Neil S. Arnold

Field‐portable, automated pyrolysis‐GC/IMS system for rapid biomarker detection in aerosols: A feasibility study

A prototype automated pyrolysis-gas chromatography/ion mobility spectrometry (Py-GC/IMS) instrument was developed for (bio)aerosol characterization. The system combines a commercially available, hand-held GC/IMS device with a specially built platinum wire grid heater, a 1-in.-diam. quartz microfiber filter and a 60-l/min air pump. The prototype Py-GC/IMS system can be operated in stand-alone mode or in series with a particle concentrator. Fully automated collection/desorption/pyrolysis of aerosols and other particulate matter can be performed at repetition rates of up to 60 h−1 by means of a special remote control and display software package. The feasibility of detecting submicrogram quantities of Bacillus endospores with the use of picolinic acid and pyridine as biochemical marker compounds for the characteristic dipicolinic acid moiety in spore cell walls was demonstrated by laboratory experiments as well as preliminary field tests. Other particulate matter that could be collected and analyzed includes allergens such as pollen or home dust, as well as a broad range of bioaerosols and reaerosolized organics like explosives or drugs. In addition to its potential use as a screening device for the presence of specific classes of aerosol components, the Py-GC/IMS system has been demonstrated to retain its capability to detect and identify a broad range of volatile and semivolatile organic compounds.

Performance advances in ion mobility spectrometry through combination with high speed vapor sampling, preconcentration and separation techniques

Abstract Rugged, low weight, hand-held ion mobility spectrometry devices, initially developed for chemical warfare detection purposes, possess attractive characteristics as field-portable instruments for paramilitary (treaty verification, chemical demilitarization, drug interdiction, counterterrorism operations) and civilian (environmental monitoring, forensic characterization, process control) applications. Generally, however, such devices tend to exhibit limited resolution, narrow dynamic range, nonlinear response and long clearance times which severely limit their usefulness for qualitative and quantitative analysis of mixtures. To overcome these restrictions a prototype combined gas chromatography-ion mobility spectrometry (GC-IMS) unit was constructed by replacing the membrane inlet of a military IMS device known as the CAM (chemical agent monitor) with suitable front-end modules. These modules enable high speed automated vapor sampling (AVS), microvolume preconcentration/thermal desorption, and isothermal GC preseparation of analytes using a short capillary column while operating the IMS source and cell at subambient pressures as low as 0.5 atm. The AVS-GC-IMS methodology sharply reduces competitive ionization and facilitates identification of mixture components, thereby enabling quantitation of volatile and semivolatile compounds over a broad range of concentration in air. At higher concentration levels (e.g. > 1 ppm) using the AVS inlet in automatic attenuation control (AAC) mode maintains excellent linear response. At ultralow concentration levels, e.g.

Advances in field-portable mobile GC/MS instrumentation

Both vehicle-portable and man-portable GC/MS techniques and applications are currently undergoing rapid further development. Vehicle-portable GC/MS technology is expanding into the area of temporally and spatially resolved mapping, formerly accessible only to stand-off methods. Also, unmanned vehicles such as UAVs and drones are expected to play an increasingly important role as experimental platforms for the future. Man-portable instrumentation appears to be evolving from the unwieldy “luggables” to the readily wearable backpacks and, possibly, even handhelds. In the next 5 to 10 years this may open the way for the “chemical Geiger counter” or “definitive electronic nose”. The above developments are discussed from the perspective of field-portable, mobile GC/MS, with special emphasis on performance breakthroughs leading to potentially new applications, rather than from a strictly historical point of view.

Field-portable hyphenated instrumentation: The birth of the tricorder?

Abstract Since hyphenated methods have all but replaced one-dimensional methods in the laboratory and demands for definitive field methods are increasing, hyphenated methods may well represent the next generation of field-portable analytical instruments. Gas chromatography—mass spectrometry (GC-MS) and GC-ion mobility spectrometry (GC-IMS) are discussed in terms of their current capability and potential for further development as powerful field-portable instruments.

Roving GC/MS: Mapping VOC gradients and trends in space and time

Obtaining representative VOC (volatile organic compound) measurements in ambient environments which exhibit complex concentration gradients and/or trends is difficult when relying upon limited numbers of analyses obtained by simple pooling or averaging techniques. A more effective approach is to perform large numbers of analyses over a period of time to permit detailed mapping or profiling of local gradients and trends. Until recently, use of GC/MS (gas chromatography/mass spectrometry) techniques for rapid profiling or mapping operations was not feasible because of sample speed limitations. This article describes a roving GC/MS system based on the combination of a Hewlett-Packard model 5972 MSD (mass selective detector), a FemtoScan Enviroprobe repetitive vapor sampling inlet with short capillary GC column, an Alcatel Micro HV olliess vacuum pump stack, and a Pentium notebook PC running under Windows 95. The roving system is further equipped with differential GPS (global positioning system) and radio transceiver capabilities thereby permitting remote tracking of vehicle location and local VOC concentrations. Laboratory tests demonstrate lower detection limits of approx 4 ppb for BTX (benzene, toluene, and xylene), corresponding to minimum detectable quantities of approximately 25 femtograms, and maximum analysis speeds of up to 5 GC peaks per second on a mixture of volatile ketones. Demonstrated outdoor performance, using a zero-emission electric vehicle, includes measurement of low ppb BTX levels along a 6 km urban route at 15 s (∼150 m) intervals while moving at an estimated average speed of 35 km h−1. Indoor measurements of toluene concentrations in the low to mid ppm range at 6 s (∼5 cm) intervals along a 6 m long path reveal a high degree of spatial and temporal variability in VOC concentrations. Mobility, specificity, sensitivity, and speed of the roving GC/MS method make this a promising candidate method for rapid outdoor and indoor screening, monitoring, and mapping of VOCs.

Fast, Repetitive GC/MS Analysis of Thermally Desorbed Polycyclic Aromatic Hydrocarbons (PAHs) from Contaminated Soils

Abstract A syslem for on-line analysis of organic vapors by short column gas chromatography/mass spectrometry (GC/MS) has been used to monitor products from a thermal soil desorption reactor. The system consists of a unique air sampling inlet with a 1 meter long capillary column coupled directly to a modified Ion Trap Mass Spectrometer (Finnigan MAT) with demonstrated detection limits for alkylbcn-zencs in the low ppb range. In this work the mobile instrument is used for repetitive GC/MS and GC/MSn (tandem MS) analysis at 30 to 60s intervals of PAH products from coal tar contaminated soils in a bed characterization reactor. Results for naphthalene through dibenzanthracenes arc compared to conventional. more detailed GC/MS analyses of extracts from soil before and after thermal treatment.

Extended theoretical considerations for mass resolution in the resonance ejection mode of quadrupole ion trap mass spectrometry

Proceeding from the pseudopotential-well approximation for ion motion in a quadrupole ion trap, mathematical expressions are derived to describe the excitation amplitude of an ion packet at a given mass-to-charge ratio. Ion-neutral collisions are incorporated to describe the damping of ion trajectories and to describe the distribution of individual ion trajectories about a mean amplitude for the ion packet. The rate of increase of the amplitude during scanning is related to expressions that describe the amplitude dispersion of the ions at the time of ejection from the trap, which is operating in a resonance ejection scanning mode to describe the temporal line width of the ejected ion packet. The temporal line width is related to mass resolution under a number of different scanning conditions. Included in the discussion are considerations of the effect on resolution of the resonance excitation voltage, temperature, pressure, noise, and buffer-gas composition. An expression for the maximum possible resolution at high ion mass-to-charge ratios is developed, and these results are compared to an existing theoretical construction. The expressions derived under the pseudopotential-well approximation are further extended to high qz values and compared to experimental data previously published by two other researchers.

Design considerations in field-portable GC-based hyphenated instrumentation

The use of hyphenated GC-based methods in the development of portable chemical-monitoring instruments can offer considerable advantages to the instrument maker. Foremost among these advantages are specificity, speed, and lower costs. In this article, the authors describe the basis for achieving these advantages using examples of three prototype and breadboard instruments developed in their laboratories and give an extended theoretical discussion of the basis for what has been called “transfer-line GC” or TLGC. This TLGC approach to fixed pressure drop chromatography can be used to illustrate overall theoretical limitations of various approaches to high-speed GC for real-time monitoring applications. The three example instruments are a “roving” automated vapor sampling (AVS) TLGC/MS instrument, a breadboard AVS-TLGC/IMS (ion mobility spectrometry) instrument, and a breadboard AVS-TLGC/GC instrument. Discussion will include the application of TLGC theory to instrument design and will use example analyses that focus on the eventual application of this technology to the near real-time detection of highly toxic chemical vapors.

High speed, two-wavelength radiation thermometry of single micro particles during CO2 laser heating

Abstract A high speed, two-wavelength radiation thermometer that is capable of monitoring surface temperatures of 50–150 μm diameter particles in the 600–2000 K range and at heating rates of up to 106 K/s, characteristic of pulverized coal combustion, was designed and constructed. To meet the above characteristics, special attention was paid to detector wavelength range and speed, detection electronics and optical system alignment. The thermometer was calibrated using an in-house constructed, black cavity radiation source. Spherocarb model particles, which have more uniform size, physical properties and emissivity than coal particles, were used to demonstrate the level of short term reproducibility attainable. Consistent, reproducible temperature—time profiles obtained for particles from different coals indicate that non-grey body effects do not dominate these measurements.