The applicability of matrix-free surface-assisted laser desorption/ionization mass spectrometry using porous silicon (DIOS-MS) and TiO₂ nanoparticles (TiO₂-SALDI-MS) was examined by analyzing an antioxidant added to insoluble polypropylene (PP) materials. The optimized solvent extraction, which involved freezing and crushing the PP, results in a good recovery, 89±4 and 97±8% for 0.5 wt% of the added antioxidant, tetrakis[methylene3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane (commercial name; Irganox1010) in laboratory-produced and commercial PP composites, respectively. The positive ion measurements in DIOS and TiO₂-SALDI-MS yielded sodium adduct ions without any interference from other signals. Using an internal standard with a similar structure, a good calibration curve linearity was established in the Irganox1010 concentration range of 0.01-2.0 mg/mL in PP, which was sufficient for monitoring the plastic additives. However, TiO₂-SALDI-MS was superior to DIOS-MS in terms of mass resolution and easiness of sample preparation. A solvent extraction method followed by TiO₂-SALDI-MS appears to be a anticipated method for the analysis of additives in insoluble plastics.