Laser vaporization sample

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). 

In Surface Analysis by Laser Ionization (SALI), a probe beam such as an ion beam, electron beam, or laser is directed onto a surfiice to remove a sample of material. An untuned, high-intensity laser beam passes parallel and close to but above the sur-fiice. The laser has sufficient intensity to induce a high degree of nonresonant, and hence nonselective, photoionization of the vaporized sample of material within the laser beam. The nonselectively ionized sample is then subjected to mass spectral analysis to determine the nature of the unknown species. SALI spectra accurately reflect the surface composition, and the use of time-of-flight mass spectrometers provides fast, efficient and extremely sensitive analysis. 

Method. The laser vaporization source eliminates the material constraints inherent in conventional oven sources. This is accomplished by localizing the heating to a very small area at the surface of the sample and by entraining the vapor produced in a rapid flow of high pressure gas. 

Figure. 1. Schematic of essential components of the Exxon group cluster laser vaporization source and fast flow tube chemical reactor. On the far left is a 1 mm diameter pulsed nozzle that emits an -200 ysec long pulse of helium which achieves an average pressure of approximately one atmosphere above the sample rod. Immediately before the sample rod position the tube is expanded to 2 mm diameter. The length of this extender section can be varied form 6 mm to 50 mm depending upon the desired integration time for cluster growth. The reactor flow tube is 10 mm in diameter and typically 50 mm long. The reactants diluted in helium are added and mixed with the flow stream via the second pulsed valve.

Ions produced in an ICP torch interfaced to a quadrupole mass spectrometer. Sample introduction by nebulizer, laser vaporization or electrical heating. 

There are several preparative methods for the production of bare metal clusters including the fast flow reactor (PER), the fast flow tube reactor (FTR), the SIDT (24), the GIB (23), and a supersonic cluster beam source (SCBS) (198). Essentially, all of these methods are similar. The first process is to vaporize the metal sample producing atoms, clusters, and ions. Laser vaporization is generally favored although FAB or FIB may be used. The sample is located in a chamber or a tube and so vaporization generally takes place in a confined environment. An inert gas such as helium may be present in the vaporization source or may be pulsed in after the ionization process. 

Volume 1 consists of chapters covering the development. Instrumentation, and results of a wide range of materials, including background correction lasers, inductively coupled-mass sp>ectroscopy plasmas, electrothermal vaporizers, sample introduction, and Fourier transform atomic spectrocopy. 

Alternate Sample Introduction — Obviously, elimination of the sample dissolution stage would greatly reduce analytical time, as it is the slowest step in the analytical scheme. Pulsed-laser vaporization using a CO2—TEA laser seems promising(63, 64). Another possibility is the introduction of a suitable prepared slurry of the sample into the nebullzer(65). Thermal vaporization studies using heated substrates such as tanta-lum(66), carbon filaments(67), or carbon rods(39) have been reported. Silvester(39) de fined the problems of vapor transport, carrier gas expansion, and solid phase chemistry associated with electrothermal sample introduction to an ICP. 

Brain Fixing and embedding in polymer matrix sectioning and staining to visualize Al deposits laser vaporization of selected sample surface into mass spectrometer LAMMA low g/g range no data Lovell et al. 1993... 

Hankin et al. [133] demonstrated femtosecond ionization following 266 nm desorption of solid samples of trinitrobenzene (TNB), TNT, and trinitrophenol (TNP). They confirmed the advantages of ultrafast ionization, namely, the formation of characteristic precursor and structure-specific fragment ions. The optimum intensities for efficient LD without ionization were determined for the compounds studied. Differences between femtosecond ionization of vapor samples of explosives [131,132] and laser desorbed molecules were also discussed. 

Combustion of coal produces ash that can be transported through the air. Slagging and fouling problems can also be predicted from elemental analysis. Therefore, elemental analysis of both the coal as well as the ash are important. Procedures for dissolution and analysis of coal and combustion products of coal have been reported [334-336]. Laser ablation sampling has been successfully used for coal and combusted materials [337,338]. The direct introduction of slurries has also been used [339]. Comparison of ICP-MS and PIXE analysis of coal combustion aerosols showed that analysis errors can occur in ICP-MS if particle vaporization is incomplete in the ICP [340]. 

A brief outline of a time-of-ffight mass spectrometer (TOF mass) and a femtosecond laser system are described. Figure 2.2 shows a TOF system with a mass resolution of to/Ato = 2000. Vaporized samples are introduced at a pressure of 10 5 Pa under a background pressure of 10 7 Pa. The aperture size of the slit of extraction plate was 1 mm or 0.5 mm. 

Figure 2. FT-ICR mass spectrum of hot toluene extract of fullerene material produced by laser vaporization of a 10% La203/graphite composite rod. The sample was exposed to air and moisture.

The laser vaporization approach allows the use of even the most refractory target materials. The source configuration used in Fig. 1 involves a target rod that is rotated and translated in a continuous screw motion to expose fresh metal to the laser beam. This has been found necessary to provide acceptable pulse-to-pulse reproducibility. Target rods of refractory metals, semiconductors, carbon, polyethylene, alumina, and alloys have all been vaporized successfully to make clusters in many laboratories. For some materials a disk target is preferred due to the ease in sample preparation. Molecular solids, liquids, and solutions could also be used, though care must be taken to consider the additional complex plasma chemistry one is likely to encounter. 

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