Effects of Particle Size Distribution on Inductively Coupled Plasma

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Anal. Chem. 1999, 71, 5123-5130

Effects of Particle Size Distribution on Inductively Coupled Plasma Mass Spectrometry Signal Intensity during Laser Ablation of Glass Samples S. H. Jeong,† O. V. Borisov, J. H. Yoo, X. L. Mao, and R. E. Russo*

Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720

The relation between laser-generated particles and ICPMS signal intensity was investigated using single-pulse laser ablation sampling of solids. The particle size distribution of glass samples was measured using an optical particle counter for different laser ablation conditions. Ablation of a new surface produced fewer particles and lower ICPMS signal intensity than a preablated surface. Laser power density of 0.4-0.5 GW/cm2 was found to be a threshold value, across which particle size distribution changed. Laser beam diameter was a more influential parameter than power density in efficient particle generation. Particle loss during transport from the ablation chamber to the ICPMS was significant for a low carrier gas flow rate of 0.1 L/min, while almost no loss was observed for a higher flow rate of 0.26 L/min. The onset of ICPMS intensity time profiles decreased as more large particles were generated. ICPMS intensity data were calibrated with respect to the particle mass entering the ICPMS. Particle entrainment efficiency of the LA-ICPMS system was estimated and found to be a strong function of laser power density. Laser ablation (LA) with inductively coupled plasma (ICP), mass spectrometry (MS), or atomic emission spectroscopy (AES) provides unique advantages in solid sampling for chemical analysis. Unlike most other sampling methods, laser ablation requires very little or no sample preparation, eliminating timeconsuming dissolution procedures and issues related to contamination. Also, laser ablation sampling consumes only a very small amount of material, typically on the order of nanogram or micrograms per pulse, which is especially advantageous for characterizing toxic or radioactive samples.1 For ICPMS or ICPAES analysis, a solid sample is ablated inside a chamber, generating particles of various sizes. These particles are entrained into the carrier gas flowing through the chamber and transported to the ICP for analysis. The mass transported to ICP should have the same chemical composition as the sample. However, fractional ablation of different elements may occur during laser-sample * To whom correspondence should be addressed: (e-mail) [email protected]. † Present address: Mechatronics, Kwangju Institute of Science and Technology, Kwangju, Republic of Korea. (1) Alexander, M. L.; Smith, M. R.; Hartman, J. S.; Mendoza, A.; Koppenaal, D. A. Appl. Surf. Sci. 1998, 127-129, 255-261. 10.1021/ac990455a CCC: $18.00 Published on Web 10/07/1999

© 1999 American Chemical Society

interaction and the composition may be altered during transport.2-4 Laser ablation of solids involves processes of heating, melting, and evaporation of the sample material at extremely high temperature and pressure. Mass may be removed from the target in the form of atoms, molecules, vapor, droplets, solid flakes, large particulates, or a mixture of these forms. The size of these particles is the primary parameter that determines the transport efficiency of laser-ablated sample to the ICP. Large particles may not be completely vaporized even if they are successfully transported to the ICP, which may result in elemental fractionation in the ICP itself. The amount of mass removed from the sample per each laser pulse, entrainment of particles in the chamber, transport of particles through the tube, and atomization and ionization of particles in the ICP may all influence the efficiency of a LA-ICPMS system. The number and size distribution of particles generated during laser ablation will be dependent on the laser power density, beam diameter, wavelength, and properties of the sample. The dependency of particle size distribution and transport on laser parameters for LA-ICPMS have been reported in several papers,1,5,6 but these issues are not yet fully understood. In addition, the chemistry of the particles can be influenced by their sizes, affecting accurate analysis.7,8 In the present work, we measured the ICPMS signal intensity during single-pulse laser ablation of solids with respect to laser parameters, including power density and laser beam diameter. The size distribution and number of particles were also measured. We emphasized single-pulse experiments to investigate the relation between ICPMS signal and particle size distribution. With single-pulse measurements, the influence of particle size distribution on the temporal profile of ICPMS signal intensity can be measured. Also, single-pulse measurements enable the investigation of differences between unablated and preablated surfaces. Several issues related to the particle size distribution and ICPMS (2) Leung, A. P. K.; Chan, W. T.; Mao, X. L.; Russo, R. E. Anal. Chem. 1998, 70, 4709-4716. (3) Mao, X. L.; Ciocan, A. C.; Russo, R. E. Appl. Spectrosc. 1998, 52, 913-918. (4) Outridge, P. M.; Doherty, W.; Gregorie, D. C. Spectrochim. Acta Part B 1997, 52, 2093-2102. (5) Arrowsmith, P.; Hughes, S. K. Appl. Spectrosc. 1988, 42, 1231-1239. (6) Figg, D. J.; Cross, J. B.; Brink, C. Appl. Surf. Sci. 1998, 127-129, 287291. (7) Outridge, P. M.; Doherty, W.; Gregorie, D. C. Spectrochim. Acta Part B 1996; 51, 1451-1462. (8) Thompson, M.; Chenery, S.; Brett, L. J. Anal. At. Spectrom. 1990, 5, 4955.

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Figure 1. Schematic diagram of the experimental system for LA-ICPMS.

intensity were investigated in this work. The onset time and time for peak intensity of ICPMS signal was related to the size distribution of particles; transport of particles from the ablation chamber to the ICPMS via a tube was examined for different flow conditions; and total mass ablated was measured and compared with the corresponding mass of the particles entering the ICPMS, to determine entrainment and transport efficiency. EXPERIMENTAL SECTION The experimental system for laser ablation sampling ICPMS is shown in Figure 1. Particles produced from laser ablation of a solid sample inside the ablation chamber are entrained and transported out of the chamber by argon gas for size measurement and chemical analysis. To investigate the relation between ICPMS signal and particle size distribution, an optical particle counter (Particle Measuring Systems, LAS-X) and an ICPMS (VG Elemental, PQ3) were linked to the laser ablation sampling chamber. A Nd:YAG laser with wavelength and pulse duration of 266 nm and 5 ns, respectively, was used to ablate the samples. The laser beam was focused onto the sample surface using a lens with a focal length of 200 mm. The laser pulse energy for each ablation was measured using a pyroelectric detector and a joulemeter. The diameter and length of the ablation chamber were 3.2 and 12.7 cm, respectively. The laser beam diameter on the sample surface was determined by measuring the ablation mark produced on a polished crystalline silicon surface with a laser fluence just above the melting threshold. The distance between the sample and the chamber exit port was 11.0 cm. Glass from the Savannah River Site Vitrification Facility was used as the sample. The approximate composition of the sample (in wt %) is as follows: 50% SiO2, 11% Fe2O3, 9% Na2O, 8% B2O3, 4% Li2O, 4% Al2O3, 3% K2O, 2% MnO2, 1.5% MgO, 1% TiO2, and 1% CaO. These oxides comprise about 95% of the total sample composition. This glass sample is a prototype for vitrified radioactive waste products. The surface of the glass sample was polished to a roughness of ∼10 µm. 5124

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The optical particle counter can measure a range of particle diameter from 0.09 to 3.0 µm, over 16 channels. Particles were transported into the optical particle counter via a 2.3-mm-diameter stainless steel tube at a flow rate of 0.03-0.09 L/min. The flow rate of the argon gas through the chamber was adjusted to ensure isokinetic conditions at the sampling location (outlet of the chamber) and was set to 0.26 L/min. To measure particles for single-pulse laser ablation, the optical particle counter was set to start counting particles just before ablating the sample. After a laser pulse ablates the sample, the optical particle counter starts counting particles until there is no further increase in particle counts on each channel, implying that all the particles were swept out of the chamber. The argon gas itself had imbedded particles and those particles were detected on channel 1 of the optical particle counter; corresponding size range was 0.09-0.11 µm. The number density of these background particles was measured prior to each ablation and subtracted from the channel 1 data. The counting efficiency of channel 1 of the optical particle counter was 50% while that of other channels was 100%. Therefore the data from channel 1 had more uncertainty compared to that from the other channels. The flow rate of argon gas through the ablation chamber was 0.26 L/min, which was limited by the optical particle counter. The transport tube between the ablation chamber and the ICP had a diameter and length of 4.4 mm and 1.4 m, respectively, and was placed in the horizontal position. For the flow rate of 0.26 L/min, the argon gas flow inside the ablation chamber and the transport tube was laminar. To ensure optimum sensitivity of the ICPMS, additional argon gas flow of 0.74 L/min was added to the carrier flow before the ICP torch as shown in Figure 1. Other ICPMS operating conditions were optimized to achieve maximum sensitivity for 55Mn. Time-resolved ICPMS signal intensities for 7Li, 11B, 23Na, 24Mg, 28Si, 48Ti, 55Mn, and 138Ba were measured during singlepulse laser ablation of the sample with dwell time of 12 ms and three points per peak. Summation of signal intensities for these

isotopes was defined as the total signal intensity, which is proportional to the total mass ablated and transported to the ICPMS. Time-resolved signal intensities for the above specified isotopes were measured for each laser pulse. Depending on laser ablation parameters, such as spot size and pulse energy, it took ∼1 min for signal intensities to return to the background levels. The laser pulse was fired following a delay of 3 s after starting the ICPMS data acquisition. Intensity data recorded during the time delay were used in signal background calculations and subtractions. The total integrated intensity was defined as the ICPMS intensity integrated over acquisition time with background subtraction. Each sampling spot was ablated with five single-pulses and ICPMS data were collected separately for each of these experiments. Data for the first pulse correspond to the ablation of a polished surface while those for the second to the fifth pulse represent repeated ablation at the same spot. The variation of laser pulse energy among these five pulses was typically within 3%. These experiments were done in triplicates on different randomly selected surface areas to improve analytical precision. The volume of the craters produced in the sample by laser ablation was measured using a white-light interferometric microscope (New View 200, Zygo). The crater volume was defined as the volume below a reference surface minus a volume above the reference surface, where the reference surface was unablated sample surface. RESULTS AND DISCUSSION First Laser Pulse versus Following Pulses. Time profiles of ICPMS total signal intensity for selected elements, acquired by single-pulse ablation, are shown in Figure 2. Laser pulse energy for the data in panels a and b of Figure 2 was 0.04 and 0.71 mJ, respectively. Laser beam diameter for both experiments was ∼60 µm. The ICPMS signal intensity time profiles for single-pulse ablation consist of numerous irregular spikes.9 Despite the numerous spikes, the underlying shape of the ICPMS time profiles remains similar. The background-corrected and integrated signal intensity data are determined by laser ablation conditions. As shown in Figure 2, time profiles for the first pulse are always significantly different from those for the following pulses at the same surface location. Time profiles for the second through the fifth pulses were similar and therefore were averaged. The difference between the first and the following pulses is clearly seen when the integrated ICPMS signal intensities are compared (Figure 3). Integrated total intensity for the first pulse is always lower than that for the following pulses and does not significantly depend on laser energy. This is in contrast to metals and alloys in which the first pulse generally provides greater signal intensity.10 The second through the fifth pulses produced a similar number of total counts. Each data point in Figure 3 represents the averaged number of counts from three locations on the sample surface, indicating good pulse-to-pulse reproducibility. These data demonstrate that the amount of ablated mass is determined by laser conditions, not by the location on the sample surface, after the first pulse conditions the surface. To use laser ablation as a solid sampling method in chemical analysis, it is important that the amount of sampled mass be reproducible. Also, the statistical (9) Chan, W. T.; Russo, R. E. Spectrochim. Acta B 1991, 46, 1471-1486. (10) Mao, X. L.; Chan, W. T.; Russo, R. E. Appl. Spectrosc. 1997, 51, 10471054.

Figure 2. Time profiles of ICPMS total signal intensities for different laser pulse energies, (a) 0.04 and (b) 0.71 mJ, with spot size of 60 µm (dotted line, first pulse; solid line, average of second to fifth pulses). Insets represent the particle size distributions for the same conditions; numbers correspond to the laser pulse.

pulse-to-pulse deviations in elemental ratios in ablated mass must be minimal. When the ratios of integrated counts for 138Ba to 55Mn are plotted as a function of pulse number, significant deviations in the ratio were observed for the first pulse (Figure 3b). After the first pulse, particles with much more reproducible composition were produced, which is evident from the smaller error bars. This lower error, along with much lower total integrated signal intensity measured for the first pulse, indicates that the first pulse data are not useful and surface “conditioning” is required prior to chemical analysis. Data from the particle size distribution measurements, shown in the subsets of Figure 2, compliment the ICPMS data presented in Figures 2 and 3, for the same laser ablation conditions. The lower total number of particles produced by the first pulse results in a significantly lower integrated number of counts in the ICPMS. The number of particles generated by the first pulse was always smaller than the following pulses in all size categories, except very small particles. The reason for the small number of particles, low intensity in ICPMS signal, and large error in the chemical analysis data for the first pulse is not well understood. Surface effects including roughness, contamination, differences in reflectivity between ablated and unablated surfaces, etc., may be responsible for the differences. Once the glass surface was irradiated by the incident laser radiation, it melted and resolidifed to form a different surface morphology. From images of ablated Analytical Chemistry, Vol. 71, No. 22, November 15, 1999

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Figure 3. (a) Total integrated ICPMS signal intensities and (b) ratio of integrated counts for 138Ba and 55Mn for the first and successive pulses. Laser pulse energies for these data were 0.04 (b), 0.41 (9), and 0.71 mJ (2).

surfaces measured using a scanning electron microscope, droplets and wavy patterns were found on the glass surface after ablation. Also cracks in random directions were found over the ablated area. These changes in the sample surface can result in higher laser energy coupling to the sample for later pulses, through a smaller surface reflectivity or multiple reflection of laser lights between surface structures such as cracks and/or droplets. Because the measured ICPMS signal intensity and particle size distributions for the first pulse were very different from those due to the second through the fifth pulses, data for the first pulse were excluded from the following experimental work in this paper; the data from the second through the fifth laser pulses were used to represent the results for a specific experimental condition. Particle Size Distribution under Different Laser Ablation Conditions. The size distribution of laser-generated particles, with respect to the laser pulse energy, for the same laser beam diameter (60 µm) is shown in Figure 4. The laser pulse energy was varied from 0.04 to 0.71 mJ; corresponding laser power density ranges from about 0.3 to 4.8 GW/cm2. The error bars in the data represent data from four single-pulse measurements, excluding the first pulse data. Figure 4a shows that the size distribution changes with increasing laser power density. The relative number of large particles decreases for increasing power density up to about 0.4-0.5 GW/cm2. For laser power density above this value, the size distribution remains the same while the total number of 5126 Analytical Chemistry, Vol. 71, No. 22, November 15, 1999

Figure 4. Size distribution of particles with respect to the laser pulse energy. Laser beam diameter was ∼60 µm.

particles increases for increasing power density (Figure 4b). A measurable change in particle size distribution was found only at laser power density of about 0.4-0.5 GW/cm2. The power density of 0.4-0.5 GW/cm2 was also reported as the threshold for plasma shielding for many materials11-13 and is the same region where we have repeatedly measured a dramatic change in mass ablation rate.14 Even though the detailed processes of particle generation during laser ablation are not well understood, these data support that there exist fundamental differences in ablation mechanisms across this power density. Particle generation conditions may change across this power density. For power densities below this threshold value, melting and instability of the melt flow due to surface tension or lateral pressure gradients may be responsible for generation of particles.15 For power densities above this threshold, direct desorption of sample materials or energy transfer between a plasma and particles that can further heat to evaporate or break up particles is considered to affect the measured particle size distribution. The effect of laser beam diameter on the particle size distribution is shown in Figure 5. When the laser beam spot size was (11) Mao, X. L.; Chan, W. T.; Shannon, M. A.; Russo, R. E. J. Appl. Phys. 1993, 74, 4915-4922. (12) Mao, X. L.; Chan, W. T.; Caetano, M.; Shannon, M. A.; Russso, R. E. Appl. Surf. Sci. 1996, 96-98, 126-130. (13) Mao, X. L.; Russo, R. E. Appl. Phys. A 1997, 64, 1-6. (14) Shannon, M. A.; Mao, X. L.; Fernandez, A.; Chan, W. T.; Russo, R. E. Anal. Chem. 1995, 67, 4522-4529. (15) Brailovsky, A. B.; Gaponov, S. V.; Luchin, V. I. Appl. Phys. A 1995, 61, 81-86.

Figure 6. Computed transport time of spherical particles from the sample to the ICP for three initial ejection velocities.

Figure 5. (a) Size distribution of particles with respect to the laser beam diameter when laser pulse energy was ∼0.85 mJ. (b) Size distribution of particles for different spot sizes at similar power density.

varied with constant laser pulse energy (changing power density) (Figure 5a), both the size distribution and number of particles changed. For the data in Figure 5a, laser beam diameter was varied from about 60 to 140 µm with a constant laser pulse energy of 0.85 mJ. The laser power density at these conditions varies from about 1.1 to 5.8 GW/cm2, which is above the threshold value described previously. Even though the laser power density decreases due to increasing laser beam diameter, the number of particles of all sizes increased for increasing laser beam diameter. For approximately the same power density, larger beam diameter simply increased the total number of particles with no changes in the distribution (Figure 5b). Both graphs show that the number of particles, including large particles, increases substantially with increasing laser beam diameter. This result indicates that laser beam diameter is a more influencing parameter than the laser power density on the number of particles produced during laser ablation, for these glass samples. Correlation between Particle Size Distribution and ICPMS Temporal Profile. ICPMS signal intensity does not provide direct information about the particle size distribution. However, it is expected that particle size distribution can influence the temporal ICPMS signal intensity profile. When particles are ejected from the sample as a result of laser ablation, they will have different angles from the normal to the sample surface as well as different

ejection velocities, which are approximately 104-105 cm/s.16,17 In general, larger particles that have greater momentum travel a longer distance from the sample surface before they are entrained into the argon gas flow; smaller particles will be entrained much sooner. Therefore, particles of different sizes separate within the laser ablation chamber. After being entrained into the argon flow, particles of all sizes will travel with approximately the same velocity determined by the gas flow velocity. The distance a particle travels from the sample until entrainment, defined as entrainment distance, was calculated by assuming that all the particles are spherical and have the same ejection velocity normal to the target surface.18 Entrainment distance of particles increases linearly with particle diameter for the same ejection velocity. The carrier-gas flow velocity is several orders of magnitude smaller than the ejection velocity and will not affect the entrainment distance. Particles smaller than 0.5 µm will be entrained into the gas flow before traveling a few millimeters for ejection velocities between 104 and 105 cm/s. The entrainment distance of large particles, however, differs greatly according to their ejection velocity. For example, a 3-µm-diameter particle with ejection velocity of 104 cm/s travels only ∼1.7 cm before entrainment while the same particle with a velocity of 105 cm/s travels 7.6 cm. From the entrainment distance of different size particles and the velocity of argon gas flow, transport time of particles from the sample to the ICP can be estimated. Figure 6 shows this transport time for three different ejection velocities, i.e., 104, 5 × 104, and 105 cm/s. Using this transport time and measured particle size distribution, the actual onset time of ICPMS signal can be estimated. Particles separated in the ablation chamber due to differences in their initial momentum maintain almost the same separation distance during transport to the ICPMS. Diffusion of particles in the direction of flow during transport is negligible compared to (16) Geohegan, D. B. Appl. Phys. Lett. 1993, 62, 1463-1465. (17) Schenck, P. K.; Hastie, J. W.; Paul, A. J.; Bonnell, D. W. SPIE Proc. 1995, 2403, 26-38 (Laser-Induced Thin Film Processing). (18) Rudinger, G. Fundamentals of Gas-Particle Flow; Elsevier: Oxford, U.K., 1980.

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Figure 8. Time profiles of ICPMS total signal intensities for different laser beam diameters at constant laser power density of ∼0.3 GW/ cm2.

Figure 7. (a) Time profiles of ICPMS total signal intensities for different laser beam diameters. Laser pulse energy was 0.85 mJ. (b) Measured onset time of ICPMS time profiles with respect to laser beam diameter.

this initial separation. The sedimentation of large particles due to gravity in a vertical section of the transport tube is negligible; the sedimentation distance of a 3-µm-size particle during transport is only ∼3 mm, even if the transport tube was completely vertical; the original separation from the small particles due to initial momentum is ∼50 mm. Therefore, large particles that are in the front of the particle cloud enter the ICP first, determining the signal onset time. Considering separation of particles inside the chamber, the onset time of the ICPMS signal was estimated for the measured particle data in Figure 5a. Measured ICPMS time profiles for the same experimental conditions show significant changes in onset time and signal intensity Figure 7a. An increase of ICPMS signal intensity for a larger laser beam diameter is attributed to the increase of overall number of particles as shown in Figure 5a. The changes in signal onset time for the ICPMS time profiles are less straightforward. To understand these data, we need to consider both the measured particle size distribution and the separation of different size particles inside the ablation chamber. In Figure 5a, the measured particle size distribution shows not only the overall increase of particle numbers for increasing laser beam diameter but also the increase of absolute size of the largest particles. For example, the measured diameter of largest particles for a laser beam diameter of 60 µm is ∼1 µm while that for laser beam diameter of 140 µm is about 3 µm. Assuming that the ejection velocity of particles is equal to 5 × 5128 Analytical Chemistry, Vol. 71, No. 22, November 15, 1999

104 cm/s, the difference in transport time from sample to the ICPMS between 1- and 3-µm-size particles becomes ∼6 s (Figure 6). Therefore, the generation of relatively more large particles, using a larger laser beam diameter, should result in faster transport and shorter ICPMS signal onset time. The ICPMS signal intensity produced from these large particles is strong because these large particles contain most of the ablated mass, despite their low numbers in the size distribution data. The measured onset time of the ICPMS signal decreases linearly as the laser beam diameter increases (Figure 7b). The difference in measured onset time is reasonably close to the computed transport time for particle ejection velocity of to 5 × 104 cm/s in Figure 6. A similar approach can be applied to explain the difference in time when the maximum signal intensity occurs in Figure 2. The increase in laser pulse energy (with the same laser beam spot size) did not result in an increase in the maximum signal intensity. The maximum signal intensity remained almost the same for laser ablation with pulse energies in the range of 0.04-0.71 mJ. The shapes of the time profile, on the other hand, are different; the time profiles become wider as laser energy increases, with a shift of the maximum signal intensity to a later time, at about 12 and 24 s for 0.04 and 0.71 mJ, respectively. The relative number of small particles increased significantly when the laser pulse energy was increased from 0.04 to 0.71 mJ. This increase of smaller particles will result in a shift of the maximum ICPMS signal intensity time profile toward a later time, due to longer transport time as described above. The relatively unchanged particle size distribution during ablation with 0.04 mJ (cf. Figure 2a) results in similar ICPMS time profiles for the first and the following pulses. On the other hand, the significant increase of smaller particles, from the first pulse, for laser pulse energy of 0.71 mJ (cf. Figure 2b) is reflected by the shift of maximum ICPMS signal intensity time profiles. Increasing the laser beam diameter, while maintaining constant laser power density, results in a significant increase in ICPMS signal intensity. Figure 8 compares two ICPMS intensity time profiles for laser ablation with spot diameters of 60 and 130 µm

Figure 9. Measured particle size distributions before and after transport through a 2-m long tube, with 0.44-cm diameter, at a flow rate of (a) 0.1 and (b) 0.26 L/min.

at constant power density of ∼0.3 GW/cm2. An order of magnitude improvement in the total integrated number of counts was measured. For the same experimental conditions, particle measurement data showed ∼10 times increase in the number of particles over all sizes (cf. Figure 5b). Onset time of the ICPMS signal did not change very much for these data because no significant shifts in particle size distribution were observed between these two data sets. Particle Loss during Transport. The particles generated inside the ablation chamber are transported by the argon gas to the ICPMS via a transport tube. During the transport, particles can be lost on the tube walls due to inertial impact, gravitational settling, laminar and/or turbulent diffusion, or electrostatic attraction, if the tube wall has static charges. Particle loss in the transport tubing and the accompanying changes in size distribution at the end of the tube were investigated by comparing the particle data measured at the outlet of the ablation chamber and at the exit of the transport tube. A 2-m-long polyethelene tube was utilized as the transport tube. The tube was wound in a circle with a diameter of ∼30 cm and placed horizontally. Two different flow rates of the argon gas, i.e., 0.1 and 0.26 L/min, were utilized to examine the effect of the flow velocity on particle transport. Figure 9a shows the results for the flow rate of 0.1 L/min for which losses should be attributed to laminar diffusion. Compared to the number of particles at the outlet of the chamber, the total number of particles of all sizes decreased ∼64% after the transport.

Measured particle loss in mass for this flow rate ranged from about 30 to 60%. When the flow rate was increased ∼3 times, to 0.26 L/min, almost no particle loss was found after the 2-m transport tube (Figure 9b), indicating that almost all particles were transported to the end of the tubing for this flow condition. In general, loss of small particles inside a tube is due to diffusion of particles to the tube walls while that of large particles is mostly due to gravitational setting or inertial impact to a curved wall.19-21 Assuming that diffusion and gravitation are the only mechanisms for particles loss, the theoretical transport efficiency of particles computed for the flow rate of 0.1 and 0.26 L/min is only about 20 and 60%, respectively. These data indicate that the transport efficiency predicted by theory is much less than the measured efficiency. The loss of particles with diameter greater than 1 µm is theoretically significant even for the flow rate of 0.26 L/min, while the measured data (Figure 9b) show very little difference before and after the transport. One possible reason for the difference between the measured and computed transport efficiencies may be the shape of the particles. The particles generated by laser ablation, especially from glass samples, are not necessarily spherical, as was assumed in theory. Particles with irregular shape have a smaller settling velocity that in turn results in less loss of particles due to gravitation.21 The effect of particle shape on particle loss for a tube flow requires further investigation to better understand the transport efficiency during LA-ICPMS. Particle Entrainment Efficiency in the Laser Ablation Chamber. ICPMS signal intensity is proportional to the total mass ablated and transported to the ICP. If the particle mass entering the ICPMS were known, it would be possible to calibrate ICPMS intensity with respect to that mass. To obtain the particle mass entering the ICPMS, it is assumed that all the particles generated during laser ablation are spherical in shape. It is also assumed that all the transported particles are completely evaporated in the ICP. Using the absolute number of particles measured in each channel, with the volume of a spherical particle of corresponding size, the total volume of all particles entering the ICPMS was calculated. Total mass of the particles was obtained by multiplying the total volume by the density of the glass, which is 2.7 g/cm3 for the samples used in this work. The measured ICPMS total counts showed a linear increase with measured particle mass. The correlation coefficient and the error of this calibration curve were about 0.6 and 12%, respectively. This calibration curve and the measured crater volume were used to calculate the entrainment efficiency of the system. The particle entrainment efficiency is defined as the ratio of the mass entering the ICPMS to the total mass ablated from the sample. The total ablated mass can be obtained by measuring the volume of the laser ablation craters, which then are multiplied by the density of the sample. Measurement of the crater volumes using an interferometric microscope showed that an average of about 4-5 ng was ablated per pulse from the glass sample for laser beam diameter and pulse energy of 60 µm and 0.21 mJ, respectively. With the calibration curve described above, the mass (19) Hesketh, H. E. Fine Particles in Gaseous Media; Lewis Publishers Inc.: New York, 1986. (20) Thomas, J. W. J. Air Pollut. Control Assoc. 1958, 8, 32-34. (21) Hinds, W. C. Aerosol Technology; Properties, Behavior, and Measurement of Airborne Particles; John Wiley & Sons: New York, 1982.

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Figure 10. Particle entrainment efficiency of the laser ablation sampling system with respect to laser power density (b, variable laser pulse energy with constant beam spot size; 0, variable beam diameter with constant laser pulse energy).

entering the ICPMS can be obtained directly from the ICPMS intensity. Figure 10 shows particle entrainment efficiency at different laser power densities. Entrainment efficiency is ∼25% at low power density and decreases to ∼5% at high power density. Note that the entrainment efficiency of a concentric liquid nebulizer, defined as the ratio of the mass entering the ICP to the mass aspirated, was reported to range from about 1.4 to 5%.22 The data points in Figure 10 are from two different experimental conditions, i.e., variable laser pulse energy with fixed spot size and variable laser beam diameter with constant pulse energy. These data demonstrate that the entrainment efficiency of particles is a strong function of laser power density. The reason for the lower entrainment efficiency at high power densities could be the change of ablation conditions on the sample. At very high irradiance, the formation of excessively large particulates (>∼10 µm) due to phase explosion during laser ablation has been reported, where phase explosion represents a rapid boiling of the molten liquid near the thermodynamic critical temperature.23 These particlulates are much bigger than the maximum size measured by the optical particle counter. Also, the removal of large fractured pieces (>∼50 µm), possibly due to increased thermal stress and pressure on the sample surface, was observed on the ablation spot after ablation at high power densities (cf. Figure 11). These big particulates and fractured pieces cannot be entrained into the argon gas flow, especially at the low flow rate of 0.26 L/min, but fall onto the ablation chamber due to gravity. The optical particle counter and the ICPMS only measure particles that are entrained into the transport tube. CONCLUSION Particle size distribution and ICPMS signal intensity were investigated during laser ablation sampling. For single-pulse laser (22) Montaser, A.; Minnich, M. G.; Liu, H.; Gustavsson, A. G. T.; Browner, R. F. In Inductively Coupled Plasma Mass Spectrometry; Montaser, A., Ed.; WileyVCH: New York, 1998; Chapter 5. (23) Yoo, J. H.; Jeong, S. H.; Greif, R.; Russo, R. E. Submitted to J. Appl. Phys.

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Figure 11. Scanning electron microscope image of the ablated glass surface. A large piece (∼50 µm) of the cracked sample was removed from the ablation spot. Laser pulse energy and the beam diameter were about 0.85 mJ and 120 µm, respectively.

ablation of glass samples, fewer particles were produced for the first pulse than successive pulses that repeatedly irradiated the same surface location; ICPMS signal intensity corresponding to the first pulse was lower compared to successive pulses. Size distribution of laser-generated particles changed with laser power density and beam diameter. Laser power density of about 0.40.5 GW/cm2 was found to be a region across which particle size distribution shifts toward fewer large particles. The total particle number increased with increasing laser beam diameter even when the laser pulse energy remained constant. Laser beam diameter was a more influential parameter than power density in generating significantly more particles. The onset time of the ICPMS signal was closely related to the particle size distribution; large particles were correlated to shorter onset times. The onset time showed a linear decrease with increasing laser beam diameter. Particle loss during transport from the ablation chamber to the ICPMS was significant for a low flow rate of 0.1 L/min. At a higher flow rate of 0.26 L/min, almost no particle loss was measured. Particle entrainment efficiency in the laser ablation chamber was estimated from the measured ICPMS data, particle size distribution data, and data for the crater volume. Entrainment efficiency was found to decrease linearly with increasing laser power density. ACKNOWLEDGMENT This research was supported by the Environmental Waste Management Science Program, funded jointly by the Assistant Secretary for Environmental Management and by the Director, Office of Energy Research, of the U.S. Department of Energy, under Contract DE-AC03-76SF00098.

Received for review April 28, 1999. Accepted August 27, 1999. AC990455A