A flow-injection extraction (FIE) detection system based on the simultaneous determination of absorbance and conductance is described. The absorbance is read radially on a 380 um bore PTFE tube with an optical aperture of ~0.5 mm, resulting in an effective detector volume of ~60 nL, small enough to resolve signals for each aqueous or organic segment. The conductance measurement probes are located immediately after the optical aperture. Absorbance and conductance are respectively measured by light emitting diode (LED) based photometry and bi-polar pulse conductometry using a single personal computer (PC) for the operation of the dual detectors, data acquisition and processing. The phases are discerned based on the conductance signal and the analyte concentration is monitored by the absorbance signal. Accurate and reliable phase recognition is achievable with conventional T-segmentors and peristaltic pumping. When the system is used to determine C-12 linear alkylbenzene sulfonate (LAS), an anionic surfactant, by ion-pairing with methylene blue (MB) and extraction into chloroform, a linear response is observed in 0-2.5 ppm range. The limit of detection (LOD) for a 65 uL injected sample is 0.03 ppm; in comparison, the standard manual method that uses several hundred mL sample for extraction reports an LOD of 0.025 ppm.

Fig. 2. (a) Horizontal (b) and vertical cross sectional views of the flow cell.