Comparison of Ultraviolet Femtosecond and Nanosecond Laser

Oct 22, 2003 - improvement was achieved by switching from infrared to ultra- violet lasers ... lasers.22 The Q-switched mode delivers pulses typically...
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Anal. Chem. 2003, 75, 6184-6190

Comparison of Ultraviolet Femtosecond and Nanosecond Laser Ablation Inductively Coupled Plasma Mass Spectrometry Analysis in Glass, Monazite, and Zircon Franck Poitrasson,*,† Xianglei Mao,‡ Samuel S. Mao,‡ Re´mi Freydier,† and Richard E. Russo‡

Laboratoire de Ge´ ochimie, UMR 5563 “Me´ canismes de Transfert en Ge´ ologie”, Centre National de la Recherche ScientifiquesUPSsIRD, 38, rue des 36 Ponts, 31400 Toulouse, France, and Lawrence Berkeley National Laboratory, M/S 70-108B, One Cyclotron Road, Berkeley, California 94720

We compared the analytical performance of ultraviolet femtosecond and nanosecond laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The benefit of ultrafast lasers was evaluated regarding thermalinduced chemical fractionation, that is otherwise well known to limit LA-ICPMS. Both lasers had a Gaussian beam energy profile and were tested using the same ablation system and ICPMS analyzer. Resulting crater morphologies and analytical signals showed more straightforward femtosecond laser ablation processes, with minimal thermal effects. Despite a less stable energy output, the ultrafast laser yielded elemental (Pb/U, Pb/Th) and Pb isotopic ratios that were more precise, repeatable, and accurate, even when compared to the best analytical conditions for the nanosecond laser. Measurements on NIST glasses, monazites, and zircon also showed that femtosecond LA-ICPMS calibration was less matrixmatched dependent and therefore more versatile. Since the first experiments of Gray in 1985,1 laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) has become a widely adopted analytical technique, notably from the mid-1990s.2-4 Nevertheless, LA-ICPMS analysis still requires careful adjustment of operating parameters and data evaluation for high-quality results. The main limitation is the chemical fractionation that may occur during laser ablation,5,6 aerosol transport through preferential redeposition, and partial digestion in the plasma of the Ar torch.7 These latter phenomena are directly * Corresponding author. E-mail: [email protected]. Fax: +33-5-6155-81-38. † CNRSsUPSsIRD. ‡ Lawrence Berkeley National Laboratory. (1) Gray, A. L. Analyst 1985, 110, 551-556. (2) Durrant, S. F. J. Anal. At. Spectrom. 1999, 14, 1385-1403. (3) Gu ¨ nther, D.; Jackson, S. E.; Longerich, H. P. Spectrochim. Acta 1999, B 54, 381-409. (4) Russo, R. E.; Mao, X.; Liu, H.; Gonzalez, J.; Mao, S. S. Talanta 2002, 57, 425-451. (5) Chenery, S.; Hunt, A.; Thompson, M. J. Anal. At. Spectrom. 1992, 7, 647652. (6) Fryer, B. J.; Jackson, S. E.; Longerich, H. P. Can. Mineral. 1995, 33, 303312. (7) Guillong, M.; Gu ¨ nther, D. J. Anal. At. Spectrom. 2002, 17, 831-837.

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correlated to the particle size distribution,7 and therefore ultimately rely on the laser ablation process as well. The use of laser ablationmulticollector-ICPMS (LA-MC-ICPMS) on simple cases demonstrated that high precision in situ isotope measurements are possible if neither father/daughter ratio determinations nor isobaric interference corrections are required.8-10 However, laserinduced chemical fractionation is especially problematic for in situ geochronological applications involving U-Th-Pb radiometric systems, which require precise and accurate Pb/U and Pb/Th ratio determination. Fractionation is also deleterious for precise in situ isotope measurements, when isobaric interference corrections are required, such as 87Rb on 87Sr for Sr isotopes. Laser ablation therefore remains the weak component in LA-ICPMS analysis, which needs further development. Various approaches have been used in the past decade to reduce chemical fractionation to tolerable levels. A significant improvement was achieved by switching from infrared to ultraviolet lasers, typically quadrupling Nd:YAG lasers.11 Reducing the wavelength further proved to be of some additional benefit in this respect, but chemical fractionation effects are still observed at 193 nm12 and even down to 157 nm.13 Other approaches to minimize fractionation involved continuous laser refocusing,14 short ablation times,15,16 beam defocusing,17 or larger spot sizes.18,19 These (8) Christensen, J. N.; Halliday, A. N.; Godfrey, L. V.; Hein, J. R.; Rea, D. K. Science 1997, 277, 913-918. (9) Christensen, J. N.; Halliday, A. N.; Lee, D. C.; Hall, C. M. Earth Planet. Sci. Lett. 1995, 136, 79-85. (10) Willigers, B. J. A.; Baker, J. A.; Krogstad, E. J.; Peate, D. W. Geochim. Cosmochim. Acta 2002, 66, 1051-1066. (11) Chenery, S.; Cook, J. M. J. Anal. At. Spectrom. 1993, 8, 299-303. (12) Eggins, S. M.; Kinsley, L. P. J.; Shelley, J. M. G. Appl. Surf. Sci. 1998, 127-129, 278-286. (13) Russo, R. E.; Mao, X. L.; Borisov, O. V.; Liu, H. J. Anal. At. Spectrom. 2000, 15, 1115-1120. (14) Hirata, T.; Nesbitt, R. W. Geochim. Cosmochim. Acta 1995, 59, 2491-2500. (15) Poitrasson, F.; Chenery, S.; Bland, D. J. Earth Planet. Sci. Lett. 1996, 145, 79-96. (16) Poitrasson, F.; Chenery, S.; Shepherd, T. J. Geochim. Cosmochim. Acta 2000, 64, 3283-3297. (17) Knudsen, T. L.; Griffin, W. L.; Hartz, E. H.; Andresen, A.; Jackson, S. E. Contrib. Mineral. Pet. 2001, 141, 83-94. (18) Horn, I.; Rudnick, R. L.; McDonough, W. F. Chem. Geol. 2000, 167, 405425. (19) Bruguier, O.; Te´louk, P.; Cocherie, A.; Fouillac, A. M.; Albare`de, F. Geostand. Newslett. 2001, 25, 361-373. 10.1021/ac034680a CCC: $25.00

© 2003 American Chemical Society Published on Web 10/22/2003

solutions remain application-specific and cannot be adapted to every sample. Other authors found that raster scanning the laser beam across the sample reduced elemental fractionation, thus resulting in more accurate elemental ratios.20,21 This rastering method, though, leads to a significant degradation of the spatial resolution, one of the key advantages of LA-ICPMS over conventional techniques involving wet chemistry. Recently, the use of homogenized laser beams yielded prolonged ablations with chemical fractionation essentially observed during the first few tens of seconds of ablation only.7 Filtering out the biggest particles reduced fractionation to negligible levels, but at the price of a drastic loss of signal, by a factor of 5 or more. Other alternatives should therefore be explored. Early analysts using laser ablation rapidly realized the benefit of Q-switched lasers against N-mode (normal-mode, long-pulse) lasers.22 The Q-switched mode delivers pulses typically several thousand times shorter than the N-mode, drastically reducing thermal effects, which are largely responsible for chemical fractionation.5 More recently, Russo et al.23 have shown that picosecond lasers permit more stoichiometric ablation than the nanosecond Q-switched lasers commonly used in LA-ICPMS analysis. There is, however, a theoretical frontier in the ablation regime estimated to be close to 1 ps.24 Below the picosecond pulse length, the ablation process becomes significantly less thermal. Comparative experiments using nanosecond and femtosecond ablation illustrated that the heat-affected zone around a femtosecond-induced crater became undetectable by transmission electron microscopy.25 An initial study involving infrared femtosecond laser ablation26 showed promising capabilities for ICPMS analysis. For this paper, we performed a further investigation of femtosecond laser ablation by comparing nanosecond and femtosecond laser pulses using the same ultraviolet wavelength, the same optical arrangement, and the same ICPMS analyzer. In addition to standard NIST glasses, this study was carried out on monazite ((Ce,La,Nd,Th)PO4) and zircon (ZrSiO4) with known chemical and isotopic compositions. These two minerals are the principal choice for U-Th-Pb geochronology.27 EXPERIMENTAL SECTION The experimental configuration, outlined elsewhere,26 involves a femtosecond laser system consisting of a Spectra Physics Mai Tai Ti:sapphire seed laser and Spitfire regenerative amplifier. The system is pumped by Quanta Ray nanosecond Nd:YAG lasers at 532 nm. The resulting 100-fs pulses at 800 nm were frequency tripled to obtain a 266-nm wavelength. The Nd:YAG laser (New Wave Research) used for this comparative study delivers 6-ns pulses. It was frequency quadrupled to provide 266 nm as well. (20) Kosler, J.; Tubrett, M. N.; Sylvester, P. J. Geostand. Newslett. 2001, 25, 375-386. (21) Li, X. H.; Liang, X. R.; Sun, M.; Guan, H.; Malpas, J. G. Chem. Geol. 2001, 175, 209-219. (22) Moenke-Blankenburg, L. Laser micro analysis; John Wiley and Sons: New York, 1989. (23) Russo, R. E.; Mao, X.; Borisov, O. V. Trends Anal. Chem. 1998, 17, 461469. (24) von der Linde, D.; Sokolowski-Tinten, K.; Bialowski, J. Appl. Surf. Sci. 1997, 109-110, 1-10. (25) Le Harzic, R.; Huot, N.; Audouard, E.; Jonin, C.; Laporte, P.; Valette, S.; Fraczkiewicz, A.; Fortunier, R. Appl. Phys. Lett. 2002, 80, 3886-3888. (26) Russo, R. E.; Mao, X.; Gonzalez, J.; Mao, S. S. J. Anal. At. Spectrom. 2002, 17, 1072-1075. (27) Poitrasson, F.; Hanchar, J. M.; Schaltegger, U. Chem. Geol. 2002, 191, 3-24.

Both lasers had a Gaussian energy beam profile and were operated at 10 Hz. The femtosecond beam diameter was ∼10 mm. The original nanosecond beam diameter was 2 mm, and it was expanded to 10 mm as well. The optical setup was such that the same beam properties (spot size, energy, fluence) for both lasers were delivered to the sample. The femtosecond and nanosecond lasers were run alternatively using the same custom-made ablation system, consisting of a plano-convex lens with a 10-cm focal length and a glass ablation cell mounted on an XYZ stage.28 The output energy was monitored continuously during the experiments and adjusted to reach 16 J/cm2 on the sample surface. The nanosecond beam energy was stable with (e2.5% relative standard deviation (RSD), whereas the femtosecond laser beam displayed about twice that RSD, at (e5%. The resulting oval craters had a width and length of approximately 40 × 60 µm. The ICPMS instrument was a PQ3 (Thermo elemental) operated at a forward rf power of 1350 W and Ar flow rates of 14, 1, and 0.9 L/min. for coolant, auxiliary, and carrier, respectively. The dwell time at each isotope was 8 ms. For each analysis, an ∼10-s Ar blank was acquired followed by 4 min of signal acquisition while pulsing the laser at the same spot (crater formation, no rastering). Each crater was separated on the sample by at least 500 µm. The samples were SRM NIST610 and 612 glasses, two monazites (Manangotry from Madagascar and Moacyr from Brazil), and the CNRS-CRPG zircon standard 91500. These samples were either in the form of polished slabs or grains mounted in epoxy and polished. RESULTS AND DISCUSSION Ablation Characteristics. Typical ICPMS response for 238U during femtosecond and nanosecond ablation sampling of NIST610 glass are reported in Figure 1A. The femtosecond signal intensity initially increases and then continuously decreases as the crater is formed in the sample. In contrast, the nanosecond signal intensity shows a large spike when ablation first starts, followed by the characteristic slow increase/decrease. The general shape of the signal intensity versus time represents the change in ablation efficiency (amount of material ablated per pulse) and particle size distribution entering the ICP as the crater is formed. The first minute of femtosecond ablation leads to a much more intense signal than the nanosecond laser, rejecting the initial spike. The initial spike is commonly observed using nanosecond laser ablation with transparent samples. Although both laser beams were at the same fluence of 16 J/cm2 and energy beam geometry, the large difference of pulse width (100 fs versus 6 ns) translates into a much higher irradiance for the femtosecond laser (160 TW/cm2) compared to the nanosecond one (2.7 GW/cm2). This key parameter led to the higher femtosecond ablation rate. In the transparent NIST612 glass, femtosecond-induced craters were ∼550 µm deep, which is typically 20% deeper than nanosecond craters. This higher ablation rate partly explains the higher femtosecond intensity on the first third of the time-resolved signal (Figure 1A), although smaller particle sizes may be an additional explanation. Direct observation of the particles produced by both lasers is currently under study. The contrasting nanosecond and femtosecond behavior visible in Figure 1A, typical for NIST610 and 612 glasses, (28) Leung, A. P. K.; Chan, W. T.; Mao, X. L.; Russo, R. E. Anal. Chem. 1998, 70, 4709-4716.

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Figure 1. (A) Representative ICPMS signals obtained during in situ nanosecond and femtosecond laser ablation of NIST610 glass. (B) Evolution of atomic 206Pb/238U ratio obtained by ICPMS during in situ nanosecond and femtosecond laser ablation of NIST610 glass. Data smoothing (sliding average) was applied to highlight the main trends.

was much less apparent for the monazites and zircon studied in this work. It is well known that the change in temporal response represents a convolution of laser and material properties. The actual time response is not as critical analytically as the accuracy and stability of the ratio of ablated elements during repetitive ablation and crater formation. The change in elemental ratio with time demonstrates how fractionation occurs/changes with crater formation. The raw 206Pb/238U ratio reported Figure 1B show that the femtosecond value remains nearly constant and is close to the actual value (see below). In contrast, the nanosecond 206Pb/238U ratio is initially close to the femtosecond ratio, but then departs significantly, ending with 3 times the initial value. In principle, the effect of crater aspect ratio on fractionation must be stronger for the femtosecond than the nanosecond laser because the crater is actually deeper (higher aspect ratio) for the femtosecond case (see above). For the nanosecond ablation, a crater aspect ratio of 6 was found to cause significant fractionation.29,30 Assuming a constant ablation rate and considering a measured nanosecond crater depth of ∼460 µm, significant nanosecond crater-induced chemical fractionation should occur ∼150 s after the start of ablation in these experiments. Accordingly, nanosecond 206Pb/ 238U departs increasingly from femtosecond ratios from about that (29) Mank, A. J. G.; Mason, P. R. D. J. Anal. At. Spectrom. 1999, 14, 11431153. (30) Borisov, O. V.; Mao, X.; Russo, R. E. Spectrochim. Acta 2000, B55, 16931704.

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time (Figure 1B). Plotting 206Pb/238U against 206Pb or 238U intensities confirms the contrasted nanosecond and femtosecond fractionation evolution (Figure 2). Whereas the ratio remains the same at any intensity for femtosecond ablation, the 206Pb signal is much higher and scattered than 238U toward the end of the nanosecond ablation, and the 206Pb/238U increases. The key aspect of this study is the comparison of two laser pulse lengths while maintaining other experimental conditions as much as possible identical, including the laser wavelength. Comparison with literature data thus cannot be as rigorous given the varied experimental conditions used (laser energy profile, optical arrangement, wavelength, ICPMS type and settings). Nevertheless, in situ ablations involving 193- (ArF) and 157-nm (F2) excimer lasers12,13,18 and using experimental conditions approaching those used in this study yielded Pb/U fractionation effects of the magnitude of that observed here with our nanosecond 266-nm laser (Figure 1B), that is, much larger than the variations observed by UV femtosecond laser ablation. Besides a possible more favorable (smaller) particle size distribution during femtosecond laser ablation, it is likely that the less thermal femtosecond laser ablation is responsible for the lack of chemical fractionation despite a less favorable femtosecond crater aspect ratio (see above). Electron microscopic examination reveals that nanosecond craters display melting features (Figure 3), which are not observed from femtosecond craters. Ejecta around the femtosecond crater are essentially made of well-

Figure 2. Atomic 206Pb/238U ratio against 206Pb intensity (top) and 238U intensity (bottom) obtained by ICPMS during in situ nanosecond and femtosecond laser ablation of NIST610 glass shown in Figure 1. The nanosecond signal, after an initial intensity spike, shows a second signal increase at nearly constant 206Pb/238U. The signal then decreases, and this is accompanied by a 206Pb/238U increase. Note the contrasted behavior of Pb and U, especially in the last part of the path. In contrast, the femtosecond 206Pb/238U ratio remains constant whatever the intensity.

separated particles with rough shapes (Figure 3A), suggesting that they condensed quickly and were not subjected to protracted heating. In contrast, the nanosecond crater is surrounded by heaps resulting from the agglomeration of melted matter (Figure 3C). Partly melted particles can be observed on the surface of these heaps, indicating that they probably make up the material forming these accumulations. It is also noteworthy that these knolls cover unmelted particles, suggesting that the melting effect occurred toward the end of the nanosecond ablation sequence, in agreement with the observed time evolution of chemical fractionation (Figures 1B and 2). Such melted ejectus deposited around Gaussian nanosecond laser ablation craters is a well-known effect.5,6,15,31 An other noticeable feature is that whereas the femtosecond laser ablation crater walls display the ripples (Figure 3B) typical of interferences between the incident laser light and the scattering light,32,33 these ripples are largely erased on the nanosecond crater walls (Figure 3D), likely as a result of melting subsequently to (31) Jackson, S. E.; Longerich, H. P.; Dunning, G. R.; Fryer, B. J. Can. Mineral. 1992, 30, 1049-1064. (32) Guosheng, Z.; Fauchet, P. M.; Siegman, A. E. Phys. Rev. 1982, B26, 53665381.

initial ablation or melted material redeposition. A similar observation was also made recently by an other group.34 In agreement with theoretical considerations,24 these features indicate that femtosecond laser ablation is less prone to thermal effects on the material ablated and results in more accurate 206Pb/238U ratios. These elements were therefore not fractionated according to their oxide melting temperatures, as observed for nanosecond ablation.30 It is apparent from Figure 1 that the best conditions for nanosecond laser ablation is in the 20-50-s time slice, after the initial spike and before chemical fractionation becomes significant. Since our purpose is to evaluate whether femtosecond laser ablation is actually analytically superior over nanosecond ablation, we use the most favorable situation for nanosecond results in the quantitative comparison that follows. Precision. The 206Pb/238U ratio of each individual quadrupole mass scan was calculated from the 20-50-s acquisition time slice for femtosecond or nanosecond ablation. No background correc(33) Bonse, J.; Baudach, S.; Kru ¨ ger, J.; Kautek, W.; Lenzner, M. Appl. Phys. Lett. 2002, A74, 19-25. (34) Niemax, K. Fresenius J. Anal. Chem. 2001, 370, 332-340.

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Table 1. 206Pb/238U Atomic Ratio Measured by Femtosecond and Nanosecond Laser Ablation ICPMSa femtosecond measured (SD) NIST610 NIST612 monazite Manangotry monazite Moacyr zircon 91500

0.21832(90) 0.2487(36) 0.0726(21) 0.0608(22) 0.1761(78)

nanosecond

calibrated (SD)

RSD (%)

measured (SD)

0.0858(25) 0.0718(26) 0.2042(95)

0.41 1.44 2.96 3.63 4.63

0.2321(30) 0.2631(85) 0.0704(14) 0.0621(26) 0.183(20)

known value

calibrated (SD)

RSD (%)

mean

RSD (%)

0.0782(18) 0.0690(30) 0.201(23)

1.27 3.22 2.35 4.37 11.4

0.25799(86) 0.2884(16) 0.08992(83) 0.07658(9) 0.17924(37)

0.33 0.56 0.93 0.12 0.21

a The monazites were calibrated with NIST610 and the zircon with NIST612. Uncertainties apply to the last digit place and were calculated on 3 repeated analyses of NIST glasses, 10 repeats of monazites, and 5 repeats of the zircon. Relative standard deviation values apply to the calibrated value, if given. “Known values” are based on NIST-recommended concentrations for the glasses, recalculated from unpublished data of R. Parrish, A. M. Seydoux-Guillaume, et al.40 and M. Wiedenbeck, et al.39 for Manangotry monazite, Moacyr monazite, and zircon 91500, respectively.

Figure 3. Scanning electron microscope images, secondary electron mode, of typical femtosecond (B is a detail of A) and nanosecond (D is a detail of C) laser ablation craters in NIST610 glass. The nanosecond crater walls (D) display more obvious melting effects compared to the femtosecond’s (B). In particular, the ripples visible on the femtosecond crater walls are nearly erased for the nanosecond crater. There are also abundant melted ejecta around the nanosecond crater (darker gray knolls in C) that are essentially absent around the femtosecond crater (A). Scale bar is 5 µm. See text for discussion.

tion was applied given its negligible influence for the isotopes measured. The RSDs of these ratios are compared in Figure 4. The RSD of the 206Pb/238U ratio obtained by femtosecond laser ablation is approximately 10-12%, whereas the nanosecond results are less precise, typically between 15 and 20%. These figures could be improved by data filtering or smoothing. The purpose here was to have a relative comparison, however. The improved precision of femtosecond LA-ICPMS measurements is even more remarkable considering that the laser itself had about double the pulse-to-pulse instability compared to the nanosecond laser beam. 6188 Analytical Chemistry, Vol. 75, No. 22, November 15, 2003

Counting statistics expected from the total number of counts (Figure 4) show that the femtosecond laser should yield results with a 2 times better precision than the nanosecond data, simply because of higher signal intensity on the 20-50-s time slice (Figure 1A), supporting the relative difference of internal precision measured on the 206Pb/238U ratio. Furthermore, the 208Pb/206Pb isotopic ratio, which should not be influenced by thermal effects, displayed a precision similar to 206Pb/238U. Taken together, these observations show that in the 20-50-s ablation time slice, thermally induced fractionation during ablation became minor and does not have a key influence on the precision. Hence, the source of uncertainty lies elsewhere for nanosecond ablation, and it is proportional to the signal intensities. Figure 4 also shows that femtosecond internal precisions from spot to spot were much less variable compared to nanosecond LA-ICPMS analyses, a feature that may be attributed to a more controlled and repeatable ablation process, despite a less favorable femtosecond laser energy stability. Repeatability. Repeatability was evaluated on the basis of 10 successive separate analyses for each of the samples; NIST610 and 612 glasses, Manangotry and Moacyr monazites, and zircon 91500. The 20-50-s time slice was again used for data evaluation. The results in Figure 5 and Table 1 demonstrate that femtosecond ablation systematically produced a better repeatability than nanosecond results. Only the Manangotry monazite did not show this trend. Repeated experiments revealed that we are probably limited by sample heterogeneity for this monazite. Improved femtosecond repeatability is especially obvious for the zircon, on which only five repeats could be made due to the limited sample size. Although 206Pb/238U and 208Pb/232Th displayed a comparable uncertainty, as expected given the similar behavior of U and Th under nanosecond Gaussian beam ablation,6 the 208Pb/206Pb isotopic ratios were more repeatable than the elemental ratios (Figure 5). This indicates that whereas no difference could be detected between elemental and isotopic ratio by internal precision comparisons (Figure 4), this is no longer the case with our repeatability experiments. It should be noted, however, that this improved repeatability of isotopic ratio is observed for both nanosecond and femtosecond laser ablation. This effect is thus not pulse-length specific. Finally, although femtosecond laser ablation ICPMS permits better repeatability than the nanosecond laser, the results obtained during our experiments are not yet to the level of uncertainty typically reached by isotope dilution thermal ionization mass spectrometry (