Gas sensors have been widely used in the field of applications such as industrial safeties, environmental monitoring systems, food industries, medical services and other fields. A kind of gas sensors that senses only certain gases were mainly used in the prior art, but electronic nose technologies where gas sensors are configured in an array pattern to sense a pattern of responses to smells and odors have developed in recent years.
In order to analyze gas samples, for example, volatile organic compounds (VOCs) by using the gas above-mentioned analysis systems such as a gas sensor, an electronic nose and gas chromatography, it is necessary to require a sample preconcentrator which can absorb and desorb the gas samples.
As one example of the sample preconcentrator, Korean Patent Publication No. 10-2005-0059364 discloses a preconcentrator for absorbing and desorbing gases using an absorbent that is composed of carbon nanotubes, and a method for fabricating the same.
More particularly, the absorbent disclosed in Korean Patent Publication No. 10-2005-0059364 is formed by directly growing carbon nanotubes on electrodes. However, the preconcentrator has problems in that it is impossible to exactly measure the gas flow rate due to the pressure drop caused by the compact growth of carbon nanotubes, and it is difficult to concentrate a variety of mixed gases to a suitable concentration since the very low growth density results in small adsorption capacity of the carbon nanotubes.
As another example of the sample preconcentrator, Korean Patent Publication No. 10-2006-0061629 discloses a sample preconcentrator having the ability of removing water vapor.
The sample preconcentrator disclosed in Korean Patent Publication No. 10-2006-0061629 includes a sample concentrating unit that has the similar configuration to the most standardized type of the conventional gas absorbing/desorbing unit [see C. J. Lu and E. T. Zellers, Analyst, Vol. 127, pp 1061-1068, 2002], where a solid absorbent, for example, Tenax or Carbotrap, which has generally been used since it has the lower absorption efficiency than other absorbents but shows the low affinity to water, which leads to a decrease in the side effects caused by water moisture, has been used as the absorbent in the sample concentrating unit, and the preconcentrator is further provided with a moisture removing unit to couple a gas sensor to the rear end of the sample preconcentrator.
Here, the sample concentrating unit serves to concentrate a sample and primarily remove water moisture, and the moisture removing unit serves to secondarily remove water moisture. In order to completely remove the water moisture, the moisture removing unit uses an absorbent, for example silica beads, which can selectively absorb water as one of the polar molecules.
As another example of the sample preconcentrator, U.S. Pat. No. 7,430,928 discloses “Method and Apparatus for Concentrating Vapors for Analysis.”
In the U.S. patent, the sample concentrating unit is configured by filling the inner part of porous metal foam, which is made of nickel, with an absorbent which may absorb volatile organic compounds, and fixing the metal foam and the absorbent with a wire mesh screen. Here, the porous metal foam serves to carry out the effective heat transfer of the absorbent.
However, the metal foam has a problem in that since it is very difficult to form pores of uniform size, the absorbent which the pores are filled are not uniformly distributed, and the pores which may be formed in the metal foam have a very limited pore size.
Meanwhile, in addition to the carbon nanotube and the 2,6-diphenylene oxide porous polymer-type Tenax, the volatile organic compound absorbent, which may be used, includes graphitized carbon-type Carbopack, carbon molecular sieve-type Carbosieve, Carbopack, Carboxen, etc. However, the above-mentioned absorbents have their limits in effectively concentrating a trace of volatile organic compounds.