Which is the Better Internal Standard for FTIR Conversion Analysis?

Method: Different blends were prepared mixing BisGMA/BISEMA and BisGMA/TEGDMA at the proportions 0 to 100%, in weigth. Canphoroquinone and ethyl-4-dimethylamino-benzoate were used to permit the light activation of blends. Degree of conversion was evaluated using FTIR equipped with an attenuated total reflectance (ATR) device. Sample was dispensed onto ATR crystal and light activated by 40s. Conversion was calculated by the relation of the intensity of the peak 1640 cm^-1 to the intensity of different internal standards peaks (1715cm^-1, 1608cm^-1 and 1582cm^-1), before and after polymerization. Data was compared by ANOVA at 5% of significance, linear regression was used to analyze the conversion behavior within different monomers ratios.

Result: The degree of conversion measured range from 33 to 79%. DC obtained from the aromatic peaks showed similar values, superior to the DC measured by carbonyl peak as internal standard. Degree of conversion diminished with the increases of viscosity of sample. Linear regression showed similar pattern of DC for blends in which all monomers contain aromatic rings independent of internal standard used. Groups containing monomers with aliphatic and aromatic chemical bonds showed different patterns of DC depending of the peak used as standard, measures from carbonyl peak had less variation with the viscosity increases, compared with data measure by aromatic peaks.

Conclusion: For samples containing monomers with aromatic rings the internal standard does not influence the DC. To blends containing high concentrations of monomers without aromatic rings, the use of carbonyl peak as internal standard could be avoid.