The complete crystallographic distribution of field-evaporated W3+ and W4+ as well as field-desorbed hydrogen has been observed by using a new technique called field desorption spectrometry. The distributions are obtained during a single evaporation event in a modified field-ion microscope by recording the desorption image displayed on a spherical CEMA detector time gated with the arrival of an ion species of preselected mass-to-charge ratio. Preliminary observations show no unique crystallographic origin for W3+ or W4+ at 78 or 27 K during field evaporation in vacuum or imaging gas. Hydrogen appears to originate at random insterstitial locations at the surface and is not uniquely associated with the protruding atoms imaged in a conventional ion micrograph. The general lack of a one-to-one correspondence between gated desorption and ion micrograph images suggests that using the atom probe to identify individual preselected ion micrograph image spots may be much more difficult than originally anticipated.