1.  Field of the Invention
The present invention relates generally to sensors for volatile compounds in the gas phase, and to sensitive sensors for sensing the presence of target volatile organic compounds contained in fugitive emissions, for example in the environment where inks, toners, and volatiles are generated in imaging processes. 
2.  Description of Related Art
There is an ever-increasing desire to reduce gaseous emissions from industrial sources, given the environmental and health threats such emissions may pose.  Most of the industrial air pollutant emissions are generated by two types of companies, namely chemical manufacturing companies and oil refineries.  Emissions from these types of industries are of two basic types: (1) stack gas emissions, and (2) fugitive emissions (i.e., leaks), the latter of which represent about one-half of the total emissions.  Therefore, a major decrease in total emissions from these industrial sources will require a significant reduction of fugitive emissions.  Of greater local interest tends to be emissions in buildings or rooms where concentration of emissions may tend to be high.  It is has been found that offices can readily exceed safety standards for volatile organic compounds because of the large use of printers and copiers.  These imaging systems generate volatile solvents whenever liquid inks, toners or developers are used. 
Originally, personnel would smell out issues of excess environmental solvents (which some would say exists when any solvent is present in the ambient office space).  The detection could be done by direct smell detection by employees or with sniffers carried through the office.  These devices would be chemical sensors that could be carried by hand as employees walk through facilities.  An old-fashioned hand-held sniffer typically consisted of a long tube through which air samples are drawn to reach a flame ionization detector component. 
A problem with the use of sniffers for leak detection is that this method does not provide continuous, around-the-clock monitoring.  Human smell detection suffers from too ready detection saturation, wherein the person's olfactory senses lose their ability to recognize an odor to which they are regularly exposed over long periods of time. 
The detection of volatile compounds in gaseous emissions include the detection of pollutants (such as hydrocarbons, aromatics, and the like) in what has been referred to as “office exhaust” from copiers and printers, and is a potentially significant health issue.  Sensors for these office spaces would require sensors with increased sensitivities over those used to detect fugitive chemical plant emissions.  Specifically, sensors to be used in the ambient air in which target vapors are greatly diluted with air may require sensitivities of at 100 ppm and less, and often in the sub-ppm regime. 
An inexpensive sensor that may be easily and flexibly employed in chemical plant environments to continuously monitor fugitive emissions is disclosed in U.S. Pat.  No.  5,417,100, entitled “Reversible Sensor for Detecting Solvent Vapors.”  Specifically, this patent discloses a fugitive emissions sensor for reversibly detecting solvent vapors that comprises a pair of electrically-conductive interdigitated electrodes disposed on the surface of a dielectric substrate and a composite coating covering the interdigitated electrodes comprising (1) a conductive polymer having a conductivity within the range of about 10.sup.-6 to 1 S/cm, and (2) a dielectric polymer with an affinity for the solvent vapors of interest.  The dielectric polymer is the major component of the composite coating and serves as an attractant for the targeted solvent vapor.  The ratio of conductive polymer to dielectric polymer is about 1:1 to 1:5.  It is contemplated that readings from such sensors would be monitored and recorded by a computer, which would then notify operators of the occurrence and location of any detected leaks. 
It is known to use conductive polymers as the active material in sensors for detecting a variety of compounds, such as acids, alcohols, complex metal ions, and proteins.  Stable conductive polymers include polypyrrole, polythiophene, and polyaniline (PANi); such materials may be readily modified by the attachment of particular functional groups and/or the incorporation of appropriate counterions during synthesis in order to detect, by selective interaction, specific compounds.  Conductive polymer sensors are advantageous because they are compact, simple, inexpensive, and easy to make. 
Another monitoring device using conductive polymers is known which indicates the presence of acid.  This device is disclosed in U.S. Pat.  No.  5,331,287, entitled “Device and Method for Sensing Water and/or Acid in the Presence of Water in Non-Aqueous Media.”  Specifically, a sensor is disclosed therein for monitoring the water and acid content that is particularly suited for monitoring the quality of nonaqueous fluids in equipment or vehicles.  The sensor comprises an insulating substrate, interdigitated electrodes formed on the substrate, and a conductive polymer deposited over the interdigitated electrodes that bridges between adjacent digits thereof.  The conductive polymer reversibly increases conductivity in measurable amounts with increasing acid and/or water content due to protonation by the acid or hydration by the water. 
European Patent Application 596,973, entitled “Device for Sensing Volatile Materials” discloses a device for sensing volatile material in the gas phase comprising a pair of electrical contacts with a semi-conductive polymer extending between the contacts.  A charge balance in the polymers is achieved using counterions in a proportion of about one counterion to four monomer units.  Typical polymers used include polypyrrole, poly-N-methylpyrrole, poly-3-methylthiophene, polyaniline, poly-5-carboxyindole, poly-3-methyl-phenylamine, polybithiophene, polythiophene, poly-3-thiopheneacetic acid, and polyfuran.  Typical counter-ions that may be used to synthesize the polymers may be tetrafluoroborate, alkyl sulfonates, tetramethylamimonium chloride, chlorates, and perchlorates.  The sensors are used in the form of an array to develop a fingerprint of a particular “odor.”  Specifically, the reference teaches the use of an array of eleven sensors to distinguish between different brands of lager beer and between two samples of the same beer that had been stored differently. 
The issue of volatile organic compounds within developing systems (e.g., printers) is an obviously dangerous situation, with combustible or flammable vapors airborne in an electrical environment.  Published U.S. patent application Ser.  No.  20040047646 describes a wet-type electrophotographic printer having a printer body.  The printer comprises a discharge passage through which air inside the printer body is discharged out to an outside of the printer body, at least one discharge fan positioned inside the discharge passage to guide the air inside the printer body to the outside of the printer body, and a photocatalystic filter positioned inside the discharge passage, and having a photocatalystic body coated with a photocatalyst, a plasma electrode disposed on the photocatalystic body, and a plasma generator coupled to the plasma electrode to filter and deodorize the air inside the printer body.  This construction assists in reducing room odor.  Published U.S. patent application Ser.  No.  20040047646 and U.S. Pat.  No.  6,744,996 are hereby incorporated by reference in their entirety.