Influence of the internal gas flow distribution on the efficiency of a μ-preconcentrator
Introduction
The monitoring of volatile organic compounds (VOCs), that can cause undesirable health effects, has received a great deal of attention nowadays. This application needs sensitive, selective, and rapid detection as well as quantification of the chemicals of interest. Benzene, in particular, is an important carcinogen factor as stated in human health studies. It is present in outdoor and indoor environments as well. Benzene vapors are within the components of automobile exhaust being the main source of benzene emissions (more than 80%). It is obvious that their concentration in air depends on time and location, which makes important the continuous monitoring of benzene concentration in air. The maximum exposure limit per day to benzene fixed by the directive of the European Communities is 1 ppm [1]. The Air Quality Directive, which is to be implemented in 2010, has set a much lower limit for benzene exposure: 1.6 ppb [2].
Benzene is included in the known chemical group of aromatic compounds. For the detection of these types of gases, different techniques mainly based on gas chromatography, were developed. Several miniaturized devices for VOC detection were developed recently. Ueno and co-workers introduced micro-fluidic devices including a photo-ionization detector and a surface acoustic wave detector. They used amorphous silicon dioxide powder and mesoporous silicate powder as adsorbents [3]. Other authors have further developed conducting polymer films to detect toluene, benzene, xylenes and other aromatic hydrocarbons [4], [5]. Different materials such as polymer films, used to coat piezoelectric quartz crystal sensor arrays in order to selectively detect organic vapors, were also studied [6]. Calvo-Muñoz reported on the use of inorganic (tetramethoxysilane) and hybrid (methyltrimethoxysilane) polymers prepared by a sol–gel method to optically detect benzene vapors [7].
Most of these chemical detection methods are either time consuming, expensive, or they have a limited sampling frequency and detection range [8]. In this way, a micro-system based on a preconcentrator device is a promising approach to low-power, low-cost, and real-time analysis of gases and gaseous mixtures. Special attention deserves the MicroChemLab system developed by Sandia National Laboratories, composed by a micro-machined preconcentrator, a gas chromatography channel, and surface acoustic wave sensors for selective detection of gas-phase chemical analytes [9], [10]. Another approach is the development of high-performance, silicon-glass micro-gas chromatography columns having integrated heaters and temperature sensors for temperature programming, and integrated pressure sensors for flow control [11], [12]. The study presented by Tian et al. [13], [14] is part of an effort to develop a sophisticated monolithic micro-scale GC for the analysis of complex vapor mixtures. Ruiz et al. [15] reported miniaturized gas preconcentrator for chromatographic analytical systems fabricated by silicon technology. Pham et al. [16] proposed a new technique for preconcentrating gaseous samples for high-speed field-portable GC instruments, which provides an homogeneously enriched sample flow.
The usefulness of metal oxide gas sensors for VOC detection was also established. In particular, the good sensitivity of vanadium doped tin oxide deposited by pulsed laser ablation was reported [17]. Unfortunately, the sensor sensitivity normally stays above the target threshold detection levels. For this reason, it could be considered very effective to incorporate a preconcentrator unit to the system with the aim to detect lower concentrations in the sub-ppm range.
Different studies have shown that adsorption of benzene can be significantly improved by using graphitized carbons such as Carbotrap, Carbograph, Carboxen, Carbopack B and Carbopack X [18]. These adsorbents offer excellent thermal stability and their extremely high porosity gives them greater adsorption strength. Their hydrophobic properties minimize the influence of humidity, enabling us to obtain accurate measurements. Furthermore, trapped compounds can be desorbed by thermal pulses at almost 100% efficiency.
In this work we present additional studies related to different air-flow input/output conditions for the device reported in Ref. [19]. Two different meshes 40/60 and 60/80 of Carbopack X were tested and the importance of the gas distribution through the device structure was analyzed considering parallel and crossed configurations.
Section snippets
Micro-preconcentrator structure
The μ-preconcentrator was designed to have a 3.8-mm3 housing volume which was achieved by defining a 3 mm × 3 mm silicon grid in which 40-μm wide and 3000-μm long bars were spaced at 230 μm. It was fabricated using Si technology, starting with a double-side polished Si wafer. A bilayer of 400-nm thick thermal silicon oxide and 300-nm thick LPCVD silicon nitride films was used to define the membrane that surrounds the grid. Afterwards, the heating element of 480-nm thick phosphorus doped polysilicon
Experimental
The experimental procedure consisted of adsorbing benzene and then desorbing it into the entrance of GC–MS system. A continuous flow of 150 ppb of benzene diluted in CO2 was injected into the μ-preconcentrator. Gas flows of 100 or 200 sccm were passed through the active material during 10, 30, 60 and 90 min. At the end of each measurement the gas lines were purged with He. Then, a 30-s electrical pulse was applied to the heater of the device in order to reach a desorption temperature of 250 °C. The
Electrical characterization
A temperature ramp was applied in order to calibrate the working temperature of the preconcentrator. The power consumption was less than 600 mW for an operating temperature of 250 °C. The structure reached the 90% of its operating temperature in only 8.2 s. The thermal uniformity of the μ-preconcentrator was evaluated by recording an infrared image of the silicon grid. Temperature uniformity higher than 95% was obtained along the whole structure.
Desorption measurements with GC–MS
A GC/MS Shimadzu QP 5000 equipment was used for the
Conclusions
A silicon μ-preconcentrator for chromatographic analytical systems was fabricated and characterized. Two different meshes 40/60 and 60/80 for Carbopack X were tested, being the 60/80 mesh the most favorable for benzene adsorption as lower diameter allows an easier diffusion of benzene vapors inside the particle. The importance of the gas distribution through the device structure was also analyzed considering parallel and crossed configurations. The best option, due to a better flow distribution
Acknowledgements
Part of this work has been financially supported by the Spanish Ministry of Education programs JC2007-00173 and TEC2007-67962-C04-01/MIC.
Isabel Gràcia joined the National Microelectronic Center (CNM) in 1986 working on photolithography. In 1993, she received the PhD degree in physics from Autonomous University of Barcelona (UAB), Spain working on chemical sensors, that is also her current research field.
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Isabel Gràcia joined the National Microelectronic Center (CNM) in 1986 working on photolithography. In 1993, she received the PhD degree in physics from Autonomous University of Barcelona (UAB), Spain working on chemical sensors, that is also her current research field.
Peter Ivanov graduated in Telecommunications from the Technical University of Sofia (Bulgaria) in and received his PhD in 2004 from the Universitat Rovira i Virgili (Tarragona, Spain). His thesis has been focused on the design, fabrication and characterization of screen-printed gas sensors and micro-concentrators for gas-sensing microsystems. He is currently a researcher in the Gas Sensors Group at the National Microelectronic Centre (Barcelona, Spain).
Fernando Blanco graduated in electronic engineering from the Technological Institute of Durango, Mexico in 2002. Since 2003 he has been a PhD student in the Electronic Engineering Department at the Universitat Rovira i Virgili (Tarragona, Spain). His work focuses on the design of micro-concentrators for gas-sensing microsystems.
Neus Sabaté received her B.Sc. degree on physics from Barcelona University (Spain) in 1998. In 1999 she joined the Microsystems department of CNM and she obtained her PhD in Physics in 2003. In 2004 she joined the Electronics Department of the University of Barcelona to work in MEMS applications for the gas-sensing field. She is currently working in thermal and mechanical analysis and simulation of silicon-based structures.
Xavier Vilanova graduated in telecommunication engineering from the Universitat Politècnica de Catalunya (UPC), (Barcelona, Spain) in 1991, and received his PhD in 1998 from the same university. He is currently an associate professor in the Electronic Engineering Department at the Universitat Rovira i Virgili (Tarragona, Spain). His main areas of interest are in semiconductor chemical sensors modeling and simulation.
Xavier Correig graduated in telecommunication engineering from the Universitat Politècnica de Catalunya (UPC), (Barcelona, Spain) in 1984, and received his PhD in 1988 from the same university. He is a full professor of Electronic Technology in the Electronic Engineering Department at the Universitat Rovira i Virgili (Tarragona, Spain). His research interests include hetero-junction semiconductor devices and solid-state gas sensors. Dr. Correig is a member of the Institute of Electrical and Electronic Engineers.
Luis Fonseca received his B.S. and PhD degrees in Physics from the Autonomous University of Barcelona in 1988 and 1992, respectively. In 1989 he joined the National Center of Microelectronics working as a process engineer, leading the diffusion and deposition areas of the CNM. In 2001 he joined the Microsystem group as a full senior researcher being his actual research area focused on technological developments for gas sensing and more specifically on optical gas sensing.
Eduard Figueras received his PhD in Physics from the Universitat Autonoma de Barcelona, Spain, in 1988. Until 1998 as Clean Room Manager at CNM he supervised all technological process developed at CNM and was responsible of the standardization of all new fabrication process. Since 1999, he works at the Microsystems and Silicon Technologies Department in the field of gas sensors. He is currently working on the developing of micromachining resonant devices to be used as gas sensors.
Joaquín Santander received his PhD degree in physics from the Autonomous University of Barcelona, Spain, in 1996. He is currently working at the Microelectronics National Center in Barcelona, and is responsible of the electrical characterization laboratory. His main research areas are related to different microelectronic technologies (CMOS, MCM, sensors, micro-systems), and electrical parametric characterization using mainly test structures.
Carles Cané received his BSc degree in telecommunication engineering in 1986 and the PhD degree in 1989 from the Universitat Politècnica de Catalunya (UPC) in Barcelona, Spain. Since 1990 he has been a permanent researcher at the CNM in Barcelona. He is currently working in the fields of sensors and micro-systems and their compatibility with standard CMOS technologies.