The high-temperature SmBa2Cu3O7-δ (Sm-123) superconducting system, which is characterised by a high critical transition temperature (Tc) and a high critical current density (Jc), suffers severely from the effects of Sm/Ba substitution in the superconducting Sm-123 phase matrix, and especially so for large, single grains grown in air, resulting in a significant variation in Tc at different positions within a single grain. As a result, the suppression of Sm/Ba substitution in the Sm1+xBa2-xCu3O7-δ phase matrix (SmBCO, where x represents the Sm/Ba substitution level in the SmBCO system) is critical to achieving good superconducting properties in this material. Here we report the use of Electron Probe Micro-Analysis (EPMA) to investigate, adjust and optimise the composition of mechanically-stabilised standard Ag-SmBCO bulk single grains. We show that the substitution levels within these samples changes linearly within increasing distance from the vicinity of a single crystal seed used to nucleate the single grain growth process. In addition, we identify a constant value of x of – 0.080 for the composition-adjusted Ag-SmBCO bulk single grain. This is the first time that the quantification of the Sm/Ba substitution level in the SmBCO system has been measured accurately and directly using EPMA, and suggests clearly that the Sm/Ba substitution can be suppressed effectively in air. This research will provide significant insight into the development of a process to suppress Sm/Ba substitution even further in superconducting SmBCO single grains in the future.