Bodo Hattendorf Christopher Latkoczy Detlef Günther Swiss Federal Institute of Technology–Zurich
magine you want to determine the elemen-
I
tal composition down to the sub-parts-permillion range of a beautiful blue gemstone
It’s the aerosol size that really matters in this high-throughput technique for ultratrace analysis of solids.
to clarify its origin or to determine whether it 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
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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 condiFIGURE 1. Principal setup of most commercially available instrumentation for spations for the origin and formation of mineral phastially resolved elemental analysis. es in geology, the association of elements with proteins after being separated by gel electrophoresis, the impact of µm spot size, 193 nm, 10-Hz repetition rate, 25 J/cm2, helimetal or ceramic implants on bones and tissues, and the identifi- um carrier gas). The scan frequency, representing the number cation of microscopic pieces of evidence based on their elemental of mass scans that can be recorded in a multielement approach fingerprint. Being aware of this situation, it is obvious to conclude for the mass range 6 amu (lithium) to 238 amu (uranium) with that in a wet chemical approach the low limit of determination 40 isotopes, is 2–50 Hz for quadrupole, 1–5 Hz for sector-field, is constrained by contamination during dilution and digestion, and 20 kHz for TOF ICPMS. The linear dynamic range of modwhich, together with the sampling process, will determine sample ern instrumentation can expand over 9 orders of magnitude, althroughput. Although laser ablation (LA) in combination with lowing for the detection of major, minor, and trace elements. ICPMS is not the only technique that can circumvent these problems (1), it is the most routinely used technique that provides suf- Quantitative analysis ficient sensitivity for ultratrace determinations in solids combined Quantifying analyte concentrations is mostly based on glass refwith a high sample throughput. The possibilities of accessing al- erence materials (e.g., the National Institute of Standards and most 80% of the elements in the periodic table, having the capa- Technology [NIST] 61X series), in-house prepared materials for bility to conduct in situ local and bulk analysis, and performing specific applications, and a combination of solution nebulization quantitative analysis using non-matrix-matched calibration stan- (with or without desolvation) with LA or direct liquid ablation, dards are major advantages of this technique, which can solve the which are based on internal standardization. Approaches using gemstone problem and many others in solid sample analysis. 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 Principles and instrumentation The sample is irradiated by a pulsed, high-energy laser beam, absolute quantification procedures (5–7). To obtain accurate and which releases particles, atoms, electrons, and ions from the precise quantitative analytical results, the method must meet the sample surface (2). This ablation is performed inside an air- following criteria—the amount of sample transported to the tight ablation chamber that is flushed with carrier gas to trans- ICPMS instrument must be explicitly known or be discernable port the aerosol to the ICPMS system for detection. Depend- from an independent parameter; the composition of the aerosol ing on the laser wavelength, laser fluence, and sample material, represents the stoichiometry of the original sample, or a deviathe laser pulse removes material from a depth of 0.02–5 µm tion is present in the calibration standards and the sample to the (3). The aerosol generated during the laser pulse is subse- same degree (8); the aerosol is completely vaporized and atomquently transported to the ICP torch where the particles are ized in the ICP; and the degree of ionization for an element is vaporized, atomized, and ionized in the plasma (4). In most identical for the calibration standards and samples (9). Knowing the amount of sample transported to the ICPMS is cases, mass analysis of the ion beam is performed by quadrupole mass filters, but an increasing number of applications are rare, even for samples that are uniform in composition (3). If samdone with sector-field instruments because of their greater de- ples have a similar composition, one might expect that a laser pulse tection power and mass resolution or with TOF instruments would ablate the same amount of material. However, even similar materials often exhibit different ablation rates. To avoid such a combecause of their quasi-simultaneous ion detection. Figure 1 shows the principal setup of most commercially plication, an internal standard (often a minor isotope of a major available instrumentation for spatially resolved analysis by LA- element) is used, and this step has significantly improved the qualICPMS. The sensitivities of the mid-mass isotope 139La are ity of data obtained by LA-ICPMS over the past two decades 103–5 103 cps/µg/g for quadrupole, 103–104 cps/µg/g for (7, 10, 11). However, a stoichiometric aerosol composition is still sector-field, and 102–5 102 cps/µg/g for TOF instruments crucial for the success of quantitative analysis by LA-ICPMS. using identical conditions for laser sampling (glass sample, 40- When an internal standard is used to compensate for changes in 342 A
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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
