(3) N. C. Clampltt and G. M. Hieflje, A n d . Chem., 46, 382 (1974). (4) N. C. Clampttl, Ph.D. Thesis, Indiana Universky, Blooniington, Ind., 1974. (5) D. R. Jenkins, R o c . R . Soc. London, Ser. A , 306, 413 (1968). (6) D. R. Jenkins, Spectrochlm. Acta, 258, 47 (1970). (7) J. D. Wlnefordner, V. Svoboda, and L. J. Cline, Crn. Rev. Anal. Chem., 1, 233 (1970). (8) G. M. Hieftje, Appl. Spectrosc., 2 5 653 (1971). (9) A. G. Gaydon and H. G. Wolfhard, Flames”, Chapman and Hall, Ltd., London, 1970. (10) F. J. Weinberg, “Optics in Flames”, Butterworth and Co. Ltd., Washington, D.C., 1963. (1 1) W. Snelleman, P h D Thesis, University of Utrecht, 1965, (12) G. M. Hieftje and R. J. Sydor, Appl. Spectrosc., 26, 624 (1972). (13) R. Lewis and G. Von Elbe, ‘Combustion, Flames and Explosions of Geses”, Academlc Press, New York, N.Y., 1951. (14) K. M. Aldous, B. W. Bailey, and J. M. Rankin, Anal. Chem., 44, 191 (1972). (15) R . F. Suddendorf and M. B. Denton, Appl. Spectrosc., 2 8 , 8 (1974). (16) S.B. Reed, Combust. Name, 13, 583 (1969). (17) E. Bartholm6, Z . Electrochem., 54, 169 (1950).
(18) P. J. Wheatley and J. W. Llnnett, Trans. Faraday Soc., 48, 338 (1952). (19) K. M. Aldous, R. F. Browner, D. Clark, R. M. Dagnall, and T. S.West, Talanta, I S , 927 (1972). (20) C. Th. J. Alkemade, Ph.D. Thesis, University of Utrecht, 1954. (21) M. D. Amos and J. B. Willis, Spectrochlm. Acta, 2 2 , 1325 (1966). (22) G. F. Klrkbrlght, M. K. Peters, M. Sargent, and T. S. West, Telenta, 15, 663 (1968). (23) A. G. Oaydon, “The Spectroscopy of Flames”, Chapman and Hall, London, 1974.
RECEIVED for review April 18,1977. Accepted August 5,1977. Presented in part at the second and third national meetings of the Federation of Analytical Chemistry and Spectroscopy Societies. The authors acknowledge the National Science Foundation and the Office of Naval Research for support of this work.
Atomization from a Platform in Graphite Furnace Atomic Absorption Spectrometry D. C. Grigolre and C. L. Chakrabarti’ Metal Ions Group, Department of Chemistry, Carleton University, Ottawa, Ontario, Canada K I S 5B6
Studies on the atomlzatlon of Mol V, Cu, Zn, and Cd from a rectangular pyrolytlc graphlte platform Inserted Into an HGA-2100 pyrolytlcallytoated graphtte tube Is reported. No Improvement In peak-helght sensltlvttles Is observed wtth the exceptlon of Mo and V, for whtch up to 1.8-fold enhancement in the sensltlvlty Is observed. Integrated sensltlvltles are generally enhanced by a factor of 1.2 to 1.4. The effect of platform mass and geometry on the temperature-time characterlstlcs of the platform has been studled. Increaslng the mass of the platform slgnlflcantly decreases the rate of heatlng of the platform at the appearance tlme of any glven metal. Peak absorbance for copper Is severely dlmlnlshed as the platform mass Is Increased. Integrated absorbances, however, are virtually unaffected by even a 5-fold Increase In the platform mass.
Studies done with commercially available atomizers, Varian Techtron Carbon Rod Atomizer 63 (CRA-63) and the Perkin-Elmer Heated Graphite Atomizer 2100 (HGA-2100),show that the conditions necessary for the theoretically predicted maximum attainable sensitivity are not fulfilled (1-3). The basic limiting factors include the geometry of these atomizers as well as their heating characteristics. Transient absorption pulses (2,3) are produced by rapid heating of electrothermal atomizers. The absorbance values can be measured by two methods-the peak absorbance method and the integrated absorbance method ( 4 ) . L’vov ( 4 ) has shown that the peak-height sensitivity is proportional to , the residence time, the ratio of the atomization time, T ~ and 72, of the analyte atoms in the analysis volume. If the atomization is completed in a time much smaller than the residence time (i.e., 7 1 / 7 2 3000 K) attained by the graphite tube surface. Therefore, the platform will still increase in temperature a t a high rate, and especially for the more volatile elements (Cd, Zn), the appearance time of these metals with the platform technique may occur a t a time when the graphite tube is still increasing in temperature and is far from equilibrium conditions. According to heat transfer theory, if a surface absorbs some fraction of the energy incident upon it from some other source, this surface wiU emit the Same fraction of energy incident upon it from within. The radiant emittance, W , from the surface of any body a t any given temperature is then given by
(7) where Wbb is the radiant emittance of a perfect blackbody and t is a constant varying from 0 in the case of a perfect reflector to 1 in the case of a perfect blackbody. Any method of making the surface of the emitter (graphite tube) and the receiver (platform) more reflective would be effective in decreasing the energy emitted by the graphite tube and absorbed by the platform. A reflective surface would then aid in further shifting the temperature-time curve of the platform relative to that of the heated graphite tube. A graphite surface of greater reflectivity can be produced by coating the tube with a layer of pyrolytic graphite (8). Pyrolytic graphite has a much smoother surface than conventional graphite. This has been confirmed by electron photomicrographs (9) of the surfaces of the graphite tubes both with and without a coating of pyrolytic graphite. Further, pyrolytic graphite has the added advantage of increasing both peak and integrated sensitivities by up to a factor of five in the most favorable cases (9). Figure 1is an illustration of the effect of the predicted shift of the appearance time on a typical atomic absorption pulse. Curve a is that of an analyte atomized from the surface of a graphite tube; curve b is that of the same analyte atomized from a platform inside the graphite tube under the same experimental conditions; curve c shows the temperaturetime characteristics of the graphite tube surface. For atomization from the tube surface (curve a), it is seen that except for the tail of the absorption pulse, the entire absorption pulse will be produced under non-isothermal ANALYTICAL CHEMISTRY, VOL. 49. NO. 13, NOVEMBER 1977
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-
A
B
Figure 1. Effect of atomization from a platform on the position of the absorption pulses relative to t h e temperature-time characteristics of the graphfie tube. (a)Atomization from a graphite tube. (6)Atomization from a platform. (c)Temperature-time characteristics of a graphite tube
conditions. Curve b shows that the shifted absorption pulse will occur under conditions of a reduced thermal gradient across the length of the graphite tube; hence, losses due to the diffusion of the analyte atomic vapor to the cold end cap windows will be diminished. In addition, the absorption pulse will be produced in a time when the inert gas contained in the graphite tube will have expanded to its final volume. Both of these will result in enhanced analytical sensitivities. In the above theory, other mechanisms of heating of the platform such as thermal conduction, which contributes a small part of the heat gained or lost by a platform, have been omitted for the sake of a simplified treatment. The simplified treatment, however, correctly shows the major mechanism of heat transfer, and is therefore useful for predictive purposes. EXPERIMENTAL Apparatus. The apparatus used in this study has been described in an earlier publication (3) from this laboratory. The atomizer unit is a modified Perkin-Elmer HGA-2100 fitted with larger cooling chambers to allow for more efficient temperature control during long or high-temperature atomization periods. The graphite cones and tubes are those commercially available from The Perkin-Elmer Corporation for the HGA-2100. The power supply, also designed and built in this laboratory, is capable of delivering variable levels of power and of dividing the atomization stage into two separate stages. The first stage provides a variable initial rate of heating of the graphite tube up to some predetermined temperature, and the second stage maintains this temperature to the end of the atomization period. This facility makes possible the study of the rates of heating of the graphite tube and its effect on the analytical parameters of the absorption pulses. All temperatures were measured with an automatic optical pyrometer, series 1100 (Ircon Inc., Niles, Ill.), which had been calibrated by the manufacturer and again in this laboratory by measurements with thermocouples and by the melting points of selected pure metals to cover the entire range of temperatures. Perkin-Elmer and Varian Techtron single-elementhollow cathode lamps were used as narrow line sources. Reagents. All reagents used were of ACS reagent-grade purity. Stock solutions containing lo00 fig/mL of each metal studied were prepared from pure metals dissolved in the appropriate acids or bases with the exception of vanadium which was prepared from vanadium pentoxide, and molybdenum, which was prepared from molybdenum trioxide. All test solutions were prepared immediately prior to their use with ultrapure water obtained directly from a Milli-Q water system (Millipore Corporation). Argon gas used to sheath and to purge internally the atomizer was of 99.95% purity (Roncar Oxygen Company). Pyrolytic graphite used in the manufacture of platforms was obtained from Ultra Carbon Corporation, Bay City, Mich. Platforms. Platforms were cut from a rectangular block of pyrolytic graphite. Slabs were cut along an axis parallel with the lamellar planes of the graphite. Sections cut perpendicular to this axis proved too weak and brittle for use. Platforms of varying 2020
ANALYTICAL CHEMISTRY, VOL. 49, NO. 13, NOVEMBER 1977
Figure 2. Geometry and position of the platform in an HGA-2100 graphite tube. (A) Rectangular platform. (B) Position of platforms in the graphite tube. (C) Semicircular platform thickness were cut to a standard length of 15 mm and width of 5 mm. Figure 2 illustrates the positioning of platforms within graphite tubes. To prevent arcing from the graphite cones or the tube ends during atomization, platforms were placed in such a way that they did not have contact with the end-grooves of the Perkin-Elmer graphite tubes. Platforms rested only on the raised inner surface of the graphite tube, and intimate contact occurred only along the lower edges of each platform. The semicircular platform was cut to a length of 15 mm from a rod of pyrolytic graphite and placed in the center of the graphite tube (Figure 2C). For a given mass, the shape of this platform afforded greater physical contact between the platform and the wall of the tube. To assure that the atomization of samples occurred on identical surfaces throughout this study, all platforms and tubes were coated with a layer of pyrolytic graphite. This was accomplished by heating the tube and the platform to 2400 K and maintaining an internal gas flow of 50 mL/min of a mixture of 10% methane and 90% argon for a period of 10 min. Sampling. For each experiment, a 5-hL aliquot of the analyte solution was delivered by means of an Eppendorf pipet fitted with disposable plastic tips. Care was taken to assure that the entire volume of the test solution was reproducibly delivered to the center of the platform and that none of the sample was lost to the edge of the platform or to the surface of the graphite tube. The drying stage of the temperature program was modified to accommodate the longer drying period required. Accordingly, the time for the 100 "C drying stage was prolonged from 30 to 90 s.
RESULTS AND DISCUSSION Mechanism of Heat Transfer. Heating of the platforms was due primarily to the radiational transfer of heat from the hot graphite tube and to a much lesser extent by thermal conduction where physical contact existed between the graphite tube and the platform surface. Radiational transfer of heat was the major mechanism of heating since only a very small fraction of the total platform surface area was in intimate contact with the graphite tube surface. In addition to this, it was observed that, at the termination of the atomization stage, the platform remained red hot for extended periods of time even after the graphite tube surface was cooled below incandescent temperature, which showed that thermal conduction of heat was a minor mechanism of heat transfer. Effect of Platform M a s s on Heating Rate. The temperature-time characteristics of a graphite tube and several platforms are presented in Figure 3. All platforms were of standard dimensions; therefore, a change in mass represents a proportional change in thickness. The attenuation of the intensity of incident radiation from the hollow cathode lamp even by the thickest platform (1.5 mm) was only a few percent. This reduction in throughput of the incident radiation was
Table I. Heating Rates for Graphite Tube and Platform as a Function of Platform Mass and Geometry
3100-
2900-
2700-
0.030 0.099
Y y \
2500-
0.119b 0.160
a
$ 2300Y
3.7 3.7 3.7 3.7
2.5
1.4 1.2 1.1
a Heating rate of the platform at the appearance time of Semicircular platform. the coDDer absorDtion signal.
I 2100-
1900L
, 1.25 X lo-'' g Cu Table 11. Atomization Time, T ~ for Atomized from Platforms of Various Masses
1700-
l500c 200
300
,
,
,
400
500
600
700
800
TIME
900 /
IO00
1100
1200
I300
1400
rns
Figure 3. Temperature-time characteristics for tne pyrolyticaliy-coated graphite tube and platforms. (A) Graphite tube only, no platform. (0) 0.030 g, rectangular platform. (A)0.1 19 g, semicircular platform. (0) 0.099 g, rectangular platform. ( 0 )0.160 g, rectangular platform
easily compensated for by increasing the gain on the photomultiplier tube. The uppermost curve in Figure 3 is the temperature-time plot of the pyrolytically-coated graphite tube heated at a rate of 3.7 K ms-I to a final temperature of 3200 K. The lower set of curves are those of several platforms of different masses heated in the graphite tube under the above conditions. Some of the features of these temperature-time curves warrant close examination. First, since for every tubeplatform combination, the temperature programs were set to give identical dry, ash, and atomization temperatures, all curves should have a common point of intersection when extrapolated to t = 0. The temperature at this point corresponds to the ashing temperature. The curves for the tube and the platform do not intersect a t this point. This discrepancy can be explained by examining the factors that govern the function of the optical pyrometer. The lens of the optical pyrometer was focused onto the surface of the platform through the sample injection port of the graphite tube. As the entire target area of the lens that was used to gather light was smaller than the aperture of the sample injection port, no stray light from the hot graphite surrounding the port would be viewed by the pyrometer. However, as mentioned earlier, pyrolytic graphite has a much smoother surface than conventional graphite. T h e reflected light from the hot pyrolytic graphite tube was viewed by the pyrometer. Thus, higher-than-expected temperatures were observed with the result that all temperature-time curves recorded for the platforms were shifted upwards by several hundred degrees. Since the present authors are not concerned with the absolute temperatures of the platforms, but rather their rate of heating a t the appearance time of each element, this effect will have little influence on the interpretation of the results. Second, for the initial 700 ms into the atomization stage, all three rectangular platforms show identical temperature-time characteristics. During this time, the graphite tube is rapidly heated to its maximum temperature, and thus the inert gas inside the tube is rapidly expanding. This expanding hot gas may be partly responsible for the efficient heating of the platform during this initial time. After this initial time, heating of the platform is accomplished mainly by the radiational transfer of heat. The semicircular platform acquires heat at the same initial rate as the rectangular platform and its temperature-time curve generally maintains the same shape as that of the rectangular platforms throughout the atomization stage. However, relative to the curve of the rectangular platform of lighter mass, the entire curve for the semicircular platform
M a s ~ / l O Kg -~
Relative mass
0.030 0.098 0.160
1.0 3.3 5.3
T , / ~ s
200 230 2-70
0 26' 0 24 -
0 22-
0 20-
018W
z
2 cc 2m
016-
014-
U 012-
OIO-
7 008-
O 0 6 k E k & - & n H h O
o l i o ole6
PLATFORM M A S S I I O - 3 ~ g
Flgure 4. Peak and integrated absorbance for 1.25 X lo-'' g Cu as a function of platform mass and geometry. (0)Peak absorbance, rectangular platform. (0)Peak absorbance, semicircular platform. (A) Integated absorbance, rectangular plalfom,. (A)Integrated absorbance, semicircular platform
is shifted upwards by about 70 to 100 degrees. This is attributed to the greater physical contact of the semicircular platform with the raised centrai portion of the graphite tube. Generally, the more massive the platform, the greater the shift to longer times at which the maximum temperature of the graphite tube is attained. For low-volatility elements such as V and Mo, atomization can be made to take place in compietely isothermal conditions, relative to the surface temperature of the tube wall. Effect of Platform Mass on Peak and Integrated Absorbance of Cu. The rates of heating of the platforms a t the appearance time of copper relative to the heating rate of the graphite tube itself are shown in Table I. The appearance time is defined as the time after the beginning of the atomization stage when the analyte signal leaves the baseline. About a fivefold increase in the mass decreases the heating rate of the platform a t the appearance time of copper by a factor of about 2.2. The effect of the platform mass and geometry on the peak and the integrated absorbances is shown in Figure 4. A decrease of about 50% in peak absorbance is observed with a fivefold increase in platform mass. This , increase in which is explained by the atomization time T ~the parallels the increase in the mass, whereas the residence time, T ~ is, undergoing little change. Thus, the T ~ / T *ratio decreases giving a diminished peak absorbance. Table I1 presents r1 ANALYTICAL CHEMISTRY, VOL. 49, NO. 13, NOVEMBER 1977
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Table 111. Comparison of the Peak Sensitivity of a Graphite Tube Atomizer (HGA-2100) with and without a Platform
Table IV. Comparison of the Integrated Sensitivity of a Graphite Tube Atomizer (HGA-2100) with and without a Platform
Absolute sensitivitylf dT/dt/
Metal K m - l Mo
0.8
3.7 V
0.8
3.7 Cu
1.7
3.7 A1 Zn Cd
1.8
3.: 2.2 3.7 2.2 3.7
Absolute sensitivity/$
ImproveWith platWithout ment formb platformb factor 1.6 x 3.0-10 3.0 x 1 0 - l o 1.8 7.2 X lo-" 0.4 1.2 7.9 x lo-" 9.3 x lo-" 4.3 x lo-" 0.6 5.1 x 10-l2 5.4 x 10-12 1.1 3.0 x 10-l2 0.6 4.7 x 10-12 3.6 x 0.8 2.7 x 10-12 0.6 3.2 x 10-l3 3.0 x 10-i3 0.9 2.6 x 10-l3 0.8 7.8 x 6.8 x 0.9 6.3 x 10-13 0.8
a Defined as the weight in grams of an element that gives a peak absorbance of 0.0044. Each value represents the average of at least four individual determinations.
values measured over the range of a fivefold increase in the mass of the platform. Figure 4 shows that there is no difference in the absorbance values given by the semicircular platform and those given by the rectangular platform of the same mass. It is evident from Figure 3 and Table I that the temperature-time characteristics and the rate of heating of the semicircular platform are identical with those of a rectangular platform of the same mass. The only observable difference is a slightly higher temperature which appears to have little or no effect on analyte atom populations. Integrated absorbance was found to be independent of the platform mass. This is probably due to the fact that by the time copper is atomized from the surface of the platform, the tube wall has already attained thermal equilibrium; therefore, all mechanisms of loss which are temperature-dependent (6) have attained a steady state, making r 2 constant. Hence, integrated absorbance is independent of the platform mass. Integrated absorbance values are low when the platform atomization technique is used because the T~ values, although independent of the platform mass, are quite small, giving small values of integrated absorbance, Q.V, according to Equation 2. The reason for small values of 7 2 are that the main factors governing the residence time, T ~of, atoms in the tube are not effectively altered when the platform is used. For example, there still exists a large temperature difference of approximately 2500 degrees between the tube center and the ends, thus creating a large, though constant, temperature gradient. Atomization from a Platform. The effect of a platform on the integrated and peak sensitivities of Mo, V, Cu, Al, Zn, and Cd was studied. These six metals were chosen as representative of each of the three groups of elements generally referred to ( 2 , 3) as the low (Mo, V), medium (Cu, Al), and high-volatility (Zn, Cd) elements. In order to make the results comparable, a single platform of standard dimensions (15 mm X 5 mm) and mass (0.84 g) was used. The heating rate of the graphite tube was set at 3.7 K ms-l with a maximum atomization temperature of 3200 K. T h e results of this experiment are summarized in Tables 111and IV. Both peak and integrated sensitivities are given for each metal atomized from the surface of the pl tform and the surface of the graphite tube. Tables I11 and I show the heating rates of the platform surface at the appearance time of each metal, the heating rate of the graphite tube surface in all cases being 3.7 K ms-'. Also given are the peak and the integrated absorbance of each metal atomized from the surface
$
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ANALYTICAL CHEMISTRY, VOL. 49, NO. 13. NOVEMBER 1977
dT/dt/ Metal K ms-' Mo
0.8
3.7
With platformb
Without platformb
1.3 x 10-lo
1.9 x 10-10 1.3 X lo-''
lo-"
V
0.8
4.3 x
Cu
3.7 1.7 3.7
7.7 x 10-12
AI
Zn Cd
1.8
3.7 2.2 3.7 2.2 3.7
1.1 x 10-11
1.4 x l o - ' * 3.6 x 10-l2
4.5 x 5.5 x 1.0 x 7.7 x 1.1 x 1.5 X
lo-" 10-1' 10-11
Improvement factor 1.4 0.9 1.0 1.3
1.3
10-12
1.0
10-11
1.0
lo-"
1.6 X 1.7 X 2.3 x 3.1 x lo-'*
1.3 1.2 1.2
0.6 0.9
a Defined as the weight in grams of an element that gives an integrated absorbance of 0.0044 absorbance x second. Each value represents the average of at least four individual determinations.
of the graphite tube, which is heated at a rate equal to the heating rate of the platform measured at the appearance time for each metal. The results in Table I11 indicate that the peak sensitivities with the platform are lower than those obtained without the platform. For the high-volatility elements (Cd, Zn) and the medium-volatility elements (Al, Cu), little or no improvement in the sensitivity was observed even when comparing the performance of the platform with atomization from the surface of the graphite tube heated a t a rate equal to the platform heating rate a t the appearance time of each metal; however, this heating rate of the graphite tube is so low as to give poor analytical sensitivity for the graphite tube alone. Under these same conditions, the low-volatility elements, Mo and V, shows a significant increase in the peak sensitivity. Two mechanisms may be operative in producing the increase in the peak sensitivity of Mo and V. First, at the appearance time of Mo and V, when the platform is used, the graphite tube has reached its maximum temperature and the thermal gradient across the length of the tube has greatly diminished as compared to the thermal gradient prevailing a t the high heating rate of the graphite tube alone. Second, Mo and V are carbide-forming elements (Mo forming carbide more easily). A reduced rate of heating over the temperature range where the analyte signal appears allows a longer time for atomization of these metals before conditions are attained such that the carbide formation becomes a major source of loss of the analyte atomic vapor. Higher residence times coupled with diminished loss due to the formation of involatile compounds (carbides) are probably responsible for the observed increases in the peak sensitivity of Mo and V. Because of the predicted increase in residence time of atoms produced from the surface of a platform, integrated sensitivities are expected to show the greatest improvement. When the integrated sensitivities obtained from a graphite tube heated at the rate of 3.7 K ms-' are compared with those obtained from a platform (Table IV), some improvement is observed, especially with the more difficult-to-atomize elements. However, when a comparison is made of the sensitivities obtained with a graphite tube heated at rates equal to that of the platform, little or no improvement is observed. When the platform was used, the appearance time of the metals studied were shifted by 85 ms for Zn and Cd, 160 and 290 ms for Cu and Al, respectively, and 430 ms for V and 530 ms for Mo. These shifts in the appearance time corresponded
to shifts in the temperature of the graphite tube surface, a t the appearance time, of 310 K for Cd and Zn; 570 K for Cu; 800 K for Al; 630 K for V; and 720 K for Mo.
CONCLUSION This study has shown that atomization from a platform has not brought the commercially available atomizers a great deal closer to the ideal case described by L’vov (7). Although some improvements are observed, both in peak and integrated sensitivities, the improvements are modest, probably because the factors governing the loss of analyte atomic vapor from the analysis volume have not been greatly altered. Again, the design of the commercial atomizer is probably responsible for this. Sturgeon and Chakrabarti (5) have shown that the primary cause of loss of the analyte atomic vapor from the commercial graphite atomizer is the condensation of analyte atomic vapor on the cold end-caps of the atomizer. With the use of a platform, atomization does occur under conditions of a somewhat reduced thermal gradient across the tube length. This reduction, however, is not effective in substantially preventing the transport of the analyte atomic vapor to the colder parts of the atomizer. I t can be seen that peak sensitivities for some metals such as Mo and V can be improved.under conditions where the rate of heating of the platform is sufficiently high to provide a high rate of atom production in a graphite tube that has attained thermal equilibrium. Assisted heating (by an independent power supply) of a platform or the use of a platform in conjunction with a tube heated by capacitive discharge could make atomization under the above mentioned conditions possible. The most efficacious application of the platform technique may not be in atomic absorption but in atomic emission. A recent paper by Ottaway and Shaw (10)reported that in a commercial graphite furnace, and for most metals, low atom
populations are observed when the maximum furnace temperature is attained. At this point in time, atom populations have passed their peak values and are generally rapidly decaying toward zero. Since atomic emission is very temperature-dependent and, in thermal equilibrium, is governed by the Boltzmann Distribution Law, a method that would provide high atom populations in the graphite tube when high temperatures prevail would yield high atomic emission. Atomization from a platform effectively shifts the occurrence of higher atom populations to a time when the maximum temperature hns been attained, and thus potentially provides a method for greatly increasing atomic emission from a graphite furnace. Work is currently under way in this laboratory to investigate these possibilities as well as the influence of matrix components on the atomization characteristics of metals atomized from a platform.
LITERATURE CITED R. Woodriff, Appl. Spectrosc., 28. 413 (1974). R. E. Sturgeon, C. L. Chakrabarti, and P. C. Berteis. Anal. Chem., 47. 1250 (1975). R. E. Sturgeon, C. L. Chakrabarti, I. S. Maines, and P. C. Berteis, A n d . Chern., 47, 1240 (1975). B. V . L’vov, “Atomic Absorption Spectrochemical Analysis”. translated by J. H. Dixon, Adam Hiigen Ltd., London, 1970. R. E. Sturgeon and C. L. Chakrabarti, Spectrochim. Acta., in press. R. E. Sturgeon and C. L. Chakrabarti, Anal. Chem.. 49, 1100 (1977). B. V. L’vov, “Electrothermal Atomization-The Way Towards Absolute Methods of Atomic Absorption Analysis”, presented as an invited paper at the 3rd FACSS meeting arid the 6th Internatbna Conference on Atomic Spectroscopy, Philadelphia, Pa., Nov. 15-19, 1976. G.W. Autio and E. Scala, Carbon, 4, 13 (1966). R. E . Sturgeon and C. L. Chakrabarti, Anal. Chern., 49, 90 (1977). J. M. Onaway and F. Shaw, Appl. Spectrosc.. 31, 12 (1977).
RECEIVED for review May 25, 1977. Accepted July 19, 1977. The authors are grateful to the National Research Council of Canada for financial support.
Evaluation of a Cesium Positive Ion Source for Secondary Ion Mass Spectrometry H. A. Storms,” K. F. Brown, and J. D. Stein General Electric Company, Vallecitos Nuclear Center, Pleasanton, California 94566
An experlmental Cs’ Ion source was evaluated for secondary Ion mass spectrometry appllcatlons uslng an Ion Mlcroprobe Mass Analyzer as a test Instrument. The duoplasmatron Ion source normally employed wlth the Ion probe was replaced wlth the experlmental Cs’ Ion source for the evaluatlon. I n terms of maxlmum beam Intensity, mlnlmum beam diameter, and current stablllty, the Cs’ source Is comparable to duoplasmatron performance with 02+.Detectlon sensltlvttles for certaln elements are slgnlflcantly enhanced with Cs’ relatlve to other commonly used prlmary Ions. Among the elements wlth Increased detection sensltlvltles are hydrogen; carbon; Groups 5, 6, and 7 elements; and the noble metals. The Cs’ Ion source provides a powerful tod for extending the analytkal capabllltles of secondary ion mass spectrometry.
Previous secondary ion mass spectrometry (SIMS) studies (1-4) have demonstrated strong secondary ion yield depen-
dence on the electronic and chemical properties of the surface of solids. Purposeful modification of the surface chemistry to obtain desired results has been achieved by the judicious selection of the bombarding ion species. In particular, Andersen ( I , 2) demonstrated dramatically higher positive secondary ion yields were obtained under bombardment by ions of an electronegative element, such as oxygen, than by inert gas ions, such as argon. Andersen attributed this chemical enhancement to the increased surface work functions of oxidized metals. Several authors (1-4) have observed marked decreases in secondary positive ion yields with increasing ionization potential regardless of bombarding species. Because elements with high ionization potentials generally possess high electron affinities, Andersen ( I , 2 ) and others (5-11) were able to take advantage of increased secondary negative ion yields resulting from bombardment by ions of an electropositive metal. With a primitive Cs+ thermal ionization source, constructed by depositing a bead of cesium compound onto a tungsten filament, dramatic improvements ANALYTICAL CHEMISTRY, VOL. 49, NO. 13, NOVEMBER 1977
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