107 respectively 207 by wedging against the corresponding quartz cylinder a joint or seal 105 respectively 205 and a joint 106 respectively 206 having a bevelled form the tip of the bevelled portion of the said joint 106 respectively 206 is wedged between the quartz and the end of the body 108 respectively 208 that is cut in a bevelled form in the opposite direction similarly the tip of the bevelled portion of the joint 105 respectively 205 is wedged between the quartz and the other end of the joint 106 respectively 206 that is shaped in a bevelled form in the opposite direction these joints 105 and 106 respectively 205 and 206 are made of polytetrafluoroethylene ptfe and ensure the sealing to pressure since the two quartz cylinders are positioned in such a manner as to obtain the desired optical trajectory interquartz distance their positions are fixed through tightening of the sockets 107 and 207 upon the bodies 108 and 208 after introduction of a wedge of a suitable thickness through one of the orifices 110 or 111 this sealing is completed by a ring made of polytetrafluoroethylene 104 respectively 204 that is applied against the bottom of the quartz cylinder 101 respectively 201 through screwing of the base 103 respectively 203 upon the main body this screwing is rendered fool proof by blocking in counter bolt 109 against the base 103 a perfect sealing to pressure and to temperature is thus obtained with as short an optical trajectory as desired the product to be analyzed enters through the hole 110 having axis xx 39 and provided in the main body passes between the two quartz cylinders that determine the optical trajectory and exits.