## 4 Conclusions  
Powder analysis revealed that carboxylates and carbonates can be present as by-product species in considerable amounts in superparamagnetic \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) prepared by SFS. As these species are located on particle surfaces, their mass contents can be normalized to the particle surface area, yielding particle surface loading (_PSL_) values that are independent to the samples' particle size. A dependence of the organic _PSL_ on the burner type was demonstrated, with SpraySyn2 showing lower loading values (\(\sim\)0.69-0.73 mg m\({}^{-2}\)) compared to SpraySyn1 (\(\sim\)0.87 mg m\({}^{-2}\)). This indicates a slightly more complete combustion for SpraySyn2, as carboxylates represent non-combusted organic species from the gas phase. The methodology used to determine the _PSL_ values can be transferred to other SFS systems, allowing a more systematic comparison of the surface chemistry of SFS-made materials. In VSM measurements, the \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) samples synthesized here exhibit superparamagnetism at room temperature with \(M_{\rm S}\) values (9.7-20.8 emu g\({}^{-1}\)) well correlating with the particle size (5.0-7.2 nm). Since the measured magnetic responses are attenuated by the amount of non-magnetic species present on the particle surface (\(\sim\)17-21 wt.%), materials with even stronger magnetic responses could potentially be obtained by applying surface purification techniques.  
In addition, local particle investigations using time-resolved thermophoretic sampling followed by TEM (TS-TEM) revealed that carbon evolves instantaneously in flame pulses when synthesizing \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) using SpraySyn1. There, core-shell structures and amorphous aggregates can be readily identified as co-existing with crystalline \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\). Downstream of the flame and in the final particle product, no evidence of carbon deposition can be found, demonstrating a low relevance of this temporal carbon formation for the final particle purity.  
However, the local/temporal presence of carbon in the visible flame has the potential to bias in situ diagnostics.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app15063294/s1](https://www.mdpi.com/article/10.3390/app15063294/s1)]([https://www.mdpi.com/article/10.3390/app15063294/s1](https://www.mdpi.com/article/10.3390/app15063294/s1)), Figure S1: BET N\({}_{2}\) adsorption and desorption isotherms. Samples are prepared by using SpraySyn1 and SpraySyn2 as burner types and 0.2 M INN in EtOH + EHA as precursor solution; Figure S2: Magnetic responses measured by VSM at room temperature. Samples are prepared by using SpraySyn1 and SpraySyn2 as burner types and 0.1 M INN and 0.2 M INN in EtOH + EHA as precursor solution; Figure S3: Complex-binding configurations of carboxylates (A) and carbonates (B) on Fe\({}_{x}\)O\({}_{y}\).  
Conceptualization, R.T. and H.-J.S.; methodology, R.T. and K.D.; software, R.T. and K.D.; validation, R.T. and K.D.; formal analysis, R.T.; investigation, R.T. and K.D.; resources, R.T., H.-J.S., K.D. and M.S.; data curation, R.T. and F.F.; writing--original draft preparation, R.T.; writing--review and editing, K.D., F.F., M.R., R.K., H.P., M.S. and H.-J.S.; visualization, R.T.; supervision, R.K., H.P., M.S. and H.-J.S.; project administration, R.T., F.F., M.R., R.K., H.P. and H.-J.S.; funding acquisition, R.T., F.F., M.R., R.K., H.P. and H.-J.S. All authors have read and agreed to the published version of the manuscript.  
We kindly thank the DFG (German Research Foundation) for funding (grant number 375857587 including subproject numbers: SCHM 1429/13-1, KN 764/20-1, PI 368/19-1).  
Not applicable.  
Entire data (powder analysis data and high-speed recordings) are available on an open-source database (www.spraysyn.org, accessed on 1 February 2025). Additionally, we provide the data and the TS sampler on request.  
The authors declare no conflicts of interest.