The performance of nanoporous silicon (pSi) and ultra-thin layer chromatography (UTLC) plates as surfaces for desorption electrospray ionization (DESI) was compared with that of polymethyl methacrylate (PMMA) and polytetrafluoroethylene (PTFE), both popular surfaces in previous DESI studies. The limits of detection (LODs) and other analytical characteristics for six different test compounds were determined using all four surfaces. The LODs for the compounds were in the fmol-pmol (pg-ng) range. The LODs with the pSi surface were further improved for each of the compounds when heat was applied to the surface during sample application which gave LODs as low as or lower than those achieved with PMMA and PTFE. The UTLC plates were successfully used as a rapid means of chromatographic separation prior to DESI-MS analysis. Another advantage achieved using the newer pSi and UTLC surfaces was increased speed of analysis, associated with drying of solution-phase samples. This took place immediately at the UTLC surface and it could be achieved rapidly by gently heating the pSi surface. The presence of salts in the sample did not cause suppression of the analyte signal with any of the surfaces.

The feasibility of ultra thin-layer chromatography atmospheric pressure matrix-assisted laser desorption ionization mass spectrometry (UTLC-AP-MALDI-MS) has been studied in the analysis of small molecules. Because of a thinner adsorbent layer, the monolithic UTLC plates provide 10 -100 times better sensitivity in MALDI analysis than conventional high performance thin-layer chromatography (HPTLC) plates. The limits of detection down to a low picomole range are demonstrated by UTLC-AP-MALDI-MS. Other advantages of UTLC over HPTLC include faster separations and lower solvent consumption. The performances of AP-MALDI-MS and vacuum MALDI-MS have been compared in the analysis of small drug molecules directly from the UTLC plates. The desorption from the irregular surface of UTLC plates with an external AP-MALDI ion source combined with an ion trap instrument provides clearly less variation in measurements of m/z values when compared with a vacuum MALDI-time-of-flight (TOF) instrument. The performance of the UTLC-AP-MALDI-MS method has been applied successfully to the purity analysis of synthesis products produced by solid-phase parallel synthesis method. (J Am Soc Mass Spectrom 2005, 16, 906 -915)

The feasibility of ultra-thin-layer chromatography (UTLC) and atmospheric pressure matrix-assisted laser desorption/ionization mass spectrometry (AP-MALDI-MS) for bioanalysis was studied with benzodiazepines as model substances in human urine. Two-dimensional (2D) UTLC was shown to be an efficient technique for the separation of benzodiazepines. Separations occurred in 4-12 min, and the separated compounds were identified by AP-MALDI-MS. The limits of detection with AP-MALDI-MS and AP-MALDI-MS/MS were in the picomole range and thus low enough for bioanalysis. The applicability of the 2D UTLC-AP-MALDI-MS was demonstrated in detection of metabolites with an authentic biological urine sample.

A high-performance thin-layer chromatography (HPTLC) method was developed for fast evaluation of the purity of solid-phase synthesis products. The results obtained were in good agreement with results obtained by the LC-MS method (r(2) = 0.8404) or by the LC-UV method (r(2) = 0.8053), confirming the suitability of HPTLC for purity analysis of combinatorial syntheses. The synthesis products can be quantified and identified by measuring UV densitograms or in situ UV spectra or by ESI-MS after isolation of the zone of interest. A new, simple, and fast method for transferring the zone of the analyte from the plate to the ESI-MS equipment is described. The new HPTLC method enables rapid and efficient analysis of approximately 40 samples in parallel. As such, it offers a cheaper and easier way to analyze the purity of synthesis products than the commonly used LC-UV-MS.

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