Dependence of signal on depth in transmission Raman spectroscopy.

Recently, transmission Raman spectroscopy has been shown to be a valuable tool in the volumetric quantification of pharmaceutical formulations. In this work a Monte Carlo simulation and experimental study are performed to elucidate the dependence of the Raman signal on depth from the viewpoint of probing pharmaceutical tablets and powders in this experimental configuration. The transmission Raman signal is shown to exhibit a moderate bias toward the center of the tablets and this can be considerably reduced by using a recently developed Raman signal-enhancing concept, the "photon diode.

Confocal Raman microscopy: common errors and artefacts.

Confocal Raman microscopy is a powerful tool for research and analysis in the chemical, materials and life sciences, particularly for non-destructive depth profiling of transparent systems. Unfortunately, many Raman microscopes are not optimally configured for this purpose, and so yield unnecessarily low signal-to-noise spectra with poor spatial resolution and grossly incorrect depth scales. This review discusses the aberrations and artefacts that can arise and describes how these can be avoided by adhering to a few basic principles that are well known to optical microscopists but which were largely ignored in the spectroscopic community for many years.

Measurement of spatial resolution and sensitivity in transmission and backscattering Raman spectroscopy of opaque samples: impact on pharmaceutical quality control and Raman tomography.

A practical methodology is described that allows measurement of spatial resolution and sensitivity of Raman spectroscopy in backscatter and transmission modes under conditions where photon migration dominates, i.e., with turbid or opaque samples.

Temporal and spatial resolution in transmission Raman spectroscopy.

Picosecond time-resolved transmission Raman data were acquired for 1 mm thick powder samples of trans-stilbene, and a Monte Carlo model was developed that can successfully model the laser and Raman pulse profiles. Photon migration broadened the incident (approximately 1 ps) probe pulse by two orders of magnitude. As expected from previous studies of Raman photon migration in backscattering mode, the transmitted Raman pulse was broader than the transmitted laser pulse and took longer to propagate through the sample.

Preliminary investigations into macroscopic attenuated total reflection-fourier transform infrared imaging of intact spherical domains: spatial resolution and image distortion.

This paper describes preliminary investigations into the spatial resolution of macro attenuated total reflection (ATR) Fourier transform infrared (FT-IR) imaging and the distortions that arise when imaging intact, convex domains, using spheres as an extreme example. The competing effects of shallow evanescent wave penetration and blurring due to finite spatial resolution meant that spheres within the range 20-140 microm all appeared to be approximately the same size ( approximately 30-35 microm) when imaged with a numerical aperture (NA) of approximately 0.2.

The influence of out-of-focus sample regions on the surface specificity of confocal Raman microscopy.

This paper considers the quantitative implications of out-of-focus regions on the lateral and depth resolution of Raman microscopy, with special regard for the surface specificity of the technique. It builds on work that has recently appeared in the literature which shows that with transparent samples, signals can originate throughout a large extended illumination volume, even though most of this region is out of focus with regard to the confocal aperture. This gives rise to weak but readily detectable spectral contributions from regions that are tens of micrometers from the point of tightest focus, an effect that is easily demonstrated if the laser is focused far above the sample surface.

Optimizing depth resolution in confocal Raman microscopy: a comparison of metallurgical, dry corrected, and oil immersion objectives.

Spherical aberration is probably the most important factor limiting the practical performance of a confocal Raman microscope. This paper suggests some simple samples that can be readily fabricated in any laboratory to test the performance of a confocal Raman microscope under realistic operating conditions (i.e.

Characterization of TiO2 nanoparticles surface modified with aluminum stearate.

This paper uses measurements of adsorption and vibrational spectra (DRIFTS, ATR, and Raman) to characterize TiO2 (rutile) nanoparticles that have been surface treated with aluminum and stearate, "aluminum stearate". From these measurements, we have developed a model of titania particles covered by patches of "alumina". Vibrational spectra, particularly the spectra of the carboxylate headgroups, show that the stearate then adsorbs on both the titania and the alumina.

Photon migration in Raman spectroscopy.

Monte Carlo simulation has been applied to study time-resolved Raman and Tyndall photon migration in opaque samples under isotropic and forward scattering conditions. For isotropic scattering, Raman and Tyndall intensities are predicted to decay according to t((1-n)) and t(-n), respectively, where the value of n depends on the ratio of the optical collection aperture to the mean scattering length. The simulation correctly reproduced the analytical results of n = 3/2 and n = 5/2 for a point source in infinite and semi-infinite media, respectively.