Investigation of matrix effects and isotope dilution in SIMS measurement of uranium in soils

578

Nuclear Instruments and Methods in Physics Research B26 (1987) 578-584 North-Holland, Amsterdam

INVESTIGATION OF MATRIX EFFECTS OF URANIUM IN SOILS *

AND ISOTOPE DILUTION

J.M.R. HUTCHINSON National Bureau

IN SIRIS MEASUREMENT

and K.G.W. INN

of Standards, Gaithersburg, MD 20899, USA

J.E. PARKS, D.W. BEEKMAN and M.T. SPAAR Atom Sciences, Inc., Oak Ridge, TN 37830, USA

W.M. FAIRBANK Colorado State University, Physics Department,

Fort Collins, CO 80523, USA

Received 28 July 1986 and in revised form 27 December 1986

The characteristics of sputter-initiated resonance ionization spectroscopy (SIRIS) for determining trace uranium concentrations in soils are investigated. Although, presently, the most formidable problem in the assay of environmental materials with SIRIS is the low sensitivity of the system, we have chosen to focus on matrix effects as possible limiting factors in this paper. Separate active efforts are being made to improve sensitivity and isotopic selectivity in RIS systems. Electrically conducting solid samples compatible with high vacuum are made by compacting soil with 15% or greater graphite binder. It is found that matrix effects on the absolute

uranium SIRIS signals can be as large as two orders of magnitude,precludinga direct comparison of uranium concentrations through uranium SIRIS signal levels. The method of isotope dilution is also explored. Systematic errors caused by different molecular forms or different microscopic physical locations of the two isotopes are less than 30% in this method. It is found that for samples in which these effects are minimized, the capability of the instrument is such that uranium concentrations can be determined sucCessftrllyto better than 10% accuracy.

1. Introduction Sputter-initiated resonance ionization spectrometry (SIRIS) [l] (also known as MPRI [2] and SARISA [3]) is a promising new technique for ultrasensitive elemental and isotopic analysis of solid samples. It uses resonant laser ionization to selectively detect neutral atoms sputtered from a sample by a pulsed ‘ion beam. Since SIRIS monitors the neutrals rather than the secondary ions, as in secondary ionization mass spectrometry (SIMS) it offers several potential advantages: high efficiency, a lack of molecular interferences, and minimal matrix effects. The first two properties of SIRIS have been demonstrated for a variety of atoms mostly in simple hosts such as silicon and stainless steel [l-3]. Nevertheless, at this time the major problem with this technique when applied to the assay of radionuclides in environmental materials rests in the presently unattainable extremely high sensitivity and isotopic selectivity that is required. This problem is being very actively * Supported by US Department DE-AI-05-ER-60446.

of Energy,

Contract

No.

0168-583X/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

pursued, at NBS and other laboratories. Improvements in the ion beam current (presently 50 pA but ultimately planned for 5 mA), higher pulse repetition rate lasers (presently 30 Hz) and the development of Doppler-free spectroscopy are some of the considered solutions to the sensitivity problem. Other important questions remain before the feasibility of the method is established, and this paper addresses what we consider to be the most significant of these: namely, the possible changes in the calibration of the system response associated with matrix effects. There has been relatively little work to date in SIRIS on this problem. Previous work has shown that the response of SIRIS for vanadium, aluminum, boron and silicon in standard steel samples is essentially linear with concentration, in the ppm range [4,5]. It was also found that the absolute silicon signal in two different hosts, niobium and tungsten, had the same proportionality to concentration as in steel [6]. Similar results were found comparing the SIRIS signal for boron in steel and silicon [5]. A discrepancy in the boron in silicon data of ref. [5] at low concentrations was shown later to be due to accumula-

J. M. R. Hutchinson et al. / SIRIS measurements

tion of boron near the surface [7]. These measurements suggest that matrix effects for metallic matrices may be minimal in SIRIS. In this paper, we present the results of SIRIS measurements on eight samples which are intended to simulate extreme conditions encountered in environmental materials and to test the influence of more complex matrices. These measurements focus on the assay of uranium as an environmentally important radionuclide. We find for the first time major matrix effects in the absolute SIRIS signals from trace uranium in soils. At the same time, however, isotopic ratios are found to be relatively free of matrix effects, which has the important implication that isotope dilution calibration for untreated samples may be possible, assuming other problems are solved. Because of the present lack of required sensitivity of the SIRIS instrument to perform measurements at typical environmental levels (less than ppm), it was necessary to perform the measurements at considerably higher concentrations (parts per thousand). Generally speaking, the samples were spiked with known concentrations under controlled chemical and physical conditions so as to simulate extreme environmental conditions. Chemical and physical effects at the high concentrations are expected to have the same response on the SIRIS signal as at typical environmental concentrations. To obtain quantitative results from SIRIS measurement of soils and sediments, several problems not present in metal or semiconductor measurements must be overcome: (1) Samples must be prepared, from the powdery soils and sediments, that can be mounted in the SIRIS instrument for measurement and have low vapor pressure. If an electron flood gun for sample neutralization is not used, the samples must be electrically conducting. We have solved both problems by adding a conducting binder such as graphite or KBr to the samples and pressing them into thin wafers. (2) In a complex system such as uranium in soils, one expects to find more problems due to matrix effects than in simpler systems. For example oxidized uranium may be predominantly sputtered as UO, UO,, and UO, rather than as atomic U. For this reason we have investigated isotope dilution as a calibration method. In our experiments a sample containing 238U is “spiked’ with a known quantity of 235U. The amount of 238U in the sample is then determined by isotope ratio measurements. (3) Soil samples may have surface nonuniformities or contamination which cannot be allowed to distort the results of the assay. The response must represent the average value of the concentration of the trace element in the sample. In the isotope dilution method the doped isotope may reside in a different physical location, e.g., on the surface of particulates, and have a different

of uranium in soils

579

chemical form than the isotope to be measured. This may lead to different matrix effects for the different isotopes. We have investigated such effects in this paper.

2. Description of the SIRIS apparatus The SIRIS apparatus with which these measurements were made has been described in detail previously [l]. The basic components are illustrated in fig. 1. A pulsed argon ion beam (approximately 50 PA, 18.5 keV, Ar+) is focused onto a solid sample, producing a cloud of vapor immediately above the target. The pulsed RIS lasers then selectively ionize atoms of the chosen element (in this case U, B, or V) in the cloud of sputtered particles. The ions are accelerated through energy and mass filters to an electron multiplier. Secondary ions produced by the impact of the ion beam are discriminated from SIRIS ions by electrostatic fields near the sample, by the energy analyzer, and through their different arrival times at the detector. The sample chamber is designed for high vacuum with ion-and cryo-pumping used. The base pressure was typically lOmE Torr during these measurements. The system is controlled by a microprocessor and has a minicomputer for data acquisition and analysis. Data can be taken in an analog mode when more than one SIRIS ion per pulse is detected or in a single ion counting mode when there is less than one count per pulse. Both were used here depending on the experiment. The resonance ionization scheme used for uranium is illustrated in fig. 2. A 2459.8 A UV photon excites the atoms from the ground state to the 40642.1 cm-’ state and subsequent absorption of 1.06 pm photon from the intense Nd: YAG laser fundamental, which is also directed into the chamber, ionizes the uranium atom.

3. Description of the experiments All experiments described in this paper were performed at Atom Sciences, Inc. The samples were prepared at NBS from standard reference materials with known trace element content. Each sample was made by thoroughly mixing with a mortar and pestle a few milligrams of a source material with a binder, either spectroscopic grade graphite or KBr, and placing it in a press at 2000 psi for approximately six minutes. Disc samples, 1 cm in diameter and roughly 1 mm thick, were thus produced. For a given experiment one to six samples were mounted on a carousel and rotated into position as desired under computer control. We have found in previous experience with SIRIS that trace uranium concentrations are more difficult to measure than any other element we have worked with

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J.M.R. Hutchinson et al. / SIRIS measurements of uranium in soils

ENERGY ANALYZER

MASS ANALYZER

Q

TUNABLE

DYE LASER

.

< I

I

--

u DETECTOR

SAMPLE

(YACUUMLOCII

Fig. 1. Schematic diagram of SIRIS.

[5]. One of the major problems is that the uranium signal decreases rapidly within minutes after a sample has been cleaned with a continuous sputter beam. The SIRIS signal for other elements decreased less than a factor of three after cleaning. With uranium, the signal may almost completely disappear. The decrease is particularly pronounced for uranium metal samples. One possible explanation of these observations is that the uranium, being an excellent getter material, reacts with oxygen and other gases in the chamber after cleaning. Upon sputtering it may then come off predominantly as uranium molecules such as UO, U02, etc. rather than as neutral uranium atoms [8.9]. A study of the secondary ion (SIMS) yield indicated that the

molecular ion peaks do change after cleaning but are not particularly enhanced relative to U+ (table 1). Thus we still do not have a good explanation for the observed decrease. These qualitative observations of difficulties in SIRIS with uranium do generate some concerns about the possibility of quantitative measurements using a technique which compares absolute SIRIS signals for uranium. For this reason it was decided to explore isotopic dilution in our next set of experiments. In the isotopic dilution method a known amount of 235U is added in the sample. By measuring the relative SIRIS response for 235U and 238U in the sample, the 238U content of the original material can be determined

RIS SCHEME FOR URANIUM

7

7

/

10641

A

Table 1 Effect of cleaning on the SIMS and SIRIS 238U signals in uranium metal and Rocky Flats Soil and sample RF-4 [lo]. The units of the results are in volts per 100 laser shots. The SIRIS data were taken at a separate time and should not be compared absolutely to the SIMS data. Sample

2459.8

ii

Uranium metal

RF-4

Fig. 2. RIS scheme for uranium - simplified.

Observed peak

Signal before cleaning

Signal immediately after cleaning

(v/(100 laser shots))

(V/W laser shots))

U+ SIMS uo+ SIMS uo; SIMS uo: SIMS U (neutral) SIRIS

1.5 6.6 5.3 0.2 0.13

1.0 1.9 1.6 0.0 3.9

U+ uo+ uo;

0.46 0.25 1.2

0.73 0.05 6.0

SIMS SIMS SIMS

J.M.R. Hutchinson et al. / SIRIS measurements

581

of uranium in soils

known amounts of uranyl-235 and -238 nitrate solutions. The shuries were allowed to air dry and the resulting cakes were ground with mortar and pestle to disperse the agglomerates. The literature on the decomposition of uranyl nitrate hexahydrate indicates melting at 65OC, loss of four water molecules at 11O’C [16], complete dehydration by 205O C [17], den&ration by 300 o C to UO,, and partial conversion of U03 to U,O, 500° C [18]. Based on this information we felt that heating the uranyl nitrate spiked samples for four hours at 300° C would convert the uranium to the trioxide form. Selected uranyl nitrate hexahydrate spiked samples were heated to provide uranium in the oxide form. Various sample parameters are listed in table 2. Samples A and B were graphite and Rocky Flats soil samples (with 15% graphite binder). Neither sample was heated, so that the chemical form for both isotopes should be uranyl nitrate in these two samples. Samples Aiii and Biii were similar except they were heated after the 238U spike but before the 235U spike. Thus we expect to find the 238U as U03 and the 235U as uranyl nitrate in these samples. Samples Aiv and Biv were similar graphite ancc Rocky Flats soil samples which were heated after each spiking. One expects that both isotopes are in the UO, molecular form in these samples. Samples C and C’ were phosphate ores which contain large amounts of natural uranium. The samples A and B, which were not heated, served as a test of the ability of the SIRIS apparatus to do accurate isotope ratio measurements in graphite and Rocky Flats soil samples. The group Aiii, Biii, Aiv, and Biv were used to test the effect of chemical form on the isotope ratio measurements with SIRIS. In the phosphate ore group, C, and C’, the natural uranium (predominantly 238U) comprises the individual particles while the 235U from the doping procedure which is quantitatively far in excess of the natural 235U, should

without having to compare absolute 238U signals between samples. Theoretically, if neutral atoms of two sputtered isotopes of the same element are delivered of the laser interrogation region with the same concentration ratio as in the bulk sample, with each isotopic component having the same energy and angular distribution as the other, then the measurement sensitivities should be equal. To ensure the condition that sputtered isotopes come off the sample in the bulk ratios, the sample must be sputtered long enough so that any initial preferential sputtering of the light isotope over the heavy will ultimately disappear [ll]. Furthermore, not necessarily all the emitted material will be in the neutral elemental form. It has been estimated that ground state yields can be decreased by as much as a factor of 50 [12-141. If molecules are ejected directly from inside the sample, it could bias the results because for the samples studied here, the chemical forms of the two isotopic species of uranium are usually different and, in general, have different sputtering yields. The neutral isotopic ratio will be affected oppositely assuming an equilibrium sputtering condition has been reached in which the total yields (atomic and molecular) of the isotopes have the bulk ratios. However, as has also been suggested by Oechsner [15], if the sputtered molecules are formed just outside the surface of the source, mixing of the isotopes could be expected to occur, and the observed isotopic ratios may well be the same as in the bulk sample. In view of the above considerations, and the relatively crude requirements in environmental assays (+ lo%), it is interesting to inquire whether SIRIS can be used directly on field samples using a calibration obtained by spiking the sample with a known amount of an isotope other than the one of interest. Samples shuried with ethyl alcohol were spiked with

Table 2 Results of isotope dilution experiments. Samples with letters A, B, and C are graphite, Rocky Flats soil, and phosphate ore, respectively. Conversion of the natural uranium and uranium-235 spikes to oxides by heating are designated as iii and iv, respectively. Sample

A B Aiii Biii

No. of meas.

25 4 6 1

AiV

1

Biv C C’

1 6 9

Molecular form

23sU

SIRIS response a)

Actual

235U

238”

(ppm)

235”

23sU/238U

Graphite percent by weight

nitrate nitrate nitrate nitrate oxide oxide nitrate nitrate

nitrate nitrate oxide oxide oxide oxide ore ore

1310 904 1249

1.181 1.177 1.110 1.126 1.110 1.126 6.922 6.922

100 15.2 100 19.2 100 23.1 39.7 74.8

938 1249 894 630 263

1.22 0.31 0.24 0.06 0.18 0.06 0.01 0.04

*) SIRIS counts at the mass peak per 100 laser shots per ppm 235U. b, Standard error of the mean in percent.

235U/238U (s.e.) b, 1.17(2) 1.21(4) 1.28(7) 1.17 1.23 1.31 6.6(3) 5.0(2)

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et al. / SIRIS

be located on the surface of the particulates. Thus this group tests the effect of different microscopic physical locations for the isotopes on the SIRIS response. There are several factors which could possibly bias an isotope ratio measurement with SIRIS. These include errors in setting the laser wavelength and finding the mass peaks, variation in system response with time, differences in optimum timing of the laser pulse due to different velocity distributions, and selective sputtering effects. The rest of the discussion in this section describes the procedures used for minimizing errors due to the first two effects. The systematic errors due to the latter two effects are expected to be small, on the order of 1%. Two SIRIS spectra representing 235U and 238U as a function of UV laser wavelength are presented in fig. 3. Note that the peak does not occur at the same wavelength for the two isotopes, i.e., there is an isotope shift. One might think that the logical way to make isotopic ratio measurements would be to read off the peak values or integrated areas from graphs such as these. Unfortunately, the UV laser power is not constant over this region due to the wavelength dependence of the UV generation process. One could correct this problem by normalizing the signal to the UV pulse or by installing tracking on the doubling and mixing crystals. However, this was not done in our experiments. We chose instead to carefully set the wavelength at the position indicated by the arrow in fig. 3, where the response for each isotope is down by the same percentage from the peak. Of course the crystals had to be optimized near this wavelength when making this determination. We also found that focusing the laser beams with a 1 m lens broadened these spectra somewhat through power broadening of the resonance transition. This reduced

measurements

of uranium in soils

the sensitivity to wavelength choice. Most of the data taken on these samples was done with the focused laser beams. The general procedure for making isotope ratio measurements on a sample consisted of alternating mass scans over masses 235 and 238, after the laser wavelength was set properly. The quoted SIRIS signals for a run were determined by averaging six data points, with each point representing 100 or 400 laser shots, depending on the magnitude of the signal. The most important time-dependent effect in the system response was the gradual decline in uranium SIRIS signal after continuous beam cleaning of the samples. There are two different approaches to avoiding this problem: taking the data in a continuous or quasicontinuous beam mode, or operating in the pulsed mode and waiting sufficiently long after cleaning that the signal is approximately steady. We explored the second alternative in the results reported in this paper. The overall signal decline after cleaning for the graphite, soil, and ore sample of table 2 was a factor of three or less. Most of the decline occurred within the first two minutes. Generally data was taken only after a delay of ten minutes following cleaning. Any further decline was then small, about 4 percent per four-minute run or less. Systematic errors on the isotopic ratio due to this effect were minimized by taking data in the order 235, 238, 238, 235 or vice versa and averaging the two results for each isotope, or in the order 235, 238, 235 and vice versa and averaging the two measurements for the first isotope. Nevertheless, systematic errors as large as 5-10% in individual measurements are still possible since the residual decay is more nearly exponential than linear. We have estimated correction factors for errors in each run due to this effect. However, when the correction are incorporated, the averages and standard deviations of table 2 are changed negligibly since some of the runs are corrected upwards and some downwards. Due to the uncertainties in correction factors the uncorrected data are given in table 2.

4. Isotope dilution results

*q/ 2459.6 DYE

Fig. 3. SIRIS

2459.7 LASER WAVELENGTH

2459.6 (ANGSTROMS)

spectra of 235U and 238U as a function of UV laser wavelength. (See text.)

The results of SIRIS measurements on the three groups of isotopically spiked samples are given in table 2. The quoted experimental results represent averages of all isotope ratio data taken with the exception of six runs where the apparatus was not working properly. The error limits given in parentheses are standard deviations only. A total of 25 measurements were made on sample A on four separate days. The reproducibility of the results is very good; the standard deviation of the measurements is less than 10%. The average value measured, 1.17, is very close to the actual isotope ratio, 1.1806.

J.M.R. Hutchinson et al. / SIRIS measurements

The difference is less than the standard error ( f 0.02) of the measurements. Only a few measurements were made on sample B and the heated samples. Here we also find good agreement with the actual values, with the largest discrepancy being only 16%. Thus the different chemical form of the uranium isotopes in the different samples does not under these conditions significantly affect the ratio of signals from different isotopes. We had considerable difficulties measuring the first two phosphate ore samples which had low graphite content (about 13%). When hit with a continuous beam, they charged up, causing arcs to occur, which sometimes shut down the apparatus. Hence we were not able to sputter clean these samples. The charging problem did not occur for a second two phosphate ore samples, which are listed in table 2, C and C’, which contained 40% and 75% graphite, respectively. With these samples the agreement with known values was reasonable, although the results for sample C’ were 28% percent low. The concentrations of 235U are given in table 2. We conclude that there are no large matrix effects in the isotopes ratios due to different microscopic physical locations for the isotopes in this rather complex sample. This is an important positive result suggesting that the isotope dilution method may work for real world samples. On the other hand, looking at the magnitude of the 235U SIRIS signals in column six of table 2, we see that uranium concentrations cannot be determined in these types of samples merely by comparing the SIRIS signals between samples. In previous work, we have had some success with this method on other atoms in simpler hosts. For uranium in soils, however, we see that major matrix effects in the absolute signals do exist. There is a difference of more than two orders of magnitude between the SIRIS response for unfired graphite and that for phosphate ore samples. There are some general trends in this data which are not understood at this time and warrant further investigation. The Rocky Flats samples consistently had about 20% of the response of the corresponding graphite samples. Heating of the samples further reduced the SIRIS signals by a factor of 5 in both graphite and Rocky Flats soil. The second heating had little effect. The spiked ore samples had the lowest uranium response of all. The factor of two or three decline of the uranium signal after cleaning in these samples is not particularly unusual compared to our experience with other atoms and host matrices. This contrasts sharply with the orders-of-magnitude loss observed in uranium metal. Thus problems with uranium metal should not be taken as indicative of problems with uranium as a trace component in other hosts. It would be interesting to repeat these measurements in a continuous or quasi-continu-

of uranium in soils

583

ous mode to see if surface oxidation significantly alters these results.

5. Conclusions We have investigated the characteristics of SIRIS as a tool for determinin g uranium concentrations in powdered soil sources. Solid samples have been made by mixing with graphite binder and compressing. These samples are electrically conducting and compatible with high vacuum. For most soils 20% graphite is adequate, although 40% graphite was needed for phosphate ore. It was found that measurements of absolute SIRIS signals could not be used to determine uranium concentrations in these samples. Variations of more than two orders of magnitude in SIRIS signal between samples of similar uranium content were observed. In spite of these large matrix effects, it was shown that concentration measurements at the 16% level of accuracy were possible if the isotope-dilution method is employed. Tests were made for effects of differing isotopic molecular form and microscopic physical location on the isotope-dilution method. Contributions of these effects were found to be less than 30%. The highest detection sensitivity of the system was observed for sources prepared by depositing uranium nitrate solution onto graphite, drying, and pelletizing.

References [l] J.E. Parks, H.W. Schmitt, G.S. Hurst, and W.F. Fairbank Jr., Thin Solid Films 108 (1983) 69. [2] N. Winograd, J.P. Baxter, and F.M. Kimock, Chem. Phys.

Lett. 88 (1982) 581; F.M. Kimock, J.P. Baxter, and N. Winograd, Surf. Sci. 124 (1983) L41. [3] M.J. Pellin, C.E. Young, W.F. Calaway, and D.M. Gruen, Surf. Sci. 144 (1984) 619. [4] J.E. Parks, H.W. Schmitt, G.S. Hurst, and W.M. Fairbank Jr., Laser Based Ultrasensitive Spectroscopy and Detection, ed., R.A. Keller, Proc. SPIE 426 (1983), p. 32; J.E. Parks, H.W. Schmitt, G.S. Hurst, and W.M. Fairbank, Jr., in: Resonance Ionization Spectroscopy 1984, eds., G.S. Hurst and M.G. Payne, Inst. of Phys. Conf. Series, no. 71 (Inst. of Phys., Bristol, UK, 1984) p. 167. [5] J.E. Parks, D.W. Be&man, H.W. Schmitt, and E.H. Taylor, Nucl. Instr. and Meth. BlO/ll (1985) 280. [6] J.E. Parks, D.W. Beckman, H.W. Schmitt, and M.T. Spaar, presented at the Materials Research Society Spring Meeting, San Francisco (April, 1985). [7] The SIRIS data agrees well with depth profile measurements made with SIMS by John Newman of Perkin-Elmer Physical Electronics Laboratories. [8] A.E. Morgan and H.W. Werner, Surf. Sci. 65 (1977) 687. [9] D.L. Donahue, W.H. Christie, D.E. Goeringer, and H.S. McKown, Anal Chem. 57 (1985) 1193.

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[lo] K.G.W. Inn, W.S. Liggett, and J.M.R. Hutchinson, Nucl. Instr. and Meth. 223 (1984) 443. [ll] G. Betz, and G.K. Wehner, Sputtering of Multicomponent Materials, in: Topics in Applied Physics, vol. 52, Sputtering by Particle Bombardment 11, ed., R. Behrisch, (Springer, Berlin, 1983) p. 64. [12] H. Oechsner, H. Schoof, and E. Stumpe, Surf. Sci. 76 (1978) 343. [13] E. Dullni, Appl. Phys. A38 (1985) 131. [14] W. Husinsky, J. Vat. Sci. Technol. B3 (1985) 1546.

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of uranium in soils

[15] H. Oechsner, Secondary Ion Mass Spectrometry, SIMS III, in: Chemical Physics 19, Springer Series, eds., A. Benninghoven, .I. Giber, J. Las&o, M. Riedel, and H.W. Werner (Springer, Berlin, 1982) p. 106. [16] W. Lodding and L. Ojamaa, J. Inorg. Nucl. Chem. 27 (1965) 1261. [17] W.H. Smith, J. Inorg. Nucl. Chem. 30 (1968) 1761. [18] R.S. Ondrejcin and T.P. Garrett Jr., J. Phys. Chem. 65 (1961) 470.