Ablation diameter

Recently, a loop antenna for the 915 MHz device was assessed in an in vivo porcine liver model by Shock et al. (2004). Loop antennae were described in combination with an electrocautery device for wire deployment. The loop antenna was reported to create a larger ablation zone (mean ablation diameter 4.3+-0.6 cm) as compared to a single straight MW antenna. However, viable tissue was found in the center of the loop for the single configuration. Complete ablation in the center was described for an orthogonal configuration of two loop antennae. 

The size of both primary and metastatic lung tumors often exceeds 3 cm in diameter at the time of diagnosis. The RF electrodes of the three leading companies currently in use are designed for a maximum ablation diameter of 5 cm. Therefore, the tumor to be ablated should not exceed 3 cm in maximum diameter, as a 1-cm safety ablation margin surrounding the tumor should ideally be achieved. 

There are reports that a threshold temperature required to produce thermal injury at the pad site is between 45°C and 47°C (Berber et al. 2000). Whilst rapidly advancing RFA technology allows a large ablation diameter, the factors that control the ablation size on the active electrodes also play an equally important role in the dispersive electrodes site, as the same current flows through both these electrodes. These factors include the current density, the amount of power delivered and the factors that affect heat distribution such as blood flow and conduction and convection of heat. 

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. 

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. 

The applications of the unsaturated polyester resins were increased in the late 1960s by the introduction of water-extended polyesters. In these materials water is dispersed into the resin in very tiny droplets (ca 2-5 p.m diameter). Up to 90% of the system can consist of water but more commonly about equal parts of resin and water are used. The water component has two basic virtues in this system it is very cheap and because of its high specific heat it is a good heat sink for moderating cure exotherms and also giving good heat shielding properties of interest in ablation studies. 

Optimisation of SWCNT production has been attempted within the framework of the arc-discharge method in which anode and cathode were made of graphite rods, a hole in the anode being filled with metal catalysts such as Y (1 at.%) and Ni (4.2 at.%) [7]. A dense collar deposit (ca. 20% of the total mass of graphite rod) formed around the eathode under He (ca. 500 Torr), with 30 V and 100 A de eurrent. It was eonfirmed that this optimal eollar eontained large amounts of SWCNT bundles eonsisting of (10, 10) SWCNTs (diameter 1.4 nm). Such morphology resembles that produced by the laser ablation teehnique [4,5]. 

In ICP-AES and ICP-MS, sample mineralisation is the Achilles heel. Sample introduction systems for ICP-AES are numerous gas-phase introduction, pneumatic nebulisation (PN), direct-injection nebulisation (DIN), thermal spray, ultrasonic nebulisation (USN), electrothermal vaporisation (ETV) (furnace, cup, filament), hydride generation, electroerosion, laser ablation and direct sample insertion. Atomisation is an essential process in many fields where a dispersion of liquid particles in a gas is required. Pneumatic nebulisation is most commonly used in conjunction with a spray chamber that serves as a droplet separator, allowing droplets with average diameters of typically <10 xm to pass and enter the ICP. Spray chambers, which reduce solvent load and deal with coarse aerosols, should be as small as possible (micro-nebulisation [177]). Direct injection in the plasma torch is feasible [178]. Ultrasonic atomisers are designed to specifically operate from a vibrational energy source [179]. 

In non-highly focussed laser desorption ionisation, employing spot sizes in the range of 50-200 pm in diameter, the surface is deformed by an ablation volume of about 1 pm3 per pixel per laser pulse. But this ablated volume is spread over a large desorption area leading to ablation depths of the order of a few nanometres. In laser microprobing, the same ablation volume leads to ablation crater depths in the micrometer range. 

The SP-ablator allows higher aerodynamic loads with lower surface/mass ratio for heat shields, and should be ideally suited for moon, mars, or other interplanetary return missions. These shields are also suitable for cost-effective flight models of winged reentry capsules. A large application potential can be seen for nozzles and combustion chambers or housings of rocket engines. Dornier plans to manufacture a heat shield for the Mirka capsule one meter in diameter. The C/SiC-cover will be fabricated in one piece. 

Mass spectrometric measurements of ions desorbed/ionized from a surface by a laser beam was first performed in 1963 by Honig and Woolston [151], who utilized a pulsed mby laser with 50 p,s pulse length. Hillenkamp et al. used microscope optics to focus the laser beam diameter to 0.5 p,m [152], allowing for surface analysis with high spatial resolution. In 1978 Posthumus et al. [153] demonstrated that laser desorption /ionization (LDI, also commonly referred to as laser ionization or laser ablation) could produce spectra of nonvolatile compounds with mass > 1 kDa. For a detailed review of the early development of LDI, see Reference 154. There is no principal difference between an LDI source and a MALDI source, which is described in detail in Section 2.1.22 In LDI no particular sample preparation is required (contrary to... 

Laser ablation is another important development, allowing solid samples to be directly analysed by ICP, since it offers the possibility of spatially-resolved microanalysis of solid samples, in a manner similar to electron microscopy, but with greater sensitivity and the potential for isotopic analysis. In this technique, a high-energy pulsed laser is directed onto a solid sample, with a beam diameter of less than 25 pm. The pulse vaporizes about 1 pg of material, to leave a crater... 

If the mass load on the electrode is not uniform, a calibration is then necessary to account for the radial sensitivity of the vibrating device (Lazare, S. Granier, V., unpublished results). The maximum of sensitivity is obtained at the centre of the electrode. This allows, for instance, etching over surface areas as small as a 2 mm diameter disc, with a minimum detectable mass of one nanogram. The calibration is performed in this case by using a fluence at which the ablation rate is known, in order to determine the sensitivity factor. 

Table 4. Tabulation of A Mg vs. A O for various chondrite components. A O laser ablation data are shown explicitly to demonstrate that averages are reflective of the bulk objects. Where data are few, the objects are small in comparison to laser beam diameter.

Young et al. (2002a) showed that ultraviolet laser ablation combined with MC-ICPMS (LA-MC-ICPMS) can offer advantages over other methods of spatially resolved Mg isotopic analysis of meteorite materials. They collected data for chondrules and a CAI from the Allende meteorite. Each datum in that study represents approximately 2.8 pg of material (based on a laser spot diameter of 100 pm and laser pit depth of 30 pm depths are uncertain to + 20%). 

CNTs are prepared by either laser ablation from graphite target, arc discharge, or chemical vapor deposition. In either case, to grow they need the presence of Co or Ni catalyst. Typically, outer diameter of CNTs prepared by a carbon arc process ranges between 20 and 200 A, and inner diameter ranges between 10 and 30 A. An aspect ratio (length-to-diameter ratio) is typically of 10 -10. ... 

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. 

Fig. 3. Graph showing Cr, Al, V, Co, Zn, Ga, Ru, Rh, Os, and Ir profiles from a laser ablation analysis of chromite with counts per second vs time. Apart from a small Ru ( Os, lr)-bearing PGM inclusion (diameter 2 pm), the chromites do not contain IPGE. Chromite sample is from a podiform chromitite from TMO.

The method of laser ablation from a metal has been used to prepare nanoparticles dispersed in a solution [202-207]. Recently, colloidal gold nanoparticles in water having an average diameter of 5.5 nm were prepared by laser ablation at 1064 nm (800 mJ/cm ) from a gold metal plate [208]. The final nanoparticle size depends on the laser fluence and the stabilizer concentration (Fig. 17). 

Figure 17 Average diameter of gold nanoparticles after laser ablation at 532 nm and a laser fluence of ( ) 320, ( ) 480, and ( ) 1200 mJ pulse as a function of the SDS concentration in the solution. (From Ref 208.)...

Injection of species into the stratosphere associated with these launches includes emissions not only from the rocket exhaust but also from ablation of the solid rocket motors, the paint on the outer hulls, and hardware from satellites and discarded portions of rockets in the atmosphere (Zolensky et al., 1989). The increase in launches of such vehicles has led to a significant increase in particles associated with solid rocket use. Figure 12.10, for example, shows the concentration of large (>l-/xm diameter) solid stratospheric particles in the 17- to 19-km altitude region from 1976 to 1984,... 

---