Experimental development and analysis of a continuous flow-through trace gas preconcentrator

A new type of continuous flow-through trace gas preconcentrator for rarefied trace gas analysis, which has been proposed by Muntz et al. (Muntz et al. 2008, Muntz et al. 2004, Han et al. 2009) has been built and consists of a main flow channel, pumping chambers, and separation membranes between the main flow channel and the pumping chambers. In this case, preconcentration is not from stop, adsorption, and release; but is caused by the continuously changing cross section of the main flow channel until released through the detecting system such as gas chromatography, mass spectrometry, or optical diagnostics. This has the possibility of achieving concentration increase of various gases in a carrier gas by using relatively simple micro/mesoscale mass diffusion separation stages, without interrupting the gas flows, and is suitable for improving the time accuracy of analytical systems. The present study focused on the design and characterization of a new type of preconcentrator that enables use with various gas detection units. The research described here addressed for heavier molecules, at atmospheric pressure and room temperature optimized conditions and possibilities of the preconcentrator.;The shape equation was introduced for the main flow channel and the concentration ratio of the trace gas was determined using a set of coefficients including; the fractional open area, the transmission probability, and the ratio of pressure between the main flow channel and pumping chambers. The fractional open area (porosity) and transmission probability were obtained from a pore diameter, thickness, and pore density of a membrane used. According to these equations and coefficients, it was possible to determine the shape and size of the main flow channel, and type of the separation membrane approximately. In order to fabricate a preconcentrator prototype, the variables were limited with reasonable boundary conditions, and could be predicted with available numerical data. Properties of the membrane were used as main factors to decide the type of the separation membranes. The pumping chambers located above and below the main flow channel maintained to maintain a constant gas flow speed in the main flow channel during the experiment, and were designed to create relatively lower pressure than that in the main flow channel.;A series of experiments were conducted in an attempt to validate the available numerical data, such as the concentration and gas flow speed of the newly continuous preconcentration technology. This study involved experimental investigations using argon mixed with xenon to obtain a base-line comparison of the existing numerical predictions provided by the prototype preconcentrator. The concentration was calculated by pressure differences between the main flow channel and pumping chambers, and by mass flow rates obtained at exit of the main flow channel.;The numerical models calculated an increase in concentration under the following conditions; 5cm/s for the helium gas flow speed, 0.000042 for the porosity, 0.0066 for the transmission probability, and 0.5 for the gas density ratio. Under these conditions, it is expected that the concentration of xenon gas in a mixture of the argon and xenon gases increases about 32 times at the end of the main flow channel. However when the shape equation was reflected in the concentration equation, the concentration ratio of the xenon gas increases about 32 times for the 10nm membrane. However the xenon gas concentration of the experimental results decreased approximately 10 percent to the concentration of xenon gas in an initial sampled gas for the all cases, and these concentrations increased about 1.07 times for some experiments used the 50nm membrane. From these experimental results and numerical models, the prototype of preconcentrator can be changed the concentration ratio of various gases.;Easily available gases in this study had a molecular mass ratio that is representative of typical, more complicated molecular structures that are encountered in actual situations. Subsequently, trace gas mixtures more typical of the complicated molecular structures found in heavy molecules, will be studied. With the experimental results it is possible to evaluate the actual performance, and thus the potential of the proposed preconcentrator and detecting system. Miniaturization, based on the improvements made to theoretical models, offers the further advantage of enabling the use of inexpensive, disposable substrates, and also enhances the analytical performance of the device. The miniaturization of analysis systems reduces the quantities of sample required, allows assays to be performed more quickly, and enables portability of the system.