Carbon Nanotube (CNT) Stationary Phase for Micro Gas Chromatograph Columns (2020)

Introduction Microfabricated gas chromatography systems based upon MEMS fabrication technology are under development for portable chemical vapor analysis and chemical detection. In particular, Sandia National Laboratories has pioneered development of these miniature systems (1). Current methods for coating microfabricated gas chromatography columns are based upon deposition from a saturated mixture of siloxanes in a solvent, which are adapted from techniques developed for coating of open-tubular capillary columns (2). This static method is more successful with silica open tube columns which have a uniform cross-section area, however in a microfabricated column, a rectangular cross-section is produced in the channel. The square corners tend to have a thicker layer of material deposited using this process and hence these thicker layers provide a different retention time for the analytes which results in peak broadening and reduction in resolution and peak capacity of the column. Alternative stationary phases have been investigated (3), and using chemical vapor deposition (CVD) they can be grown with uniform thickness, for example carbon nanotubes (CNTs) (4) The CNTs have high surface area and are stable at high temperatures. Method A silicon substrate is used for the fabrication of a microfabricated spiral column, 6m in length using standard photolithography and deep reactive ion etching fabrication processes (5). An ultra-thin layer of iron catalyst is deposited by sputtering into the etched silicon channel. Using a Black Magic plasma enhanced CVD growth a layer of CNTs are formed thickness 0.5 to 1 micrometer. Figure 1 shows an SEM micrograph of the deposited CNTs layer which coats the sides of channel and base with uniform thickness. Next a layer of gold is deposited for eutectic bonding of the silicon lid in order to enclose the channel making the column. Finally high temperature annealing is carried out at 450 oC to increase bond strength. Results The column is mounted inside a commercial Agilent 6890 GC system oven with a short length of guard column, approximately 0.5m length, connect between the split injector and the flame ionization detector. A liquid injection of three compounds, hexane, octane and decane, volume of 0.02uL, was carried out with a 100:1 split and injector temperature of 275°C. The helium carrier gas flow rate was 0.1 ml/min. A typical chromatogram is shown in Figure 2. The McReynolds probes for polarity were also injected, and the Kovats indices were evaluated for comparison with OV-1 (Ohio Valley, OH) polymer stationary phase. Conclusions The use of CNT’s for a stationary phase may offer some advantages with micro-GC column fabrication because the quality of the layer can be inspected prior to bonding. We can observe the low polarity of the CNT stationary phase compared to the poly-siloxane However, it may be possible to chemically functionalize the surface for the CNTs for more polar compound separation and analysis. References [1] in J. J. Whiting, E. B. Myers, R. P. Manginell, M. W. Moorman, K. Pfeifer, J. M. Anderson, C. S. Fix, C. Washburn, A. Staton, D. Porter, D. Graf, D. R. Wheeler, J. Richards, K. E. Achuythan, M. Roukes, R. J. Simonson, "μChem Lab: twenty years of developing CBRNE detection systems with low false “μChemLab: twenty years of developing CBRNE detection systems with low false alarm rates” Proceedings of SPIE, Vo.l. 11010, Chemical, Biological, Radiological, Nuclear, and Explosives (CBRNE) Sensing XX, 1101012 (17 May 2019); doi: 10.1117/12.2518778. [2] C. F. Poole, The Essence of Chromatography, Elsevier, 2003. [3] I. Azzouz, J. Vial, D. Thiébaut, R. Haudebourg, K. Danaie, P. Sassiat, J. Breviere, “Review of stationary phases for microelectromechanical systems in gas chromatography: feasibility and separations,” Analytical and Bioanalytical Chemistry, Vol. 406, pp. 981–994 (2014). [4] W. Intrchom, S. Mitra, “Analytical sample preparation, preconcentration and chromatographic separation on carbon nanotubes,” Current Opinion in Chemical Engineering, Vol. 16, pp. 102-114 (2017). [5] M. Navaei, A. Mahdavifar, J.-M. Dimandja, G. McMurray, P. J. Hesketh “All silicon micro GC column temperature programming using axial heating,” Micromachines, Vol 6, pp. 865-878 (2015). Figure 1