between the probing evanescent fields at analyte, and the highest potential sensitivity. Analyte liquids were introduced into the device by syringe pump with separate solutions connected to each inlet (Figure ). We alternatively admitted Na2HPO4 solution (0.1 M) and water into the channel through one of the two inlets, and sequentially obtained the respective spectra (Figure ). The bands at 1072 and 990 cm -1 belong to the stretching vibration of v1(PO4) and v1(PO3), respectively. Next, we acquired data while flow manipulations were underway. In order to scan as quickly as possible, we took advantage of the device high sensitivity so that we could minimize the number of scans to observe rapidly changing signals. In the next experiment, we alternatively admitted water and Na2HPO4 solution into the channel at a high flow rate (10 mL•h -1 ) for 90 for each liquid. Simultaneously, spectra were collected at a high acquisition rate. The instrument base time to obtain a spectrum was 5 s due to overheads in communication, calculations, and data management. As we could scan twice in this time, we set the number of scans to 2, and obtained time-series spectra with a time-resolution of 5 s. Integration of the peak for Na2HPO4 provided the composition of flow in channel versus time (Figure ). Based on Eqn 1, the velocity of liquids in channel was 5 mm•s -1 . In the 5 s between measurements, the fluid can move 25 mm, which is greater than the 9 mm length of the downstream dimension of the ATR crystal. Therefore, it expected that once the new liquid reaches the ATR crystal, its spectral signature will jump from zero to maximum intensity between 2 consecutive data points. The result indicated the process of flow switching in device characterization zone was less than 5 s at this flow rate.