Recently the essential amino acid tryptophan has attracted attention in cancer research, as its metabolism regulates antitumor immune responses and tumor-intrinisic properties. Measurement techniques to determine tryptophan concentrations of aqueous solutions are therefore vastly important for ongoing research in this field. Recently, Terahertz spectroscopy has illustrated its high potential to be utilized for the characterization of bio-crystals and bio-molecules. We have developed a method to detect and quantify tryptophan based on the parallel-plate waveguide (PPWG) technology together with a commercially available terahertz time domain spectroscopy (TDS) system called “T-SPECTRALYZER F” providing a spectral bandwidth from 0.1 THz to 5 THz. As Terahertz waves are strongly absorbed by water, a measurement of aqueous solutions is a challenging task. In our setup, parallel-plate waveguides are used to detect low tryptophan concentrations, in principle, in solution. Drop-casting the solution into the waveguide forms a dry homogeneous film after evaporation of the solvent. A spectroscopic analysis of the transmission spectrum of the waveguide allows for a determination of the tryptophan concentration as the detection limit is drastically improved by the use of waveguides. In order to increase the detection sensitivity of this measurement technique the terahertz setup was encapsulated in a dry air box to reduce water vapor effects. Here we introduce the working mechanism of “TSPECTRALYZER F” and present the spectral evaluation procedures applied. Finally, we show the improvement of the detection sensitivity using a terahertz time-domain spectroscopy system together with PPWG technology.