## 1 Introduction  
The metrological investigation of sprays is of great interest for a number of technical applications such as fuel injectors or drying processes in the food and pharmaceutical industries, including spray and freeze drying, with regard to the evaporation and particle formation process. However, since it is hardly possible to track and analyze the ongoing mechanisms for a single droplet over the entire spray process, studies of the drying process of single droplets have proven to be particularly suitable. In general, single droplet experiments can be divided into techniques with and without physical contact of the experimental apparatus to the evaporating droplet. Typical contact techniques, including the sessile droplet method [1] or suspension from a glass filament [2], bring some disadvantages such as the lack of free rotation, bubble formation or heat conduction between the droplet and the fixture. To circumvent these problems, levitation methods such as optical levitation [3], electromagnetic levitation [4] and acoustic levitation via an ultrasonic field are applied. In particular, the latter method, where a liquid sample is levitated in one of the nodal points of a standing acoustic wave, which is established between an ultrasonic transducer and a reflector, has proven to be particularly suitable and is frequently used for single droplet analysis [2; 5; 6]. This is due to its comparatively low acquisition and operating costs, as well as the fact that acoustic levitation does not require any special physical properties of the sample [7]. The investigation of the evaporation of acoustically levitated single droplets is usually performed using imaging techniques. Here, the evaporation rate is determined by measuring the droplet diameter, via images of the drying droplet, and applying the so-called \(d^{2}\) law [2]. According to this law, the normalized squared equivalent diameter \(d(t)^{2}/d(0)^{2}\) decreases linearly during a single droplet drying process [8], with \(d(t)\) being the momentary equivalent diameter and \(d(0)\) being the initial equivalent diameter of the injected droplet. This technique is used for the analysis of pure liquid droplets [2; 9], multicomponent droplets [5], as well as for droplets from solutions [6; 10]. However, the imaging procedure also has disadvantages. For example, the optical system must be calibrated in advance for absolute measurements [2]. Furthermore, the imaging determination of the volume decrease for a complete drying of solution droplets is limited. The reason for this is that the drying progress of solutions can be divided into two phases, which are separated by the so-called critical point of drying. This point marks the formation of a solid particle surface and, therefore, no changes in volume are detectable anymore while the evaporation of the solvent proceeds [11]. In principle, the evaporation rate can be calculated from the vertical position of the particle in the standing ultrasonic wave, but, to the best of our knowledge, this method is neither reliable nor easily reproducible [12; 13]. To monitor the entire drying process, even after the critical point of drying, [PERSON] et al. [14] used a dew point hygrometer to measure the moisture content of the exhaust air of the levitator chamber. With this setup, they could measure water concentrations down to a few 100 ppm [14]. In order to determine low species concentrations in general and low water concentrations in particular, tunable diode laser absorption spectroscopy (TDLAS) has proven to be a particularly suitable measurement method. For example, TDLAS has already been used to detect water concentrations down to 10 ppm in internal combustion engines [15], for atmospheric humidity sensing [16], for studying drying processes of wood [17], and for water transpiration rate measurements on plant leaves [18]. Moreover, [PERSON] et al.