Terahertz time-domain spectroscopy (THz-TDS) is a promising non-destructive technique for probing dielectric and interfacial properties of complex materials. However, aqueous samples remain challenging because water-rich media strongly attenuate THz radiation. This work proposes a reflection-mode THz-TDS strategy for assessing dye-contaminated aqueous samples using a custom polytetrafluoroethylene (PTFE) liquid cell compatible with the internal reflection configuration of the spectrometer. Measurements are performed with a TDS1008 spectrometer operating over 0.05–4.5 THz and using the SHR reflection mount, where the THz beam is redirected toward the sample plane at a fixed angle of 30°. The custom PTFE liquid cell is positioned directly on the SHR sample plane so that its lower planar base acts as the interrogated surface. Following the reflection protocol, a baseline is first recorded with the shutter closed, then a metallic mirror reference is acquired at the same sample plane, and finally measurements are taken for the PTFE cell filled with deionized water and aqueous dye solutions of different concentrations under unchanged acquisition parameters. The analysis focuses on robust observables from the reflected waveform, including temporal shift, peak-to-peak amplitude, RMS amplitude, and frequency-domain magnitude variations. Preliminary results indicated measurable differences in peak-to-peak amplitude, temporal shift, and spectral magnitude between the reference medium and dye-contaminated aqueous samples, supporting the feasibility of reflection-mode THz screening with the proposed PTFE cell. This study is designed to evaluate whether measurable differences in peak-to-peak amplitude, temporal shift, and spectral magnitude can be observed between the reference medium and dye-contaminated aqueous samples.