## 3 Results and Discussion
### QCL Coupled to the RobustATR Assembly with and without Hollow-Core Fiber  
The developed sensor assembly has already been successfully used during previous studies [16]. Here, the RobustATR is coupled to a QCL (uniMir, mirSense, Palaiseau, France)  
Figure 4: Measurements were executed with the hollow waveguide coupled RobustATR assembly of (**a**) ethanol and (**b**) 1 w% tryptophan deposited at the IRE surface. Data were treated using a five-point FFT filter; (**c**) measurements were performed with the QCL directly coupled into the sensor assembly (i.e., without fiber coupling); no data treatment was used.  
pack aged within a miniature housing facilitating future portable sensing applications. As shown in Figure 2a, the laser is coupled via a hollow-core fiber to the developed sensor assembly and the detector is directly located at the distal end of the transducer. To test the feasibility of this setup, analytes with vibrational signatures around 1300 cm\({}^{-1}\) have been selected. As an example of environmentally relevant analytes, ethanol was selected as the model constituent. Although ethanol does not have its most characteristic peaks in this wavelength regime, it exhibits some broad features. Characteristic ethanol signatures in this region are the peaks at 1391 cm\({}^{-1}\)[54], which corresponds to the OH bending mode, and 1242 cm\({}^{-1}\)[54], which is associated with the CH\({}_{3}\) bending mode. To record the signal change between the background and the analyte, the analyte was added onto the IRE while data was recorded (see Figure 4a). Applying ethanol onto the crystal leads to a decrease of the signal output. However, the ethanol signal is still higher than the background noise at a value of approximately 0.03 V.  
As an example of a medically relevant analyte, tryptophan was used. The same measurement procedure was followed as for the ethanol measurements. Here, a significant dampening of the laser signal was also observed after applying the analyte onto the IRE.  
Regarding the level of noise during the measurements, two sets of experiments were used. In Figure 3, the laser was in operation during the entire measurement time (i.e., blank measurement, analyte measurement, and cleaning cycle), while in Figure 4, the laser was turned off after each measurement, yielding similar results. Using an Allan variance plot, the optimum averaging time for each measurement routine can be derived.  
While the optical setup used herein was already quite compact, an even more integrated version can be achieved if the laser is placed directly in front of the sensor assembly without additional coupling fiber. Using this setup without beam-shaping lenses in between (Figure 3b), the intensity shows the expected exponential decrease with distance. Placing the laser directly in front of the sensor assembly ensures that a sufficient fraction of the emitted radiation is directly coupled into the RobustATR assembly. Figure 4c shows an exemplary recording of the laser signal propagated through the device without analyte present at the IRE surface, and in presence of an analyte (i.e., either ethanol or methanol). Methanol has a broad vibrational absorption at 1346 cm\({}^{-1}\)[54], which is characteristic of the OH vibration.