for digestion, mixed with acids, and diluted to determine the element concentrations by inductively

Transcription

1 Bodo Hattendorf Christopher Latkoczy Detlef Günther Swiss Federal Institute of Technology Zurich Imagine you want to determine the elemental composition down to the sub-parts-permillion range of a beautiful blue gemstone to clarify its origin or to determine whether it It s the aerosol size that really matters in this high-throughput technique for ultratrace analysis of solids. is natural or synthetic. Traditionally, a part as small as possible might be drilled out, placed in a beaker for digestion, mixed with acids, and diluted to determine the element concentrations by inductively coupled plasma (ICP) MS or an equivalent method. Now you face the problem that several potential sources of error contribute to the uncertainty of your results. You may not know which elements have been lost during sample preparation or were entrained from external sources, which is especially critical in a dilution of more than 1000-fold, which dramatically increases the error caused by only minute 341 A

2 amounts of contaminants. You may further assume that it is not feasible to mechanically drill out a large piece of sample within a region that may be only several tens of micrometers in size. Some might ask, Does anyone really need such information? Actually, many applications other than gemstone analysis require a highly sensitive, spatially resolved determination of elements and/or their isotopes in solid samples for example, the stoichiometric composition of a newly synthesized inorganic compound, the reconstruction of conditions for the origin and formation of mineral phases in geology, the association of elements with proteins after being separated by gel electrophoresis, the impact of metal or ceramic implants on bones and tissues, and the identification of microscopic pieces of evidence based on their elemental fingerprint. Being aware of this situation, it is obvious to conclude that in a wet chemical approach the low limit of determination is constrained by contamination during dilution and digestion, which, together with the sampling process, will determine sample throughput. Although laser ablation (LA) in combination with ICPMS is not the only technique that can circumvent these problems (1), it is the most routinely used technique that provides sufficient sensitivity for ultratrace determinations in solids combined with a high sample throughput. The possibilities of accessing almost 80% of the elements in the periodic table, having the capability to conduct in situ local and bulk analysis, and performing quantitative analysis using non-matrix-matched calibration standards are major advantages of this technique, which can solve the gemstone problem and many others in solid sample analysis. Principles and instrumentation The sample is irradiated by a pulsed, high-energy laser beam, which releases particles, atoms, electrons, and ions from the sample surface (2). This ablation is performed inside an airtight ablation chamber that is flushed with carrier gas to transport the aerosol to the ICPMS system for detection. Depending on the laser wavelength, laser fluence, and sample material, the laser pulse removes material from a depth of µm (3). The aerosol generated during the laser pulse is subsequently transported to the ICP torch where the particles are vaporized, atomized, and ionized in the plasma (4). In most cases, mass analysis of the ion beam is performed by quadrupole mass filters, but an increasing number of applications are done with sector-field instruments because of their greater detection power and mass resolution or with TOF instruments because of their quasi-simultaneous ion detection. Figure 1 shows the principal setup of most commercially available instrumentation for spatially resolved analysis by LA- ICPMS. The sensitivities of the mid-mass isotope 139 La are cps/µg/g for quadrupole, cps/µg/g for sector-field, and cps/µg/g for TOF instruments using identical conditions for laser sampling (glass sample, 40- FIGURE 1. Principal setup of most commercially available instrumentation for spatially resolved elemental analysis. µm spot size, 193 nm, 10-Hz repetition rate, 25 J/cm 2, helium carrier gas). The scan frequency, representing the number of mass scans that can be recorded in a multielement approach for the mass range 6 amu (lithium) to 238 amu (uranium) with 40 isotopes, is 2 50 Hz for quadrupole, 1 5 Hz for sector-field, and 20 khz for TOF ICPMS. The linear dynamic range of modern instrumentation can expand over 9 orders of magnitude, allowing for the detection of major, minor, and trace elements. Quantitative analysis Quantifying analyte concentrations is mostly based on glass reference materials (e.g., the National Institute of Standards and Technology [NIST] 61X series), in-house prepared materials for specific applications, and a combination of solution nebulization (with or without desolvation) with LA or direct liquid ablation, which are based on internal standardization. Approaches using the normalized total count rates or total mass for calibration have also been reported and are good indicators for the potential of truly independent calibration techniques with a trend toward absolute quantification procedures (5 7). To obtain accurate and precise quantitative analytical results, the method must meet the following criteria the amount of sample transported to the ICPMS instrument must be explicitly known or be discernable from an independent parameter; the composition of the aerosol represents the stoichiometry of the original sample, or a deviation is present in the calibration standards and the sample to the same degree (8); the aerosol is completely vaporized and atomized in the ICP; and the degree of ionization for an element is identical for the calibration standards and samples (9). Knowing the amount of sample transported to the ICPMS is rare, even for samples that are uniform in composition (3). If samples have a similar composition, one might expect that a laser pulse would ablate the same amount of material. However, even similar materials often exhibit different ablation rates. To avoid such a complication, an internal standard (often a minor isotope of a major element) is used, and this step has significantly improved the quality of data obtained by LA-ICPMS over the past two decades (7, 10, 11). However, a stoichiometric aerosol composition is still crucial for the success of quantitative analysis by LA-ICPMS. When an internal standard is used to compensate for changes in 342 A ANALYTICAL CHEMISTRY / AUGUST 1, 2003

3 the ablation rate and transport efficiency, it must exhibit behavior identical to that of the analyte elements. Complete vaporization, atomization, and ionization require an ICP ion source with sufficient energy. Because ionization efficiency depends on the residence time of the atoms in the ICP, the highest ion number density can be achieved when atomization is completed at an early stage in the ICP. From sample to aerosol In the early years of LA, observations indicated that the ion signals recorded by the ICPMS often did not represent the sample composition, a phenomenon subsequently called elemental fractionation (12). Fractionation may occur during ablation, aerosol transport, or the atomization and ionization processes within the ICP (Figure 1). Recent studies have led to a better understanding of the processes involved, and the focus on laser sample interaction (which has been considered the major source of fractionation for the past 17 years) is now shifting toward aerosol transport phenomena and the ICPMS. Almost all laser sources have been used for ablating samples. The physical and optical properties of the sample together with the laser s irradiance have a significant influence on elemental fractionation (13). Many studies have determined the influence of the ablation parameters (laser energy and pulse duration, beam profile and delivery, ratio of depth to diameter, number of pulses) on fractionation, and they all point to interaction among the various parameters. This interaction indicates the need for a multidimensional optimization procedure improving one single parameter will not eliminate elemental fractionation. There is some agreement, however, that fractionation at the sample surface is strongly dependent on the laser wavelength together with irradiance. Changing the wavelength from 1064 nm (Nd:YAG) to 266 nm (fourth harmonic Nd:YAG; 14), and more recently to 213 nm (fifth harmonic; 15) and 193 nm (ArF excimer; 16, 17), has successively reduced this effect. Recent studies on silicate samples show that the particle size distribution obtained from a laser pulse is a function of the wavelength (9). Smaller mean particle sizes were obtained with decreasing laser wavelengths in the order 1064 nm > 532 nm > 266 nm > 213 nm > 193 nm; this trend is also supported at 157 nm (18). Changes in the absorption of the material also affect the particle size distribution for a given wavelength (3). Figure 2 shows the particle size distributions measured during ablation at 213 nm for four different samples of similar matrix, where absorption increases with the concentration of transition metals. Comparing the results at 193 nm and 266 nm at identical fluency revealed that the ablation rates increase for the NIST 61X glass samples, proceeding from opaque to transparent samples, only at 266 nm. Because of the almost constant absorption at 193 nm for these materials, the ablation rates are independent of color. At least for nonmetallic samples, the ablation rate and particle size distribution are closely related to each other (i.e., larger ablation rates lead to larger average particle sizes), which can qualitatively be explained by the penetration depth of the laser beam into the sample body. At a smaller penetration depth, more energy is deposited within this volume, and that energy can be converted into chemical energy to break bonds. When more bonds can be broken in a given volume, a larger fraction of vapor and particles <200 nm can be released. This primary particle size distribution, however, does not necessarily reflect what is introduced to the ICP. The size of an individual particle may change in any direction by vaporization or condensation and coalescing particles after initial release (19). These processes are controlled by the density, temperature, and lifetime of the plasma plume that develops above the sample surface during the laser pulse; the rate of energy exchange with the gas environment; and the kinetic energy of the particles. If the results of ablation experiments performed in argon or helium are compared, the first visible difference is the intensity Volume (µm 3 ) Particle size (µm) and lateral extension of the plasma plume. A large and bright plasma is visible in argon, whereas it is much smaller and less intense in helium. Particle size distributions of glass samples, measured after the ablation cell, change significantly for 193 nm in helium compared to argon. Particles with a size below 100 nm show a 700-fold increase in helium. This effect is much less pronounced for the 266-nm laser, showing only a 30% increase for particles <200 nm (20). These observations can be explained by the higher thermal conductivity of helium, which may allow the thermal energy to spread away from the sampling position faster and quickly end condensational growth. The initial particle size distribution from the laser pulse seems to be preserved to a greater extent in helium than argon. From aerosol to ions Eggins and co-workers first observed the influence of a helium environment and the resulting change of the particle size dis FIGURE 2. Particle size distribution measured in glass reference materials using a particle measurement system. The particle size distributions demonstrate the effect of absorption on particle size distribution. NIST 610, blue; 612, light blue; 614, colorless; U.S. Geological Survey basalt Columbia River 2G, black. 2G 343 A

4 tribution on sensitivity (21). They observed a 3 5-fold increase in signal intensity in the ICPMS and a reduced deposition of material in the vicinity of the ablation site when the ablation was performed in helium instead of argon with a 193-nm ArF excimer laser. A direct comparison between the results obtained at 266 nm and 193 nm indicated that signal intensity increased minutely for 266 nm (10 20 % increase), and the results for the 193-nm laser were verified (22). Taking into account that small particles do not contribute as much to the total transported mass, it becomes clear why the signal increases only 3 5 times for 193 nm and almost no improvements are observed using 266 nm. Although the ablation rate for 266 nm was twice as high as the rate for 193 nm, only half the sensitivity was obtained, which can only be explained by selective loss of large particles in the ablation cell or transport system or by particle-size-dependent vaporization and atomization efficiency of the ICP ion source. Even though the ICPMS is a powerful and efficient ion source, the vaporization, atomization, and ionization efficiency (i.e., the ratio of ions produced from a given concentration of an element in the ICP) change with the local temperature of the ICP. Differences in the sample load may therefore change the plasma temperature and hence change vaporization and ionization efficiencies, especially for elements with high first ionization energies (23). Recent studies have proven that the aerosol structure itself (i.e., the average particle size and distribution) has a significant impact on elemental fractionation (24 26). Figure 3a illustrates this effect using a 193-nm LA with two different ICPMS instruments that were individually optimized. Uranium and thorium have very similar ionization energies, and their concentration in the SRM NIST 610 glass sample used in the experiment is almost identical, which should result in an intensity ratio of close to 1, as observed for ICPMS 1. On the other hand, the relative fraction of Th + to U + in ICPMS 2 appears to be reduced, especially at the beginning of the signal, when the mean particle size and density in the ICP is highest. Therefore, it must be assumed that selective vaporization occurs, which depends on the amount of energy the ICP can deliver for full vaporization, atomization, and ionization of all particles within a time frame of several milliseconds (from the time the particles enter the ICP to extraction at the MS). This deviation is much more pronounced for 266 nm and was observable for this wavelength on every ICPMS in our lab. It disappears, however, when the abundance of large particles in the aerosol is removed, for example by filtering with mineral wool, before reaching the ICPMS. Figure 3b shows the contribution of larger particles and indicates their influence on elemental fractionation. Furthermore, only a small change in elemental sensitivity is observed as the size of the particles reaching the ICP decreases continuously. Even when the filtering process removes ~80% of the total material, sensitivity is decreased by only 30%, which implies that the fraction of particles that can be completely vaporized and atomized is comparably small. Comparing different ablation cell designs (size, geometry, and carrier gas supply) and transport tubes of variable diameter and length did not lead to significant changes in ion signal intensities (27), which also indicates that the influence of the transport system on fractionation is rather small. It is therefore very important to understand that not only generation of the aerosol, but also the transport, vaporization, atomization (which all depend on the particle size distribution), and ionization within the ICP significantly affect elemental fractionation. (a) 3.0 (b) ICPMS 2 80 U + /Th + intensity ratio ICPMS 1 Th + signal (kcps) Ablation time (s) U + signal (kcps) FIGURE 3. Thorium and uranium. (a) Transient signal ratios for Th + and U + measured on two different instruments using identical ablation conditions. The significant offset and variation of this ratio for ICPMS 2 is an indication of incomplete atomization and ionization of Th +. (b) Correlation of thorium and uranium signal intensities during single-spot ablation at 266 nm. Open circles represent the intensities for a nonfiltered aerosol, where a significant deviation occurs. The filled circles indicate the higher correlation obtained after filtering the aerosol. 344 A ANALYTICAL CHEMISTRY / AUGUST 1, 2003

5 Table 1. Detection limits for different laser spot diameters analyzing MPI-DING reference glass T1-G (Italian Alps quartz diorite glass). 4 µm 16 µm 30 µm 60 µm Sodium 79 µg/g 3.4 µg/g 1.3 µg/g 0.22 µg/g Particle-size-related elemental fractionation The laser-produced aerosol may be subdivided into three parts a fraction of particles that remains inside the ablation cell or transport system (referred to as immobile); a fraction that can be transported to the ICP but is not completely vaporized, atomized, and ionized (referred to as incomplete); and a fraction consisting of vapor and particles below a certain size, which is completely transported, vaporized, atomized, and ionized (referred to as complete). Whether the ICP causes elemental fractionation will then depend on the mass ratio of incomplete to complete particles. When the ratio is small, almost every particle is fully vaporized and atomized, and the ion number density in the ICP will reflect the stoichiometry of the aerosol. When the incomplete fraction increases, however, an increasing number of particles will not be fully vaporized and atomized, and a preferential vaporization of compounds with higher volatility will alter the relative composition of atoms and ions, leading to ICPinduced elemental fractionation. On the other hand, ablationinduced elemental fractionation would be indicated by the alteration of the bulk sample caused by diffusion and preferential vaporization during the laser shot and the relative amount and composition of the mass ratio immobile/(incomplete + complete). ICP-induced elemental fractionation can be reduced when the operating parameters are optimized to generate a sufficient gas temperature in the central channel of the ICP. The influence of ICP forward power, which depends on the carrier gas flow rates and composition, is shown in Figure 4. Although the ICP forward power (kw) U + /Th + Normalized Th + signal Carrier gas flow (ml/min) FIGURE 4. The influence of plasma power and carrier gas flow on the intensity ratio for uranium and thorium and the normalized Th + signal. The diagrams indicate that the most robust plasma conditions are achievable under compromised sensitivity conditions (loss of 50% thorium intensity). plasma operating conditions are usually optimized to generate the highest possible sensitivity (normalized signal for Th + 1, blue region in Figure 4, right), these conditions may not be appropriate to minimize ICP-induced fractionation (U + /Th + 1, blue region in Figure 4, left). After an extensive study of plasma operating conditions, we concluded that maximizing only Aluminum Silicon Titanium Cobalt Copper Zinc Barium Europium Thulium Lutetium Tungsten Thallium Lead Uranium S/N sensitivity is not an ideal optimization criterion. In fact, the operating conditions that eliminate ICP-induced fractionation in this example are accompanied by as much as 50% reduction in sensitivity for Th +. The LA process is responsible for the initial particle size distribution. Most of the particle loss occurs in the ablation cell, and the transported particle size fraction is vaporized, atomized, and ionized to a highly variable degree within the ICP. Therefore, all observations of elemental fractionation, and especially contradictory results for different instrumentation, have at least some relation to the particle size distribution and its treatment in the ICP. Figures of merit The lack of reference materials matching the composition of the unknown sample seems to be the Achilles heel for most direct solid-sampling techniques. In contrast, in LA-ICPMS, it has often been demonstrated that nonmatrix-matched calibration provides accurate results in a wide variety of applications (10, 11, 15, 17). However, because the concentration of one element in the standard and the sample must be known, the universal and routine use of this technique has been limited. Therefore, finding an absolute quantification procedure has become a major focus of research. Even though the sensitivity of LA-ICPMS is still limited, it has demonstrated single-digit parts-per-billion detection limits for solids (Table 1). These limits are comparable with or better than those achieved by a variety of established analytical techniques. Further improvement in sensitivity is of major importance for LA-ICPMS in order to determine trace-element concentrations at high spatial resolution. In the example of a typical ablation of 345 A

6 Table 2. SiO 2 /TiO 2 -based nanoparticles doped with manganese and iron analyzed using LA and solution nebulization ICPMS. Sample LA (µg/g) RSD % SN (µg/g) Iron SiTi SiTi SiTi Manganese SiTi SiTi SiTi LA analyses used NIST 610 glass calibration standard and silicon as an internal standard. Composition of SiO 2 /TiO 2 particles is 98 wt%/1.5 wt%. glasses, the detection efficiency can be estimated as follows: At an ablation rate of 100 nm per pulse and using a spot size of 40 µm, the laser beam removes ~380 pg/shot (density = 3 g/cm 3 ). The sensitivity for a mid-mass isotope, obtained under these conditions for a laser frequency of 10 Hz, is usually cps/µg/g, corresponding to an absolute detection efficiency of 0.1 and 1%. This leaves 2 3 orders of magnitude for improvement. The transport efficiency in LA-ICPMS is currently estimated to be 5 10% of the ablated material, depending on the carrier gas and wavelength used for ablation. This is a significant limitation of this technique because, apart from potential fractionation between the mobile and immobile fractions, 90% of the sample is not analyzed by the ICPMS. At least another 2 orders of magnitude improvement in efficiency could be gained by improving the ion source, interface, and ion optic design of ICPMS instrumentation. The most successful approaches to enhancing elemental sensitivity have used helium as the carrier gas (20) and modified the interface to achieve higher ion extraction efficiency (smaller sampling cone orifice and lower background pressure) (28). Improvements in sensitivity, especially for isotopes of low m/z, have also been reported after adding molecular gases (N 2, H 2 O) to the carrier gas (29, 30). However, this technique is of limited use because the abundance of polyatomic interferences increases, complicating the spectrum. The role of molecular gases on sensitivity is not fully understood yet. One hypothesis is that an increased energy transfer for the ICP to the aerosol leads to more efficient atomization and ionization. Another hypothesis is that changes in the ion extraction process lead to higher transmission in the interface and to an increased secondary discharge in the ICP, which would reduce space charge effects. Although solvent-based interferences are significantly reduced under dry plasma conditions when compared to conventional solution-based applications, a wide variety of polyatomic and isobaric interferences are still present in the mass spectrum. Recent developments in ICPMS instrumentation offer new opportunities to determine isotopes that have not previously been accessible and to eliminate some of these interferences. These advances include quadrupole-based instruments equipped with collision and/or reaction cells, where spectral interferences are resolved by ion molecule reactions in a multipole ion guide (31). Sector-field instruments with higher mass resolution and high scanning frequency capabilities over the entire mass range are now available; these instruments are suitable for fast transient signal acquisition (32, 33). Applications The linear dynamic range of modern ICPMS instrumentation allows determination of major, minor, and trace element concentrations from a single spot analysis. Based on these capabilities, LA-ICPMS is used in a variety of applications, including determining the stoichiometric composition of highly refractory ceramic materials, trace element compositions of inorganic materials, archaeological or forensic fingerprints, and the origin of paintings (10), as well as analyzing meteorites, minerals or fluid inclusions, optical materials, cement, and fish ears. In the last couple of years, there has been a lot more interest in LA-ICPMS for analyzing gemstones. Studies have shown that the mass needed from valuable sapphires for their elemental fingerprint can be reduced to <100 ng per analysis per sample (34). The method has also been used to determine the element distribution in sapphires that have undergone a specific thermal treatment, and it was shown that beryllium, which is difficult to determine using other solid sampling techniques, is enriched at the outer boundary of the stone. A new application is the determination of trace element distributions in polymer beads used in chemical catalysis. Different polystyrene beads, used to catalyze the enantioselective epoxidation of styrene to the epoxide, were doped with manganese and lithium and analyzed at certain points in the catalytic process. Manganese is homogeneously distributed within freshly prepared and loaded beads. The high-performance polymer leaches manganese down to a constant value of ~25% of the original content after 15 sequential uses. The low-performance polymer loses more than 95% of manganese after 6 uses. Reloading the beads increased the manganese content only in the outer layers of the beads. Therefore, the elemental distribution in such samples can be successfully studied with a spatial resolution down to 40 µm (35). Nanomaterials based on SiO 2 are usually difficult and time consuming to digest due to their chemical resistance. The concentration of a dopand in the structure of nanoparticles has a significant influence on its physical and chemical properties. Pressed to thin pellets, the nanoparticles were analyzed using a crater diameter of 80 µm to measure the level of dopants. An external calibration using synthetic glass standards resulted in identical concentrations compared with those obtained after digestion of the sample and solution-based analysis (Table 2). The technique allows for fast production process control without the extensive sample preparation required for solution analysis. 346 A ANALYTICAL CHEMISTRY / AUGUST 1, 2003

7 Lasers and ICPs LA-ICPMS is the most powerful technique for quantitative in situ solid sampling at high spatial resolution. Our steadily increasing understanding of the ablation process, the transport of the aerosol, and the vaporization, atomization, and ionization in the ICP is dramatically improving the range of applications and makes LA-ICPMS the most important complementary technique to ICP-optical emission spectrometry, ICPMS, X-ray fluorescence, electron microprobe, and secondary ion MS for major, minor, and trace element analysis. However, the reservoir of suitable wavelengths is almost exhausted. Shorter pulse width (femtosecond lasers) can help to overcome laser-induced elemental fractionation (36), especially for the analysis of metal samples. Some of the particle-related problems, however, might remain (37). The laser sample interaction is not the only source of elemental fractionation because particle-size-related processes inside the ICP also contribute to this effect. However, processes in the ICP are not well understood, especially for dry sample introduction systems, and careful optimization of the ion source is required. The aerosol structure must be optimized because it also plays a dominant role. An aerosol with a significant fraction of large and refractory particles will likely lead to ICP-induced fractionation even under optimized conditions. To achieve high sensitivity and reduced elemental fractionation, analytical instrumentation research needs to focus on laser systems that produce small particle sizes independent of matrix properties; robust ion sources capable of complete vaporization, atomization, and ionization of particles independent of their composition; and enhanced aerosol transport efficiency for complete sample transport. Improvements in these three areas will minimize matrix effects and improve quantification. If complete matrix independence can be achieved, determining relative mass transport efficiency will allow absolute quantification. Would you prefer to dissolve a gemstone to determine its composition and origin? Most likely not. But LA-ICPMS, successfully applied, can keep you from spending a lot of money on an artificial stone. The research discussed in this article was supported by ETH Zurich, the Swiss National Science Foundation, and the Federal Office for Education and Science, Switzerland. Detlef Günther is assistant professor and Bodo Hattendorf and Christopher Latkoczy are scientists in the research group for trace element and microanalysis in the Laboratory of Inorganic Chemistry at the Swiss Federal Institute of Technology in Zurich (ETH). The authors are interested in fundamentals and applications of LA-ICPMS. The special interests of Günther are spatially resolved microanalysis and accompanying instrumental developments. 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