Spatial resolution laser microprobe

The spatial resolution of the Raman microprobe is about an order of magnitude better than that obtainable using an infrared microscope. Measurement times, typically of a few seconds, are the same as for other Raman spectrographs. To avoid burning samples, low (5—50-mW) power lasers are employed. 

There are two principal sources of reliable partitioning data for any trace element glassy volcanic rocks and high temperature experiments. For the reasons outlined above, both sources rely on analytical techniques with high spatial resolution. Typically these are microbeam techniques, such as electron-microprobe (EMPA), laser ablation ICP-MS, ion-microprobe secondary ion mass spectrometry (SIMS) or proton-induced X-ray emission (PIXE). 

Microprobe laser desorption laser ionisation mass spectrometry (/xL2MS) is used to provide spatial resolution and identification of organic molecules across a meteorite sample. Tracking the chemical composition across the surface of the meteorite requires a full mass spectrum to be measured every 10 p,m across the surface. The molecules must be desorbed from the surface with minimal disruption to their chemical structure to prevent fragmentation so that the mass spectrum consists principally of parent ions. Ideally, the conventional electron bombardment ionisation technique can be replaced with an ionisation that is selective to the carbonaceous species of interest to simplify the mass spectrum. Most information will be obtained if small samples are used so that sensitivity levels should be lower than attomolar (10—18 M) fewer than 1000 molecules can be detected and above all it must be certain that the molecules came from the sample and are not introduced by the instrument itself. 

Microanalytical techiuques such as laser microprobe (Kelley and Fallick 1990 Crowe et al. 1990 Hu et al. 2003 Ono et al. 2006) and ion microprobe (Chaussidon et al. 1987, 1989 Eldridge et al. 1988, 1993) have become promising tools for determining sulfur isotope ratios. These techniques have several advantages over conventional techniques such as high spatial resolution and the capability for in-situ spot analysis. Sulfur isotopes are fractionated during ion or laser bombardment, but fractionation effects are mineral specific and reproducible. 

Beaudoin G, Taylor BE (1994) High precision and spatial resolution sulfur-isotope analysis using MILES laser microprobe, Geochim Cosmochim Acta 58 5055-5063 Beaudoin G, Taylor BE, Rumble D, Thiemens M (1994) Variations in the sulfur isotope composition of troUite from the Canyon Diablo iron meteorite, Geochim Cosmochim Acta 58 4253 255... 

Table 1 summarizes the capabilities of laser and ion microprobe analysis in comparison to the time-honored conventional techniques. Figure 1 shows the advantages and trade-offs involved in the newer techniques spatial resolution vs. accuracy and precision vs. cost of analysis. These factors will be discussed further under Microanalysis. Continuous flow mass-spectrometry (Merritt and Hayes 1994) IR-spectroscopy (Kerstel et al. 1999, Esler et al. 2000) large radius, multi-collector ion probes (McKeegan and Leshin, this volume) automation and shorter wavelength ElV lasers (Young et al. 1998, Farquhar and Rumble 1998, Fiebig et al. 1999, Jones et al. 

Figure 1. Advantages and trade-offs of new analytical techniques for stable isotope analysis (see Table 1). At present, the best accuracy and precision is achieved for 5 0 by IR-laser fluorination of chips or powdered samples the fastest and least expensive analyses are made by automated pyrolysis systems with continuous flow mass-spectrometers (CFMS) and the smallest samples and best in situ spatial resolution is attained by ion microprobe. The capabilities of in situ UV-laser fluorination are intermediate.

In situ analysis by UV laser is more precise than in situ analysis by IR because there is less heating (see Rumble and Sharp 1998, Farquhar and Rumble 1998, Young et al. 1998, Fiebig et al. 1999, Jones et al. 1999). The spot diameter of a UV laser and the sample size of a continuous flow mass-spectrometer can rival that presently used for oxygen isotope analysis by ion microprobe, however, the spatial resolution by laser is limited by the quantity of oxygen necessary for accurate purification during conversion of mineral to gas. At present, the analytical precision and the spot size attainable by in situ lasers are intermediate between the capabilities of the ion microprobe (best spatial resolution) and the CO2 laser (best precision and accuracy. Fig. 1). For some projects this offers the best analytical compromise. 

Conventional Raman spectroscopy has a poor spatial resolution as the laser spot is normally ca. 0.5 mm. However, with the introduction of a Raman microscope, a spatial resolution as high as the diffraction limit is attainable [41, 42]. Recently, with the further development of the Raman microscope into the confocal microprobe Raman instrument, three-dimensional resolution is achievable. The quite high vertical spatial resolution ensures elimination of the signal from the bulk solution. This is certainly a great help for detecting relatively weak surface Raman signals from weak-SERS or non-SERS active electrodes. 

Laser micropyrolysis gas chromatography-mass spectrometry A laser microprobe is used to target selectively microscopic samples of toner, for subsequently GC-MS detection. Fused toners are directly analyzed in situ avoiding the traditional separation of toner from the paper substrate. This method offers a high spatial resolution and selectivity but further studies may be necessary to improve its reproducibility. 

SR p-XRF offers a number of advantages compared to other microprobe techniques it combines high spatial resolution with high sensitivity, can be used in atmospheric conditions, and is relatively insensitive to beam damage to the sample. As we will show further in this section, the simplicity of the method and the quite good understanding of the physics of the processes involved make it more adaptable for quantitative analysis than a number of other beam methods of analysis. The method is now well accepted as an important tool for microscopic elemental analysis, complementing other beam methods of analysis that rely on electron, ion, photon, or laser beams. 

The diameter of the laser beam used to probe the sample surface typically determines the effective spatial resolution of a measurement performed in microprobe mode. Obviously, the laser beam diameter can be reduced by focusing the beam to smaller dimensions. However, as the laser beam diameter is reduced, it illuminates a smaller area, fewer molecules of each analyte are present within the probe beam, and so fewer molecules are ionized at each location. Therefore, smallest diameter beams are rarely practical because the amount of analyte that can be desorbed and ionized from a smaller sample area is not sufficient for detection and high-accuracy mass measurement. Consequently, the laser probe diameter for the analyses of proteins and peptides usually is larger than 10 xm. 

Laser-microprobc mass spectrometry has an unusually high sensitivity (down to 10 g), is applicable to both inorganic and organic (including biological) samples, has a spatial resolution of about I pm, and produces data at a rapid rate. Some typical applications of laser-microprobe mass spectrometry include determination of Na/K concentration ratios in frog nerve fiber, determination of the calcium distribution in retinas, classification of asbestos and coal mine dusts, determination of fluorine distributions in dental hard tissue, analysis of amino acids, and study of polymer surfaces. ... 

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