Thin MALDI samples can perform differently than thicker samples, on metal substrates. Divergent results and models for the effect have been presented. Positive and negative yields are investigated here for three matrixes (2,5-dihydroxybenzoic acid (DHB), sinapinic acid, and alpha-cyano 4-hydroxycinnamic acid) on stainless steel and gold substrates. Samples were electrosprayed for uniformity and thickness control and imaged across a metal-metal boundary. Thin sample enhancement is found in both polarities for all three matrixes on a steel substrate. On gold, only alpha-cyano-4-hydroxycinnamic acid shows enhancement. These and earlier data are used to evaluate two models. The first is based on one-photon photoelectron emission from the metal; the second one, on two-photon matrix ionization at the metal interface. The surface-enhanced matrix photoionization model best fits the evidence, including the fluence dependence of electron emission from DHB on steel.