Topological insulators are to this day an extensively studied group of materials. Their topologically protected metalic states on the surface, spanning across the bulk band gap, show many interesting phenomena such as universally quantised conductance and Farady rotation. Consequently, studying these systems provides an ideal platform to gain insights into fundamental physical phenomena. In my work, I use Landau level spectroscopy in an infrared region, to show that we have found an ideal candidate (Sn-BiSbTe2S) for optical investigations of topological surface states, without an influence of the bulk charge carriers, that usually dominate the physical properties of the previously studied materials. Additionally, I demonstrate how analyzing bulk properties can shed light on the long-debated topological characteristics of ZrTe5.