Nitrogen laser, ionization

Both LDI and MALDI make use of the absorption of laser light by a solid sample layer. The energy uptake upon laser irradiation then causes evaporation and ionization of the sample. Wavelengths ranging from ultraviolet (UV) to infrared (IR) have been employed, e.g., nitrogen lasers (337 nm), excimer lasers (193, 248, 308 and 351 nm), Q-switched, frequency-tripled and quadrupled Nd Yag lasers (355 and 266 nm, respectively), [24] Er Yag lasers (2.94 pm) [24,25] and TEA-CO2 lasers (10.6 pm). [16,26]... 

The application of electrospray ionization (ESI) to macromolecules was first described by Yamashita and Fenn in 1984, and he later applied this technique to the study of proteins in 1988. In ESI, charged droplets of protein were produced and the solvent was stripped away, leaving only the free protein ion. In 1987, Tanaka demonstrated that laser pnlses from a low-energy nitrogen laser could be used to ionize proteins from a surface. As described in a paper presented during the Second Japan-China Joint Symposium on Mass Spectrometry, proteins are desorbed from a glycerol matrix containing cobalt powder. The cobalt powder was necessary to increase absorption of the laser photons. ... 

Matrix assisted laser desorption ionization time-of-flight (MALDI-TOE) mass spectrometry was carried out with a PerSeptive Biosystems Voyager-DE-RP MALDl-TOF mass spectrometer. A 337-nm UV nitrogen laser producing 3-ns pulses was used in the reflectron mode. The samples were prepared by mixing 10 pi of a 0.1 M HAc solution of the sample with 20 pi of a solution of 3 mg/1 a-cyano-4-hydroxy cinnamic acid in wafer. One pi of that solution was loaded on the gold-sample plate. 

After introduction of the target into the vacuum, an UV laser pulse is used to desorb and ionize the sample. Nitrogen laser emitting at 337 nm and Nd YAG laser emitting at 355 nm are the most widely used. MALDI is a very powerful technique for the analysis of synthetics and natural biopolymers. It has completely replaced former techniques such as fast atom bombardment (FAB). In... 

In the MALDI technique a pulsed laser beam strikes a solid sample and heats, vaporizes, and ionizes compounds with little decomposition.201-209 Proteins or other biopolymers are mixed with a "matrix" Fiat absorbs the heat of Fie laser beam. The protein sample together with Fie matrix is dried. Most proteins form crystals and Fie laser beam is directed toward individual protein crystals or aggregates. Various materials are used for the matrix. Compounds as simple as glycerol, succinic acid, or urea can be used with an infrared laser. For proteins an ultraviolet nitrogen laser tuned to 337 nm is usually employed with an ultraviolet light-absorbing matrix such as hydroxy-benzoic acid, 2,5-dihydroxybenzoic acid, a-hydroxy-... 

Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer with UV laser (i.e., 337-nm nitrogen laser), MALDI probe, and optional delayed extraction and postsource decay... 

The 13C NMR data were confirmed with MALDI mass spectral data. MALDI is a soft ionization technique able to transfer analytes into the gas phase as intact molecules without fragmentation. 2,5-Dihydroxybenzoic acid was used as the matrix to absorb nitrogen laser energy at 337 nm and assist in the ionization. The MALDI data verified the prolinol assignment (see Fig. 6). The data also showed that a minor amount of other pyrrolidines was formed including 2-methylpyrrolidine and pyrrolidine. 

MALDI TOP (7) mass spectra were recorded on a Bruker Reflex instrument (Billerica, MA). The rp-HPLC fractions containing PMP labeled oligosaccharides were dried in a SpeedVac concentrator (Farmingdale, NY) and redissolved in water/acetonitrile (75/25, v/v). 2,5-Dihydroxybenzoic acid (DHB) was used as a matrix. Normally, 0.3 uL of a half-saturated solution of DHB in water/acetonitrile/trifluoroacetic acid (70/30/0.1, v/v) was mixed on the sample dish with 0.3 pL of the sample solution. Desorption and ionization was done with a nitrogen laser (337 nm) adjusted to minimum laser attenuation. Mass spectra (10 to 20) were accumulated in linear negative ionization mode with an acceleration voltage of 25 kV. Calibration was done externally with bovine insulin and angiotensin I. 

Matrix Assisted Laser Desorption Ionization-Mass Spectrometry (MALDI-MS) Mass spectra of native and denatured antibodies were obtained with a PerSeptive Biosystems (Farmingham, MA.) Voyager Elite mass spectrometer operated in the linear mode with a Laser Sciences Inc., 337 nm nitrogen laser. hAB-1 was denatured by boiling the sample in 1.0 M guanidine-HCl, 50 mM Tris pH 7.5 buffer. Native and denatured samples were diluted with 20 mM Tris, 10 mM octylglucoside (Tris/OG) pH 6.8 buffer prior to MALDl-MS analysis. Proteins were spotted on the sample plate as a sandwich between two layers of the matrix. The bottom layer consisted of 100 mM sinapinic acid in acetonitrile and the top layer consisted of 50 mM sinapinic acid in 30% acetonitrile / 70% H2O / 0.07% TFA. The m/z scale of the instrument was calibrated using a Hewlett-Packard protein standard mixture. 

The samples were analyzed on the VG TofSpec-SE MALDI TOP mass spectrometer in the reflectron mode with positive ion detection. The samples were spotted on the sample plate in acetonitrile water (60 40) or chloroform methanohTFA (1 1 0.1) mixture plus ammonium suliate on alpha-C (a-cyano-4-hydroxycinnamic acid) matrix. The ionization of the samples was carried out with Nd YAG laser at 355 nm or nitrogen laser at 337 nm. Some of the fractions were analyzed by SIMS on a Kratos 890 mass spectrometer equipped with a Phrasor Scientific SIMS source. The mass spectral data were analyzed by the MSFIT program at the University of California, San Francisco. 

In MALDI, the sample is deposited on a target and co-ciystallized with a solid matrix [14-15]. The target is transferred to vacuum and bombarded by photon pulses from a laser, in most cases a nitrogen laser (337 nm) nowadays. The ionization results from efficient electronic excitation of the matrix and subsequent transfer of the energy to the dissolved analyte molecules, which are desorbed and analysed as protonated or cationized molecules [7]. The ionization process is not fully understood. Extremely high molecular-mass compounds, e.g., in excess of 200 kDa, can be analysed using the MALDI, if performed on a time-of-flight mass spectrometer (Ch. 2.4.3). 

Since the second laser ionizes the excited atom in DLI, this step may be accomplished by an off-resonant photon (see Fig. 1). If a nitrogen laser-pumped dye laser provides the resonant photon, a fraction of the nitrogen laser beam can conveniently ionize the atom from the laser-excited state 14,15). It is theoretically possible to photoionize every atom in the periodic table except helium and neon using five RIS ionization schemes involving stepwise and/or multiphoton excitation4). Presumably these... 

FiCURE 4.3 Matrix-assisted laser desorption ionization source, (a) Gas phase ions are formed when the nitrogen laser produces photons at 337 nm that strike the solid matrix-analyte sample. The ions above the sample plate are accelerated to 25,000 eV of kinetic energy and fly through a stainless steel tube (not shown) for TOP m/z analysis, (b) Three commonly used matrix molecules for MALDI of peptides and proteins. 

Multiphoton ionization combined with mass spectroscopy has been proposed as a sensitive tool for analysis of trace gases to a detection limit of 2 parts in 10 . Flashlamp- and nitrogen-laser-pumped dye lasers have been used for REMPI detectors for gas chromatography. Aromatic hydrocarbons could be detected at the 10 pg level. The necessity for constant peak energy as well as CW power, for multiphoton ionization studies has been discussed. ... 

Toluene (IE = 8.83 eV) is ionized by two color R2PI multiphoton ionization scheme. The first photon is derived from a 337-nm nitrogen laser. What should be the wavelength of the second photon ... 

A MALDI-FT-MS-based automated system for rapid screening of a large array of compounds is also available [109]. In this approach, the library compounds were mixed with a suitable matrix and were deposited on an auto-indexed multiple-sample disk. The matrix-sample mixture was ionized by irradiation with UV nitrogen laser (337 nm), and mass calibrant ions were generated by electron ionization of perfluorotributyl amine. 

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