Laser ablation variables

Prohaska, T., Stadlbauer, C., Wimmer, R., et al. (1998). Investigation of element variability in tree rings of young Norway spruce by laser-ablation-ICPMS. Science of the Total Environment 219 29-39. 

We will illustrate this quantitative characterization method of the microstructure with a study [BOU 03] conducted on a sample comprised of a lithium niobate film which is which is about 500 nm thick, deposited on a sapphire substrate cut parallel to the (0006) planes. This film was produced by laser ablation and is epitaxial so as to have the (0006) planes of LiNbOs parallel to the interface and the in-plane orientation is characterized by two variables. The epitaxy relations can be written ... 

Abstract. Thermally-assisted grafting of linear alkene molecules either in the liquid phase (ethyl undecylenate) or in the gas phase (perfluorodecene), has been performed on atomically flat amorphous carbon (a-C) films with variable average surface hybridization, sp3/(sp2+ sp3), as obtained from X-ray photoelectron spectroscopy. In contrast with the sp2-rich sputtered a-C, optimized sp3-rich a-C films obtained by Pulsed Laser ablation of a glassy carbon target do not require surface preparation before hquid phase grafting. 

N. W. 2003. Determination of intratest variability of trace elements in foraminifera by laser ablation inductively coupled plasma mass-spectrometry. Geochemistry, Geophysics, Geosystems, 4, 8408. 

Fig. 4. Element to calcium ratios in Globigerinoides sacculifer and G. ruber determined by laser ablation ICPMS indicating the spatial heterogeneity in Mg concentrations in G. sacculifer but not in G. ruber (modified after Eggins et al. 2003). The Mg concentrations are higher in the inner part of G. sacculifer but constant in G. ruber which lacks Mg-poor gametogenic calcite. In contrast, Sr concentrations are relatively stable throughout the test and very similar for both species. Note that the 1 pm layer with very high Mg values, which we interpret as contamination on the outside of the test, has been excluded from this plot to highlight the internal variability. The different duration of the analysis is determined by the thickness of the foraminiferal test.

Other techniques are based on sample sputtering followed by mass spectrometry of the vaporized products, including secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Beam sizes are in the few to tens of pm range. Elemental sensitivities for SIMS are highly variable depending on ion yield, and quantification can be difficult because of matrix effects in the ion production process. SIMS and LA-ICP-MS have very high sensitivities for some elements and low sensitivity for others. These and other microanalytical techniques used in earth science research are described in Potts et al. (1995). 

Vacuum is effective in collecting ISF firom skin that has been mechanically penetrated or had the stratum comeum removed. As with blood collection studies, large site-to-site and person-to-person variability is observed. A large vacuum headspace in the collection devise results in a decrease in volume due to evaporation and an increase in analyte concentrations. Outgassing of dissolved gas in the ISF and leaks between the skin and vacuum source lead to the formation of bubbles in the ISF stream over time. The application of one-half atmosphere vacuum through a harvesting head with an aperture of 2.5 mm over a 1 mm pattern of micropores produced by laser ablation of a black dye yields straw-colored interstitial fiuid at fiow rates of 5-15 pL/h for several days [4]. Vacuum collection over a much larger area (about 1 cm ) is required to collect similar amounts of ISF from low-frequency ultrasound-treated skin [8]. 

The advent of laser ablation MC-ICP-MS technology allows the rapid in situ determination of the stable isotope ratios of heavy metals commonly found in sulfide ore deposits (e.g., Cu, Zn, Fe, Sb, Ag) providing important information on the source, transport, and depositional mechanisms of these metals. Pb, Pb, and Pb are formed as the end product of radioactive decay and the isotopic variability of lead results because the elements from which the isotopes form were not evenly distributed in ore bodies. Hence, the analysis of stable lead isotopes in annually laminated lake-sediments is a useful method to study lead pollution history as the relative contribution of pollution and natural lead in sediment samples can be calculated. The analysis of lead isotopes by SIMS has also been used to identify the geographical origin of bullets. 

Kontak, D.J., and Jackson, S. (1995). Laser-ablation ICP-MS micro-analysis of calcite cement from a Mississippi-Valley-type Zn-Pb deposit. Nova Scotia Dramatic variability in REE content. Can. Mineral 33(2), 445. 

Clearly, it would be more efficient to have a conceptual framework to guide development, or at least show where fundamental limits might lie. A complete theory of MALDI should quantitatively predict the observed mass spectrum as a function of variables such as matrix choice, analyte physical and chemical properties, concentrations, preparation method, laser characteristics (wavelength, spatial and temporal properties), local environment (such as ambient pressure or substrate temperature), and ion extraction method. Here we focus only on ionization mechanisms and do not address all factors affecting a MALDI experiment. Some of these are discussed in other contributions to this volume. Only mechanisms involving molecular matrices and laser excitation are included, methods that depend primarily on properties of the substrate, such as nanoparticles or stmctured surfaces (such as DIOS) are not Hybrid methods, such as laser ablation into electrosprays, are also out of our scope, but vacuum and higher-pressure (e.g., atmospheric pressure) MALDI are both considered to have the same underlying mechanisms discussed here. 

The large variability in elemental ion yields which is typical of the single-laser LIMS technique, has motivated the development of alternative techniques, that are collectively labeled post-ablation ionization (PAI) techniques. These variants of LIMS are characterized by the use of a second laser to ionize the neutral species removed (ablated) from the sample surface by the primary (ablating) laser. One PAI technique uses a high-power, frequency-quadrupled Nd-YAG laser (A, = 266 nm) to produce elemental ions from the ablated neutrals, through nonresonant multiphoton ionization (NRMPI). Because of the high photon flux available, 100% ionization efflciency can be achieved for most elements, and this reduces the differences in elemental ion yields that are typical of single-laser LIMS. A typical analytical application is discussed below. 

The potential of LA-based techniques for depth profiling of coated and multilayer samples have been exemplified in recent publications. The depth profiling of the zinc-coated steels by LIBS has been demonstrated [4.242]. An XeCl excimer laser with 28 ns pulse duration and variable pulse energy was used for ablation. The emission of the laser plume was monitored by use of a Czerny-Turner grating spectrometer with a CCD two-dimensional detector. The dependence of the intensities of the Zn and Fe lines on the number of laser shots applied to the same spot was measured and the depth profile of Zn coating was constructed by using the estimated ablation rate per laser shot. To obtain the true Zn-Fe profile the measured intensities of both analytes were normalized to the sum of the line intensities. The LIBS profile thus obtained correlated very well with the GD-OES profile of the same sample. Both profiles are shown in Fig. 4.40. The ablation rate of approximately 8 nm shot ... 

Ablation can be carried out with lower cost laser diodes when a broadband energy absorber is applied to the skin. For example, shallow pores are created by application of twenty to thirty 20 ms pulses of 810 nm radiation at a power of250 mW to a 50 pm thick carbon containing black tape on the skin.50 A pattern of these shallow pores is easily generated on the skin by movement of the laser. Application of about one-half atmosphere of vacuum extracts about 0.2 pL ISF per minute with a relative standard deviation of 65%, which includes person-to-person and site-to-site variabilities. Larger volumes of ISF ( 0.5 pL/min) can be exuded from the ablated site by application of positive pressure to the tissue around the site. In general, the volume exuded increases as the number of micropores at the site increases from one to... 

Some of the problems encountered in using the LA technique arise from so-called elemental fractionation, a non-sample related change in the analyte response during the ablation process. This effect has been observed to some extent with all types of lasers, but other variables including energy density, focal point and the crater diameter-to-depth ratio also appear to exert a strong influence [9-13,39,51]. 

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