Constant-Fraction Discrimination/Boxcar Integrator for Plasma Source Time-of-Flight Mass Spectrometry

A constant-fraction discrimination (CFD) system has been combined with a boxcar integrator for detection in inductively coupled plasma/time-of-flight mass spectrometry. The discriminator provides gating logic for the boxcar integrator when an incoming ion signal occurs, but discriminates against electronic or background noise of lower amplitude. As a result, the combination can effectively reject noise and accumulate analyte signal, rather than relying on an averaging process to reduce noise levels. The signal-to-noise ratio is therefore enhanced in this operation compared with the conventional boxcar method. The dynamic range of the detection system is at least five orders of magnitude.

Temperature and Gatewidth Dependence of the Solid-Matrix Fluorescence Spectra of Stereoisomeric Tetrols

A unique laser-based method was developed for the solid-matrix fluorescence characterization and identification of tetrols, which are the hydrolysis products from benzo[ a]pyrene-DNA adducts. By employing different gatewidths with a boxcar integrator, we obtained the fluorescence spectra of the four tetrol isomers adsorbed on 10% α-cyclodextrin/NaCl at several temperatures. The data showed that the fluorescence spectral characteristics of the tetrols were both gatewidth- and temperature-dependent. The experimental conditions were optimized to identify the tetrols which had only minor structural differences. Applications of the gatewidth-dependence method both at room temperature and low temperature were considered. The low-temperature approach has the potential for the application to several species over a range of fluorescence lifetimes. The differences in the fluorescence spectral properties were partially explained by the rotation of functional groups in the tetrols and the solid matrix during the time frame of obtaining the spectra. Comparisons were made among the solid-matrix fluorescence spectra and the solution fluorescence spectra of the tetrols, and the solution fluorescence spectra of pyrene and the solid-matrix fluorescence spectra of pyrene.

Double Modulation Technique in Pulsed Jet Experiments with Tunable Diode Lasers

A double modulation technique was used to achieve higher sensitivity in absorption measure­ ments with a pulsed supersonic jet. The wavelength of a lead-salt diode laser was modulated at a frequency of 10 kHz and detected at twice the frequency by a lock-in amplifier, reducing significantly the influence of the low frequency excess laser noise. The output of this lock-in amplifier was measured by a boxcar integrator, driven at 80 Hz, the frequency of the pulsed jet. In case the duration of the jet pulse is long enough, a second lock-in amplifier can also be used instead of the boxcar integrator. The optical path of the diode laser radiation through the jet was increased 16 times by using White type multireflection optics. The achieved value for the minimum detectable absorption of Ar - CO complexes in the jet was 2 x 10-5 in relative absorption, limited by the excess noise of the diode laser.

Time-Resolved Luminescence Spectroscopy: Computer Interfacing and Software Development for a Boxcar-Based Laser-Luminescence Spectrometer

The undesirable resistance-capacitance (RC) distortion coming from the exponential moving averaging process of a boxcar integrator/averager acquisition system in kinetic and spectral luminescence measurements could be effectively eliminated by interfacing it to a microcomputer. Accurate acquisition of luminescence decay curves and time-resolved spectra was provided by a developed software that allows synchronous scan control of the gate delay time in the boxcar integrator with the emission wavelength in the monochromator. Some simple mathematical calculation routines, including excited-state lifetime extraction from luminescence decay curves and normalization of time-resolved spectra, were added. The main characteristics of both the software and the interfacing hardware are reported.