Development of an optimized sorbent for direct HPTLC-FTIR on-line coupling

By use of a combination of silica gel 60 and a weak infrared-active, reflection-enhancing material, the quality of DRIFT (diffuse reflectance infrared Fourier transform spectroscopy) spectra was improved and the signal-to-noise ratio of adsorbed substances increased. Of the reflection-enhancers tested, magnesium tungstate proved the most suitable; the optimum proportion was 50%. For this type of layer the signal-to-noise ratios for caffeine, paracetamol, and phenazone were increased by factors of 3.4, 3.1, and 2.3, respectively, compared with those obtained on pure silica gel 60. Use of material of small mean particle-diameter and narrow particle-size distribution provided an almost optimum diffuse reflecting surface. addition of strongly reflecting metallic powders to the silica gel had the effect of increasing diffuse Fresnel reflection in particular; this did not, however, contain desired spectral information. The same effect was also achieved with reflecting sorbent supports. In contrast with aluminum-coated glass plates, normal glass plates absorb most of the LR radiation, a small proportion only being reflected. When aluminum-coated glass plates mere used as supports small fluctuations in layer thickness resulted in artefact peaks in the chromatograms, Layer thickness sensitivity was less for normal glass plates. A layer thickness of 100 mu m for a 50:50 (m/m) mixture of silica gel 60 and magnesium tungstate on a normal glass plate proved to be the most suitable combination. The chromatographic, analytical, and spectroscopic performance of the optimized sorbent layer were investigated using separation numbers, detection limits, and the quality of the DRIFT spectra of selected substances. addition of 50% magnesium tungstate resulted in sorbent layers with separation performance practically identical with that of silica gel 60 F-254s, even though the retention properties were different, By use of this optimized sorbent layer, detection limits were reduced by a factor of 3.7 for caffeine and 2.3 for both paracetamol and phenazone. In addition, the evaluable IR range was extended by approximately 100 wavenumbers to approximately 1270 cm(-1) in the 'fingerprint' range. This enabled the detection of mo additional caffeine bands which, if a conventional silica gel layer had been used, would hare been superimposed by the wide interference bands occurring at approximately 1340 cm(-1).