Robust and fast absolute quantification of a colored wood surface coating by scanning densitometry

Abstract A scanning densitometry-based method was used to determine absolute surface concentrations and homogeneity of a spray-coating treatment with a model dye, 5,8-dihydroxy-[1,4]-naphthoquinone (DHNQ), on hardwood veneers (Beech, Fagus sylvatica; Birch, Betula pendula; Oak, Quercus alba). An automatic TLC sampler sprayed accurately known amounts of several concentrations of DHNQ as rectangles on the surface. Scanning was then used to acquire the color intensities of each rectangle. After an adequate data treatment, three models – Michaelis-Menten and two logarithmic – were selected to fit the obtained intensities to the deposited surface concentrations. Based on the quality of fit and the residuals, the logarithmic model is examined in detail in this work. The method was validated by comparing expected concentrations to back-calculated ones, and produced correct results with acceptable error within ±20%. The applicability of the method was shown by a sprayed specimen. By comparing our method to two widely applied techniques, CIELAB color system and x-ray fluorescence, we demonstrated that densitometry is advantageous as it measures an absolute surface concentration, also of coatings containing atoms lighter than silicon. In production, densitometry can be adapted from current technologies and would quickly control the quality of the coated wood, or any other surface, with good sensitivity.