Given growing concerns over the observed relationship between ultrafine particles and adverse human health effects, there is a major need in the community performing human/animal exposure studies for methods that can be used for the generation of high concentrations of ultrafine particles (<100 nm) with controllable compositions. The Palas spark discharge generator (Palas GFG 1000) is commonly used to generate “soot-like” particles for such studies. However, before such methods can be used routinely in the lab, it is important to assess the chemical variability and reproducibility of the ultrafine particles produced using such techniques. The goal of this study involves performing the on-line assessment of the chemical variability of individual ultrafine and fine (50-300 nm) particles produced by a Palas generator. The aerodynamic size and chemical composition of ¹²C and ¹³C elemental carbon (EC), composite iron-carbon (Fe-¹²C), and welding particles were analyzed using aerosol time-of-flight mass spectrometry, and in general highly reproducible single-particle mass spectra were obtained. When using pure graphite (¹²C) electrodes, EC particles were produced with sizes peaking in the ultrafine mode and 96% of the mass spectra containing distinct C_n⁺ (n = 1-3) envelopes at m/z 12, 24, and 36. In contrast, the size mode of the particles generated from isotopically labeled ¹³C graphite electrodes peaked in the accumulation mode, with 73% of the particles producing EC carbon ion cluster patterns at m/z 13 (¹³C⁺), 26(¹³C₂⁺), and 39 (¹³C₃⁺), with additional organic carbon species at m/z 15 (CH₃⁺), 27 (C₂H₃⁺/CHN⁺), 43 (C₃H₇⁺/CH₃CO⁺), m/z 58 (C₃H₈N⁺), and 86 (C₅H₁₂N⁺). Observed differences between the ¹²C and ¹³C particle spectra are most likely due to their different surface properties, with ¹³C particles more effectively adsorbing semivolatile organic species originating in the particle-free dilution air. Homogeneous metal particles were also generated from Fe-¹²C and welding rods with almost all (92% and 97%, respectively) of the spectra showing reproducible Fe/Mn/Cr and Fe/¹²C ion ratios.