A new detection technique for flow -injection extraction (f.i.e.) is introduced. The absorbance is read radially, on the same PTFE tube that constitutes the reaction coil, near its distal terminus. With an effective illuminated detector volume of ~60 nl, the optical aperture is much smaller than the segment length, enabling the detector to reliably measure signals for each phase. The detector used is a light emitting diode (LED) based dual-wavelength photometric system utilizing personal computer (PC) based data acquisition and processing. One non-specific wavelength is used to recognize the phases and the other wavelength monitors the analyte concentration. Accurate and reliable phase recognition is achievable with conventional segmentors and peristaltic pumping. Applied to the determination of anionic surfactants by ion-pairing with methylene blue (MB) and extraction into chloroform, a linear response is observed with a limit of detection (LOD) of 0.03 ppm C-12 linear alkylbenzene sulfonate (LAS) for a 65 ul injected sample, compared to an LOD of 0.025 ppm quoted for the standard manual method attainable by subjecting several hundred ml of the sample to extraction.

Fig. 6. Reconstructed fiegram for various concentrations of C-12 LAS, time scale refers only to the accumulated time elapsed during organic phase detection.