## 3 Results and Discussion
### Final Particle Characteristics of Samples Obtained by SpraySyn1 and SpraySyn2
#### 3.1.3 ATR-FTIR  
Due to their measurement principles, TEM and Raman spectroscopy provide only limited information on whether carbonates/carboxylates and precursor/solvent residues may be present, making other techniques mandatory. By characterizing all samples by ATR-FTIR, qualitatively similar spectra are obtained, and two exemplary spectra are shown in Figure 6 (SpraySyn1 samples). As depicted, a global maximum is revealed at 575 cm\({}^{-1}\), which is typical for \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)[51] (Fe-O vibrations).  
Apart from the \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) signals, all spectra exhibit prominent signals in the mid to upper wavenumber region (1000-3000 cm\({}^{-1}\)), evidencing the presence of side products. To verify whether these dominant signals indicate residues of the precursor and/or the solvents used, a comparison with the spectra of EtOH, INN, and EHA can be conducted (Figure 6a). As shown, the spectra of INN, EtOH, and EHA differ crucially from the synthesized samples' spectra in the mid-wavenumber regime (fingerprint region), which refutes the assertion of substantial precursor/solvent residues.  
In order to identify the side products in greater detail, literature references are mandatory: Given the extant literature, the vibration quartet at 2962, 2935, 2877, and 2868 cm\({}^{-1}\) (magnified in Figure 6c) reveals stretching vibrations due to methyl and methylene groups (CH\({}_{2}\), CH\({}_{3}\)) [52; 53]. Thus, the presence of aliphatic species is evidenced. In addition, as magnified in Figure 6b, very prominent double peaks exist at 1420 and 1520 cm\({}^{-1}\). According to several references, this indicates C-O stretching vibrations caused by complexly bound carboxylates (R-COO') and/or carbonates (CO\({}_{3}\)\({}^{2-}\)) [15; 54]. Thus, ATR-FTIR measurements reveal the presence of carboxylates and, presumably, carbonates. From a structural point of view, the above-mentioned aliphatic species are likely incorporated in the molecular rest (R) of surface-bond carboxylates, explaining the particles' hydrophobic character. It is worth mentioning that carboxylates and carbonates usually show only _one_ C-O stretch at _one_ wavelength in ATR-FTIR measurements; however, when these species are complex-bonded to some ligands, their C-O stretches split into _two_ energy states, yielding _two_ signals [55]. The SFS community has already postulated that carbonates and carboxylates may form surface complexes together with surface Fe atoms on \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\), and some possible binding configurations are schematically depicted in the supplement (Figure S2) [16; 17].  
\begin{table}
\begin{tabular}{c c c c c} \hline INN conc. & 0.1 M & 0.2 M & 0.1 M & 0.2 M \\ \hline Burner type & \multicolumn{2}{c}{SpraySyn1} & \multicolumn{2}{c}{SpraySyn2} \\ \hline \(d_{\text{PP,BET}}\)/nm & 6.2 & 7.2 & 5.0 & 5.3 \\ \(M_{\text{S}}\)/emu g\({}^{-1}\) & 16.4 & 20.8 & 9.7 & 11.4 \\ \hline \end{tabular}
\end{table}
Table 2: High-field magnetizations (\(M_{\text{S}}\) at \(H=70\) kOe) determined by VSM. Given their difference in \(d_{\text{PP,BET}}\), \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) samples exhibit different magnetic response strengths. Samples prepared by SFS using SpraySyn1 and SpraySyn2 by processing 0.1 M INN and 0.2 M INN in EtOH + EHA.