The concept of the chemical space was initially introduced within the field of drug discovery, where the central role was the exploration of drug like structures. Those efforts were 5 based on using brute-force and known organic chemistry rules to generate all possible structures within set boundaries, for example, the number and the type of elements. 11, This approach results in an extremely large number of possible structures ranging between 10 20 to 10 60 for molecules containing 30 atoms or less. Chemical databases such as GDB-20 or Zinc are generated using this strategy and they also contain a few estimated molecular descriptors such as logP (i.e. partitioning coefficient between water and an organic phase). 10, The main objective of such databases is to be able to query them for structural similarity and/or a specific functional group. Given the approach used for generating such databases, they consist of structures that go beyond drug-like chemicals. The chemical space contains a myriad of structures that may or may not be relevant to a specific application case. 12, For example, ozonation degradation products of a natural product, even though highly relevant to exposomics, may not be relevant to drug discovery. These selected subspaces of the chemical space are defined as "relevant chemical space" (Figure ), which are field/question dependent. Another chemical subspace is the "measurable chemical space" (Figure ). This sub-space represents the chemicals within the chemical space that can be measured using current analytical techniques. The measurable chemical space focuses only on whether a chemical can be separated and be ionized via one of the existing mass spectrometry ionization technologies. The measured chemical space, is the chemical subspace where all the structures have been previously measured (Figure ). Being part of the measured chemical space does not imply that these chemicals have been identified (i.e. structurally confirmed). As an example, features in chromatograms that are not identified during NTA assays are part of the measured chemical space. The most well-known chemical subspace is the structurally confirmed chemical space. This is comprised of chemicals that are well known and studied, for example pharmaceuticals. Except the identified chemical space, other subspaces are mostly unexplored and thus unknown. The relevant and measurable subspaces may overlap, depending on the field. For example, in the field of exposomics, the relevant chemical space may be larger than the measurable one, while in drug discovery, this may not hold true.