## 2 Materials and Methods
Optical components: fiber collimator (C1, C2), parabolic mirror (PM), mirror (M), photo diode (PD1, PD2, PD3). card (National Instruments PCIe-6351, Austin, TX, USA) connected to a personal computer. In order to minimize the influence of the unavoidable adsorption of water on surfaces, appropriate materials were used and constant experimental conditions were ensured. For example, hydrophobic PTFE tubing was used for the connection between the levitator chamber and the MPC, and the connections were kept as short as possible. All further, not explicitly mentioned optics and glass fibers were optimized for the specific wavelength. For the measurements, which are all carried out at ambient pressure, the levitation cell is first set to the desired drying temperature at a constant nitrogen flow rate of 1.0 nL/min until a stationary condition regarding the cell temperature and remaining amount of water vapor is reached. Simultaneously, the Perspex chamber is purged with nitrogen at a flow rate of 1.0 nL/min. At reaching stationary conditions a measurement of the background signal \(I_{0}(v)\) is carried out, since the drying gas flow is not completely free of water. Subsequently, a droplet is injected by hand into the acoustic levitator using a microvolume syringe, and the measurement signal \(I(v,t)\) is recorded continuously over the whole drying process. In order to obtain the water mole fraction of the evaporating droplet from \(I(v,t)\), it is necessary to correct the absolute measured water mole fraction by the corresponding proportions of (i) the remaining water mole fraction at the beginning of the measurement (background signal \(I_{0}(v)\)) and (ii) the water mole fraction in the Perspex chamber in front of the MPC (reference signal \(I_{ref}(v,t)\)). The background water mole is a constant value determined by 30 consecutive averaged background signals \(I_{0}(v)\) prior to a droplet drying experiment, whereas the water mole fraction in the Perspex chamber is continuously co-determined via the reference signal \(I_{ref}(v,t)\). The injection volume of the pure water and protein solution droplets is about 4 uL, which is determined gravimetrically with an accuracy of 1.7%. For the drying of the protein solution droplets, the model protein lysozyme was used. This protein was chosen because it has already been used several times in the literature to investigate different drying processes [28; 29]. The samples were freshly prepared by dissolving a corresponding amount of lysozyme, in distilled water, to a concentration of 10 wt%. Figure 2 shows typical unprocessed single scan detector signals from the measurement path through the multipass cell (blue) at different times of the drying process (drying time: 250 s, 500 s, 750 s and 1000 s), the reference path through the Perspex chamber (red), the interference signal from the Fabry-Perot-etalon (yellow), as well as a simulated baseline (black), to demonstrate the baseline slope caused by the triangular laser current modulation. Figure 2: Typical raw single scan detector signals of the measurement path through the multipass cell (blue) at different times of the drying process (drying times: 250 s, 500 s, 750 s and 1000 s), the reference path through the Perspex chamber (red) and the etalon signal (yellow) as a function of time. The signals of the multipass-cell and the signal of the Perspex chamber clearly show the sloping background baseline (visualized as black line), caused by the triangular laser current modulation and the water absorption line at 1370 nm. The etalon signal shows the transmission maxima caused by the wavelength modulation of the laser and clearly shows an increase in the temporal occurrence of the interference maxima. The spectral spacing of the maxima results from the free spectral range (FSR), which is determined by the design of the etalon and is 0.800 GHz (approx. 0.005 nm) in this work. Knowing the wavelength at the beginning of the modulation, the emission frequency of the DFB laser can be determined at the points in time where the maxima occur.