What determines the variation in chemical composition of each breath sample?

The variation in the chemical composition of each breath sample is determined by the interactions between the respiratory tract and blood supply. This is because the blood acts as a carrier for alcohol and other substances, which diffuse into the alveoli of the lungs and are then expelled during exhalation. Factors such as how alcohol is metabolized in the body, the concentration in the blood, and the efficiency of diffusion from the bloodstream to the lungs all contribute to the specific chemical makeup of each breath sample. Therefore, variations can occur based on an individual's physiology, including aspects like the volume of blood flow to the lungs and individual differences in lung function.

In contrast, while the machine's calibration is essential for accuracy, it does not inherently cause variation in each breath sample's chemical composition. The subject's temperature can affect overall metabolism but does not directly account for the variability in breath samples. Similarly, the duration of breath sample collection can influence how much air is sampled but does not determine the inherent chemical composition of what is being measured. Thus, it is the physiological interactions underlined in the correct response that lead to the observed variations in breath sample composition.