Abstract
The purpose of the present study was to visualize the sorption dynamics of small compounds, such as propylene glycol (PG) in cellulose acetate (CA) film, by deuterium (d) labeling-aided confocal Raman spectroscopy (CRM). Substitution of hydrogen atoms in the target molecule with deuterium caused a marked shift of C-H bond-related Raman bands to low wavenumbers, while the number of deuterium did not affect the magnitude of the shift. Raman bands derived from the stretching vibration of C–H near 2900 cm−1 for PG and ethanol were shifted to approximately 2100 cm−1 for PG-d6 and ethanol-d5 in the silent region of the CA Raman spectrum. When PG-d6 was dissolved in glycerol triacetate (GTA), the observed Raman intensity ratio at 2123 cm−1 of PG-d6 against 1739 cm−1 of GTA (C=O bond-related) showed a linear relationship between the molar and intensity ratios, indicating that the observed Raman intensity can be used for quantitative assay of the target in polymer film. The depth profiling experiments by CRM revealed that the distribution (or sorption) of PG-d6 in CA film was successfully visualized as a function of Raman band at the characteristic 2123 cm−1 intensity ratio.
Highlights
The purpose of the present study was to visualize the sorption dynamics of small compounds, such as propylene glycol (PG) in cellulose acetate (CA) film, by deuterium (d) labeling-aided confocal Raman spectroscopy (CRM)
It was found that the deuterium substitution of PG, namely PG-d6 and -d8, allowed a significant Raman shift from the C-H stretching vibration region of 2700–3100 cm−114 to the silent region of 2000–2400 cm−1 for CA (Fig. 2A)
For the discriminant CRM visualization of CA polymer film from the targeting penetrants in this study (Fig. 1), a candidate Raman band would be 1739 cm−1 by stretching vibration of C=O ester bonds[17] (Fig. 2A). This suggests that the sorption dynamics of small penetrants in polymer films may be evaluated by their own characteristic Raman bands
Summary
The purpose of the present study was to visualize the sorption dynamics of small compounds, such as propylene glycol (PG) in cellulose acetate (CA) film, by deuterium (d) labeling-aided confocal Raman spectroscopy (CRM). A matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS)[8] method was applied for sorption visualization, and the local distribution of penetrants in laminated films was successfully visualized. The non-destructive MALDI-IMS visualization, still has serious disadvantages due to limited targets with low volatility and high ionization efficiency in the mass spectrometry (MS) vacuum chamber. By considering the importance and benefit of non-destructive visualization of volatile flavor compounds in film polymers for food quality evaluation, a confocal Raman spectroscopy (CRM) was applied to a sorption study for the first time, since the CRM technique requires no molecular properties, such as less volatility and fluorescence, for targeting volatile flavors.
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