A breath test system comprising: - an air inlet configured to receive an air flow for a breath sample; - a tube cell mounted to a backplane surface , the tube cell enclosing a detection cavity connected to the air inlet through which the air flow passes, the detection cavity comprising: - a first electromagnetic detection path configured to generate a first signal based on the presence of a reference gas in the air flow; and - a second electromagnetic detection path configured to generate a second signal based on the presence of a volatile substance in the air flow; and - logic in signal communication with the first detection path and the second detection path configured to determine a concentration of the volatile substance in the air flow based on the first signal and the second signal; - wherein the first electromagnetic detection path comprises a first electromagnetic emitter and a first electromagnetic detector , and the second electromagnetic detection path comprises a second electromagnetic emitter and a second electromagnetic detector ; characterized in that - the first electromagnetic emitter , the first electromagnetic detector , the second electromagnetic emitter and the second electromagnetic detector are each located on separated, thermally isolated surfaces , and further wherein the thermally-isolated surfaces comprise electronic circuitry in electrical connection with the backplane surface via flexible connectors having low heat transfer, such that the first electromagnetic emitter , the first electromagnetic detector , the second electromagnetic emitter and the second electromagnetic detector are thermally isolated from one another. - an air inlet configured to receive an air flow for a breath sample; - a tube cell mounted to a backplane surface , the tube cell enclosing a detection cavity connected to the air inlet through which the air flow passes, the detection cavity comprising: - a first electromagnetic detection path configured to generate a first signal based on the presence of a reference gas in the air flow; and - a second electromagnetic detection path configured to generate a second signal based on the presence of a volatile substance in the air flow; and - a first electromagnetic detection path configured to generate a first signal based on the presence of a reference gas in the air flow; and - a second electromagnetic detection path configured to generate a second signal based on the presence of a volatile substance in the air flow; and - logic in signal communication with the first detection path and the second detection path configured to determine a concentration of the volatile substance in the air flow based on the first signal and the second signal; - wherein the first electromagnetic detection path comprises a first electromagnetic emitter and a first electromagnetic detector , and the second electromagnetic detection path comprises a second electromagnetic emitter and a second electromagnetic detector ; characterized in that - the first electromagnetic emitter , the first electromagnetic detector , the second electromagnetic emitter and the second electromagnetic detector are each located on separated, thermally isolated surfaces , and further wherein the thermally-isolated surfaces comprise electronic circuitry in electrical connection with the backplane surface via flexible connectors having low heat transfer, such that the first electromagnetic emitter , the first electromagnetic detector , the second electromagnetic emitter and the second electromagnetic detector are thermally isolated from one another. <EOS>