Presented are the results of the first mass spectrometry study of impurities at JET with the ILW. Measurements are performed with the newly-installed RGA system that allows for data acquisition in all stages of machine operation. Impurities are found in the 16­20AMU range, at 28AMU (N2) and 40AMU (Ar). The main contaminants are water and nitrogen, which are present only in low amounts. During seeded discharges, the impurity content increases, however it falls back to non-seeded quantities in subsequent non-seeded discharges, exhibiting only slight legacy levels.