Dual thermostating in flow analysis

An advanced strategy involving concentric tubes is proposed for fast and controlled heating (or cooling) of the reaction medium in flow analysis. Different temperatures are set by sequentially circulating two thermostated water streams through the outer larged bore (2.0 mm i.d.) silicone tube, which acted as a water-jacket of the inner (0.8 mm i.d.) PTFE tube, and directing the sample zone to flow through it. Each end of the outer tube is connected to a three-way valve that selects the stream to flow inside it. For 25-85 cm tube lengths and a 12.0 mL min(-1) flow rate, the time interval required for temperature attainment, and the uniformity of temperature along the tube were evaluated. For the 85-cm tube, low differences in temperatures along the coil (1.1-8.7 degrees C) and between programmed and attained values (2.3-13.4 degrees C) were noted within a wide range of pre-set temperatures (15-75 degrees C). The feasibility of the innovation in flow analysis was demonstrated in a model system relying on the iodide-nitrite reaction. The strategy allows fast (15-120 s) thermostating of the reaction medium in a versatile and simple way, and is especially attractive when two controlled temperatures are set during the analytical course. Potentialities and limitations of the innovation are discussed.