Flow-through sensors for enhancing sensitivity and selectivity of FTIR spectroscopy in aqueous media

The principle of novel flow-through sensor systems with FTIR spectroscopic detection is presented on the example of the determination of organic acids in aqueous solution. The constructed flow-through sensor system is based o n trapping of derivatized porous polymer beads in a conventional IR transmi ssion cell and integration of the flow cell into a sequential injection (SI ) manifold. By the SI-manifold sample pre-conditioning, sample-sensor inter action and sensor regeneration were performed in an automated and highly re producible way. The polymer beads used in this study contained anion exchan ger groups so that negatively charged molecules such as organic acids prese nt in the anionic form could selectively interact with the polymer beads. U pon pumping a sample through the sensor cell organic acids were retained on the polymer beads whereas non-ionic matrix molecules passed hence allowing to separate the target analytes form the matrix. Apart from that the organ ic acids were also concentrated onto the polymer beads so that absolute ana lyte amounts in the low mu g range could easily be detected. Linear calibra tion curves from 0 to 1 mmol l(-1) were recorded for acetic and malic acid using a sample volume of 500 mu l (s(x0): 0.032 mmol l(-1) acetic acid and 0.031 mmol l(-1) malic acid). Mixtures of both acids were analyzed as well and it could be shown that by application of multivariate data evaluation p rocedures (PLS) simultaneous quantification of both acids could be performe d successfully using the developed flow-through sensor system. (C) 1999 Els evier Science B.V. All rights reserved.