A novel method of peptide sequencing by mass spectrometry is described. Metastable decay of laser-desorbed ions, taking place in the first field-free drift region of a reflection time-of-flight mass spectrometer, has been monitored to get structural information from larger peptides. Fragment ions from metastable decay are mass analysed by adjusting the potentials of the ion reflectron according to the kinetic energies of the ions. The features of the technique and its significance for future applications are outlined.
In matrix-assisted laser desorption ionization (MALDI) a large fraction of analyte ions undergo post-source decay (PSD) during flight in the field-free drift path. By means of a modified two-stage reflectron, product ion time-of-flight spectra of medium-sized linear peptides (up to 2800 u) were recorded, containing full sequence information. Precision, accuracy and mass resolution of fragment ions were almost as good as obtained in high-energy collisionally activated dissociation (CAD) studies performed in four-sector instruments. Instrumental sensitivity was better by at least one order of magnitude. In reflectron time-of-flight mass spectrometry (RETOFMS) the cleavage pattern of PSD products is different from that obtained by high-energy and low-energy CAD. Activation mechanisms of PSD were found to be largely determined by collisional events (ion/neutral) occurring in the acceleration field during early plume expansion. Future potentials of PSD analysis after MALDI are discussed.
Fragmentation of protein and peptide ions generated by matrix-assisted laser desorption has been investigated using a modified LAMMA 1000 reflecting time-of-flight (TOF) mass spectrometer. Whereas fragmentation of covalent bonds prior to ion acceleration (i.e., within several ns after the laser pulse) in general is not observed using the matrix technique, extensive fragmentation on a longer time scale can be studied in our instrument. The high mass resolution (MIAM= 1200-1800 for insulin and peptides) permits the investigation of even small mass losses from parent molecular ions (occurring in the first section of the field-free drift region) by measuring flight time differences of daughter ions acquired during passage through a two-stage reflectron. The kind and extent of this metastable decay has been found to depend strongly on the substance under investigation. Typical fragmentations are loss of ammonia and parts of the amino acid side-chains. The large abundances of peaks due to such metastable fragmentation, observed for most of the peptides and proteins investigated, may, at least in part, explain peak broadening (and, hence, poor mass resolution) typical in matrix-assisted laser-desorption TOF mass spectra.One of the main goals in matrix-assisted laser desorption of high-mass biopolymers, a technique first described by Karas and Hillenkamp, ' is the achievement of high mass resolution and high mass accuracy for the full mass range (up to ca 300 000 Da) accessible to date. For several proteins 'having molecular weights below 30 000 Da, Beavis and Chait reported a mass resolving power, inlAm, of 300-500 leading to an accuracy in mass measurement of about ?0.01% by means of internal calibrants.' However, mass spectra of proteins in the very high mass region as reported in the literature have degraded mass resolutions, reaching values of not higher than ca 50 in the range above ca 100 000 Da. Since, in this mass range, the detection of the ions is governed by processes of secondary-ion rather than secondary-electron formation at the instrument's target surfaces (such as a conversion dynode, an ion detector or field grids3-'), spread in flight times of these secondary species has been suspected to cause substantial peak b r~a d e n i n g .~ However, even under instrumental conditions that carefully avoid such secondary-ion formation at or near the detector, mass resolution remains much lower than expected. We report here on another mechanism that contributes to peak broadening, resulting from the lack of stability of high-mass ions during their flight through the time-of-flight (TOF) mass spectrometer. It is shown that a substantial number of ions decay after acceleration and prior to detection, even under threshold conditions of laser irradiation. This leads to peak broadening by two mechanisms: (i) flight-time differences between neutrals, parent ions and fragment ions resulting from their passage through electric fields (e.g., ion lenses, fringe fields in front of channel-plate detectors, ion converters etc....
A new instrument is presented allowing in situ single-particle analysis by pulsed laser-induced ionization and time-of-flight mass analysis. Particles are introduced into the mass spectrometer through a differentially pumped interface system, detected by light scattering caused by a He-Ne laser beam and ionized by a pulsed UV laser. Instrumental performance is demonstrated using artificial aerosols. Application to on-line mass analysis of ambient aerosol particles is reported for the first time.The importance of single-particle analysis for environmental research, industrial hygiene, or occupational medicine has not been fully recognized yet. There is no doubt that, due to heterogeneity, both chemical activity and toxicity of particulate matter can only be sufficiently understood by additionally employing a single-particle approach rather than by using bulk analytical techniques only.1-2 3Atmospheric chemistry, on the other hand, is known to involve both gas-phase chemistry and processes controlled by physicochemical properties of individual airborne particles (either solid or liquid) ? Chemical reactions in the atmosphere are thus influenced by many factors such as humidity, temperature, pressure, particle size, particle composition (surface and volume), and particle shape. Two examples for particle-assisted chemistry in the atmosphere should be mentioned:1. The catalytic activity of aerosol particles and the importance of the aqueous phase surrounding each particle (as a result of air humidity) was, e.g., confirmed for the S02 oxidation at soot particles.42. The ozone depletion in antarctic stratosphere is thought of as being catalyzed by chlorine activated by particle surface reactions (with e.g. ice).5-6A realistic description of atmospheric chemical processes therefore requires an analytical method capable of investigating single airborne particles in their native state, i.e. by conserving and coanalyzing any volatile surface layer. Sampling and preparation of particles for off-line analysis is thus usually not a sufficient method, since it gives access to the nonvolatile part of airborne particles only.10 In addition to that, off-line methods for single-particle analysis often suffer from physical and chemical interactions of the particles with(1) Tourmann,
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