Assessing the reliability of wall-coated microfabricated gas chromatographic separation columns

This study addresses several aspects of etched-silicon/glass microfabricated channels that affect their performance as micro-gas chromatographic (μGC) separation columns, including the consistency of stationary phase deposition, deactivation of surface-adsorption sites on the microcolumn walls, and the stability of the stationary phase following repeated thermal cycling. Convolved square-spiral microcolumns 0.5–3-m long with cross sections of 150 μm × 240 μm consisting of deep-reactive-ion-etched (DRIE) Si with anodically bonded Pyrex caps are used. Replicate devices with wall-coated films of the non-polar polydimethylsiloxane (PDMS) or the moderately polar poly(trifluoropropylmethyl)siloxane (PTFPMS) deposited statically to a nominal thickness of 0.15 μm provide minimum plate heights that are reproducible to 5% in air or helium carrier gases. Using split-flow injection, 4900 theoretical plates/m can be produced and a 19-vapor mixture can be separated on a 0.5-m PDMS-coated column in air in less than 3.5 min. Significant reductions in peak tailing of polar analytes are afforded by surface pre-treatment or post-treatment with the deactivation agent hexamethyldisilazane (HMDS). In addition, retention times remain stable and peak widths remain stable or decrease after prolonged pre-conditioning at 200 °C in air. Non-pretreated PTFPMS-coated microcolumns can produce 2300 plates/m and also retain high resolution after pre-conditioning at 200 °C in air. Marginal improvements in separation efficiency are achieved by pre-treatment with trifluoropropylmethylcyclotrisiloxane. Results demonstrate that efficient μGC columns with polar and non-polar stationary phases can be made reproducibly and operated at elevated temperatures in air.