Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, R01-OH-003692, 2010 Aug; :1-138
This report describes the development and implementation of a high-performance portable gas chromatograph (GC) employing novel approaches to the capture, preconcentration, separation, and detection of volatile organic compounds (VOC) at trace levels in complex mixtures. The key analytical components of the instrument are an on-board multi-stage adsorbent preconcentrator/focuser (PCF), two series-coupled low-thermal-mass separation columns with pressure and temperature tunable retention control, and a detector consisting of a microfabricated sensor array whose responses patterns can be used together with chromatographic retention times to identify and differentiate eluting vapors. Ambient air is employed as the carrier gas. This project is an extension of that performed over the preceding three years under NIOSH funding, and it concerns two 'generations' of the prototype instrument. The polymercoated surface-acoustic wave (SAW) sensor array used in the 1st-generation (Gen-1) prototype was replaced in the 2nd-generation (Gen-2) prototype with an array of four chemiresistor (CR) sensors coated with thiolate-monolayer-protected gold nanoparticles (MPN), which provide greater sensitivity. Other refinements made to improve the performance of the Gen-2 instrument concern the control software, system fluidic layout, split-flow injection, thermal control of individual components, on-board internal standard, and a wireless interface for remote interrogation of the instrument. Field testing in an office building and a printing services workplace environment that was performed with the Gen-1 prototype provided results that were critical to the refinements made to the Gen-2 prototype. These led to significant improvements in performance and reliability. The Gen-2 instrument was extensively characterized in the laboratory, with a focus on the tradeoffs in performance associated with thermal and fluidic operating variables. It was shown that the Gen-2 prototype could analyze mixtures containing more than 30 VOCs in < 16 minutes with limits of detection (LOD) as low as 0.05 parts-per billion. The application of the instrument to the analysis of markers of environmental tobacco smoke in ambient air, biomarkers of lung cancer in breath, and other applications of interest in occupational health has also been demonstrated. In the course of developing the Gen-2 prototype, a number of additional investigations were completed that focused on the design and operation of the key components of the analytical subsystem of the instrument, as well as on the development and application of chemometric analyses for assessing performance and extracting useful information from the output of the prototype detector. Many of these studies go beyond the aims of the project as originally proposed. Our studies of preconcentrators have led to an improved understanding of how to design these devices and the factors that influence their capacity and desorption efficiencies as well as their use as focusers/injectors for the separation module. Methods for optimizing preconcentrators for this application were developed. Our studies of separations have focused on conventional capillary columns with so-called 'at-column', low-thermal-mass heaters as well as on microfabricated columns with integrated heaters. Separation tuning by means of a band accelerator device at the mid-point between the two columns in the separation module was achieved, as well as by means of pressure-modulation at the column junction point. Several germinal studies explored the use of microcolumns interfaced to conventional FIDs, to miniature differential mobility spectrometers, and to microsensor arrays. We performed groundbreaking work on the chemiresistor array technology used for the prototype detector. For example, we synthesized several new nanoparticle materials that serve as interfaces for the CR sensors, we eveloped a theoretical model describing the dependence of MPN-coated sensors on the physicochemical properties of the analytes and MPN ligands, we demonstrated how MPNs can be combined with novel charge-transfer complexes to create sensors with unusually high selectivity for olefinic VOCs, among which are numerous carcinogens, and we established the feasibility of patterning nanoparticle films on integrated CR arrays using electron-beam crosslinking of the nanoparticle ligands. We also performed exhaustive testing to determine the limits of performance of sensor arrays, such as those used in this project, with regard to the complexity of mixtures and relative compositions that can be analyzed effectively. Finally, this project contributed indirectly, but significantly, to the development of the first uGC employing microfabricated components for preconcentration, separation, and detection and that is capable of multi-vapor determinations. This microsystem has undergone several design revisions in the ensuing years and we are now embarking on a field study with several prototypes that will illustrate the capabilities of uGC technology for trace-level VOC determinations in real-world applications. These investigations have produced numerous important findings that have improved our understanding of the scientific basis for preconcentration, separation, and detection of VOCs, as well as the design and operation of devices for performing these functions in a portable instrument. The ultimate challenge of integrating all devices and operations into a functional system has been met and has resulted in a functional prototype. The performance of the Gen-2 prototype is unprecedented and represents a significant advancement in the state-of-the-art in direct-reading instrumentation for on-site analysis of complex VOC mixtures. Transfer of this technology to the private sector for production of instruments modeled after the Gen-2 prototype would add a key capability currently lacking in the repertoire of exposure assessment tools available to occupational health professionals. This, in turn, would enhance the quality and quantity of data that can be collected to characterize and control worker exposures to complex VOC mixtures.
Edward T. Zellers, PhD, Professor, Department of Environmental Health Sciences, University of Michigan, Ann Arbor, MI