under control of the operator so that their positions relative to the common fixed body 40 can be altered simultaneously the valves 30 32 are independent of the valves 34 36 the components shown and the plumbing that interconnects them remain the same throughout fig2 5 however in carrying out the steps of the method described above the operator manipulates the valves 30 32 34 and 36 to alter the flow paths of the liquids reference will be made to the specific components in the course of the discussion below fig2 shows the flow paths during the step of the method shown in block 12 of fig1 with the valves 30 32 and 34 36 in the positions shown a sample pump 42 loads a volume v s1 of a standard having known concentration k s1 through the conduits 62 and 64 and into concentrator column 44 to provide a flow path from the sample pump through the concentration column 44 the sample liquid must be routed by the valve 32 to a waste receptacle 46 in the working example v s is 20 ml and k s1 is 100 parts per billion 10 1 mg l after the concentrator column 44.