Mass analysis time-of-flight

ToF analysers are able to provide simultaneous detection of all masses of the same polarity. In principle, the mass range is not limited. Time-of-flight mass analysis is more than an alternative method of mass dispersion it has several special qualities which makes it particularly well suited for applications in a number of important areas of mass spectrometry. These qualities are fast response time, compatibility with pulsed ionisation events (producing a complete spectrum for each event) ability to produce a snapshot of the contents of the source volume on the millisecond time-scale ability to produce thousands of spectra per second and the high fraction of the mass analysis cycle during which sample ions can be generated or collected. 

This desorption ionisation technique leads to weak fragmentation. The analyte is incorporated into a solid organic matrix (such as hydroxybenzoic acid) and the mixture is placed on a sample holder that is irradiated with UV laser pulses (e.g. N2 laser, A = 337 nm, pulse width = 5 ns). The laser energy is absorbed by the matrix and transferred to the analyte, which becomes desorbed and ionised (Fig. 16.18c). Although MALDI is considered to be a soft ionisation technique, a substantial amount of energy is involved. Because the technique involves pulsed ionisation, it is well suited for time-of-flight mass analysis of biomolecules. The analysis of small molecules (M < 500 Da) is limited because the matrix decomposes upon absorption of the laser radiation. However, solid supports such as silicone can be used as the matrix to overcome this disadvantage. 

Laser time-of-flight mass analysis is a preferred method for solution of difficult analytical problems, because of its basic physical characteristics,... 

MSP Mass-spectrometer ion source—pulse technique. TOP Time-of-flight mass analysis. LP Low-pressure experiment < 10 /xm. MP Medium-pressure experiment < 100 /im. HP High-pressure experiment > 100 /im. ICR Ion cyclotron resonance. CR Constant repeller field—variable reaction time. IMP Impulse technique. PPE Pulsed product ion ejection. MS Mass-spectrometer ion source. 

The advantages of SIMS are (1) surface sensitivity ( 2 monolayers) (2) detection limits (as low as parts per billion (ppb) in dynamic SIMS) (3) molecular specificity and high degree of structural information from both organic and inorganic materials (in static SIMS). The main disadvantage is the inherent lack of quantitation, that is there is no direct relationship between peak intensity and species concentration (as in XPS and AES) Since about 1990, static SIMS has been transformed by the introdnction of time-of-flight mass analysis, so that the term TOF-SIMS has become synonymons with modem static SIMS. 

The photolysis of the allenyl-metal complex (202) in EtzO/HzO yielded the novel compound (203), which lacked the Cr(CO)5 moiety, while the photolysis in the presence of dimethyl fumarate led to the adduct (204). ° A detailed pathway for the photochemical ring closing reaction of benzylallene was proposed by using resonate two-photon ionization spectroscopy coupled with time-of-flight mass analysis and DFT calculation. ... 

Clayton, E. Bateman, R. H. Time-of-flight mass analysis of high-energy collision-induced dissociation fragment ions. Rapid Common. Mass Spectrom. 1992, 6, 719-720. 

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. 

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-... 

Sputter-Initiated Resonance Ionization Spectroscopy Surface Analysis by Resonant Ionization Spectroscopy Time-of-Flight Mass Spectrometer... 

Isotope shifts for most elements are small in comparison with the bandwidth of the pulsed lasers used in resonance ionization experiments, and thus all the isotopes of the analyte will be essentially resonant with the laser. In this case, isotopic analysis is achieved with a mass spectrometer. Time-of flight mass spectrometers are especially well-suited for isotopic analysis of ions produced by pulsed resonance ionization lasers, because all the ions are detected on each pulse. 

An advantage of the microbore gas chromatrography/time-of-flight mass spectrometry (GC/TOFMS) method over the other two approaches is that separation efficiency need not be compromised for speed of analysis. The rapid deconvolution of spectra ( scan rate ) with TOFMS makes it the only MS approach to achieve several data points across a narrow peak in full-scan operation. However, the injection of complex extracts deteriorates performance of microbore columns quickly, and an increased LOD and decreased ruggedness result. Microbore columns may be used in water analysis if the LOD is sufficiently low, but they can rarely be used in real-life applications to complicated extracts. 

Choudhary, G., Chakel, J., Hancock, W., Torres-Duarte, A., McMahon, G., and Wainer, I., Investigation of the potential of capillary electrophoresis with offline matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for clinical analysis examination of a glycoprotein factor associated with cancer cachexia, Anal. Chem. 71, 855, 1999. 

In mass spectrometers, ions are analysed according to the ml7. (mass-to-charge) value and not to the mass. While there are many possible combinations of technologies associated with a mass-spectrometry experiment, relatively few forms of mass analysis predominate. They include linear multipoles, such as the quadrupole mass filter, time-of-flight mass spectrometry, ion trapping forms of mass spectrometry, including the quadrupole ion trap and Fourier-transform ion-cyclotron resonance, and sector mass spectrometry. Hybrid instruments intend to combine the strengths of the component analysers. 

An FIM may be modified so that the imaged atom chosen for analysis can be positioned over a small aperture in the phosphor-coated screen. If the electric field is raised to a sufficiently high value, material may be removed from the surface by field evaporation. The specimen is subjected to a high-voltage pulse, which causes a number of atoms on the specimen surface to field evaporate as positive ions. Only the atom that was imaged over the aperture (or probe hole ) passes into a time-of-flight mass spectrometer, all the other atoms being blocked off by the screen. The applied... 

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