Anal. Chem. 1002, 64, 1643-1649
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Determination of Trace Elements in Uranium by Inductively Coupled Plasma-Atomic Emission Spectrometry Using Kalman Filtering Eric H.van Veen' and Margaretha T. C. de Loos-Vollebregt Atomic Spectrometry Unit, Laboratory of Materials Science, Delft University of Technology, Rotterdamseweg 137, 262% AL Delft, The Netherlands Alex P. Wassink and Hans Kalter Chemical Laboratory, Urenco Nederland Operations B.V.,Drienemansweg 1, 7600 AG Almelo, The Netherlands
When uranium hexafluoride Is analyzed for trace ImpurItles udng lnducthdy coupled plama-atomlc emission rpectrometry, the Kalman fitter approach to data reductlon has been applled. Thirty elements lncludlng boron and rare-earth elementscan be dlrectly determlned In hydrolyzedUF8 under compromise ICP condttbns,wlthout any chemkalseparation. Sbc vdatlk fluorides can be detected but not quantifiedat the required concentratlon level, due to IhItatknr of the ICPAES technique Itself. The setup of a routlne analysb by one rlngle, rknph and fast method k described. I n contrast, the conventional UI. of ICP-AES requlres the dlmlnatlon Ot the huge spectral Interlerences through at least four extractlon schemes to determine the dements Ikted In the UF6feed and product speclflcatlonr for an enrkhment plant (ASTM C787 and CSS6) and some elements required by customers.
INTRODUCTION Urenco is involved in the enrichment of uranium hexafluoride (UF6). Uranium, used as a fuel in reactors, has stringent specifications with respect to its composition. Apart from the isotope distribution, to be determined by mass spectrometry, the levels of trace impurities are of paramount importance. Impurities, like boron and rare-earth elements, have a high neutron-capture cross-section. At a bg/g level, they hinder the fission chain reaction and may alter the metallurgical characteristics of the uranium metal. In addition, volatile fluorides in UF6 affect the separation efficiency of 235U. One of the elements forming volatile fluorides is boron, which is used in the reactor as a neutron absorber. Therefore, analyzing uranium for its boron content is essential. About 35 trace metala have been listed in the C787 specification,issued by the American Societyfor Testing and Materials (ASTM), for UF6 containing uranium of any 235U concentration, that is intended for a feed material to a gaseous diffusion plant. A total of 21 elements, that form nonvolatile fluorides, shall not exceed 300 pg/g of uranium, when added up. A separate lit specifiesthe volatile fluorides, one by one. The ASTM C996 specificationapplies to nuclear grade UF6, that has been processed through an enrichment plant to obtain uranium of any 235U concentration below 5 7% and that is intended for fuel fabrication. It only lists the elements boron and silicon. Some 10years ago, six or even more techniques were needed to determine over 40 metallic impurities in UF6.l Techniques range from carrier distillation methods, separation by ex(1) Floyd, M. A.; Morrow, R. W.; Farrar, R. B. Spectrochim. Acta 1983,38B, 303-308. 0003-2700/92/0364-1643$03.00/0
traction and precipitation to dc arc emission spectrometry, atomic absorption spectrometry,and spectrophotometry.The classical methods suffer from a lack of sensitivity. The direct determination of impurities in the uranium matrix often is hampered by matrix and spectral interferences. A more versatile and less time consuming single procedure was presented by liquid-liquid extraction of uranium with a tris(2-ethylhexy1)phosphate(TEHP)-hexane mixture, followed by detection using inductively coupled plasma-atomic emission spectrometry (ICP-AES). Unfortunately, results have been given for only 17 elementa, covering about five volatile fluorides. It was stated that the time required for sample preparation and analysis was approximately 2 days, which was considerablyless than the time required for determining the same elements involving six different procedures. In the next years, several approaches using different techniques were developed. An electrothermal atomizationatomic absorption spectrometric (ETA-AAS) method was described in which the low sensitivity for uranium allows direct analysis.2 However, matrix-matching of standards was required.2~3Together with the determination of one element at the time and the short analytical range, the technique is less attractive. Rare-earth elements were determined by neutron activation y-spectrometry.* Quantitative separation from uranium was essential and was realized by ion exchange on preconditioned columns of Chelex-100 resin. The technique is very sensitive for rare-earth elements, but the procedure required a lot of manipulation and time. Experiments using ICP atomic fluorescence spectrometry (AFS) were conducted? but results were given in too little detail to draw conclusions. In X-ray fluorescence spectrometry (XRFS), the high mass-absorption coefficient of uranium affected the various lines, and precision and accuracyprobably suffered from interfering lines of other elements.6 Several investigations were reported using the ICP in combination with either AES or mass spectrometry (MS). In attempts at direct ICP-AES analysis, the high uranium concentration (up to 35 g/L) leads to an enormous amount of more or less strong emission lines, completely burying the trace element emissions. The wings of all the lines add up to a high continuum background level compared to the aqueous reference level. Moreover, the lines result in direct (2) Goyal, N.; Purohit, P. J.; Dhobale, A. R.; Patel, B. M.; Page, A. G.; Sastry, M. D. J. Anal. At. Spectrom. 1987,2,459-461. (3) Santoliquido, P. M. J. Res. Natl. Bur. Stand. 1988, 93, 462-464. (4) Kayasth, S. R.; Desai, H. B.; Sundaresan, M. Anal. Chim. Acta 1989,219, 313-315. (5) Frbmy, L.; Dall'Ava, D.; Bergey, C.;Klok,A. Presented at the second Karlsruhe International Conference on Analytical Chemistry in Nuclear Technology, Karlsruhe, FRG, June 5-9,1989; paper 46. (6) Ribeiro Salvador, V. L.: Imakuma, K. Anal. Chim. Acta 1986,188, 67-72.
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line overlap for many prominent trace element lines, thereby causing line selection to fail. Due to the heavily structured background in the vicinity of the line, also the problem of selectionof background correctionpositionsarises. Therefore, separation and preconcentration are required and are done by column or solvent extraction. A cellulose column filled with a nitric acid-diethyl ether mixture' was employed. Liquid-liquid extraction used hydrochloric acid with triisooctylamine in xylene* or tributyl phosphate (TBP) in a nitric acid medium.3 Several more papers give extractions based on TEHP"1 or tri-n-octylphosphine oxide (TOP0).12 Comparing MS detection with respect to AES detection, spectral interferences are reduced and better detection limits are obtained. The high mass range is not expected to give any problems. However, the mass range form = 16-80 suffers from polyatomic ion interferences. Due to compromise operating conditions and space charge effects, the lowest masses exhibit low sensitivities. Ionization suppression effect on analytes are large in the presence of uranium. Therefore, matrix separation still is required. With liquid-liquid extraction in NJV-dihexylacetamide (DHA) a laborious procedure was followed, but excellent detection limits were attained.13 Uranium solutions were directly aspirated into the ICP. Multiple standard additions were employed and three internal standards were used.14 Equal accuracy in comparison to ICP-AES was found.16 In the above papers, usually small sets of elements, without reference to the ASTM C787 specificiation, were examined. This limits the prospect of general applicability. Moreover, it is remarkable that the determination of boron, considered to be essential, was not reported at all. ICP-AES has grown into a major technique to analyze uranium for trace metals. As studied in the laboratories of Urenco and British Nuclear Fuels with respect to hydrolyzed UF6solutions, multiple U extraction by using TBP in carbon tetrachloride (dry or wet) or DHA in toluene or extraction of trace elements by using benzoylphenylhydroxylamine (BPHA) in chloroform is needed. Although one instrumental technique is employed, four extraction schemes have to be followed to determine all elements listed in the ASTM specifications at the required concentration levels. Two elements (P and Si) are even determined by classical colorimetric methods. The above approach may be conclusive for uranium analysis; however, it is desirable to avoid excessive sample pretreatment. There are several disadvantages associated with extraction. As mentioned, no single method can be applied for all elements. No extractant is fully selective for uranium. Small amounts of uranium still may lead to spectral interferences. Small sample volumes of a few milliliters are obtained. Extraction is time consuming and causes contam(7) Murty, P. S.; Barnes, R. M. J . Anal. At. Spectrom. 1986, 1 , 145148. (8) Seshagiri, T. K.; Babu, Y.;Jayanth Kumar, M. L.; Dalvi, A. G. I.; Sastry, M. D.; Joshi, B. D. Talanta 1984,31, 773. (9) Coleman, C. J. Anol. Chem. Spectrosc. Symp. Ser. 1984,19, 195. (10) Halouma, A. A.; Farrar, R. B.; Hester, E. A.; Morrow, R. W. Anal. Chem. Spectrosc. Symp. Ser. 1984, 19, 201. (11) Short, B. W.; Spring, H. S.; Grant, R. L. Determination of Trace Impurities in Uranium Hexafluoride by an Inductively Coupled Argon P l a s m Spectrometer; Goodyear Atomic Corp.: Piketon, OH, 1983 (CATT-3184). (12) Bear,B. R.;Edelson,M. C.;Gopalan,B.; Fasse1,V.A. Anal. Chem. Spectrosc. Symp. Ser. 1984,19, 187. (13) Palmieri, M. D.; Fritz, J. S.; Thompson, J. J.; Houk, R. S.Anal. Chim. Acta 1986,184,187-196. (14) Vijayalakshmi, S.; Krishna Prabhu, R.; Mahalingam, T. R.; Mathews, C. K. Presented at the second Karlsruhe International Conference on Analytical Chemistry in Nuclear Technology, Karlsruhe, FRG, June 5-9, 1989; paper 43. (15) Dall'Ava,D.;FrBmy,L.;Bergey,C.;Batel, A.;Borsier,M.Presented at the 1990Winter Conferenceof PlasmaSpectrochemistry,St. Petersburg, FL, January 8-13, 1990; paper WP5.
Table I. Default Operating Conditions of the Perkin-Elmer Plasma I1 ICP Spectrometer lo00 power (W) observn height (mm) 15 21 frequency (MHz) nebulizer argon flow (L/min) 1 auxiliary argon flow (L/min) 1 plasma argon flow (L/min) 15 1 sample uptake rate (mL/min)
nebulizer
cross flow
ination. Finally, extraction of uranium yields radioactive Waste.
The need for one single and simple method remains. In recent publications,16-'8 it was demonstrated that the Kalman filter approach to data reduction in ICP-AES is able to mathematically separate trace element information from all kinds of background signals. As opposed to conventional background correction techniques, the approach can handle large and heavily structured background. The filter is a multicomponent analysis technique, modeling emission based on full scans of samples and pure components and using criteria to eliminate the optical instability inherent to scanning systems. By remedying the problem of spectral interference, it allows direct measurement in the matrix. In a preliminary investigation, it was shown that boron can be detected below 1 pg/g in a 1.6% uranium hydrolysate.l8 This paper investigates whether ICP-AES analysis,without chemical separation, but using Kalman filtering, will meet the ASTM specifications for all trace metals, including rareearth elements. Spectral scans around the analysis lines are measured in a fast scanning mode. Data have been obtained from 1.6%-3.4% uranium hydrolysates a t high resolution (7-pm spectral bandwidth). The setup of a routine analysis is described. EXPERIMENTAL SECTION
Sample Preparation. High-purity UFe was obtained through 3-fold sublimation. Next, it was hydrolyzed in an appropriate Kel-F flask. The water used in this procedure was deionized (conductivitybelow 1&/cm at 293 K). To the hydrolysate were added 36 elements as nitrates in order to keep any interaction of ions to a minimum. Instrumentation. The measurements were performed with a Perkin-Elmer Plasma I1 system equipped with a 1-m Ebert monochromator. In the configuration used for these experiments, the monochromatorresolution is high using a holographic grating with 3600 lines/", resulting in a practical spectral bandwidth of about 7 pm. The spectral window, in which the scans were taken, was in the range of 100pm, the analyte emission showing up in the center of the window. The step size amounts to about 1.0 pm and is dependent on wavelength. The integration time was adjusted to 0.1 s/step. Since a large variety of elements was measured, no attempt was made to optimize the ICP parameters for each element. Instead, the default operating conditions of the Plasma I1 system were used, as specified in Table I. To check for proper operation of the equipment, the detection limit in the boron pure component solution was determined to equal the value reported in 1iterat~re.l~ The spectraldata from the Plasma I1 system were transmitted to an IBM PS/2, Model 70 computer with mathematical coprocessor via the RS232 communications port. Software written in Turbo Pascal V receives the RS232 input. As soon as the data (16) Van Veen, E. H.; De Loos-Vollebregt, M. T. C. Spectrochim. Acta 1990, &E, 313-328. (17) Van Veen, E. H.; Oukes, F. J.; De Loos-Vollebregt,M. T. C. Spectrochim. Acta 1990,45B, 1109-1120. (18) Van Veen, E. H.; De Loos-Vollebregt,M. T. C. Anal. Chem. 1991, 63, 1441-1448. (19) Winge,R. K;Fassel,V. A.;Peterson,V. J.; Floyd, M. A. Inductively Coupled Plasma-Atomic Emission Spectrometry; An Atlas of Spectral Information; Elsevier: Amsterdam, 1985.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUQUST 1, 1992
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have been transferred, the Kalman filter calculations were automatically performed. The implementation of the Kalman filter for ICP-AES has been described elsewhere.16Js
RESULTS AND DISCUSSION Determination of Boron. The determination of boron will serve as an example for the examinations to be performed for accurate analysis by Kalman filtering. In addition, it will show how to proceed in the Kalman filter approach. In method development, the emission over the spectral window of the most prominent analyte line has to be modeled. Which species in the sample are emitting is simple to answer in the present case: boron and uranium. They show structured emissions which need to be modeled in an experimental way, since theoretically modeling by Voigt profiles does not lead to quantitation. The model contains the pure component sensitivities and some parameter in order to locate the sensitivities at the proper wavelength position with respect to each other. To be sure that modeling proceeds in an accurate way, the next procedure was followed. First, three synthetic solutions were prepared. A high-purity UFs hydrolysate, containing 1.57% uranium, was spiked with about 18 pg of boron/g of uranium. This spiked hydrolysate serves as sample in the method development. The amount of boron has been chosen in such a way that it leads to a clearly observable increase in intensity with respect to the huge intensity emitted by the 1.57% uranium. The hydrolysate itself and a 2 mg/L boron solution are the pure component solutions for uranium and boron. The boron concentration has been chosen to give a well-definedsignal with respect to the continuum background. Then, three scans were measured in the 100-pm-widespectral window of B 249.773 nm, the most prominent line of boron.19 The choice of the window width usually is a compromise between a smaller width to speed up measurements and a larger width to get a better description of the background signal. The pure component scans are given in Figure l a as sensitivities: the intensities have been divided by the respective concentrations. The sample scan is displayed in Figure Ib. Since a scanning monochromator is subject to drift, the intensity in each data point is not integrated for 1 s or more to obtain a good signal-to-noise ratio (SNR). Instead, an integration time of 0.1 s was applied, and 10 scans were taken to improve the SNR. Since each scan contains a well-defined signal, a peak search routine based on the first derivative of the scan can locate the top of the largest peak in the scan. All points in the replicate scans were shifted for coincidence of the top by a quadratic interpolation routine, and the intensities were averaged. In this way, the effect of possibly occurringdrift during the pure components or sample scan is minimized. If the two pure component scans (sensitivities) are simply used to estimate the sample scan (intensities) by Kalman filtering, the resulting weighting coefficients (concentration estimates) usually are inaccurate, again due to drift. Multicomponent analysis always fails, when the abscissas do not coincide. This obviously is the case for the wavelength axes of the three scans, shown in Figure 1: the boron signal seems to be located in between two uranium peaks (Figure la), whereas the boron enhancement in Figure l b shows up at about the top of the first peak. The search for a proper location of the scans with respect to each other is not straightforward. If the distance between peaks in the pure component scans is known from literature, it has usually been specified up to an accuracy of 1 pm, whereas multicomponent analysis appears to require an accuracy of 0.1 pm. If hyperfine structure is present, the position of the top of a peak depends on the resolution of the spectrometer. In the
40
20
0
60
data point
9 ,
“
I
0
40
20
60
data point Figure 1. Scans In a 100-pmwMe spectral window for B I 249.773
nm: (a,top) pure component senslthritlesdetermined from (-) 2 mg/L B and (- -) 15 680 mg/L U; (b, bottom) sample scan of 1.57% U hydrolysate spiked with 18 pg of B/g of U.
-
sample scan, the top of the composite structure is somewhere in between the maxima of the pure component emissions, and its location depends on the (yet unknown) concentration of the componenta. In order to solvethis problem, the solution has been found in software, rather than in hardware improvements. Two procedures have been designed to eliminate the effect of drift. First, the maxima in the two pure component scans are located at some reasonable position with respect to each other, e.g. the distance just measured, and the sample scan is shifted with respect to this fixed pure component model. For each shifted position, the Kalman filter calculation is done and the difference between the estimated and measured sample scan is determined, as expressed by the so-called innovation number: Nin
= 100(
:g [
X”(X,)
- ‘“hk”]”” rmese(Xk)
where z,,(hk) is the measured intensity at hk, ~ , , t ( h k )iS the estimated intensity at hk, based on k data points, and hk is the kth data point in the scan, containing n data points. Ni, is expressed as a percentage. By an iteration procedure, based on successive interval halving, a minimum innovation number is attained at some
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUGUST 1, 1992
a
0.3
0.2
E
.-0
E E8
0.1
0
-1 n
b n
E
2
E
-2
C
.-e
P3
. -18
.
. -16
.
. -14
.
. -12
.
. -10
.
. -8 -5
3
-1
1
3
relative peak position (steps) Flgurr 2. Mlnlmum innovation numbers and boron concentration estimates as a function of the distance between the maxima of the largest peaks In the B and U pure component scans: (.I measured dlstance; (- - -) optimum dlstance.
Flgurr 3. Innovatlon numbers and boron concentratlon estlmates as a function of the shift of the sample scan with respect to the pure component scans, flxed at the optimum relathre peak posltlon: measured shift; (- - -) optlmum shift.
position of the sample scan. Then, the two pure component scans are shifted with respect to each other and the iteration procedure is repeated, resulting in another minimum innovation number. By an optimization procedure, also based on successive interval halving, the "lowest" minimum innovation number is found, indicating that we eventually ended up with the model, estimating the sample scan at best. The corresponding relative peak position of the largest peaks in the pure component scans is the required model parameter. The procedure is illustrated with Figures 2 and 3. The measured relative peak position for the boron and uranium scans is -15.1 data points or steps of the stepping motor, driving the grating (see Figure la). Through the above procedure with-15.1 steps as the startingvalue, the optimum relative peak position was found at the expected lower value of -12.1 steps or -14.1 pm, when converted to wavelength scale. This means that within a few minutes the spectrometer may occasionally drift over about 3 steps or picometers. The boron concentration estimate is also displayed in Figure 2. Without drift compensation, the result is biased by about 30%. The concentration estimate happens to reach a maximum at the optimum relative peak position; it should be noted that this behavior is not general. Figure 3 shows the results of the shift of the sample scan with respect to the pure component scans, fiied at the optimum relative position. This figure demonstrates the sensitivity of the innovation number for shifting scans. The sample scan had to be shifted over about 1 pm with respect to its measured position. The sample scan not only contains boron and uranium emission but also contains continuum background emission. Over a rather small spectral window, this unstructured emission exhibits at most a parabolic behavior due to an unmodeled wing of a nearby emission line. In the first calculations, performed in the above described procedure, the continuum background was theoretically modeled by a
parabola, built up from a constant, a linear, and a quadratic term. The weighting of the quadratic term, however, appeared to be almost zero, whereas the linear coefficient definitely differed from zero. Therefore, all subsequent calculations were done with a linear model for the continuum background. Modeling the continuum background in this way, one can automatically compensate for blank levels, varying over a series of samples. And, no single scan needs continuum background correction prior to application of the Kalman filter. For instance, the pure component sensitivities are obtained by dividing the gross instead of the net intensities by the concentration value. The uranium pure component scan (sensitivity), the optimized relative peak position, and the fact that the continuum background can be described by a constant and a linear term are stored in a data base, together with the central wavelength and width of the spectral window, the peak search window, and the window over which sample scans will be filtered. Now, we are entering the analysis mode. When boron is determined in uranium hydrolysates, first a scan from a boron standard, based on 10 replicate scans, is measured according to the width of the spectral window in the data base. The other data are retrieved from the data base: the boron standard scan is located with respect to the uranium scan according to the relative peak position; the continuum background model is added, thereby completing the modeling over the spectral window. Then, 1.57%U hydrolysates spiked with 4, 9, and 18 pg of B/g of U are measured by 10 scans/ sample in the boron window. Each sample scan is properly located with respect to the model through the iteration procedure demonstrated in Figure 3. The resulting recoveries were 3.8, 8.6, and 18.1 pg/g, respectively, with a precision better than 5%.
shift (steps)
(-e)
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUGUST 1, 1992
Table 11. Elements Forming Volatile Fluorides: Anslyte Wavelength, Width of Spectral Window, and Kalman Filter Parameters
B Cr Mo Nb
P Ru Sb Si Ta Ti V W
relative peak positiono wavelength window measured optimized converted continuum (pm) bckgdb (steps) (steps) (nm) (pm) 1 -12.11 -14.1 -15.1 249.173 100 C 48.10 58.2 205.559 150 46.9 C -23.74 -29.0 202.030 150 -19.8 C 30.08 32.1 125 30.0 316.340 C 11.22 13.5 11.6 213.618 150 C 14.37 16.9 100 14.5 240.272 C -42.36 -50.9 -41.7 206.833 125 I 39.37 45.8 150 41.0 251.611 1 -51.29 -60.7 -51.3 228.916 155 1 6.48 6.7 334.941 150 5.1 C -8.91 -9.6 150 -8.6 309.311 C 42.47 51.8 207.911 150 43.0
Largest uranium emission in the window with respect to maximum of analyte peak. b Shows constant (c) or linear (1) behavior over the spectral window.
Table 111. Elements Forming Volatile Fluorides: Recoveries and Detection Limits in Uranium Hydrolysates with Kalman Filtering
Cr
Mo Nb Ru Sb Si Ta Ti V
w
9.5
2.0 5.7
10.7 8.9 7.5 11.4 8.6 9.2 7.9 8.9
4.8
-
6.0 4.7
4.8 4.9
19.7 16.9 15.7 16.4 19.8 16.3 17.8
8.3
0.8 0.3
10
0.9 1.7
1 1
1 100 1 1
13.8
1.0 1.0 0.2 0.9
18.5
1.3
15.0
40 38
P
80 79
(rg/g) 4.2
0.2 0.2 1.2 0.9 0.9
1.4
1.2
recovery of spike (rg/g)
a
For boron we used one standard of 1mg/L, corresponding to 63.8pg/g of U. In view of the recoveries, an aqueous solution is adequate for obtaining the pure component sensitivities and for single-point calibration to determine the boron concentration levels in the samples. No matrix effects are present. Since the uranium emission was not calibrated, its concentration outcome was biased. However, this is not important, because one is not interested in its accurate value. From the standard deviation in the concentration values a t low boron spikes, the detection limit can be determined.16 This detection limit is a so-called Yrue’’limit, since it is based on direct measurements of the analyte signal in the matrix. A 3.44% U hydrolysate was used for the determination of the detection limit. Its value is 0.4 pg/g. The ASTM feed and product specifications for boron amount to 1 and 4 pg/g, respectively. Therefore, our result shows not only that boron can be determined from uranium samples but also that it can be detected at the required level from direct measurements in the uranium matrix, without chemical separation. When using three-point background correction, this technique will fail in selection of the three points. Actually, in the 3.44% U hydrolysate the detection limit amounts to 10 pg/g in the three-point mode. The boron window can be changed so as to include the second prominent boron line at 249.678nm,19 located on the edge of a uranium line. Due to the effect of multiple line analysis,18the detection limit is expected to be slightly lower than 0.4 pg/g. This option, however, has not been explored here. Although the results do not invoke line selection, we have checked for the Kalman detection limit in one single spectral window of two other boron lines a t 208.959 and 208.893 nm. The first line suffers from direct line overlap, the second one lies on the edge of a uranium line. The resulting detection limit is almost equal to the detection limit at 249 nm. Determination of Elements Forming Volatile Fluorides. In the method development mode, described above, the model parameters for all elements forming volatile fluorides were determined. Three scans were determined for the two pure components and the synthetic sample. Then, the software was checked for near zero values of the linear and quadratic terms in the continuum background model and automatically optimized for relative peak position. The relevant data were stored in the data base and are given in Table 11. This full procedure takes some 10 min for each element but is performed once. Only in the rare case of changing monochromator parts may peak profiles change, and one needs to enter the method development mode again.
3.3 4.0
~
1.4 1.4
feed (wz/g) 50
0.3
~
0.9 0.1 0.1 0.9
ct:?l
(fig/g) 2.2
a 1.57% U hydrolysate. Recoveries have been based on three measurements, performed on different days. 3.44% U hydrolysate. Limits have been calculated from data of six measurements, each consisting of ten replicates. Reference detection limit calculated from the value in the pure component solution1eusing 34.4g/L U.
Elements like Cr and Mo hardly suffer from line overlap, although the continuum background level has been increased with respect to the aqueous background level. The P line lies on the wing of a uranium line. The Ti line is buried in a structure consisting of several uranium lines, but there is a second Ti line in the spectral window. At the most prominent lines of Nb and Ta, analysis is not possible. For Nb the second most prominent line was selected; for Ta, the fifth line. According to Winge et al.’9 detection limits in the pure component solution hardly differ for the selected and the most prominent line of both Nb and Ta. From comparison of the measured and optimized relative peak positions in Table 11,it is clear that the wavelength drift in the spectrometer is in the range 0-4 steps of the stepping motor. After conversion to picometers, by different factors dependent on wavelength, the drift turns out to be 1.3 pm on average and 5 pm at most. This agrees well with the findings at Perkin-Elmer Corp.,20that the thermal, mechanical, and mathematical errors are generally less than 3 pm. In the analysis mode, single high calibration standards and spiked 1.57% U hydrolysates were measured. Parameters and uranium scans were retrieved from the data base, and the sample scans were filtered to automatically yield the analyte concentration estimates and their relative standard deviations. For each scan the filter calculation takes 1 or 2 s, depending on the width of the window and the number of continuum background terms. This means that the computing time is shorter than the scanning time. The resulta in Table I11show good recoveries,and no matrix effects appear to be present. The latter is a nice circumstance, since the Kalman filter approach cannot correct for nonspectroscopic matrix effects. However, in this analysis tailored to uranium, a possibly occurring matrix effect would not lead to problems, because the amount of uranium in the samples can be kept at an almost constant level, allowing determination and use of a constant correction factor. Kalman detection limits were determined from six measurements of ten replicates and are given in Table I11together with the ASTM C787 specification. Only boron (4pglg) and silicon (250pg/g) are listed in the C996 product specification, which is fully satisfied with this method. With respect to the ~
(20)Grosser, 2.A.;Collins, J. B. Appl. Spectrosc. 1991,45,993-998.
~
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUQUST 1, 1992
I-
Table IV. Elements Forming Nonvolatile Fluorides: Recoveries and Detection Limits in Spiked Uranium Hydrolysate. with Kalman Filtering
Ag A1 As Ba
Be Bi Ca Cd co cu
DY
I
Eu Gd Fe Mg Mn Ni
i 20
0
40
60
eo
100
data point
Pb Sm Sn Sr Th
Zn Zr
wavelength (nm) 338.289 237.312 193.759 233.527 234.861 223.061 393.366 214.438 228.616 224.700 353.170 381.967 342.247 238.204 279.553 257.373 231.604 216.999 359.260 189.989 407.771 401.913 213.856 339.198
amt added
amt recovered
(rglg)
(rglg)
10 10 100 20 10 10 10 10 10 10 10 10 10 10 10 10 10 30 10 10 10 20 10 10
8.8 10.6 101 15.3 8.4 17.0 10.2 7.4 9.1 8.9 11.3 7.9 12.3 12.8 6.6 8.9 6.9 34 12.7 11.4 7.6 23 7.6 5.7
detn limit (rglg) 7.9 0.6 9.2 1.4 0.1 3.5 0.1 0.9 1.7
0.5 1.0 0.8 3.4 2.4 0.5 0.8 0.5 4.3 5.0 6.5 0.5 10 0.7 1.2
3.08% U hydrolysate.
d
20
0
40
60
80
100
data points processed Flgurr 4. Resut of data reduction in the 100-pmwide spectral window of Eu I1 381.967 nm: (a) pure component sensltivitles, determined
-
from (-) 2 mg/L Eu and (- -) 30 840 mg/L U, and (. - .) constant continuum background model; (b) sample scan of 3.08% U hydroiysate spiked wlth 10 pg of Eu/g of U; (c) concentration estimates for (-) Eu and (- -) U, and (. -) contlnuum background coefflclent; (d) innovations eequence. The vertlcai dashed llne Indicates the top of the Eu peak.
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feed specification, half the elements on the list in Table I11 can be determined. Five elements, including Nb, Sb, Ta, V, and W,can only be detected at the required concentration level. For Ru the detection limit is worse, but it is too high by less than a factor of 2. These results can be interpreted as follows. Uranium shows not only structured interfering emission but also an increased background level up to a factor of 10, due to the same interfering emission and the wings of other emission lines. Both effede degrade the detection limit. By elimination of optical instability in Kalman filtering, the degradation due to structured emission is removed. Owing to noise averaging and multiple line analysis over the full scan, a gain up to a factor of 4 in detection limit is attained. These effects more or less counterbalance the effect of the increased total background level. Therefore, the Kalman detectionlimits in uranium are expected to equal the detection limits in the pure component solutions, times some factor, which magnitude is smaller than the increase in the background level. In Table 111, detection limits are given, calculated from the values in the pure component solution in correspondence to the uranium contents in the sample. Since our equipment yields limits similar to those of ref 19, these values have been used in this comparison. From comparison to these reference limits (asif uranium does not
influence the pure component detection limit), the detection limits measured in uranium are worse by a factor of 1-4. Then, two conclusions can be drawn. First, notwithstanding the large and structured background, the Kalman detection limit in uranium often is marginally degraded as compared to limit in the pure component solution. Second, it is not the presence of uranium which causes some elements to be hardly detectable at the required level, but it is the limit of the ICPAES technique. There are several ways open to improve detection. A uranium concentration as high as possible may be introduced into the plasma, but the plasma cannot be loaded with more than 5 % uranium. The ICP parameters may be optimized for elements like Ru and Sb. With these two measures we end up with the fact that all volatile,fluoride-formingelements meet the ASTM specification;however six of them can only be detected and not determined. If one has to determine some of these elements, extraction of uranium from a highconcentration hydrolysate is necessary. Tributyl phosphate in carbon tetrachloride may be used. This situation of using at most one extraction scheme favorably contrasts with the starting situation of using at least four extraction schemes and colorimetry. Determination of Elements Forming Nonvolatile Fluorides. The above two procedures of method development and analysis have been automated and applied to the determination of elements that form nonvolatile fluorides. According to the ASTM C787 specification,the total of these elements shallnot exceed 300 pglg of uranium. Also, elements like the Dy, Eu,Gd, and Sm rare earths have been measured. Recoveries and detection limits are given in Table IV. In this more or less routinely performed analysis, the recoveriesat low anal@ concentrations are good for a sample as difficultas the uranium hydrolysate. Not only the detection limits themselves but also the sum of the detection limits are well below the overall specification of 300 pg/g. Therefore, determination of the elements in Table IV according to the ASTM specification is straightforward. As a consequence, a routine analysis can be set up for all elements forming
ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUGUST 1, lQQ2 1649
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the measured and the estimated sample scan: the innovation sequence. Since at the f i i t data point the recursive calculation starta with concentration estimates equal to zero, the difference is large and has been omitted from the figure. After about three to four data points, the innovations sequence shows its white noise behavior, and the innovation number, based on this sequence, amounts to 1.2 % ,both sequence and number indicating proper modeling. In Figure 5a the quality of the estimate of the total background signal can be judged. Subtracting all sensitivities and continuum background terms according to their weights from the measured sample scan, except for the analyte sensitivity,yields the analyte intensities over the sample scan. In Figure 5b, these intensities are compared to the analyte sensitivity,multiplied by the analyte concentration estimate. It is obvious that the K h a n filter approach eliminates wavelength drift and performs noise averaging.
CONCLUSIONS
data point Flgure 5. Quality of the Kalman filter estimates: (a, top) (-) sample scan of 3.08% U hydrolysate, spiked with 10 pg of Eu/g of U, (- -1 filter estlmate of total backgroundslgnal;(b,bottom)(-) filter estimate of analyte signal In sample scan, (- -) Eu senslthrltles tlmes the Eu concentratlon estlmate.
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nonvolatile fluorides, including half the elements forming volatile fluorides, under compromise ICP conditions. A typical result of the routine analysis is shown in Figures 4 and 5. Figure 4a displays the sensitivities for Eu at 381.967 nm, obtained from calibration, and for uranium and the constant continuum background, retrieved from the data base. The distance between the largest peaks in both scans is -35.3 data points. Note the hyperfine structure in the Eu signal.21 Figure 4b displaysthe sample scan, corresponding to the entry in Table IV,which is located at the proper position with respect to the emission model through iteration in Kalman filtering. Since the filter is recursive, it yields concentration estimates as afunction of the data points processed. In Figure 4c, it can be seen that the uranium concentration estimate is stable within 20 data points, at about the top of the first uranium peak in the spectral window. Over the remainder of the scan, the estimate remains stable. Then, any significant deviation from the uranium profile at the position of the analyte emission is interpreted by the filter as the analyte signal. During the first 40 data points, the Eu concentration cannot be properly estimated, since no Eu signal is present. As soon as the Eu signal shows up, the Eu concentration estimate converges and attains a quite stable value, already before passing the top of the Eu peak. At the same time the background coefficientestimate stabilizes. The concentration estimate calculatedfor the last data point is the value reported for Eu in Table IV. Figure 4d gives the difference between (21)Boumane, P.W.J. M.; Vrakking, J. J. A. M. Spectrochim. Acta 1986,41B,1235-1275.
With the Kalman fiiter approach for data reduction of scam, recorded in ICP-AES at high resolution in a fast scanning mode, routine analysis can be performed directlyin hydrolyzed UF6,containing up to 3.5% U, without chemical separation. Uranium pure component scans, a continuum background model, and relative peak positions have been determined once and stored in a data base. Pure analyte scans are measured daily for calibration. Kalman filter data reduction of the sample scans immediately yields the analyte concentrations. Under compromise ICP conditions, one single, simple and fast method allows determination of all elements forming nonvolatile fluoridesand of half the elements formingvolatile fluorides, as given in the ASTM C787 feed specifications for UF6. The method also meets the C996 product specification. Besides, some of the rare-earth elements have been included in this study, resulting in the direct and accurate determination of a total of 30 trace impurities in uranium. Six volatile fluorides cannot be determined; however, they can be detected at the required concentration level. This shortcoming is not caused by the presence of uranium but by the ICP-AES technique itself. If determination is necessary, extraction should be done from highly concentrated uranium hydrolysates. With the analyticaltechnique of ICP-AES only, the w e of at most one extraction scheme is needed. This favorably contrasts with the starting situation of using at least four extraction schemes and colorimetry.
RECEIVED for review December 19, 1991. Accepted May 4, 1992. Registry No. B, 7440-42-8;Cr, 7440-47-3;Mo, 7439-98-7;Nb, 7440-03-1;Ru, 7440-18-8;Sb, 7440-36-0;Si, 7440-21-3;Ta, 744025-7;Ti, 7440-32-6;V, 7440-62-2;W, 7440-33-7;P, 7723-14-0; Ag, 7440-22-4;Al,7429-90-5; As,7440-38-2; Ba, 7440-39-3;Be, 902210-0;Bi, 7440-69-9;Ca, 7440-70-2;Cd, 7440-43-9;Co, 7440-484; Cu, 7440-50-8;Dy, 7429-91-6;Eu, 7440-53-1; Gd, 7440-54-2;Fe, 7439-89-6;Mg,7439-95-4;Mn, 7439-96-5;Ni, 7440-02-0;Ph, 743992-1; Sm, 7440-19-9;Sn, 7440-31-5;Th, 7440-29-1;Zn,7440-66-6; Zr,7440-67-7; UFB,7783-81-5.
