Method for Measuring Absolute Optical Properties of Turbid Samples in a Standard Cuvette

Abstract

Many applications seek to measure a sample’s absorption coefficient spectrum to retrieve the chemical makeup. Many real-world samples are optically turbid, causing scattering confounds which many commercial spectrometers cannot address. R1C1Using diffusion theory and considering absorption and reduced scattering coefficients on the order of 0.01/mm and 1/mm, respectively, we develop a method which utilizes frequency-domain to measure absolute optical properties of turbid samples in a standard cuvette ( 45x10x10mm). Inspired by the self-calibrating method, which removes instrumental confounds, the method uses measurements of the diffuse complex transmittance at two sets of two different source-detector distances. We find: this works best for highly scattering samples (reduced scattering coefficient above 1/mm); higher relative error in the absorption coefficient compared to the reduced scattering coefficient; accuracy is tied to knowledge of the sample’s index of refraction. Noise simulations with 0.1 amplitude and 0.1=1.7mrad phase uncertainty find errors in absorption and reduced scattering coefficients of 4 and 1 , respectively. We expect that higher error in the absorption coefficient can be alleviated with highly scattering samples and that boundary condition confounds may be suppressed by designing a cuvette with high index of refraction. Further work will investigate implementation and reproducibility.

Calibration-free measurement of absolute optical properties of scattering samples in a standard cuvette

“…Previous work utilized this feature to develop a fit for µ a and µ s based on DR{ T } measurements. 5…”

“…II, Figure 1(a)(b)). 5 Config. I achieves source-detector distances (ρs) of 15 mm and 25 mm and Config.…”

“…We utilize a diffusion theory derived expression for the FD complex Transmittance ( T ) in a box geometry. 5,7 Considering a box with the size of a standard cuvette (i.e., 45 mm × 10 mm × 10 mm) (Figure 1) we generate forward data for a range of µ a and µ s within a transmittance based SC / DS arrangement. An example of the forward data generated with this diffusion theory model is found in Figure 2.…”

“…We introduced the Dual-Ratio of the T (DR{ T }) as an auto-calibrated data-type similar to SC / DS to create a measurement insensitive to optical coupling and instrumental drifts. 5 DR{ T } is defined as follows for the optode configurations in Figure 1:…”

“…1 In this work, we investigate a previously published method to measure absolute optical properties of small sample volumes, targeting the standard cuvette geometry (45 mm × 10 mm × 10 mm). 5 This is an extension of the work we presented where SC Frequency-Domain (FD) NIRS and the Dual-Slope (DS) technique with broadband Continuous-Wave (CW) NIRS were combined to achieve calibration-free absolute broadband measurements of µ a . 6 Herein we apply this method to the standard cuvette geometry allowing for easier optical assessment of various samples, biological and beyond.…”