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Diode laser absorption measurement of uranium isotope ratios in solid samples using laser ablation
Spectrochimica Acta Part B 57 (2002) 1611–1623
Diode laser absorption measurement of uranium isotope ratios in solid samples using laser ablation夞 H. Liu, A. Quentmeier*, K. Niemax Institute for Spectrochemistry and Applied Spectroscopy (ISAS), Bunsen-Kirchhoff-Strasse 11, D-44139 Dortmund, Germany Received 25 April 2002; accepted 29 July 2002
Abstract The isotopes 235U and 238U were simultaneously measured by absorption in a laser-induced plasma ignited by a Nd:YAG laser. Two separate diode lasers were used which probed almost the same plasma volume. The diode lasers were tuned to the absorption lines 682.6736 nm for 235U and 682.0768 nm for 238U. The 235U y 238U isotope ratio was measured on a pulse-to-pulse basis evaluating the transient absorption peaks as analytical signals. Three uranium oxide pellets were used for calibration with natural (0.714 wt.%) and depleted 235 U concentration (0.407 and 0.204 wt.%). The measured 235Uy238 U mass ratio in an uranium containing mineral sample was (0.686"0.119)=10y2 and found to be in good agreement with the mass ratio of naturally occurring isotopes (0.719=10y2 ). The observed accuracy (-5%) and precision (;17% R.S.D.) are encouraging if it is taken into account that the mineral sample was probed directly without any preceding preparation step. Spatially and temporally resolved emission spectroscopic measurements were performed to investigate the kinetics of the laser plasma and to optimize the operating conditions of the laser ablation with regard to sensitivity and selectivity of the 235 U isotope detection. Optimal conditions were obtained at a distance of ;0.3 cm from the sample surface, an argon pressure of ;30 mbar and for 7 mJ pulse energy of the Nd:YAG laser. The limit of detection of the 235 U isotope, evaluated on the basis of the 3s criteria, was estimated to be 47 mg gy1 and improved by a factor of two compared with the sequential measurement with a single laser diode used in a previous work. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser ablation; Diode laser; Atomic absorption; Isotope ratio; Uranium
1. Introduction 夞 This paper is published in the special issue of Spectrochimica Acta Part B dedicated to the 50th anniversary of the ISAS. *Corresponding author. Fax: q49-231-1392-120. E-mail address: [email protected] (A. Quentmeier).
Over the past three decades laser ablation (LA) has become a versatile tool in analytical chemistry. The capability of LA as an easy to use and rapid sampling technique has opened up new analytical applications especially for the direct analysis of solids w1x. Apart from the well approved coupling
0584-8547/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 2 . 0 0 1 0 5 - 2
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of LA to optical emission spectrometry (LA-OES), also known as laser-induced breakdown spectroscopy (LIBS) w2x, the combination of LA and inductively coupled plasma with optical (LA-ICPOES) or mass spectrometric detection (LA-ICPMS) is widely used for in situ and microprobe analysis w3,4x. The outstanding sensitivity and detection power of the MS techniques makes them preferable for the determination of isotopic abundances which can be measured even at trace concentration levels if a sample preparation step like matrix separation is applied. The goal of our recent investigations was to develop an approach for isotope selective detection of 235U and 238U 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. The approach was based on laser ablation of a solid sample followed by the optical detection of free uranium atoms in the expanding laser plume. The possibility of using emission rather than mass spectrometric techniques is highly advantageous in view of minimizing the contamination of instrumentation. The determination of the 235Uy 238U isotope ratio by OES is a challenging task which requires an extremely high spectral resolving power of the optical spectrometer system. In addition, suitable excitation conditions are necessary to ensure that the widths of the emission lines are significantly smaller than their corresponding isotope shift. Several attempts have been reported to analyze the isotopic composition of solids by means of laser-induced breakdown spectroscopy w5,6x, but the quality of the results (although encouraging) cannot compete in respect to precision and sensitivity to those obtained with MS detection techniques w7,8x. A possible alternative could be the application of high-resolution laser spectroscopy. For example, diode lasers (DL) are easy to use light sources with narrow line widths (approx. 20 MHz). Therefore, the spectral resolution is appropriate to measure the isotope components of spectral lines either by atomic absorption spectroscopy (AAS) w9–12x, laser-induced fluorescence (LIF) w13x, optogalvanic spectroscopy w14,15x or ionization spectroscopy w16x.
In recent works it has been demonstrated that laser ablation can be combined successfully with DL based techniques probing free sample atoms in the expanding plasma plume. Smith et al. w12x and Quentmeier et al. w13x reported on results obtained with the detection of the 235U and 238U isotopes by LA-DL-LIF and LA-DL-AAS, respectively. In both investigations only one DL was used as the light source. In the LA-DL-LIF experiments the wavelength of the DL was tuned rapidly across the profiles of the absorption lines of both isotopes so that the fluorescence signals of 235U and 238U could be registered in every laser shot w12x. In LA-DL-AAS the DL wavelength was tuned sequentially to the respective line centers of both isotope lines and the corresponding absorption signals were detected on a shot-to-shot basis w13x. The optimum conditions for the detection of the 235 U isotope by LA-AAS differed greatly from the conditions required for LA-LIF. In LA-LIF a low pulse energy (;0.5 mJ) of the ablation laser (Nd:YAG laser) and a low buffer gas pressure (;1 mbar) had to be used. Furthermore, the optimum excitation region was approximately 0.8 cm from the sample in order to reduce the background by continuous plasma emission. The widths of the uranium lines were smaller than 1 pm and enabled even the observation of the hyperfine structure of the 235U transition w12x. In case of LA-AAS, the optimal experimental parameters were found to be at a higher gas pressure (;30 mbar) and closer (;0.2 cm) to the sample surface. These operating conditions ensured relatively high number densities of uranium atoms in the laser plume, which favored the measurement of AAS with regard to sensitivity and detection limit. As a consequence, the detection limit of 100 mg gy1 obtained with AAS was approximately one order of magnitude lower than obtained with LIF w12x. The wing of the broadened absorption line of the 238 U isotope was found to be the main source of the relevant analytical background signal which was responsible for limiting the precision of the sequential technique used so far. The measurement of the isotope ratio also suffered from poor reproducibility, since the absorption of the main isotope had to be measured on the wing of the absorption
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Fig. 1. Schematic set-up of the experimental system.
line instead of at the line center due to optical thick conditions. The main objective of the present work is the detailed characterization of the spatial and temporal evolution of the laser plume to improve the sensitivity and selectivity of the 235U isotope detection by LA-AAS. Plasma parameters such as temperature and electron number density, which strongly influence the absorption line profiles were investigated thoroughly to achieve optimum operating conditions. Furthermore, the absorption signals of both uranium isotopes were detected simultaneously with two separate DLs and evaluated on a shot-to-shot basis to improve the reproducibility, and thus the analytical capability of LA-DL-AAS. 2. Experimental The main components used in the present experimental system are shown schematically in Fig. 1. Sampling was performed by a Nd:YAG laser (Model SL401, Spectron Laser Systems) operating at the fundamental wavelength of 1064 nm with a typical pulse energy of 7 mJ and a repetition rate of 2 sy1. The ablation was performed in a sealed chamber with reduced argon pressure. All meas-
urements were performed under continuous gas flow (approx. 0.5 l miny1). The argon pressure was carefully adjusted in the range of 0.4–200 mbar by a needle valve in the by-pass of the pumping line and controlled by a capacitance manometer (Baratron, MKS Instruments). In contrast to our previous absorption measurements with only one laser diode w13x, two 50 mW single mode laser diodes (Mitsubishi Electric Corp., type ML1013R-01) were used. The DL beams formed an angle of approximately 48 and intersected each other on the axis of the ablating Nd:YAG laser beam at a well defined distance from the sample surface. This enabled simultaneous measurements of the absorption signals of both uranium isotopes at two different wavelengths and under similar plasma conditions because both DLs probed almost the same volume of the laser plasma. Obviously, a strict collinear alignment of the two diode laser beams would have been even better. However, in this case the laser intensities and absorption signals have to be modulated in the MHz range, due to the fast absorption, in order to separate the absorption signals. Modulation as well as demodulation is not easy. The diodes were powered by a combined temperature and current controller (Model ITC502,
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Profile Optische Instrumente GmbH, Germany). The DLs could be tuned continuously without mode hops over typical ranges of approximately 30–50 pm, which allowed tuning across the complete U line with its isotope components. Extended fine-tuning of the DLs was achieved with a short Etalon-type external laser resonator w17x, which yielded a weak optical feedback and thus an improved wavelength stability. The single mode structure and wavelength stability of the DLs were monitored by means of an interferometric wavemeter (ATOS Lambdameter, model LM007). After passing the chamber the DL radiation was measured by photodiodes (type S1223-01, Hamamatsu Photonics) which have a rise time of less than 20 ns. The photodiode signals were amplified by a low gain 100 MHz bandwidth integrated amplifier stage (type OPA603, Burr–Brown). The effective bandwidth of the detection system was decreased slightly to reduce pick-up noise from the power supply of the Nd:YAG laser in the early phase of plasma formation. The diodes were positioned approximately 50 cm behind the sample chamber to minimize the influence of background emission of the laser plasma. The solid angle of observation was further reduced by two apertures (⭋s0.3 cm) in the beam path of each diode. A first aperture was positioned near to the exit window of the sample chamber and a second one in front of the photodiode. The output signals of both photodiode circuits were registered simultaneously by a 4channel 500 MHz digital storage oscilloscope (Tektronik, model 714L). The trigger signal was provided by a PIN photodiode which monitored the Nd:YAG laser output pulse. The absorption was evaluated in the usual way, the peak transmission (or the transmission at any time) of the transient absorption signal was related to the full intensity of the laser diode as measured without ablation. Detailed information on the relevant excitation parameter and the expansion properties of the laser plasma was obtained by additional emission spectroscopic measurements with spatial and temporal resolution. A 1-m monochromator (SPEX M1000, aperture fy8) was used for this purpose. The spectrometer was equipped with a 2400-lines mmy1 grating and offered a typical linear dispersion of approximately 0.4 nm mmy1. An intensi-
fied charge coupled device (ICCD, Princeton Instruments Inc., model TEyCCD-576) was used as the detection system. It allowed time resolved observations with a gate width as short as 50 ns. In combination with the detector array (576=384 pixels of approx. 25 mm in size) the practical dispersion of the optical system was equivalent to approximately 11 pm pixely1 at 682 nm. The instrumental width of the spectrometer was determined with a light source of narrow line width (e.g. a diode laser or a hollow cathode lamp) and was approximately 2 pixel which corresponds to approximately 22 pm. Three uranium oxide samples (UO2qx; 0.05FxF0.15) were used in the present experiments. The samples were prepared and specified by the Institute of Transuranium Elements (Karlsruhe, Germany) in form of pellets with a specific gravity of rs10.4 g cmy3. One of the samples contained uranium in the natural isotopic composition (0.714 wt.% 235U), whereas the concentration of the minor isotope 235U of the two other samples was depleted to 0.407 and 0.204 wt.%. The homogeneity of the pellets was investigated by secondary electron micrographs (SEM) which indicated an average size of the micro structure of approximately 10 mm, and EDX line scans proved the uniform distribution of uranium oxide particles in the samples. Table 1 gives several U I and II lines with lower and upper energy levels, Log (gf)-values, and isotope shifts 238-235IS w18,19x which can be used in DL-AAS. Although most of them were tested in the experiments, the final measurements for the 238 U isotope were performed with the U I lines 682.0768 and 682.6913 nm, whereas the 235U isotope was detected with the absorption line 682.6736 nm. 3. Results and discussion The best operation parameters for the detection of the 235U isotope, i.e. ;30 mbar argon and 0.2– 0.3 cm distance from the sample surface, represent a compromise between a sufficiently large number density of absorbing uranium atoms and moderate plasma conditions which ensure narrow widths of the uranium absorption lines. As demonstrated in
H. Liu et al. / Spectrochimica Acta Part B 57 (2002) 1611–1623 Table 1 Spectroscopic constants of atomic and ionic uranium lines with large isotope shift (IS) Wavelength (lair, nm)
Lower level (cmy1)
Upper level (cmy1)
Log (gf)
II-405.0041 II-424.4373 II-425.2426 I-682.0768b I-682.4463 I-682.6913a I-683.2719
0 0 4420.872 4275.707 7103.921 0 7000.532
24684.135 23553.975 27930.242 18932.767 21753.047 14643.867 21636.957
y0.713 y1.278 y1.251 y1.690 y1.803 y1.679 y2.117
238-235
IS
(pm) y9.5 y25.1 y15.5 y12.8 y8.6 y17.7 y16.8
Spectroscopic data from Smith et al. w23x, isotope shift data for neutral uranium from Engleman and Palmer w18x, and for singly ionized uranium from Blaise and Radziemski w19x. a Used for detection of 235U and 238U isotopes. b Used as reference line for determination of 235Uy238U isotope ratio.
previous LIF measurements w12x, the absorption of the 238U isotope could also be measured at greater distances (;0.5–0.8 cm) because of the relatively high abundance of approximately 99.3 %. However, the detection of the 235U isotope required a much higher number density of absorbing uranium atoms to yield an adequate SyN ratio. The measurement near to the sample and at an early stage in the plasma evolution led to a very strong absorption by the 238U isotope and a significant broadening of its corresponding absorption line. Therefore, the absorption of the 238U isotope had to be measured in the line wing at a definite wavelength distance (5 pm) from the line center, which resulted in poor reproducibility. The precision, i.e. the relative standard deviation (R.S.D.f10–15%) obtained with the uranium oxide pellets was not adequate for the precise measurement of the 235Uy 238U isotope ratio. The measurement of the isotopes at different positions in the plume, 235U at a short sample distance, 238U at a greater sample distance with reduced atom density, cannot be recommended due to poor correlation between the absorption signals of both isotopes. 3.1. Plasma diagnostics The evolution of the laser plasma was thoroughly investigated by time-resolved emission measure-
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ments to study the influence of the plasma parameters on the spectral line widths and thus on the analytical capability of LA-DL-AAS. For this purpose the plasma was imaged on to the entrance slit of the spectrometer by a single lens (image ratio 1:1) which probed a well-defined plasma region of 30 mm width equivalent to the geometrical slit width. The plasma emission was registered at a definite sample distance which was varied by means of a precise linear translation stage providing detailed information on the different plasma zones. The temporal evolution of the plasma was monitored with a gated ICCD detector (gate widths0.1 ms). The evolution of the emission spectra in the range 545–553 nm during plasma expansion is presented in Fig. 2a–d. The spectra were measured under the standard conditions used for the detection of isotope absorption (0.2 cm sample distance and 30 mbar argon pressure) and are shown after background correction. The weak emission observed with zero delay was probably due to recombination background from the low pressure Ar plasma (Fig. 2a), which was superimposed by argon lines at 0.1 ms delay (Fig. 2b). Ionic uranium lines dominated the spectrum after a delay of 1 ms (Fig. 2c), whereas atomic uranium lines were not prominent before 10 ms (Fig. 2d). The dominating role of uranium ion lines, which were still present at 10 ms after plasma ignition, implied that the uranium atoms were completely ionized in the early phase of plasma evolution. The presence of ionic lines enables the detection of the uranium isotopes by means of their ionic absorption lines if the lines have a sufficiently large isotope shift. In principle, the measurement of isotope ratios by ionic lines should be better than by atomic lines, because of the relatively high number density of uranium ions shortly after plasma ignition. However, one serious obstacle of ion detection is the Stark broadening of the absorption lines, which affects the isotope selectivity of the measurement. The excitation temperature was deduced from Boltzmann plots obtained on uranium lines in the wavelength range 547–550 nm (Table 2). Various operating conditions (pressuress1–30 mbar, observation heightss0.1–0.4 cm above the sample surface) and different delay times were used to
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investigate their influence on the excitation temperature. The results are presented in Fig. 3 at two different buffer gas pressures. As shown the excitation temperature decreased rapidly from approximately 8=103 K within the first microseconds of plasma formation to moderate values of approximately 3.5–4=103 K, which were observed in the expanding plasma after approximately 20 ms. This finding confirms our experimental results that a delay time of ;15–25 ms was most favorable for the detection of atomic absorption. The argon lines measured at short distance from the sample and early in the plasma formation revealed a remarkable broadening and a distinct red shift. Line broadening and shift were clearly emphasized in the dense center of the plasma
Table 2 Spectroscopic constants of the uranium ion lines used in Boltzmann plot temperature determination Wavelength Lower level Upper level E upper (cmy1) (cmy1) (eV) (lair, nm) II-547.5706 9241.966 II-548.0265 12513.884 II-548.1203 6445.033 II-548.2526 15392.416 II-548.7004 5790.641 II-549.1222 8510.866 II-549.2952 0 II-550.4128 6445.033
27499.379 30756.109 24684.135 33627.116 24010.461 26716.691 18200.092 24608.170
3.41 3.81 3.06 4.17 2.98 3.31 2.26 3.05
level
Log (gf) y1.276 y0.934 y1.677 y0.773 y2.060 y1.727 y2.110 y1.924
Spectroscopic data from Smith et al. w23x.
Fig. 2. Temporal evolution of the laser plasma in the wavelength range 545–553 nm. The ionic uranium lines shown were used for the determination of the excitation temperature. Gate widths0.1 ms. The delay times used are given in (a)–(d): 0 ms (a); 0.1 ms (b); 1.0 ms (c); and 10 ms (d). Sample distances0.2 cm, and pressures30 mbar. Sample, uranium pellet with 0.714 wt.% 235U.
H. Liu et al. / Spectrochimica Acta Part B 57 (2002) 1611–1623
Fig. 3. Excitation temperature of the laser plasma as derived from Boltzmann plots using ionic uranium lines (see Table 2). Sample distances0.2 cm, pressures30 mbar (h) and 8 mbar (d). Sample, uranium pellet with 0.714 wt.% 235U. The insert shows Boltzmann plots derived with 30 mbar pressure and with different delay times: 1 ms (a); and 10 ms (b).
when compared to the outer and cooler plasma region. The line broadening of the Ar I-696.54 nm emission line was measured at different plasma zones. For this purpose, the ‘binning mode’ was applied, i.e. a fixed number of pixels of the CCD were grouped together and evaluated separately with respect to the corresponding distance from the plasma center. The number density of electrons, ne, can be estimated if one assumes that the Stark effect was the main mechanism of the observed line broadening. The full width at half maximum (FWHM) of the Stark broadened line, Dl1y2, and the corresponding line shift, DlShift, are related to ne (my3) in the following form according to Lochte-Holtgreven w20x: Dl1y2s2w1q5.53=10y6ne1y4a .x=10y22wne =Ž1y0.0068ne1y6Ty1y2 e
(1)
DlShiftswŽdyw.q6.32=10y6ne1y4a y1y2 .x =Ž1y0.0068n1y6 e Te y22 =10 wne
(2)
where (dyw) is the ratio of shift to width. The Stark broadening parameters w and a of the Ar I696.54 nm line were taken from Griem w21x. They are weakly dependent on the electron temperature
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Te, which is of the order of 104 K in low-pressure plasmas. The FWHM were deduced from Lorentzian fitting of the argon line profiles and corrected for instrumental broadening. The contribution by Doppler broadening DlDs1.67lyc (2kTyAm)1y2 is small. It does not exceed 7 pm (for 8=103 K). The instrumental broadening of the spectrometer (;16 pm at 696 nm) was determined in a separate measurement with the neutral Ar line 696.54 nm emitted from a low current uranium hollow cathode lamp (HCL). The Stark width Dl1y2 was deduced from the measured line width after correction for Doppler width and instrumental width. The argon line shift was measured relative to the argon line from the HCL, and ne was estimated according to Eqs. (1) and (2) as a function of the excitation temperature (see Fig. 3). 3.2. Double beam technique As already mentioned above, the approach of our previous DL-AAS experiment was the sequential detection of the uranium isotopes with only one diode laser. The wavelength of the DL was tuned to the absorption line of the 235U isotope and the absorption was measured by accumulating the results of successive laser shots. After tuning the DL wavelength to the wing of the 238U line the measurements were repeated under controlled and stable operating conditions. The relevant background absorption signal was determined in a third independent measurement. Here, the DL wavelength was tuned to the opposite side of the 238U line wing without the minor isotope component. It is quite clear that the reproducibility of the measurement was dependent on the wavelength setting of the DL. The temperature of the DL was stabilized within 10y3 8C and the DL wavelength was continuously monitored by the interferometric wavemeter to avoid any wavelength shift. The most prominent error source, however, was due to the ablation process itself. The typical shot-to-shot fluctuations and the drift effects, which were caused by successive sampling from the same target spot, deteriorate the reproducibility of laser sampling. Therefore, it can be expected that the situation would improve if the 235U and 238U
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Fig. 4. Measurement of the 235U and 238U absorption within the same laser plasma. The diode laser was tuned rapidly across the profiles of both isotopes by the linear current ramp applied to the diode laser controller. Sample distances0.2 cm, and pressures30 mbar. Sample, uranium pellet with 0.714 wt.% 235 U.
isotope absorption were measured in each laser pulse. This has been demonstrated already in our recent DL-LIF measurements where the DL wavelength was scanned by means of a linear current ramp pulse which was triggered by an external modulation frequency (;1.5 kHz) applied to the DL controller. The result of such a fast scan mode in LA-DL-AAS is presented in Fig. 4. The absorption signal of the 238U isotope was separated from the 235U component by 76 ms and is of the same order of magnitude as the 235U absorption due to the expansion of the plasma. The FWHM of the 238 U absorption line as measured in the diluted laser plume was estimated to be approximately 1.6 pm. The reproducibility of the isotope ratio determination obtained in the fast scan mode was worse than in the measurement with fixed wavelength mode because of shot-to-shot variations in the plasma size. Therefore, two individual DLs were used which enabled the simultaneous detection of both isotopes in the same volume of the laser plume. The simultaneous detection technique should overcome the shot-to-shot variations inherent in the ablation process because of the correlation between the absorption signals of both isotopes. As a consequence, one could expect a significantly improved reproducibility irrespective of the size or shape of the laser plume. Even an
inhomogeneous distribution of uranium atoms in the sample plume should have no effect. The two DL beams were carefully aligned to probe almost the same region of the laser plume. For this purpose the beams intersected each other on the axis of the Nd:YAG laser beam forming an angle of 48. For the correct alignment both DLs were tuned to the 235U isotope component providing the same absorption within an experimental error of approximately 5%. To avoid the measurement of the optically thick 238U component on the line wing, other weaker uranium absorption lines were investigated as possible reference lines for the 235U-682.6736 nm component. The 238U682.0768 nm line (see Table 1) was selected as the most suitable reference line. However, as shown in Table 1, both lines do not share the same lower level. The alternative absorption line starts at 4275.707 cmy1 above the ground level and should therefore be dependent on the argon pressure and the plasma temperature. This was studied by changing the pressure as well as the distance of the DL beams from the sample. Within the limits of uncertainty the ratios of the absorption peaks of 235U and 238U lines were found to be slightly dependent on the pressure near to the sample but almost independent of the distance in the ranges up to 2, 3 and 4 mm at a fixed pressure of 50, 30 and 10 mbar, respectively, as shown in Fig. 5. The increases in the 235Uy 238U absorption ratio are due to reduced Boltzmann populations of the 238U energy level at 4275.707 cmy1. 3.3. Optimization of the experiment The plasma diagnostic results obtained so far confirmed that the operating conditions which were selected on an experimental basis were appropriate for the detection of isotope absorption. The plasma temperature at 0.2 cm above the sample and 20 ms after the laser pulse was high enough to ensure good atomization of uranium dioxide ablated from the samples. For dissociation of the uranium dioxide molecules the temperature should exceed 2400 K w22x. At 7 mJ pulse energy the Nd:YAG laser ablated only a minute amount of material in a single laser shot. The mass removal was determined with 10
H. Liu et al. / Spectrochimica Acta Part B 57 (2002) 1611–1623
Fig. 5. Ratio of the peak absorption of the 235U-682.6736 nm line and the 238U-682.0768 nm reference line as obtained for different sample distances and buffer gas pressures equal to: 10 mbar (%); 30 mbar (h); and 50 mbar (j). Sample, uranium pellet with 0.714 wt.% 235U.
consecutive laser shots delivered on the polished surface of the uranium pellets (rs10.4 g cmy3). The crater profiles were measured by means of an interferometric microscope (model: NewView 5000, Zygo Corporation). The mass ablation of ;50 ngyshot corresponded to a mean number density of n(235U);1013 cmy3 provided that the atomization was complete and the uranium atoms were homogeneously distributed in a spherically shaped plume of ;0.5 cm diameter at 30 mbar pressure. Generally, these assumptions are not met and the number density of sample atoms detected at a certain delay is dependent on the position of observation. Therefore, the influence of the temporal and spatial distribution of the sample atoms on the isotope detection was investigated to achieve an optimum analytical capability. The diameter of the DL beam (1ye 2;0.15 cm) was reduced by an aperture (f;0.05 cm) in the beam path to ensure the required spatial resolution. The spatial distribution of the absorption signals measured with varying axial distance from the Nd:YAG laser beam at a distance of 0.3 cm from the sample is shown in Fig. 6. The optimum absorption was obtained within a range of "0.05 cm indicating the inhomogeneous distribution of sample atoms in the laser plume.
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The radiation of the Nd:YAG laser was focused onto the sample by a lens of 12 cm focal length forming a spot size of approximately 100 mm under best focus conditions. The depth of the evolving crater increased with increasing number of laser shots. As a result the laser plume changed from a spherical shape to a conical form. Therefore, only a limited number of laser shots were delivered to the same sample spot to improve the reproducibility of absorption measurements. The most satisfying conditions were found to be with a total number of shots less than 50 and the focus position just below the sample surface. In this case, the absorption signals were reproducible within ;2–3% R.S.D. The absorption signals obtained with the very first shots on the sample were less reproducible probably due to surface roughness. Therefore, the sample surface was preconditioned with approximately 10–20 laser shots before the measurements. The simultaneous measurements with two DLs could explain the double peak structure of the absorption signals which has already been discussed in our previous paper w13x. The separation into two distinct components was interpreted as the effect of two different phases of absorbing uranium atoms. Both components were most clear-
Fig. 6. 235U isotope absorption signals observed at different axial positions relative to the Nd:YAG laser beam. Sample distances0.3 cm, and pressures30 mbar. Sample, uranium pellet with 0.714 wt.% 235U.
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Fig. 7. Simultaneous determination of the 235U isotope absorption (solid line) and of the background signals (dotted line) observed with different buffer gas pressures of 30 mbar (a) and 50 mbar (b). Sample distances0.2 cm. Sample, uranium pellet with 0.714 wt.% 235U.
ly separated if the absorption was detected in the range of 0.2–0.3 cm above the sample and merged with increasing sample distance and pressure. The investigation was performed with the first DL tuned to the 235U absorption line, whereas the second DL was tuned to the ‘mirror’ position, i.e. the wavelength position in the red wing of the 238 U line, which corresponds to the isotope shift. In such a measurement the relevant analytical background of the 235U signal can be determined, since the line profile of the 238U absorption line is symmetrical. The results of the measurement are shown in Fig. 7 for two different buffer gas pressures. The first component of the double peak structure coincides with the background signal at 30 mbar (Fig. 7a), while at 50 mbar buffer gas
pressure (Fig. 7b) the first peak is not clearly visible. The contribution of the background is indicated by the rising edge of the 235U absorption signal. The background signal contributes significantly to the analytical absorption signal and has therefore to be taken into account. The distinct peak of the background signal observed at the rising edge of the absorption signal clearly indicated that the evaluation of the first component of the double peak structure is not favorable for the detection of the 235U isotope. This can clearly be seen in the results shown in Fig. 8 where the DL was tuned in a step-by-step fashion across the absorption lines of both uranium isotopes evaluating the first as well as the second component of the double peak structure. The first component yielded only a small 235U absorption superimposed on the wing of the 238U absorption line, whereas the evaluation of the second component resulted in a well detectable 235U absorption line. As has already been discussed in Quentmeier et al. w13x, the contribution by Lorentzian broadening is larger in lines obtained from the first component than in the profiles from the second. Obviously, the different line profiles reflect different conditions during the expansion of the laser plasma. Fig. 9 shows the FWHM of the 238U absorption line measured at different delay times in the range from 400 ns
Fig. 8. Line wings of the 235U-682.6736 nm absorption line derived from evaluation of the first (j) and the second peak (s) of the transient 235U isotope absorption signal. Sample distances0.2 cm, pressures30 mbar. Sample, uranium pellet with 0.714 wt.% 235U.
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Fig. 9. Full width at half maximum (FWHM) of the 238U682.6913 nm absorption line evaluating the transient absorption signals at different delay times. The line profiles were derived from successive measurements tuning the diode laser across the uranium absorption line. Sample distances0.1 cm, and pressures30 mbar. Sample, uranium pellet with 0.714 wt.% 235U.
to 40 ms. The significant broadening of the 238U absorption line observed at short delay times can be attributed, in particular, to Stark broadening which is also responsible for the broadening of the argon lines early in plasma formation. The relevant absorption was determined in two different ways evaluating either the peak values of the transient signals or the integrated signal areas. Both methods were tested and found to be nearly equivalent. However, the evaluation of the peak values was preferred because the background was measured at the same time as the absorption peak of the 235U isotope which takes into account the time dependence of the background signal shown in Fig. 7. 3.4. Calibration The ratios of the absorption peaks of the 235U682.6736 nm line and the 238U-682.0768 nm reference line were measured with approximately 120 laser shots and plotted vs. the certified 235Uy 238U ratios of the samples. The isotope ratios were measured with a precision of typically 2–4% R.S.D. indicating that the uranium oxide pellets used are sufficiently homogeneous. The relevant
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background signals were measured separately in the red wing 17.7 pm from the center of the 238U682.6913 nm line assuming a symmetric line profile. The background signal was equivalent to an absorption signal of ;0.4 %. The comparison with ;6% absorption obtained with the sample of natural 235U isotope concentration (0.714 wt.%) yields a corresponding SyB ratio of ;15. The relative standard deviation of the background signal which is responsible for the detection limit for 235 the U isotope was estimated to R.S.D.;2=10y4. Based on the 3s criterion an LOD of 47 mg gy1 was found, which is approximately a factor of two lower than the LOD of 100 mg gy1 obtained by the sequential technique with only a single DL w13x. The noise of the detector system was not relevant for the detection limit. The noise level was estimated to be equivalent to an absorption signal of ;10y2% corresponding to an SyN ratio )400. The improvement resulting from the simultaneous detection technique is demonstrated by the following data which were derived from repetitive measurements of the sample with 0.714 wt.% 235U concentration. The sequential technique yielded a precision of ;3.7% R.S.D. for the absorption measurement of the 235U isotope, and of ;1.6% R.S.D. for the 238U isotope evaluating the reference line. From these data the precision of the 235Uy 238 U isotope ratio was calculated by the law of error propagation to ;4.1% R.S.D. This result can then be compared to the 1.1% R.S.D. obtained by the simultaneous measurement of the isotope ratio on a shot-to-shot basis. 3.5. Isotope ratio measurement The detection of the isotope ratio was based on the previous calibration step. In order to prove the analytical capability of DL-AAS technique, we determined the 235Uy 238U isotope ratio in a natural geological mineral sample (type pitchblende) of unknown chemical composition and unknown uranium concentration. A small sample piece was broken from the mineral, and the resulting fragment (;20 g) was transferred into the ablation chamber without any further sample preparation. Material from different parts of the heterogeneous
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material was ablated to check the distribution of uranium. The poor shot-to-shot reproducibility (;70% R.S.D.) of the 235U absorption signal indicated a non-uniform distribution of uranium in the mineral sample. The net absorption was found to be relatively small and only a factor of approximately three above the background equivalent absorption. This finding was attributed to the noncomplete atomization of sample material, which partially covered the output window of the ablation chamber. No attempt was made to improve the efficiency of sample ablation by an increase of the pulse energy of the Nd:YAG laser in order to assure the same operating conditions as used for calibration. The measured 235Uy238U mass ratio of (0.686"0.119)=10y2 obtained on 10 different sample positions with a total of 200 laser shots, is in good agreement with the isotope ratio of natural abundant uranium (0.719=10y2). The accuracy of approximately 5% is encouraging, but the precision of approximately 17% R.S.D. is relatively poor if compared to the precision of typically 2– 3% R.S.D. obtained with the homogeneous uranium pellets. Obviously, a much larger number of laser shots is required in case of the mineral sample to achieve a higher precision. 4. Conclusion Diode laser atomic absorption spectrometry (DL-AAS) was applied to measure the 235Uy 238U isotope ratio in solid samples by laser ablation. Spatially and temporally resolved OES measurements were performed to optimize the operating conditions of laser ablation with regard to selectivity and sensitivity of the uranium isotope detection. The double peak structure of the transient absorption signals already mentioned in a recent paper was studied by the combination of DL-AAS and OES measurements and could be explained by the background due to the wing of the 238U absorption line. It was shown that the measurement of the 235 Uy 238U isotope ratio can be improved if two independent diode lasers are used. The simultaneous measurement of both isotopes enables the correlation of the absorption signals on a shot-toshot basis and thus the improvement of the precision and accuracy of the DL-AAS technique. The
method was applied to measure the 235Uy 238U isotope ratio of a natural mineral sample. The measurement approach described, using a simple sample geometry, diode laser excitation and laser sampling with a modest pulse energy Nd:YAG laser, should have application 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 favorably with alternative detection techniques having a similar goal, such as laser-induced plasma emission and optogalvanic spectroscopy. Acknowledgments Financial support of the Deutsche Forschungsgemeinschaft (Project Qu 102y1-1) is gratefully acknowledged. Furthermore, we thank Dr J.P. Glatz of the Institute of Transuranium Elements, Karlsruhe, for providing us with the uranium pellet samples. References w1x K. Niemax, Laser ablation—a reflection on a very complex technique of solid sampling, Proceedings of the Ninth Solid Sampling Spectrometry Colloquium, Merseburg, Germany, September 11–15, 2000. w2x F. Leis, W. Sdorra, J.B. Ko, K. Niemax, Basic investigations for laser microanalysis: I. Optical emission spectrometry of laser-produced sample plumes, Mikrochim. Acta II (1989) 185–190. w3x L. Moenke-Blankenburg, Laser-ICP-spectrometry, Spectrochim. Acta Rev. 15 (1993) 1–37. w4x R.E. Russo, Laser ablation, Appl. Spectrosc. 49 (1995) A14–A28. w5x 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 Part B 53 (1998) 751–761. w6x C.A. Smith, M.A. Martinez, D.K. Veirs, D.A. Cremers, 239 Puy 240Pu isotope ratios can be determined using LIBS, The Actinide Research Quarterly: 4th Quarter 2000, http:yywww.lanl.govyorgsynmtynmtdoyAQarchivey00winteryLIBS.html. w7x 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. At. Spectrom. 11 (1996) 57–60. w8x S. Uchida, R. Garcıa-Tenorio, ´ ´ K. Tagami, M. Garcıa´ Determination of U isotope ratios in environmenLeon,
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w9 x
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tal samples by ICP-MS, J. Anal. At. Spectrom. 15 (2000) 889–892. H.D. Wizemann, K. Niemax, Isotope selective element analysis by diode laser atomic absorption spectrometry, Mikrochim. Acta 129 (1998) 209–216. H.D. Wizemann, K. Niemax, Cancellation of matrix effects and calibration by isotope dilution in isotopeselective diode laser atomic absorption spectrometry, Anal. Chem. 69 (1997) 4291–4293. H.D. Wizemann, K. Niemax, Measurement of 7Liy 6Li isotope ratios by resonant Doppler-free two-photon diode laser atomic absorption spectroscopy in a lowpressure graphite furnace, Spectrochim. Acta Part B 55 (2000) 637–650. B.W. Smith, A. Quentmeier, M. Bolshov, K. Niemax, Measurement of uranium isotope ratios in solid samples using laser ablation and diode laser-excited atomic fluorescence spectrometry, Spectrochim. Acta Part B 54 (1999) 943–958. A. Quentmeier, M. Bolshov, K. Niemax, Measurement of uranium isotope ratios in solid samples using laser ablation and diode laser-excited atomic absorption spectrometry, Spectrochim. Acta Part B 56 (2001) 45–55. 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 (1994) 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 (1995) 3814–3818.
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w16x B.A. Bushaw, B.D. Cannon, Diode laser based resonance ionization mass spectrometric measurement of strontium-90, Spectrochim. Acta Part B 52 (1997) 1839–1854. w17x 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 Part B 52 (1997) 1215–1221. w18x R. Engleman, B.A. Palmer, Precision isotope shifts for the heavy elements. I. Neutral uranium in the visible and near infrared, J. Opt. Soc. Am. 70 (3) (1980) 308–317. w19x J. Blaise, L.J. Radziemski, Energy levels of neutral atomic uranium (UI), J. Opt. Soc. Am. 66 (1976) 644–659. w20x W. Lochte-Holtgreven, Plasma Diagnostics, North-Holland Publishing Co, Amsterdam, 1968. w21x H.R. Griem, Plasma Spectroscopy, McGraw-Hill, New York, 1964. w22x 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 Part B 50 (1995) 803–814. w23x P.L. Smith, C. Heise, J.R. Esmond, R.L. Kurucz, Atomic spectral line database, Harvard-Smithsonian Center for Astrophysics, http:yycfa-www.harvard.eduyamdatay ampdatayKurucz23ysekur.
Diode laser absorption measurement of uranium isotope ratios in solid samples using laser ablation夞 H. Liu, A. Quentmeier*, K. Niemax Institute for Spectrochemistry and Applied Spectroscopy (ISAS), Bunsen-Kirchhoff-Strasse 11, D-44139 Dortmund, Germany Received 25 April 2002; accepted 29 July 2002
Abstract The isotopes 235U and 238U were simultaneously measured by absorption in a laser-induced plasma ignited by a Nd:YAG laser. Two separate diode lasers were used which probed almost the same plasma volume. The diode lasers were tuned to the absorption lines 682.6736 nm for 235U and 682.0768 nm for 238U. The 235U y 238U isotope ratio was measured on a pulse-to-pulse basis evaluating the transient absorption peaks as analytical signals. Three uranium oxide pellets were used for calibration with natural (0.714 wt.%) and depleted 235 U concentration (0.407 and 0.204 wt.%). The measured 235Uy238 U mass ratio in an uranium containing mineral sample was (0.686"0.119)=10y2 and found to be in good agreement with the mass ratio of naturally occurring isotopes (0.719=10y2 ). The observed accuracy (-5%) and precision (;17% R.S.D.) are encouraging if it is taken into account that the mineral sample was probed directly without any preceding preparation step. Spatially and temporally resolved emission spectroscopic measurements were performed to investigate the kinetics of the laser plasma and to optimize the operating conditions of the laser ablation with regard to sensitivity and selectivity of the 235 U isotope detection. Optimal conditions were obtained at a distance of ;0.3 cm from the sample surface, an argon pressure of ;30 mbar and for 7 mJ pulse energy of the Nd:YAG laser. The limit of detection of the 235 U isotope, evaluated on the basis of the 3s criteria, was estimated to be 47 mg gy1 and improved by a factor of two compared with the sequential measurement with a single laser diode used in a previous work. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser ablation; Diode laser; Atomic absorption; Isotope ratio; Uranium
1. Introduction 夞 This paper is published in the special issue of Spectrochimica Acta Part B dedicated to the 50th anniversary of the ISAS. *Corresponding author. Fax: q49-231-1392-120. E-mail address: [email protected] (A. Quentmeier).
Over the past three decades laser ablation (LA) has become a versatile tool in analytical chemistry. The capability of LA as an easy to use and rapid sampling technique has opened up new analytical applications especially for the direct analysis of solids w1x. Apart from the well approved coupling
0584-8547/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 2 . 0 0 1 0 5 - 2
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of LA to optical emission spectrometry (LA-OES), also known as laser-induced breakdown spectroscopy (LIBS) w2x, the combination of LA and inductively coupled plasma with optical (LA-ICPOES) or mass spectrometric detection (LA-ICPMS) is widely used for in situ and microprobe analysis w3,4x. The outstanding sensitivity and detection power of the MS techniques makes them preferable for the determination of isotopic abundances which can be measured even at trace concentration levels if a sample preparation step like matrix separation is applied. The goal of our recent investigations was to develop an approach for isotope selective detection of 235U and 238U 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. The approach was based on laser ablation of a solid sample followed by the optical detection of free uranium atoms in the expanding laser plume. The possibility of using emission rather than mass spectrometric techniques is highly advantageous in view of minimizing the contamination of instrumentation. The determination of the 235Uy 238U isotope ratio by OES is a challenging task which requires an extremely high spectral resolving power of the optical spectrometer system. In addition, suitable excitation conditions are necessary to ensure that the widths of the emission lines are significantly smaller than their corresponding isotope shift. Several attempts have been reported to analyze the isotopic composition of solids by means of laser-induced breakdown spectroscopy w5,6x, but the quality of the results (although encouraging) cannot compete in respect to precision and sensitivity to those obtained with MS detection techniques w7,8x. A possible alternative could be the application of high-resolution laser spectroscopy. For example, diode lasers (DL) are easy to use light sources with narrow line widths (approx. 20 MHz). Therefore, the spectral resolution is appropriate to measure the isotope components of spectral lines either by atomic absorption spectroscopy (AAS) w9–12x, laser-induced fluorescence (LIF) w13x, optogalvanic spectroscopy w14,15x or ionization spectroscopy w16x.
In recent works it has been demonstrated that laser ablation can be combined successfully with DL based techniques probing free sample atoms in the expanding plasma plume. Smith et al. w12x and Quentmeier et al. w13x reported on results obtained with the detection of the 235U and 238U isotopes by LA-DL-LIF and LA-DL-AAS, respectively. In both investigations only one DL was used as the light source. In the LA-DL-LIF experiments the wavelength of the DL was tuned rapidly across the profiles of the absorption lines of both isotopes so that the fluorescence signals of 235U and 238U could be registered in every laser shot w12x. In LA-DL-AAS the DL wavelength was tuned sequentially to the respective line centers of both isotope lines and the corresponding absorption signals were detected on a shot-to-shot basis w13x. The optimum conditions for the detection of the 235 U isotope by LA-AAS differed greatly from the conditions required for LA-LIF. In LA-LIF a low pulse energy (;0.5 mJ) of the ablation laser (Nd:YAG laser) and a low buffer gas pressure (;1 mbar) had to be used. Furthermore, the optimum excitation region was approximately 0.8 cm from the sample in order to reduce the background by continuous plasma emission. The widths of the uranium lines were smaller than 1 pm and enabled even the observation of the hyperfine structure of the 235U transition w12x. In case of LA-AAS, the optimal experimental parameters were found to be at a higher gas pressure (;30 mbar) and closer (;0.2 cm) to the sample surface. These operating conditions ensured relatively high number densities of uranium atoms in the laser plume, which favored the measurement of AAS with regard to sensitivity and detection limit. As a consequence, the detection limit of 100 mg gy1 obtained with AAS was approximately one order of magnitude lower than obtained with LIF w12x. The wing of the broadened absorption line of the 238 U isotope was found to be the main source of the relevant analytical background signal which was responsible for limiting the precision of the sequential technique used so far. The measurement of the isotope ratio also suffered from poor reproducibility, since the absorption of the main isotope had to be measured on the wing of the absorption
H. Liu et al. / Spectrochimica Acta Part B 57 (2002) 1611–1623
1613
Fig. 1. Schematic set-up of the experimental system.
line instead of at the line center due to optical thick conditions. The main objective of the present work is the detailed characterization of the spatial and temporal evolution of the laser plume to improve the sensitivity and selectivity of the 235U isotope detection by LA-AAS. Plasma parameters such as temperature and electron number density, which strongly influence the absorption line profiles were investigated thoroughly to achieve optimum operating conditions. Furthermore, the absorption signals of both uranium isotopes were detected simultaneously with two separate DLs and evaluated on a shot-to-shot basis to improve the reproducibility, and thus the analytical capability of LA-DL-AAS. 2. Experimental The main components used in the present experimental system are shown schematically in Fig. 1. Sampling was performed by a Nd:YAG laser (Model SL401, Spectron Laser Systems) operating at the fundamental wavelength of 1064 nm with a typical pulse energy of 7 mJ and a repetition rate of 2 sy1. The ablation was performed in a sealed chamber with reduced argon pressure. All meas-
urements were performed under continuous gas flow (approx. 0.5 l miny1). The argon pressure was carefully adjusted in the range of 0.4–200 mbar by a needle valve in the by-pass of the pumping line and controlled by a capacitance manometer (Baratron, MKS Instruments). In contrast to our previous absorption measurements with only one laser diode w13x, two 50 mW single mode laser diodes (Mitsubishi Electric Corp., type ML1013R-01) were used. The DL beams formed an angle of approximately 48 and intersected each other on the axis of the ablating Nd:YAG laser beam at a well defined distance from the sample surface. This enabled simultaneous measurements of the absorption signals of both uranium isotopes at two different wavelengths and under similar plasma conditions because both DLs probed almost the same volume of the laser plasma. Obviously, a strict collinear alignment of the two diode laser beams would have been even better. However, in this case the laser intensities and absorption signals have to be modulated in the MHz range, due to the fast absorption, in order to separate the absorption signals. Modulation as well as demodulation is not easy. The diodes were powered by a combined temperature and current controller (Model ITC502,
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Profile Optische Instrumente GmbH, Germany). The DLs could be tuned continuously without mode hops over typical ranges of approximately 30–50 pm, which allowed tuning across the complete U line with its isotope components. Extended fine-tuning of the DLs was achieved with a short Etalon-type external laser resonator w17x, which yielded a weak optical feedback and thus an improved wavelength stability. The single mode structure and wavelength stability of the DLs were monitored by means of an interferometric wavemeter (ATOS Lambdameter, model LM007). After passing the chamber the DL radiation was measured by photodiodes (type S1223-01, Hamamatsu Photonics) which have a rise time of less than 20 ns. The photodiode signals were amplified by a low gain 100 MHz bandwidth integrated amplifier stage (type OPA603, Burr–Brown). The effective bandwidth of the detection system was decreased slightly to reduce pick-up noise from the power supply of the Nd:YAG laser in the early phase of plasma formation. The diodes were positioned approximately 50 cm behind the sample chamber to minimize the influence of background emission of the laser plasma. The solid angle of observation was further reduced by two apertures (⭋s0.3 cm) in the beam path of each diode. A first aperture was positioned near to the exit window of the sample chamber and a second one in front of the photodiode. The output signals of both photodiode circuits were registered simultaneously by a 4channel 500 MHz digital storage oscilloscope (Tektronik, model 714L). The trigger signal was provided by a PIN photodiode which monitored the Nd:YAG laser output pulse. The absorption was evaluated in the usual way, the peak transmission (or the transmission at any time) of the transient absorption signal was related to the full intensity of the laser diode as measured without ablation. Detailed information on the relevant excitation parameter and the expansion properties of the laser plasma was obtained by additional emission spectroscopic measurements with spatial and temporal resolution. A 1-m monochromator (SPEX M1000, aperture fy8) was used for this purpose. The spectrometer was equipped with a 2400-lines mmy1 grating and offered a typical linear dispersion of approximately 0.4 nm mmy1. An intensi-
fied charge coupled device (ICCD, Princeton Instruments Inc., model TEyCCD-576) was used as the detection system. It allowed time resolved observations with a gate width as short as 50 ns. In combination with the detector array (576=384 pixels of approx. 25 mm in size) the practical dispersion of the optical system was equivalent to approximately 11 pm pixely1 at 682 nm. The instrumental width of the spectrometer was determined with a light source of narrow line width (e.g. a diode laser or a hollow cathode lamp) and was approximately 2 pixel which corresponds to approximately 22 pm. Three uranium oxide samples (UO2qx; 0.05FxF0.15) were used in the present experiments. The samples were prepared and specified by the Institute of Transuranium Elements (Karlsruhe, Germany) in form of pellets with a specific gravity of rs10.4 g cmy3. One of the samples contained uranium in the natural isotopic composition (0.714 wt.% 235U), whereas the concentration of the minor isotope 235U of the two other samples was depleted to 0.407 and 0.204 wt.%. The homogeneity of the pellets was investigated by secondary electron micrographs (SEM) which indicated an average size of the micro structure of approximately 10 mm, and EDX line scans proved the uniform distribution of uranium oxide particles in the samples. Table 1 gives several U I and II lines with lower and upper energy levels, Log (gf)-values, and isotope shifts 238-235IS w18,19x which can be used in DL-AAS. Although most of them were tested in the experiments, the final measurements for the 238 U isotope were performed with the U I lines 682.0768 and 682.6913 nm, whereas the 235U isotope was detected with the absorption line 682.6736 nm. 3. Results and discussion The best operation parameters for the detection of the 235U isotope, i.e. ;30 mbar argon and 0.2– 0.3 cm distance from the sample surface, represent a compromise between a sufficiently large number density of absorbing uranium atoms and moderate plasma conditions which ensure narrow widths of the uranium absorption lines. As demonstrated in
H. Liu et al. / Spectrochimica Acta Part B 57 (2002) 1611–1623 Table 1 Spectroscopic constants of atomic and ionic uranium lines with large isotope shift (IS) Wavelength (lair, nm)
Lower level (cmy1)
Upper level (cmy1)
Log (gf)
II-405.0041 II-424.4373 II-425.2426 I-682.0768b I-682.4463 I-682.6913a I-683.2719
0 0 4420.872 4275.707 7103.921 0 7000.532
24684.135 23553.975 27930.242 18932.767 21753.047 14643.867 21636.957
y0.713 y1.278 y1.251 y1.690 y1.803 y1.679 y2.117
238-235
IS
(pm) y9.5 y25.1 y15.5 y12.8 y8.6 y17.7 y16.8
Spectroscopic data from Smith et al. w23x, isotope shift data for neutral uranium from Engleman and Palmer w18x, and for singly ionized uranium from Blaise and Radziemski w19x. a Used for detection of 235U and 238U isotopes. b Used as reference line for determination of 235Uy238U isotope ratio.
previous LIF measurements w12x, the absorption of the 238U isotope could also be measured at greater distances (;0.5–0.8 cm) because of the relatively high abundance of approximately 99.3 %. However, the detection of the 235U isotope required a much higher number density of absorbing uranium atoms to yield an adequate SyN ratio. The measurement near to the sample and at an early stage in the plasma evolution led to a very strong absorption by the 238U isotope and a significant broadening of its corresponding absorption line. Therefore, the absorption of the 238U isotope had to be measured in the line wing at a definite wavelength distance (5 pm) from the line center, which resulted in poor reproducibility. The precision, i.e. the relative standard deviation (R.S.D.f10–15%) obtained with the uranium oxide pellets was not adequate for the precise measurement of the 235Uy 238U isotope ratio. The measurement of the isotopes at different positions in the plume, 235U at a short sample distance, 238U at a greater sample distance with reduced atom density, cannot be recommended due to poor correlation between the absorption signals of both isotopes. 3.1. Plasma diagnostics The evolution of the laser plasma was thoroughly investigated by time-resolved emission measure-
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ments to study the influence of the plasma parameters on the spectral line widths and thus on the analytical capability of LA-DL-AAS. For this purpose the plasma was imaged on to the entrance slit of the spectrometer by a single lens (image ratio 1:1) which probed a well-defined plasma region of 30 mm width equivalent to the geometrical slit width. The plasma emission was registered at a definite sample distance which was varied by means of a precise linear translation stage providing detailed information on the different plasma zones. The temporal evolution of the plasma was monitored with a gated ICCD detector (gate widths0.1 ms). The evolution of the emission spectra in the range 545–553 nm during plasma expansion is presented in Fig. 2a–d. The spectra were measured under the standard conditions used for the detection of isotope absorption (0.2 cm sample distance and 30 mbar argon pressure) and are shown after background correction. The weak emission observed with zero delay was probably due to recombination background from the low pressure Ar plasma (Fig. 2a), which was superimposed by argon lines at 0.1 ms delay (Fig. 2b). Ionic uranium lines dominated the spectrum after a delay of 1 ms (Fig. 2c), whereas atomic uranium lines were not prominent before 10 ms (Fig. 2d). The dominating role of uranium ion lines, which were still present at 10 ms after plasma ignition, implied that the uranium atoms were completely ionized in the early phase of plasma evolution. The presence of ionic lines enables the detection of the uranium isotopes by means of their ionic absorption lines if the lines have a sufficiently large isotope shift. In principle, the measurement of isotope ratios by ionic lines should be better than by atomic lines, because of the relatively high number density of uranium ions shortly after plasma ignition. However, one serious obstacle of ion detection is the Stark broadening of the absorption lines, which affects the isotope selectivity of the measurement. The excitation temperature was deduced from Boltzmann plots obtained on uranium lines in the wavelength range 547–550 nm (Table 2). Various operating conditions (pressuress1–30 mbar, observation heightss0.1–0.4 cm above the sample surface) and different delay times were used to
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investigate their influence on the excitation temperature. The results are presented in Fig. 3 at two different buffer gas pressures. As shown the excitation temperature decreased rapidly from approximately 8=103 K within the first microseconds of plasma formation to moderate values of approximately 3.5–4=103 K, which were observed in the expanding plasma after approximately 20 ms. This finding confirms our experimental results that a delay time of ;15–25 ms was most favorable for the detection of atomic absorption. The argon lines measured at short distance from the sample and early in the plasma formation revealed a remarkable broadening and a distinct red shift. Line broadening and shift were clearly emphasized in the dense center of the plasma
Table 2 Spectroscopic constants of the uranium ion lines used in Boltzmann plot temperature determination Wavelength Lower level Upper level E upper (cmy1) (cmy1) (eV) (lair, nm) II-547.5706 9241.966 II-548.0265 12513.884 II-548.1203 6445.033 II-548.2526 15392.416 II-548.7004 5790.641 II-549.1222 8510.866 II-549.2952 0 II-550.4128 6445.033
27499.379 30756.109 24684.135 33627.116 24010.461 26716.691 18200.092 24608.170
3.41 3.81 3.06 4.17 2.98 3.31 2.26 3.05
level
Log (gf) y1.276 y0.934 y1.677 y0.773 y2.060 y1.727 y2.110 y1.924
Spectroscopic data from Smith et al. w23x.
Fig. 2. Temporal evolution of the laser plasma in the wavelength range 545–553 nm. The ionic uranium lines shown were used for the determination of the excitation temperature. Gate widths0.1 ms. The delay times used are given in (a)–(d): 0 ms (a); 0.1 ms (b); 1.0 ms (c); and 10 ms (d). Sample distances0.2 cm, and pressures30 mbar. Sample, uranium pellet with 0.714 wt.% 235U.
H. Liu et al. / Spectrochimica Acta Part B 57 (2002) 1611–1623
Fig. 3. Excitation temperature of the laser plasma as derived from Boltzmann plots using ionic uranium lines (see Table 2). Sample distances0.2 cm, pressures30 mbar (h) and 8 mbar (d). Sample, uranium pellet with 0.714 wt.% 235U. The insert shows Boltzmann plots derived with 30 mbar pressure and with different delay times: 1 ms (a); and 10 ms (b).
when compared to the outer and cooler plasma region. The line broadening of the Ar I-696.54 nm emission line was measured at different plasma zones. For this purpose, the ‘binning mode’ was applied, i.e. a fixed number of pixels of the CCD were grouped together and evaluated separately with respect to the corresponding distance from the plasma center. The number density of electrons, ne, can be estimated if one assumes that the Stark effect was the main mechanism of the observed line broadening. The full width at half maximum (FWHM) of the Stark broadened line, Dl1y2, and the corresponding line shift, DlShift, are related to ne (my3) in the following form according to Lochte-Holtgreven w20x: Dl1y2s2w1q5.53=10y6ne1y4a .x=10y22wne =Ž1y0.0068ne1y6Ty1y2 e
(1)
DlShiftswŽdyw.q6.32=10y6ne1y4a y1y2 .x =Ž1y0.0068n1y6 e Te y22 =10 wne
(2)
where (dyw) is the ratio of shift to width. The Stark broadening parameters w and a of the Ar I696.54 nm line were taken from Griem w21x. They are weakly dependent on the electron temperature
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Te, which is of the order of 104 K in low-pressure plasmas. The FWHM were deduced from Lorentzian fitting of the argon line profiles and corrected for instrumental broadening. The contribution by Doppler broadening DlDs1.67lyc (2kTyAm)1y2 is small. It does not exceed 7 pm (for 8=103 K). The instrumental broadening of the spectrometer (;16 pm at 696 nm) was determined in a separate measurement with the neutral Ar line 696.54 nm emitted from a low current uranium hollow cathode lamp (HCL). The Stark width Dl1y2 was deduced from the measured line width after correction for Doppler width and instrumental width. The argon line shift was measured relative to the argon line from the HCL, and ne was estimated according to Eqs. (1) and (2) as a function of the excitation temperature (see Fig. 3). 3.2. Double beam technique As already mentioned above, the approach of our previous DL-AAS experiment was the sequential detection of the uranium isotopes with only one diode laser. The wavelength of the DL was tuned to the absorption line of the 235U isotope and the absorption was measured by accumulating the results of successive laser shots. After tuning the DL wavelength to the wing of the 238U line the measurements were repeated under controlled and stable operating conditions. The relevant background absorption signal was determined in a third independent measurement. Here, the DL wavelength was tuned to the opposite side of the 238U line wing without the minor isotope component. It is quite clear that the reproducibility of the measurement was dependent on the wavelength setting of the DL. The temperature of the DL was stabilized within 10y3 8C and the DL wavelength was continuously monitored by the interferometric wavemeter to avoid any wavelength shift. The most prominent error source, however, was due to the ablation process itself. The typical shot-to-shot fluctuations and the drift effects, which were caused by successive sampling from the same target spot, deteriorate the reproducibility of laser sampling. Therefore, it can be expected that the situation would improve if the 235U and 238U
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Fig. 4. Measurement of the 235U and 238U absorption within the same laser plasma. The diode laser was tuned rapidly across the profiles of both isotopes by the linear current ramp applied to the diode laser controller. Sample distances0.2 cm, and pressures30 mbar. Sample, uranium pellet with 0.714 wt.% 235 U.
isotope absorption were measured in each laser pulse. This has been demonstrated already in our recent DL-LIF measurements where the DL wavelength was scanned by means of a linear current ramp pulse which was triggered by an external modulation frequency (;1.5 kHz) applied to the DL controller. The result of such a fast scan mode in LA-DL-AAS is presented in Fig. 4. The absorption signal of the 238U isotope was separated from the 235U component by 76 ms and is of the same order of magnitude as the 235U absorption due to the expansion of the plasma. The FWHM of the 238 U absorption line as measured in the diluted laser plume was estimated to be approximately 1.6 pm. The reproducibility of the isotope ratio determination obtained in the fast scan mode was worse than in the measurement with fixed wavelength mode because of shot-to-shot variations in the plasma size. Therefore, two individual DLs were used which enabled the simultaneous detection of both isotopes in the same volume of the laser plume. The simultaneous detection technique should overcome the shot-to-shot variations inherent in the ablation process because of the correlation between the absorption signals of both isotopes. As a consequence, one could expect a significantly improved reproducibility irrespective of the size or shape of the laser plume. Even an
inhomogeneous distribution of uranium atoms in the sample plume should have no effect. The two DL beams were carefully aligned to probe almost the same region of the laser plume. For this purpose the beams intersected each other on the axis of the Nd:YAG laser beam forming an angle of 48. For the correct alignment both DLs were tuned to the 235U isotope component providing the same absorption within an experimental error of approximately 5%. To avoid the measurement of the optically thick 238U component on the line wing, other weaker uranium absorption lines were investigated as possible reference lines for the 235U-682.6736 nm component. The 238U682.0768 nm line (see Table 1) was selected as the most suitable reference line. However, as shown in Table 1, both lines do not share the same lower level. The alternative absorption line starts at 4275.707 cmy1 above the ground level and should therefore be dependent on the argon pressure and the plasma temperature. This was studied by changing the pressure as well as the distance of the DL beams from the sample. Within the limits of uncertainty the ratios of the absorption peaks of 235U and 238U lines were found to be slightly dependent on the pressure near to the sample but almost independent of the distance in the ranges up to 2, 3 and 4 mm at a fixed pressure of 50, 30 and 10 mbar, respectively, as shown in Fig. 5. The increases in the 235Uy 238U absorption ratio are due to reduced Boltzmann populations of the 238U energy level at 4275.707 cmy1. 3.3. Optimization of the experiment The plasma diagnostic results obtained so far confirmed that the operating conditions which were selected on an experimental basis were appropriate for the detection of isotope absorption. The plasma temperature at 0.2 cm above the sample and 20 ms after the laser pulse was high enough to ensure good atomization of uranium dioxide ablated from the samples. For dissociation of the uranium dioxide molecules the temperature should exceed 2400 K w22x. At 7 mJ pulse energy the Nd:YAG laser ablated only a minute amount of material in a single laser shot. The mass removal was determined with 10
H. Liu et al. / Spectrochimica Acta Part B 57 (2002) 1611–1623
Fig. 5. Ratio of the peak absorption of the 235U-682.6736 nm line and the 238U-682.0768 nm reference line as obtained for different sample distances and buffer gas pressures equal to: 10 mbar (%); 30 mbar (h); and 50 mbar (j). Sample, uranium pellet with 0.714 wt.% 235U.
consecutive laser shots delivered on the polished surface of the uranium pellets (rs10.4 g cmy3). The crater profiles were measured by means of an interferometric microscope (model: NewView 5000, Zygo Corporation). The mass ablation of ;50 ngyshot corresponded to a mean number density of n(235U);1013 cmy3 provided that the atomization was complete and the uranium atoms were homogeneously distributed in a spherically shaped plume of ;0.5 cm diameter at 30 mbar pressure. Generally, these assumptions are not met and the number density of sample atoms detected at a certain delay is dependent on the position of observation. Therefore, the influence of the temporal and spatial distribution of the sample atoms on the isotope detection was investigated to achieve an optimum analytical capability. The diameter of the DL beam (1ye 2;0.15 cm) was reduced by an aperture (f;0.05 cm) in the beam path to ensure the required spatial resolution. The spatial distribution of the absorption signals measured with varying axial distance from the Nd:YAG laser beam at a distance of 0.3 cm from the sample is shown in Fig. 6. The optimum absorption was obtained within a range of "0.05 cm indicating the inhomogeneous distribution of sample atoms in the laser plume.
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The radiation of the Nd:YAG laser was focused onto the sample by a lens of 12 cm focal length forming a spot size of approximately 100 mm under best focus conditions. The depth of the evolving crater increased with increasing number of laser shots. As a result the laser plume changed from a spherical shape to a conical form. Therefore, only a limited number of laser shots were delivered to the same sample spot to improve the reproducibility of absorption measurements. The most satisfying conditions were found to be with a total number of shots less than 50 and the focus position just below the sample surface. In this case, the absorption signals were reproducible within ;2–3% R.S.D. The absorption signals obtained with the very first shots on the sample were less reproducible probably due to surface roughness. Therefore, the sample surface was preconditioned with approximately 10–20 laser shots before the measurements. The simultaneous measurements with two DLs could explain the double peak structure of the absorption signals which has already been discussed in our previous paper w13x. The separation into two distinct components was interpreted as the effect of two different phases of absorbing uranium atoms. Both components were most clear-
Fig. 6. 235U isotope absorption signals observed at different axial positions relative to the Nd:YAG laser beam. Sample distances0.3 cm, and pressures30 mbar. Sample, uranium pellet with 0.714 wt.% 235U.
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Fig. 7. Simultaneous determination of the 235U isotope absorption (solid line) and of the background signals (dotted line) observed with different buffer gas pressures of 30 mbar (a) and 50 mbar (b). Sample distances0.2 cm. Sample, uranium pellet with 0.714 wt.% 235U.
ly separated if the absorption was detected in the range of 0.2–0.3 cm above the sample and merged with increasing sample distance and pressure. The investigation was performed with the first DL tuned to the 235U absorption line, whereas the second DL was tuned to the ‘mirror’ position, i.e. the wavelength position in the red wing of the 238 U line, which corresponds to the isotope shift. In such a measurement the relevant analytical background of the 235U signal can be determined, since the line profile of the 238U absorption line is symmetrical. The results of the measurement are shown in Fig. 7 for two different buffer gas pressures. The first component of the double peak structure coincides with the background signal at 30 mbar (Fig. 7a), while at 50 mbar buffer gas
pressure (Fig. 7b) the first peak is not clearly visible. The contribution of the background is indicated by the rising edge of the 235U absorption signal. The background signal contributes significantly to the analytical absorption signal and has therefore to be taken into account. The distinct peak of the background signal observed at the rising edge of the absorption signal clearly indicated that the evaluation of the first component of the double peak structure is not favorable for the detection of the 235U isotope. This can clearly be seen in the results shown in Fig. 8 where the DL was tuned in a step-by-step fashion across the absorption lines of both uranium isotopes evaluating the first as well as the second component of the double peak structure. The first component yielded only a small 235U absorption superimposed on the wing of the 238U absorption line, whereas the evaluation of the second component resulted in a well detectable 235U absorption line. As has already been discussed in Quentmeier et al. w13x, the contribution by Lorentzian broadening is larger in lines obtained from the first component than in the profiles from the second. Obviously, the different line profiles reflect different conditions during the expansion of the laser plasma. Fig. 9 shows the FWHM of the 238U absorption line measured at different delay times in the range from 400 ns
Fig. 8. Line wings of the 235U-682.6736 nm absorption line derived from evaluation of the first (j) and the second peak (s) of the transient 235U isotope absorption signal. Sample distances0.2 cm, pressures30 mbar. Sample, uranium pellet with 0.714 wt.% 235U.
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Fig. 9. Full width at half maximum (FWHM) of the 238U682.6913 nm absorption line evaluating the transient absorption signals at different delay times. The line profiles were derived from successive measurements tuning the diode laser across the uranium absorption line. Sample distances0.1 cm, and pressures30 mbar. Sample, uranium pellet with 0.714 wt.% 235U.
to 40 ms. The significant broadening of the 238U absorption line observed at short delay times can be attributed, in particular, to Stark broadening which is also responsible for the broadening of the argon lines early in plasma formation. The relevant absorption was determined in two different ways evaluating either the peak values of the transient signals or the integrated signal areas. Both methods were tested and found to be nearly equivalent. However, the evaluation of the peak values was preferred because the background was measured at the same time as the absorption peak of the 235U isotope which takes into account the time dependence of the background signal shown in Fig. 7. 3.4. Calibration The ratios of the absorption peaks of the 235U682.6736 nm line and the 238U-682.0768 nm reference line were measured with approximately 120 laser shots and plotted vs. the certified 235Uy 238U ratios of the samples. The isotope ratios were measured with a precision of typically 2–4% R.S.D. indicating that the uranium oxide pellets used are sufficiently homogeneous. The relevant
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background signals were measured separately in the red wing 17.7 pm from the center of the 238U682.6913 nm line assuming a symmetric line profile. The background signal was equivalent to an absorption signal of ;0.4 %. The comparison with ;6% absorption obtained with the sample of natural 235U isotope concentration (0.714 wt.%) yields a corresponding SyB ratio of ;15. The relative standard deviation of the background signal which is responsible for the detection limit for 235 the U isotope was estimated to R.S.D.;2=10y4. Based on the 3s criterion an LOD of 47 mg gy1 was found, which is approximately a factor of two lower than the LOD of 100 mg gy1 obtained by the sequential technique with only a single DL w13x. The noise of the detector system was not relevant for the detection limit. The noise level was estimated to be equivalent to an absorption signal of ;10y2% corresponding to an SyN ratio )400. The improvement resulting from the simultaneous detection technique is demonstrated by the following data which were derived from repetitive measurements of the sample with 0.714 wt.% 235U concentration. The sequential technique yielded a precision of ;3.7% R.S.D. for the absorption measurement of the 235U isotope, and of ;1.6% R.S.D. for the 238U isotope evaluating the reference line. From these data the precision of the 235Uy 238 U isotope ratio was calculated by the law of error propagation to ;4.1% R.S.D. This result can then be compared to the 1.1% R.S.D. obtained by the simultaneous measurement of the isotope ratio on a shot-to-shot basis. 3.5. Isotope ratio measurement The detection of the isotope ratio was based on the previous calibration step. In order to prove the analytical capability of DL-AAS technique, we determined the 235Uy 238U isotope ratio in a natural geological mineral sample (type pitchblende) of unknown chemical composition and unknown uranium concentration. A small sample piece was broken from the mineral, and the resulting fragment (;20 g) was transferred into the ablation chamber without any further sample preparation. Material from different parts of the heterogeneous
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material was ablated to check the distribution of uranium. The poor shot-to-shot reproducibility (;70% R.S.D.) of the 235U absorption signal indicated a non-uniform distribution of uranium in the mineral sample. The net absorption was found to be relatively small and only a factor of approximately three above the background equivalent absorption. This finding was attributed to the noncomplete atomization of sample material, which partially covered the output window of the ablation chamber. No attempt was made to improve the efficiency of sample ablation by an increase of the pulse energy of the Nd:YAG laser in order to assure the same operating conditions as used for calibration. The measured 235Uy238U mass ratio of (0.686"0.119)=10y2 obtained on 10 different sample positions with a total of 200 laser shots, is in good agreement with the isotope ratio of natural abundant uranium (0.719=10y2). The accuracy of approximately 5% is encouraging, but the precision of approximately 17% R.S.D. is relatively poor if compared to the precision of typically 2– 3% R.S.D. obtained with the homogeneous uranium pellets. Obviously, a much larger number of laser shots is required in case of the mineral sample to achieve a higher precision. 4. Conclusion Diode laser atomic absorption spectrometry (DL-AAS) was applied to measure the 235Uy 238U isotope ratio in solid samples by laser ablation. Spatially and temporally resolved OES measurements were performed to optimize the operating conditions of laser ablation with regard to selectivity and sensitivity of the uranium isotope detection. The double peak structure of the transient absorption signals already mentioned in a recent paper was studied by the combination of DL-AAS and OES measurements and could be explained by the background due to the wing of the 238U absorption line. It was shown that the measurement of the 235 Uy 238U isotope ratio can be improved if two independent diode lasers are used. The simultaneous measurement of both isotopes enables the correlation of the absorption signals on a shot-toshot basis and thus the improvement of the precision and accuracy of the DL-AAS technique. The
method was applied to measure the 235Uy 238U isotope ratio of a natural mineral sample. The measurement approach described, using a simple sample geometry, diode laser excitation and laser sampling with a modest pulse energy Nd:YAG laser, should have application 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 favorably with alternative detection techniques having a similar goal, such as laser-induced plasma emission and optogalvanic spectroscopy. Acknowledgments Financial support of the Deutsche Forschungsgemeinschaft (Project Qu 102y1-1) is gratefully acknowledged. Furthermore, we thank Dr J.P. Glatz of the Institute of Transuranium Elements, Karlsruhe, for providing us with the uranium pellet samples. References w1x K. Niemax, Laser ablation—a reflection on a very complex technique of solid sampling, Proceedings of the Ninth Solid Sampling Spectrometry Colloquium, Merseburg, Germany, September 11–15, 2000. w2x F. Leis, W. Sdorra, J.B. Ko, K. Niemax, Basic investigations for laser microanalysis: I. Optical emission spectrometry of laser-produced sample plumes, Mikrochim. Acta II (1989) 185–190. w3x L. Moenke-Blankenburg, Laser-ICP-spectrometry, Spectrochim. Acta Rev. 15 (1993) 1–37. w4x R.E. Russo, Laser ablation, Appl. Spectrosc. 49 (1995) A14–A28. w5x 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 Part B 53 (1998) 751–761. w6x C.A. Smith, M.A. Martinez, D.K. Veirs, D.A. Cremers, 239 Puy 240Pu isotope ratios can be determined using LIBS, The Actinide Research Quarterly: 4th Quarter 2000, http:yywww.lanl.govyorgsynmtynmtdoyAQarchivey00winteryLIBS.html. w7x 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. At. Spectrom. 11 (1996) 57–60. w8x S. Uchida, R. Garcıa-Tenorio, ´ ´ K. Tagami, M. Garcıa´ Determination of U isotope ratios in environmenLeon,
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