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 minimize 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. 0046L will differ less than 10% from the exact value if L/r**3 less than 1. 5 multiplied by 10**1**2. 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. Experimental data in corroboration of the model are presented.