Pressure ionization, with laser irradiated

Experimentally the UV sources are provided by two dye lasers giving a minimum of 2 x 250 pJ UV-light of 10 ns pulse duration in the range 260 - 285 nm. The benzene pressure was between 10 pbar and 200 pbar. The electrodes were biased with 16 V and the ion current following laser irradiation was collected by a gated integrator. Details can be found in Fig. 3. In thi way a complete ionization spectrum for several selected intermediate ( vibronic levels could be recorded fjom slightly ne-... 

Luosujarvi L, Kanerva S, Saarela V, Franssila S, Kostiainen R, Kotiaho T, Kauppila TJ (2010) Environmental and food analysis by desorption atmospheric pressure photoionization-mass spectrometry. Rapid Commun Mass Spectrom 24 1343-1350 Manicke NE, Kistler T, Ifa DR, Cooks RG, Ouyang Z (2009) High-throughput quantitative analysis by desorption electrospray ionization mass spectrometry. J Am Soc Mass Spectrom 20 321-325 Martynov IL, Karavanskii VA, Kotkovskii GE, Kuzishchin YA, Tsybin AS, Chistyakov AA (2011) Ion mobility spectrometer with ion source based on laser-irradiated porous silicon. Tech Phys Lett 37 15-18... 

A laser-induced acoustic desorption (LIAD) device combined with a chemical ionization source was employed for the analysis of crude oil distillates under atmospheric pressure. In general, LIAD, a matrix-free and laser-based approach, is usually performed under vacuum conditions. The desorption process in LIAD is induced by the action of a shockwave that is generated as a pulsed laser irradiated on the backside of a metal foil. As the energy is transferred from the metal foil to the sample, which is deposited on another side of the foil, it induces the desorption of analytes. By the interaction of the analyte with an ion cloud generated by a chemical ionization (Cl) process, analytes with a wide range of polarity are successfully ionized. Marshall et ah have combined an atmospheric pressure AP-LIAD/ Cl with a 9.4 T FT-ICR/MS to perform high resolution chemical analyses imder ambient conditions. It was demonstrated that not only polar but also non-polar compounds in the crude oil distillates could be successfully characterized by this AP-LIAD/Cl/FT-ICR/MS approach. 

Where thermal desorption is inadequate to remove an analyte from the surface, a laser beam can be directed, focused or unfocused, against a solid, and compounds on surfaces of solids can be vaporized and ionized at ambient pressure in air. In one application of laser-based IMS to environmental analyses, soils contaminated with petroleum products were assayed for PAHs. In this, a laser was used to irradiate soil, vaporizing PAHs into the gas phase. This provided a direct, fast, extraction-free method for soil analyses. 

A secmid popular soft ionization method is MALDI. In this technique, analytes are co-crystallized with a matrix (typically a small, acidic, organic molecule), with an absorption maximum close to the wavelength of a laser used to irradiate the substrate. This process is typically performed in vacuum (although it has been shown to be feasible at atmospheric pressure) after the analytes have been desorbed and ionized (in MALDI, it is believed that fast heating caused by the laser pulse desorbs analytes into gas phase however, the process by which... 

Probe electrospray ionization (PESI) and TOF-MS were used for direct profiling of phytochemicals in different parts of a fresh tulip bulb [80], which emphasized the possibility of conducting in-vivo MS analysis of less sensitive biological matrices such as plant tissues. Recently, Pan et al. [81] demonstrated single-probe MS which can conduct metabolomic analysis of individual living cells in real time. The diameter of this probe is < 10 pm which makes the device compatible with eukaryotic cells. Atmospheric pressure ion sources are particularly suitable for analysis of live biological specimens. Cellular metabolism does not need to be quenched before analysis. For instance, in laser ablation electrospray ionization (LAESI)-MS, cells are irradiated by a laser beam in order to extract small amounts of cytosolic components, and to transfer them to the ESI plume [82]. 

An experiment of this kind was performed (Bagratashvili et al. 1983) with anthracene (C14H10) molecules (5 = 66, Dq = 4.8 eV), for which the estimate in eqn (10.1) gives — Eg c 3Eg. With so strong an overexcitation, ionization of the molecule (I = 7.4 eV) is quite possible. In fact, the formation of anthracene molecular ions was observed when the molecules were irradiated by sufficiently powerful (about 10 W/cm ) short (70 ns) C02-laser pulses under collisionless conditions (pressure 4 X 10 Torr) in a time-of-flight mass spectrometer. These ions can be believed to appear as a result of the IR multiphoton ionization of molecules in accordance with the scheme of Fig. 10.5(b). Of course, such a multiphoton ionization technique is applicable only to large polyatomic molecules, since for polyatomic molecules with a small number of atoms the maximum possible degree of overexcitation above the dissociation threshold is comparatively low. 

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