# Is there any “law” that states that particles must have different $\frac{e}{m}$ ratios?

In class, we talked about the discovery of electrons. The book stated, "..results all gave same value for charge to mass ratio. This showed that there was some (particle) common to all materials in a discharge tube" Is this true that particles must necessarily have different values for $\frac{e}{m}$? Is there a law for this?