This study investigates the formation of by-product species during flame spray synthesis (SFS) of superparamagnetic maghemite (γ-Fe₂O₃) nanoparticles. Four samples are synthesized by utilizing two standardized burner types (SpraySyn1 and SpraySyn2) and varying the iron (III) nonahydrate (INN) concentration (0.1 M and 0.2 M) in the precursor feed while using ethanol and 2-ethylhexanoic acid as solvent. Conducting complementary powder analysis revealed a predominant presence of carboxylates and carbonates as by-product species (~14–18 wt.%), while no strong indications for elemental carbon and precursor/solvent residues can be found. Carbonates/carboxylates are located on particle surfaces, and the particles’ surface loadings by these species are independent of the precursor concentration but depend on burner type, with SpraySyn2 exhibiting lower values, indicating a more complete combustion for this burner. Through time-resolved thermophoretic sampling, we further demonstrate that carbon forms temporally in the visible flame center when using SpraySyn1. Since carbon solely forms momentarily within large flame pulses and decomposes further downstream, its temporal formation is of minor relevance for the final particle purity. However, its local co-existence aside from γ-Fe₂O₃ in the flame has potential to bias in situ diagnostics.

Particles generated by spray flame synthesis (SFS) can exhibit impurities, with surface adsorbates and soot being the best-known examples. Unfortunately, the extent to which the formation of these impurities can be characterized, and how their formation can be avoided is not fully understood yet. In order to contribute to the understanding of the formation of carbonaceous impurities, we investigated their formation using a standardized burner type (SpraySyn burner) while synthesizing maghemite by the gas-to-particle conversion. Two approaches were followed to characterize the formation of soot and surface adsorbates in this paper. Firstly, fabricated powders were analyzed by complementary powder analysis. In this term, the detection of soot was negative, but powders contained carboxylates and carbonates, which were bound to particle surfaces. The methodological procedure used allowed a reliable quantification of those adsorbates, and it was possible to normalize their relative mass fractions (up to ~16 %) to the powders' respective specific surface areas. The normalization approach yielded a surface loading values of 0.84–0.85 mg m^-2. The methodical procedure and the data provided can be used in the future to denote/compare SFS materials in sensitivity studies. Secondly, particles were thermophoretically extracted from the process in situ and subsequently characterized by transmission electron microscopy (TS-TEM). Interestingly, particles extracted at 5 cm height above the burner (HAB) (representing the center of the visible flame) showed amorphous core-shell structures and amorphous aggregates, indicating a coexistence of soot alongside maghemite particles. Via reducing the sampling time of the TS system to 1.5 ms samples were taken either from flame pulses (a state of high flame activity) or from flame flickering (lower flame activity). A comparison of these samples showed that heterogeneous particle structures formed predominantly in flame pulses. As samples extracted at 15 cm HAB neither showed amorphous coating nor amorphous aggregates, it was indicated that soot fractions oxidize between 5 and 15 cm HAB.