SUMMARY
Precision of spectroscopic methods were frequently affected by the identity of the inhomogeneous materials, especially for direct scanning. This research aimed to investigated effects of waxy types, naturally founded on cane surface, on spectral characteristic. A portable Vis/SWNIR instrument with interactance mode across wavelength of 570-1031 nm were used for direct scanning on cane stalk. Principle component analysis (PCA) was applied to examine the differences of spectra scanned from 180 samples including 3 types of waxy type; white, black, and mixed black and white. Seven widespread pretreatments were employed to reduce the effect of waxy types. Results show that spectra of samples with each waxy type was separated in groups and SNV pretreatment gave the best results but was not able to eliminate the effect compared to the wax-removed samples. Meanwhile, the standard deviation of absorbance values, at the wavelength of 760, 904 and 970 nm of 3 samples, was used for assessing the repeatability and reproducibility. The samples with removing waxy cover provided lower the standard deviation of absorbance values of spectra than the best pretreated spectra using standard normal variate (SNV) of the samples without removing waxy cover by one to six times. Thus, the waxy material on cane surface should be removed before collect spectra.