Optical Sectioning Raman Microscopy

Colombo

et al. 2015

Analyst

We compare several basic embodiments of a recently proposed and demonstrated micrometer-scale Spatially Offset Raman Spectroscopy (micro-SORS). Micro-SORS is a recently introduced analytical method for noninvasive characterisation of the chemical composition of subsurface, micrometre-scale-thick diffusely scattering layers at depths beyond the reach of conventional confocal Raman microscopy. The technique is applicable, for example, in nondestructive subsurface analysis of highly scattering stratified matrices such as painted layers in cultural heritage or in noninvasive analysis of stratified polymer systems or biological samples. Using Monte Carlo simulations, we analysed two defocusing variants of micro-SORS and a variant involving a full separation of illumination and collection zones on the sample surface. Both the penetration depth into the sample and relative enhancement of sublayer Raman signals were studied as a function of layer thickness and type of technique and their parameters. The model predicts that the most effective method by far is the variant with fully spatially separated illumination and collection zones. On the other hand, the defocusing micro-SORS, where both the laser and Raman collection zones are defocussed and overlapped, yielded the lowest performance although its key benefit lies in its simplicity as the concept can be practiced on existing conventional Raman microscopes without any modifications. A basic experimental verification of the theoretical findings contrasting two extreme modalities, the fully separated micro-SORS variant with the defocusing one, is also presented.

Section: Micro-sors Variantsmentioning

confidence: 99%

Comparison of key modalities of micro-scale spatially offset Raman spectroscopy

Colombo

et al. 2015

Analyst

“…5 The concept of scaled subtraction was first applied to confocal Raman microscopy with transparent samples by the Morris group. 7,8 Micro-SORS uses similar approaches but instead of direct imaging it utilises diffuse component of light.…”

Section: Introductionmentioning

confidence: 99%

Development of portable defocusing micro-scale spatially offset Raman spectroscopy

Botteon

et al. 2016

Analyst

We present, for the first time, portable defocusing micro-Spatially Offset Raman Spectroscopy (micro-SORS). Micro-SORS is a concept permitting the analysis of thin, highly turbid stratified layers beyond the reach of conventional Raman microscopy. The technique is applicable to the analysis of painted layers in cultural heritage (panels, canvases and mural paintings, painted statues and decorated objects in general) as well as in many other areas including polymer, biological and biomedical applications, catalytic and forensics sciences where highly turbid stratified layers are present and where invasive analysis is undesirable or impossible. So far the technique has been demonstrated only on benchtop Raman microscopes precluding the non-invasive analysis of larger samples and samples in situ. The new set-up is characterised conceptually on a range of artificially assembled two-layer systems demonstrating its benefits and performance across several application areas. These included stratified polymer sample, pharmaceutical tablet and layered paint samples. The same samples were also analysed by a high performance (non-portable) benchtop Raman microscope to provide benchmarking against our earlier research. The realisation of the vision of delivering portability to micro-SORS has a transformative potential spanning across multiple disciplines as it fully unlocks, for the first time, the non-invasive and non-destructive aspects of micro-SORS enabling it to be applied also to large and non-portable samples in situ without recourse to removing samples, or their fragments, for laboratory analysis on benchtop Raman microscopes.

“…). The concept of scaled subtraction was first applied to confocal Raman microscopy (CRM) with transparent samples by the Morris group …”

Section: Introductionmentioning

confidence: 99%

“…The concept of scaled subtraction was first applied to confocal Raman microscopy (CRM) with transparent samples by the Morris group. [8,9] The classical SORS spatial offset [6] is present in this arrangement only at the level of individual photons; each detected Raman photon can be tracked back to its originating laser photon. With enlarged illumination and collection zones, the exit point of a detected Raman photon can be spatially separated at sample surface from its originating laser photon when entering sample.…”

Section: Introductionmentioning

confidence: 99%

Contrasting confocal with defocusing microscale spatially offset Raman spectroscopy

Colombo

et al. 2015

J Raman Spectroscopy

The study compares and contrasts conventional confocal Raman microscopy/spectroscopy (CRM) with a recently developed micrometer scale defocusing spatially offset Raman spectroscopy (micro-SORS), a method providing a new analytical capability for investigating non-destructively the chemical composition of subsurface, micrometer-scale-thick diffusely scattering layers at depths beyond the reach of CRM. Because of close similarities between the two techniques and comparable embodiment of the instrumentations, but radically different interpretations of data, it is crucially important to recognise which type of method is pertinent to a specific measurement. The distinction comes principally from the nature of sample, whether turbid (micro-SORS measurement) or transparent (CRM measurement) on the spatial scale of the axial (z-)scan of the measurement. Which type of sample one deals with may not always be easily recognisable with micro-scale thick layers, and the study therefore also presents a simple method for suggesting whether CRM or micro-SORS methodology applies. This test relies on an axial (z-)scan performed through the sample in both the positive and negative directions from the normal, imaged sample surface position using conventional CRM instrument. The absence or presence of symmetry or asymmetry of the intensity profiles of measured Raman signals around the imaged sample surface position as a function of sample axial displacement then suggests which interpretation could apply. The study paves a way for the development of micro-SORS as a widely applicable analytical tool deployable on conventional Raman microscopes.