Time-of-flight Spectrometry

There have been a number of positron experiments over the past twenty-five years which have benefited from the advantages of the time-of-flight (TOF) method. In very low signal-to-background beam experiments monoenergetic positrons all have essentially the same TOF and appear as a peak on a time spectrum, whereas the very large number of unrelated, randomly-occurring events are spread over all times. 

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Arnold, R. J. Reilly J. P. Fingerprint matching of E. coli strains with matrix-assisted laser desorption/ionization time-of-flight spectrometry of whole cells using a modified correlation approach. Rapid Comm. Mass Spectrom. 1998,12, 630-636. 

LC under critical conditions of adsorption (LACCC) and SEC methods with matrix-assisted laser desorption/ioniza-tion-time-of-flight spectrometry (MALDI-TOF) and postcolumn derivatization. The critical conditions of polymer adsorption in the LC and an optimal matrix system for MALDI-TOF are reported. The changes of molar mass distribution and chemical heterogeneity are said to be due to the simultaneous processes of degradation and recombination. ... 

The time-of-flight spectrometry for ERD analysis has been discussed under Sect. 3.4.2 of Chap. 3. The TOF-MS system is employed in AMS, because of the reason that the energy resolution of a silicon detector or an ionization chamber is often insufficient to resolve neighbouring masses of heavier ions. Better separation is possible by combining the energy measurement with a... 

Before ehding this presentation on mass spectrometry, we should cite the existence of spectrometers for which the method of sorting ions coming from the source is different from the magnetic sector. These are mainly quadripolar analyzers and, to a lesser degree, analyzers measuring the ion s time of flight. 

Guilhaus M 1995 Principles and instrumentation in time-of-flight mass spectrometry physical and instrumental concepts J. Mass Spectrom. 30 1519-32... 

TOF-SARS (time-of-flight scattering and recoiling spectrometry) no 13 1.2... 

A) TIME OF FLIGHT SCATTERING AND RECOILING SPECTROMETRY (TOF-SARS)—SHADOW CONE BASED EXPERIMENT... 

Masson F and Rabalais J W 1991 Time-of-flight scattering and recoiling spectrometry (TOF-SARS) analysis of Pt 110. I. Quantitative structure study of the clean (1 x 2) surface Surf. Sc/. 253 245-57... 

Orthogonal TOP is the name commonly given to orthogonally accelerated time-of-flight mass spectrometry. It is sometimes referred to by the acronym oa-TOF, especially in official publications, but it is more usual for it to be referred to simply as orthogonal TOP this abbreviation is used here. 

Laser-desorption mass spectrometry (LDMS) or matrix-assisted laser desorption ionization (MALDI) coupled to a time-of-flight analyzer produces protonated or deprotonated molecular ion clusters for peptides and proteins up to masses of several thousand. 

Py/GC/MS. pyrolysis, gas chromatography, and mass spectrometry used as a combined technique Py/MS. pyrolysis and mass spectrometry used as a combined technique oa-TOF. orthogonally accelerated time of flight Q. quadrupole field or instrument... 

Cotter, R.J., Time-of-Flight Mass Spectrometry, ACS Symposium Series, Vol. 549, American Chemical Society, Washington, DC, 1994. 

MALDI = matrix assisted laser desorption, ftms = Fourier transform mass spectrometry TOF = time of flight. 

The use of separation techniques, such as gel permeation and high pressure Hquid chromatography interfaced with sensitive, silicon-specific aas or ICP detectors, has been particularly advantageous for the analysis of siUcones in environmental extracts (469,483—486). Supercritical fluid chromatography coupled with various detection devices is effective for the separation of siUcone oligomers that have molecular weights less than 3000 Da. Time-of-flight secondary ion mass spectrometry (TOF-sims) is appHcable up to 10,000 Da (487). 

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. 

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-... 

Molecular orbital calculations indicate that cyclo C-18 carbyne should be relatively stable and experimental evidence for cyclocarbynes has been found [25], Fig. 3B. Diederich et al [25] synthesised a precursor of cyclo C-18 and showed by laser flash heating and time-of flight mass spectrometry that a series of retro Diels-Alder reactions occurred leading to cyclo C-18 as the predominant fragmentation pattern. Diederich has also presented a fascinating review of possible cyclic all-carbon molecules and other carbon-rich nanometre-sized carbon networks that may be susceptible to synthesis using organic chemical techniques [26]. 

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