purposes the capillary is arranged between two stabilizing bodies 3 and 4 which stabilize the capillary both dimensionally and positionally the cylindrical cover surface longitudinal wall of the capillary sample vessel 1 completely consists of a semi permeable membrane 5 depicted in the sectional view of fig1 as a circular dotted line fig2 also shows cross sections through four capillary sample vessels 6 7 8 9 with different cavity cross sectional areas 10 11 12 13 in contrast to fig1 in which the capillary is designed independently as a cylindrical cover surface by a semi permeable membrane i e independently of the adjacent stabilizing bodies 3 4 each of the capillary sample vessels 6 7 8 9 is a cavity formed in a stabilizing body 14 15 16 17 that provides positional and dimensional stabilization for the capillary with said cavity being produced for example by molding pressing deforming or machining processes each cavity in the stabilizing body 14 15 16 17 is covered for example by an adhered or bonded semi permeable membrane 18 again represented as a dotted line and thus the cavities of the capillary sample vessels 6 7 8 9 are laterally closed in this way a capillary sample vessel 6 with a triangular cavity cross sectional area 10 is formed in the stabilizing body 14 a capillary sample vessel 7 with.