Anal. Chem. 1993, 65, 2999-3003
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Micro Laser Ablation-Inductively Coupled Plasma Mass Spectrometry. 1 Instrumentation and Performance of Micro Laser Ablation System Yieru Huang,fYasuyuki Shibata, and Masatoshi Morita' National Institute for Environmental Studies, P.O.16-2 Onogawa, Tsukuba, Ibaraki 305, Japan
A micro laser ablation system for the elemental analysis of a small area down to several microns in diameter was constructed, and its performance was investigated in combination with an inductively coupled plasma (ICP)mass spectrometry technique. A Q-switched, low-power Nd-YAG laser was used to ablatethe samples. The products formed by laser ablation processes were trapped on a filter and then examinedby scanningelectron microscope. Entrainment and transport of ablated particulates were evaluated experimentally, and the detection limits for elementsin an alloy sample were determined. INTRODUCTION The introduction of solid sample into an analytical excitation source is a major challenge of atomic spectroscopy. To analyze solid samples directly, various techniques such as direct insertion, spark discharge ablation, powder injection, slurry nebulization, and electrothermal vaporization have been developed. In particular, the application of laser ablation (LA) sampling has been increasing because of the minimum sample preparation, the elimination of solvent introduction, and the independence from conductivity. The microprobe capability of the system, on the other hand, has not yet been fully investigated. Laser ablation is caused by an interaction of laser light at high-powerdensity with solid material; the laser shot produces ablation, vaporization, and excitation of the test material, a process first proposed in the early 1960s.' When a laser beam interacts with solid material, a plume of luminous plasma is produced. Initially, this plume provoked considerableinterest as an excitation source for analytical spectrometry. More recently, several laser ablation sampling systems have been designed for elemental analysis via conjunction with an auxiliary excitation source such as atomic absorption spectrometry (AAS),u microwave-induced plasma (MIP),5+3inductively coupled plasma (ICP),7-12 and direct current plasma ~
~~
~~
(DCP).13J4 The technique, which has been applied to ICPAES, was first coupled with ICP mass Spectrometry (ICPMS) by Gray.16 Laser ablation systems currently used for most LA ICPMS instruments employ a Nd-YAG laser (1064 nm) operated a t Q-switched mode with more than 100-mJ output energy at repetition rates from 10 to 20 Hz. The diameter of the crater created on the sample surface is about 100 pm. In order to apply the laser ablation technique to local analysis down to the micron range, it is necessary to decrease the crater size and improve the repeatability of the system to obtain good precision. Although there have been several laser ablation microprobe systems capable of producing craters as small as 20-40 pm,lJ6J7this dimension is still somewhat too large to meet the needs of local microanalysis. For the purpose of local analysis a t the micron level, a new type of microlaser ablation system based on an optical microscope and a lowenergy laser source was developed in our laboratory.18 Laser radiance is characterized by the wavelength, the mode of operation, and the power. The amount of material evaporated and the shape of the craters on the sample are all influenced by these laser parameters. It was found that laser wavelength affected the laser plasma formation and ionization process.lg A shorter wavelength laser is preferred for increasing the interaction between laser beam and sample and for facilitating laser focusing at the micron level. For these purposes, a micro laser ablation system was built with a small Q-switched Nd-YAG laser operated in the second harmonic mode (4mJ at 532 nm).18 The present paper describes the characteristics of the micro laser ablation system as a local microanalysis of solid material for ICPMS.
EXPERIMENTAL SECTION Laser. A Nd-YAG laser (MYL-lMD, Laser Photonics Inc.) at a wavelength of 532 nm ( the second harmonic)with a maximum pulse energy of 4 mJ (Q-switched mode), combined with a modified opticalmicroscope,was used for sampling.ls The output energy of the laser beam can be attenuated in a stepwise manner with the aid of two turret-type filters beneath the laser source. The sample surface can be monitored by a charge coupled device (CCD, Sony) camera which is installed in the microscope and catches a surface image through an objective lens.
~
* To whom correspondence should be addressed.
+On leave from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Department of Analytical Chemistry, Lanzhou, China 73oooO. (1)Denoyer, E.R.;Fredeen, K. J.;Hager,J. W. Anal. Chem. 1991,63, 445A. (2)Wennrich, R.;Dittrich, K. Spectrochim. Acta 1982,37E,913. (3)Schron,W.; Bomback,G.; Benge, P. Spectrochim.Acta 1983,%E, 1269. (4)Wennrich, R.;Dittrich, K. Spectrochim. Acta 1987,42B,995. (5)Iehizuka, T.;Uwamino, Y. Anal. Chem. 1980,52,125. (6)Richner, P.;Borer, M. W.; Brushwyler,K. R.; Hieftje, G. M. Appl. Spectrosc. 1990,44,1290. (7)Thompson, M.; Goulter, J. E.; Sieper, F. Analyst 1981,106, 32. (8)Carr, J. W.; Horhck, G. Spectrochim. Acta 1982,37B,1. (9)Ishizuka, T.;Uwamino, Y. Spectrochim. Acta 1983,38B, 519. 0003-2700/93/0365-2999$04.00/0
(10)Mochizuki, T.; Sakashita, A.; Akiyoshi, T.; Iwata, H. Anal. Sci. 1989,5,535. (11)Furuta, N.Appl. Spectrosc. 1991,45,1372. (12)Lin, S.;Peng, C. J . Anal. At. Spectrom. 1990,5,509. (13)Mitchell, P. G.;Sneddon, J.; Radziemski, L. J. Appl. Spectrosc. 1987,41,141. (14)Mitchell, P. G.; Sneddon, J.; Dadziemeki, L. J. Appl. Spectrosc. 1986,40,274. (15)Gray, A. L. Analyst 1986,110,551. (16)Pearce, N.J. G.; Perkins, W. T.; Abell, I.; Duller, G. A. T.;Fuge, R. J. Anal. At. Spectrom. 1992,7,53. (17)Jackson, S.E.;Longerich,H. P.; Dunning, G. R.; Fryer, B. J. Can. Mineral. 1992,30,1049. (18) Shibata, Y.; Yoshinaga, J.; Morita, M. Anal. Sci. 1993,9,129. (19)Dittrich, K.;Wennrich, R. h o g . Anal. At. Spectrosc. 1984,7,139. Q 1903 Amerlcen Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 21, NOVEMBER 1, 1993
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Effectof laser energy on the amount of sampled material for alloy sample.
The ablation cell is an enclosed cell made of Teflon, and the volume is about 5.6 cm3. The height of the cell is limited by the working distance of objective lenses. The position of ablation on the sample surface is adjusted by controlling a stepper motordriven transitional x-y stage (with a minimum step resolution of 0.5 wm) (XY-C210, Sanyo Electronic Co. Ltd.) on which the sample chamber is mounted. The objective lens (ULWD MS Plan X20, X50, and XI00 with working distances of 11.0,8.1, and 3.18 mm, respectively) is mounted on the microscope. The laser beam is projected onto the sample surface so that the radius of the laser spot at the focusing point is inversely proportional to the magnifying power of the objective lens; for example, the spot
radius is 6.25 or 2.5 Wm if an objective lens with a magnifying power of 20 or 50 is used, respectively. The objective lens of X20 was used throughout the present study. ICPMS Instrument. Elemental detection was performed with a Yokogawa Electric PMS-2000 inductively coupled plasma mass spectrometer. Tuning of the instrument was carried out with (38Ar)z+and''Ge+ (UsingaGeHdHesupply). The operating conditions for laser and ICPMS are given in Table I. The carrier gas flow was controlled independentlyby a mass flow controller (SEC-400MK3 mass flow controller,Stec Ltd.). The gas stream, passing through the LA chamber and transfer tube, which is a smooth-walled Teflon tube (2-mm i.d., overall length 120 cm)
ANALYTICAL CHEMISTRY, VOL. 65, NO. 21, NOVEMBER 1, 1993
Table I. Apparatus and Operating Parameters Laser Q-switchedenergy (mJ) 4 pulse width (ns) 8 repetition rate 1pulse/5 s objective lens MS plan X20
I
ICPMS
rf power (W) argon gas flow rate (L/min) plasma gas auxiliary gas nebulizer gas carrier gas flow rate (mL/min) helium
argon load coil sampling aperture distance (mm) sampling aperture diameter (mm) skimmer aperture diameter (mm) data acquisition
1200
3001
H 0.16m~ IOAmJ I1.88mJ
H3mJ
14
0.80 0.80 400 250 3.7 1 0.5
V Cr Mn Fe Co Mo W Flgure 2. Effect of laser energy on the relative ion response with NI as reference element for alloy sample.
time-analysis modea
a A display of the response to the selected ion against time, the time scale being chosen to suit the experiment.
connected to the center tube of the ICP torch via a T-shaped connector,was mixed with a nebulizer gas (nebulizer and spray chamber were removed) controlled by the ICPMS system. Particle Size and Transport Efficiency Measurement. The test material was ablated in the sample chamber, and products were swept out of the chamber with an argon gas flow rate of 250 mL/min or a helium gas flow rate of 400 mL/min. At the exit of the chamber, a Millipore membrane filter (with 0.3pm pore size) was connected directly to capture the products. The trapped material was then observed by a scanning electron microscope (JMS-840,JEOL Ltd.). Mass transport efficiency was estimatedby accuratelyweighing a small piece of metal sheet or mesh before and after repeated ablation. A t the outlet of the transfer tube was set a trap bottle containing 10% HN03 solution. The outlet of the trap bottle was directly connected to the ICPMS for the determination of gas-phase material if it was present. The HN03 solution s t m d overnight and was analyzed by conventional ICPMS.
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Flgure 3. Variation of micro laser ablation crater area (square of crater radius) in tungsten as a function of laser pulse energy (4 ml of full energy).
RESULTS AND DISCUSSION Effect of Laser Energy. Laser energy is an important factor in the vaporization of samples. The energy of the laser beam can be attenuated in a stepwise manner with the aid of filters in the present system. The relationship between ion response (peak area/counts) detected by ICPMS and the energy used is shown in Figure 1. All of the elements in a Hastelloy alloy sample (Nilaco 583261) show almost linear relationships between laser energies and signal responses. The slopes, however, apparently change at around 0.4 mJ and are steeper below 0.4 mJ. The relative response of each element normalized by SNi, the major constituent in the sample, varied little irrespective of the change of laser energy (Figure 2), indicating that selective vaporization depending on laser power does not occur for any element in the alloy sample examined. Sampling Crater Size. Laser energy also affects the size of the crater produced. According to Denoyer et al.,1 the analytical signal depended on area rather than volume of the crater in the Q-switched mode. An example of the variation in crater area produced by a single laser irradiation as a function of laser energy is shown in Figure 3. Crater dimensionswere determined by scanning electron microscope (SEM). The relationship again is apparently biphasic with a inflection at around 0.4 mJ. The crater produced by laser irradiation at the smallest energy (0.16 mJ) was only 3-5 pm in radius for many metals, which was similar to or a little smaller than the spot size of a laser beam on the sample surface.
Flgure 4. Wide-field view of ablation productsfrom stainless steel with full laser energy (4 mJ).
Products of Laser Ablation. Solid ablation products were collected from a variety of materials by filtration as described above. Figure 4 showsawidefield SEM micrograph of a Millipore filter with the products of laser ablation from a stainless-steel target. There were a large number of spherical particles ranging in size from 3 pm to smaller than 0.2 pm. Similar ranges of particles were observed in other metals (indium, copper, nickel, gold, tungsten, etc.) but lead, the easiest one to be ablated, showed a tendency to give larger particles (Figure 5a and Figure 6). Figure 7 shows a typical metal particle (from the ablation of lead) with a diameter of approximately 6 pm. This is a virtually perfect sphere with a certain amount of amorphous material. There are several possibilities as to how the spherical material formed. The spherical material could either have condensed from the gas stage via a liquid to a solid stage or could have simply cooled from a liquid. On the basis of the observation of the shape of material adhering to the glass window of the sample chamber after ablation (Figure 8), it seems that the bulk of
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 21, NOVEMBER 1, 1993
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Flgwe 6. (a) Particle size distribution obtained by ablating stainless steel (SS) and Pb metal at 0.16 and 4 ml of laser energy; (b) an enlargement of (a) from 0 to 2.5 pm.
ablated material was mainly melted and then removed explosivelyfrom the crater. Only a minor amount of material
Flgure 8. Adhered material (Cu)on the glass window of the sample chamber.
in the fluffy forms might be condensed from the vapor phase. The products produced by the smallest laser energy (0.16 mJ) were also examined (Figure 5b). No particle bigger than 3 p m in diameter could be found by SEM. Figure 6 indicates that the smaller the laser energy used, the finer the particles produced. According to the results of slurry ICPAES and ICPMS,20-23 it was known that particles smaller than 3 pm could be vaporized, atomized, and ionized in the plasma more efficiently than large particles. Figures 1 and 3 seem to support the occurrence of two different ablation mechanisms, a less efficient one dominating in the higher (than 0.4 mJ) power mode and a more efficient one dominating in the lower power mode, respectively. The crater sizes produced by a laser pulse of higher than 0.4 mJ were larger than the calculated laser spot size on the surface (see Experimental Section), indicating the Occurrence of an indirect ablation mechanism in the higher laser power mode, such as heat conduction or laser shocks. Such a mechanism dominating with the higher laser power probably was less efficient to ablate or transfer materials than the direct interaction between laser and samples and thus gave the responses shown in Figures 1and 3. The difference in particle sizedistribution shown in Figure 6b, Le., the increaseof relative (20) Williams, J. G.; Gray, A. L.; Norman, P.; Ebdon, L. J . Anal. At. Spectrom. 1987,2, 469. (21) Mochizuki, T.; Sakashita, A.; Iwata, H.; Ishibaahi, Y.; Gunji, N. Anal. Sci. 1989, 5, 311. (22) Ebdon, L.; Foulkes, M. E.; Parry,H. G. M.; Tye, C. T. J. Anal. At. Spectrom. 1988,3, 753. (23) Ebdon, L.; Foulkes, M. E.; Hill, S. J . Anal. At. Spectrom. 1990, 5, 67.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 21, NOVEMBER 1, 1993
Table 11. Transport Efficiencies for Different Metals*b amt of material transport element removed (ng/pulse) effic (76)
AI
cu In Ni Pb Sn 0
4 3.5 28 5 30 15
3003
Table 111. Detection Limits of Elements in an Alloy Sample (hastelloy) by Micro-LA-ICPMS detection limit (ppm)
20 21 64 10 13 17
*
Ar as carrier gas, flow rate 250 mL/min. Laser energy 4 mJ.
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Flgute 9. A copper-mesh surface.
abundance of larger particles in higher laser power (4 mJ) compared with lower power (0.16 mJ) in stainless steel, may also reflect the difference of mechanism. For many metals, the critical point between the two mechanisms seems to be between 0.16 and 0.4 mJ; the laser fluence on the sample surface is estimated to be between 130 and 330 J/cm2. Experimental Transport Efficiency. The experiments were carried out in an effortto determine the mass of material ablated from metal meshes or sheets during a laser ablation event. More than loo0 shots of laser a t 4 mJ were irradiated on a sample and ablated material was adsorbed by 10% HN03. The transport efficiencies for different metals are calculated and listed in Table 11. The efficiency of the trap was evaluated. By comparison of the data before and after absorption by 10% HNOs solution, it was estimated that nearly 100% of the sample particles that could contribute to the signalwere trapped in the solution. Part of the material ablated from the sample surface was attached to the chamber wall, window (Figure 8),or transfer tube, which thus lowered the transport efficiencies. Some other part, on the other hand, was redeposited to the sample surface (Figure 9), which amount could not be estimated by the present experiment. The amount of removed material listed in Table I1 equals the difference between the ablated amount and the amount redeposited on the sample surface. Thus, the transport efficiencies in Table I1 may be overestimated values of the "true" transport efficiencies,the ratios
laser energy (mJ)
crater radius (pm)
Co
Mn
V
W
4 3 2 0.4 0.16
9 7
3 5 8 31 78
3 8 10 45 136
0.3 0.5 0.8 3
1 2 3 12 22
6 5 3
7
of the material amount reaching the ICP to that ablated from the sample surface. The amounts of removed materials in Table I1 seem to be related to the melting point; i.e., metals with lower melting points (In, Pb, Sn) show higher removed amounts than those with higher melting points (Al, Cu, Ni). The meaning of the variance of transport efficiency values is less clear at this stage. Detection Limit. The detection limit of the method was evaluated for a Hastelloy sample (Table 111). The detection limit is defined as the concentration of element that gives a signal equal to 3 times the standard deviation of the blank signals. It was calculated from the measurements on the gas background and sample. A widely used method for local elemental analysis will be electron probe X-ray fluorescence microanalysis (EPMA). EPMA has a spatial resolution of 2-5 pm and a detection limit down to 100 ppm. The present method has a comparable spatial resolution and an elevated detection limit. The high sensitivity of the method along with its capability for isotope ratio determination will find wide application in a variety of analytical fields.
CONCLUSION The characteristics of micro laser ablation as a solid sampling method for ICPMS have been demonstrated. With the microlaser ablation system, the signal intensity of ICPMS increases almost linearly with the laser output energy. The particles that leave the ablation chamber are in the range of 50.2-3 pm for many metals. This size is much smaller than that achieved by a pulse with much higher laser energy, which is usually larger than 6 pm, and is easier to be transferred by carrier gas. The smallest ablation crater size of 3-5 pm in radius and high sensitivity makes it easy to carry out sensitive local microanalysis.
ACKNOWLEDGMENT This work was supported by the project of the special coordination funds for promoting science and technology (Science and Technology Agency of Japan) entitled "Development and Application of Accurate Elemental and Isotopic Measurement Method for Micro Samples".
RECEIVED for review October 27, 1992. Accepted July 28, 1993.@ e Ahtract
published in Advance ACS Abstracts, September 1,1993.
