A method of assaying a solid or liquid material, the method comprising: illuminating a sample of the material with pulses of light at a plurality of different wavelengths at which atoms and/or molecules in the material are ionized in multiphoton ionization (MPI) process; generating a value responsive to charge produced in the ionization process for each wavelength to provide an MPI spectrum for the material; and processing the MPI spectrum to assay an atom or molecule in the material.