A breath test system comprising: - an air inlet configured to receive an air flow for a breath sample; - a tube cell (110, 202) 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 (218, 220, 222, 224), and further wherein the thermally-isolated surfaces (218, 220, 222, 224) comprise electronic circuitry in electrical connection with the backplane surface via flexible connectors (230, 232, 234, 236) 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 (110, 202) 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 (218, 220, 222, 224), and further wherein the thermally-isolated surfaces (218, 220, 222, 224) comprise electronic circuitry in electrical connection with the backplane surface via flexible connectors (230, 232, 234, 236) 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.