Ablation, laser

Laser sources make use of population inversion. When radiation enters a medium both absorption or stimulated emission of radiation can occur as a result of the interaction with matter and the change in flux at the exit is given by  

When a laser beam with a small divergence impinges on a solid surface, part of the energy is absorbed (10-90%) and material evaporates locally [223]. The energy required therefore varies between about 104 W/cm2 (for biological samples) and 109 W/cm2 (for glasses). Hence as a result, a crater is formed, the smallest diameter of which is determined by the diffraction of the laser radiation, and can be approximated by  

An interesting development is the system where the laser beam is moved over the sample surface by swinging the focussing lens (Fig. 69). In this way material sampling from a larger part of the sample is possible. Bulk information from the sample is then obtained and the method could be an alternative to conventional spark emission spectrometry. The high precision of the approach, which again can 

Ihe laser beam is focused on the graphite target and is allowed to scan across the target smface to maintain a smooth, uniform surface for vaporization. Vaporized carbon forms various carbon nanostructures, which are taken to a water-cooled copper collector by the inert gas flow. 

In laser ablation, a solid sample is irradiated with a laser pulse that ablates the point of laser-solid contact to produce a plume of ions and neutrals in the vapor space just above the point of laser-solid contact with the sample surface. If this plume is swept into an ionization source or if reactant ions are electrically focused into the ablated sample plume, product ions are formed. These ions can be electrically focused into an IMS for ion mobility analysis. Direct laser ablation followed by ionization from 

02 ions formed from electrons during the ablation process has been demonstrated. Both corona ionization and ESI have been used to assist the laser ablation process in creating analyte ions for IMS. 

Borsdorf, H. Rammler, A. Schulze, D. Boadu, K.O. Feist, B. Weiss, H., Rapid on-site determination of chlorobenzene in water samples using ion mobility spectrometry. Anal. Chim. Acta 2001, 440, 63-70. 

Sielemann, S. Baumbach, J.L Schmidt, H. Pilzecker, R, Detection of alcohols nsing UV-ion mobility spectrometers. Anal. Chim. Acta 2001, 431, 293-301. 

Karasek, F.W., Plasma chromatography—a new dimension for gas chromatography and mass spectrometry, J. Chromatogr. Sci. 1970, 8, 330-337. 

By the interaction of the radiation from high-power laser sources with solid matter, the latter can be volatilized. This occurs partly as a result of thermal evaporation 

As the interaction of laser radiation with solids very much depends on the wavelength of the radiation, frequency doubling resulting from nonlinear effects, for example in LiNbOs or quartz, is often used. By doubling the frequency, the degree of 

With advanced Nd YAG lasers at atmospheric pressure, as utilized when coupling with ICP-OES or MS, selective volatilization is moderate. However, in the case of brass, it is as high as in spark ablation and causes problems in calibration [257]. In recent work, favorable working conditions in laser ablation studies were also shown to apply at reduced pressures of around 10-100 mbar [255, 258, 259]. In a number of cases, analytes in very different matrices were found to give signals which fitted astonishly well with the same calibration curves from OES, and a nearly matrix-independent calibration could be applied. This would be very welcome in the analysis of compact ceramics, for which no other direct analysis methods exist. By careful optimization of the laser working conditions, it is now possible to obtain very reproducible sample material from plastics, as shown by Hemmerlin and Mermet [260]. 

To obtain SWNT alone, a certain amount of catalyst ( l-2 at. %) must be added to the graphite target. Without this additive, multiwalled tubes are formed (Section 

The concentration of catalyst metal at the surface of the graphite target increases during exposure to the laser, which reduces the rate of SWNT formation. Hence a technique simultaneously irradiating a neat and a catalyst-bearing target has been developed that leads to markedly better yields. The amount of SWNT obtained is further influenced by the temperature. The higher it is, the more nanotubes are produced-below 200°C on the other hand, no SWNTs are observed any more. Usually the apparatus is run at a temperature of 1200°C. 

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Figure Bl.22.7. Left resonant seeond-hannonie generation (SHG) speetnimfrom rhodamine 6G. The inset displays the resonant eleetronie transition indueed by tire two-photon absorption proeess at a wavelength of approximately 350 mn. Right spatially resolved image of a laser-ablated hole in a rhodamine 6G dye monolayer on fiised quartz, mapped by reeording the SHG signal as a fiinetion of position in the film [55], SHG ean be used not only for the eharaeterization of eleetronie transitions within a given substanee, but also as a mieroseopy tool.

Akkermans R P, Wu M, Bain C D, Fidel-Suerez M and Compton R G 1998 Electroanalysis of ascorbic acid a comparative study of laser ablation voltammetry and sonovoltammetry E/eofroana/ys/s 10 613... 

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). 

Laser desorption to produce ions for mass spectrometric analysis is discussed in Chapter 2. As heating devices, lasers are convenient when much energy is needed in a very small space. A typical laser power is 10 ° W/cm. When applied to a solid, the power of a typical laser beam — a few tens of micrometers in diameter — can lead to very strong localized heating that is sufficient to vaporize the solid (ablation). Some of the factors controlling heating with lasers and laser ablation are covered in Figure 17.2. 

Aerosols can be produced as a spray of droplets by various means. A good example of a nebulizer is the common household hair spray, which produces fine droplets of a solution of hair lacquer by using a gas to blow the lacquer solution through a fine nozzle so that it emerges as a spray of small droplets. In use, the droplets strike the hair and settle, and the solvent evaporates to leave behind the nonvolatile lacquer. For mass spectrometry, a spray of a solution of analyte can be produced similarly or by a wide variety of other methods, many of which are discussed here. Chapters 8 ( Electrospray Ionization ) and 11 ( Thermospray and Plasmaspray Interfaces ) also contain details of droplet evaporation and formation of ions that are relevant to the discussion in this chapter. Aerosols are also produced by laser ablation for more information on this topic, see Chapters 17 and 18. 

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. 

For solids, there is now a very wide range of inlet and ionization opportunities, so most types of solids can be examined, either neat or in solution. However, the inlet/ionization methods are often not simply interchangeable, even if they use the same mass analyzer. Thus a direct-insertion probe will normally be used with El or Cl (and desorption chemical ionization, DCl) methods of ionization. An LC is used with ES or APCI for solutions, and nebulizers can be used with plasma torches for other solutions. MALDI or laser ablation are used for direct analysis of solids. 

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 three techniques — laser desorption ionization, laser ablation with secondary ionization, and matrix-assisted laser desorption — are all used for mass spectrometry of a wide variety of substances from rock, ceramics, and bone to proteins, peptides, and oligonucleotides. 

IKES. ion kinetic energy spectroscopy IRMS. isotope ratio mass spectrometry ISDMS. isotope dilution mass spectrometry ITMS. ion trap mass spectrometry LA. laser ablation... 

The requirements of thin-film ferroelectrics are stoichiometry, phase formation, crystallization, and microstmctural development for the various device appHcations. As of this writing multimagnetron sputtering (MMS) (56), multiion beam-reactive sputter (MIBERS) deposition (57), uv-excimer laser ablation (58), and electron cyclotron resonance (ECR) plasma-assisted growth (59) are the latest ferroelectric thin-film growth processes to satisfy the requirements. 

Laser ablation systems hold considerable promise if restenosis (reblocking of the arteries) rates are reduced. The rate as of 1995 is 30%, typically within six months. Mechanical or atherectomy devices to cut, shave, or pulverize plaque have been tested extensively in coronary arteries. Some of these have also been approved for peripheral use. The future of angioplasty, beyond the tremendous success of conventional balloon catheters, depends on approaches that can reduce restenosis rates. For example, if appHcation of a dmg to the lesion site turns out to be the solution to restenosis, balloon catheters would be used for both dilating the vessel and deUvering the dmg. An understanding of what happens to the arterial walls, at the cellular level, when these walls are subjected to the various types of angioplasty may need to come first. 

Alternative Thin-Film Fabrication Approaches. Thin films of electronic ceramic materials have also been prepared by sputtering, electron beam evaporation, laser ablation, chemical beam deposition, and chemical vapor deposition (CVD). In the sputtering process, targets may be metal... 

VOLTAMMETRY WITH AN ELECTRODE SURFACE PERIODICALLY RENEWED BY LASER ABLATION... 

Laser based mass spectrometric methods, such as laser ionization (LIMS) and laser ablation in combination with inductively coupled plasma mass spectrometry (LA-ICP-MS) are powerful analytical techniques for survey analysis of solid substances. To realize the analytical performances methods for the direct trace analysis of synthetic and natural crystals modification of a traditional analytical technique was necessary and suitable standard reference materials (SRM) were required. Recent developments allowed extending the range of analytical applications of LIMS and LA-ICP-MS will be presented and discussed. For example ... 

Liquids, directly solids, following dissolution solids, surfaces, and thin films with special methods (e.g., laser ablation)... 

Direct sampling of solids may be carried out using laser ablation. In this technique a high-power laser, usually a pulsed Nd-YAG laser, is used to vaporize the solid, which is then swept into the plasma for ionization. Besides not requiring dissolution or other chemistry to be performed on the sample, laser ablation ICPMS (LA-ICPMS) allows spatial resolution of 20-50 pm. Depth resolution is 1-10 pm per pulse. This aspect gives LA-ICPMS unique dit nostic capabilities for geologic samples, surface features, and other inhomogeneous samples. In addition minimal, or no, sample preparation is required. 

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. 

ICP-OES is a destructive technique that provides only elemental composition. However, ICP-OES is relatively insensitive to sample matrix interference effects. Interference effects in ICP-OES are generally less severe than in GFAA, FAA, or ICPMS. Matrix effects are less severe when using the combination of laser ablation and ICP-OES than when a laser microprobe is used for both ablation and excitation. 

The crater surfaces obtained in the LA-TOF-MS experiment on the TiN-TiAlN-Fe sample were remarkably smooth and clearly demonstrated the Gaussian intensity distribution of the laser beam. Fig. 4.45 shows an SEM image of the crater after 100 laser pulses (fluence 0.35 J cm ). The crater is symmetrical and bell-shaped. There is no significant distortion of the single layers. Fig. 4.45 is an excellent demonstration of the potential of femtosecond laser ablation, if the laser beam had a flat-top, rather than Gaussian, intensity profile. 

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