Growing concerns about the impact of microplastics (MPs) on the environment and health have fueled research into their lifecycle and environmental behavior. Single-particle inductively coupled plasma-mass spectrometry (sp-ICP-MS) has emerged as a promising technique for the identification and sizing of MPs. In this study, we developed a new, matrix-independent calibration method using CO₂ as the calibration gas for sp-ICP-MS. By optimizing the sample introduction and gas flow, the method significantly improves the signal-to-noise ratio and minimizes matrix interference. MPs ranging from 2 to 7 μm were accurately sized, and the results were cross-validated using scanning electron microscopy (SEM). The CO₂-based calibration curve showed high linearity and reproducibility in different matrices, making it a robust tool for accurate MPs measurement.