Incorporating Non-NIR Absorbing Agent into Packed Powder Samples in Diffuse Reflectance NIR Measurement to Improve Representation of Sample Composition and Accuracy of Concentration Determination.

A strategy to improve accuracy of near-infrared (NIR) quantitative analysis of powder samples by alleviating the subsampling problem is demonstrated. The approach is to increase spectroscopic sampling volume for more accurate representation of its composition by filling the void space in a packed sample with liquid to reduce differences in refractive index between particle and unoccupied space. By this way, NIR radiation can propagate deeper into the packed powder due to decreased degree of reflection at the interfaces, leading to greater sampling volume. Perfluorohexane (PFH) was chosen as the space-filling liquid since it does not readily absorb NIR radiation and does not disintegrate sample particles due to its strong hydrophobicity. For evaluation, binary mixtures composed of ambroxol and lactose at different concentrations were prepared, and their diffuse reflectance spectra were collected in two ways: conventional direct powder measurement and PFH-incorporated powder measurement. The accuracy of ambroxol concentration determination improved in the PFH-incorporated measurement due to acquisition of more composition-representative spectra by the greater sampling volume. Also, the sample morphology did not change in PFH medium based on XRD analysis. In parallel, Monte Carlo simulation was executed to track NIR photons in the samples and explain the improved accuracy. Improvement in accuracy was also realized when analyzing real pharmaceutical samples containing an active pharmaceutical ingredient in granular form, demonstrating expandability of the proposed scheme.

Influence of interfering co-appearing container peaks on the accuracy of direct quantitative Raman measurement of a sample in a plastic container.

The axially perpendicular offset (APO) scheme was previously demonstrated as a versatile scheme able to minimize or eliminate the glass background in the direct and non-sampling Raman measurement of an ethanol sample housed in a glass bottle. Alternatively, when directly analyzing a sample housed in a plastic container, another typical container yielding strong Raman peaks itself, the Raman peaks of both the container and the housed sample are unavoidably present together in a collected spectrum. Therefore, a crucial issue to investigate under this situation is how the magnitude of the co-appearing container peaks influences the accuracy for quantitative analysis of the housed sample. For the evaluation, a non-sampling Raman spectroscopic measurement of the urea concentration in a urea gel housed in a circular polypropylene (PP) container was attempted by employing two axially perpendicular offset (APO) schemes with detection windows of different sizes (25.4 and 10.0 mm, referred to as the wide-window APO (WW-APO) and narrow-window APO (NW-APO), respectively), and transmission and back-scattering schemes incorporating a 25.4 mm detection window. The intensity ratios between the container and urea peaks in the collected spectra were different depending on the adopted measurement scheme. The intensity ratio was greatest (smallest container peak) in the NW-APO measurement due to the narrowed detection window, making the generated container Raman photons at the side-wall less detectable to the bottom-positioned detector. A spectral acquisition scheme allowing the maximal suppression of the container peaks, while still maintaining the sample features, was a key requirement to secure an accurate measurement of the sample concentration. In addition, a Monte Carlo simulation was used to visualize the distributions of the container and urea photons inside the sample-housed container.

Fast and non-destructive Raman spectroscopic determination of multi-walled carbon nanotube (MWCNT) contents in MWCNT/polydimethylsiloxane composites.

A versatile Raman spectroscopic method to determine the contents of carbon nanotubes (CNTs) in CNT/polydimethylsiloxane (PDMS) composites is demonstrated, and important issues directly related to the accuracy of the measurement have been investigated. Initially, Raman microscopic mappings over an area of 6.0 × 6.0 mm2 were carried out on CNT/PDMS composites, which revealed the existence of the partial localization of CNTs on a microscopic scale. Therefore, a laser illumination scheme covering a large sample area of 28.3 mm2 was employed to acquire a sample spectrum representative of the whole CNT concentration. The peak area ratio between the CNT and PDMS peaks clearly varied with the CNT concentration, whereas the reproducibility of measurements was degraded for the composites containing more than 3.0 wt% CNTs because of the decreased Raman sampling volume arising from the absorption of laser radiation by the CNTs. The laser penetration depth was semi-quantitatively investigated by observing the spectra of thin-sliced samples collected by positioning a Teflon disk behind the sample, and Monte Carlo simulations were employed to examine the internal photon propagation as well as explain the experimental observation. In summary, the fundamental issues affecting the Raman measurement of the CNT containing polymer matrix have been clearly addressed, and the finding here will be a beneficial basis for successful Raman spectroscopic analysis of different CNT-containing composites.

A mechanical pencil lead-supported carbon nanotube/Au nanodendrite structure as an electrochemical sensor for As(iii) detection.

A mechanical pencil lead (MPL), an easily obtainable carbon-based material with a consistent size, was used as a frame to construct an MPL-supported carbon nanotube/Au nanodendrite (MPL-CNT/AuND) sensor through simple electrodeposition of Au onto the MPL in the presence of CNTs. A nanodendrite structure was adopted to ensure large numbers of active electrochemical sites because of its hierarchical structure with well-aligned terraces; the CNTs were used to firmly adhere the fabricated Au nanodendrites to the MPL surface to ensure ruggedness. The MPL-CNT/AuND structure was used to measure As3+ samples in a concentration range from 0.5 to 80 ppb using anodic stripping voltammetry (ASV). The variation in peak intensities was linear (R2: 0.997), and the limit of detection (LOD) was 0.4 ppb. The average relative standard deviation (RSD) of the peak intensities from the voltammograms of each sample collected using three separately prepared MPL-CNT/AuNDs was 8.7%, thereby demonstrating good sensor-to-sensor reproducibility. Furthermore, when three As3+ samples prepared in tap water were measured, the accuracy was maintained without noticeable degradation and the response was steady up to 50-cycle measurements.

Acquisition of reproducible transmission near-infrared (NIR) spectra of solid samples with inconsistent shapes by irradiation with isotropically diffused radiation using polytetrafluoroethylene (PTFE) beads.

A bead-incorporated transmission scheme (BITS) has been demonstrated for collecting reproducible transmission near-infrared (NIR) spectra of samples with inconsistent shapes. Isotropically diffused NIR radiation was applied around a sample and the surrounding radiation was allowed to interact homogeneously with the sample for transmission measurement. Samples were packed in 1.40 mm polytetrafluoroethylene (PTFE) beads, ideal diffusers without NIR absorption, and then transmission spectra were collected by illuminating the sample-containing beads using NIR radiation. When collimated radiation was directly applied, a small portion of the non-fully diffused radiation (NFDR) propagated through the void space of the packing and eventually degraded the reproducibility. Pre-diffused radiation was introduced by placing an additional PTFE disk in front of the packing to diminish NFDR, which produced more reproducible spectral features. The proposed scheme was evaluated by analyzing two different solid samples: density determination for individual polyethylene (PE) pellets and identification of mining locality for tourmalines. Because spectral collection was reproducible, the use of the spectrum acquired from one PE pellet was sufficient to accurately determine the density of nine other pellets with different shapes. The differentiation of tourmalines, which are even more dissimilar in appearance, according to their mining locality was also feasible with the help of the scheme.

Development of a particle-settling tolerant transmission Raman scheme for analysis of suspension samples.

We have demonstrated a simple and effective strategy, the so-called axial illumination scheme, that is able to obtain representative Raman spectra of suspension samples with minimal influence from internal particle settling. In a partially settled suspension sample, since particle concentrations at given points throughout the sample differ, the acquisition of Raman spectra representative of the entire sample composition is critically important for accurate quantitative analysis. The proposed scheme used axially irradiated laser radiation in the same or opposite direction of settling, thus allowing laser photons to migrate through the settling-induced particle-density gradient formed in the suspension and to widely interact with particles regardless of their settled locations. Therefore, transmitted Raman signals gathered opposite to the illumination could be more representative of the overall suspension composition even with partial settling. In this study, the performance of axial illumination schemes (TB (Top-to-Bottom) and BT (Bottom-to-Top) illumination) was evaluated for the determination of the aceclofenac (a non-steroidal anti-inflammatory drug) concentration in suspensions. Although the spectral features exhibited minute variations during settling, settling did not significantly degrade the accuracy of the concentration determination, thereby indicating effective acquisition of settling-tolerant Raman spectra. In addition, the characteristics of photon migration in a partially settled suspension sample were studied using a simulation based on Monte-Carlo method.

Diffuser-incorporated transmission NIR measurement for reliable analysis of packed granular samples.

A diffuser-incorporated transmission near-infrared (NIR) scheme that enables direct spectral collection of packed granular samples with reliable sample representation and reproducibility has been demonstrated. The analytical utility of this method has been evaluated for the determination of polyethylene (PE) pellet density and the discrimination of the geographical origin of rice samples. Based on the preliminary observation of transmission spectral features acquired from spherical polyoxymethylene (POM) packings composed of different particle sizes as well as packing thickness, a portion of the radiation was propagated through the void space in the packing without fully interacting with the POM pellets. This type of radiation, so-called non-fully interacted radiation (NFIR), adversely affected the sample representation as well as the reproducibility of transmission measurements. To maximize the interaction of NIR radiation with granular samples, a polytetrafluoroethylene (PTFE) diffuser was positioned in front of the sample packing to introduce isotropically diffused radiation into the sample. This diffuser-incorporated scheme resulted in highly reproducible transmission spectra for both packed granular samples. Consequently, the density determination of PE pellets as well as discrimination of rice samples according to geographical origin was more accurate using the proposed scheme.

Development of hydrophobic surface substrates enabling reproducible drop-and-dry spectroscopic measurements.

We investigated several spectroscopic substrates with hydrophobic surfaces that were able to form reproducible droplets of aqueous samples for reliable high throughput drop-and-dry measurements. An amine-coated substrate, a polytetrafluoroethylene (PTFE) disk, and a perfluorooctyltrichlorosilane (FTS) coated substrate were prepared and initially evaluated for use in the determination of fat concentrations in milks using near-infrared (NIR) spectroscopy. Since the dried milk spots were not compositionally uniform due to the localization of components during sample drying, NIR spectra were collected by fully covering each spot to ensure a correct compositional representation of the sample. The amine-coated substrate yielded more reproducible dried milk patterns because its hydrophobicity was optimal for loading an appropriate amount of milk with decreased component localization after drying. The relative standard deviation (RSD) of the absorbance at 4330cm(-1) was 1.0%, thereby resulting in the more accurate determination of fat concentration. In addition, infrared (IR) spectroscopic discrimination between wild and transgenic tobaccos using their extracts was attempted. The extracted metabolites had a low concentration, so an FTS-coated CaF2 substrate that maximized sample loading was used to improve measurement sensitivity and produce reproducible droplets. The RSD of the absorbance at 1070cm(-1) was only 0.8%. Our strategy produced droplets that had consistent sizes and provided reproducible IR spectral features, which enabled the differentiation between wild and transgenic tobacco groups in the principal component (PC) score domain.