Correction method for absorption-dependent signal enhancement by a liquid-core optical fiber

Abstract

The enhancement of a dissolved chemical's Raman scattering by a liquid-core optical fiber (LCOF) geometry is absorption dependent. This dependence leads to a disruption of the usual linear correlation between chemical concentration and Raman peak area. To recover the linearity, we augmented a standard LCOF Raman spectroscopy system with spectrophotometric capabilities, permitting sequential measurements of Raman and absorption spectra within the LCOF. Measurements of samples with identical Raman-scatterer concentrations but different absorption coefficients are described. Using the absorption values, we reduced variations in the measured Raman intensities from 60% to less than 1%. This correction method should be important for LCOF-based Raman spectroscopy of sample sets with variable absorption coefficients, such as urine and blood serum from multiple patients.

Rapid and accurate determination of tissue optical properties using least-squares support vector machines

“…For the PLS regression analysis, the regression vectors for prediction are constructed by employing the number of loading vectors which corresponds to the minimum error in inner cross-validation in the calibration data set. In all cases, the number of factors is observed to vary between 4 and 7, which ensures that the rank of the calibration model is more than three times smaller than the size of the calibration data set [26]. Finally, the LS-SVM computations are performed using a LS-SVM MATLAB toolbox [27].…”

Multicomponent analysis using a confocal Raman microscope

“…LCOFs greatly enhance the number of scattered photons that can be collected over a given integration time, with the additional advantage of requiring a small sample volume in the order of 1 µL [23]. Building on the work of Altkorn et al [24,25], the experiments of Qi and Berger [19,23,26,27] demonstrated the application of LCOFs for multicomponent analysis. An illustration of the LCOF Raman system used by Qi and Berger in 2007 [27] for quantifying different analytes in blood serum and urine samples is given in Fig.…”

Chemical concentration measurement in blood serum and urine samples using liquid-core optical fiber Raman spectroscopy

“…1; detailed information can be found in previous publications. 20,24,26 Briefly, the biofluid sample resides within the LCOF and is illuminated sequentially by an 830 nm laser L and a broadband thermal source W through fiber OF 1 . Backscattered Raman and transmitted white-light spectra are recorded over the same wavelength range by a spectrograph and CCD detector.…”

“…In previous studies, we found the approaches gave similar values of s . 26 Here we calculated s by optimizing predictions of blood urea nitrogen (BUN) in blood serum samples and urine urea nitrogen (UUN) in urine samples, arriving at values of 0.005 and 0.007 mm Ϫ1 , respectively. The same optimized s value was then used for all the chemicals in the sample set; i.e., s was not a free parameter for each chemical.…”

“…The linearity between Raman intensity and chemical concentration is distorted in a LCOF, however, because the collection enhancement is absorption dependent. 25,26 To correct this effect, we recently augmented a LCOF system to measure a sample's absorption coefficient in the LCOF as well. 26 With this information and a characterization of scattering losses within the LCOF, a 60% variance in 890 cm Ϫ1 Raman peak areas from 20% aqueous ethanol was reduced to 1% in a set of samples with different concentrations of India ink.…”