Noise in Determination of Degree of Conversion of Composites using FTIR-ATR

FTIR is widely used in determining the degree of conversion (DC) of dental composites. FTIR with an attenuated total reflectance attachment (ATR) greatly simplifies the sample operation. It also extends the measuring capability onto the top surface as well as into the deep bulk (by grinding away the surface). Unfortunately extraneous absorbance bands, i.e. “noise,” often appear in the absorbance region of aliphatic double bond, rendering its quantification inaccurate. OBJECTIVES: To examine the origin of the noise and find an effective way to minimize it. METHODS: Uncured and cured dental composites were placed on the ATR prism and their spectra were taken. The peak height was used to represent the double bond concentration. RESULTS: Only the FTIR spectra of cured composites showed evidence of noise, the absorbance of which appeared consistently at the same frequencies. Similar absorbance bands were also observed in the spectra of other solids, becoming stronger with hydrophilic materials. For this reason it was hypothesized that the water adsorbed on the solid was the source of noise. Based on the finding, a background subtraction method was developed, which removes the absorbance bands of water in the wave number region of interest.  The following is a DC comparison of a light cured dimethacrylate resin with or without the subtraction of a background spectrum of SiO₂.

CONCLUSIONS: The noise observed in the spectra is the absorbance bands of adsorbed water. Appropriate background subtraction can reduce or even remove the noise, yielding an improvement in the accuracy of FTIR-ATR measurements.