A photometric probe for in situ analysis : principle, method, first trials
Various dissolved compounds are mot stables in surface conditions. We realized a prototype to collect chemical data related to redox sensitive species without any contact with the atmospheric oxygen. The principle of this probe for in situ measurements is to produce colorimetric reaction in depth. A photometric cell and a horizontal coiled glass tube for fluid mixing are immersed and connected with two optic fibers to a spectrophotometer on boat (fig. la, b, c). Reagent is injected continuously from surface and sample is sucked up with a peristaltic pump through a tubular filter. Every species which can be analysed by colorimetric method should be determined, in deep river or in lake, with this simple equipment. Nevertheless, during preliminary trial, we detected a lack of reproducibility in the mixing ratio of the sample with the complexing agent. The problem is solved by adding an auxiliary dye with reagent and measuring optical densities at two different wavelengths (λ1 and λ2). In the system, with a the sample proportion, absorbance A at λ is expressed as : A=ɛe∙l∙Ce∙α+ɛr∙l∙Cr∙(1-α) We suggest to calibrate directly the cell of the probe and work in a calibration graph. It is built with first, marking on an A1λ1=f(A2λ2)graph (fig. 2), the « pivot » point (P) (when α= 0), second, plotting the « calibration curve » A2=A1 (ɛe2/ɛe1) (when α= 1), third, plotting the different S1 (A1i, A2i) measured from standards. Therefore, if sample signal Re at two wavelengths is plotted in this graph, by joining P and Re, the straight line intersects with calibration curve at C. On this curve, interpolation of C between two standards determine the concentration of the analyte. Experimental verification of the mixing rule has been clone independently of chemical reaction, with different heliantine solutions in pH 7 buffer as samples and chlorophenol red as reagent, bath in laboratory and at 15 meters depth (Beffes lake, France) (fig. 4 and 5). Furthermore, chlorophenol red, previously tested, is used as auxiliary dye for total iron measurement, in Aydat lake (Puy de Dôme, France) for a first in situ trial. Results are compared to those got from oceanographic bottle sampling and laboratory analysis (fig. 6). Data from the probe are in good agreement with data from the laboratory method. Next development of this chemical sensor will consist in adding to the system an osmotic pump which should allow measurements without intervention during several months.