Development of a new particulate 4-μm adsorbent layer for ultrathin-layer chromatography (miniaturized chromatogram).

The performance of particulate layers of a smaller particle size than given HPTLC plates was investigated. However, reducing the particle size and thus interporous space, the capillary flow is substantially reduced. For the first time, optimizations led to the successful use of a layer made of mainly 4-μm irregular silica gel particles as stationary phase in planar chromatography. Layer thicknesses of 50 and 70 μm were studied to suit especially for ultrathin-layer chromatography (UTLC). Along with it, the chromatographic fingerprint was miniaturized by 75%. The required development time and developing distance were reduced up to 50%. A typical 35-mm separation took less than 10 min. The limit of detection was improved by a factor of up to 3. Further chromatographic performance data, such as separation number (peak capacity), resolution and analytical response proved the suitability of the particulate UTLC layer. A direct transferability and miniaturization of established HPTLC methods to the UTLC dimension was aimed at. This was successfully demonstrated for eight quite different separations, i.e. food dyes, polycyclic aromatic hydrocarbons, steviol glycosides in cough drops and sweetener tablets, preservatives in shrimp-surimi salad, gingerol in ginger-containing products, 5-hydroxymethylfurfural in honey, tanshinones in Salvia miltiorriza extracts and monacolin K in fermented red rice food supplements. The direct method transfer, mainstream usability and fast analyses at low running costs are crucial features of the developed UTLC plate. Profiting from such advantages, the miniaturized fingerprint shows potential to shift the field of HPTLC towards UTLC. Nevertheless, the full separation potential will first be exploited with tailored, miniaturized instrumentation capable to operate with the reduced layer thickness and UTLC format.