Cyril V. Thompson

Design, optimization and initial performance of a toroidal rf ion trap mass spectrometer

Abstract The design, optimization, and initial performance of a novel ion trap mass analyzer is reported. This analyzer geometry is based on the edge rotation of an ion trap cross-section into the shape of a torus. The advantages of this design are that for a given device analyzer radius (r0), significantly higher ion storage capacity than that obtained with a traditional three-dimensional quadrupole ion trap may be possible. Initial performance of this device was poor however, due to the significant contribution of additional nonlinear fields introduced by the rotation of the symmetrical ion trap cross-section. These nonlinear fields contributed to poor mass resolution and sensitivity as well as erratic ion ejection behavior. Using field analysis and ion trajectory simulation computer programs to guide the optimization, the geometry of this toroidal rf ion trap analyzer was modified in an attempt to correct for these nonlinear fields. These programs revealed that the original, symmetric toroid analyzer trapping field had a significant, sublinear component. Increasing the endcap separation and intentionally skewing the cross-sectional symmetry of the device minimized the field faults. Ion trajectory simulations indicated that the mass resolution and sensitivity of this asymmetric analyzer should be dramatically improved. Based on these results, an asymmetric version of the toroidal rf ion trap analyzer was constructed and this device has demonstrated unit mass resolution performance.

Review of direct MS analysis of environmental samples

Direct sampling mass spectrometry (DSMS) is an increasingly popular technique for rapid and sensitive measurement of organic pollutants in air, water, soil, and wastes. Detection limits are typically in the range of 1 ppb for volatile organic compounds (VOCs in water) with little or no sample preparation required and sample analysis times of less than 3 min. Despite the fast sample analysis time and consequent large sample throughput capability, there are numerous situations that would benefit from the ability to continuously monitor the concentration of targeted VOCs in real time. These include incinerator stack emissions, industrial wastewater streams, chemical process streams, vent emissions, exhaust emissions, groundwater and soil remediation process systems, waste-site off gas during remediation, fugitive emissions, hazardous workplace atmospheres, and soil gas analysis with the use of a dynamic depth profiling probe such as a cone penetrometer. During the past year, several pilot studies have been conducted with a DSMS field instrument for various real-time continuous monitoring applications. These studies have included the monitoring of VOCs in an incinerator stack, monitoring of VOCs in pilot-scale photolytic groundwater remediation systems, measurement of automobile exhaust in a moving vehicle, soil gas measurements in conjunction with a cone penetrometer, and in situ measurement of VOCs in groundwater with the use of a special sampling probe. In each instance, the DSMS instrument was configured with special sampling probes that extracted the VOCs from the sample matrix and transported them through an appropriate transfer line into a direct capillary restrictor interface to the ion trap. Targeted compounds were monitored based on unique peaks in the electron impact and proton transfer chemical ionization mass spectra. The real-time detection limits for VOCs in aqueous systems are in the range of 1 ppb, and in gaseous streams detection limits are in the range of approximately 10 ppb by volume. Temporal resolution ranges up to a maximum of 10 full-scan mass spectra per second, which provides the ability to monitor transient events in a sample stream that might be missed by discrete sample collection and analysis.

Effects of Silcosteel® transfer line on the sampling of volatile organic compounds

A 150-ft section of 1/16-in, o.d. x 0.030-in i.d. fused-silica-lined stainless tubing (Silcosteel®) was studied to determine its suitability as a transfer line in the sampling and analysis of vapor samples of volatile organic compounds (VOCs), principally in subsurface soil gases. A variety of test atmospheres of individual VOCs at concentrations ranging from 0.01 to 10,000 ppm were used to challenge the tubing. The adsorptive and desorptive properties of the tubing were characterized under conditions [14 °C (57 °F) and 90% RH] comparable to subsurface environments and also at temperatures of 2 °C (35 °F) and 46 °C (115 °F). Clearance rates of the selected VOCs from the tubing were also determined. Direct-sampling ion-trap mass spectrometry (DS-ITMS) real-time air monitoring was used to measure the effects of the transfer line on the vapor samples.