Spatial offset Raman spectroscopy

C. Ricci, C. Eliasson, N.A. MacLeod, P.N. Newton, P. Matousek and S.G. Kazarian, Characterization of genuine and fake artesunate anti-malarial tablets using Fourier transform infrared imaging and spatially offset Raman spectroscopy through blister packs. Anal. Bioanal. Chem., 389, 1525-1532 (2007). 

C. Eliasson and P. Matousek, Noninvasive authentication of pharmaceutical products through packaging using spatially offset Raman spectroscopy, Arml Chem., 79, 1696-1701 (2007). 

This section will outline the developments of deep Raman spectroscopy from the use of time gating to spatially offset Raman spectroscopy to transmission Raman a sequence of increasing practical probing depth as advances have been made. This is counterbalanced by a reduction in depth selectivity with each new technique. An exploration of the potential use of deep Raman for breast cancer diagnostics will be used to illustrate the potential here. 

Spatially Offset Raman Spectroscopy (SORS) for Deep Probing of Calcifications... 

N. Stone, R. Baker, K.D. Rogers, A.W. Parker, P. Matousek, Future possibilities in the diagnosis of breast cancer by subsurface probing of calcifications with spatially offset Raman spectroscopy (SORS). Analyst 132, 899-905 (2007)... 

Spatially offset Raman spectroscopy (SORS) Conventional Raman Spectroscopy is limited to the near-surface of diffusely scattering objects and to the first few hundred micrometers depth of surface material. Spatially Offset Raman Spectroscopy (SORS) is a variant of Raman Spectroscopy that allows highly accurate chemical analysis of objects beneath obscuring surfaces. This is done by making at least two Raman measurements one at the surface and one at an offset position of t3q>ically a few millimeters away. To do this without using an offset measurement would be severely restricted by photon shot noise generated... 

Although Raman spectroscopy does not employ absorption of infrared radiation as its fundamental principle of operation, it is combined with other infrared spectroscopies into a joint section. Results obtained with various Raman spectroscopies as described below cover vibrational properties of molecules at interfaces complementing infrared spectroscopy in many cases. A general overview of applications of laser Raman spectroscopy (LRS) as applied to electrochemical interfaces has been provided [342]. Spatially offset Raman spectroscopy (SORS) enables spatially resolved Raman spectroscopic investigations of multilayered systems based on the collection of scattered light from spatial regions of the samples offset from the point of illumination [343]. So far this technique has only been applied in various fields outside electrochemistry [344]. Fourth-order coherent Raman spectroscopy has been developed and applied to solid/liquid interfaces [345] applications in electrochemical systems have not been reported so far. 

Ma, K., et al. (2011). In vivo, transcutaneous glucose sensing using surface-enhanced spatially offset Raman spectroscopy multiple rats, improved hypoglycemic accuracy, low incident power, and continuous monitoring for greater than 17 days. Analytical Chemistry, 83(23), 9146-9152. 

Matousek, P. et al (2005) Subsurface probing in diffusely scattering media using spatially offset Raman spectroscopy. Appl Spectrosc., 59 (4), 393-400. 

Xie, H.N. et al (2012) Tracking bispho-sphonates through a 20 mm thick porcine tissue by using surface-enhanced spatially offset Raman spectroscopy. Angew. Chem. Int. Ed., 51 (34), 8509-8511. 

The basic concept is known as spatially offset Raman spectroscopy (SORS) [17], and it utilizes the differences in the spatial distribution of Raman photons emerging at the surface from different depths of the probed sample. In this approach, Raman spectra are collected from regions on the sample surface that is spatially offset by different amounts from the point of laser incidence (Figures 13.1 and 13.2). Such spectra contain varying relative Raman contributions from layers located at different depths within the sample. This difference is brought about by the wider lateral diffusion of photons emerging from greater depths and the effect of photon loss at sample-to-air interface [3,11,12]. 

Stone and co-workers reported surface-enhanced spatially offset Raman spectroscopy for detecting NPs deeply in tissue (Stone et al. 2010). Ag NPs labelled with RRMs were injected into the centre of fresh porcine tissue. SERS spectra from these Ag NPs were obtained by spatially offset Raman spectroscopy from a depth of 4.5-5 cm. Later, the same group improved this approach for multiplexed detection and imaging using four different commercial SERS probes (Stone et al. 2011). Each NP suspension was injected into one of the comers of a 10 mm square, from where multiplexed spatially offset Raman spectroscopy imaging of the NPs was obtained according to the characteristic bands of each RRM. 

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