Christie G. Enke

The correlation of membrane permselectivity with Hildebrand solubility parameters

Abstract Experimental and theoretical permselectivities for a filled elastomeric membrane have been correlated utilizing Hildebrand solubility parameters in a solution-diffusion model. Equilibrium partitioning of substances between the feed and membrane phases may be modelled as solutions. These two phases may be thermodynamically characterized by means of the Hildebrand solubility parameters. In addition, the effect on diffusive transport of the partitioning of a substance between a filler and polymer phase in a membrane may be correlated to solubility parameters. A variety of substances was examined in this study using a silicone elastomer membrane, composed by 69 wt% poly (dimethylsiloxane) and 31 wt% fumed silica. The model is demonstrated to have good predictive value over three or four orders of magnitude for the permselectivities of substances including permanent gases, alkanes, chlorinated and brominated hydrocarbons, alcohols, and aromatic hydrocarbons.

Models for mass-independent space and energy focusing in time-of-flight mass spectrometry

Abstract Various models for improved resolving power in time-of-flight mass spectrometry have been considered. A model for dynamic post-source acceleration has been developed in an attempt to achieve mass-independent space and energy focusing. The model incorporates all pertinent parameters including the extraction voltage, the accelerating and field-free region lengths, and the dynamic acceleration function. Mathematical analysis and computer simulations based on equations from this model have been used to develop a new post-source acceleration technique. Simulations indicate that concurrent space and energy focusing is possible, and that unit resolving power can be achieved up to at least 2000 u using typical gaseous source conditions and readily attainable instrumental parameters.

Design, construction, and evaluation of an integrating transient recorder for data acquisition in capillary gas chromatography/time‐of‐flight mass spectrometry

An integrating transient recorder (ITR) has been designed, constructed, and evaluated to accomplish time‐array detection in gas chromatography time‐of‐flight mass spectrometry (GC‐TOFMS) applications. The ITR consists of a 200‐MHz flash analog‐to‐digital converter, 16 high‐speed 100K emitter‐couple logic (ECL) summing boards, three parallel processors for real‐time data reduction, instrument control and routing functions, and a 300‐Mbyte mass storage device. The ITR is capable of recording 80 μs bursts of transient information with a time resolution of 5 ns. For each transient, up to 16 384 sequential time‐resolved channels may be recorded. An operator‐selectable number of sequential transients may be summed in a locked time registry creating a summed scan file while maintaining the integrity of the transient time resolution. The information from each transient is read, summed, and stored in one of two summing registers (16×1024×24 bits). While incoming information is being stored in one summing register,...

Beam deflection for temporal encoding in time-of-flight mass spectrometry.

The pulsed ion sources used in conventional time-of-flight mass spectrometry (TOFMS) generally do not provide adequate resolving power across the mass range required for applications such as gas chromatography combined with mass spectrometry (GC/MS). Theoretical and experimental aspects of beam deflection techniques, which provide time encoding for TOFJMS with continuous ions sources, are explored here. In this approach, ion source conditions do not affect resolving power, allowing for a greater variety of ionization modes to be used. Theoretical predictions for the resolving power attainable with beam deflection, which are satisfactory for GC/MS applications, agree well with experimentally determined values. The combination of GC-beam deflection-TOFMS with time-array detection is evaluated, and the capabilities of this system are compared to those of scanning mass spectrometers.

System for simultaneous count/current measurement with a dual‐mode photon/particle detector

A microcomputer‐based control system for a dual‐output Channeltron (TM) detector has been developed and implemented. This system is capable of collecting ion current and ion‐counting information simultaneously. Ion current and ion‐counting data are scaled in software and provide consistent ion intensity values from both outputs. Calculation of the ion current/ion count rate ratio at each peak in the mass spectrum allows the system to compensate for variations in gain from peak to peak. The data collection system maximizes the data collection rate and extends the useful dynamic range of electron multiplier‐based detection to nine orders of magnitude with no change in any of the multiplier conditions. Measurement of such a wide range of ion intensities in a single spectrum is particularly useful for applications in tandem mass spectrometry.

Fast atom bombardment tandem mass spectrometry employing ion-molecule reactions for the differentiation of phospholipid classes

Fast atom bombardment tandem mass spectrometry, employing ion-molecule reactions with ethyl vinyl ether in a triple-quadrupole mass spectrometer, is used to differentiate classes of phospholipids. The phospholipids are desorbed and ionized by fast atom bombardment, mass-selected by the first quadrupole, and reacted with ethyl vinyl ether in the second quadrupole; the resulting product ions are analyzed by the third quadrupole. The protonated molecules and reaction product ions observed permit the differentiation of various phospholipid classes. The pattern of addition reaction products formed is shown to depend solely on the functionality of the lipid polar head group and not on the fatty acyl constituents. Neutral gain scans that are specific for each phospholipid class are performed. Ion dissociation products are observed in the same scan as the ion reaction products to provide data on the fatty acyl composition and position on the glycerophosphate COre along with the phospholipid class. Although this method is less sensitive than neutral loss scanning for most phospholipid classes, it can (1) identify phospholipids that do not readily lose their head group as a neutral fragment and (2) detect phospholipids in mixtures containing species that give interfering neutral losses.

High Efficiency Photo-Induced Dissociation of Precursor Ions in a Tandem Time-of-Flight Mass Spectrometer

High efficiency photo-induced dissociation (PID) has been demonstrated in a tandem time-of-flight mass spectrometer. This instrument focuses isomass ion packets to temporal and spatial dimensions similar to those of the focused laser pulses from a high power excimer laser. This high density overlap of photons and ions yields highly efficient fragmentation and also provides high resolution selection of specific precursor ion mass-to-charge ratio values. Using 193 nm photon excitation of the molecular ion of bromobenzene (m/z = 1561, fragmentation, collection, and PID efficiencies af 79%, 132%, and 104%, respectively, were obtained. Characteristic fragmentations of toluene, nitrobenzene, acetophenone, triethylamine, N,N-diethylformamide, N-methylacetamide, and cyclohexene have also been demonstrated.

Critical study of temperature effects in stopped-flow mixing systems

Abstract A comparative study of temperature effects in three different stopped-flow mixing systems is reported. Special emphasis was placed on the determination of the signs and magnitudes of the temperature changes which occur before and during mixing when the modules are maintained at 20–30°C. Temperature changes were found to occur under almost all experimental conditions, but two of the stopped-flow modules exhibited reproducible temperature changes, provided that thermal equilibrium was reached between pushes. Based on the results of the experiments that are presented, guidelines are suggested for minimizing the effects of the temperature changes which occur during mixing, particularly when the modules are utilized routinely in reaction-rate methods of analysis.

Mass dependence of time-lag focusing in time-of-flight mass spectrometry—an analysis

Abstract When using mass spectral detection strategies in which wide ranges of m/z values are monitored, it is important for ions of all masses of interest to be in focus during the data collection interval. In conventional time-of-flight mass spectrometry of gaseous samples using dual field source extraction for spatial compensation, ion focus is a function of the time lag and the mass-to-charge ratio of the ion. The mass dependence of ion focus in a CVC 2000 time-of-flight mass spectrometer has been studied using a computer simulation in order to determine the mass ranges over which the losses in mass resolution and peak intensity are acceptable. As few as three different values of time lag are required to cover the m/z range from 50 to 700 u. Selection of optimum extraction voltages involves a trade-off between the width of the mass windows for any given time lag setting and the total mass range available with unit-mass-resolution.

Method for the design of broad energy range focusing reflectrons

A novel method for the design of reflections capable of focusing large kinetic energy ranges is presented. The design method itself is a numerical approach that provides a geometrically flexible alternative to traditional analytical design solutions. This design method has been used to produce a reflectron that provides unit mass resolution for product spectra in a tandem reflectron time-of-flight (TOF) mass spectrometer despite a kinetic energy range of 1950–2700 eV. In this application, the systematic progression of reflectron design results in a practical, nonlinear field reflectron with the use of only two grids. Design improvements are proposed for more flexible systems, although geometric constraints in the current instrument limit their experimental evaluation.