Solid sample analysis using laser ablation inductively coupled plasma mass spectrometry

Trends in Analytical Chemistry, Vol. 24, No. 3, 2005

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Solid sample analysis using laser ablation inductively coupled plasma mass spectrometry Detlef Gu¨nther, Bodo Hattendorf Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is a highly accepted, widely used method for the determination of major, minor and trace elements in solids, as well as isotope-ratio measurements. However, it suffers from non-stoichiometric effects occurring during sampling, aerosol transport, vaporization, atomization and ionization within the ICP, described as elemental fractionation. This matrix-dependent phenomenon has been the limitation for quantitative analysis without matrix-matched calibration standards. Significant insights into the processes responsible for elemental fractionation have been obtained and, with improved understanding of LA-ICP-MS, a variety of strategies for more precise, more accurate quantitative analyses have been developed. This review aims to summarize recent developments in LA-ICP-MS based on the fundamental understanding of the LA process and particle formation but also includes the importance of the ICP and its operating conditions. We discuss figures of merit and new trends in quantification in order to demonstrate the capabilities of this direct solid-sampling technique. We present a few selected applications to underline why LA is a fast-expanding analytical technique. ª 2005 Elsevier Ltd. All rights reserved. Keywords: Elemental fractionation; Figure of merit; Inductively coupled plasma; Laser ablation; Mass spectrometry

1. Introduction Detlef Gu¨nther*, Bodo Hattendorf Swiss Federal Institute of Technology (ETH) Laboratory of Inorganic Chemistry, WolfgangPauli-Str. 10, HCI Ho¨nggerberg, CH-8093 Zurich, Switzerland

*

Corresponding author. E-mail: [email protected]

Soon after the realization of lasers in 1960 [1], laser micro-optical emission spectrometry (LM-OES) and laser ablation mass spectrometry (LA-MS) [2,3] had been investigated for qualitative and quantitative elemental analyses in solids. It has taken more than 20 years to explore the combination of laser sampling with a separated excitation source capable of multi-element analysis LA inductively coupled plasma atomic emission spectrometry (LA-ICP-AES) [4] and LA-ICP-MS [5] for sensitive trace-element analysis as well as isotope-ratio analysis. The general set-up of LA-ICP-MS has not changed significantly in past decades

0165-9936/$ - see front matter ª 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.trac.2004.11.017

(Fig. 1). The sample is placed in an airtight, closed ablation chamber, which is flushed with argon or helium as carrier gas and the laser beam is focused or imaged onto the sample surface through a cell window. Provided the irradiance is sufficiently high [6], the material will be ablated (generating vapor, particles, and agglomerates) and transported to the plasma of the ICP-MS. The apparent ‘‘simplicity’’ of LA-ICP-MS (sample handling, sensitivity, simple spectra, . . .) attracted a lot of attention. It offered immediate access to the qualitative and, in some applications, the quantitative composition of any solid sample with high sensitivity. The ICP serves as a separate excitation source, where the laser-generated particles are vaporized, atomized and ionized. Subsequently, ions are extracted by the vacuum interface and guided into the mass analyzer, separated by mass to charge ratio and finally detected. Different ICP-mass analyzer combinations are available and some of the pros and cons for coupling ICP-MS and LA will be discussed in the following sections. One disadvantage of LA-ICP-MS (and basically all laser-based sampling techniques) is the occurrence of nonstoichiometric effects in the transient signals, defined as elemental fractionation [7]. The great advantage of LA-ICP-MS, compared to recording optical emission spectra from the laser plume at the ablation spot, is the fact that sampling and excitation/ detection can be optimized separately and that the generated signal depends only on the ablated mass. Due to the lack of reference materials for the wide variety of 255

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Trends in Analytical Chemistry, Vol. 24, No. 3, 2005

Figure 1. Schematic set-up of LA inductively coupled plasma Lmass spectrometry (LA-ICP-MS).

samples of interest, quantitative analysis was often restricted to a few elements of identical fractionation behavior. In early studies, LA-ICP-MS was as matrix dependent as laser-induced breakdown spectroscopy (LIBS), albeit at four orders of magnitude higher sensitivity. Recent developments and future trends in LA-ICP-MS are linked to developments in laser technologies and optics as well as ICP-MS instrumentation. Only recently established is the current prospective of considering (a) the ablation process, (b) composition of transported material, and (c) atomization and ionization within the ICP all as equally important for optimization and fundamental understanding of the technique. All these three domains, together with the physical properties of

the samples (Table 1), are important considerations for successful use of this technique. The range of applications for LA-ICP-MS has increased significantly due to the improved understanding of factors that influence quantitative analysis. The use of laboratory-produced or matrix-matched reference materials has demonstrated the potential of this technique for specific applications. The first paper showing the suitability of glass reference standards (NIST 61X) for quantitative analysis of rare-earth elements in minerals by using Ca as internal standard [8] was a breakthrough for the acceptance of LA-ICP-MS in geological applications. To date, geochemical analysis is the field where LA-ICP-MS is most widely spread, which led to more

Table 1. Summary of relevant parameters and their effects in LA-ICP-MS

Instrumental parameters

Have influence on

a

Sample material

Laser

Transport system

ICP-MS

Absorbance Reflectivity Heat capacity Heat conductivity

Wavelength Pulse length Spot size Fluency Irradiance Repetition rate Ablation mode Gas environment

Cell volume Tubing diameter Tubing length Gas composition

Rf-Power Plasma potential Gas flows Gas composition Torch position Torch configuration Interface pressure Ion lens settings Data Acquisition

fl Ablation rate Surfacea composition Surfacea morphology

fl Penetration depth Surface temperature Ablation rate Vapor composition Particle-size distribution Aerosol composition Transported material

fl Gas velocity Signal dispersion Transport efficiency

fl Plasma temperature Vaporization Atomization Ionization Ion extraction Ion transmission Sensitivity Mass Bias Duty cycle

Surface in this context extends to the volume, which is affected by the laser radiation and vaporized or melted material.

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than 500 papers published just within this field. This trend is paralleled by the increasing sales of LA equipment, with three manufacturers currently in the market. This review aims to describe the status of LA-ICP-MS in the transition from a period dominated by the problematic ‘‘elemental fractionation’’ into its application for quantitative analyses in many different sample types. It includes a summary of current instrumentation and capabilities, combined with a critical review of the apparent ‘‘simplicity’’ of the technique. We present selected applications in order to demonstrate that the current activities and the emerging fields of interest have made LA-ICP-MS into a mature, solid-sampling technique. Our final outlook highlights areas of further development and future fields of research.

2. Instrumentation 2.1. Lasers Significant improvements in LA-ICP-MS have been achieved due to the rapid development in laser technology. In the past 20 years, almost all available laser wavelengths have been tested in combination with ICPMS (Table 2). As can be seen from the year of introduction, the wavelengths have changed from visible and IR to todayÕs dominant UV wavelengths (266, 213 and 193 nm), which was generally driven by the demonstrated advantages of shorter wavelengths in the ablation behavior [7,9–19]. We also need to mention that most of the fundamental studies reported were carried out to improve traceelement determinations in minerals, silicates and oxides or age determination in zircons (U, Pb, Th), samples where absorbance plays a major role in coupling the laser into the sample. However, during ablation of dif-

Table 2. Year of first report on the different laser types and wavelengths for micro-scale analysis with ICP-MS Year

Laser

k (nm)

Ta

References

1985 1992 1993 1995 1996 1997 1997 1998 2002 2003 2003 2003

Ruby Nd:YAG Nd:YAG ArF KrF Nd:YAG XeCl Nd:YAG Ti:Sapphire F2 Nd:YAG Ti:Sapphire

694 1064 266 193 248 532 308 213 800 157 193 260

ns ns ns ns ns ns ns ns fs ns ns fs

[5] [8] [20] [21] [22] [23] [24] [10] [25] [16] [15] [26]

Many systems were also used in other applications, such as LA-ICP-OES or LIBS, before being introduced into LA-ICP-MS. a Pulse duration.

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ferent homogenous samples, it has been observed that the intensity ratios of different elements ratios change upon progressive ablation at a single spot. In order to describe these non-stoichiometric effects in LA-ICP-MS, Fryer et al. [7] introduced the term ‘‘elemental fractionation’’ and calculated a relative fractionation index. The fractionation index allows elements to be classified into groups with similar behavior during ablation and ionization [27]. U and Pb showed great differences in their fractionation index but were important for geochronological studies in zircons. Accordingly, these elements were widely studied by LA-ICP-MS [28,29]. The fractionation of U versus Pb in silicates was significantly reduced by using shorter laser wavelengths. The advantages of the shorter wavelengths are a result of reduced thermal alteration of the sample material during the ns-laser pulse [30]. Improvements are related to an increased coupling of the laser energy into the sample, leading to higher energy densities. This increased absorptivity directly translates to smaller ablation rates in transparent materials at a given irradiance [31,32] and reduces the heat-affected zone in the sample. Besides the improved ablation behavior, it has further been shown that the composition of filter-collected aerosols of the NIST 610 glass standard, generated by LA using 193 and 266 nm in argon and helium, were in agreement with the composition of the bulk sample. Except for Cd, Be and Al, more than 40 elements in the collected aerosol represented the stoichiometry of the original sample within the uncertainty of the measurements [33]. However, analysis of individual particle-size fractions from the 266-nm laser showed enrichment of specific elements (Cu, Zn, Cd, Ag, Pb) in the smaller particles, corresponding to the observed fractionation trends. However, for metallic samples, the wavelength has only little effect on the ablation characteristics [34] and the pulse duration appears to be the dominant parameter affecting formation of molten material in the ablation spot and the stoichiometry of the generated aerosol. Russo et al. [25] and recently Koch et al. [17] showed that selective vaporization during LA of brass can be significantly reduced by using femtosecond (fs) instead of nanosecond (ns) laser pulses. These improvements are again attributed to reduced thermal alteration of the sample material, as the dissipation of energy into the sample body is reduced. It has been shown that the overall aerosol composition, collected by an impactor, represents the stoichiometry of the bulk sample [17]. A first indication for reduced elemental fractionation using fs-LA at 266nm in comparison to ns LA for silicates and zircons was reported by Poitrasson et al. [26]. A clear trend towards shorter pulse width and wavelength is observable today. However, more widespread use of fs-laser systems is http://www.elsevier.com/locate/trac

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currently limited by the price of such instrumentation. It further needs to be clarified whether fs-lasers ensure stoichiometric ablation and permit use of non-matrixmatched calibration for all types of samples. 2.2. Optics Significant improvements have been achieved by various techniques for homogenizing the laser-energy density at the sample surface. An ‘‘even’’ or ‘‘flat-top’’ energy distribution throughout the ablation spot ensures that the sample removal is identical across the entire spot. Spatial filtering using apertures or lens-array optics in combination with an aperture are common procedures to homogenize laser-beam profiles and have been described elsewhere [13,14,29]. The advantage of a homogenized laser beam has been shown for depth-profile analysis [35]. 2.3. Ablation cells The most important requirement for the ablation cell was and is to accommodate the samples. Many different ablation cells have been designed, and differ mainly in internal volume (0.25–100 cm3). Various studies have been carried out to improve transport efficiency, which has been reported to be of the order of 10–20% [31,36] and 40% [37,38]. The relative transport efficiency of different ablation cells and transfer tubing, measured as signal intensity in LA-ICP-MS, has been reported not to change significantly for different volumes of ablation cell and tubing [39]. Nonetheless, it has been shown that the volume of the ablation cell mainly affects the dispersion of the signal and thus the magnitude of the signal/ background ratios. Recently, Koch et al. [40] proposed numerical simulations of aerosol transport, which should allow description of the dispersion of the aerosol and eventually indicate parameters that affect analyte loss within the ablation cell. 2.4. ICP-MS The major developments in ICP-MS include operation of the ICP source at low plasma potential and ‘‘soft-extraction’’ mode [41] and the use of collision or reaction cells for interference suppression [42,43]. Sector-field instruments [44] and time-of-flight mass spectrometers (TOF-MS) [45] have been used in addition to the original quadrupole instruments and, despite the limited ion-storage capabilities, even ion-trap instruments are available today [46]. The great advantage of the ICP-MS for LA analysis is the high sensitivity, wide dynamic range (typically greater than six orders of magnitude in pulse-counting mode and nine orders in combined pulse-analog detection) and relatively simple spectra. Recognizing these features, almost immediately after ICP-MS had been installed in the laboratories as prototypes or commercial 258

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instruments, they were coupled to LA sampling systems [5,8,47–51]. Currently, the majority of ICP-MS systems used with LA employ quadrupole mass filters, which allow very fast scanning over the mass spectrum and achieve a temporal resolution of several spectra per second, even for extended element suites. Typically, the acquisition of the laser-generated aerosol is performed by transient signal acquisition, so that heterogeneity and/or changes of the ablation conditions (e.g., defocusing the laser) can be monitored during the measurement [52]. Double-focusing instruments are becoming increasingly popular – a direct result of the increase in the scan speed that these instruments achieve today [53,54]. Sector-field instruments often provide higher sensitivity and can be operated at higher mass resolving powers than quadrupoles, and they can account for a variety of the spectral interferences, especially in the low and the mid-mass ranges. Other means to control spectral interferences were recently introduced in the form of reaction or collision cells [42,55]. These devices reduce the level of selected interferences by specific chemical reactions inside a gasfilled multipole ion guide, which can provide separation of analyte–interference pairs, where the mass resolution of double-focusing instruments is still insufficient [43]. However, when used in multi-element applications with LA sampling, the use of highly reactive gases may be problematic due to the fact that analytes may react with the gas, which can lead to significant signal suppression, and sequential operation with and without the gas is impossible due to the transient nature of the LA signals [56]. Operation with hydrogen appears the best compromise with respect to multi-element operation and suppression of the dominating Ar-based interferences [57,58]. TOFinstruments were successfully used in LA-ICP-MS for the acquisition of short transient signals because the simultaneous sampling of all isotopes at a frequency of typically 20 kHz allows much better signal correlation than scanning-type instruments [59,60]. Despite this principal advantage, current ICP-TOF-MS instruments do not exhibit adequate sensitivity for LA analysis and their application in trace and ultra-trace analysis is restricted. The understanding of the fundamental processes that influence ion formation from an aerosol and their transmission through the interface and ion optics has significantly increased in recent years [61–64]. This has led to enhanced reliability, robustness, sensitivity, lower instrumental background, increased data-acquisition bandwidth and speed, making ICP-MS a routine tool in many laboratories [65–67]. However, most of these improvements are based on investigations using solution nebulization, which, even when desolvating nebulizers are employed, appears not to be directly transferable to the dry aerosol generated with LA.

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The major causes of these differences are: (a) the highly variable and wide particle size distribution generated by laser ablation and, (b) the fact that laser ablation must be optimized to match the requirements of the ICP-MS rather than considering LA-ICP-MS an independent technique. Nebulizers can be operated to create an almost mono-disperse aerosol that can be atomized and ionized within a narrow region inside the ICP. Furthermore, nebulizer droplets are sufficiently small to become fully atomized and ionized inside the ICP. In contrast, LA typically creates particles whose sizes are significantly larger and may vary by orders of magnitude. Thus, even if atomization and ionization of all particles can be achieved in the ICP, larger particles will continue to atomize further downstream in the ICP. Figg et al. [68] reported a first indication of the influence of the ICP in combination with the particle-size distribution on elemental fractionation. Later, it was demonstrated that the particle-size distribution of lasergenerated aerosols had a significant impact on the ion signals of ICP-MS [69]. Selective removal of large particles from the aerosol was not accompanied by a proportional reduction in these ion signals, which suggests that a large portion of the aerosol is not completely transferred into ions. Using high-speed digital photography Aeschliman et al. [70] showed that large particles can survive passage through the ICP when ablating pressed pellets of Y2O3. Other work, using a particle-separation device [71], and studies on ion-formation processes in the ICP by Rodushkin et al. [72] also indicated that ICP-induced elemental fractionation occurs. The extent of these fractionation effects was found to be directly related to the absorptivity of laser light in the different materials. Comparing different laser wavelengths (266, 213, 193 nm obtained as 4th and 5th harmonic and by an optical parametric oscillator (OPO) of a Nd:YAG), under otherwise constant conditions, revealed that high absorption (low penetration depth) reduced the fraction of large particles in the aerosol and is accompanied by a reduction of fractionation effects [15,32]. The particle-formation process is further influenced by the gas environment used for aerosol transport. Eggins and co-workers [12] demonstrated that using helium instead of argon as carrier gas leads to a five-fold signal enhancement for 193-nm lasers and smaller fractionation effects in silicate matrices. In order to assess the effect of particle-size-related fractionation effects in detail, Kuhn [73] successively removed large particles from the aerosol. These studies indicated that complete vaporization and ionization occurs at a particle size of 90–150 nm (for a NIST 610 glass sample). It was also shown that the vaporization efficiency of large particles varied between different

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elements and the magnitude of the different efficiencies was defined by a vaporization index. Volatile elements exhibit lower vaporization indices, indicating a higher degree of atomization, while refractory elements such as the REE and Ca, have a similar, but higher, vaporization index. Reduced elemental fractionation using 193-nm LA compared to 266-nm ablation in helium are thus a direct result of the different particle-size distributions and the particle sizes that can be fully atomized in the ICP. However, the size limit for effective atomization depends on ICP-operating parameters. Due to the complex interaction of sample material, laser wavelength and ablation environment on the resulting particle-size distribution, the ICP-operating conditions need to be optimized specifically to eliminate fractionation effects that result from incomplete atomization. In ICP-MS with solution nebulization, such ‘‘robust’’ plasma-operating conditions are typically adjusted by minimizing the abundance of temperaturesensitive species (e.g., oxide ions) in the mass spectrum [74]. Monitoring complete atomization and ionization is also possible by comparing the measured ion-signal intensities for U+ and Th+ with their respective concentrations in a reference material [75,76]. Both ions have similar ionization energies and mass, and their major isotope has an abundance of >99%. If full atomization is achieved, one should therefore expect an intensity ratio close to the concentration ratio in the ablated material. Fig. 2 shows the dependence of the intensity ratio on ICP-operating conditions when ablating the SRM NIST 610 glass standard. Concentrations of U and Th in this sample are 450 and 457 lg/g, respectively, which should result in an intensity ratio of approximately 1. However, the measured U+/Th+ ratio increases with increasing carrier-gas flow rate (i.e., lower plasma

Figure 2. Dependence of analyte sensitivity, ThO+ formation and U+/Th+ intensity ratios (summed ion signals of M+ and MO+) on carrier-gas flow rate during laser-ablation sampling. Helium carrier gas; 193 nm ArF-Excimer laser; 10 Hz, 60-lm spot size, Agilent 7500a ICPMS.

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temperature and residence time of the aerosol inside the ICP). This behavior can be explained only by different atomization efficiencies of U and Th. However, the ion signals themselves usually maximize at a gas flow rate, where the measured ratio is far from 1. This behavior has significant implications for the operation of ICP-MS. In order to adjust the operating parameters, it has been common practice to monitor the formation of refractory oxide ions (e.g., ThO+, which is easily measured in LA-ICP-MS) and adjust the operating parameters (rf-power, sampling distance and gas-flow rates) for minimum abundance of oxide ions while keeping signal-to-background ratios as high as possible. However, considering Fig. 2, the formation rate of ThO+ is still low when the atomization is incomplete, indicating that ThO+/Th+ (and other oxide-based indicators) is not a suitable indicator for robust plasma-operating conditions [71,76]. The current status of research cannot fully explain the formation of particles and their agglomeration after LA and, until now, particle-size distributions have been measured for few materials only. Incomplete atomization and ionization of particles inside the ICP have been shown for large particles. Optimization of ICP-MS alone cannot overcome this limitation completely and further studies need to determine which parameters control atomization and ionization in order to improve the ion source. Since particle-size distribution appears to be one of the most important parameters, it is of great value to gain further insight into the formation processes by numerical simulations, as recently presented by Bogaerts et al. [77].

3. Figures of merit Despite some of the aforementioned limitations, the range of applications where LA-ICP-MS is successfully used is steadily increasing. Nonetheless, the figures of merit strongly depend on the application, the instrumentation and the operating conditions of the LA system and the ICP-MS connected to it. Variations in sensitivity, instrumental backgrounds, abundance of spectral interferences and plasma robustness make it difficult to define the figures of merit for LA-ICP-MS. One of the main attributes of LA-ICP-MS is sensitivity. However, the detection efficiency, given by the ratio of ions detected by the ICP-MS to the number of atoms present in the ablated material, is of the order of 10 3 (depending on instrumentation and operating parameters). Significant differences in the detection efficiency can be obtained when different LA instrumentation is used with the same ICP-MS (Fig. 3). However, limits of detection (LODs) can usually compete with solution-based analyses, taking into account that dilution is generally required to ensure stable operating conditions of ICP-MS. Depending on the 260

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Figure 3. Typical detection efficiency for the NIST 610 glass reference material versus isotope mass in LA-ICP-MS, showing the great variability that can be obtained under different operating conditions of LA using the same ICP-MS instrument. Spot size: 40 lm; energy density: 15 J/cm2, 10 Hz, 266 nm with 1.2 L/min argon as carrier gas, 193 nm with 1.0 L/min helium admixed to 0.7 L/min argon after the ablation cell. The ICP-MS was optimized to balance sensitivity with atomization efficiency.

desired spatial resolution, which determines the maximum amount of sample that can be analyzed, LA-ICPMS can achieve sub-ng/g LODs for spot sizes above 100 lm (‘‘bulk-analyses’’, Fig. 4), which increase with increasing spatial resolution, according to the smaller sample uptake and the given detection efficiency. Solvent-based spectral interferences are absent in LA-ICP-MS and that improves the accuracy that can be obtained significantly, while short dwell times, required to account for the transient nature of the ablation signals, limit the attainable precision. Typical results for multielement analyses are presented in Table 3 and Fig. 5. Matrix dependency is still the major concern in LA-ICP-MS, especially for metals and semi-metals that exhibit significant fractionation during the ablation using ns-lasers. However, in many applications, LA systems can generate a stoichiometric aerosol with

Figure 4. Typical LODs (calculated according to [52]) for the determination of selected elements in industrial CaF2 pressed powder pellets. 193-nm ablation, 120-lm spot size, 10-Hz repetition rate, helium carrier gas.

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Table 3. Analytical results for MPI GOR128 and T1 silicate reference samples using LA-ICP-MS, calibrated with NIST 610 reference standard GOR128

Li Be B Na2O MgO Al2O3 SiO2 P2O5 TiO2 V Cr MnO FeO Co Ni Cu Zn Rb Sr Y Zr Nb Cd Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Tm Yb Lu Tl Pb Bi Th U

T1

Units

Reference

Measured

Reference

Measured

9 0.04 20 0.557 25.8 9.87 46.1 0.028 0.28 200 2180 0.179 9.78 86 1070 70 74 0.39 31 11.3 10.2 0.11 <0.5 0.25 1.09 0.124 0.46 0.105 0.78 0.54 0.27 1.21 0.25 1.97 0.44 0.2 1.39 0.21 – 0.42 0.0009 0.007 0.013

11 ± 1 0.03 ± 0.02 25 ± 2 0.61 ± 0.05 22.7 ± 0.5 9.70 ± 0.08 51 ± 5 0.029 ± 0.003 0.26 ± 0.00 184 ± 6 2031 ± 110 0.180 ± 0.009 9.85 ± 0.72 100 ± 10 1165 ± 121 78 ± 8 87 ± 10 0.41 ± 0.02 28.4 ± 0.5 10.9 ± 0.1 9.3 ± 0.1 0.09 ± 0.01 0.45 ± 0.34 0.25 ± 0.03 1.00 ± 0.09 0.13 ± 0.01 0.45 ± 0.02 0.10 ± 0.01 1.08 ± 0.53 0.53 ± 0.03 0.26 ± 0.01 1.04 ± 0.05 0.23 ± 0.01 1.82 ± 0.07 0.42 ± 0.02 0.18 ± 0.01 1.23 ± 0.04 0.18 ± 0.00 0.06 ± 0.10 0.32 ± 0.04 0.025 ± 0.024 0.012 ± 0.003 0.011 ± 0.002

20 2 5 3.14 3.74 17 58.5 0.176 0.73 190 22 0.131 6.42 19 13 21 84 80 283 23.2 147 9.1 <30 2.9 382 69 127 12.1 40.7 6.5 1.21 5.2 0.82 4.44 0.83 0.35 2.32 0.35 – 13 0.09 30 1.67

20 ± 0.7 1.8 ± 0.1 4.3 ± 0.5 2.59 ± 0.16 3.10 ± 0.03 17 ± 0.41 57.8 ± 4.89 0.141 ± 0.02 0.69 ± 0.01 186 ± 5.17 20 ± 2 0.127 ± 0.01 5.95 ± 0.29 19 ± 1 11 ± 2 20 ± 3 64 ± 22 73 ± 5 253 ± 5 23.4 ± 0.4 139 ± 2 7.6 ± 0.3 0.1 ± 0.06 2.6 ± 0.4 350 ± 10 82 ± 2 129 ± 5 11.9 ± 0.4 40.6 ± 1.1 6.4 ± 0.2 1.12 ± 0.02 4.5 ± 0.1 0.69 ± 0.02 3.91 ± 0.09 0.78 ± 0.03 0.29 ± 0.01 1.93 ± 0.08 0.29 ± 0.01 0.16 ± 0.08 12 ± 2 0.08 ± 0.01 27 ± 1 1.50 ± 0.13

lg/g lg/g lg/g lg/g wt% wt% wt% wt% wt% lg/g lg/g wt% wt% lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g lg/g

193 nm ablation, 15 J/cm2, 31-lm spot size, 5-Hz repetition rate, Ca as internal standard. Bold data, certified reference values; other data, indicative values. All reference data from Jochum et al. [79].

particle-size distributions that can be fully atomized in the ICP and matrix-independent calibrations could be demonstrated, even using solution-based calibration with desolvating nebulizers [65,78].

4. Applications 4.1. Geology As mentioned above, some of the greatest successes of LA-ICP-MS are in the geological sciences, where the high

spatial resolution of LA sampling and the high sensitivity allows trace-element determinations at the lm scale. Trace and ultra-trace analyses in a wide variety of minerals, most often without matrix-matched calibration, have been reported [8]. In this early work, it had already been shown that the use of a homogeneously distributed element as an internal standard (e.g., Ca) from a glass sample permits accurate determination of rare-earth element concentrations within different mineral phases. Improvements in sensitivity, background signals, and linear dynamic range provide access to http://www.elsevier.com/locate/trac

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major, minor and trace elements. Precision and accuracy of the analyses are comparable with complimentary techniques, such as l-XRF or EPMA (Fig. 5). In addition, LA-ICP-MS has proved to be a powerful screening tool in geochronological studies (U/Th/Pb series in zircons) [29,80] and results published on age determinations in zircon are in agreement with established techniques, such as SHRIMP and TIMS. Even though the precision of the latter is still better in many cases at the expense of spatial resolution, these techniques can hardly match the sample throughput of LA-ICP-MS. Another field in geology, where LA-ICP-MS is one of the dominating techniques is the elemental analysis of fluid inclusions for studies of ore-forming processes. Quantitative determination of elements in inclusions in the range 10–25 lm in diameter within quartz is one of these typical applications where LA-ICP-MS is an ideal tool to generate quantitative, multi-element data. Heinrich et al. [81] recently published a comprehensive overview. One of the growing areas, in terms of instrumentation, is the coupling of LA to multi-collector ICP-MS for spatially resolved isotope-ratio determinations [82]. LA studies on Cu isotopes have shown that the isotope fractionation can be significantly reduced by introducing the lowest possible particle sizes into the ICP. Further applications in this area are various isotoperatio measurements (e.g., in the W and Hf system [83]) and combined element and isotope studies in minerals [75]. 4.2. Proof of authenticity, and forensic Within this field, the fingerprinting of gemstones is probably one of the fastest growing applications of LA-ICP-MS. Quasi-non-destructive analysis on cut gem-

25

wt % LA-ICP-MS

GSD-1G

Si

20 4

15 3

Na

Fe 10

Ca 5

Al

K

Mg

2

Ti

1

1

2

3

4

stones (sampling with the laser using a beam of 120 lm in diameter · 1 lm depth) is sufficient to determine the concentration of more than 40 elements within such stones [84]. The element concentrations make it possible to distinguish between natural and synthetically produced gems within a few minutes. However, LA-ICP-MS is an expensive technique and the availability of appropriate instrumentation in the field is rather limited. Nonetheless, the unmatched sensitivity compared to other established (XRF) or competing techniques (LIBS) is likely to leave LA-ICP-MS as the arbiter. Various forensic studies on glass have been reported and fingerprinting is becoming more important in crime studies [85], including studies on cannabis crops [86] and gold [87]. 4.3. Materials sciences LA-ICP-MS is impacting on the materials sciences, especially low-level, trace-element determinations in refractory materials, such as CaF2, CeO2, TiO2 or other raw materials, and end products in high-end optical materials and ceramics are studied in great detail [65,88,89]. The interest is mostly based on the fact that sample-preparation procedures for common solution-based analyses (AAS, ICP-OES, ICP-MS) are time-consuming, prone to contamination and require hazardous chemicals (e.g., HF). Furthermore, it is becoming increasingly important to study the local distribution of elements in different sample types [90], where LA-ICP-MS can provide the desired information at an acceptable effort in terms of instrumentation, availability and speed, when compared to highly specialized approaches, such as synchrotron XRF or SIMS. 4.4. Biology and medicine A fairly recent field of applications of LA-ICP-MS can be found in the bio-medical sector. After the pioneering work by Neilsen et al. [91], studies of element distributions in 2D electropherograms, the metal-binding properties of proteins or their S and P concentrations are more frequently carried out using LA-ICP-MS [92–95]. However, in many cases, the applicability of such studies is still limited by contamination of the levels of the reagents used and the need for internal standards, for quantitative analysis. Qualitative element distributions in tissues can be readily determined by scanning the laser beam over the area of interest and subsequently correlating the measured ion signals with one of the matrix elements, if accessible (e.g., carbon) [96].

0 0

5

10

15

20

25

wt % EPMA Figure 5. Comparison of major element concentrations determined by EPMA and LA-ICP-MS in synthetic glass standards (GSE-1G, USGS, Denver, USA, provided by S. Wilson).

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4.5. Future applications It is very difficult to predict future directions of elemental analysis applications for LA-ICP-MS, given the current widespread usage. Geological studies are most likely to remain a major area because the technique is already

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established and accepted in many laboratories. Forensic studies and investigations of isotope variations will be subject to further research activities, together with biomedical research. It is likely that the composition of new materials (fast quality control) will more often be characterized by LA-ICP-MS in order to avoid time-consuming and labor-intensive sample-preparation steps for solution-based analysis. With the growing understanding, increased robustness and ease of use, more and more laboratories will consider this technique at least a powerful sample-introduction system for existing ICP-MS instrumentation; this is already visible in the growing interest from chemical industries in implementing LA-ICP-MS in product control and research laboratories [97].

5.2. Transport systems Seek 100% transport efficiency. On-line determination of transported aerosol mass.

5. Trends and topics for further development

6. Conclusion

Progress in LA-ICP-MS will depend strongly on developments in the two major, interconnected fields of spatial resolution and matrix tolerance. Currently, the best lateral resolution achievable with LA instruments is in the range of several micro-meters, which is in the same order of magnitude in comparison to most other micro-beam techniques (X-ray-, electron- or proton micro-probes). Nonetheless, in order to satisfy the everincreasing requirements in spatial resolution and especially the range of elements to be detected, detection efficiency needs significant improvements, in comparison with that available in current instrumentation. However, recent studies on widely used glass reference standards using synchrotron l-XRF indicate that any improvement in spatial resolution requires verification of the micro-homogeneity of the calibration standard [98]. Considering the analystÕs dream of determining trace elements at the lg/g level from a single laser shot (e.g., at a spot size of 1-lm crater diameter and a depth resolution of 0.1 lm, corresponding to a sample volume of 0.08 lm3), the ideal instrument would be capable of generating sufficient signal/background level from only a few hundred atoms! Basically, every single atom must be detected independently of the composition of the sample analyzed, and that would also allow a full nonmatrix matched calibration by any type of external standard. Under these premises, it is obvious that, LA-ICP-MS requires further developments in the following areas.

LA-ICP-MS is well established in the geological fields and is making inroads in other key areas, such as forensics, materials science, and medical and biological applications as well as chemical analysis in general. Current features of this technique include the flexibility to perform spatially resolved analyses at the lm scale and also bulk analyses. Even though sensitivity is still a major field of research, the technique already provides superior LODs in comparison with other solid-sampling techniques. Furthermore, short analysis times and minimal sample preparation make it especially attractive. Current research is focused on reduction of matrix dependency, ensuring reproducible quantitative determinations using non-matrix matched calibrations. These studies need to identify the factors controlling the stoichiometry of the ablated material in the vapor phase, and its nucleation, condensation and agglomeration to larger particles. Wavelength-tunable LA systems and fs-lasers are promising approaches to improve ablation characteristics and reduce fractionation effects. Furthermore, the role of ICP-MS with respect to atomization and ionization, in depending on particle size, needs further investigation in order to reduce matrix effects and to improve detection efficiency. Understanding the complex interaction of laser wavelength, pulse duration, gas environment, on the one hand, and the detection efficiency, affected by particle-size distribution, carrier-gas composition and ICP-operating conditions, on the other hand, will benefit from numerical simulations of the processes involved.

5.1. Laser ablation Seek to reduce the matrix dependence of the LA event in terms of particle-size distribution and composition, including use of fs-laser technology [17,19,26]. Develop a physical model of the particle formation and agglomeration processes.

5.3. ICP-MS Complete atomization and ionization in the ICP, independent of particle size. Increased ion transmission from ICP to detector. Truly simultaneous detection of all isotopes of interest. Wider dynamic range in a single detection mode. 5.4. Automation Integration of data-acquisition protocols and datareduction software. Autosamplers for unattended operation over hours.

Acknowledgements The constructive comments from H. Longerich, C. McLeod and R.S. Houk on this work are greatly appreciated. Further, we thank D. Bleiner, M. Guillong, http://www.elsevier.com/locate/trac

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K. Hametner, I. Horn, H.R. Kuhn, C. Latkozcy and M. Ramseier for their contributions.

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