Solid sample analysis using laser ablation

Gunther D, Hattendorf B (2005) Solid sample analysis using laser ablation inductively coupled plasma mass spectrometry. Trends Anal Chem 24 255-265... 

Guenther, D. Hattendorf, B. Solid Sample Analysis Using Laser Ablation Inductively Coupled Plasma Mass Spectrometry. Trends Anal. Chem. 2005, 24, 255-265. 

Digestion of samples is reaUy only necessary for solid materials (unless using laser ablation). Urine and blood samples can simply be diluted and then analysed. Urine and serum samples are commonly diluted in acidic solutions and whole blood samples in alkaline media. It would be possible to analyse urine and blood in the same analysis if the blood samples were acid digested prior to determination by ICP-MS. The need for matrix-matched blood standards makes it difficult to dilute the blood sample and analyse in situ with urine samples. 

Fundamentally, introduction of a gaseous sample is the easiest option for ICP/MS because all of the sample can be passed efficiently along the inlet tube and into the center of the flame. Unfortunately, gases are mainly confined to low-molecular-mass compounds, and many of the samples that need to be examined cannot be vaporized easily. Nevertheless, there are some key analyses that are carried out in this fashion the major one i.s the generation of volatile hydrides. Other methods for volatiles are discussed below. An important method of analysis uses lasers to vaporize nonvolatile samples such as bone or ceramics. With a laser, ablated (vaporized) sample material is swept into the plasma flame before it can condense out again. Similarly, electrically heated filaments or ovens are also used to volatilize solids, the vapor of which is then swept by argon makeup gas into the plasma torch. However, for convenience, the methods of introducing solid samples are discussed fully in Part C (Chapter 17). 

The matrix is the major component of the solution (or of the solid if using laser ablation or SEM). Matrix differences between standards and samples or between samples may result in differences of elemental sensitivity. Therefore, it is desirable that all the blanks, calibration standards, and samples have the same matrix (i.e., are matrix matched). This is relatively easy to achieve in solution, but can be a major problem with the analysis of solid samples. 

Quantitative analysis MS is used extensively for the quantitative determination of the organic components of liquid and gas samples. Solid samples can be analyzed using laser ablation. 

The laser typically used for laser ablation is the frequency quadrupled Nd YAG (266 nm) with a pulse width of a few nanoseconds. With this laser, some fractionation will occur. Unless a good external standard is used, quantification of the elements in the sample is difficult if not impossible. This is not an impediment to use of laser ablation in the field of radiochemistry or radionuclides. One of the most important aspects of radionuclide analysis is determining isotope ratios of elements of interest. All isotopes of a particular element behave the same when removed from a solid sample by a laser pulse. Ionization of those isotopes can depend on the laser bandwidth and polarization so care must be used when using lasers for direct ionization of atomic species. 

Some solid materials are very intractable to analysis by standard methods and cannot be easily vaporized or dissolved in common solvents. Glass, bone, dried paint, and archaeological samples are common examples. These materials would now be examined by laser ablation, a technique that produces an aerosol of particulate matter. The laser can be used in its defocused mode for surface profiling or in its focused mode for depth profiling. Interestingly, lasers can be used to vaporize even thermally labile materials through use of the matrix-assisted laser desorption ionization (MALDI) method variant. 

Approximately 70 different elements are routinely determined using ICP-OES. Detection limits are typically in the sub-part-per-billion (sub-ppb) to 0.1 part-per-million (ppm) range. ICP-OES is most commonly used for bulk analysis of liquid samples or solids dissolved in liquids. Special sample introduction techniques, such as spark discharge or laser ablation, allow the analysis of surfaces or thin films. Each element emits a characteristic spectrum in the ultraviolet and visible region. The light intensity at one of the characteristic wavelengths is proportional to the concentration of that element in the sample. 

There are several sample introduction methods that are used in conjunction with ICP, including nebulization, electrothermal evaporation, gas chromatography, hydride generation, and laser ablation [30]. Laser ablation combined with ICP (LA-ICP) is useful for analysis of solids. In such a source the sample is positioned in a chamber prior to the ICP source, the ablation cell. Argon gas at atmosperic pressure flows through the cell towards the ICP source. The sample is irradiated by a laser beam and... 

Laser ablation ICP-MS (LA-ICP-MS) was established in the early 1990s as a potential routine tool for the measurement of trace and ultra-trace elements in silicate systems for geology. Early studies (Perkins et al. 1993) used sample preparation techniques identical to that used to prepare rock samples for WDXRF, i.e., either a pressed powder disk or a glass bead fusion method (see Appendix VIII). Such studies concluded that LA-ICP-MS had the potential to surpass XRF in terms of the limits of detection achieved and INAA in terms of the speed of analysis (Perkins et al. 1993 481). It has long been recognized that the main limit on the quantitative performance of LA-ICP-MS is the homogeneity at the trace and ultra-trace level of the solid calibration standards available. Subsequent work (e.g., Hollecher and Ruiz 1995, Norman et al. 1996) has demonstrated that some of the international... 

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