The whole VIP system consists of several units: the submersible voltammetric probe based on a unique micro-sensor, a submersible on-line O2 removal module, a submersible Ocean Seven 316 multiparameter probe, a calibration deck unit, a surface deck unit and a management software running on WINDOWS PCs.
It allows simultaneous measurements of Cu(II), Pb(II), Cd(II) and Zn(II) with a sensitivity at the ppt level, as well as Mn(II) and Fe(II) with a sensitivity at the ppb level (extension to other elements and organic compounds are foreseen).
It is usable in sea and fresh water down to 500 m. It is controlled either by an operator from the surface or in autonomous mode, under pre-set sequences. When interfaced with an automatic Buoy Profiler, the VIP System allows the whole water column to be automatically
monitored for at least one week, at prefixed periods.
The VIP System is a no-compromise, state-of-the-art analytical system to address the requirements of every environmental laboratory interested by real-time monitoring of aquatic ecosystems.
:: Unique micro-sensor
The heart of a VIP is its sensor. It measures a signal intensity, produced by the chemical reaction at its surface, which is proportional to the concentration of the analytes. However, to perform automatic measurements over extended periods in complex media such as natural waters, most of the currently available sensors are not reliable nor sensitive enough for monitoring very low concentration of chemical compounds. In addition, fouling problem, due to the adsorption of organic and inorganic matters at the sensor surface, is an important limitation of direct voltammetric measurements in complex matrices. The VIP sensor is a unique micro-sensor which has been developed by CABE and IMT (University of Neuchatel-Switzerland) to solve all the above problems. It is produced by thin film technology on chips and photolithographic technique. It consists of an array of 5 x 20 interconnected Iridium microdiscs, having a diameter of 5 µm and a centre to centre spacing of 150 µm, coated by Hg layers and covered by an hydrophilic protective gel membrane. Measurements with this gel-integrated micro-sensor are performed in two successive steps:
a) equilibration of the gel with the sample (typically 5 minutes for a membrane thickness of 300 µm);
b) voltammetric analysis inside the gel. |
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| The main key features of the VIP micro-sensors for in situ measurements are: |
| 1) high sensitivity and reliability; |
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2) organic and inorganic colloidal and particulate material, which might adsorb on the surface sensor (fouling problem), are efficiently excluded from the gel and do not interfere with voltammetric measurement;
3) the protective gel membrane prevents irreproducibility of the measured signal due to external ill-controlled convection;
4) external medium is not modified by the voltammetric measurements (i.e. the voltammetric diffusion layer is small compared with the gel membrane thickness (see figure gel principle);
5) effects of temperature variation on the current intensity measured can be readily corrected (note that peak current variations of 3 to 8% °C-1, depending on the analyte, were observed which are of significant concern for concentration profiling in water columns where temperature may vary from typically 4 to 25°C);
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6) signals are independent of the pressure in the range 1 to 600 bars;
7) microsized-electrodes allow direct measurements in low ionic strength fresh waters without the addition of an electrolyte;
8) current intensity measured at micro-sized electrodes is controlled by a spherical diffusion and reaches a non-zero steady-state value at constant potential, i.e. stirring is not required during the preconcentration step of stripping techniques (Note: this is absolutely required to allow anodic stripping voltammetric measurements inside the protective gel membrane);
9) signals measured at micro-sized electrodes are proportional to the diffusion coefficient values of the target compounds and negligible for species larger than a few nm.
:: Points 1) to 6) are unique key features of the VIP gel-integrated micro-sensors. They are required to enable rigorous interpretation of voltammetric data obtained from direct measurements in complex media as well as reliable operation of chemical sensors in complex media for a long period of time (typically standard deviations of max. 10% were observed for continuous trace metal measurements, at the nanomolar level, over two weeks using the same Hg layers ).
:: Points 7) to 9) are features of voltammetric techniques coupled to micro-electrodes. Point 9) is a key feature for trace metal speciation studies, i.e. the VIP voltammetric probe measures in situ specifically the dissolved fraction of trace metals (i.e. free metal ions and small labile complexes with size of few nanometers) without any sample pre-treatment. Additional determination of the total metal concentrations in acidified, filtered and raw samples permits calculation of colloidal and particulate metal species by difference.
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| The distinction between these three different fractions is important to understand the role and the fate of vital or harmful trace elements. In particular, the dissolved fraction is the most important one for bioavailability and ecotoxicity interpretation. The fraction is also the most difficult to measure without analytical artifact (due to sample degradation and risk of contamination) and thus it requires direct in situ measurements. The mobile and the particulate forms play different important roles in metal circulation and residence time, e.g. fast sedimentation followed by accumulation in sediments of the particulate species and slow coagulation/sedimentation of the colloidal species. |
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