Lateral translation means 96, as shown in FIG 9, is coupled to and is controlled by controller 28 to move flow cell assembly 94′ between {HEAD ~ a first position} wherein {TAIL ~ compensator window} 110 is optically intermediate IR source/lens 84/86 and detector assembly 88, and a second position wherein sample cell 104 is optically intermediate IR source 84/lens 86, and detector assembly 88 
Lateral translation means 96 , as shown in FIG 9 , is coupled to and is controlled by controller 28 to move flow cell assembly 94′ between { HEAD ~ a first position } wherein { TAIL ~ compensator window } 110 is optically intermediate IR source / lens 84/86 and detector assembly 88 , and a second position wherein sample cell 104 is optically intermediate IR source 84 / lens 86 , and detector assembly 88
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