Technologies are generally described for a method and system configured effective to detect a defect in a sample including graphene. An example method may include receiving a sample, where the sample may include at least some graphene and at least some defects in the graphene. The method may further include exposing the sample to a gas under sufficient reaction conditions to produce a marked sample, where the marked sample may include marks bonded to at least some of the defects. The method may further include placing the marked sample in a detector system. The method may also include detecting at least some of the marks with the detector system.