Rationale It has been demonstrated that micro- and nanoplastics (MNPs) can be found in soil, and that MNPs can be taken up by plants. In order to conduct a risk assessment for human consumption, it is necessary to have an estimate of the mass concentration of plastics in crops. A new thermal extraction and desorption coupled with gas chromatography and mass spectrometry (TED-GC/MS) method has been developed for the analysis of polystyrene in pak choi. Methods In this study, a thermogravimetric analyzer (TGA) equipped with a thermal absorption unit (TAU), was coupled with a GC system equipped with a thermal desorption unit (TDU2), a (5%-phenyl)-methylpolysiloxane GC column and a GC/MSD single quadrupole mass spectrometer. The systems were connected via an Multi Purpose Sampler (MPS). Samples were pyrolyzed in the TGA, the pyrolysis products were trapped on a PDMS polymer bar, desorbed in the TDU, separated and analysed on the GC/MS system. Results PS MNPs of different sizes were qualitatively and quantitatively determined in pak choi, The determined LOD is 0.09 µg and the LOQ is 0.28 µg PS absolute. In the plants treated with 100 nm particles 19.0 ± 6.7 µg/g DM PS and in the plants treated with 500 nm particles 64.1 ± 8.6 µg/g DM PS have been found. Conclusions This study was the first to use a TED-GC/MS method for the detection of polystyrene (PS) nanoplastics of different sizes in pak choi and thus provides an important basis for the determination and risk assessment of PS in vegetables.