Estimation of CDA content
A 1H NMR analysis of CDA was performed to estimate the content among the dimer acids mixture. To establish the feasibility of the proposed method, a quantitative analysis of mesitylene was also performed with maleic acid. A known quantity of maleic acid (0.5 mg) was added to the NMR solvent with various amounts of mesitylene. A standard curve of the mesitylene concentration was constructed by these results and it showed high linearity (from 0.1 to 40 lg of mesitylene), as presented in Fig. 6a. The ratio of the peak area between mesitylene and IS was determined by the peak of mesitylene at 6.90 ppm to the peak of IS at 6.20 ppm. A regression analysis of the data from the mesitylene shows an R2 value of 0.995, indicating a highly linear relationship between the mass and the average integral of the peak in the 1H NMR spectra. Based on this curve, we compared the quantity of mesitylene in the prepared sample and confirmed that the predicted value was consistent with the added amount. This result indicates that cyclic compounds can be reliably quantified using NMR. Furthermore, this curve could also be applied to quantify the CDA level in dimer acid mixtures due to the structural similarity of mesitylene with CDA. The spectra of various dimer acid mixtures are similar to those in Fig. 5 except for the peak intensity from the cyclic proton due to the different concentration of CDA. A series of samples with identical IS and various CDA concentrations were analyzed. Similar with the mesitylene case (Fig. 6a), the curve in Fig. 6b represents the integrated area of CDA specific peak (6.8 ppm) referenced to IS. The intensity ratio of CDA to IS was obtained by averaging the peak area of the CDA signal at 6.81 ppm to the specific signal of the IS at 6.20 ppm. A linear regression with a correlation coefficient of 0.995 implied an excellent linear relationship between the concentration of IS and the integrated area of CDA peaks, as presented in Fig. 6b