Ultrathin-layer chromatography on SiO2, Al2O3, TiO2, and ZrO2 nanostructured thin films

• Study of anisotropic SiO2, Al2O3, TiO2, and ZrO2 GLAD UTLC stationary phases.
• Chromatographic merits evaluated using carotenoid and food dye mixtures.
• Separation performance depended on material, oxidation heat treatments, and UV irradiation.
• First successful GLAD UTLC–ESI-MS recording of dyes on inorganic ultrathin films.
• New GLAD UTLC layers may separate samples better than traditional miniaturized sorbents.

We explored four different inorganic oxides and determined their merits in miniaturized planar chromatography. Despite progression of chromatographic techniques over several decades, such alternatives to traditional planar silica gel stationary phases have not been fully evaluated. Glancing angle deposition (GLAD) provided an excellent platform for engineering nanostructured thin films in these materials for ultrathin-layer chromatography (UTLC). Separations of carotenoids and synthetic food dyes were used to investigate the attributes of SiO2, Al2O3, TiO2, and ZrO2 GLAD UTLC media. These anisotropic high surface area thin films possessed similar channel-like features but different chromatographic properties. TiO2 and ZrO2 media were especially interesting since analyte retention could be modified through simple oxidation heat treatments and UV irradiation. Generally, oxidation reduced analyte retention while UV exposure increased retention. Changes in retention factor as large as ΔhRF ∼ 40 (for Acid Red 14 on titanium oxide) were achieved. Food dye mixtures were applied using consumer inkjet printers as per the Office Chromatography concept and separation performance was quantified using advanced video instrumentation designed for miniaturized plates. Enhanced time-resolved UTLC methods were used to calculate figures of merit from recorded dye separation videos. Small theoretical plate heights (<4 μm) and low limits of detection (<2 ng per zone for the food dye tartrazine) were measured. The combination of engineered GLAD UTLC plates, inkjet application of analyte spots, time-resolved UTLC, and custom analysis algorithms enabled some of the best performance achieved on GLAD UTLC layers. Separations on the inorganic oxide thin films were also successfully hyphenated with electrospray ionization mass spectrometry for the first time. This investigation demonstrates the utility of alternative inorganic oxide GLAD UTLC media and probes avenues of expanding the capabilities of miniaturized planar chromatography.