Background： Chemical properties of projectile residues are important in studying the symmetry energy using isobaric yield ratio （IYR） and isoscaling methods. Purpose： The correlation between chemical properties of intermediate mass residues and reaction systems is to be studied. It is hoped that the symmetry energy information can be extracted more precisely. Methods： In the framework of the grand-canonical ensemble theory, the △μ/T difference between two heave-ion reactions, which was labeled as △u21/T, was extracted by using isobaric yield ratio difference （IBD） method. Results： By comparing the △μ21/T among six different combinations ^48Ca＋^9Be/^40Ca＋^9Be, ^48Ca＋^9Be/^58Ni＋^9Be, ^64Ni＋^9Be/^40Ca＋^9Be, ^64Ni＋^9Be/^58Ni＋^9Be,^58Ni＋^9Be/^40Ca＋^9Be, ^48Ca＋^9Be/^64Ni＋^9Be, it was proved that △u/T depends on the reaction system. In different reactions, the values of A,tdT were not fixed. When the double neutron-to-proton ratios （N/Z）21=1. 17, △u21/T can be approximately a constant. Conclusion： Symmetry energy can be more reasonable in isoscaling and IBD methods, when double neutron-to-proton ratios （N/Z）21=1.17 between two similar reaction system.