THz ATR Spectroscopy for Inline Monitoring of Highly Absorbing Liquids Amin Soltani 1 & Stefan F. Busch 1 & Patrick Plew 2 & Jan C. Balzer 1 & Martin Koch 1 Received: 14 January 2016 /Accepted: 2 May 2016 # Springer Science+Business Media New York 2016 Abstract We present a THz attenuated total reflection (ATR) setup which allows for inline measurements of highly absorbing liquids. As a proof of principle, we investigate a mixture of water and ground calcium carbonate (GCC) from 5 to 40 wt%. Inline measurements prove that our THz ATR setup allows for the distinction of various concentrations. As an example, we show inline THz ATR measurements for 30 to 40 wt% for GCC watery solution, as this concentration range is of technical relevance. We obtain a sensitivity better than 2 wt%. Keywords ATR THz- TDS . Inline measurements . Industrial application 1 Introduction Paper coating is an important step in the paper production process by adding value to the final product. It improves several characteristics of the end result, like surface uniformity of fiber- based paper, better white tint, and reduced transparency [1]. Here, we want to concentrate on the so-called coating color. The coating color is a mixture of minerals, including clay, ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), titanium dioxide (TiO 2 ), and talc [1]. The coating procedure poses a considerable percentage of the overall production costs in the paper industry. In order to save valuable resources and to keep production costs low, it is necessary to find a suitable inline technique to monitor the quality of the coating process. A promising technique to address this problem is THz time-domain spectroscopy (TDS) [2, 3]. There are already several examples for inline measurements in industrial applications [4–7]. Particularly, Banerjee et al. reported on THz TDS measurements for determining the moisture J Infrared Milli Terahz Waves DOI 10.1007/s10762-016-0285-6 * Amin Soltani soltanim@physik.uni-marburg.de 1 Faculty of Physics and Material Sciences Center, Philipps-Universität Marburg, Renthof 5, 35032 Marburg, Germany 2 Papiertechnische Stiftung, Pirnaer Straße 3, 01809 Heidenau, Germany