The influence of tube geometry on the performance of long sampling tubes in respiratory mass spectrometry.

Long sampling tubes allow remote patient monitoring by mass spectrometry. The choice of adequate tube geometry and material, which may minimise signal distortion and signal transmission delay, needs quantification of the relations between tube dimensions and factors determining tube performance (sample flow, residence time, response time). A mathematical model is presented which considers ideal tubes (no gas-tube interaction) with various geometries. It is shown for tubes with length L and uniform radius r that the response time for nitrogen estimated by the expression 0.0046 L will differ less than 10% from the exact value if L/r3 less than 1.5 X 10(12). The limited number of commercially available radii prevents free choice of sample flow for uniform tubes of a given length. However, tubes consisting of two parts with different diameters can provide any desired sample flow. The model indicates that residence and response times are effectively reduced if optimum radii are used for these two parts, and that a tube with an exponentially rising radius as a function of distance may halve residence and response times, compared with a uniform tube providing identical sample flow. Model predictions can be used to judge experimental results obtained with real tubes made of various materials. Experimental data in corroboration of the model are presented.