It appears that this thin-film method may be good enough to replace many of the wet chemical methods now employed for analysis of pigments separated from paint. Indeed, the method looks so promising that, given a dry pigment powder mixture, it would appear that making a paint out of the sample prior to analysis might be proper. This was done for some of the examples given in Table I. However, in doing this, special attention needs to be given to redispersing the pigments, as agglomerates of undispersed pigments which are much greater than one micron in diameter would be expected to have as severely reduced X-ray intensities as those observed for iron and zinc particles of only 8-micron size (9). When in doubt about the particle size ranges involved or the degree of dispersion, the technique of microscopic examination is recommended (9). If particle sizes are too large, then it would appear critical that standards be prepared from pigments of similar particle size to those observed in the unknowns.
working curves and the results shown in Table I were developed on this basis. Although the method is described as thin film method, one mil (or one-half mil dry film thickness) is not really thin in the trLle sense of the word. However, on the basis that the paint is diluted 60 times with respect to heavier pigment elements, it is, with regard to X-ray absorption, below infinite thickness and approaching a true thin film. Fortunately, for the X-ray spectroscopist in the paint industry, most of the pigments used in paints are chosen or manufactured to have particles of 0.2-micron size. According to some recent work of Gunn (9) particles of this size should provide X-ray intensities which approximate the intensity of the pigment elements in solution. The fact that standards prepared here from insoluble pigment dispersions fell on the same working curve as standards prepared from soluble metal organics tends to verify this. Also fortunate is the fact that when larger particle size pigments or extenders are used in paints, they are usually compounds of light elements such as Mg, Al, Si, and Ca. Again based on Gunn’s recent work and his experience with Si02 particles suspended in liquid hydrocarbons, these materials would not be expected to produce X-ray intensities which deviated from those expected for solution standards until the particle sizes ranged well above the onemicron size considered to be limiting for suspensions of metallic particles of iron and zinc (9).
The authors acknowledge the assistance of the SherwinWilliams Resin Research Dept. in preparation of a special fast drying varnish vehicle and the valuable background information developed by Roger L. Harper during the period that he was assigned to the Analytical Research Dept. as an X-ray spectroscopist.
(9) E. L. Gunn, “Advances in X-Ray Analysis,” Vol. 11, J. B. Newkirk, G. R. Mallett, and H. G. Pfeiffer, Eds., Plenum Press, New York, 1967, p 164.
RECEIVED for review June 6, 1969. Accepted September 5 , 1969.
ACKNOWLEDGMENT
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Diffusion-Thermal Ionization Source for Mass Spectrometric Assay of Trace Metals W. G . Myers and F. A. White Department of Nuclear Science, Rensselaer Polytechnic Institute, Troy, N . Y. 12181
THEREARE many types of mass spectrometer ionization sources, none of which can be considered to be ideal. For quantitative mass spectrometric analysis of solids and liquids, the surface or thermal ionization source has distinct and important advantages over several other types, in addition to a high sensitivity. Although multiple-filament thermal ionization sources (I-3) are in wide use today, the single-filament source, especially the “V” configuration (4,offers many advantages [i.e., simplicity, superior geometric and ion optical properties to provide a higher transmission (5, 6), lower background emission ( I , 6), less warpage, etc.] to warrant its use for general analytical purposes. However, the usefulness of single-filament thermal ionization sources is severely limited by the fact that the rate of evaporation of a sample and the temperature of the ionizing surface cannot be varied inde(1) M. G. Inghram and W. A. Chupka, Rea. Sci. Instr., 24, 518
(1953). (2) H. Patterson and H. W. Wilson, J. Sci. Instr., 39, 84 (1962). (3) W. E. Duffy and R. E. Carr, Fourteenth Anriiial Conf. Mass Spectry. & Allied Topics, May 22-27 1966, p 54. Rea.. (4) . , F. A. White, T. L. Collins, and F. M. Rourke,. Phvs. . 101, 1786 (1956). ( 5 ) L. A. Dietz. Rev. Sei. Instr.. 30. 235 (1959). (6j C. M. Stevens “Analysis of Essential Nuclear Reactor Materials,” c. J. Rodden, Ed., U. S . AEC, 1964, pp 1023-4.
pendently. The chemical form of the sample used on this type of ionization source is obviously very important. Consequently, in the analysis of most elements, a single-filament thermal ionization source is usually constrained to operate well below its potential efficiency, in a temperature range which will provide a reasonable degree of ionization, without an excessively rapid depletion of the sample via evaporation. A new technique has been conceived for thermally ionizing small quantities of the chemical elements on single-filaments, and for minimizing the production of spurious ions that give rise to errors in isotopic ratio measurements. The concept of this new single-filament source is to utilize a diffusion mechanism in addition to thermal surface ionization, in order to obtain a higher ionization efficiency and a mass spectrum which is not obscured by impurities or superimposed molecular spectra. The sample, together with its isotopic spike, is encapsulated within an appropriate filament that is made of a refractory metal of high work function. The filament is then operated at a high temperature, which causes the sample atoms to diffuse to the exterior surface where a fraction becomes ionized by thermal ionization. This approach provides a number of advantages while still retaining those previously cited for single-filament sources. These include : prompt loss of the sample in the form of neutral vapor is minimized; the diffusion time required for the sample atoms to reach the
ANALYTICAL CHEMISTRY, VOL. 41, NO, 13, NOVEMBER 1969
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ionizing surface allows this surface to become relatively free of impurities (i.e., hydrocarbons) and to stabilize with a work function that is characteristic of the pure metal; the diffusion controlled release of the sample provides a longer time to focus the spectrometer and conduct the analysis; the elevated filament temperature minimizes the mean residence time and reduces the probability that atoms of the sample will undergo surface recombination prior to ionization; and the diffusion process promotes the generation of an atomic ion beam by dissociating virtually all the sample molecules. Therefore, the chemical form of the sample plays a minor role. The diffusion-ionization concept also shows potential with respect to the quantitative mass spectrometric analysis of monoisotopic elements. A number of techniques or filament configurations were investigated to test the diffusion-thermal ionization source concept, including physical vapor deposition, spot-welding, and tubular containment. The ion filament material used was high purity (99.99-99.995 %) rhenium, while enriched NBS uranium isotopes were employed primarily as the diffusant atoms. All filament source materials employed were mass spectrometrically analyzed for their natural uranium impurity content, and where required, received an extended outgassing prior to use. The fabricated filaments were operated over a temperature range of 2200-2480 OK (measured with a precision optical pyrometer, 1 accuracy), using rhenium cover to 8.75 X lo-* centimeters. thicknesses from 5 X
-*
EXPERIMENTAL
In order to correlate theoretical and experimental diffusion and ion emission rates, and to determine practical sample cover thicknesses, measurements of the diffusion coefficients of uranium atoms through polycrystalline rhenium, at high temperatures, were required. This diffusion data were obtained using the mass spectrometric procedure developed by Schwegler and White (7). The method of physical vapor deposition as a means of plating a metallic covering over a sample placed on a singlefilament substrate was given considerable attention in this study. Here, nanogram g) quantities of uranium (NBS U930 dissolved in nitric acid) were covered, under vacuum, by condensing rhenium vapors which originated from an adjacent incandescent rhenium filament. In most instances, the substrate ion source was a well outgassed, “V” single-filament. Here the vapor deposition process was carried out in a vacuum of mm Hg, with the vapor source filament (dimensions 0.001 X 0.030 X 0.050 inch) maintained at a temperature of 2100 OK and positioned approximately 3 mm above the substrate filament. Under these conditions, approximately 2 ~ 3hours were required to produce a vapor deposit of 500 A thickness. Deposition thicknesses were determined gravimetrically and estimated to be accurate to within *200 A. In another technique, the uranium samples were deposited within “V” filaments (made from 0.00054.001-inch thick X 0.030-inch wide ribbon) and then sandwiched and fused between the two rhenium walls using a spot welder equipped with high purity graphite electrode tips. Generally, only one spot weld was made at the center of the filament, after which it was etched in a dilute nitric acid solution for 15 minutes to remove any sample remaining on the surface. A third method of encapsulation employed thin walled (0.002 X 0.015-inch O.D.) rhenium tubing formed by a chemical vapor deposition process (San Fernando Labs, Calif.). Liquid samples were drawn into these tubes and the ends (7) E. C . Schwegler, Jr., and F. A. White, Int. J . Moss Spedry. Ion Phys., 1, 191 (1968). 1862
10
0
’
2
’
4
TIME
‘
6
‘
e
10
12
1
I4
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Figure 1. Ion emission rates of identical uranium samples (-3 x lO-*g) having different rhenium vapor deposited cladding thicknesses (temperature = 2300 OK)
sealed by spot welding tapered, high purity, molybdenum plugs to the rhenium walls. Other methods considered, but not fully explored, include electron beam welding of thin ribbons and sputtered coverings. RESULTS AND DISCUSSION
The diffusion coefficient analysis indicated a dilute impurity diffusion coefficient equation for uranium in polycrystalline rhenium of: D(cm2/sec) = (0.78) exp
- (107,000RT* 20,000) cal/mole
in the temperature range of 2270-2520 O K . This information was instrumental in developing practical diffusion-thermal ionization sources and interpreting ion emission profiles. Figure 1 shows the 2asUion emission profile from four different filaments having the same sample size (3 X lo-* gram 2aW, NBS U930) but different vapor deposited rhenium cladding thicknesses. Filament temperatures were maintained at approximately 2300 OK for all four runs. The curves on this figure and those following are best line fits to count rates recorded over a 10-second interval, every 12 seconds. Curve A clearly demonstrates the result of operating an ordinary single-filament (no vapor cladding) at high temperatures. Here, most of the uranium sample is promptly lost by evaporation, leaving only a fraction of the sample on or impregnated within the surface of the filament. This low and rapidly decaying ion emission rate is obviously undesirable. Curves B, C, and D (representing cladding thicknesses of approximately 400,500, and 600 A, respectively) show the effects that a vapor
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
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95
J
30
X = 0 001 inch T : 3600.F
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R,,IUITIl:
Figure 2. Uranium ion emission through a 0.0005-inch thick rhenium spot-welded source (temperature = 2300 "K)
IOI! I0
,
. 20
,
,
,
30
,
40
,
,
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0 933 ,
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deposited covering has on the retention and ultimate ionization efficiency of the sample. The shape of these ion emission curves has been interpreted to be a result of both sample diffusion and changes in the ionization efficiency of the emitting surface during the re-evaporation of the vapor deposited material. There appears to be an optimum cladding thickness (Curve C) for the particular elemental system studied here. This thickness, estimated in the order of 500 A, is apparently heavy enough to retain and prevent most of the sample from evaporating, yet thin enough to re-evaporate away before too much of the sample can diffuse deep into the crystalline lattice of the substrate filament. At these operating temperatures (-2300 OK) uranium atoms would diffuse into the rhenium filament to a depth of 0.0001 inch (-25,000 &in this period of time (4 minutes), producing a shallow doped filament. The experimentally observed ion emissipn decay from those filaments having vapor deposits of 500 A or thicker (Curves C and D),followed a rate characteristic of the depletion of a doped, thin plate. Figure 2 shows a typical semi-log plot of the observed ion emission from a 10-7 gram 236U sample encapsulated within a rhenium filament by the spot-welding technique. Here the sample was sandwiched between two 0.0005-inch thick, high purity, rhenium ribbons and operated at a temperature of 2270 "K. The particular experimental results shown here provide good agreement with theoretical calculations pertaining to diffusion time and ion emission rate. The initial emission from the source is attributed to surface impurities and that portion of the sample which failed to be sealed between the two walls of rhenium when they were spot welded together. Both theoretical calculations and experimental results indicated that the transit times (and ultimate ion emission rates) of uranium atoms through the 0.002-inch walled rhenium tubing were such as to make this encapsulation material unattractive, on a trace analysis basis, for this elemental system. Thinner walled tubings could be made by the chemical vapor deposition process but they became extremely fragile and would fail at high operating temperatures. Precise quantitative, mass spectrometric analysis of monoisotopic elements, uia thermal ionization, has generally been prevented due to the inapplicability of the sensitive isotopic dilution procedure (8) to these elements. Many investigators have attempted to obtain quantitative information on the abundance of mono-isotopic elements in a sample by using the ion intensity levels of adjacent elements as reference. These (8) M. G . Inghram, Ann. Reu. NUC.Sci., 4, 81 (1954).
Figure 3. Emission of thorium and uranium ions from a 0.001inch thick rhenium spot-welded source efforts have been unsuccessful primarily because of the difference in volatility of each element, and the subsequent uncontrolled surface evaporation of the sample. The diffusionthermal ionization source concept presented here offered means to overcome these problems. Figure 3 shows the emission rates of 2a5Uand zazThions from a 0.001-inch thick, spot welded rhenium filament operating at 2250 OK. The encapsulated sample contained microgram quantities of uranium and the mono-isotopic element, thorium, having a true isotopic ratio of 0.933 (U/Th). If the sample is prevented from evaporating away by the encapsulation condition, then the observed emission rate of each ion specie, at some time and source temperature, will be a function of the element's diffusion coefficient and ionization potential. By using another element, in this instance uranium, as a spike or tracer, one can determine the relationship between true and observed isotopic ratios (as a function of time and temperature) of these two ion species, from which a quantitative measure of the monoisotopic element present in the sample can be calculated. Preliminary experiments using this approach on the U-Th system were encouraging (within a factor of 2) in light of the crude spot welded sources fabricated for this purpose. Refinement of sample encapsulation methods and fabrication reproducibility should improve the accuracy of the procedure. CONCLUSIONS
An improved thermal ionization source for the mass spectrometric assay of trace metals has been developed, based on the concept of implanting sample and tracer atoms within the crystalline lattice of a refractory material. There is some latitude in the method or technique that is employed to achieve this implanted or doped condition, for, subsequently, the temperature dependent diffusion and thermal ionization processes can be controlled to approach both desired source characteristics as well as optimum ionization efficiencies. The diffusion controlled release of the sample, regardless of its original chemical form, from within the source material, provides ion emission currents that allow precise isotopic ratio measurements to be made. On the basis of the experimental results obtained, it would appear that the advantages of high temperature operation of single-filament thermal ionization sources were achieved, while either reducing or eliminating their inherent disadvantages. The sample encapsulation techniques investigated in this study, while not fulfilling
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theoretical boundary conditions, did function to retain most of the sample long enough so that crystalline lattice “impregnation” conditions could be achieved before prohibitive loss, via evaporation, had occurred. For highest sensitivity, the encapsulation or covering thickness should be kept to minimum values to reduce diffusion times and maintain most of the sample close to the ionizing surface (vapor deposition or sputtering techniques would offer the best approach for such purposes). At the same time, the cover thickness should be great enough to allow a thorough cleaning of its surface to take place before the appearance of the sample atoms. This cleaning of the surface ensures the maintenance of a high and constant work function, essential for high ionization efficiencies. In
those instances where a stable, long sustained ion beam is desired rather than maximum sensitivity, the spot and electron-beam welding methods provide means of obtaining the required heavy sample coverings. For uranium diffusion in rhenium, cover thicknesses should be a maximum of 0.0005 inch when operating in the temperature range of 2200 to 2450 OK. For the case of the lighter elements, diffusion rates allow the use of considerably thicker coverings.
RECEIVED for review June 30, 1969. Accepted August 25, 1969. The New York State Department of Health, Division of Air Resources, provided financial support for this investigation.
Spectrophotometric and Visual Titrations of Milligram Amounts of Strontium with EDTA Jean Salomon,l John E. Vance, and Walter A. Baase2 Department of Chemistry, New York University, Washington Square, New York, N . Y . 10003
THEEXPERIMENTS reported here were needed in a study of the kinetics of precipitation of strontium sulfate in solutions of hydrochloric acid and sodium chloride; it was necessary to titrate about 1 to 10 mg of strontium in the presence of 10 mmol of sodium chloride, formed from the neutralization of the acid-salt mixture. There is little information on the direct titration of strontium, Korbl and Pribil (1) reported ten titrations of 4 to 90 mg of strontium using thymolphthalexone (not the preferred indicator today) but did not give an explicit procedure; they noted an adverse effect of neutral salts on the color change of that indicator. Phthalein purple (sometimes called phthalein complexone or metalphthalein), in combination with a green dye and methyl red was introduced by Anderegg, et d.(2), for the titration of alkaline earths but no data were given for strontium. Phthalein purple was used also by McCallum (3) in a few titrations of calcium, magnesium, and barium, and by Cohen and Gordon (4) for the visual and spectrophotometric titration of barium. The same indicator was used by Ogawa and Musha (5) who studied the spectrophotometric titration of strontium and found that the optimum pH was between 10 and 12. All reports on the use of phthalein purple note that the color change is sharpened by the addition of 30-50x ethanol. The effect of salts on the quality of the end-point color change in chelometric titrations has been reported by others, e.g., Flaschka and Mann (6)who found difficulties with murexide. The present experiments show that the presence of 10 mmol of NaCl does decrease 1
Present address, Gulf Research and Development Co., Pitts-
burgh, Pa. 15230.
2 Present address, Department of Chemistry, University of California, Berkeley, Calif. 94700.
(1) ~, J. Korbl and R. Pribil, Collect. Czech. Chem. Commun., 23, 873 (1958). (2) G. Anderegg. H. Flaschka. R. Sallman. and G. Schwarzenbach, Helc.. Chim.Acta, 37, 113 (1954). (3) J. R. McCallum, Can. J. Chem., 34, 921 (1956). CHEM., 28, 1445 (1956). (4) A. I. Cohen and L. Gordon, ANAL. (5) K. Ogawa and S. Musha, Bull. Uniu. Osaka Prefect., Ser. A , 8, 63 (1960). (6) H. Flaschka and J. Mann, Anal. Lett., 1, 19 (1967). \
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both the precision and accuracy of the determinations but does not impair their utility; the visual titration is fully as satisfactory as the more time-consuming spectrophotometric method. EXPERIMENTAL
Apparatus. Spectrophotometric titrations were made with a Hitachi Perkin-Elmer 139 instrument with the standard attachment. A 10-ml microburet with a platinum tip was used in all titrations; readings could be estimated to +0.005 ml. All volumetric ware was calibrated at intervals during the experiments. Visual titrations were made in light from a fluorescent illuminator. Reagents. Deionized water was used for all reagents; water and reagents were stored in plastic containers. Approximately 0.01M strontium chloride was prepared from Merck Reagent, “Low in Barium”; the solution was standardized by evaporation with sulfuric acid in platinum and heating to constant weight (7). Eastman EDTA di-sodium salt was recrystallized; solutions were approximately 0.01 M and were standardized against the strontium chloride, using amounts within the range being studied. Other solutions were 1.023N HC1, approximately 1N and 0.1N NaOH, carbonate free, and NH3. The same lots of NaOH, HCl, and NH3were used in all experiments. The indicator was a Fisher Certified Reagent and was made up as follows: 0.1 gram phthalein purple, 0.005 gram methyl red, 0.05 gram “Erie Green Mt Conc 110 %” (Allied Chemical Co.), dissolved in a few drops of NH3 and diluted to 100 ml. The choice of green dye is not critical but its presence is necessary; the dye cited (Color Index unknown) was satisfactory. The indicator solution was prepared daily because the color faded over a period of hours; the rate of fading did not affect the titrations which took less than 30 min. Procedure. Three series of titrations were carried out with varying amounts of strontium chloride : visual titrations in the absence of NaC1, with addition of ethanol; visual titrations in the presence of 10 mmol of NaC1, with addition of ethanol; and spectrophotometric titrations in the presence of 10 mmol of NaC1, without addition of ethanol. In the visual titrations, varying amounts of the strontium chloride (7) W. F. Hillebrand and G. E. F. Lundell, “Applied Inorganic Analysis,” John Wiley and Sons, New York, N. Y.,1929, p 503.
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