that the free atoms would then be able to contribute to absorption of the signal. However, although vanadium absorbed slightly,
no absorption by AI, Mol or W occurred even a t this elevated temperature. Although it is possible that these elevated temperatures do not break down these oxides, it raises the question that insensitivity of these metala may be due to another cause. As an alternative method to the use of a high temperature flame for breaking down metal oxides, a spark may be used in conjunction with the flame. Experience in spectrography would indicate that this should be a means of breaking down metal oxides formed in the flame. Under these circumstances it is possible that metal atoms will be formed in the region of the spark and immediately above it. Hitherto, no work has been done on this proposal, but i t is hoped that results will be available shortly. CONCLUSIONS
The results to date indicate that atomic absorption spectroscopy is a technique which should have many uses in the field of analytical chemistry. There is a high degree of freedom from interference of other metals present and high sensitivity is usually encountered in all cases wherp absorption takes place. It is hoped that problems associated with solvent, feed rate, etc., will be controlled to a considerable
degree by the design of a better burner for use in this work. I t is anticipated that with increasing amount of equipment becoming available commercially, this field will grow and expand and take its place among the other major fields of analyticnl chemistry. ACKNOWLEDGMENT
The author thnnks Esso Standard, Division of Ilumblc Oil & Refining Co., for pcrmission to publish this paper. LITERATURE CITED
(1) Box, (f. F., W~llih, A., Speclrochim. Acta 16,255 (1060). (2) Crosswhite, 13. M.,Ijickc, G. H., Jxanrrneiir, ,J., .I. ODL Soc. A m . 45, 270
(I$L5g.
'
(3) Fost,er W. IT., Jr., &me, D. N., ANAL.&EM. 31,2028 (1059). (4) Fiiwa, I
