Laser ablation in analytical chemistry—a review

R Russo
2002 Talanta: The International Journal of Pure and Applied Analytical Chemistry  
Laser ablation is becoming a dominant technology for direct solid sampling in analytical chemistry. Laser ablation refers to the process in which an intense burst of energy delivered by a short laser pulse is used to sample (remove a portion of) a material. The advantages of laser ablation chemical analysis include direct characterization of solids, no chemical procedures for dissolution, reduced risk of contamination or sample loss, analysis of very small samples not separable for solution
more » ... ysis, and determination of spatial distributions of elemental composition. This review describes recent research to understand and utilize laser ablation for direct solid sampling, with emphasis on sample introduction to an inductively coupled plasma (ICP). Current research related to contemporary experimental systems, calibration and optimization, and fractionation is discussed, with a summary of applications in several areas. * Corresponding author Compared to conventional dissolution techniques, laser ablation has many advantages. Most analytical techniques involve removing a portion of the solid sample, which is then dissolved in acid solutions. With this procedure, there is a greater chance of exposure to hazardous materials and there is a risk of introducing contaminants or losing volatile components during sample preparation. For laser ablation, any type of solid sample can be ablated for analysis; there are no sample-size requirements and no sample preparation procedures. Chemical analysis using laser ablation requires a smaller amount of sample (< micrograms) than that required for solution nebulization (milligrams). Depending on the analytical measurement system, picogram to femtogram sample quantities may be sufficient for laser ablation analysis. In addition, a focused laser beam permits spatial characterization of heterogeneity in solid samples, with typically micron resolution both in terms of lateral and depth conditions. 2 Interest in analytical optical and mass spectrometry associated with laser ablation for sample introduction has increased markedly during the past few years. Laser ablation optical and mass spectrometry with inductively coupled plasma (LA-ICP-AES/MS) appears to be the only analytical approach for nearly non-destructive determination of a large number of elements with very low detection limits [1] [2] [3] [4] [5] [6] . Applications of LA-ICP-AES/MS now span a great range of academic and industrial fields that include environmental, geological, archaeological, forensic, and semiconductor manufacturing sectors. A primary goal of laser ablation for analytical chemistry is quantitative analysis. Calibration typically requires matrix-matched standards, which, however, can be difficult to obtain or fabricate. Developing reliable, possibly universal calibration standards will be essential for laser ablation to become more widely adopted in analytical chemistry. Without matrixmatched calibration, fractionation (mass removal based on thermal properties) becomes an issue during laser ablation sampling. Fractionation limits accurate chemical analysis, as the ablated mass composition is not the same as the actual sample composition. These challenges (calibration and fractionation) along with perspectives of laser ablation as a viable analytical tool will be discussed in this review. In addition to the challenges of calibration and fractionation, the fundamental physical processes of laser ablation are not fully understood. Understanding fundamental laser ablation mechanisms is necessary in order to efficiently couple the laser beam into the sample, ablate a reproducible quantity of mass, minimize fractionation and plasma shielding, control ablated particle transport, and produce stoichiometric ablation. A thorough review of the fundamental 3 issues of laser ablation (e.g., ultrafast laser-solid interactions, vapor and plasma dynamics) falls beyond the scope of this article. We direct readers to several recent articles and conference proceedings devoted to these topics [7] [8] [9] [10] . This review will summarize current laser ablation research in analytical chemistry, focusing on standard laser ablation experimental systems, calibration, fractionation, and numerous analytical applications. LA-ICP-AES and LA-ICP-MS will be emphasized in this review. As evident from a large number of publications in the field of laser ablationapproximately two thousand published papers over the past ten years, interest in laser ablation continues to flourish. Experimental Systems A typical laser ablation system consists of a laser, an ablation stage, and a detection system (Figure 1) [1]. Pulsed lasers are used most often to produce coherent light for ablation. The samples are usually placed on a mechanically adjustable ablation stage. The detection system is often an ICP-MS or ICP-AES. Argon or other inert gases typically carry the ablated sample into the ICP. Lasers for ablation Many types of pulsed lasers have been used for ablation. Ruby lasers were among the first applied to ablate solid materials for chemical analysis applications [2, 11,12]. Currently, most laser ablation experiments utilize Nd:YAG or excimer lasers [4]. Solid state Nd:YAG systems have been widely employed because they are relatively inexpensive, require little 4 maintenance, and are easily incorporated into small commercial ablation systems. Excimer lasers utilize halogen gas-filled chambers rather than solid-state crystals. Ablation is affected by the laser wavelength. In general, the shorter the laser wavelength, the higher the ablation rate and lower the fractionation [13-18]. For Nd:YAG lasers, the fundamental wavelength is in the near-IR at 1064 nm. Optical frequency doubling, tripling, quadrupling, and quintupling (wavelengths at 532, 355, 266, and 213 nm) of the Nd:YAG lasers have been achieved and employed for laser ablation chemical analysis [4,18,19]. For excimer lasers, the operating gas determines the lasing wavelength. An excimer laser produces a choice of output wavelengths at 308, 248, 193, or 157 nm, using XeCl, KrF, ArF, or F 2 , respectively. Generally speaking, shorter wavelengths offer higher photon energies for efficient bond breaking and ionization of the solid sample. For laser wavelengths at 266, 213, and 157 nm, the equivalent photon energy is 4.66, 5.83, and 7.90 eV. Absorption of laser energy by target material and by the laser-induced plasma varies significantly with laser wavelength. Ablation may involve thermal and/or non-thermal mechanisms, depending on the wavelength. For a thermal process, electrons directly absorb the laser light, transferring this energy into the atomic lattice. Melting and vaporization of the target material occurs. Because of the difference in latent heat of vaporization for different chemical elements, a thermal mechanism may induce strong fractionation. If the photon energy is higher than the bonding energy between neighboring atoms in the solid (a few eV), the electromagnetic laser radiation can directly breaks the atomic lattice, inducing ion and atom ejection without traditional heating effects [20-21]. 5 Excimer lasers generally have 'flat-top' beam profiles. With appropriate imaging optics, both Nd:YAG and excimer lasers can generate flat-bottom craters (figure 2). The shape of the crater walls will influence depth resolution and fractionation may increase with the development of the ablation crater [31]. However, the degree of fractionation is not strictly related to the beam profile; fractionation is not eliminated by having a flat-top beam profile. Ablation Stage 7 A typical laser ablation stage consists of a lens, an ablation chamber, and an adjustable platform. The lens may be incorporated into an optical microscope so that optical and visual focusing coincides. Often the sample surface can be viewed remotely via a CCD camera. The sample is located inside the ablation chamber that has a fused silica window. The adjustable platform, typically under computer control, allows positioning of the sample in the X, Y, and Z directions. Displacement as small as a few microns can be achieved without difficulty. Depending on the timing between individual laser pulses and platform movement, different tasks such as depth profiling, spatial profiling, surface and bulk analyzes are possible. For applications using LA-ICP (figure 1), the chamber is flushed with an inert gas to transport the ablated sample to the ICP. Argon and helium have been employed with the latter gas providing improved ablation and transport rates [2, 15]. An arrangement for chamber purging must be included, usually by a three-way valve. Since the 1980s, a variety of different ablation chamber designs have been developed and tested [4, [33] [34] [35] [36] [37] [38] for LA-ICP-AES and LA-ICP-MS. The volume of the chamber and the transport tube can influence the sample density in the ICP. Smaller volumes (below 1 cm 3 ) may reduce "sample-washout" time. A high carrier gas flow can reduce sample deposition in the chamber and transfer tube, thereby decreasing memory effects and increasing transport efficiency. To improve transport efficiency, the sample and or chamber can be placed directly under the ICP torch [37] . In general, chamber designs need to be further studied and optimized for improved laser ablation analysis. Detection Systems ICP-MS 8
doi:10.1016/s0039-9140(02)00053-x fatcat:gz5ulqyrvfgb5hi7jmhj6azkwy