Abstract
Highly reproducible fabrication of porous layer open tubular (PLOT) structures in fused silica capillaries is often challenging; thus, methods to measure layer thickness growth in real time represent a powerful tool for the production of such columns. The work presented herein demonstrates the application of optical absorbance in the near-infrared (near IR) range for the in-process measurement of polymer layer growth inside fused silica capillaries during the fabrication of PLOT columns. The proposed technique can be used for both on- and off-line measurements of layer thickness for thermal- and photo- initiated polymerisation methods, performed in either polytetrafluoroethylene (PTFE)- or polyimide-coated capillaries. Measurements of layer thickness were carried out at λ 700 nm, using 100 μm and 8 μm optical fibres, yielding relative standard deviation (%RSD) values of 27% and 22%, respectively.
Highlights
Several methods exist for the measurement of thin film thickness, ranging from β- and X-ray techniques [24,25], electrical impedance and conductivity [26,27], to tactile methods such as atomic force microscopy (AFM) [28,29,30] and scanning acoustic microscopy techniques [31]
Since the intensity of the incident light drops due to absorption within the polymer layers and remaining polymerisation mixture in capillary, and the level of absorption is relative to the thickness, average density, and porosity of the layer formed, it is possible to relate absorption to the physical properties of the polymer layer
Optical fibres for optical absorbance measurements were purchased from Thor Labs (Munich, Germany), fibre couplers were purchased from L-Com (North Andover, MA, USA), and the light sources and UV-IR spectrometer were purchased from Ocean Optics (Dunedin, FL, USA)
Summary
Porous layer open tubular (PLOT) columns (see Figure 1) are a type of open tubular column used in a variety of chromatography applications, and they find some use in liquid chromatography (LC) [1,2,3,4,5,6,7,8,9], capillary electrochromatography [10,11,12,13,14], and micro solid phase extraction (μ-SPE) [15,16], their main area of application is in gas chromatography (GC) [17,18]. Several works describing the fabrication of another more stable type of PLOT column—the monolithic porous layer open tubular (monoPLOT) column—have been published [16,17,18,19] In this case, the layer is formed through the polymerisation of homo- or co-polymers in the presence of a porogen, giving the material a porous structure. MonoPLOT columns can be produced through thermal- or photo-initiated polymerisation methods; in the majority of these techniques, it can be extremely difficult to precisely control layer thickness and morphology. The control of these parameters is crucial for these types of columns, as loading capacity and separation efficiency will depend on the volume and Separations 2016, 3, 34; doi:10.3390/separations3040034 www.mdpi.com/journal/separations. In-process thickness growth monitoring suitable different processes, layer thickness growth monitoring method, suitable for different fabrication processes, remains unmet
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