This application relates to isotopically labeled affinity markers of the formula (II) ##STR00001## for mass spectrometric analysis of proteins. In formula (II), the groups A, PRG, S, Z, L', Z', R, R', k, l, m, and n are as defined in the claims. The application also provides a process for preparing these materials, a method for analyzing proteins using such materials, and a kit containing one or more of these materials.