Ablation precision

Additionally, Fig. 13 suggests a higher vertical and lateral ablation precision when shorter pulses are employed. Figure 14 depicts the absolute vertical ablation precision Ad vs pulse duration t. Ad describes the shot-to-shot deviation of the ablation depth per pulse d from its mean value for a 99% confidence interval [42]. The d-values were extracted from [24]. For both fused silica (FS) and barium aluminum borosilicate glass (BBS), a tendency of an increasing vertical ablation precision with decreasing pulse duration is clearly evident. This trend is expected to become even more pronounced if data obtained at F0/F = const rather than F0=const could be compared. Figure 14 visualizes that with pulses shorter than 10 fs, an absolute vertical ablation precision of the order of 10 nm can be achieved. 

For thermally sensitive or undoped polymers with low linear absorption at the laser wavelength, the use of femtosecond laser pulses can improve the ablation precision in contrast to long-pulse treatment. Further, the thermal load to the samples is minimized. For these reasons, femtosecond laser pulses were chosen to perforate a polyethylene membrane serving as a diffusion-discriminating element on a miniaturized biosensor for the measurement of glucose concentration [78]. 

In order to visualize the limits of the application of longer laser pulses for the structuring of a nonabsorbent composite material, a result of ablation experiments on human enamel with a 7-ns dye laser (2=600 nm) is presented in Fig. 29. The lateral ablation precision is unsatisfactory (Fig. 29a). Collagen fiber bundles were disrupted (Fig. 29b). The application of high laser fluences above 20 J cm-2 is necessary to achieve a relatively uncontrolled material removal. The constituents of enamel, water, collagen, and the hydroxyapatite matrix, are practically nonabsorbent for wavelengths in the visible region at 2=600 nm. 

Other projects are using fs pulsed lasers with microscopy to ablate precisely small stractures and/or organelles from inside living cells attached to glass slides. This technique provides an entirely new level of control for studying cell physiology in vitro, and will be used in conjunction with the above project on photodynamic therapy for refractory epilepsy by examining the role of mitochondria in the metabolism of ALA. 

The previous discussion has centered on how to obtain as much molecular mass and chemical structure information as possible from a given sample. However, there are many uses of mass spectrometry where precise isotope ratios are needed and total molecular mass information is unimportant. For accurate measurement of isotope ratio, the sample can be vaporized and then directed into a plasma torch. The sample can be a gas or a solution that is vaporized to form an aerosol, or it can be a solid that is vaporized to an aerosol by laser ablation. Whatever method is used to vaporize the sample, it is then swept into the flame of a plasma torch. Operating at temperatures of about 5000 K and containing large numbers of gas ions and electrons, the plasma completely fragments all substances into ionized atoms within a few milliseconds. The ionized atoms are then passed into a mass analyzer for measurement of their atomic mass and abundance of isotopes. Even intractable substances such as glass, ceramics, rock, and bone can be examined directly by this technique. 

The sensitivity, accuracy, and precision of solid-sample analysis have been greatly improved by coupling LA with ICP-OES-MS. The ablated species are transported by means of a carrier gas (usually argon) into the plasma torch. Further atomization, excitation, and ionization of the ablated species in the stationary hot plasma result in a dramatic increase in the sensitivity of the detection of radiation (LA-ICP-OES) or of the detection of ions (LA-ICP-MS). 

Surgical or electrolytic lesions of a particular pathway or nucleus. Even with precise location and subsequent histological verification this approach rarely ablates all the axons (or neurons) in a pathway using the NT under study and can also affect other... 

D microfabricated reactor devices are typically made by fabrication techniques other than stemming from microelectronics, e.g. by modern precision engineering techniques, laser ablation, wet-chemical steel etching or pEDM techniques. Besides having this origin only, these devices may also be of hybrid nature, containing parts made by the above-mentioned techniques and by microelectronic methods. Typical materials are metals, stainless steel, ceramics and polymers or, in the hybrid case, combinations of these materials. 

Horn I, Rudnick RL, McDonongh WF (2000) Precise elemental and isotope ratio determination by simultaneous solution nebnlization and laser ablation-ICP-MS Application to U-Pb geochronology. ChemGeol 164 281-301... 

Stirling CH, Lee DC, Christensen JM, Halliday AN (2000) High-precision in situ U-238-U-234-Th-230 isotopic analysis using laser ablation multiple-collector ICPMS. Geochim Cosmochim Acta 64 3737-3750... 

Cousin, H. and Magyar, B. (1994). Precision and accuracy of laser ablation-ICP-MS analysis of rare earth elements with external calibration. Mikrochimica Acta 113 313-323. 

The photoablation behaviour of a number of polymers has been described with the aid of the moving interface model. The kinetics of ablation is characterized by the rate constant k and a laser beam attenuation by the desorbing products is quantified by the screening coefficient 6. The polymer structure strongly influences the ablation parameters and some general trends are inferred. The deposition rates and yields of the ablation products can also be precisely measured with the quartz crystal microbalance. The yields usually depend on fluence, wavelength, polymer structure and background pressure. 

Hirata T, Hayano Y, Ohno T (2003) Improvements in precision of isotopic ratio measurements using laser ablation-multiple collector-ICP-mass spectrometry reduction of changes in measured isotopic ratios. J Anal At Spectrom 18 1283-1288... 

Figure 18. Magnesium three-isotope plot (relative to SRM 980 0) showing laser ablation data for Allende CAI 3576-1 (after Young et al. 2002a). Ellipses represent the 95% confidence for each datum. The shaded datum is the analysis that included alteration material in the CAI. This point is related to the ofiiers by mass fractionation (dashed line) at constant 5 Mg where 5 Mg is the horizontal deviation from a terrestrial mass-fractionation curve. The 5 Mg value depends on the Al/Mg ratio, indicating in situ decay of A1 in the CAI. The high precision of the MC-ICPMS analyses makes it possible to resolve mass-dependent fractionation from excesses in Mg at the sub-per mil level.

Future studies should make use of the combination of rapid analysis time and high precision afforded by laser ablation in order to compile larger data sets for the myriad of different objects found in meteorites. 

The high precision with which Mg isotope ratios can be measured using MC-ICPMS opens up new opportunities for using Mg as a tracer in both terrestrial and extraterrestrial materials. A key advance is the ability to resolve kinetic from equilibrium mass-dependent fractionation processes. From these new data it appears that Mg in waters is related to mantle and crustal reservoirs of Mg by kinetic fractionation while Mg in carbonates is related in turn to the waters by equilibrium processes. Resolution of different fractionation laws is only possible for measurements of Mg in solution at present laser ablation combined with MC-ICPMS (LA-MC-ICPMS) is not yet sufficiently precise to measure different fractionation laws. 

Norman M, McCulloch M, O Neill H, Brandon A (2004) Magnesium isotopes in the Earth, Moon, Mars, and Pallasite parent body high precision analysis of olivine by laser ablation multi-collector ICPMS. Lunar and Planetary Science Conference XXXV 1447... 

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