Gel-surface enhanced fluorescence sensing system coupled to a continuous-flow assembly for simultaneous monitoring of benomyl and carbendazim Original Research Article

A novel, versatile and sensitive continuous-flow on-line solid phase fluorescence based system is proposed for the simultaneous determination of benomyl and carbendazim. The continuous-flow system is based on the on-line preconcentration and resolution of the pesticides on a solid sensing zone, followed by the sequential measure of their native fluorescence, monitored at 235/306 and 293/398 nm (λexc/λem for carbendazim and benomyl, respectively), and later desorption of these analytes (from the flow-through cell filled with C18 silica gel) using aqueous methanol mixtures as carrier and eluent solutions. A double discrimination is used for the simultaneous monitoring of these analytes: (1) the usage of two pair of excitation/emision wavelengths, performed by the use of a multiwavelength fluorescence detection mode and (2) a temporary sequentiation in the arrival of the analytes to the sensing system by on-line separation due to the different kinetics showed by the analytes in the sorption–desorption process performed just in the solid support placed in the flow-through cell. Carbendazim is determined the first, because it shows a weaker retention in the C18 bonded phase silica beads, while benomyl is strongly fixed. Then, benomyl is conveniently eluted from the flow-through sensing zone and its native fluorescence signal is measured (at 398 nm). The sensor was calibrated for two different injection volumes: 400 and 2000 μl. Using a 2000 μl sample volume, the analytical signal showed linearity in the range 0.050–1.0 and 0.020–0.50 μg ml−1 with detection limits of 3.0 and 7.5 ng ml−1 for carbendazim and benomyl, respectively, and R.S.D. values smaller than 2% for both analytes. A recovery study was performed on four different spiked environmental water samples at concentration levels from 0.05 to 0.35 μg ml−1. The recovery percentage ranged from 97 to 104%, and from 98 to 104%, for benomyl and carbendazim, respectively.