| Micro gas chromatographic(GC) column as a new generation of GC column combines the technology of micro electro mechanical system(MEMS) and GC column fabrication process. Device has a small size, high column efficiency. In the future micro GC column would be integrated with other devices to formed micro gas chromatography system. Most micro GC columns are still in the development stage and are not been widely used in business. One of the problems is the separation efficiency of MEMS GC column is still poor compare with traditional GC column, especially for the rapid separation of the sample mixture. Meanwhile the limitation of column capacity is problem about samples separation in high concentration condition. According to previous reports, this paper focuses on rapid separation of the sample mixture by applying MEMS GC columns and carries out a series of exploratory and innovative work. The following conclusions:(1) The single-channel columns were fabricated by MEMS technology and were designed with different aspect ratio. By separating the gas mixture in rapid separation condition, the issue of column efficiency for single-channel columns can be find out. Through simulation of HETP for MEMS single-channel column, the curve of HETP was bending dramatically. It indicated that a change in the velocity of carrier gas would influence the column efficiency and caused the separation efficiency of single-channel column reduced. Meanwhile ANSYS FLUENT software was used to simulate gas fluid after moved into the single-channel column. In the mobile phase, when gas travelled through the open-tube capillary columns, laminar flow was observed. This process led to parabolic velocity profile across the width of the channel because the solute molecules in the centre of the column moved faster than those at the wall. When gas mixture was rapid separated from single-channel column, the width of column was needed to re- narrowing in order to improve the column efficiency. Thus, based on the structure of single-channel column, multi-capillary columns and semi-packed columns were designed.(2) The multi-capillary columns were designed with high aspect ratio to achieve high separation efficiency. In this paper the multi-capillary columns not only maintained high column efficiency comparing with previous report, but also improved the resolution at the rapid separation condition. The multi-capillary columns were fabricated by MEMS technique. The range of column width was designed from 100μm~40μm and the column depth was etched at 450μm which formed 1:10 aspect ratio. Through simulation of HETP for MEMS multi-capillary column, when average carrier gas velocity increased, the curve drops dramatically and then trends to flat. In the real world, the multi-channel columns can work stable with variable velocity. When the velocity of carrier gas was 27 cm / s, the highest number of theoretical plates can reach a value of 14437 plates / m. The N-alkanes mixture(C5-C10, C12) was passed through 40μm wide multi-capillary column. All components were perfectly separated. Meanwhile the flow splitters were designed to divide the sample gas equally in each channel. The effectiveness of the flow splitter was simulated by ANSYS FLUENT. After the addition of the splitters, the sample gas flows equally in each channel. According to the result, by Narrowing the channel width increased the column efficiency because when gas travels through the open tubular capillary columns, laminar flow occurs and the parabolic velocity profile across the width of the channel due to the solute molecules in the centre of the column moves ahead of those at the wall. Band broadening can be reduced by decreasing the width because the mass transfer distances in the mobile phase are relatively short. This finding indicates that at higher velocities, the column efficiency is not affected significantly with any further increase in the average flow-rate of carrier gas. In practical applications, MCCs exhibit stability with variable velocity.(3) Due to the effect of laminar flow occurs in open-tube column, the semi-packed column was designed. By applying Computer Aided Design(CAD), the posts were designed with regular arrangement. The plate number of two-post semi-packed column can reach at 55366 plates /m, which was two times greater than previous reports. Because of the arrangement of three-posts semi-packed column, it caused the velocity of carried gas was uneven in each path which reduced separation efficiency. Based on that structure, two-post semi-packed column was designed and was simulated by ANSYS FLUENT. When gas passed through semi-packed column, the band broadening was reduced to 1/3 of its present in the single channel column. The velocity profile was composed of three continue small peaks. By adding posts in open-tube column, the surface of stationary phase was increase by 100%. In this experiment the N-alkanes mixture(C5-C10, C12) was passed through semi-packed column and HP-5 commercial capillary column. The column efficiency of semi-packed column reached at 55366 plates /m, which was 10 times higher than HP-5 commercial capillary column. The retention time of components was two times shorter than traditional column. Four kinds of ester additive components were perfectly separated within 90 second in semi-packed column. According to the result, the effect of laminar flow was reduced in packed columns. Due to regular arrangement of posts, the eddy diffusion effects can be ignored. It indicated that the semi-packed columns obtained high column efficiency. Both Band broadening and mass transfer distances were decrease in channel.(4) To rapidly separate chemical warfare agents(CWA) at low concentration condition needs to do more work. Today few reports presented about this research. In this experiment, the 40μm wide multi-capillary column was used to separate chemical warfare agents. Sarin, Soman and sulfur mustard were chosen as the CWA analytes. The concentration of gas mixture was lower than 100 ppm and was separated within two minute. In addition methyl phosphonate and 2-chloroethylethyl sulphide were used as CWA simulants and mixed with real components. All of the samples were separated within 120 s. The Agilent HP-5 commercial capillary column was used to compare with 40μmwide multi-capillary column. The plate number was 3 times higher than commercial capillary column. The retention time of components was 3 times shorter than HP-5 commercial capillary column. Based on the structure of multi-capillary and semi-packed column, the multi-capillary packed columns were designed. The column efficiency was increase 15%. The GC system not only can detect CWA at low concentration condition, but also can identify CWA. In the future, these MEMS columns can be used to separate harmful components in public spaces. |
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