Vapor monitor, organic detection

While these features greatly facilitate monitoring worker exposures, most instruments suffer from limitations associated with the types of sensing conponents enployed. For exeunple, the majority of the truly portable Instruments used for monitoring organic gases and vapors detect contaminants by catalytic combustion, semiconductor charge-transfer, or photo-ionization. 

Fixed-bed adsorbers may be operated in either intermittent or semicon-tinuous mode. A typical removal system is a semicontinuously operated dnal-bed system one bed is in adsorption mode while the other is being re generated (Fig. 13.23). " The adsorption performance of the bed can he monitored by analyzing the outlet gas. Once organic vapors are detected in the gas stream, the incoming gas stream is routed to the parallel adsorber, and the exhausted bed is regenerated. The adsorption and desorption cycles can also be fixed. 

By controlling the structural and electronic properties of sNPS which are related to the nanocrystallite dimensions and porosity, their surface selectivity and sensitivity to different gases (nitrogen and carbon oxide, vapors of water and organic substances) can be adjusted. This approach for the effective detection of acetone, methanol and water vapor in air was described in [13-15].The minimal detectable acetone concentration was reported to be 12 pg/mL. Silicon sensors for detection of SO2 and some medicines such as penicillin were created [16-18]. sNPS were used for the development of a number of immune biosensors, particularly using the photoluminescence detection. Earlier we developed similar immune biosensors for the control of the myoglobin level in blood and for monitoring of bacterial proteins in air [19-23]. 

The detection performance of the molecular sieve coated sensor is examined from the measurement of frequency variation while different concentrations of organic vapor contained air are contacted to the sensor sur e. While organic vapor contained air flows continuously with constant flow rate of 0.4 IVmin., the variation of frequency is monitored and the outcome is converted to the organic concentration. In order to examine the process of adsorption and desorption of the organic vapor on the molecular sieve coated on the sensor surface, fresh air and organic substance contained air are alternately provided. 

If continuous monitoring is required, vapor sensors can be used. The most common type of vapor sensor is the metal oxide sensor (MOS). In general, a MOS responds to virtually all organic vapors and provides information that a vapor release has occurred with little or no identification capability. These types of sensors have the requisite sensitivity to detect vapors at parts per million to parts per billion levels and do not require pre-concentration. Some chemical warfare agents are resistant to oxidation so they are undetectable at low concentrations by metal oxide sensors, unless the sensors are operated at elevated temperatures. 

Preconcentrators used in detection of complex organic vapors employ a granular, highly porous sorption material which provides a large collection area. Some preconcentrators are packed with up to three different absorbent materials to enhance the efficiency of collection and concentration of various organic compounds. Silica gel, polyurethane foam (PDF), and other sorbents can be used for these purposes. Sorbents usually used for atmospheric air monitoring are shown in Table 12.3. 

To date, direct UV absorption sensing has been used mainly in environmental applications to monitor pollutants in the atmosphere such as ozone and NO (Wu et al. 2006), hydrocarbons, and volatile organic compounds (VOCs) (Lin et al. 2004). Fiber optic UV systems for gas and vapor analysis have been reviewed by Eckhardt et al. (2007). The strong absorbance of vapors and gases in the UV region is advantageous and has resulted in a compact detection system of good accuracy. 

Laser PAS has been used to monitor air samples. Minimum detectable concentrations of gaseous pollutants are often in the parts per billion (ppb) or sub-ppb range depending on the molecular absorption cross section and on possible absorption interferences. Most studies have been devoted to investigations on collected air samples of different origin. A CO laser-based PAS has been used for analysis of vehicular exhausts. A number of compounds such as nitric oxide, nitrogen dioxide, H2O vapor, and other volatile organic compounds such as alkenes, aromatic hydrocarbons, and aldehydes have been reported. A mobile PAS system has been developed for analysis in the field. 

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