Atomic Absorption Analysis by Flameless Atomization in a Controlled Atmosphere H. M. Donega and T. E. Burgess Research and Development Center, Sprague Electric Company, North Adams, Mass.
An atomic absorption method is described which employs electrothermal heating of the sample in an enclosed chamber to produce the atomic sample vapor. The atomization takes place in a selected atmosphere which may be maintained at any desired pressure, usually in the range of 1-300 Torr. This system increases absorption sensitivity by reducing foreign gas effects such as the loss of ground state sample atoms through chemical reaction with ambient gases. The method is capable of detecting lo-’* gram of an element in a few microliters of solution. In addition to significant improvement in absolute sensitivity as compared to flame methods, it is simple to operate and more adaptable to a variety of special techniques.
THESENSITIVITY which is theoretically available in atomic absorption makes this technique potentially ideal for trace analysis. The greatest limitation of commercial atomic absorption instruments is their use of the inefficient flameatomizer system. Primarily because of physical limitations, the amount of the element which must be supplied to the burner system is large compared to that which is actually in the absorbing part of the flame at any time. This requires that the sample be in solution and that the volume of solution per determination be in the order of 1 or 2 ml which results in absolute detection limits of lod6 to lo-’ gram for most elements (1-3). Other methods of atomization have been investigated. Some recent work describes the use of electrothermal heating in graphite furnace tubes (4-8). These studies demonstrate that detection limits may be reached which are well below those obtained by flame atomization methods and that only a few microliters of liquid sample are required. Atmospheres in these furnaces are controlled to eliminate oxidation of the atomized sample. By such a device, the atomic absorption spectrophotometer may be used to detect nanogram to picogram quantities of elements in a fraction of a milliliter of solution and possibly to determine elements in microgram quantities of solids. West and Williams have used a flameless carbon filament atomic absorption technique to determine gram of silver and magnesium in a few microliters of soluand tion (9). These authors also have determined gram, respectively, of silver and magnesium using the same equipment in the fluorescent mode. The atomic ab(1) J. Ramirez-Munoz, “Atomic Absorption Spectroscopy,” Elsevier Publishing Co., New York, N. Y., 1968. (2) J. W. Robinson, “Atomic Absorption Spectroscopy,” Marcel Dekker, Inc., New York, N. Y., 1966. (3) J. A. Dean and T. C . Rains, “Flame Emission and Atomic Absorption Spectrometry,” Vol. 1 , Marcel Dekker, Inc., New
York, N. Y., 1969. (4) H. Massman, Spectrochim. Acta, 23B, 215 (1968). ( 5 ) B. V. L’vov, Inzh.-Fiz. Zit., Akad. Nauk. Belones S.S.R. 2 (2), 44 (1959). (6) B. V . L’vov, Spectrochim. Acta, 17, 761 (1961). (7) B. V. L’vov, “Atomic Absorption Analysis,” Nauka, Moscow, USSR, 1966. (8) R. Woodriff, R. W. Stone, and A. M. Held, Appl. Spectros., 22, 408 (1968). (9) T. S . West and X . K. Williams, Anal. Chim. Acta, 45, 27 (1969).
sorption sample chamber described here is similar to these systems, but it is simpler to operate, uses lower operating currents, is more readily adaptable to a variety of techniques such as vacuum UV spectrometry, and yet retains extreme sensitivity for most elements. EXPERIMENTAL
A schematic drawing of the atomic absorption sample chamber appears in Figure 1. The chamber consists of a brass base plate which contains an O-ring seal, a quartz window, a gas/vacuum port, and two feed-through insulators. Two l/r-inch copper rods which are used to conduct the current to the sample boat are sealed into the insulators. The inner ends of these rods were fitted with small clamps which support the sample boat. The rest of the chamber is fabricated from a 50-mm i.d. quartz tube which has an optical quartz window sealed on one end and an O-ring flange at the other. When this tube is in position over the sample boat, the air within the chamber can be evacuated and replaced with any desired operating gas at a pressure of 1 to 760 Torr. The alignment of the windows in the chamber and the sample boat were such that light from a hollow cathode lamp passes along the axis of the chamber directly over the sample boat. The sample boats used in these experiments were cut from 5-mil graphite sheet, 1-mil high-purity tantalum or tungsten foils and were about 50 mm long by 6 mm wide. The tantalum and tungsten boats were shaped to provide a small indentation which could hold about 100 pl of solution. In the case of the more brittle graphite, the clamps were cut with a 5.5-mm radius which produced a transverse curvature in the clamped graphite strip. About 50 p1 of solution can be easily accommodated on these strips. Power is supplied to these boats from a step-down transformer-variac source which can provide about 150 A. The normal operating range is 30-50 A at 12 V. This current is sufficient to heat the above boats to -2200 “C in less than 0.1 second. The relative positions of sample boat and light path are important. The largest absorption signal is obtained when the light beam passes directly over the heated surface. Since the boats tend to expand on heating and drop away from the light path, some loss of sensitivity is observed. This loss can be eliminated by boat design in the case of tantalum and tungsten. If the boats are bent to give a longitudinal profile similar to that in Figure 1, then expansion on heating is relieved by the small verticle portions of the boat and the sample-bearing surface tends to remain adjacent to the light path. This type of compensation cannot be achieved with boats made from the brittle graphite sheet. To operate the chamber, the quartz envelope is removed from the base plate and a measured volume of the sample solution is placed on the boat. The sample is dried on the boat by passing a small current (4 A) through the circuit and the quartz envelope is replaced. The chamber is evacuated to about 50 p through a mechanical pump attached to the gas port. A series of valves permits the chamber to be back-filled with the desired background (foreign) gas through the same port. When the operating conditions have been reached, the sample is atomized by passing a large current through the boat. The complete operation from sample drying to atomization takes about two minutes. The
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Figure 4. Oscilloscope- trace showing absorption by 300 ng of molybdenum at 3133 A Vertical scale: per cent absorption, 12.5 %/em. Horizontal scale: sweep speed, 0.5 seclcm RESULTS AND DISCUSSION
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per cent absorption, 12.5 %/cm. Horizontal scale: sweep speed, 0.2 sec/cm actual atomic absorption signal usually has a duration of a few tenths of a second. Figure 2 shows a schematic drawing of the complete atomic absorption spectrophotometer. Light from the hollow cathode lamp is focused on a 1000-Hz mechanical chopper. The light beam then passes through the absorption chamber and into a Zeiss Model M4QIII monochromator. The detector attached to the exit of the monochromator is a 1P28 photomultiplier tube. The signal from the photomultiplier is passed into a PAR Model HR-8 lock-in amplifier which is phase-locked to the mechanical chopper, and the amplifier signal is read out on a Model R564B Tektronix storage type oscilloscope. This combination of the
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accurate measurement of the brief absorption signals observed in the sample chamber. 1522
Figures 3 and 4 show typical oscilloscope traces obtained for absorption produced by aluminum and molybdenum, respectively. Three separate samples each containing 3 ng of aluminum were run to produce the data in Figure 3. The sweep rate of the oscilloscope trace (left to right) was set at 0.2 sec/div for these measurements. The average height of the absorption signal is 70.8% with one complete absorption cycle taking about 0.1 second. The normal background noise for this system is about 2%; however, in this figure it appears larger since many oscilloscope sweeps were made during the preparation of these samples and a gradual drift of the zero setting has obscured the true background noise. Figure 4 shows the absorption trace obtained for a 300-ng samule of molvbdenum. The slower atomization rate of the high melting molybdenum causes a broadening of the absorption peak for this element. Slow vaporization occurred even though the temperature of the tantalum boat was 2400 "C. In all the work reported here, the height of the absorption signal was used to determine the concentration of the element being determined. Although useful data can be obtained in the same manner from broad absorption signals, integration of such curves should improve sensitivity. Five experimental conditions, temperature of atomization, boat material, chemical composition of the deposited sample, and foreign gas pressure and composition, are important in making absorption measurements, The rate at which a sample is vaporized and the element of interest converted to ground state atoms is directly related to the temperature of the sample boat. If the magnitude of the observed absorption signal is taken as the quantitative measure of the absorbing element, then the greatest sensitivity would be obtained if all of the element were present in the light beam as ground state atoms at the same time. However, loss of atoms by diffusion out of the light path also occurs and a combination of these processes determines the shape of the absorption curve.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
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Figure 3 is an example of absorption signal obtained for very rapid vaporization and atomization of the sample. The fast rise in absorption is followed by an equally fast decline as the atomized element rapidly diffuses out of the light path. Figure 4 is an example of the broad absorption signal obtained where low boat temperature results in a slow sample vaporization rate. An example of the effect of temperature on ground state atom population is shown in Figure 5. Here equal amounts of standard solution of sodium silicate were deposited on the tantalum boat. After each deposition, the boat was heated to the desired temperature and the absorption measured. It is quite evident that under conditions used for these measurements, 300-Torr argon and silicon present as sodium silicate, that 2300 "C boat temperature produced the best results. The difference in ground state atom population was easily shown by deposition of a single sample followed by heating first at 1500 "C then 2300 "C. Since no absorption was observed in either case, it was assumed that all sodium silicate was vaporized at 1500 "C without atomization of the compound to give neutral ground state silicon atoms. Since thermal decomposition of the sample followed by atomization of the element in question are the required steps before absorption can take place, the chemical composition of the sample is also important. The use of volatile compounds would certainly result in errors in analysis. For example, loss of aluminum as aluminum chloride would be expected during the initial drying step if chlorides were present in the original sample solution. Compounds which would resist decomposition at the temperatures available in the system or which would volatilize in the molecular form also should be avoided. Although many compounds yield usable absorption signals, improved sensitivity may be obtained through the choice of a more suitable matrix. Atomization by electrothermal heating requires that the sample boat used in the experiments be fabricated from a high melting point material capable of being resistance heated with a reasonable sized power supply. Tantalum, tungsten, and
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graphite, the three materials used in this work, fitted this requirement very well. It is possible that chemical reaction could take place between the element being measured and the boat material which would result in loss of sensitivity. This has not been observed in this work. Perhaps the most important of the five conditions mentioned above are those of foreign or background gas, composition and pressure. Examples of the effect of changes in these conditions are shown in Figures 6 and 7, where absorption for the same size samples of sodium (Figure 6) and aluminum (Figure 7) were measured in different background gases at various pressures. An explanation of these results is not obvious at this time. It is proposed that the rapid rise in ob-
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Table I. Absolute Sensitivity of Flameless Atomic Absorption Method WavelFngth, Operating Element A Sensitivity, gram conditionsa Aluminum 3093 4 x 10-10 (1) Chromium 3594 6 x 10-l2 (2) Copper 3248 3 x 10-12 (2) Manganese 2795 3 x 10-12 ( 2) Molybdenum 3133 3 x 10-9 (3) Nickel 3415 3 x 10-9 (2) Platinum 2659 3 x 10-7 ( 2) Silicon 2516 3 x 10-9 (2) Sodium 5890 3 x 10-11 (4) Vanadium 3184 6 x 10-lo (1) a (1) Tantalum boat, 300 Torr hydrogen; (2) tantalum boat, 300 Torr argon; (3) tungsten boat, 1 Torr argon; (4) graphite boat, 100 Torr argon.
served absorption with initial increase in foreign gas pressure is due to a gas buffer effect. The presence of the foreign gas reduces the diffusion rate of the absorbing species from the light path and results in a stronger absorption signal. This effect would be expected to vary also with gas composition. The observed decrease in absorption at higher foreign gas pressures observed with sodium vapor and shown in Figure 6 cannot be explained at this time. It is clear from these two examples that the use of the correct foreign gas composition and pressure is important in order to obtain optimum sensitivity for each element. Sensitivities for several elements using this flameless atomic absorption method are given in Table I. Improvement on these sensitivities can be expected since optimum operating conditions, chemical composition of sample, etc., have not been found for all elements. This is no doubt true in the case of platinum where the sensitivity is poor as compared with
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those obtained for some of the other elements. The excellent results obtained for most of the elements investigated combined with the small sample size required for analysis demonstrate the superiority of this system over present flame methods. The advantages of this method over other systems of atomization for atomic absorption analysis are considerable. The limits of detection are well below those attained by flame atomization with sensitivity increases of 105-106 times quite common. Much smaller liquid samples may be accommodated by this system with the limit here probably in the order of a few microliters; however, volumes as large as one milliliter can be easily accommodated. The ability of this method to operate in atmospheres which can be controlled in composition and pressure permits the selection of the most favorable conditions for atomization. The closed system allows operation in the vacuum UV region of the spectrum. Control of the background or foreign gas composition as well as its pressure is difficult or impossible in flame systems and not available in most furnace systems. The fact that low power is required for electrothermal heating is a distinct advantage over most furnace systems. Solid samples could be accommodated in the sample chamber and other forms of atomization, for example, laser beam heating could be adopted. With proper control of heating current it would be possible to determine two or more elements in the same sample providing vaporization and atomization temperatures for these elements were sufficiently separated. ACKNOWLEDGMENT
The authors are indebted to Dr. Frank H. Hielscher for his valuable assistance and advice.
RECEIVED for review May 25, 1970. Accepted August 14, 1970.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
