## 3 Results and Discussion
### Final Particle Characteristics of Samples Obtained by SpraySyn1 and SpraySyn2
#### 3.1.4 TGA-DSC-MS  
By examining the samples by TGA-DSC-MS, again revealing identical measurement profiles from a qualitative point of view. The entire TGA-DSC-MS data of one sample is depicted in Figure 7. There, Figure 7a refers to the thermal treatment conducted in synthetic air while Figure 7b corresponds to argon. The samples' thermal events are discussed on a qualitative basis first. Subsequently, samples are compared by considering the TGA-DSC-MS data from a quantitative point of view (Table 3). From a qualitative point of view, all samples exhibit an initial mass loss accompanied by water emission below \(\sim\)200 \({}^{\circ}\)C. The absence of other \(m/z\) signals beyond \(m/z=18\) suggests that physical water desorption is the prevalent process in this temperature range. This is further supported by a slight positive curvature in the DSC signal. The water emission (\(m/z=18\) trace) reaches a maximum at \(\sim\)120 \({}^{\circ}\)C independent of the atmosphere applied. Under air atmosphere, between \(\sim\)200 \({}^{\circ}\)C and \(\sim\)400 \({}^{\circ}\)C, \(m/z=18\) signals emerge again, coinciding with a decrease in \(m/z=32\) signals, an increase in \(m/z=44\) signals, and exothermic DSC signals. Therefore, the CO\({}_{2}\) emission peaks at \(\sim\)225 \({}^{\circ}\)C and \(\sim\)270 \({}^{\circ}\)C, while the deviation of the sample mass (dark blue) reaches minima simultaneously. In this temperature range, the relative sample masses decrease sigmoidal, attaining a steady state after \(\sim\)400 \({}^{\circ}\)C.  
The thermal behavior in the air suggests that the present carbonaceous by-products oxidize in the range of 200-400 \({}^{\circ}\)C, yielding CO\({}_{2}\) and H\({}_{2}\)O while consuming O\({}_{2}\). Given this temperature range, the TGA-DSC-MS data rejects the probability of elemental carbon, which should yield considerable thermal events at much higher temperatures [13,20]. Hence, the samples' thermal behavior does not evidence the suggestions obtained by Raman spectroscopy but aligns with the TEM impressions. Additionally, the samples' thermal behavior also refutes the assertion that INN might be present in the materials' outcome, which aligns with the ATR-FTIR data. INN residues should yield certain mass losses at \(\sim\)135 \({}^{\circ}\)C and \(\sim\)155 \({}^{\circ}\)C [56], which is, however, not recognizable either in the mass evolution or in the DSC signal. In air, all samples exhibit an additional DSC signal at \(\sim\)490 \({}^{\circ}\)C according to the phase transfer of \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) to \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\)[4,5]. Figure 6: Exemplary ATR-FTIR data for \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) particles prepared by SpraySyn1 by processing 0.1 M INN and 0.2 M INN in EtOH + EHA. Subfigures present most important wavenumber regions with (**a**) 500–3200 cm\({}^{-1}\), (**b**) 1350–1650 cm\({}^{-1}\), (**c**) 2750–3000 cm\({}^{-1}\). At a first glance, given the fact that the boiling temperature of EHA is at \(\sim\)228 \({}^{\circ}\)C, the thermal event taking place in the air at 225 \({}^{\circ}\)C indicates EHA residues. These EHA residues could evaporate, which may cause \(m/z\) = 44 (CO\({}_{2}\)) signals in the MS fragmentation (NIST MS number: 291562). Although this claim appears to be plausible at first glance, it can be refuted by considering the samples' thermal behavior under argon. As shown by the sample's thermal behavior under argon atmosphere (Figure 7b), CO\({}_{2}\) and H\({}_{2}\)O signals occur over a wider temperature range, and especially the thermal event at \(\sim\)225 \({}^{\circ}\)C is absent. This phenomenon invalidates the evaporation-fragmentation mechanism for EHA mentioned above, which should take place independently of the atmosphere applied. Instead, the samples' thermal behavior under argon is well aligning, which the theory that carboxylates and carbonates are present as dominant by-product species.