- Email: [email protected]
Measurement of uranium isotope ratios in solid samples using laser ablation and diode laser-excited atomic fluorescence spectrometry1
Spectrochimica Acta Part B 54 Ž1999. 943]958
Measurement of uranium isotope ratios in solid samples using laser ablation and diode laser-excited atomic fluorescence spectrometry q B.W. Smith1 A. Quentmeier U , M. Bolshov, K. Niemax Institute for Spectrochemistry and Applied Spectroscopy (ISAS), Bunsen-Kirchhoff-Strasse 11, D-44139 Dortmund, Germany Received 3 November 1998; accepted 4 February 1999
Abstract A diode laser is used for the selective excitation of 235 U and 238 U in a laser-induced plasma applying Nd:YAG laser pulses to UO 2 samples. The diode laser is rapidly scanned immediately following each laser sampling and the resonance atomic fluorescence spectrum for both isotopes is obtained on a pulse-to-pulse basis. Time-integrated measurements, with the diode laser fixed at either isotope, were also made. Optimum signal-to-noise was obtained at a distance of 0.8 cm from the sample surface, a pressure of 0.9 mbar and a Nd:YAG laser pulse energy of 0.5 mJ Ž880 MW cmy2 .. Three samples with 0.204, 0.407 and 0.714% 235 U were measured. For example, for the UO 2 pellet with the natural uranium isotopic composition Ž99.281% 238 U and 0.714% 235 U., the accuracy and precision were 7% and 5% Ž460 shots., respectively, limited by the continuum emission background from the laser-induced plasma. Q 1999 Elsevier Science B.V. All rights reserved. Keywords: Laser ablation; Laser-excited fluorescence; Diode laser spectrometry; Isotope ratio; Uranium
1. Introduction The most widely used analytical techniques for isotopically selective detection tend to be large, complex and expensive, e.g. inductively coupled
q
Dedicated to Prof. Dr. Dieter Klockow on the occasion of his 65th birthday. U Corresponding author. Fax: q49-231-1392120. 1 On leave from the Department of Chemistry, University of Florida, Gainesville, FL 32611, USA.
plasma mass spectrometry ŽICPMS. and thermal ionization mass spectrometry ŽTIMS.. The goal of the present work was to develop an approach for isotopically selective detection of 235 U and 238 U which would provide a useful level of accuracy and precision in a compact, portable instrument, with the possibility of remote, or robotic measurements in hostile environments. We have therefore evaluated diode laser excited atomic fluorescence combined with pulsed laser sampling. Many years of intensive investigations of
0584-8547r99r$ - see front matter Q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 9 9 . 0 0 0 2 2 - 1
944
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
laser]material interactions has led to a wide range of applications in many fields. As an approach to microsampling in analytical chemistry, laser ablation is now widely used as a sample introduction technique for ICP emission and mass spectrometry. By far the most studied analytical use has been the direct observation of emission from the laser-induced plasma, a technique generally known as laser-induced breakdown spectroscopy, LIBS w1x. However, as was demonstrated as early as 1977 by Measures et al., the laser-induced plasma can serve as a very efficient and useful atomic reservoir for laser excited atomic fluorescence measurements w2x. Measures and Kwong, using a ruby laser for sampling and a pulsed dye laser for fluorescence excitation, reported a freedom from chemical matrix effects when the measurements were carried out at low pressure Žapprox. 10y3 mbar. w3,4x. Similar results were reported by Gornushkin et al. for the determination of Pb in copper, brass, steel and zinc samples w5x and Co in soils, steels and graphite w6x. Piepmeier and co-workers used laser excited fluorescence to study the distribution of species in the laser plasma in argon and oxygen atmospheres w7,8x. In our laboratory, we have made a direct comparison of fluorescence and emission detection for the determination of Si and Cr in steel w9x. We also have reported absolute limits of detection for seven elements, ranging from 4 to 600 fg, for a single shot measurement on steels samples w10x. Using a 193-nm excimer laser, Oki et al. w11x have measured Na in glasses using atomic fluorescence detection. They reported a limit of detection of 0.6 fg per shot with a spatial Ždepth. resolution of 1.1 nm per shot. The general conclusion to be drawn from the preceding literature is that improved accuracy and precision, as well as much lower detection limits, can be expected for laser excited fluorescence compared to emission in laser-induced plasmas. Successful atomic fluorescence results have been reported for both UV and IR laser sampling, although it is impossible to categorically recommend laser ablation in either wavelength domain because of the many environment and matrix-dependent variables. Uranium has a complex spectrum which has
been well studied w12,13x and atomic spectroscopic methods for its detection have been used for many years. Among the most compelling reasons for a spectroscopic approach to the detection of uranium is the potential for very high selectivity and especially, the capability to resolve individual isotopes. Atomic emission spectroscopy has been used for at least 50 years for uranium isotopic determinations w14x. With modern instrumentation, isotopes with low specific activity can often be detected at lower levels than achievable with radiochemical techniques. Uranium is not atomized efficiently in low energy plasmas, such as flames, and also has a poor free atom fraction in the graphite furnace. Consequently, it is not detectable to any useful degree by any flame or furnace atomic emission or absorption method. Nevertheless, isotopic selectivity was demonstrated by Goleb w15x using monoisotopic hollow cathode lamps as early as 1963. The characteristic mass for uranium by electrothermal atomization atomic absorption spectrometry is only 3 ng, worse than all other elements with the exception of phosphorous w16x. The poor atomization efficiency in the furnace is thought to be due to the formation of refractory carbides and oxides w17x. Having a relatively low ionization potential Ž6.08 eV., uranium is completely ionized when introduced into higher power plasma sources such as the inductively coupled plasma w18x. The most sensitive ion emission line Ž385.96 nm. yielded a limit of detection of 250 ng mly1 in the early comprehensive study of ICP emission spectrometry by Winge and co-workers w19x. In the tables published by Boumans and Vrakking w20x in 1987, comparing limits of detection in 27 MHz and 50 MHz plasmas, a limit of detection of 16 ng mly1 was listed for the same spectrometer spectral bandwidth used by Winge Ž15 pm.. An atlas of the ICP uranium spectrum has been produced by DeKalb and co-workers w21x. Uranium exhibits significant isotope shifts which permits isotopic selectivity by ICP emission in some cases. Edelson and Fassel w22x showed that the 238r236r235 isotopes can be resolved for the 424.4-nm uranium ion line in the ICP using a spectrometer having a resolving power of approximately 250 000. They observed a shift of approximately 10 pm
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
between the emission peaks of the three isotopes. More recently, Goodall and Johnson w23x have used both solution nebulization and laser ablation sampling with ICP emission to determine the isotopic ratio of 238 Ur 235 U. Isotopic selectivity has also been achieved using laser excited fluorescence of the uranium ion in the ICP. Using a pulsed dye laser, Vera and co-workers w24x excited the 286.57-nm ion line and observed the fluorescence at the 288.96-nm line, obtaining a detection limit of 2 mg mly1 and resolving the 235r238 isotopes in a complex matrix. The most successful use of the ICP for the detection of uranium has been in its coupling with mass spectrometric detection. Accurate isotopic selectivity was demonstrated early in the development of the method. Russ and Bazan w25x were able to determine the 235r238 ratio with a precision of better than 0.5% with only 50 ng of material. This determination is commonly done using TIMS with better precision and smaller samples but takes more than 4 h of measurement time and requires extensive sample preparation. There have been numerous applications of ICPMS to the detection of uranium in recent years. Limits of detection are among the best reported for the method, sometimes below 0.1 pg mly1 . More typical limits of detection are in the vicinity of 0.02 ng mly1 w26x. For example, Fukuda and Sayama w27x have determined traces of uranium in antimony oxide at this level. Sengupta and Bertrand w28x have reported similar detection limits for geological samples which were processed by a microwave digestion procedure. A flow injection procedure has been used for the determination of 238 U in groundwater with a limit of detection of 0.3 pg mly1 w29x. Hollenbach et al. w30x used a flow injection technique as a preconcentration mechanism and obtained a limit of detection of 0.03 ng gy1 for 234 U in soils. Gastel et al. w31x have used laser-ablation ICP-MS for the analysis of longlived radionuclides in concretes and reported a limit of detection for 233 U of 0.6 ng gy1 . Among the commercially available spectroscopic laboratory methods for uranium determination, ICPMS will undoubtedly continue to be one of the most useful. The loss of detection power caused by the need to dissolve solid samples is generally more
945
than compensated for by the uniquely low limits of detection. The primary drawback of the method, aside from the complexity of sample dissolution, is that the instrumentation is relatively expensive, and, over time will become contaminated with gradual accumulation of radioactive species. In relation to the present work, it is important to mention the use of optogalvanic measurements for the determination of uranium. The spectroscopy of uranium in low pressure discharges has been studied and applied for many years. For example, Rossi and Mol w32x used a high resolution direct reading spectrograph to determine 235 Ur 238 U ratios from hollow cathode lamp emission. The observation of the optogalvanic spectrum of uranium was reported as early as 1979 by Keller et al. w33x who investigated the applicability of the technique for standard spectroscopic measurements, the determination of oscillator strengths, the measurement of fundamental discharge characteristics, and isotopic ratio analysis. They reported that as few as 10 8 atoms cmy3 could be detected. Gagne ´ et al. w34x studied the fundamental aspects of the uranium glow discharge and its optogalvanic characteristics. The optogalvanic effect uranium spectrum has been suggested as a useful wavelength calibration device for tunable lasers w35,36x. In 1984, Gagne ´ et al. w37x demonstrated, using a cw dye laser at the well-studied 591.54-nm uranium transition, the measurement of isotopic ratios for the 235r238, 234r238 and 236r235 isotopes in natural and enriched uranium. They estimated a limit of detection of 10 6 atoms cmy3 . More recently, this approach has been successfully carried out by Barshick et al. using a diode laser as the excitation source w38,39x. A precision ranging from 2.4 to 30% was attained, depending upon the level of enrichment, for the 235r238 isotope ratio, which is sufficient for screening applications. A transition at 831.84 nm was used with a 42-mW diode laser. Spectroscopic measurements in the glow discharge Žapprox. 2.7 hPa. are appealing because of the reduced spectral linewidth and consequently improved resolution of isotopic fine structure. The approach taken by Barshick et al. has the added advantage of being simple and poten-
946
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
tially portable, although the sample must be formed into a shape which is compatible with the glow discharge device. This may involve machining of the sample or grinding and mixing it with a conductive binder such as silver which can be pressed into a cathode. Laser-induced breakdown spectroscopy has been used for the determination of uranium in aqueous solutions, without any isotopic selectivity. Wachter and Cremers w40x reported a limit of detection of 100 mg ly1 for uranium using a laser breakdown on the liquid surface. Van der Mullen et al. reported on the time-resolved emission behavior of a laser plasma on uranium in argon and air w41x. Recently, emission spectroscopic detection from a 308-nm laser-induced plasma has been used for the detection of the uranium 235r238 isotopes by Pietsch and colleagues w42,43x. At a pressure of 2.67 Pa Žair., the UII 424.437-nm transition was detected with a spectrometer having a spectral resolution of 5.5 pm. A precision of 5% in the isotope ratio determination was demonstrated for an enriched 235 U concentration Ž3.5%., by averaging of 20 000 laser samplings. Isotopically selective absorption and fluorescence detection of other elements in laser-induced plasmas have also been recently described by King et al. w44,45x. They reported the measurement of 6 Lir 7 Li with UV laser sampling and atomic fluorescence detection using a pulsed dye laser for excitation w44x. Measurements of 87 Rbr 86 Rb were made using a cw Ti:sapphire laser for atomic absorption measurements with laser sampling by a 1064-nm Nd:YAG laser w45x.
Diode lasers make particularly attractive excitation sources for high resolution spectroscopy of uranium. They can access numerous uranium transitions in the red region of the spectrum with sufficient spectral power density to saturate, they have spectral linewidths well below those of the uranium atom lines found in laser-produced plasmas, they can be rapidly and accurately tuned and they are moreover, compact, reliable and quite stable. Other actinide elements are also accessible in this spectral range we.g. PuŽI. 648.8853 nm with an isotope shift 240 Pu] 239 Pu of 0.425 cmy1 x w46,47x for which rapid, portable isotopic detection may prove valuable. In the present work, we have combined diode laser excited atomic fluorescence with laser sampling for the selective detection of 235 U and 238 U. 2. Experimental Fig. 1 shows a schematic diagram of the experimental system. A Nd:YAG laser ŽSpectron model SL801. operating at the fundamental wavelength of 1064 nm was used as the ablation sampling source. Pulse repetition rates ranging from 1 to 10 Hz, with energies of up to 10 mJ, were typically used. The Nd:YAG beam was directed to the sample chamber by several mirrors and focused by a lens of 12-cm focal length onto the sample. The laser spot size at the sample was a nominal 85 mm. The laser pulse energy at the sample was varied as necessary by using calibrated glass filters or by changing the high voltage
Fig. 1. Schematic diagram of the experimental set-up.
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
947
Fig. 2. Design of the sampling chamber.
of the laser flashlamp, over a limited range. The sample chamber, shown in Fig. 2, was a simple three-axis aluminum cell with two perpendicular paths for optical access though 2-cm diameter fused silica windows. The diode laser beam passed along the vertical axis through the center of the chamber, intersecting the Nd:YAG beam in the plane of detection. The third axis provided a window for the Nd:YAG laser beam and, opposite to it, an easily removable port for the sample probe. The sample could be independently adjusted along the axis of the sampling laser via a rotatable vacuum feedthrough, thereby allowing measurements to be made at any distance from the sample surface. Independent movement of the laser focusing lens permitted its adjustment in coordination with the displacement of the sample. The chamber was fitted with additional ports for vacuum, vacuum measurement ŽMKS Instruments capacitance manometer, model PR-2000. and introduction of 99.999 grade argon. Most of the measurements were carried out using a sample which consisted of a pressed, 6-mm diameter graphite pellet containing 60% natural UO 2 . The diode laser, with a short external cavity, has been recently described in detail elsewhere w48x. The laser diode ŽType ML1013R-01, nominal 684 nm, 50 mW, Mitsubishi Electric Corp.., with its housing can and window removed, was mounted in an aluminum case, 18 = 10 = 8 cm. A thin quartz interferometer Ž70 mm. mounted very close to the laser diode chip provided a weak optical feedback thereby suppressing the domi-
nate longitudinal mode and allowing the laser to operate in a selected nearby longitudinal mode. Operated in this way, the laser is stable over long periods of time and tunes reliably over a more extended wavelength range. After a 1-h warm-up time and initial tuning, the laser would remain within the absorption linewidth of the 238 U line for an entire working day. The laser was powered by a combination temperature controller and current source ŽModel ITC502, Profile Optische Systeme GmbH, Karlsfeld, Germany.. Preliminary optimization of the wavelengthrtemperature] current behavior was carried out using a vacuum interferometric wavemeter ŽATOS Lamdameter, model LM007.. The diode laser current source was externally modulated with a triangle wave using a function generator ŽWavetek, model FG5000. which was triggered by the master pulse generator Žmodel DG535, Stanford Research Systems. used to trigger the Nd:YAG laser. In this way, the diode laser wavelength could be scanned over an adjustable, known wavelength interval at a variable rate, synchronized with the firing of the Nd:YAG sampling laser. The diode laser output power at the 682.69-nm uranium line, at the sample chamber, was 15 mW at the typical operating conditions of 82 mA and 27.908C, which was sufficient to saturate the 238 U transition with a 0.1-cm beam diameter. The wavelength]current relationship Ž7.15 pm mAy1 . was linear over the necessary range of current Ž79]82 mA.. The atomic fluorescence was detected in two ways. In most of the experiments to be described,
948
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
the fluorescence was collected by a single lens Žfr3, 7.5-cm focal length. and imaged Ž1:1. onto the entrance slit Ž1 mm. of a small monochromator ŽJ-Y model H20. fitted with a photomultiplier tube ŽHamamatsu R928.. The current output of the photomultiplier tube was passed through a laboratory-constructed transimpediance amplifier which had two output channels, a low gain, high bandwidth output Ž10 5 VrA, 1-MHz bandwidth. and a higher gain, restricted bandwidth output Ž10 7 VrA, 100-kHz bandwidth.. This provided the necessary dynamic range for measurement of both isotope signals simultaneously when desired. The signal was then recorded with a digital oscilloscope ŽTektronix model TDS360. and transferred to a spreadsheet for final analysis. The fluorescence signal was also detected in some cases with a compact module consisting of a collimating lens Ž5-cm focal length., narrowband interference filter Ž682 nm, 1 nm FWHM, 55%T., focusing lens Ž2.6-cm focal length. and photodiode detector Žtype S2386-5K7E, Hamamatsu.. Once the experiment was optimized, the two systems were virtually identical in performance but the version with the monochromatorrPMT proved more versatile for the optimization studies. However, the compact photodiode module, which mounted directly on the port of the sample chamber, was better suited for use in a simple, portable instrument. A trigger for the oscilloscope was provided by a fast photodiode ŽLambda Physik, model LF302 UV. mounted near the Nd:YAG focusing lens. For some atomic absorption experiments, which were used to assist in the optimization, the diode laser beam, appropriately attenuated to avoid saturation of either the detector or the atomic population, was simply detected with a photodiode or PMT, placed a short distance from the sample chamber. Samples, provided by the Institute of Transuranium Elements ŽKarlsruhe, Germany., were prepared as 60% by weight, uranium oxide with graphite, pressed in pellets of 0.6-cm diameter and varying thickness. When fresh samples were inserted into the vacuum chamber, it was evacuated to below the minimum pressure which could be measured by our vacuum sensor Ž0.01 mbar., typically requiring approximately 20 s, flushed
once with argon and finally evacuated to the operating pressure. All measurements were made under conditions of continuous pumping and gas flow, at constant pressure. The complex uranium spectrum provides several good transitions in the spectral region accessible to the fundamental output of commercially available diode lasers. We selected the transition from the ground state to 14643.867 cmy1 at 682.69 nm Ž 5 L 68] 7 M 6 . based on a reasonably strong transition probability ŽgAs 2.23= 10 7 sy1 . and rather large isotope shift Žy0.380 cmy1 . w49,50x. The fluorescence spectrum consists of a single line at 682.6913 nm Žair. for 238 U, and a hyperfine structure Žhfs. multiplet with seven predominant components, centered at 682.6736 nm, for 235 U. Direct line fluorescence to the 620.320 cmy1 level, at 712.9 nm, was observed, but since this line is weaker and scattered laser radiation was negligible at 682.69 nm, the resonance fluorescence measurement was preferred. 3. Results and discussion Two different techniques were used for the detection of the LIF signals: Ži. a fast wavelength scanning of the diode laser during each individual laser sampling; and Žii. a time-integrated measurement where the diode laser wavelength was fixed at the peak of either isotope line on alternate laser samplings. In either approach, one must account for the gradual degradation of the sample with time and consequent variation in laser]sample interaction as the measurement progresses. Data must be background-corrected in some quasi-continuous fashion during the entire measurement interval. While experimentally demanding, this strategy would always be likely to provide better precision than measurements averaged over a large number of laser shots because of the inevitable variation in sampling due to the gradual modification of the sample surface by the laser, as the measurement progresses. It is possible to control this effect, to some extent, by moving the sample, either laterally or rotationally, to present a fresh sample surface to each laser pulse. However, this leads to additional complexities because of the constraints it places
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
on the size and surface character Žhomogeneity and morphology. of the sample. Our goal was to provide a simple, fixed sample presentation with few restrictions on sample geometry. By making the measurements in this way, the laser]surface interaction naturally will change as the surface is modified during the course of several tens or hundreds of laser probings. The influence of this surface modification on the isotope ratio measurement can be accounted for by acquiring data from each individual laser probing Žor a very few number of probings.. The fast wavelength scanning approach will be discussed first. 3.1. Wa¨ elength scanning technique Fig. 3 illustrates this measurement approach under conditions of optimum Nd:YAG pulse energy, pressure and measurement distance from the sample. Within a few microseconds after the arrival of the Nd:YAG pulse at the sample, one observes, at a distance of 0.8 cm from the sample surface, a background emission composed of underlying continuum and dense, broadened ura-
949
nium line emission, coincident with the production of free uranium atoms, which lasts several hundred microseconds w t Ž1re. s 134 ms at 0.8 mbarx. Triggered by the firing of the Nd:YAG laser, the current to the diode laser is increased in a linear ramp, scanning the laser wavelength over the two isotope spectral lines as the uranium atomic population within the observation zone gradually decreases. The minor isotope Žnom. 0.714%. is scanned early in the plume lifetime, when the atomic population density is relatively high, while the major isotope Žnom. 99.281%. is probed approximately 1 ms later, when the population has declined significantly. In Fig. 3, one can clearly see the hfs of the 235 U line on top of a background of decaying continuum emission. The diode laser is scanning at a rate of approximately 0.016 pm msy1 , reaching the 238 U line 1.25 ms later. During this interval, the atom population has decreased, exponentially, by approximately 15 times. Note that the diode laser intensity increases, with increasing current, by approximately 2.75%. The signal-to-noise ratio obtained from the
Fig. 3. 235 U and 238 U atomic fluorescence spectrum with background emission from the ablation plasma. The linear ramp, linked to the right abscissa, indicates the tuning of the diode laser by increasing of the current.
950
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
uranium atomic fluorescence spectrum depends primarily on three variables: the measurement distance from the sample surface, the power density of the sampling laser and the gas pressure. Because of the interdependence of these variables, and the complex spatial]temporal development of the laser plasma plume and the uranium ground state atom population, a careful optimization was required. Preliminary diode laser absorption measurements showed that the highest ground state atom density occurred in the first several hundred microseconds after the ablation event, close to the sample surface Žwithin 0.2 cm. at pressures of 2]4 mbar. However, fluorescence measurements in this region are hindered by intense continuum emission from the laser-produced plasma. Ultimately, the optimization required finding the measurement location, pressure and Nd:YAG laser power density which provided the highest ground state atomic population with the least emission background Žwhich was the limiting source of noise for the measurement of the minor isotope.. Systematic measurements were made at sample distances ranging from 0.1 to 1 cm, pressures ranging from 0.01 to 10 mbar and Nd:YAG laser pulse energies ranging from 0.3 to 10 mJ Ž; 0.6]20 GW cmy2 .. Fig. 4 shows a portion of the resulting data, emission and fluorescence intensities as a function of argon pressure at distances of 0.3 and 0.6 cm from the sample surface. Both the emission and fluores-
cence are highly pressure dependent with an optimum in the fluorescenceremission ratio at 0.8]0.9 mbar and 0.8 cm from the sample surface. Fig. 5 shows a portion of the optimization for Nd:YAG pulse energy at 0.8 cm from the sample surface and 0.95 mbar. The Nd:YAG pulse energy was varied smoothly from 0.3 to 0.93 mJ by increasing the high voltage to the laser flashlamp. The last two spectra, at 2.9 and 3.6 mJ were obtained by removing an attenuating filter at the sample chamber. The uranium fluorescence intensity remains almost constant over this range of pulse energies, beginning to decline only at pulse energies below 0.4 mJ. However, the emission intensity continuously decreases with decreasing pulse energy. This may indicate that the amount of material removed from the sample is rather constant over this range of pulse energies, the decreasing emission being due to the reduced energy available to couple into the plasma. Ultimately, the optimum SrN ratio was obtained at a distance of 0.8 cm from the sample surface, 0.86 mbar pressure and 0.5 mJ Nd:YAG laser pulse energy. Under these conditions, the peak absorbance of the attenuated diode laser at the 238 U transition was 0.056 at the measurement time, 1.8 ms after the laser ablation event, well within the range of low atom density. In addition, during the fluorescence measurements, the diode laser spectral irradiance was well above the saturation spectral irradiance for the 238 U isotope. This was
Fig. 4. Emission and fluorescence intensity as a function of pressure at 0.3 and 0.6 cm from the sample surface.
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
951
Fig. 5. Emission and fluorescence intensity as a function of Nd:YAG laser pulse energy at 0.8 cm from the sample surface and 0.95 mbar.
checked experimentally by decreasing the diode laser intensity. A 10-fold decrease of the diode laser intensity caused only a twofold decrease of the 238 U fluorescence. The saturation of the 235 U line was smaller. A sevenfold decrease in laser intensity caused a decrease of the 235 U fluorescence of approximately 2.5. This is not unexpected, considering the hfs of the 235 U transition and the collisional mixing of the hfs levels involved. For the optimum pulse energy of 0.5 mJ, an estimate was made of the amount of material removed from the sample surface for 1, 10 and 100 successive shots. The material removal in each case was estimated by laser interferometric profilometry of the resulting craters and the mass removal was 80 ng per shot, 350 ng per 10 shots and 1000 ng per 100 shots. The initial laser pulse is the most effective at removing material at a fresh spot; as a crater is formed in the sample surface, the mass removed per shot declines. Because of the necessity to correct for the plasma emission background beneath the 235 U spectrum, data were obtained by collecting an emission spectrum Ždiode laser blocked., a spectrum of the emission plus fluorescence and then another emission spectrum. The two emission
spectra were averaged to obtain an estimate of the background during the fluorescence measurement, which was then subtracted from it. Although the spectrum of 235 U was detectable for individual ablation events, the SrN ratio was inadequate to provide for accurate background correction on a pulse-to-pulse basis. A study was made to determine the optimum number of laser shots which could be averaged while still providing an accurate background correction using the average of background spectra taken prior to and subsequent to the fluorescence data acquisition. If one averages too many samplings, 256 for example, for each of the three spectra, the variation in sampling character during the entire measurement period will result in a poor background correction. An optimum was found at 27 laser samplings for each spectrum. Fig. 6 shows a representative data acquisition under optimized conditions. It is the average of eight background-corrected measurements, a total of 17 spectra Žeight of emission q fluorescence and nine of emission only, each the average of 27 laser shots. and, therefore, a total of 460 laser samplings. The net fluorescence intensity is given in a logarithmic scale. The ratio of the areas, 238 Ur 235 U, is 5.22" 0.26 Ž"5%, relative.. Be-
952
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
Fig. 6. Background-corrected atomic fluorescence spectrum for a 60% natural uranium oxide sample diluted in graphite. Total of 460 laser shots at 0.5 mJ per pulse.
cause of the exponentially declining uranium atom population during the time required to scan the diode laser from one isotopic line to the other, two corrections need to be made to obtain an isotope ratio from the ratio of peak areas. The distribution of intensities over the hf components of the 235 isotope has been distorted because the uranium atom population declined exponentially during the 600-ms required to scan the diode laser over the transition. In addition, the uranium atom population continued to decline during the interval between the measurement of the two isotopes. This overall normalization factor of approximately 27 can be estimated fairly easily, for example, by fixing the diode laser on the peak of the 238 U transition and monitoring the atomic population decline, via the atomic fluorescence signal. The SrN is adequate to make such a measurement with good precision even on a single pulse basis and it could be done, for example, on every third laser sampling, in an automatic way, with an appropriate data acquisition system. A normalization for the decline in atomic population could also be done simultaneously on a shot-
to-shot basis using a second coincident diode laser tuned to the 238 U line and arranged to monitor the absorption at that wavelength. Fig. 7 shows the result of applying such a correction to the 235 U spectrum, in this case derived from an independent measurement of the time dependence of the atomic fluorescence signal. The declining atomic population is evident in the degradation in SrN as the laser scans to longer wavelengths. Finally, if the transition is not saturated, one would need to correct for the 2.75% increase in the intensity of the diode laser as the current is increased to tune it from 682.6736 to 682.6913 nm. The possibility of optical pumping within the hf spectrum was considered by examining the 235 U spectrum using a highly attenuated diode laser. Aside from poorer SrN, no significant influence on the intensity ratios of the hfs components was observed. From this finding one can conclude that collisional mixing of the hfs levels occur at the experimental pressure of 0.8 mbar. Of course, it is also possible to simply calibrate the measurement of the isotope ratio with suitable standards. For this purpose, we obtained two
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
953
Fig. 7. Spectrum of 235 U, corrected for the decline in atomic population during the scan interval. Average of 405 laser samplings under optimized measurement conditions. The central wavelength is determined from the calibration of the diode laser tuning behavior with current using a vacuum interferometer and an absolute calibration against the 238 U transition, approximately 17 pm to the red.
samples of uranium oxide with depleted 235 U concentration. These samples were prepared and certified in the Institute of Transuranium Elements ŽKarlsruhe .. The concentrations of 235 U were measured by isotope dilution mass spectrometry to 0.204% and 0.407% with relative standard deviation below 0.1%. Fig. 8 shows spectra, each the average of 300 laser probings, for these samples and a sample of natural isotopic composition. They have not been corrected for the emission background and are scaled such that the 238 U peaks, which are offscale in the figure, are of equal areas. The limit of detection for this fast scanning approach is different for each isotope because each has a different source of limiting noise. In addition, it is important to note that this experiment has been optimized for the detection of the minor isotope. The major isotope is detected from a diminished atomic population. For the minor isotope, the limiting uncertainty is due to the background emission, while for the major isotope,
it is due to a combination of laser scatter and detector dark current. Based upon the data shown in Fig. 6 Ž460 laser shots., the limit of detection Ž3s . for 235 U is 1.3 mg gy1. 3.2. Fixed wa¨ elength technique A completely different measurement approach was also evaluated. One can simply tune the diode laser to the peak of either isotope line on alternating laser samplings. This has the advantage of providing the complete time-integrated fluorescence signal for each isotope Žavoiding the need to know the decline in atomic population as one scans the diode laser. but requires taking data on alternate laser samplings. If the variation in mass removal between two adjacent shots is small enough, good precision can be attained. The other disadvantage of this approach is that one does not obtain the spectrum of the two isotopes, but only their peak intensities. This is not so important for the single isolated line of
954
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
Fig. 8. Spectra of uranium oxide samples with 235 U concentrations of 0.204, 0.407 and 0.714%. Each is the average of 300 laser probings and has not been corrected for the emission background. The small peak at approximately 1500 ms is due to 238 U fluorescence excited by a small side mode of the diode laser. 238
U, but for the structured 235 U line, one needs to establish the relationship between the intensity at the maximum of the largest hf peak and the area of the entire ensemble of hf components. Fortunately, this relationship is easy to establish, by simple calibration with a certified reference sample, and should remain a constant for a particular measurement system. Fig. 9 shows the results of such a time-integrated measurement obtained with a total of 120 laser probings, on the sample of natural isotopic composition. Five probings were averaged for each data file acquisition, alternating measurements of emission Žemission q235 U fluorescence. and Žemission q238 U fluorescence.. The 238 Ur 235 U ratio, based on the areas of these two time-integrated peaks is 1978 " 130 Ž6.5% relative.. Obviously, the intensity of the 235 U component has been under-estimated Žby approximately 14-fold. because the laser is interacting with only one of the hf components and, in any case, is not necessarily tuned to the peak of the most intense feature of the hf spectrum.
The time-integrated measurement approach provided somewhat better precision than the fast scanning approach and was therefore applied to the determination of the isotopic composition of the two reference samples which were depleted in 235 U concentration. Sample pellets with concentrations of 0.204, 0.407 and 0.714% uranium were studied, using the 0.714% Žnatural . sample as a standard. Table 1 summarizes the results. As expected, the precision improved with increasing 235 U concentration, from 27% at 0.204% 235 U to 7% at 0.714% 235 U. Both depleted samples yielded concentration values for 235 U which were low by an average of approximately 7% which is within the precision of the experimental values. The limit of detection, for either isotope, for the time-integrated approach is determined by the uncertainty in the correction for the continuum background emission. Based upon the data shown in Fig. 9 Ž120 laser shots., the limit of detection Ž3s . for either isotope is approximately 0.6 mg gy1 . The signal for the minor isotope shown in Fig. 9 is due to 3.8 mg 235 U gy1 sample.
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
Fig. 9. Time-dependent atomic fluorescence of average of 120 laser shots at 0.5 mJ per pulse.
235
U and
238
U from a sample of natural isotopic composition. Each signal is the
One would expect the limit of detection for the minor isotope to be poorer because the laser is interacting with only one component of the hf spectrum. It was impossible, in the present experiment, to verify this behavior because the concentration of the major isotope Ž52%. in the sample pellets was so far above the limit of detection. 4. Conclusions Isotopically selective measurements of uranium by diode laser excited fluorescence in a laser-induced plasma have been demonstrated. Two experimental approaches were evaluated: Ži. fast Table 1 Results for the analysis of
235
U-depleted samples
Certified w235 Ux
Measured w235 Ux
R.S.D. Ž%.
Error Ž%.
No. laser probings
0.714% 0.407% 0.204%
Standard 0.39" 0.05 0.18" 0.05
7 13 27
] y4 y9
110 170 190
955
scanning of the diode laser wavelength across the entire spectral interval between the 235 U and 238 U isotopes Žf 17.7 pm. with the detection of LIF signals of both isotopes during every laser probing; and Žii. time-integrated separate detection of LIF signals from the individual isotopes in sequential laser probings. In the scanning mode, one has to account for the expanding plasma and hence the exponential decay of the uranium atomic population density within the measurement volume. In the time-integrated approach, the diode laser is fixed at the absorption maximum of either isotope on alternate laser probings. In either mode, one has to account for the slow degradation of the stationary sample surface with the number of laser shots. Both factors are important for accurate measurement of the isotope ratio and can be adequately accounted for by simple calibration with samples of known isotopic composition. Most of the experiments were carried out using a sample in the form of a pressed graphite pellet containing 60% natural uranium oxide. Back-
956
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
ground emission from the laser-induced plasma, which decreases with increasing number of laser shots, was found to be the limiting influence upon the precision. Optimal experimental conditions for LIF detection of uranium in the laser plasma were: 0.86-mbar Ar pressure, 0.8-cm measurement distance from the sample surface, and 0.5-mJ pulse energy Ž880 MW cmy2 . of the ablating Nd:YAG laser. An estimate of the material removal at this laser energy gave 80 ng for a single shot, 350 ng for 10 shots and 1000 ng for 100 shots. The hfs of the 235 U isotope in natural UO 2 Žnom. 0.714% 235 U. can be detected on a singleshot basis, under the optimal experimental conditions, using the scanning mode. The accuracy for the isotope ratio measurement, using the time-integrated data acquisition approach, is approximately 7%, for samples which are depleted in 235 U. The precision may be slightly better for the time-integrated approach Ž6.5% for 120 shots vs. 5% for 460 shots. and the limits of detection for either isotope are similar, f 0.6 mg gy1 which is approximately two times lower than the limit of detection obtained in the scanning mode. The accuracy and precision of either approach is expected to improve for 235 U-enriched samples and for increasing number of laser probings. The main drawback of the time-integrated approach, using sequential measurements with fixed diode laser wavelengths, is the partial excitation of a specific hf component of the 235 U by the narrow-band diode laser Ž Dl L - Dl hfs .. The easiest way to avoid the problem is the calibration of the experimentally measured intensity ratios in terms of isotope content using one sample with certified isotope concentration. This measurement approach, using a simple sample geometry, diode laser excitation and laser sampling with a modest pulse energy Nd:YAG laser, should have applicability as a portable instrument for routine screening of isotopic composition of uranium samples in the field and should be extendable to other actinide elements. It compares very favorably with alternate techniques having a similar goal, such as laser-induced plasma emission and optogalvanic spectroscopy.
Acknowledgements The authors acknowledge the valuable experimental assistance of M. Schuth. ¨ One of the authors ŽB.W.S.. acknowledges gratefully the financial support of the Deutsche Forschungesgemeinschaft for a visiting professor fellowship. We also are grateful to Dr Glatz of the Institute of Transuranium Elements ŽKarlsruhe, Germany. for providing the uranium samples. References w1x D.A. Rusak, B.C. Castle, B.W. Smith, J.D. Winefordner, Recent trends and the future of laser-induced plasma spectroscopy. TrAC 17 Ž1998. 453]461. w2x R.M. Measures, N. Drewell, H.S. Kwong, Atomic lifetime measurements obtained by the use of laser ablation and selective excitation spectroscopy. Phys. Rev. A 16 Ž1977. 1093]1097. w3x R.M. Measures, H.S. Kwong, TABLASER: trace Želement. analyzer based on laser ablation and selectively excited radiation. Appl. Opt. 18 Ž1979. 281]286. w4x H.S. Kwong, R.M. Measures, Trace element laser microanalyzer with freedom from chemical matrix effect. Anal. Chem. 51 Ž1979. 428]432. w5x I.B. Gornushkin, S.A. Baker, B.W. Smith, J.D. Winefordner, Determination of lead in metallic reference materials by laser ablation combined with laser excited atomic fluorescence. Spectrochim. Acta 52B Ž1997. 1653]1662. w6x I.B. Gornushkin, J.E. Kim, B.W. Smith, S.A. Baker, J.D. Winefordner, Determination of cobalt in soil, steel, and graphite using excited-state laser fluorescence induced in a laser spark. Appl. Spectrosc. 51 Ž1997. 1055]1059. w7x A.L. Lewis II, G.J. Beenen, J.W. Hosch, E.H. Piepmeier, A laser microprobe system for controlled atmosphere time and spatially resolved fluorescence studies of analytical laser plumes. Appl. Spectrosc. 37 Ž1983. 263]269. w8x A.L. Lewis II, E.H. Piepmeier, Chemical and physical influences of the atmosphere upon the spatial and temporal characteristics of atomic fluorescence in a laser microprobe plume. Appl. Spectrosc. 37 Ž1983. 523]530. w9x W. Sdorra, A. Quentmeier, K. Niemax, Basic investigations for laser microanalysis: II. Laser-induced fluorescence in laser-produced sample plumes. Mikrochim. Acta II Ž1989. 201]218. w10x K. Niemax, W. Sdorra, Optical emission spectrometry and laser-induced fluorescence of laser produced sample plumes. Appl. Opt. 29 Ž1990. 5000]5006. w11x Y. Oki, K. Matsunaga, T. Nomura, M. Maeda, Removal of thin layer for trace element analysis of solid surface in subnanometer scale using laser-ablation atomic fluorescence spectroscopy. Appl. Phys. Lett. 71 Ž1997. 2916]2918.
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958 w12x T. Bieniewski, Absolute oscillator strengths for neutral atomic uranium. J. Opt. Soc. Am. 68 Ž1978. 1173]1181. w13x R. Kapoor, G.D. Saksena, Relative transition probabilities of neutral uranium transitions. J. Opt. Soc. Am. B 6 Ž1989. 1623]1626. w14x L.E. Burkhart, G.L. Stukenbrocker, S. Adams, Isotope shifts in uranium spectra. Phys. Rev. 75 Ž1949. 83]85. w15x J.A. Goleb, Uranium isotope investigations by atomic absorption. Anal. Chem. 35 Ž1963. 1978. w16x Thermo Jarrell Ash Corporation, Guide to Analytical Values for TJA Spectrometers, TJA Corp., Franklin, MA, 1988. w17x D.M. Goltz, D.C. Gregoire, J.P. Byrne, C.L. Chakrabarti, Vaporization and atomization of uranium in a graphite tube electrothermal vaporizer: a mechanistic study using electrothermal vaporization inductively coupled plasma mass spectrometry and graphite furnace atomic absorption spectrometry. Spectrochim. Acta 50B Ž1995. 803]814. w18x A.R. Date, A.L. Gray ŽEds.., Inductively Coupled Plasma Mass Spectrometry, Chapman and Hall, New York, NY, 1989. w19x R.K. Winge, V.J. Petterson, V.A. Fassel, Inductively coupled plasma atomic emission spectroscopy: prominent lines. Appl. Spectrosc. 33 Ž1979. 206]219. w20x P.W.J.M. Boumans, J.J.A.M. Vrakking, Detection limits of about 350 prominent lines of 65 elements observed in 50 and 27 MHz inductively coupled plasmas ŽICP.: Effects of source characteristics, noise, spectral bandwidth } standard values for the 50 MHz ICP. Spectrochim. Acta 42B Ž1987. 553]579. w21x E.L. DeKalb, J.G. Douglas, M.C. Edelson, V.A. Fassel, An ICP Spectral Atlas for Uranium, Ames Laboratory Report, IS-4979, UC-15, Ames Laboratory, 1988. w22x M.C. Edelson, V.A. Fassel, Isotopic abundance determinations by inductively coupled plasma atomic emission spectrometry. Anal. Chem. 53 Ž1981. 2345]2347. w23x P.S. Goodall, S.G. Johnson, Isotopic uranium determination by inductively coupled plasma atomic emission spectrometry using conventional and laser ablation sample introduction. J. Anal. Atom. Spectrom. 11 Ž1996. 57]60. w24x J.A. Vera, G.M. Murray, S.J. Weeks, M.C. Edelson, Isotopic abundance determinations by inductively coupled plasma high resolution laser excited atomic and ionic fluorescence spectroscopy. Spectrochim. Acta 46B Ž1991. 1689]1700. w25x P.G. Russ III, J.M. Bazan, Isotopic ratio measurements with an inductively coupled plasma source mass spectrometer. Spectrochim. Acta 42B Ž1987. 49]62. w26x A. Montaser, D.W. Golightly ŽEds.., Inductively Coupled Plasmas in Analytical Atomic Spectrometry, VCH Publishers, Inc., New York, NY, 1992. w27x M. Fukuda, Y. Sayama, Determination of traces of uranium and thorium in antimonyŽIII. oxide by ICP-MS. Bunseki Kagaku 44 Ž1995. 719]723. w28x J.G. Sengupta, N.B. Bertrand, Direct ICP-MS determi-
w29x
w30x
w31x
w32x
w33x w34x w35x
w36x
w37x w38x
w39x
w40x w41x w42x
957
nation of trace and ultratrace elements in geological materials after decomposition in a microwave oven: Quantitation of Y, Th, U and the lanthanides. Talanta 42 Ž1995. 1595]1607. J.H. Aldstadt, J.M. Kuo, L.L. Smith, M.D. Erickson, Determination of uranium by flow-injection inductively coupled plasma mass spectrometry. Anal. Chim. Acta 319 Ž1996. 135]143. M. Hollenbach, J. Grohs, S. Mamich, M. Droft, E.R. Denoyer, Determination of technetium-99, thorium-230 and uranium-234 in soils by inductively coupled plasma mass spectrometry using flow injection preconcentration. J. Anal. Atom. Spectrom. 9 Ž1994. 927]933. M. Gastel, J.S. Becker, G. Kuppers, H.-J. Dietze, De¨ termination of long-lived radionuclides in concrete matrix by laser ablation inductively coupled plasma mass spectrometry. Spectrochim. Acta 52B Ž1997. 2051]2059. G. Rossi, M. Mol, Isotopic analysis of uranium by an optical spectral method. III: determination of uranium235ruranium 238 ratios with a hollow cathode source and a direct reading attachment. Spectrochim. Acta 24B Ž1969. 389]398. R.A. Keller, R. Engleman, Jr., E.F. Zalewski, Optogalvanic spectroscopy in a uranium hollow cathode discharge. J. Opt. Soc. Am. 69 Ž1979. 738]742. J.M. Gagne, ´ P. Pianarosa, G. Larin, J.P. Saint-Dizier, P. Bouchard, Ionization and excitation of uranium in a hollow cathode lamp. Appl. Opt. 20 Ž1981. 3770]3773. N.J. Dovichi, D.S. Moore, R.A. Keller, Use of the optogalvanic effect and the uranium atlas for wavelength calibration of pulsed lasers. Appl. Opt. 21 Ž1982. 1468]1473. C.J. Sansonetti, K.-H. Weber, Reference lines for dye laser wave-number calibration in the optogalvanic spectra of uranium and thorium. J. Opt. Soc. Am. B 1 Ž1984. 361]365. P. Pianarosa, Y. Demers, J.M. Gagne, ´ Isotope analysis by optogalvanic spectroscopy. J. Opt. Soc. Am. 1 Ž1984. 704]709. C.M. Barshick, R.W. Shaw, J.P. Young, J.M. Ramsey, Isotopic analysis of uranium using glow discharge optogalvanic spectroscopy and diode lasers. Anal. Chem. 66 Ž1994. 4154]4158. C.M. Barshick, R.W. Shaw, J.P. Young, J.M. Ramsey, Evaluation of the precision and accuracy of a uranium isotopic analysis using glow discharge optogalvanic spectroscopy. Anal. Chem. 67 Ž1995. 3814]3818. J.R. Wachter, D.A. Cremers, Determination of uranium in solution using laser induced breakdown spectroscopy. Appl. Spectrosc. 41 Ž1987. 1042]1048. E. Stoffels, P. Van de Weijer, J. Van der Mullen, Timeresolved emission from laser-ablated uranium. Spectrochim. Acta 46B Ž1991. 1459]1470. W. Pietsch, A. Petit, A. Briand, Isotope ratio determination of uranium by optical emission spectroscopy on a laser-produced plasma } basic investigations and analytical results. Spectrochim. Acta 53B Ž1998. 751]761.
958
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
w43x M.W. Pietsch, Laser-Produced Plasma at Reduced Pressure for Analytical Isotope Control, Doctoral Dissertation, Universite 1994. ´ D’Orleans, ´ w44x B.W. Smith, I.B. Gornushkin, L.A. King, J.D. Winefordner, A laser ablation-atomic fluorescence technique for isotopically selective determination of lithium in solids. Spectrochim. Acta 53B Ž1998. 1131]1138. w45x L. King, I. Gornushkin, B.W. Smith, J.D. Winefordner, Rubidium isotope measurements in solid samples by laser ablation-laser atomic absorption spectrosocopy, Spectrochim. Acta 54B Ž1999. in press. ´ w46x S. Gerstenkorn, Etude du plutonium par spectroscopie a haute resolution: I. } Contribution a la classification ´ du spectre d’arc. Ann. Phys. 7 Ž1962. 367]404.
w47x E.W.T. Richards, A. Ridgeloy, Preliminary classification in the plutonium I spectrum. Spectrochim. Acta 21 Ž1965. 1449]1466. w48x A. Zybin, K. Niemax, Improvement of the wavelength tunability of etalon-type laser diodes and mode recognition and stabilization in diode laser spectrometers. Spectrochim. Acta 52B Ž1997. 1215]1221. w49x J. Blaise, L.J. Radziemski, Jr., Energy levels of neutral atomic uranium ŽUI.. J. Opt. Soc. Am. 66 Ž1976. 644]659. w50x R. Engleman, Jr., B.A. Palmer, Precision isotope shifts for the heavy elements. I. Neutral uranium in the visible and near infrared. J. Opt. Soc. Am. 70 Ž1980. 308]317.
Measurement of uranium isotope ratios in solid samples using laser ablation and diode laser-excited atomic fluorescence spectrometry q B.W. Smith1 A. Quentmeier U , M. Bolshov, K. Niemax Institute for Spectrochemistry and Applied Spectroscopy (ISAS), Bunsen-Kirchhoff-Strasse 11, D-44139 Dortmund, Germany Received 3 November 1998; accepted 4 February 1999
Abstract A diode laser is used for the selective excitation of 235 U and 238 U in a laser-induced plasma applying Nd:YAG laser pulses to UO 2 samples. The diode laser is rapidly scanned immediately following each laser sampling and the resonance atomic fluorescence spectrum for both isotopes is obtained on a pulse-to-pulse basis. Time-integrated measurements, with the diode laser fixed at either isotope, were also made. Optimum signal-to-noise was obtained at a distance of 0.8 cm from the sample surface, a pressure of 0.9 mbar and a Nd:YAG laser pulse energy of 0.5 mJ Ž880 MW cmy2 .. Three samples with 0.204, 0.407 and 0.714% 235 U were measured. For example, for the UO 2 pellet with the natural uranium isotopic composition Ž99.281% 238 U and 0.714% 235 U., the accuracy and precision were 7% and 5% Ž460 shots., respectively, limited by the continuum emission background from the laser-induced plasma. Q 1999 Elsevier Science B.V. All rights reserved. Keywords: Laser ablation; Laser-excited fluorescence; Diode laser spectrometry; Isotope ratio; Uranium
1. Introduction The most widely used analytical techniques for isotopically selective detection tend to be large, complex and expensive, e.g. inductively coupled
q
Dedicated to Prof. Dr. Dieter Klockow on the occasion of his 65th birthday. U Corresponding author. Fax: q49-231-1392120. 1 On leave from the Department of Chemistry, University of Florida, Gainesville, FL 32611, USA.
plasma mass spectrometry ŽICPMS. and thermal ionization mass spectrometry ŽTIMS.. The goal of the present work was to develop an approach for isotopically selective detection of 235 U and 238 U which would provide a useful level of accuracy and precision in a compact, portable instrument, with the possibility of remote, or robotic measurements in hostile environments. We have therefore evaluated diode laser excited atomic fluorescence combined with pulsed laser sampling. Many years of intensive investigations of
0584-8547r99r$ - see front matter Q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 9 9 . 0 0 0 2 2 - 1
944
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
laser]material interactions has led to a wide range of applications in many fields. As an approach to microsampling in analytical chemistry, laser ablation is now widely used as a sample introduction technique for ICP emission and mass spectrometry. By far the most studied analytical use has been the direct observation of emission from the laser-induced plasma, a technique generally known as laser-induced breakdown spectroscopy, LIBS w1x. However, as was demonstrated as early as 1977 by Measures et al., the laser-induced plasma can serve as a very efficient and useful atomic reservoir for laser excited atomic fluorescence measurements w2x. Measures and Kwong, using a ruby laser for sampling and a pulsed dye laser for fluorescence excitation, reported a freedom from chemical matrix effects when the measurements were carried out at low pressure Žapprox. 10y3 mbar. w3,4x. Similar results were reported by Gornushkin et al. for the determination of Pb in copper, brass, steel and zinc samples w5x and Co in soils, steels and graphite w6x. Piepmeier and co-workers used laser excited fluorescence to study the distribution of species in the laser plasma in argon and oxygen atmospheres w7,8x. In our laboratory, we have made a direct comparison of fluorescence and emission detection for the determination of Si and Cr in steel w9x. We also have reported absolute limits of detection for seven elements, ranging from 4 to 600 fg, for a single shot measurement on steels samples w10x. Using a 193-nm excimer laser, Oki et al. w11x have measured Na in glasses using atomic fluorescence detection. They reported a limit of detection of 0.6 fg per shot with a spatial Ždepth. resolution of 1.1 nm per shot. The general conclusion to be drawn from the preceding literature is that improved accuracy and precision, as well as much lower detection limits, can be expected for laser excited fluorescence compared to emission in laser-induced plasmas. Successful atomic fluorescence results have been reported for both UV and IR laser sampling, although it is impossible to categorically recommend laser ablation in either wavelength domain because of the many environment and matrix-dependent variables. Uranium has a complex spectrum which has
been well studied w12,13x and atomic spectroscopic methods for its detection have been used for many years. Among the most compelling reasons for a spectroscopic approach to the detection of uranium is the potential for very high selectivity and especially, the capability to resolve individual isotopes. Atomic emission spectroscopy has been used for at least 50 years for uranium isotopic determinations w14x. With modern instrumentation, isotopes with low specific activity can often be detected at lower levels than achievable with radiochemical techniques. Uranium is not atomized efficiently in low energy plasmas, such as flames, and also has a poor free atom fraction in the graphite furnace. Consequently, it is not detectable to any useful degree by any flame or furnace atomic emission or absorption method. Nevertheless, isotopic selectivity was demonstrated by Goleb w15x using monoisotopic hollow cathode lamps as early as 1963. The characteristic mass for uranium by electrothermal atomization atomic absorption spectrometry is only 3 ng, worse than all other elements with the exception of phosphorous w16x. The poor atomization efficiency in the furnace is thought to be due to the formation of refractory carbides and oxides w17x. Having a relatively low ionization potential Ž6.08 eV., uranium is completely ionized when introduced into higher power plasma sources such as the inductively coupled plasma w18x. The most sensitive ion emission line Ž385.96 nm. yielded a limit of detection of 250 ng mly1 in the early comprehensive study of ICP emission spectrometry by Winge and co-workers w19x. In the tables published by Boumans and Vrakking w20x in 1987, comparing limits of detection in 27 MHz and 50 MHz plasmas, a limit of detection of 16 ng mly1 was listed for the same spectrometer spectral bandwidth used by Winge Ž15 pm.. An atlas of the ICP uranium spectrum has been produced by DeKalb and co-workers w21x. Uranium exhibits significant isotope shifts which permits isotopic selectivity by ICP emission in some cases. Edelson and Fassel w22x showed that the 238r236r235 isotopes can be resolved for the 424.4-nm uranium ion line in the ICP using a spectrometer having a resolving power of approximately 250 000. They observed a shift of approximately 10 pm
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
between the emission peaks of the three isotopes. More recently, Goodall and Johnson w23x have used both solution nebulization and laser ablation sampling with ICP emission to determine the isotopic ratio of 238 Ur 235 U. Isotopic selectivity has also been achieved using laser excited fluorescence of the uranium ion in the ICP. Using a pulsed dye laser, Vera and co-workers w24x excited the 286.57-nm ion line and observed the fluorescence at the 288.96-nm line, obtaining a detection limit of 2 mg mly1 and resolving the 235r238 isotopes in a complex matrix. The most successful use of the ICP for the detection of uranium has been in its coupling with mass spectrometric detection. Accurate isotopic selectivity was demonstrated early in the development of the method. Russ and Bazan w25x were able to determine the 235r238 ratio with a precision of better than 0.5% with only 50 ng of material. This determination is commonly done using TIMS with better precision and smaller samples but takes more than 4 h of measurement time and requires extensive sample preparation. There have been numerous applications of ICPMS to the detection of uranium in recent years. Limits of detection are among the best reported for the method, sometimes below 0.1 pg mly1 . More typical limits of detection are in the vicinity of 0.02 ng mly1 w26x. For example, Fukuda and Sayama w27x have determined traces of uranium in antimony oxide at this level. Sengupta and Bertrand w28x have reported similar detection limits for geological samples which were processed by a microwave digestion procedure. A flow injection procedure has been used for the determination of 238 U in groundwater with a limit of detection of 0.3 pg mly1 w29x. Hollenbach et al. w30x used a flow injection technique as a preconcentration mechanism and obtained a limit of detection of 0.03 ng gy1 for 234 U in soils. Gastel et al. w31x have used laser-ablation ICP-MS for the analysis of longlived radionuclides in concretes and reported a limit of detection for 233 U of 0.6 ng gy1 . Among the commercially available spectroscopic laboratory methods for uranium determination, ICPMS will undoubtedly continue to be one of the most useful. The loss of detection power caused by the need to dissolve solid samples is generally more
945
than compensated for by the uniquely low limits of detection. The primary drawback of the method, aside from the complexity of sample dissolution, is that the instrumentation is relatively expensive, and, over time will become contaminated with gradual accumulation of radioactive species. In relation to the present work, it is important to mention the use of optogalvanic measurements for the determination of uranium. The spectroscopy of uranium in low pressure discharges has been studied and applied for many years. For example, Rossi and Mol w32x used a high resolution direct reading spectrograph to determine 235 Ur 238 U ratios from hollow cathode lamp emission. The observation of the optogalvanic spectrum of uranium was reported as early as 1979 by Keller et al. w33x who investigated the applicability of the technique for standard spectroscopic measurements, the determination of oscillator strengths, the measurement of fundamental discharge characteristics, and isotopic ratio analysis. They reported that as few as 10 8 atoms cmy3 could be detected. Gagne ´ et al. w34x studied the fundamental aspects of the uranium glow discharge and its optogalvanic characteristics. The optogalvanic effect uranium spectrum has been suggested as a useful wavelength calibration device for tunable lasers w35,36x. In 1984, Gagne ´ et al. w37x demonstrated, using a cw dye laser at the well-studied 591.54-nm uranium transition, the measurement of isotopic ratios for the 235r238, 234r238 and 236r235 isotopes in natural and enriched uranium. They estimated a limit of detection of 10 6 atoms cmy3 . More recently, this approach has been successfully carried out by Barshick et al. using a diode laser as the excitation source w38,39x. A precision ranging from 2.4 to 30% was attained, depending upon the level of enrichment, for the 235r238 isotope ratio, which is sufficient for screening applications. A transition at 831.84 nm was used with a 42-mW diode laser. Spectroscopic measurements in the glow discharge Žapprox. 2.7 hPa. are appealing because of the reduced spectral linewidth and consequently improved resolution of isotopic fine structure. The approach taken by Barshick et al. has the added advantage of being simple and poten-
946
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
tially portable, although the sample must be formed into a shape which is compatible with the glow discharge device. This may involve machining of the sample or grinding and mixing it with a conductive binder such as silver which can be pressed into a cathode. Laser-induced breakdown spectroscopy has been used for the determination of uranium in aqueous solutions, without any isotopic selectivity. Wachter and Cremers w40x reported a limit of detection of 100 mg ly1 for uranium using a laser breakdown on the liquid surface. Van der Mullen et al. reported on the time-resolved emission behavior of a laser plasma on uranium in argon and air w41x. Recently, emission spectroscopic detection from a 308-nm laser-induced plasma has been used for the detection of the uranium 235r238 isotopes by Pietsch and colleagues w42,43x. At a pressure of 2.67 Pa Žair., the UII 424.437-nm transition was detected with a spectrometer having a spectral resolution of 5.5 pm. A precision of 5% in the isotope ratio determination was demonstrated for an enriched 235 U concentration Ž3.5%., by averaging of 20 000 laser samplings. Isotopically selective absorption and fluorescence detection of other elements in laser-induced plasmas have also been recently described by King et al. w44,45x. They reported the measurement of 6 Lir 7 Li with UV laser sampling and atomic fluorescence detection using a pulsed dye laser for excitation w44x. Measurements of 87 Rbr 86 Rb were made using a cw Ti:sapphire laser for atomic absorption measurements with laser sampling by a 1064-nm Nd:YAG laser w45x.
Diode lasers make particularly attractive excitation sources for high resolution spectroscopy of uranium. They can access numerous uranium transitions in the red region of the spectrum with sufficient spectral power density to saturate, they have spectral linewidths well below those of the uranium atom lines found in laser-produced plasmas, they can be rapidly and accurately tuned and they are moreover, compact, reliable and quite stable. Other actinide elements are also accessible in this spectral range we.g. PuŽI. 648.8853 nm with an isotope shift 240 Pu] 239 Pu of 0.425 cmy1 x w46,47x for which rapid, portable isotopic detection may prove valuable. In the present work, we have combined diode laser excited atomic fluorescence with laser sampling for the selective detection of 235 U and 238 U. 2. Experimental Fig. 1 shows a schematic diagram of the experimental system. A Nd:YAG laser ŽSpectron model SL801. operating at the fundamental wavelength of 1064 nm was used as the ablation sampling source. Pulse repetition rates ranging from 1 to 10 Hz, with energies of up to 10 mJ, were typically used. The Nd:YAG beam was directed to the sample chamber by several mirrors and focused by a lens of 12-cm focal length onto the sample. The laser spot size at the sample was a nominal 85 mm. The laser pulse energy at the sample was varied as necessary by using calibrated glass filters or by changing the high voltage
Fig. 1. Schematic diagram of the experimental set-up.
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
947
Fig. 2. Design of the sampling chamber.
of the laser flashlamp, over a limited range. The sample chamber, shown in Fig. 2, was a simple three-axis aluminum cell with two perpendicular paths for optical access though 2-cm diameter fused silica windows. The diode laser beam passed along the vertical axis through the center of the chamber, intersecting the Nd:YAG beam in the plane of detection. The third axis provided a window for the Nd:YAG laser beam and, opposite to it, an easily removable port for the sample probe. The sample could be independently adjusted along the axis of the sampling laser via a rotatable vacuum feedthrough, thereby allowing measurements to be made at any distance from the sample surface. Independent movement of the laser focusing lens permitted its adjustment in coordination with the displacement of the sample. The chamber was fitted with additional ports for vacuum, vacuum measurement ŽMKS Instruments capacitance manometer, model PR-2000. and introduction of 99.999 grade argon. Most of the measurements were carried out using a sample which consisted of a pressed, 6-mm diameter graphite pellet containing 60% natural UO 2 . The diode laser, with a short external cavity, has been recently described in detail elsewhere w48x. The laser diode ŽType ML1013R-01, nominal 684 nm, 50 mW, Mitsubishi Electric Corp.., with its housing can and window removed, was mounted in an aluminum case, 18 = 10 = 8 cm. A thin quartz interferometer Ž70 mm. mounted very close to the laser diode chip provided a weak optical feedback thereby suppressing the domi-
nate longitudinal mode and allowing the laser to operate in a selected nearby longitudinal mode. Operated in this way, the laser is stable over long periods of time and tunes reliably over a more extended wavelength range. After a 1-h warm-up time and initial tuning, the laser would remain within the absorption linewidth of the 238 U line for an entire working day. The laser was powered by a combination temperature controller and current source ŽModel ITC502, Profile Optische Systeme GmbH, Karlsfeld, Germany.. Preliminary optimization of the wavelengthrtemperature] current behavior was carried out using a vacuum interferometric wavemeter ŽATOS Lamdameter, model LM007.. The diode laser current source was externally modulated with a triangle wave using a function generator ŽWavetek, model FG5000. which was triggered by the master pulse generator Žmodel DG535, Stanford Research Systems. used to trigger the Nd:YAG laser. In this way, the diode laser wavelength could be scanned over an adjustable, known wavelength interval at a variable rate, synchronized with the firing of the Nd:YAG sampling laser. The diode laser output power at the 682.69-nm uranium line, at the sample chamber, was 15 mW at the typical operating conditions of 82 mA and 27.908C, which was sufficient to saturate the 238 U transition with a 0.1-cm beam diameter. The wavelength]current relationship Ž7.15 pm mAy1 . was linear over the necessary range of current Ž79]82 mA.. The atomic fluorescence was detected in two ways. In most of the experiments to be described,
948
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
the fluorescence was collected by a single lens Žfr3, 7.5-cm focal length. and imaged Ž1:1. onto the entrance slit Ž1 mm. of a small monochromator ŽJ-Y model H20. fitted with a photomultiplier tube ŽHamamatsu R928.. The current output of the photomultiplier tube was passed through a laboratory-constructed transimpediance amplifier which had two output channels, a low gain, high bandwidth output Ž10 5 VrA, 1-MHz bandwidth. and a higher gain, restricted bandwidth output Ž10 7 VrA, 100-kHz bandwidth.. This provided the necessary dynamic range for measurement of both isotope signals simultaneously when desired. The signal was then recorded with a digital oscilloscope ŽTektronix model TDS360. and transferred to a spreadsheet for final analysis. The fluorescence signal was also detected in some cases with a compact module consisting of a collimating lens Ž5-cm focal length., narrowband interference filter Ž682 nm, 1 nm FWHM, 55%T., focusing lens Ž2.6-cm focal length. and photodiode detector Žtype S2386-5K7E, Hamamatsu.. Once the experiment was optimized, the two systems were virtually identical in performance but the version with the monochromatorrPMT proved more versatile for the optimization studies. However, the compact photodiode module, which mounted directly on the port of the sample chamber, was better suited for use in a simple, portable instrument. A trigger for the oscilloscope was provided by a fast photodiode ŽLambda Physik, model LF302 UV. mounted near the Nd:YAG focusing lens. For some atomic absorption experiments, which were used to assist in the optimization, the diode laser beam, appropriately attenuated to avoid saturation of either the detector or the atomic population, was simply detected with a photodiode or PMT, placed a short distance from the sample chamber. Samples, provided by the Institute of Transuranium Elements ŽKarlsruhe, Germany., were prepared as 60% by weight, uranium oxide with graphite, pressed in pellets of 0.6-cm diameter and varying thickness. When fresh samples were inserted into the vacuum chamber, it was evacuated to below the minimum pressure which could be measured by our vacuum sensor Ž0.01 mbar., typically requiring approximately 20 s, flushed
once with argon and finally evacuated to the operating pressure. All measurements were made under conditions of continuous pumping and gas flow, at constant pressure. The complex uranium spectrum provides several good transitions in the spectral region accessible to the fundamental output of commercially available diode lasers. We selected the transition from the ground state to 14643.867 cmy1 at 682.69 nm Ž 5 L 68] 7 M 6 . based on a reasonably strong transition probability ŽgAs 2.23= 10 7 sy1 . and rather large isotope shift Žy0.380 cmy1 . w49,50x. The fluorescence spectrum consists of a single line at 682.6913 nm Žair. for 238 U, and a hyperfine structure Žhfs. multiplet with seven predominant components, centered at 682.6736 nm, for 235 U. Direct line fluorescence to the 620.320 cmy1 level, at 712.9 nm, was observed, but since this line is weaker and scattered laser radiation was negligible at 682.69 nm, the resonance fluorescence measurement was preferred. 3. Results and discussion Two different techniques were used for the detection of the LIF signals: Ži. a fast wavelength scanning of the diode laser during each individual laser sampling; and Žii. a time-integrated measurement where the diode laser wavelength was fixed at the peak of either isotope line on alternate laser samplings. In either approach, one must account for the gradual degradation of the sample with time and consequent variation in laser]sample interaction as the measurement progresses. Data must be background-corrected in some quasi-continuous fashion during the entire measurement interval. While experimentally demanding, this strategy would always be likely to provide better precision than measurements averaged over a large number of laser shots because of the inevitable variation in sampling due to the gradual modification of the sample surface by the laser, as the measurement progresses. It is possible to control this effect, to some extent, by moving the sample, either laterally or rotationally, to present a fresh sample surface to each laser pulse. However, this leads to additional complexities because of the constraints it places
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
on the size and surface character Žhomogeneity and morphology. of the sample. Our goal was to provide a simple, fixed sample presentation with few restrictions on sample geometry. By making the measurements in this way, the laser]surface interaction naturally will change as the surface is modified during the course of several tens or hundreds of laser probings. The influence of this surface modification on the isotope ratio measurement can be accounted for by acquiring data from each individual laser probing Žor a very few number of probings.. The fast wavelength scanning approach will be discussed first. 3.1. Wa¨ elength scanning technique Fig. 3 illustrates this measurement approach under conditions of optimum Nd:YAG pulse energy, pressure and measurement distance from the sample. Within a few microseconds after the arrival of the Nd:YAG pulse at the sample, one observes, at a distance of 0.8 cm from the sample surface, a background emission composed of underlying continuum and dense, broadened ura-
949
nium line emission, coincident with the production of free uranium atoms, which lasts several hundred microseconds w t Ž1re. s 134 ms at 0.8 mbarx. Triggered by the firing of the Nd:YAG laser, the current to the diode laser is increased in a linear ramp, scanning the laser wavelength over the two isotope spectral lines as the uranium atomic population within the observation zone gradually decreases. The minor isotope Žnom. 0.714%. is scanned early in the plume lifetime, when the atomic population density is relatively high, while the major isotope Žnom. 99.281%. is probed approximately 1 ms later, when the population has declined significantly. In Fig. 3, one can clearly see the hfs of the 235 U line on top of a background of decaying continuum emission. The diode laser is scanning at a rate of approximately 0.016 pm msy1 , reaching the 238 U line 1.25 ms later. During this interval, the atom population has decreased, exponentially, by approximately 15 times. Note that the diode laser intensity increases, with increasing current, by approximately 2.75%. The signal-to-noise ratio obtained from the
Fig. 3. 235 U and 238 U atomic fluorescence spectrum with background emission from the ablation plasma. The linear ramp, linked to the right abscissa, indicates the tuning of the diode laser by increasing of the current.
950
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
uranium atomic fluorescence spectrum depends primarily on three variables: the measurement distance from the sample surface, the power density of the sampling laser and the gas pressure. Because of the interdependence of these variables, and the complex spatial]temporal development of the laser plasma plume and the uranium ground state atom population, a careful optimization was required. Preliminary diode laser absorption measurements showed that the highest ground state atom density occurred in the first several hundred microseconds after the ablation event, close to the sample surface Žwithin 0.2 cm. at pressures of 2]4 mbar. However, fluorescence measurements in this region are hindered by intense continuum emission from the laser-produced plasma. Ultimately, the optimization required finding the measurement location, pressure and Nd:YAG laser power density which provided the highest ground state atomic population with the least emission background Žwhich was the limiting source of noise for the measurement of the minor isotope.. Systematic measurements were made at sample distances ranging from 0.1 to 1 cm, pressures ranging from 0.01 to 10 mbar and Nd:YAG laser pulse energies ranging from 0.3 to 10 mJ Ž; 0.6]20 GW cmy2 .. Fig. 4 shows a portion of the resulting data, emission and fluorescence intensities as a function of argon pressure at distances of 0.3 and 0.6 cm from the sample surface. Both the emission and fluores-
cence are highly pressure dependent with an optimum in the fluorescenceremission ratio at 0.8]0.9 mbar and 0.8 cm from the sample surface. Fig. 5 shows a portion of the optimization for Nd:YAG pulse energy at 0.8 cm from the sample surface and 0.95 mbar. The Nd:YAG pulse energy was varied smoothly from 0.3 to 0.93 mJ by increasing the high voltage to the laser flashlamp. The last two spectra, at 2.9 and 3.6 mJ were obtained by removing an attenuating filter at the sample chamber. The uranium fluorescence intensity remains almost constant over this range of pulse energies, beginning to decline only at pulse energies below 0.4 mJ. However, the emission intensity continuously decreases with decreasing pulse energy. This may indicate that the amount of material removed from the sample is rather constant over this range of pulse energies, the decreasing emission being due to the reduced energy available to couple into the plasma. Ultimately, the optimum SrN ratio was obtained at a distance of 0.8 cm from the sample surface, 0.86 mbar pressure and 0.5 mJ Nd:YAG laser pulse energy. Under these conditions, the peak absorbance of the attenuated diode laser at the 238 U transition was 0.056 at the measurement time, 1.8 ms after the laser ablation event, well within the range of low atom density. In addition, during the fluorescence measurements, the diode laser spectral irradiance was well above the saturation spectral irradiance for the 238 U isotope. This was
Fig. 4. Emission and fluorescence intensity as a function of pressure at 0.3 and 0.6 cm from the sample surface.
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
951
Fig. 5. Emission and fluorescence intensity as a function of Nd:YAG laser pulse energy at 0.8 cm from the sample surface and 0.95 mbar.
checked experimentally by decreasing the diode laser intensity. A 10-fold decrease of the diode laser intensity caused only a twofold decrease of the 238 U fluorescence. The saturation of the 235 U line was smaller. A sevenfold decrease in laser intensity caused a decrease of the 235 U fluorescence of approximately 2.5. This is not unexpected, considering the hfs of the 235 U transition and the collisional mixing of the hfs levels involved. For the optimum pulse energy of 0.5 mJ, an estimate was made of the amount of material removed from the sample surface for 1, 10 and 100 successive shots. The material removal in each case was estimated by laser interferometric profilometry of the resulting craters and the mass removal was 80 ng per shot, 350 ng per 10 shots and 1000 ng per 100 shots. The initial laser pulse is the most effective at removing material at a fresh spot; as a crater is formed in the sample surface, the mass removed per shot declines. Because of the necessity to correct for the plasma emission background beneath the 235 U spectrum, data were obtained by collecting an emission spectrum Ždiode laser blocked., a spectrum of the emission plus fluorescence and then another emission spectrum. The two emission
spectra were averaged to obtain an estimate of the background during the fluorescence measurement, which was then subtracted from it. Although the spectrum of 235 U was detectable for individual ablation events, the SrN ratio was inadequate to provide for accurate background correction on a pulse-to-pulse basis. A study was made to determine the optimum number of laser shots which could be averaged while still providing an accurate background correction using the average of background spectra taken prior to and subsequent to the fluorescence data acquisition. If one averages too many samplings, 256 for example, for each of the three spectra, the variation in sampling character during the entire measurement period will result in a poor background correction. An optimum was found at 27 laser samplings for each spectrum. Fig. 6 shows a representative data acquisition under optimized conditions. It is the average of eight background-corrected measurements, a total of 17 spectra Žeight of emission q fluorescence and nine of emission only, each the average of 27 laser shots. and, therefore, a total of 460 laser samplings. The net fluorescence intensity is given in a logarithmic scale. The ratio of the areas, 238 Ur 235 U, is 5.22" 0.26 Ž"5%, relative.. Be-
952
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
Fig. 6. Background-corrected atomic fluorescence spectrum for a 60% natural uranium oxide sample diluted in graphite. Total of 460 laser shots at 0.5 mJ per pulse.
cause of the exponentially declining uranium atom population during the time required to scan the diode laser from one isotopic line to the other, two corrections need to be made to obtain an isotope ratio from the ratio of peak areas. The distribution of intensities over the hf components of the 235 isotope has been distorted because the uranium atom population declined exponentially during the 600-ms required to scan the diode laser over the transition. In addition, the uranium atom population continued to decline during the interval between the measurement of the two isotopes. This overall normalization factor of approximately 27 can be estimated fairly easily, for example, by fixing the diode laser on the peak of the 238 U transition and monitoring the atomic population decline, via the atomic fluorescence signal. The SrN is adequate to make such a measurement with good precision even on a single pulse basis and it could be done, for example, on every third laser sampling, in an automatic way, with an appropriate data acquisition system. A normalization for the decline in atomic population could also be done simultaneously on a shot-
to-shot basis using a second coincident diode laser tuned to the 238 U line and arranged to monitor the absorption at that wavelength. Fig. 7 shows the result of applying such a correction to the 235 U spectrum, in this case derived from an independent measurement of the time dependence of the atomic fluorescence signal. The declining atomic population is evident in the degradation in SrN as the laser scans to longer wavelengths. Finally, if the transition is not saturated, one would need to correct for the 2.75% increase in the intensity of the diode laser as the current is increased to tune it from 682.6736 to 682.6913 nm. The possibility of optical pumping within the hf spectrum was considered by examining the 235 U spectrum using a highly attenuated diode laser. Aside from poorer SrN, no significant influence on the intensity ratios of the hfs components was observed. From this finding one can conclude that collisional mixing of the hfs levels occur at the experimental pressure of 0.8 mbar. Of course, it is also possible to simply calibrate the measurement of the isotope ratio with suitable standards. For this purpose, we obtained two
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
953
Fig. 7. Spectrum of 235 U, corrected for the decline in atomic population during the scan interval. Average of 405 laser samplings under optimized measurement conditions. The central wavelength is determined from the calibration of the diode laser tuning behavior with current using a vacuum interferometer and an absolute calibration against the 238 U transition, approximately 17 pm to the red.
samples of uranium oxide with depleted 235 U concentration. These samples were prepared and certified in the Institute of Transuranium Elements ŽKarlsruhe .. The concentrations of 235 U were measured by isotope dilution mass spectrometry to 0.204% and 0.407% with relative standard deviation below 0.1%. Fig. 8 shows spectra, each the average of 300 laser probings, for these samples and a sample of natural isotopic composition. They have not been corrected for the emission background and are scaled such that the 238 U peaks, which are offscale in the figure, are of equal areas. The limit of detection for this fast scanning approach is different for each isotope because each has a different source of limiting noise. In addition, it is important to note that this experiment has been optimized for the detection of the minor isotope. The major isotope is detected from a diminished atomic population. For the minor isotope, the limiting uncertainty is due to the background emission, while for the major isotope,
it is due to a combination of laser scatter and detector dark current. Based upon the data shown in Fig. 6 Ž460 laser shots., the limit of detection Ž3s . for 235 U is 1.3 mg gy1. 3.2. Fixed wa¨ elength technique A completely different measurement approach was also evaluated. One can simply tune the diode laser to the peak of either isotope line on alternating laser samplings. This has the advantage of providing the complete time-integrated fluorescence signal for each isotope Žavoiding the need to know the decline in atomic population as one scans the diode laser. but requires taking data on alternate laser samplings. If the variation in mass removal between two adjacent shots is small enough, good precision can be attained. The other disadvantage of this approach is that one does not obtain the spectrum of the two isotopes, but only their peak intensities. This is not so important for the single isolated line of
954
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
Fig. 8. Spectra of uranium oxide samples with 235 U concentrations of 0.204, 0.407 and 0.714%. Each is the average of 300 laser probings and has not been corrected for the emission background. The small peak at approximately 1500 ms is due to 238 U fluorescence excited by a small side mode of the diode laser. 238
U, but for the structured 235 U line, one needs to establish the relationship between the intensity at the maximum of the largest hf peak and the area of the entire ensemble of hf components. Fortunately, this relationship is easy to establish, by simple calibration with a certified reference sample, and should remain a constant for a particular measurement system. Fig. 9 shows the results of such a time-integrated measurement obtained with a total of 120 laser probings, on the sample of natural isotopic composition. Five probings were averaged for each data file acquisition, alternating measurements of emission Žemission q235 U fluorescence. and Žemission q238 U fluorescence.. The 238 Ur 235 U ratio, based on the areas of these two time-integrated peaks is 1978 " 130 Ž6.5% relative.. Obviously, the intensity of the 235 U component has been under-estimated Žby approximately 14-fold. because the laser is interacting with only one of the hf components and, in any case, is not necessarily tuned to the peak of the most intense feature of the hf spectrum.
The time-integrated measurement approach provided somewhat better precision than the fast scanning approach and was therefore applied to the determination of the isotopic composition of the two reference samples which were depleted in 235 U concentration. Sample pellets with concentrations of 0.204, 0.407 and 0.714% uranium were studied, using the 0.714% Žnatural . sample as a standard. Table 1 summarizes the results. As expected, the precision improved with increasing 235 U concentration, from 27% at 0.204% 235 U to 7% at 0.714% 235 U. Both depleted samples yielded concentration values for 235 U which were low by an average of approximately 7% which is within the precision of the experimental values. The limit of detection, for either isotope, for the time-integrated approach is determined by the uncertainty in the correction for the continuum background emission. Based upon the data shown in Fig. 9 Ž120 laser shots., the limit of detection Ž3s . for either isotope is approximately 0.6 mg gy1 . The signal for the minor isotope shown in Fig. 9 is due to 3.8 mg 235 U gy1 sample.
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
Fig. 9. Time-dependent atomic fluorescence of average of 120 laser shots at 0.5 mJ per pulse.
235
U and
238
U from a sample of natural isotopic composition. Each signal is the
One would expect the limit of detection for the minor isotope to be poorer because the laser is interacting with only one component of the hf spectrum. It was impossible, in the present experiment, to verify this behavior because the concentration of the major isotope Ž52%. in the sample pellets was so far above the limit of detection. 4. Conclusions Isotopically selective measurements of uranium by diode laser excited fluorescence in a laser-induced plasma have been demonstrated. Two experimental approaches were evaluated: Ži. fast Table 1 Results for the analysis of
235
U-depleted samples
Certified w235 Ux
Measured w235 Ux
R.S.D. Ž%.
Error Ž%.
No. laser probings
0.714% 0.407% 0.204%
Standard 0.39" 0.05 0.18" 0.05
7 13 27
] y4 y9
110 170 190
955
scanning of the diode laser wavelength across the entire spectral interval between the 235 U and 238 U isotopes Žf 17.7 pm. with the detection of LIF signals of both isotopes during every laser probing; and Žii. time-integrated separate detection of LIF signals from the individual isotopes in sequential laser probings. In the scanning mode, one has to account for the expanding plasma and hence the exponential decay of the uranium atomic population density within the measurement volume. In the time-integrated approach, the diode laser is fixed at the absorption maximum of either isotope on alternate laser probings. In either mode, one has to account for the slow degradation of the stationary sample surface with the number of laser shots. Both factors are important for accurate measurement of the isotope ratio and can be adequately accounted for by simple calibration with samples of known isotopic composition. Most of the experiments were carried out using a sample in the form of a pressed graphite pellet containing 60% natural uranium oxide. Back-
956
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
ground emission from the laser-induced plasma, which decreases with increasing number of laser shots, was found to be the limiting influence upon the precision. Optimal experimental conditions for LIF detection of uranium in the laser plasma were: 0.86-mbar Ar pressure, 0.8-cm measurement distance from the sample surface, and 0.5-mJ pulse energy Ž880 MW cmy2 . of the ablating Nd:YAG laser. An estimate of the material removal at this laser energy gave 80 ng for a single shot, 350 ng for 10 shots and 1000 ng for 100 shots. The hfs of the 235 U isotope in natural UO 2 Žnom. 0.714% 235 U. can be detected on a singleshot basis, under the optimal experimental conditions, using the scanning mode. The accuracy for the isotope ratio measurement, using the time-integrated data acquisition approach, is approximately 7%, for samples which are depleted in 235 U. The precision may be slightly better for the time-integrated approach Ž6.5% for 120 shots vs. 5% for 460 shots. and the limits of detection for either isotope are similar, f 0.6 mg gy1 which is approximately two times lower than the limit of detection obtained in the scanning mode. The accuracy and precision of either approach is expected to improve for 235 U-enriched samples and for increasing number of laser probings. The main drawback of the time-integrated approach, using sequential measurements with fixed diode laser wavelengths, is the partial excitation of a specific hf component of the 235 U by the narrow-band diode laser Ž Dl L - Dl hfs .. The easiest way to avoid the problem is the calibration of the experimentally measured intensity ratios in terms of isotope content using one sample with certified isotope concentration. This measurement approach, using a simple sample geometry, diode laser excitation and laser sampling with a modest pulse energy Nd:YAG laser, should have applicability as a portable instrument for routine screening of isotopic composition of uranium samples in the field and should be extendable to other actinide elements. It compares very favorably with alternate techniques having a similar goal, such as laser-induced plasma emission and optogalvanic spectroscopy.
Acknowledgements The authors acknowledge the valuable experimental assistance of M. Schuth. ¨ One of the authors ŽB.W.S.. acknowledges gratefully the financial support of the Deutsche Forschungesgemeinschaft for a visiting professor fellowship. We also are grateful to Dr Glatz of the Institute of Transuranium Elements ŽKarlsruhe, Germany. for providing the uranium samples. References w1x D.A. Rusak, B.C. Castle, B.W. Smith, J.D. Winefordner, Recent trends and the future of laser-induced plasma spectroscopy. TrAC 17 Ž1998. 453]461. w2x R.M. Measures, N. Drewell, H.S. Kwong, Atomic lifetime measurements obtained by the use of laser ablation and selective excitation spectroscopy. Phys. Rev. A 16 Ž1977. 1093]1097. w3x R.M. Measures, H.S. Kwong, TABLASER: trace Želement. analyzer based on laser ablation and selectively excited radiation. Appl. Opt. 18 Ž1979. 281]286. w4x H.S. Kwong, R.M. Measures, Trace element laser microanalyzer with freedom from chemical matrix effect. Anal. Chem. 51 Ž1979. 428]432. w5x I.B. Gornushkin, S.A. Baker, B.W. Smith, J.D. Winefordner, Determination of lead in metallic reference materials by laser ablation combined with laser excited atomic fluorescence. Spectrochim. Acta 52B Ž1997. 1653]1662. w6x I.B. Gornushkin, J.E. Kim, B.W. Smith, S.A. Baker, J.D. Winefordner, Determination of cobalt in soil, steel, and graphite using excited-state laser fluorescence induced in a laser spark. Appl. Spectrosc. 51 Ž1997. 1055]1059. w7x A.L. Lewis II, G.J. Beenen, J.W. Hosch, E.H. Piepmeier, A laser microprobe system for controlled atmosphere time and spatially resolved fluorescence studies of analytical laser plumes. Appl. Spectrosc. 37 Ž1983. 263]269. w8x A.L. Lewis II, E.H. Piepmeier, Chemical and physical influences of the atmosphere upon the spatial and temporal characteristics of atomic fluorescence in a laser microprobe plume. Appl. Spectrosc. 37 Ž1983. 523]530. w9x W. Sdorra, A. Quentmeier, K. Niemax, Basic investigations for laser microanalysis: II. Laser-induced fluorescence in laser-produced sample plumes. Mikrochim. Acta II Ž1989. 201]218. w10x K. Niemax, W. Sdorra, Optical emission spectrometry and laser-induced fluorescence of laser produced sample plumes. Appl. Opt. 29 Ž1990. 5000]5006. w11x Y. Oki, K. Matsunaga, T. Nomura, M. Maeda, Removal of thin layer for trace element analysis of solid surface in subnanometer scale using laser-ablation atomic fluorescence spectroscopy. Appl. Phys. Lett. 71 Ž1997. 2916]2918.
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958 w12x T. Bieniewski, Absolute oscillator strengths for neutral atomic uranium. J. Opt. Soc. Am. 68 Ž1978. 1173]1181. w13x R. Kapoor, G.D. Saksena, Relative transition probabilities of neutral uranium transitions. J. Opt. Soc. Am. B 6 Ž1989. 1623]1626. w14x L.E. Burkhart, G.L. Stukenbrocker, S. Adams, Isotope shifts in uranium spectra. Phys. Rev. 75 Ž1949. 83]85. w15x J.A. Goleb, Uranium isotope investigations by atomic absorption. Anal. Chem. 35 Ž1963. 1978. w16x Thermo Jarrell Ash Corporation, Guide to Analytical Values for TJA Spectrometers, TJA Corp., Franklin, MA, 1988. w17x D.M. Goltz, D.C. Gregoire, J.P. Byrne, C.L. Chakrabarti, Vaporization and atomization of uranium in a graphite tube electrothermal vaporizer: a mechanistic study using electrothermal vaporization inductively coupled plasma mass spectrometry and graphite furnace atomic absorption spectrometry. Spectrochim. Acta 50B Ž1995. 803]814. w18x A.R. Date, A.L. Gray ŽEds.., Inductively Coupled Plasma Mass Spectrometry, Chapman and Hall, New York, NY, 1989. w19x R.K. Winge, V.J. Petterson, V.A. Fassel, Inductively coupled plasma atomic emission spectroscopy: prominent lines. Appl. Spectrosc. 33 Ž1979. 206]219. w20x P.W.J.M. Boumans, J.J.A.M. Vrakking, Detection limits of about 350 prominent lines of 65 elements observed in 50 and 27 MHz inductively coupled plasmas ŽICP.: Effects of source characteristics, noise, spectral bandwidth } standard values for the 50 MHz ICP. Spectrochim. Acta 42B Ž1987. 553]579. w21x E.L. DeKalb, J.G. Douglas, M.C. Edelson, V.A. Fassel, An ICP Spectral Atlas for Uranium, Ames Laboratory Report, IS-4979, UC-15, Ames Laboratory, 1988. w22x M.C. Edelson, V.A. Fassel, Isotopic abundance determinations by inductively coupled plasma atomic emission spectrometry. Anal. Chem. 53 Ž1981. 2345]2347. w23x P.S. Goodall, S.G. Johnson, Isotopic uranium determination by inductively coupled plasma atomic emission spectrometry using conventional and laser ablation sample introduction. J. Anal. Atom. Spectrom. 11 Ž1996. 57]60. w24x J.A. Vera, G.M. Murray, S.J. Weeks, M.C. Edelson, Isotopic abundance determinations by inductively coupled plasma high resolution laser excited atomic and ionic fluorescence spectroscopy. Spectrochim. Acta 46B Ž1991. 1689]1700. w25x P.G. Russ III, J.M. Bazan, Isotopic ratio measurements with an inductively coupled plasma source mass spectrometer. Spectrochim. Acta 42B Ž1987. 49]62. w26x A. Montaser, D.W. Golightly ŽEds.., Inductively Coupled Plasmas in Analytical Atomic Spectrometry, VCH Publishers, Inc., New York, NY, 1992. w27x M. Fukuda, Y. Sayama, Determination of traces of uranium and thorium in antimonyŽIII. oxide by ICP-MS. Bunseki Kagaku 44 Ž1995. 719]723. w28x J.G. Sengupta, N.B. Bertrand, Direct ICP-MS determi-
w29x
w30x
w31x
w32x
w33x w34x w35x
w36x
w37x w38x
w39x
w40x w41x w42x
957
nation of trace and ultratrace elements in geological materials after decomposition in a microwave oven: Quantitation of Y, Th, U and the lanthanides. Talanta 42 Ž1995. 1595]1607. J.H. Aldstadt, J.M. Kuo, L.L. Smith, M.D. Erickson, Determination of uranium by flow-injection inductively coupled plasma mass spectrometry. Anal. Chim. Acta 319 Ž1996. 135]143. M. Hollenbach, J. Grohs, S. Mamich, M. Droft, E.R. Denoyer, Determination of technetium-99, thorium-230 and uranium-234 in soils by inductively coupled plasma mass spectrometry using flow injection preconcentration. J. Anal. Atom. Spectrom. 9 Ž1994. 927]933. M. Gastel, J.S. Becker, G. Kuppers, H.-J. Dietze, De¨ termination of long-lived radionuclides in concrete matrix by laser ablation inductively coupled plasma mass spectrometry. Spectrochim. Acta 52B Ž1997. 2051]2059. G. Rossi, M. Mol, Isotopic analysis of uranium by an optical spectral method. III: determination of uranium235ruranium 238 ratios with a hollow cathode source and a direct reading attachment. Spectrochim. Acta 24B Ž1969. 389]398. R.A. Keller, R. Engleman, Jr., E.F. Zalewski, Optogalvanic spectroscopy in a uranium hollow cathode discharge. J. Opt. Soc. Am. 69 Ž1979. 738]742. J.M. Gagne, ´ P. Pianarosa, G. Larin, J.P. Saint-Dizier, P. Bouchard, Ionization and excitation of uranium in a hollow cathode lamp. Appl. Opt. 20 Ž1981. 3770]3773. N.J. Dovichi, D.S. Moore, R.A. Keller, Use of the optogalvanic effect and the uranium atlas for wavelength calibration of pulsed lasers. Appl. Opt. 21 Ž1982. 1468]1473. C.J. Sansonetti, K.-H. Weber, Reference lines for dye laser wave-number calibration in the optogalvanic spectra of uranium and thorium. J. Opt. Soc. Am. B 1 Ž1984. 361]365. P. Pianarosa, Y. Demers, J.M. Gagne, ´ Isotope analysis by optogalvanic spectroscopy. J. Opt. Soc. Am. 1 Ž1984. 704]709. C.M. Barshick, R.W. Shaw, J.P. Young, J.M. Ramsey, Isotopic analysis of uranium using glow discharge optogalvanic spectroscopy and diode lasers. Anal. Chem. 66 Ž1994. 4154]4158. C.M. Barshick, R.W. Shaw, J.P. Young, J.M. Ramsey, Evaluation of the precision and accuracy of a uranium isotopic analysis using glow discharge optogalvanic spectroscopy. Anal. Chem. 67 Ž1995. 3814]3818. J.R. Wachter, D.A. Cremers, Determination of uranium in solution using laser induced breakdown spectroscopy. Appl. Spectrosc. 41 Ž1987. 1042]1048. E. Stoffels, P. Van de Weijer, J. Van der Mullen, Timeresolved emission from laser-ablated uranium. Spectrochim. Acta 46B Ž1991. 1459]1470. W. Pietsch, A. Petit, A. Briand, Isotope ratio determination of uranium by optical emission spectroscopy on a laser-produced plasma } basic investigations and analytical results. Spectrochim. Acta 53B Ž1998. 751]761.
958
B.W. Smith et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 943]958
w43x M.W. Pietsch, Laser-Produced Plasma at Reduced Pressure for Analytical Isotope Control, Doctoral Dissertation, Universite 1994. ´ D’Orleans, ´ w44x B.W. Smith, I.B. Gornushkin, L.A. King, J.D. Winefordner, A laser ablation-atomic fluorescence technique for isotopically selective determination of lithium in solids. Spectrochim. Acta 53B Ž1998. 1131]1138. w45x L. King, I. Gornushkin, B.W. Smith, J.D. Winefordner, Rubidium isotope measurements in solid samples by laser ablation-laser atomic absorption spectrosocopy, Spectrochim. Acta 54B Ž1999. in press. ´ w46x S. Gerstenkorn, Etude du plutonium par spectroscopie a haute resolution: I. } Contribution a la classification ´ du spectre d’arc. Ann. Phys. 7 Ž1962. 367]404.
w47x E.W.T. Richards, A. Ridgeloy, Preliminary classification in the plutonium I spectrum. Spectrochim. Acta 21 Ž1965. 1449]1466. w48x A. Zybin, K. Niemax, Improvement of the wavelength tunability of etalon-type laser diodes and mode recognition and stabilization in diode laser spectrometers. Spectrochim. Acta 52B Ž1997. 1215]1221. w49x J. Blaise, L.J. Radziemski, Jr., Energy levels of neutral atomic uranium ŽUI.. J. Opt. Soc. Am. 66 Ž1976. 644]659. w50x R. Engleman, Jr., B.A. Palmer, Precision isotope shifts for the heavy elements. I. Neutral uranium in the visible and near infrared. J. Opt. Soc. Am. 70 Ž1980. 308]317.