Ablating material

Lasers can be used in either pulsed or continuous mode to desorb material from a sample, which can then be examined as such or mixed or dissolved in a matrix. The desorbed (ablated) material contains few or sometimes even no ions, and a second ionization step is frequently needed to improve the yield of ions. The most common methods of providing the second ionization use MALDI to give protonated molecular ions or a plasma torch to give atomic ions for isotope ratio measurement. By adjusting the laser s focus and power, laser desorption can be used for either depth or surface profiling. 

With a typical ablated particle size of about 1 -pm diameter, the efficiency of transport of the ablated material is normally about 50% most of the lost material is deposited on contact with cold surfaces or by gravitational deposition. From a practical viewpoint, this deposition may require frequent cleaning of the ablation cell, transfer lines, and plasma torch. 

A somewhat related technique is that of laser ionization mass spectrometry (LIMS), also known as LIMA and LAMMA, where a single pulsed laser beam ablates material and simultaneously causes some ionization, analogous to samples beyond the outer surface and therefore is more of a bulk analysis technique it also has severe quantiBaction problems, often even more extreme than for SIMS. 

In Table 4.3, the Cetac product LSX-200 is the specialized system for coupling with the ICP customer s system. It includes the laser, optical viewing system for exact positioning of the laser focus on a sample surface, and the sample cell mounted on the computer controlled XYZ translation stage. The system is also provided with the appropriate gas tuhing for transport of the ablated material into an ICP-OES/MS. 

Different analytical techniques are used for detection of the elemental composition of the solid samples. The simplest is direct detection of emission from the plasma of the ablated material formed above a sample surface. This technique is generally referred to as LIBS or LIPS (laser induced breakdown/plasma spectroscopy). Strong continuous background radiation from the hot plasma plume does not enable detection of atomic and ionic lines of specific elements during the first few hundred nanoseconds of plasma evolution. One can achieve a reasonable signal-to-noise ra-... 

The intrinsic drawback of LIBS is a short duration (less than a few hundreds microseconds) and strongly non-stationary conditions of a laser plume. Much higher sensitivity has been realized by transport of the ablated material into secondary atomic reservoirs such as a microwave-induced plasma (MIP) or an inductively coupled plasma (ICP). Owing to the much longer residence time of ablated atoms and ions in a stationary MIP (typically several ms compared with at most a hundred microseconds in a laser plume) and because of additional excitation of the radiating upper levels in the low pressure plasma, the line intensities of atoms and ions are greatly enhanced. Because of these factors the DLs of LA-MIP have been improved by one to two orders of magnitude compared with LIBS. 

For reentry heat shields, ablator materials are well proven. These materials typically consist of reinforced plastics with a density of 0.5 - 1.0 g/cm3. The high heat loads are consumed by the carbonization or sublimation of the ablator. The carbonized material cannot withstand very high aerodynamical loads. When the aerodynamic forces exceed a specific threshold value, the initial slight erosion on the carbonized layer intensifies until the whole layer splits of. This results in an exponential rise in ablation velocity for the unprotected ablator. 

Figure 9.2 Schematic diagram of a quadrupole ICP-MS capable of working in either the solution or laser ablation mode. In the solid mode, a vertical laser ablates material from the sample which is mounted on a moveable horizontal stage. In solution mode, the liquid is sucked up into the injection chamber.

By fluorescence analyses just upon laser ablation and of ablated surface, Molecular aspects of ablation echanisa were elucidated and a characterization of ablated Materials was perforaed. Laser fluence dependence of poly(N-vinylcarbazole) fluorescence indicates the iaportance of Mutual interactions between excited singlet states. As the fluence was increased, a plasna-like eaission was also observed, and then fluorescence due to diatonic radicals was superinposed. While the polyner fluorescence disappeared Mostly during the pulse width, the radicals attained the naxinun intensity at 100 ns after irradiation. Fluorescence spectra and their rise as well as decay curves of ablated surface and its nearby area were affected to a great extent by ablation. This phenonenon was clarified by probing fluorescence under a Microscope. 

Another advantage to examine these polyaers is that characterizations of ablated materials can be Bade possible by fluorescence spectroscopy. Fluorescence is very sensitive, and such surrounding aicroenvironaental conditions around the it -chromophore as polarity and viscosity and chroaophore aggregation can be probed. 

A laser pulse can ablate material from the surface of a sample, and create a microplasma which ionizes some of the sample components. The laser pulse accomplishes both vaporization and ionization of the sample [366,534,535]. This method is called laser ionization mass spectrometry (LIMS). 

Chromite major and minor element composition was determined by microprobe. The IPGE contents of chromite were determined at UQAC by a LA-ICPMS (Thermo X7 ICP-MS coupled to a New Wave Research 213 nm UV laser, 80 pm spot diameter, 10 Hz pulse rate, 0.3 mJ/pulse power). In addition to the IPGE other elements were monitored to control the nature of ablated material and the presence of included phases. 

The influence of various gas pressure conditions within the laser ablation cell on the particle formation process in laser ablation has also been investigated.69 In LA-ICP-MS studies at low pressure (down to 2kPa) a small particle size distribution and a reduction in elemental fractionation effects was obtained. But with decreasing pressure and transport volume of ablated material, a significant decrease in the ion intensities was observed as demonstrated for uranium measurements in the glass SRM NIST 610.69 However, the laser ablation of solid materials at atmospheric pressure in LA-ICP-MS is advantageous for routine measurements due to lower experimental effort and the possibility of fast sample changing in the ablation chamber. Fractionation... 

On line additions of aqueous standard solutions for the calibration of LA-ICP-MS including a comparison of wet and dry plasma conditions are discussed by O Connor et al.ls For solution calibration of standard solutions the authors used a 100 (xl PFA nebulizer together with a cyclonic spray chamber or a MCN-6000 sample introduction system with desolvator, to study the wet and dry plasma, respectively. A polypropylene Y piece was applied to mix the laser ablated material and the nebulized standard solutions. The authors found that the on line addition of water is the preferred mode of operation for quantification by LA-ICP-MS, i.e., wet plasma is more stable (improved standard deviation of sensitivity ratios). 

A serious problem in LA-ICP-MS described in the literature on many occasions is the time-dependent elemental fraction (so-called ablation fractionation ) occurring during laser ablation and the transport process of ablated material, or during atomization and ionization processes in the inductively coupled plasma.20-22 Numerous papers focus on the study of fraction effects in LA-ICP-MS as a function of experimental parameters applied during laser ablation (such as laser energy, laser power density, laser pulse duration, wave length of laser beam, ablation spot size,... 

---