Ionization laser

Solid samples may be heated and vaporized using laser energy with the additional benefit that compounds may be ionized with the same laser, all at ambient pressure, and characterized in an IMS drift tube. While lasers were used with mobility spectrometers initially for selective photoionization of vapors only,° ° ions were observed from laser contact with solids, including metals and salts. This was not simple vaporization by heating and instead was understood as ablation and ionization. In laser desorption ionization (LDI), the laser pulse initiates a mobility spectrum with a 

FIGURE 5.8 (a) Pulsed corona ion source with high-speed ion injection without an ion 

Ablation under purified gas afmospheres provided rapid characterization of matter and had the promise to provide information on depths when materials could be eroded. Mobility spectra for a range of metals and nonmetals were distinctive for the materials however, ablation under conditions of ordinary air atmospheres was complicated by the production of excessive levels of hydrated protons. These were formed through the release of a high-energy electron during the ablation and 

FIGURE 5.9 Mobility spectrometer with laser ablation and ionization of a solid sample (a). Laser ablation and ionization are possible with a solid sample in a purified gas atmosphere. Mobility spectra (b) were generated from thin films of individual polycyclic aromatic hydrocarbons on borosilicate glass. (From Young et al. Anal. Chim. Acta 2002, 453, 231-243.) 

When laser energies are low, measurements of absorbates or thin films of molecules on surfaces could be made without the complications described, and this was demonstrated for polycyclic aromatic hydrocarbons (PAHs). ° An elegant design for laser desorption was used to measure mobility coefficients of 19 aromatic compounds, ranging from benzene to Cgo fullerene, all without an ion shutter.  

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The above direct process does not produce a high yield of ions, but it does form many molecules in the vapor phase. The yield of ions can be greatly increased by applying a second ionization method (e.g., electarn ionization) to the vaporized molecules. Therefore, laser desorption is often used in conjunction with a second ionization step, such as electron ionization, chemical ionization, or even a second laser ionization pulse. 

Laser ionization. Occurs when a sample is irradiated with a laser beam. In the irradiation of gaseous samples, ionization occurs via a single- or multiphoton process. In the case of solid samples, ionization occurs via a thermal process. 

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 ... 

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. 

In Laser Ionization Mass Spectrometry (LIMS, also LAMMA, LAMMS, and LIMA), a vacuum-compatible solid sample is irradiated with short pulses ("10 ns) of ultraviolet laser light. The laser pulse vaporizes a microvolume of material, and a fraction of the vaporized species are ionized and accelerated into a time-of-flight mass spectrometer which measures the signal intensity of the mass-separated ions. The instrument acquires a complete mass spectrum, typically covering the range 0— 250 atomic mass units (amu), with each laser pulse. A survey analysis of the material is performed in this way. The relative intensities of the signals can be converted to concentrations with the use of appropriate standards, and quantitative or semi-quantitative analyses are possible with the use of such standards. 

In Surface Analysis by Laser Ionization (SAU) ionized and neutral atoms are sputtered from the sample surface, typically using an ion beam (like SIMS) or a... 

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. 

A comparison of the various post-ionization techniques electron-gas bombardment, resonant and nonresonant laser ionization, etc. While some of the numbers are outdated, the relative capabilities of these methods have remained the same. This is a well-written review article that reiterates the specific areas where post-ionization has advantages over SIMS. 

Laser ionization mass spectrometry or laser microprobing (LIMS) is a microanalyt-ical technique used to rapidly characterize the elemental and, sometimes, molecular composition of materials. It is based on the ability of short high-power laser pulses (-10 ns) to produce ions from solids. The ions formed in these brief pulses are analyzed using a time-of-flight mass spectrometer. The quasi-simultaneous collection of all ion masses allows the survey analysis of unknown materials. The main applications of LIMS are in failure analysis, where chemical differences between a contaminated sample and a control need to be rapidly assessed. The ability to focus the laser beam to a diameter of approximately 1 mm permits the application of this technique to the characterization of small features, for example, in integrated circuits. The LIMS detection limits for many elements are close to 10 at/cm, which makes this technique considerably more sensitive than other survey microan-alytical techniques, such as Auger Electron Spectroscopy (AES) or Electron Probe Microanalysis (EPMA). Additionally, LIMS can be used to analyze insulating sam-... 

Laser Ionization Mass Spectrometry Laser Microprobe Mass Analysis Laser Microprobe Mass Spectrometry Laser Ionization Mass Analysis Nonresonant Multi-Photon Ionization... 

Surface Analysis by Laser Ionization Post-Ionization Secondary Ion Mass Spectrometry Multi-Photon Nonresonant Post Ionization Multiphoton Resonant Post Ionization Resonant Post Ionization Multi-Photon Ionization Single-Photon Ionization... 

Resonant (R-) laser-SNMS [3.107-3.112] has almost all the advantages of SIMS, e-SNMS, and NR-laser-SNMS, with the additional advantage of using a resonance laser ionization process which selectively and efficiently ionizes the desired elemental species over a relatively large volume (Eig. 3.40 C). Eor over 80% of the elements in the periodic table, R-laser-SNMS has almost unity ionization efficiency over a large volume, so the overall efficiency is greater than that of NR-laser-SNMS. Quantification is also simpler because the unsaturated volume (where ionization is incom-... 

Vol. 124. Laser Ionization Mass Analysis. Edited by Akos Vertes, Renaat Gijbels, and Fred Adams... 

Long SR, Christesen SD. 1989. Laser ionization studies of organophosphonates and phosphorus oxide radicals. Journal of Physical Chemistry 93(18) 6625-6628. 

The apparatus shown in Fig. 2 consists of three main components two rotatable molecular beam sources, laser ionization and TOF spectrometer. 

To operate the ion TOF spectrometer in the velocity mode, we adapted a single-stage TOF spectrometer as shown in Fig. 3, which consisted of a repeller, an extractor (and guard rings, not shown) and a free-drift tube. After laser ionization, ions are extracted towards the MCP detector. For an ion with an initial kinetic energy Do, the total flight time t can be written as... 

Statistical Methods in Analytical Chemistry. By Peter C. Meier and Richard Zud Laser Ionization Mass Analysis. Edited by Akos Vertes, Renaat Gijbels, and Fred Adams Physics and Chemistry of Solid State Sensor Devices. By Andreas Mandelis and Constantinos Christofides... 

Vertes, A. Gijbels, R. Adams, F. Laser Ionization Mass Analysis. New York Wiley, 1993. 

K. Tanaka, H. Waki, Y. Ido, S. Akita, Y. Yoshida, T. Yoshida and T. Matsuo, Protein and polymer analyses up to m/z 100 000 by laser ionization time of flight mass spectrometry, Rapid Commun. Mass Spectrom., 2, 151 153 (1988). 

Mass spectrometry requires that the material being studied be converted into a vapor. Great strides have been taken in recent years to address this problem, especially in enticing large, thermally fragile (bio)molecules into the vapor state. Matrix assisted laser ionization-desorption (MALDI) and electrospray ionization (ESI) are two current forefront methods that accomplish this task. Even components of bacteria and intact viruses are being examined with these approaches. John B. Fenn and Koichi Tanaka shared in the award of a Nobel Prize in 2002 for their respective contributions to development of electrospray ionization and soft laser desorption. 

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... 

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