Diffusion Controlled, Thermal Ionization Source for Mass Spectrometric Analysis of Trace Metals John R. Ret,' Willard G. Myers,* and Frederick A. White Rensselaer Polytechnic lnstltute, Nuclear fngmeering D/v/s/on,Troy, N Y
Analysis sensitivities achievable with the thermal ionization, mass spectrometric technique for trace quantities of uranium and plutonium are improved by replacement of the commonly employed direct evaporation, single-filament source with a diffusion controlled source. The sample on the single filament surface is coated with a high work function, refractory metal using a R.F. sputtering technique. The arrival of sample atoms at the ionizing surface is controlled by diffusion through the applied coaling. Nickel, tungsten and rhenium coatings were evaluated. Only the rhenium coats were found to be thermally stable, adherent, and visibly defect free. Coated and uncoated uranium and plutonium samples were mass spectrornetrically analyzed. The coated samples were at least four times more efficient in sample utilization than the uncoated samples. Analytical sensitivities of at least 1 X grams plutonium on the filament were observed with comparable sensitivities estimated by extrapolation for uranium.
Presently, alpha particle spectroscopy is the most commonly employed method of analysis of survey samples of uranium and plutonium. This method is limited, however, in several aspects. First. sensitivity is fundamentally limited by the specific activity of the isotopes of interest. In addition, several important isotope pairs, i . e . , 239Pu/240Pu and 235U/236E, cannot be resolved. Finally, sample preparation and analysis. especially for very small amounts of sample. can be quite involved and time consuming. Thermal ionization mass spectrometry (TIMS) offers an alternate method of analysis, not only for uranium and plutonium, but also for many other metals. The sensitivit y of this technique is basically limited only by the number of atoms of interest in the sample, regardless of whether they are radioactive or stable. In addition, this method is equally sensitive for all isotopes of a particular element. The major factor limiting the sensitivity of the TIMS method is the low sample ionization efficiency that is generally obtained with the commonly employed direct evaporation, single filament, thermal ionization source. The efficiency of' ionization for most metals realized with direct evaporation from the surface of a heated, high work function, refractory metal filament is typically low, because the evaporation and the ionization processes, which both depend strongly on filament temperature, cannot be controlled separately. Myers and White proposed a technique to effectively eliminate the direct relationship between evaporation rate and ionization efficiency of the direct evaporation source ( I ) . By encapsulating the sample within a matrix of high work function? refractory metal, the sample atoms thermally diffuse to the surface before they can evaporate as neutral atoms or as ions. Since thermal diffusion is generally a much slower process than direct evaporation, the Present address. Combustion Engineering, Inc., Il'uclear Phys-
ics Department, Windsor. Conn. Present address. Sargent & Lundy Engineers, Chicago, Ill. ( 1 ) W . G . Myers and F. A .
White, Ana/. Chem., 4 1 , 1861 (1969).
filament can be operated a t higher temperatures, yielding higher ionization efficiencies without rapid loss of the sample. The three encapsulation techniques evaluated in this earlier study included: sandwiching the sample between two filament ribbons, depositing the sample in a tubular filament, and coating the sample by physical vapor deposition. These methods were not entirely satisfactory, because the coating thicknesses could not be optimized and complete sample encapsulation was not consistently attained. In order to overcome these encapsulation problems, R.F. sputtering was used in this work as the method of coating the sample filaments with a high work function, refractory metal. The sputtering technique allows accurate control of the coating thickness and results in a dense, adherent coating. Three coating materials, nickel, tungsten, and rhenium, were evaluated. The ion emission characteristics of the direct evaporation, single filament source were the parameters of primary interest. Predictions of these characteristics for the diffusion controlled source were made using Fick's laws to describe the diffusion process and the Saha-Langmuir (S-L) equation to estimate thermal ionization efficiency. For a semi-infinite slab with a plane source located a distance L - b from the surface L , the current at the surface, L, is given by
where J is the current a t the surface L a t time t , D is the temperature dependent diffusion coefficient, S is the source or the number of atoms deposited on the filament, L is the total thickness of the encapsulated source, and b is the distance of the source plane from the origin boundary. The ionization efficiency as predicted by the S-L equation is given by
_ N+ NT
1
1
+ A exp(+e(IP - 4)/hT)
where N f is the relative rate of positive ions emitted to N,r total atoms evaporated from the filament surface, A is a constant assumed to be unity, IP is the ionization potential of the sample atoms, q5 is the surface work function of the filament or coating and T is the absolute temperature of the filament surface ( 2 ) . Comparisons between the predicted and observed ion emission characteristics were made to determine whether a diffusion process was actually controlling the rate of arrival of sample atoms a t the surface.
EXPERIMENTAL High purity 233U and 242Pu a s the oxides were obtained from ORNL Isotope Sales. Stock solutions of 233U and 242Pu containing 2011 X f 0.5% gram/ml and 247 x f 1% gram/ml, respectively, in > 4 M "03 were prepared and calibrated mass spectrometrically using the isotopic dilution method. All dilutions (2) M . Kaminsky, "Atomic and Ionic Impact Phenomena on Metal Sur-
faces,"Academic Press, Inc.. New York. N.Y., 1965, Chap. 1
ANALYTICAL CHEMISTRY, VOL. 46, NO. 9, AUGUST 1974
1243
were made from these solutions. Great care was taken to avoid sample contamination. All chemical laboratory apparatus used in sample preparation was plastic; thoroughly cleaned with both dilute and concentrated highest purity, redistilled "03. Those items used directly in sample preparation were discarded after use. Triple distilled water and highest purity reagents were used for all dilutions and sample preparations. Sample filaments were prepared by two methods: an evaporation technique and an electroplating method. In the evaporation technique, a known volume of 233Uor 242Pu stock solution or a dilution of it was placed in a plastic test tube and the sample evaporated to dryness in a water bath. The residue was redissolved in either 50 or 500 pl of 0.05M H N 0 3 solution depending on the desired final concentration. A portion of this volume, generally 5 or 10 PI, was transferred onto the surface of a zone refined rhenium filament which had been cleaned (outgassed) by heating to about 2000 "C in high vacuum for about 5 hours or more. The sample solution was evaporated to dryness on the filament surface by passing a low voltage current through the filament. Generally, a residue, which was visible under microscopic examination, was formed on the surface as a result of the evaporation. A companion filament was prepared in an identical manner. In the electroplating method, a known volume of 233U or 242Pu stock solution or a dilution of it was added to an acid chloride solution according to the procedure described by Mitchell(3). Electroplating of the uranium or plutonium onto an outgassed rhenium filament was done in a cell specially designed to hold sample filaments a constant distance from the platinum cathode. Electroplating was conducted for 1 hour at an applied potential of 4 volts generating a current of about 200 mA. The filament was washed with triple distilled water and dried under a heat lamp. Paired samples were prepared for initial analyses. Samples were encapsulated by sputtering metal coats over the sample on the rhenium filament using a Varian R.F. Diode Sputterer, Model 980-2403A. The sputterer is rated at 1 kW R.F.power and operates at 13.56 MHz. The three coating materials (sputtering targets), high purity nickel, tungsten, and rhenium, were obtained from MRC. Inc. A sputtering procedure was developed which resulted in very adherent, thermally stable coatings of each of the three materials on the rhenium filament substrates. The use of the following procedure resulted in very satisfactory coats: 1) The filament containing the sample was placed on the water cooled substrate table of the sputtering chamber and the chamber was evacuated to 1 x IO-* mm Hg. The distance between the filament and the sputtering target was approximately 1.75 inches. 2) Ultra-high purity argon gas was introduced into the sputtering chamber through a leak valve until a pressure of about 0.02 mm Hg was attained under dynamic pumping. The chamber was flushed in this manner for about 20 minutes. 3) The pressure was raised to about 0.1 mm Hg argon and the R.F. power increased until a glow discharge was established. 4) The R.F. power was adjusted to a value of about 30 watts (-0 reflected power). The argon pressure was reduced to and held at about 0.02 mm Hg under dynamic pumping. 5) The sample filament was coated for about 15 minutes at 30 watts, about 15 minutes at 40 watts, and 15 to 30 minutes at 50 to 100 watts, depending on the coating thickness desired and the coating material used. 6) After completing the deposition, the filament was left in the sputtering chamber under high vacuum for about 1 hour to allow stress relief in the coating-substrate matrix to occur. After removal from the sputtering chamber, the coating was microscopically examined for integrity and visible defects. The sputtered coatings completely covered the residues resulting from the sample deposition technique employed. The coating thickness was estimated to within &lo% using a technique which involved masking a glass slide, coating the exposed glass portion at the same time and location as the sample filament, and measuring the coating thickness on the slide with an interferometer. As mentioned previously, pairs of sample filaments were prepared. One of the pair of sample filaments was coated (the diffusion controlled source) and the other was left uncoated (direct evaporation source). Each of the samples was then analyzed in a two-stage, double focusing mass spectrometer. The spectrometer has a modified Dietz type source ion gun and operates in the pulse counting mode using an 18-stage EM1 electron multiplier. The instrument was designed for high transmission efficiency. Sample filaments were placed individually into the ion gun in the source chamber. The chamber was evacuated to a pressure of 1 X ( 3 ) R. F Mitchell, Ana/. Chem., 32,326 (1960)
1244
*
mm Hg with a LN2 trapped diffusion pump and two VacIon pumps. The filament temperature was raised to the operating range of 2200 to 3800 "F by passing a low voltage current through the filament. The ion emission rate at various temperatures as a function of time was recorded using an automated data acquisition system linked to a P D P 81 computer. The filament temperature, measured with a Leeds & Northrup optical pyrometer, was raised sequentially in approximately 100 "F increments until sample exhaustion occurred or until a maximum filament temperature of about 3700 "F was reached. Both the encapsulated and the uncoated filaments were analyzed identically, except for the filament temperature range employed. Since the objective of this study was to determine the overall sensitivity of the technique, the amount of sample, 233Uor 242Pu, was systematically reduced on succeeding sample filaments from the 1 X gram level to the 1 x 10-15 gram level for 242Puand 1 X 10-l2 gram level for 233U. The sample utilization efficiency was estimated by summing the number of ions counted, correcting for the time between count time intervals and the background, and dividing the sum by the number of sample atoms deposited initially on the filament. The diffusion controlled technique was applied to two environmental samples provided by the New York State Department of Health, Radiological Health Laboratory. The water samples, containing trace quantities of uranium and plutonium. were analyzed quantitatively and isotopically using the isotopic dilution procedure with 233Uand 242Puemployed as "spikes." Ten-milliliter aliquots of each water sample were used for analysis. To the 10 ml of sample were added, 4 ml concd H S 0 3 , 1 ml concd HC103 and several drops of 0.1M HF. The spike solutions were added and the samples taken to dryness in a Teflon beaker. The residue was redissolved in eoncd "03 and again brought to dryness. Ether extractions were performed t o reduce the amount of extraneous material in the sample. Aliquots of the final solution were used for electroplating the samples onto the rhenium filaments. The filaments were encapsulated with rhenium and analyzed in the mass spectrometer for the isotopic composition of the sample. Data for both uranium and plutonium were obtained at the same filament operating temperatures.
RESULTS AND DISCUSSION The development of a satisfactory sputtering procedure for the deposition of nickel, tungsten, and rhenium on rhenium substrate filaments comprised a major portion of this study. Several sputtering parameters significantly affected the structure and integrity of the coatings. R.F. power densities greater than about 1 watt/cm2 on the sputtering target resulted in a high incidence of cracking and delamination of the coatings on the filaments. Slow initial deposition of the coating a t low R.F. power with subsequent increases in the deposition rate as the coating thickness increased greatly improved the adherence and stability of the coatings. Flushing the sputtering chamber with argon prior to deposition also improved the integrity of, the coatings and reduced the amount of impurities trapped in or adsorbed on the coatings. After deposition of the coatings, the filaments were kept under high vacuum for about one hour or more. Apparently stress relief occurred in the coatings, thereby, reducing the incidences of cracking. The sputtering procedure developed resulted in coatings which were stable, very adherent to the substrate, and visibly defect free. Nickel and tungsten proved unsatisfactory as coating materials. Nickel, which has a high work function ( - 5 eV), melts at the relatively low temperature of 2647 "F. The nickel coatings evaporated from the filament surface in the operating temperature range of the thermal ionization source (2200 to 3800 O F ) . Tungsten, a highly refractory metal, remained intact as a stable coating on the filaments to temperatures greater than 4000 "F. However, tungsten coatings apparently contained numerous microcracks which permitted sample atoms to evaporate directly without diffusion through the coating. This emission of sample ions a t temperatures much lower than those expected with a diffusion process controlling sample trans-
ANALYTICAL CHEMISTRY, VOL. 46, NO. 9, AUGUST 1974
Y
-
3880.F
"
23
9
35
ic
' 5
53
51
60
65
-i
-5
83
85
30
95
T ME~MIN.TLS
Figure 2. 242Pu+ ion current as a function of time and brightness temperature Z42Pu samples evaporated on rhenium filaments and coated wlth rhenium 0 1 2 X 1 0 - 2gram -8000 A Re A 1 2 X 10-'3gram - 7 4 0 0 A Re 1 2 X 1 0 - 1 4 gram -9400 A Re 1 2 X 10-'5gram -7500 A Re
\
i 0
20
10
TINE-WNY-EZ
Figure 1 . 242Pu+ ion current as a function of time and brightness temperature
-
Samples: 1.2 X 1 0 - l 3 gram Z42Pu evaporated on rhenium filament. Top: 7400 A. Bottom: Uncoated Rhemum coated
port to the surface led to the conclusion that the coatings were defective. In addition, the observation of a low sample utilization efficiency indicates that the tungsten coats may have lower work functions than bulk tungsten. Rhenium was found to be an excellent coating material. The rhenium coats were stable a t temperatures to 4000 "F, adhered very well to the rhenium substrates, and contained very few observable defects. Representative results of analyses are presented in the following paragraphs. The results of the analyses of a set of paired sample filaments, each containing 1.2 x gram 242Pu, are presented in Figure 1. The coating thickness and filament brightness temperatures are noted on the figure. It is apparent from this result that the emission characteristics of the coated sources differ markedly from those of the uncoated filament. Ion emission from the uncoated filament occurred in the temperature range 2200 to 3000 OF. At the upper end of the temperature range, the ion emission from this filament decreased very rapidly with time. In comparison, ion emission from the coated filament occurred in the temperature range 3100 to 3600 "F, with ion emission much more constant even a t the higher temperatures. Comparing the ion count rates recorded from the coated and the uncoated filaments clearly demonstrates the higher sample utilization efficiencies achieved with the coated filaments. This fact is particularly apparent in results for samples in the range 1 x to 1 x gram. The ion emission from the uncoated filaments was quite erratic and near background levels while the coated filaments performed consistently well a t these low levels. These results support the postulate that in the direct evaporation source the majority of the sample evaporates before a fila-
Table I. Observed Sample Utilization Efficiencies of 242Pu Evaporated Samples Sample size, gram
1.2 x 1.2 x 1.2 x 1.2 x a
Efficiency, 7c
Rhenium coat thickness, A
Uncoated
Coated
10-12 10-13
0.05 0.02
0.16 0.10
7300 7400
10-14 10-15
0.01 a
0.13
9400
0.4
7500
Count rate of 242Pu mass position at background level.
ment operating temperature is reached a t which efficient thermal ionization occurs. The results of the analyses of a set of four coated filaments, ranging in size from 1.2 x to 1.2 x l O - l 5 gram, 242Pu, are summarized in Figure 2. The temperature a t which ion emission is first observed is similar for all the samples regardless of sample size. The observed count rates a t comparable filament temperatures decreased proportionately with decreased sample size as would be anticipated. The observed ion emission, as a function of time and filament temperature, followed the behavior predicted by the thermal diffusion model. The calculated sample utilization efficiencies for four sets of paired filaments are listed in Table I. The stated efficiencies include the transmission and the ion detection efficiencies as well as the thermal ionization efficiency. Since the transmission and the ion detection efficiencies were essentially the same for all the filaments, the difference in overall efficiency reflects primarily the difference in the thermal ionization efficiency of each filament. The sample utilization efficiencies of the coated filaments were a t least four times as great as those for the uncoated filaments. Assuming a 100% ion detection efficiency and a
ANALYTICAL CHEMISTRY, VOL. 46. NO. 9, AUGUST 1974
1245
.........
*
+
.........
'
..........
3035*F
++
'
++
3170'F
JI6O'F
+++I
+*
+%
....
. .
14SPF
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
lm
5
0
0
5
20
25
yi
3s
,lUE-M,NJTLS
Figure 3. 233Uf ion current as a function of time and brightness temperature Samples. 2 X 10Coated, -8800 0 Uncoated
+
qram 233Uevaporated on rhenium filaments
'2
A Re
36 0. c
,m
,
IO0 0
, 4
,
, 8
,
, 82
,
,
Id
,
,
,
20
, 24
.
,
ZB
,
,
12
,
, 15
1
' '0
,
, 4 1
,
1
,
-8
T,YE-*,NLTL$
Figure 4. Z4*Pu+ ion current ness temperature
as
a function of time and bright-
Sample: 1 2 X 1 0 - ' 2 gram Z42Pu evaporated on rhenium filament and coated with -7400 A rhenium
1% transmission efficiency, the thermal ionization efficiencies of the coated filaments were consistently better than 10%. The apparent higher efficiency a t the smallest sample size is attributed to a relatively larger uncertainty in the actual amount of sample initially deposited. A number of samples containing 233U were also analyzed. The results of the analyses of a coated and uncoated pair of sample filaments each containing about 2 x gram 233U deposited by the evaporation technique are presented in Figure 3. It is very apparent that the coated filament was much more efficient in sample utilization than the uncoated filament. The temperature range of ion emission for the coated filament is significantly 1246
higher than the temperature range of the uncoated filament. In addition, the ion current from the coated filament was much more constant with time than that from the uncoated filament. The dip a t 3120 O F was due to refocusing of the ion beam. Comparison of these results with those for the 242Pusample of smaller size shown in Figure 1 indicates that the operating temperature ranges of the coated filaments are comparable for the two elements while there is a large difference in the ranges of the uncoated filaments (compare with Figure 1). For the coated uranium and plutonium samples, the ion count rates a t similar filament temperatures are comparable. Differences can be partially accounted for by the differences in the ionization potentials between uranium and plutonium. The results of the analyses of a number of uranium samples indicate that the sensitivities of this technique for uranium and plutonium are comparable. Results of the analyses of 242Puand 233Usamples electroplated on and coated over with rhenium were comparable to evaporated samples of similar size. The temperature range of operation and the ion emission rates for these samples were comparable to the coated samples of similar size prepared by the evaporation technique. The ion emission rates of the plutonium and the uranium electroplated sample filaments are comparable and indicate a high sample utilization efficiency. The ion currents from samples were found to be very steady over the operating temperature range. Comparison of the ion emission characteristics of replicate samples demonstrated the reproducibility of results that can be achieved with the diffusion controlled source. The sample utilization efficiencies of the electroplated sample filaments were not estimated, because the electroplating deposition efficiency was not determined. Since a specific objective of this study was to develop a practical means of increasing the sensitivity of analyses for very small amounts of plutonium and uranium, a plutonium sample filament was analyzed in a manner similar to the way an actual environmental sample would be run. A rhenium filament containing 1.2 x gram 242Pu was prepared by the evaporation technique. The filament
ANALYTICAL CHEMISTRY, VOL. 46, NO. 9, AUGUST 1974
Table 11. Isotopic Concentrations, grams/mla Sample
Waste holding tank Holding lagoon a
239
""Pu
Pu
(1) 6 . 5 x 10-13 ( 2 ) 4 . 3 x 10-13 (1)5 . 2 x 10-14 (2) ND
NR* 1.4 x 10-13 NR* ND
2 3 8 ~
2 x 10-10 5 . 5 x 10-11 8 x 10-8 8 . 7 X 10"
236U
NR** 2 . 1 x 10-13 NR** 7 . 2 X 10-lo
I 1) Alpha particle spectroscopy, (2) Mass spectrometry. NR-Not Reported *2hOPu cannot be distinguished from *39Pu.**235U cannot be distinguished from
23W.
ND-Not Detected above Background (see Text).
was coated with about 7400 A rhenium and analyzed in the mass spectrometer. The desired results of this analysis were to obtain a high ion emission rate over a period of time long enough to obtain isotopic information. The results of the analysis are presented in Figure 4. The temperature of the filament was raised to 3150 "F where ion emission was first observed. The mass spectrometer ion source was then quickly focused to obtain maximum ion current. The filament temperature was then raised to 3380 "F and the 2 4 2 P ~ion + current observed. It is seen that the ion current reached a broad maximum after about 3 minutes and then slowly decreased. The ion count rate decreased a factor of two from the maximum over a period of about 11 minutes. The maximum count rate of about 2000 cps implies a sensitivity of better than 1 x gram 242Puon the filament. The time period of ion emission was more than adequate to obtain isotopic information. The filament temperature was subsequently increased to 3550 and then 3610 "F to determine if the sample had been depleted. A substantial amount of material still remains distributed in the coating matrix. The ion emission characteristics observed for this sample compare very well with the characteristics predicted by the thermal diffusion model. The above technique was applied to several environmental samples taken from a holding lagoon a t a nuclear fuels reprocessing facility and from a holding tank a t an operating nuclear power plant. The samples, containing trace amounts of uranium and plutonium, were prepared by the electroplating technique and coated with rhenium. Mass spectrometric analysis of the samples provided the isotopic and quantitative information presented in Table 11. Information obtained by alpha particle spectroscopic analysis. conducted a t the NYS Department of Health Radiological Health Labs, is also presented. Where comparisons can be made, the agreement is excellent. However, it is also apparent that alpha particle spectroscopy and mass spectrometry are not equally sensitive for the same isotopes. Only the mass spectrometric technique provided information on the ratio of 240Pu to 239Pu, as well as isotopic abundances of all the long-lived uranium isotopes. This type of information would be very useful in determining the source of detected releases. The reason that no plutonium information was obtained for the holding lagoon sample was due to the fact that the amount of 238U in the sample was so much largerethan the amounts of 239Pu and 240Pu present that ion beam scattering of 238Uresulted in a relatively large background in the 239Pu
mass position. This particular problem was experienced only because the mass spectrometer used was optimized for ion beam transmission and, thus, the abundance sensitivity was somewhat lower than desired for isotopic analysis.
CONCLUSIONS The R.F. sputtering technique proved to be a highly successful method to encapsulate very small plutonium and uranium samples in a high work function, rhenium matrix. Comparison of the ion emission characteristics of these encapsulated sources with the characteristics predicted by the thermal diffusion model verified that a diffusion process was controlling the rate of arrival of sample atoms at the filament surface. Sample utilization efficiencies achieved with the diffusion controlled sources were a t least four times greater than those achieved with the direct evaporation sources. Analytical sensitivities for plutonium of better than 1 x gram were readily obtained with equal sensitivities estimated for uranium. Replicate analyses of samples deposited by both the evaporation and the electroplating techniques confirmed the reproducibility of the encapsulation technique. The analysis of environmental samples demonstrated that the technique not only provides data complementary to that obtained by alpha particle spectroscopy, but also provides additional information which could not be gathered by any other method. Individual sample encapsulation and analysis times on the order of four hours are easily achieved. Since a number of samples can be encapsulated simultaneously and then sequentially analyzed in the mass spectrometer in less than 30 minutes per analysis, sample throughput can be high. Preliminary analyses conducted a t this laboratory have indicated that the encapsulation technique works comparably well for other metallic elements. The technique is now being evaluated for use in the analysis of particulate samples. Received for review January 7, 1974. Accepted March 15, 1974. Work performed in partial fulfillment of requirements by J. Rec for the Doctor of Philosophy degree. The authors would like to acknowledge the support of NASA, the US Air Force, and the NYS Department of Health Radiological Health Lab. J . Rec wishes to thank the USAEC for financial support in the form of an AEC Special Fellowship.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 9, AUGUST 1974
1247
