Laser ablation mounting

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.

Laser ablation can be carried out on any material without special sample preparation. The laser beam can be directed onto a defined spot of the sample or moved to different parts to analyse over a defined area. It can be moved in an XYZ plane using a stepper motor and driven in translational motions on which the cell is mounted and with more expensive models can be turned for analysis in other parts of the sample. Lasers can operate in UV, visible, and IR regions of the spectrum and a recent development in laser technology uses neodymium yttrium aluminium garnet (Nd YAG) which gives high repetition rate at a comparatively low power. This method of analysis is suited to bulk analysis of solid materials and the amount of volatility varies from sample to sample. The size of the laser spot can vary from 10 to 250 pm and little or no sample preparation is required. Errors are greatly reduced because of the simple sample preparation, and the fact that no solvents are required reduces interferences. 

In laser ablation various chambers for different types of samples have been designed to analyse rod and disc-shaped samples, and powders. For example, a disc ablation chamber system accommodates a range of disc sizes including the typical metal industry standards used in spark emission spectrometry. In order to keep the distance between the ablation chamber and the plasma as short as possible, the chamber is placed directly below the ICP. Another ablation system includes a laser microprobe which consists of a ruby laser head mounted in a binocular microscope. The target area at the sample surface is selected by the microscope, then the laser beam is focused and fired. 

As mentioned before there are two important parameters that are required for the fabrication of superhydrophobic smfaces, namely, surface roughness and low siuface energy. In order to roughen metallic surfaces in a controlled manner, particularly stainless steel smfaces, the laser ablation method has been used in this work. The stainless steel substrates were mounted on a precise, computer-controlled translation stage capable of moving in front of a fixed laser beam. 

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. 

For laser irradiation of samples we employed the p2 laser (Lambda Physik LPF 202, wavelength of 157 nm, pulse duration of 15 ns). For the irradiation with the F2 laser, the light was polarized linearly with a MgF2 prism. For homogeneous illumination of the samples, we used only the central part of the beam profile by means of an aperture (10 mm - 3.5 mm). We performed the irradiation at fluences well below the ablation threshold of PET at 157 nm (29.6 mj/cm ). The samples were mounted onto a translation stage and scanned at a speed of 14 mm/s. At a repetition rate of the laser of 11 Hz. We carried out the experiments in a flow box purged with nitrogen at a pressure of 110 kPa [41]. 

This technique uses a high-energy laser beam to remove a minute amount of the sample under investigation. A typical scheme that shows the basic components of an LA-ICPMS instrument is shown in Figure 39.2. The sample is typically placed inside a closed ablation chamber, and the laser beam is usually focused onto the surface. Generally, the ablation cell is mounted on a software-controlled translation stage that can be steered in all three spatial directions. Moreover, the cell is monitored by a video camera and/or microscope, such that it is possible to select the exact location for ablation with an accuracy of a few micrometers. 

The typical laser experimental set-up used to produce carbon nanotubes is shown in Fig. 6.13. It consists of a quartz tube inside a furnace (not needed in the case of the CO2 ablation). The tube is sealed and coimected to a vacuum system and an inert gas reservoir. The laser beam enters the quartz tube through a special window mounted in a vacuum flange. The carbon target is placed in the center of the quartz tube and is aligned to the laser beam. The distance between the laser and the carbon target is changeable due to a small quartz tube coaxial to the water-cooled metallic collector mounted at the other end of the external tube. During the synthesis process the temperature profile is measured by thermocouples collocated into the tube. 

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