Abstract: The present techniques relate to sampling devices, and in particular to breath sampling devices. We describe a computer-implemented method of designing a breath sampling device comprising a housing through which a substantial portion of an exhaled breath passes and sorbent material which extends across a cross-section of the housing. The method comprises identifying a plurality of parameters of the device which are variable with different designs of the breath sampling device and selecting values of the plurality of parameters which maximise a global fitness value. The plurality of parameters comprises at least two of mass of the sorbent material, size of the sorbent material, thickness of the sorbent material, breakthrough volume and flowrate within the device.