Comparison of a jet separator and an open splitter as an interface between a multi-capillary gas chromatographic column and a time-of-flight mass spectrometer

A gas chromatographic/time-of-flight mass spectrometric (GC/TOFMS) interface is being developed for fast on-line analysis utilizing multi-capillary column technology. A variable gap-distance jet separator has been constructed and its performance compared with that of a commercially supplied post-column open splitter recommended for use between the multi-capillary column and a mass spectrometer. Both interfaces were found to be compatible with the GC/TOFMS system at high carrier gas flow-rates, facilitating high-speed and high-resolution separations. The systems were investigated and tested with a mixture of volatile organic compounds (VOCs) with molecular masses from 85 to 166: dichloromethane, toluene, m-dichlorobenzene, o-dichlorobenzene and tetrachloroethylene. The optimum tip-to-tip gap distance corresponding to the highest efficiency of the jet separator was found to be 0.030 mm for each compound at carrier gas flow-rates of 20, 40 and 60 ml min(-1) giving, in the ion source housing, ion gauge pressure readings of 1.6 x 10(-6), 5.0 x 10(-6) and 5.8 x 10(-6) mbar, respectively. The efficiency of the jet separator (10-30% yields) was significantly higher than that of the open splitter (6-9% yields). The observation that the open splitter did not provide a constant flow-rate to the ion source was not in agreement with the manufacturer's specifications. A method for measuring the gas flow-rates in all parts of the equipment is described. The correlation between yield in the jet separator and molecular mass for the heterogeneous set of compounds studied was found to be less linear than usually reported for homologous series of compounds in jet separator studies. The result suggests that the pressure conditions in the jet may be sufficient for the separation process to be partly controlled by diffusion rather than predominately by effusion. Copyright 2000 John Wiley & Sons, Ltd.

Orthogonal acceleration time-of-flight mass spectrometry.

The principles and applications of time-of-flight mass spectrometry involving instruments with independent (orthogonal) axes for ion generation and mass analysis are reviewed. This approach, generally referred to as orthogonal acceleration time-of-flight mass spectrometry, has proved particularly advantageous for the combination of continuous ionization sources with time-of-flight mass spectrometry. The history of the technique is briefly discussed along with the instrumental principles pertaining to all the stages of the instrumentation from ion source to detector. The applications of commercial and customized instruments are discussed for several ionization methods including electrospray, matrix assisted laser desorption/ionization, electron ionization, and plasma ionization.

Resolution limitations from detector pulse width and jitter in a linear orthogonal-acceleration time-of-flight mass spectrometer.

Recent and ongoing advances in timing electronics together with the development of ionization techniques suited to time-of-flight mass spectrometry (TOF-MS) have contributed to renewed interest in this method of mass analysis. Whereas low resolving powers (m/†m < 500) were once an almost unavoidable drawback in TOF-MS, recent developments in instrument geometries have produced much higher resolving powers for many ion sources. The temporal width of detector pulses and jitter in timing electronics, however, lead to contributions to peak widths that are essentially independent of the mass-analyzer ion optics. The effective detector pulse width (†t d ≈ 1-10 ns typically) can be a limiting factor in the development of high resolution time-of-flight (TOF) instruments with modest drift lengths (∼1 m), It also reduces the mass resolution more seriously for light ions. This article presents a method for distinguishing the instrumental "ion arrival-time" resolution (R o) of a linear TOF mass analyzer from that which is locally measured at a particular mass, limited by the broadening of the detector pulse width and electronics. The method also provides an estimate of †t d, that is useful in determining the temporal performance of the detection system. The model developed here is tested with data from a recently constructed orthogonal-acceleration TOF mass spectrometer equipped with a commercially available transient recorder (a LeCroy 400-Msamplejs digital oscilloscope) from which we obtained R o = 4240 ± 100 [full width at half maximum (FWHM)) and †t d = 3.0 ± 0.1 ns (FWHM).

Spontaneous and deflected drift-trajectories in orthogonal acceleration time-of-flight mass spectrometry.

Orthogonal acceleration is a method for gating ions from an ion beam into a time-of-flight (TOF) mass spectrometer. The technique involves a pulsed electric field to apply acceleration directed orthogonally to an ion beam. This approach is useful for coupling continuous ion sources to TOF mass analyzers. Most instruments of this type, which have been described in the literature, use steering electrodes after the orthogonal acceleration step. Those velocity components of ions originating from the ion beam velocity are minimized so that the deflected drift-trajectory is parallel to a transverse flight tube. In an alternative geometry the ion beam velocity is conserved and the drift-trajectory after the orthogonal acceleration step is spontaneous. The differences between the space-time focusing ability with spontaneous and deflected drift-trajectories are discussed and investigated. Trajectory calculations indicate that deflection fields placed after the orthogonal acceleration step distort the ion packet because, in this geometry, the flight-time to the detector is dependent on the position that the ions enter the steering optics. Increasing the duty-cycle efficiency by sampling longer sections of the continuous ion beam leads to a degradation of resolving power. Employing a spontaneous drift-trajectory after orthogonal acceleration provides the advantage that the arrival time spread for isobaric ions is, in principle, independent of the length of the ion beam sampled. The major implication of these findings is that simultaneously optimized sensitivity and resolving power may not be achievable with the deflected drift-trajectory instruments. The calculations are in agreement with results from the published data of a number of groups who have built instruments based on the orthogonal acceleration principle.

Detection limits and surface interactions in quantitative negative chemical-ionization mass spectrometry Ultratrace determination of metals and organic compounds by means of their complexes.

Details are given of a selective negative-ion mass-spectrometric method appropriate for the ultratrace determination of metals and organic compounds by means of their complexes. Direct introduction of the sample into the ion-source, attachment of low-energy electrons, and selected-ion monitoring are described, and comparative data are given relating to surface effects at the tips of insertion-probes on detection limits. Detection limits for chromium and cobalt, determined as their tris(2,2,6,6-tetramethylheptane-3,5-dione) chelates, were respectively 1.0 and 0.16 pg, and that for nickel [as its bis(N,N-diethyldithiocarbamate) complex] was 1.0 pg. Detection limits of 2.0 and 1.0 ng are attainable for malathion and ethion by measurement of the nickel(II) complexes of their O,O'-dialkyldithiophosphate hydrolysis products.