## 3 Results and Discussion
### Comparison of Raman and MIR Spectroscopy
One reason for this result could be the higher spectral resolution of the recorded MIR spectra (4 cm\({}^{-1}\)) in comparison to the Raman spectra (approximately 7 cm\({}^{-1}\)). Therefore, using a Raman spectrometer with enhanced spectral resolution might improve the analytical performance of this method for fermentation monitoring. However, both spectroscopic methods have their advantages and disadvantages taking the sample constitution, as well as the measurement environment into account. Table 3 gives an overview of important criteria for process implementation of Raman and MIR-ATR spectroscopy and can help the user to select the appropriate spectroscopic method for the particular application. Although the selectivity of both spectroscopic methods is high, other criteria differ from each other. For example, Raman spectroscopy measures intensities, whereas MIR spectroscopy measures absorbances. Likewise, the optical components (light sources, probes, and spectrometer) of typical Raman and MIR setups for reaction monitoring in a liquid phase are different. In the MIR range, Fourier transform (FT) spectrometers, pyroelectric DTGS, or mercury cadmium telluride (MCT) semiconductor detectors and light sources, such as black-body radiators made of SiC (Globar), are predominantly utilized. To apply [PERSON]--[PERSON]'s law for the quantitative analysis of MIR spectra, optical path lengths in the order of micrometers are necessary to achieve absorbances A of less than two (A \(<\) 2). Such small path lengths can be realized by attenuated total reflection (ATR) [38]. The ATR prism (crystal) usually consists of Si, Ge, ZnSe, or diamond, where the latter exhibits the highest chemical resistance. In the case of ATR immersion probes, the prism is located at the probe tip. ATR immersion probes are connected to the light source and the spectrometer via \begin{table}
\begin{tabular}{c c c} \hline \hline
**Criterion** & **Raman** & **MIR** \\ \hline Selectivity & + & + \\ Sensitivity & o & + \\ Life-time of light source & o & + \\ Use of optical fibers & + & \(-\) \\ Error of calibration/prediction & + & + \\ Interference by light scattering or fluorescence & \(-\) & + \\ Interference by film formation & + & \(-\) \\ Process implementation & + & o \\ Costs & o & + \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of the functionalities of Raman versus MIR-ATR spectroscopy for bioprocess monitoring (+ advantage, o sufficient, \(-\) disadvantage/challenging). optical fibers made of mixed silver halides or chalcogenides [3; 39]. The length of optical fibers is restricted to less than 5 m due to the poor MIR transmission of these materials. The transmission losses in fibers and the light source's weak intensity require liquid nitrogen-cooled MCT semiconductor detectors to obtain absorption spectra with sufficient signal-to-noise ratios. In addition, fibers made of these materials are fragile and become opaque over time. These circumstances complicate a transfer of this measurement setup from the laboratory into a process environment. In order to avoid the disadvantages of MIR fibers, the fiber-optical ATR probe can be replaced by an ATR module, which is directly integrated into the FT spectrometer. In such a design, the measurement is often carried out in a bypass, where the sample to be analyzed must be transported to the ATR module. In contrast to the fiber-optical immersion probe, there is no more in-line measurement. However, this measurement in the bypass is a real-time analysis of the original sample. Such a solution is much more cost-effective (about US$25,000 for the spectrometer used equipped with a DTGS detector and an ATR module) and more long-term stable than the fiber-optical setup (from US$60,000 for FT spectrometers with a liquid nitrogen-cooled MCT detector and a fiber-optical immersion probe). A weak point of MIR measurement systems with ATR flow cells could be biofilm formation or the deposition of silicone antifoam on the ATR crystal.