Optical multiplexer for liquid samples in a conduit

A liquid analysis system including a plurality of sample containing units (207a-207n) comprising a multiplexer for electromagnetic radiation, said multiplexer comprising:

• a first part (253) provided with a first outward through hole (263) connectable to a source of electromagnetic radiation (203) and a first return through hole (265) connectable to a detector of electromagnetic radiation (223), wherein said first outward through hole (263) and said first return through hole (265) are spaced a distance D apart;
• a second part (255) provided with a plurality of second outward though holes (273(1)-273(n)) and a plurality of second return through holes (275(1)-275(n)) wherein said second outward through holes (273(1)-273(n)) and said second return through holes (275(1)-275(n)) are arranged in equidistantly spaced apart pairs (273(1), 275(1);
• 273(2), 275(2);
• . . . 273(n), 275(n)) of second outward and return through holes, with each second outward hole (273(x)) at a distance D from its second return through hole (275(x)),
• wherein said first part (253) is movable relative to said second part (255) from a first position P1 in which first outward through hole (263) is aligned with a second outward through hole (273(1)) and said first return through hole (265) is aligned with a second return through hole (275(1)), to a second position Px in which first outward through hole (263) is aligned with another second outward through hole (273(x)) and said first return through hole (265) is aligned with another second return through hole (275(x)).