Interestingly, the calibration curve of glucose, unlike that of Na2HPO4, does not converge with the y axis at 0 value. One possible explanation is the adsorption of analyte to the ATR crystal. Since the crystal used here is silicon, a thin layer of oxide will form spontaneously on the side contacting with the analyte, which will therefore interact with the analyte. This was the case for water as shown in our previous study, where the broad peak of O-H stretching centered at lower frequency compared to standard spectrum from NIST library5. Consequently, because D-glucose molecule is rich in hydroxyl group, the molecule can interact wit the surface via hydrogen bond.