Showing papers on "Time of flight published in 1993"

Mass spectrometric sequencing of linear peptides by product‐ion analysis in a reflectron time‐of‐flight mass spectrometer using matrix‐assisted laser desorption ionization

Abstract: 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 (RETOF-MS) 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.

A highly-segmented ΔE-time-of-flight wall as forward detector of the 4π-system for charged particles at the SIS/ESR accelerator

Abstract: At the SIS/ESR accelerator facility at GSI in Darmstadt the 4π-detector system FOPI is under construction at present. It is designed for the investigation of central collisions of heavy ions in the energy range up to 2 A GeV. As phase I of this detector a forward wall has been built and used in various experiments. It comprizes a total number of 764 scintillators with an additional shell of 188 thin ΔE -detectors in front of it and covers the full azimuth of the polar angles from 1° to 30°. The velocity and the nuclear charge of the fragments are determined by a combined time-of-flight and ΔE measurement.

Profiling of hot surfaces by pulsed time-of-flight laser range finder techniques

Abstract: The possibilities for using the pulsed time-of-flight (TOF) laser radar technique for hot refractory lining measurements are examined, and formulas are presented for calculating the background radiation collected, the achievable signal-to-noise ratio (SNR), and the measurement resolution. An experimental laser radar device is presented based on the use of a laser diode as a transmitter. Results obtained under real industrial conditions show that a SNR of 10 can be achieved at measurement distances of up to 15-20 m if the temperature of the converter is 1400 °C and the peak power of the laser diode used is 10 W. The single-shot resolution is about 60 mm (sigma value), but it can be improved to millimeter range by averaging techniques over a measurement time of 0.5 s. A commercial laser radar profiler based on the experimental laser radar device is also presented, and results obtained with it in real measurement situations are shown. These measurements indicate that it is possible to use the pulsed TOF laser radar technique in demanding measurement applications of this kind to obtain reliable data on the lining wear rate of a hot converter in a steel works.

Laser desorption ionization mass monitor (LDIM)

Abstract: A laser desorption ionization instrument for measuring the molecular weight of large organic molecules includes a time of flight (TOF) mass spectrometer. The time of flight mass spectrometer includes a sample lock for holding, under vacuum, a plurality of samples to be analyzed. A sample may be inserted into and removed from the sample lock and into the mass spectrometer without breaking vacuum in the spectrometer. Signal processing electronics of the LDIM instrument include means for identifying quasi-molecular species of a molecule being measured. The instrument includes improvements in ion optics, microchannel plate detectors, laser irradiation of samples, and preparation of samples for measurement.

Time resolved energy spectrum of the axial ion beam generated in plasma focus discharges

Abstract: The energy spectrum of the deuteron beam along the electrode axis (0 degrees ) in a plasma focus discharge has been determined with a time of flight (TOF) method and with a differential filter method in the ion energy interval E=0.3-9 MeV. The ion TOF method is applied to single-ion pulse events with an ion emission time t(E) that is only weakly dependent on the ion energy E for E>0.3 MeV. The correlation of the ion beam intensity with the filling pressure, the neutron yield and the hard X-ray intensity is also reported

Accurate digital time-of-flight measurement using self-interference

Abstract: Most ultrasonic distance measurements are based on the determination of the time of flight. This paper presents a novel way for the measurement of the time of flight, based on the detection of the envelope zero in an ultrasonic wave. A particular wave form is produced by supplying two pulse trains subsequently to an acoustic transducer. Distance information is retrieved from the zero, so a fully digital measurement is possible, reducing signal-processing time and resulting in a fast and accurate distance measurement. >

Second-order space focusing in two-field time-of-flight mass spectrometers

Abstract: Design equations are given for second-order space focusing in two-field time-of-flight mass spectrometers. Simulations and experimental tests confirm that, for diffuse sources and high-energy fragments, second-order designs give superior resolution and linearity of flight time deviations to initial ion velocities.

Time‐of‐flight investigation of infrared laser‐induced multilayer desorption of benzene

Abstract: Infrared (IR) laser‐induced resonant desorption/ablation of benzene layers condensed on liquid‐nitrogen‐cooled substrates has been investigated using the time‐of‐flight (TOF) technique. Adsorbates of up to 5 μm thickness were irradiated with a line‐tuned TEA CO2 laser. Time‐resolved detection of desorbed particles was performed with a quadrupole mass spectrometer (QMS). Measured TOF spectra can usually be well fitted with a stream‐modified Maxwell–Boltzmann distribution. Systematic deviations from these fits were observed depending on film thickness, laser fluence, and angle of detection. Spectroscopic data of the adsorbate–substrate system needed for model calculations were measured in situ. The absorption coefficients were found to be fluence dependent. Measurements of the desorption yield and kinetic energy of the desorbed particles were performed with the P932 and P934‐CO2 laser lines as a function of film thickness and laser fluence. The solid–liquid phase transition could clearly be observed in thes...

Enhancement of resolution in matrix-assisted laser desorption using an ion-trap storage/reflectron time-of-flight mass spectrometer

Abstract: A quadrupole ion-trap storage/reflectron time-of-flight mass spectrometer (IT/RETOFMS) has been used to enhance resolution in the ultraviolet matrix-assisted laser desorption/ionization (MALDI) of peptides. The MALDI of peptides is performed directly inside an ion-trap storage device using 266 nm laser radiation from a 2,5-dihydroxybenzoic acid matrix. Helium buffer gas is used to collisionally cool the hot desorbed ions and to rapidly relax the ions to the center of the ion trap. The result is that upon ejection into the TOF device, a resolution of nearly 6200 is obtained-for Substance P using a 90 ms storage time. A resolution of 3000–6000 is consistently obtained for peptides up to 2100 u used in this study. The individual ion peaks from the isotopic distribution are easily resolved under these conditions. In addition, the ion trap is found to be useful as an ion-storage device for enhancing the signal where multiple laser desorption pulses can be stored per cycle in the trap before ejection into the RETOF. Also, using the relatively high resolving power of the IT/RETOF instrument, excellent mass accuracy can be obtained using the matrix background and bradykinin as internal standards.

Angle‐resolved time‐of‐flight studies on ground‐state neutrals formed by near‐threshold excimer laser ablation of copper

Abstract: The angle‐resolved velocity distributions of neutral copper atoms created by near‐threshold ultraviolet excimer laser ablation of polycrystalline copper foils are measured as a function of the polar desorption angle and the laser fluence. The obtained time‐of‐flight spectra can be fitted by elliptical Maxwell–Boltzmann distributions on a stream velocity. Integration of the spectra allows one to determine the kinetic energy of these atoms. This kinetic energy is hyperthermal (1 eV).

Measurement of CO pressures in the ultrahigh vacuum regime using resonance‐enhanced multiphoton‐ionization time‐of‐flight mass spectroscopy

Abstract: An evaluation is made of measurements of CO pressures in the UHV regime using resonance‐enhanced multiphoton ionization coupled with time‐of‐flight mass spectroscopy (REMPI‐TOFMS). It has been found that once the REMPI‐TOFMS system has been calibrated, quantitative measurement of CO pressures as low as 10−10 Pa is possible, even in overwhelming N2 backgrounds. With compensation for laser pulse energy variations, we find measurements with uncertainties of 10%–15% are possible for pressures down to 10−7 Pa, and an ultimate detection limit for CO pressures of 10−10 Pa for our measurement system. In this study, the REMPI‐TOFMS system was calibrated using a pressure division technique along with a spinning rotor gage. The ionization of CO is achieved using 230 nm radiation to excite the B 1Σ+ state of CO at 10.8 eV via a two‐photon absorption and then ionizing some of the excited state molecules by the absorption of an additional photon from the laser beam.

A method and apparatus for surface modification by gas-cluster ion impact

Abstract: We report here a preliminary study of the effects of Ar cluster ions of relatively low energy (15 → 30 keV) on Si surfaces and on Au films deposited on Si. The cluster size distribution was measured by time of flight and retarding potential methods. The mean cluster size was varied between 150 and 600 atoms so that the mean energy per atom in a cluster was of the order of 100 eV. The irradiated samples were analyzed by RBS and channeling and their surfaces were examined with SEM.

Laser induced mass spectrometry

Abstract: Disclosed is an apparatus which can serve to detect, count, size discriminate and analyze the chemical composition of particles in the air or process gases. In a preferred embodiment, the particles enter via a capillary into a differentially pumped chamber. A pulsed laser which is continuously fired is focused at an opening in the chamber. When the particles come into the path of the laser beam, the particles are fragmented and ionized. A dual time of flight mass spectrum is produced, recorded with an oscilloscope and analyzed with a computer. The mass spectrum information enables the determination of the chemical nature and concentration of the species of the particles, the particle size and the elemental composition of airborne particles in real time. Once these parameters are determined the source of the particles can be determined and eliminated from the environment and process. Thus, the inventive apparatus is advantageously used in conjunction with a facility, i.e., a semiconductor manufacturing facility, that requires ultra-clean conditions.

A high-sensitivity method for hydrogen analysis in thin targets

Abstract: A sensitive and fast method for hydrogen analysis has been developed. The method is based on a technique where a proton beam of a few MeV impinges on the target and the elastically scattered proton and the recoiling target hydrogen nucleus are detected in coincidence. The method makes use of the difference in time of flight and requires no energy determination. The system consists of two thin, large-area plastic scintillation detectors placed at angles of 45° relative to the beam direction on either side of the beam. Evaluations of the system showed that a stable coincidence yield with respect to position in the target plane and target displacement can be obtained provided that the geometry of the setup is carefully designed. The yield as a function of target thickness was almost linear for target thicknesses up to 2000 μg/cm2. It was also found that count rates up to 500000 s−1 in each branch may be used. The minimum detection limit for the hydrogen detection system was found to be of the order of a few pg/cm2 for a proton beam energy of 3.6 MeV.

Time‐of‐flight mass spectroscopic studies on the laser ablation process

Abstract: For understanding a formation process of superconducting thin films, we have tried to identify the species ablated from a YBCO target. Mass spectroscopic studies on ionic species in a plume have been carried out using a Time-of-Flight (TOF) mass-spectrometer. Ionic materials in the plume are consisted of monatomic ions. The velocities for these ions are found to be ~3X106, 2~3X106 and 3~4X106 cm/sec for Y+, Ba+ and Cu+, respectively.

A time‐of‐flight spectrometer for low‐energy neutral and ionized alkalis

Abstract: A time‐of‐flight spectrometer has been developed for measuring velocity‐ and angle‐resolved distributions of neutral and ionized alkali atoms in the range of several electron volts to several thousand electron volts. The spectrometer is mounted on a rotating platform inside a previously existing ultra‐high‐vacuum scattering chamber equipped with a low‐energy ion beamline. A high work function platinum foil, placed after a pair of angle‐defining apertures, is used to ionize the alkali atoms, regardless of their incident charge states. After striking the platinum foil, the alkali ions are accelerated and detected with a channel electron multiplier. Velocity resolution of the scattered flux is achieved using standard time‐of‐flight techniques. Neutral and ionized alkali atoms are detected with the same high efficiency. The ion flux can be rejected by biasing a pair of rejection elements, and therefore absolute ratios of the neutral to total flux can be determined. Such a spectrometer can be used to probe the...

A paraboloidal electrostatic reflector for molecular‐beam time‐of‐flight photoelectron spectrometers

Abstract: We describe a design for a paraboloidal electrostatic reflector (PER) adapted for molecular‐beam time‐of‐flight (TOF) photoelectron spectrometers The PER offers a nearly two orders of magnitude improvement in detection efficiency over standard line‐of‐sight TOF detection The energy resolution ΔE/E is nominally about 002, but can be improved to about 0005 (to a current limit of ΔE=10 meV) at some expense in sensitivity The PER makes possible sensitive measurements for inherently weak ionization experiments We have used our spectrometer in applications involving low‐power picosecond pulses, low‐density molecular clusters, and ionization through dissociative states