Disclosed herein is a device for holding a substrate having one or more samples in a position for ionizing the samples with a light energy source. The device has a receiving plate with a receiving surface, a back surface and an edge defining an outline. One or more receiving lips project from the receiving surface. The receiving surface receives the back face of the substrate with at least one edge received by the receiving lips for positioning the substrate for ionization of the sample. The device also has at least one substrate clip that has a front finger and a back finger for capturing and aligning the substrate. The front finger extends across the thickness of the receiving plate and substrate to engage the front edge of the substrate to hold the substrate in position. The back finger engages the back surface of the receiving plate. The receiving plate cooperates with the space in a matrix assisted laser desorption ionization mass spectrometer to hold the samples in position for ionization.