Embodiments herein include a method for detecting a health condition of a subject. The method can include obtaining a biological sample from the subject and placing it into a container having a headspace surrounding the biological sample. The method can include contacting a gas from the headspace with a chemical sensor element, the chemical sensor element including one or more discrete graphene varactors. The method can include sensing and storing capacitance of the discrete graphene varactors to obtain a sample data set. Other embodiments are also included herein.