is somewhat smaller than the diameter of the cavity such that an end face 52 is formed a pre column 56 is inserted into the cavity it includes a filter 58 which touches the end face 52 arranged adjoining the rear side of the filter is a pre column packing 60 which abuts against a further filter 62 by way of its rear end said filter 62 and with it the entire pre column is fixed by a closure 64 which comprises a through channel 65 for the fluid to be analyzed to pass through the sealing element 44 can be produced from a plastics material such as polyetheretherketone peek it is possible in particular to use biocompatible materials the third portion 26 of the adapter housing 12 1 can be introduced into the pilot bore 28 of the socket unit 13 1 which in the example shown passes over into the expanded receiving opening 24 via a conical transition region 78 the pilot bore 28 comprises a radial wall 66 and an end face wall 68 a socket capillary tube 70 by means of which the fluid to be analyzed is guided to a separation column not shown connects to the end face wall 68 axially and concentrically with respect to the axis of the pre column 56 not shown is a realization variant of the socket unit where a comparable further receiving opening is provided in the housing of the socket unit on the end of the socket capillary tube 70 which is remote from the receiving opening 24.