The mass spectrometer was invented in the 1940s and began to be used in radiometric dating in the 1950s.

Why cant we use isotopic dating techniques with sedimentary rocks?

Sedimentary rocks may have radioactive elements in them, but they have been re-worked from other rocks, so essentially, there radiometric clock has not been re-set back to zero. However, sedimentary rocks can be age dated if a volcanic ash horizon or a diabase sill or dyke can be found within the sequence.