###### Abstract
Tryptophan is a metabolite of serotonin, which can be synthesized in the gut and brain, where it is relevant for several disorders including growth, mood, behavior, and immune response [24]. Furthermore, it is under consideration as a biomarker for depression, which usually appears together with other mood disorders such as anxiety [25]. Once synthesized, tryptophan is bound to plasma albumin, and part of it is taken up in the brain. Foodstuffs containing a large number of proteins, such as cheese, fish, or meat, therefore also contain tryptophan [26]. In both scenarios--environmental analysis and sensing in health-related applications--the direct analysis of selected tracer molecules in aqueous environmental systems and human secretion (e.g., saliva or urine), online and close to real-time measurements are demanded. In harsh in-field conditions (e.g., elevated pressure and/or temperature, vibrations due to portable use, etc.) only few studies have reported the utility of IR-ATR techniques at elevated pressure and temperature conditions [27, 28], e.g., based on compact Fourier transform infrared (FTIR) devices for various molecular monitoring scenarios [5, 29, 30, 31, 32, 33, 34]. Devices based on FTIR systems use broadband emitters enabling access to the entire MIR range (i.e., usually 2.5 to 25 \(\upmu\)m) [35, 36, 37, 38]. Although compact FTIR systems are commercially available, the potential for miniaturization is limited. In turn, IR sensors based on laser light sources cover only a limited spectral window, but facilitate exceedingly compact device dimensions. Therefore, some IR sensors currently utilizing broadly tunable quantum and interband cascade lasers (QCLs and ICLs) provide an improved signal-to-noise ratio (SNR) along with high energy density within a distinct spectral emission window [39, 40, 41, 42]. Fabry-Perot lasers--as applied in the present study--are based on an optical cavity placed in the middle of the waveguide, and provide a broad gain bandwidth along with significant emission powers frequently coupled to external tuning mechanisms [42, 43]. For ensuring the emission stability, such lasers are commonly integrated with a thermoelectric cooler (TEC) inside a sealed high-heat load (HHL) package for suppressing mode hopping during wavelength tuning via controlling of the operating temperature of the chip. For analytical application scenarios, these lasers are usually operated in a quasi-continuous wave (QCW) modality, delivering several tens of milliwatts of output power at room temperature. Furthermore, they exhibit low power consumption, and can therefore be integrated into portable sensor devices. Attenuated total reflection (ATR) is a sensing concept ensuring reproducible and intimate interaction of photons with a sample at the interface between a high-refractive-index waveguide and the--usually liquid or solid--sample. If light propagates within an internal reflection element (IRE)--here, a zinc selenide (ZnSe) crystal--an evanescent field is generated, which exponentially decays in intensity into the surrounding medium (i.e., sample) with a wavelength-dependent penetration of up to several micrometers [44, 45, 46]. In the present study, we report a compact and exceedingly robust IR-ATR sensor concept termed RobustATR, which was coupled to a quantum cascade laser light source, facilitating future studies in real-world in-field application scenarios. This sensor concept combines the advantage of IR-ATR spectroscopy with robust optical coupling elements based on substrate-integrated hollow waveguides ensuring permanent alignment of light source, transducer (i.e., ATR sensing interface) and detector, facilitating applications in clinical analysis as well as in environmental monitoring.