Development of a method based on laser-induced plasma spectrometry for rapid spatial analysis of material distributions in paper coatings

A rapid and low-cost analytical method for studying the material distributions of paper coatings has been developed. The method operates in the dimensional scale of a raster point in printing (≈0.1 mm ) and on the other hand it is capable of mapping areas the size of several square centimeters. The results have been used to explain functional properties of paper, especially visual appearance and printability. An instrument based on laser-induced plasma spectroscopy (LIPS) has been constructed. In this method a pulsed laser beam is focused onto the paper surface at irradiance greater than 0.1 GW/cm2. A small amount of material (10-100 ng), depending on energy of laser pulse and material, is ablated and excited by a single laser pulse to produce a plasma. The plasma emits an atomic emission spectrum which represents the elemental composition of the sample. Typically an area of 0.1 mm in diameter and 2 μm in depth is ablated by a single laser pulse from the paper surface. The depth profile of elemental compositions is obtained by consecutive measurements in the same XY-location. Larger areas have been analyzed by translating the sample with respect to the laser focus. In this way three dimensional elemental distribution maps have been constructed. In this study the LIPS-method has been utilized in the analysis of the properties characteristic of papers: coat weight, coating coverage, pigment compositions, binder/pigment-ratio, and the relative distribution of coatings in multilayer coatings. The filler distributions of uncoated papers have also been studied.