Automatic analytical methods for environmental monitoring and control

Two software solutions are available:

One software allows the analyzer to be a stand-alone unit, its main functions being:

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  management of all analytical specifications;
  
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  automatic calibration with 1 to 5 points;
  
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  archiving and data transmission;
  
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  high an low level alarms
  
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  auto dilution;
  
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  reanalyze in case of problems;
  
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  diagnostics.
  

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  full remote control;
  
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  remote diagnostics;
  
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  upload/download of methods, programs and results.
  

The SINGLE had been designed for the control and analysis of a specific parameter important to laboratory or production unit. It comprises a built-in sampling system with probe and wash receptacle (a sampler with 12 cups). One analytical unit includes: a pump; a manifold; one or two heating bath; one colorimeter with a filter in the range 340-880 nm; a compartment for reagents; flasks and wash valves; two standard solutions and micro-electronic valves. A microprocessor controlled unit which comprises a logical board for data processing and system’s control, a touch panel to access all functions, 20 columns impact printer; scrolling menu software. The SINGLE can be used by virtually anyone, with no specialist knowledge equipped for routine use: simply load in the sample and press the “analyze” key.

The calibration is made with the base line (zero) and two standards (3 points). If the correlation coefficient is out of the user defined limits, the operator is warned by an error message and a new calibration must be performed. The result is calculated from the average of 200 measurements and validated only if their coefficient of variation is below 0.5 %.

The SINGLE is mainly used for: phenols, cyanides, total organic carbon in wastewater; volatile acidity, sugars in wine; nitrates in food products.

The EVOLLUTION II distinguish itself by using a modern proportioning pump and an advanced “unique focusing” colorimeter using optical fibre technology and bichromatism. With a capacity to sample of 104 cups, the analyzer can assay 60 tests per hour for up to 8 methods, including methods where distillation is required. The system incorporates computer-generated data reporting, with a real time display of measuring signals, zooming function, selective printout and electronic archiving. The EVOLUTION II is mainly used for nutrients in seawater; phenols, cyanides and detergents in wastewater.

The INTEGRAL_Futura is the new generation in continuous flow analyzers featuring advanced microflow technology, electronic debubbling, modern electronics and comprehensive computer control and data management. Each FUTURA console can be in fact considered as an independent analyzer and it is directly connected to the computer.

Nowadays, the manipulation of samples, reagents and bead suspensions is technologically different from the initial continuous flow systems, although the methodology principles are the same: sample introduction, controlled dispersion, reproducibility of events, which result in a precise control of the physical and chemical parameters.

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III.2.3. APPLICATION OF THE FLOW TECHNIQUES OF ANALYSIS IN ENVIRONMENTAL MONITORING AND CONTROL

The emergent advances for monitoring portability (last 10-15 years) is a clear consequence of the recent development of chemical sensors, biochemical’s, gas sensors and disposable strip sensors. The combination of sensors with electronic transducers (electrodes-electrochemical, transistors-optical, semiconductors-impedance, etc.) allows the conversion of the molecular (or ionic) recognition response into an electrical output to establish the analytical concentration of the substance to be tested.

A lot of efforts resulted in the existing (commercially available or in the home-made phase) plethora of portable devices developed for such different scientific fields as bio-medical, mining, industrial, oceanographic studies, domestic water production and, in environmental monitoring and control, especially for testing water quality and atmospheric pollution. The future wide use of these portable devices is clearly connected with the manufacturing of robust, compact, low-cost, long-term and easily transportable monitoring devices. At the present, one must recognize we are far away from the ideal.

Flow procedures when are provided with miniaturized detectors, fit especially well with portable multi-sensor monitoring instrumentation. The low-cost miniaturized flow-cells containing electrochemical sensors (potentiometry, conductimetry, etc.) was the starting point to establish a radical departure from traditional analytical ways (sampling and transport) followed by the LED spectrophotometric detectors. The next natural step was the combination of different cells in the same flow manifold or multi-sensor arrays. Examples are the portable FIA manifolds with three or four ion sensors for calcium, potassium, nitrate and chloride.

Gas analyzers based on multi-cell potentiometric sensors have been also proposed and developed for many toxic gases: namely, sulfur dioxide, volatile organic compounds and carbon monoxide or nitrogen oxides. Even some portable devices have been designed to be adapted to liquid or gas samples depending on the input (type of pump) and sensors.

A flow system for complete environmental monitoring and control must contain the sampler, the flow assembly and the detector and recording devices. The central part of the system, the flow assembly in different modalities (FIA, SIA, multi-syringe, multi-commutation, etc.) fits well with the rest of the system due to the easiness to conform to the requirements of the “standard recommended procedure”, usually proposed in a batch mode. Flow methods also fit well with any kind of analytical detector with the single requirement to change the batch flow to a flow-cell; and, normally it is not a complicated task, to adjust either sample and reagent concentrations to conform to the batch empirical conditions. An advantage of flow procedures over its batch counterparts is that they improve the precision of the batch method, because of the reproducible timing and controllable sample dispersion.

Another interesting point is the capability of flow methods to modify the sample matrix (sample pre-treatment) by integrating different experimental steps like analyte pre-concentration (ion exchange, liquid-liquid extraction, and gas diffusion) or dilution, the filtration of turbid samples or containing suspended solids, etc.

These changes in the sample matrix resulted in benefits to improve the detector performance. For instance, when the sample matrix is filtered, some constituents that enhance the base line are removed; then, the filtration improves the detectability of the compound of interest. The pre-concentration of the analyte means lower detection limits.

Analytical flow procedures are relatively young; segmented-flow analysis is the “father” of the dynasty; then, the apparition of FIA supposed an explosive and widespread use of the flow methodology. New flow-methodologies are continuously expanding and appearing new modalities: in this order, mono-segmented-FIA, SIA, Multi-syringe, Lab-on a valve, Multi-commutation, etc. And last but not least, the primary advantage of the flow methods over other automatic methodologies is economic; an aspect of paramount relevance when the environmental monitoring and control means an unimaginable number of daily analyses.

Flowing stream techniques are well adapted to in-situ determination of analytes in water samples (about 2500 references up to 2003) and a lesser extent to air analyses. An overview on the flow analytical procedures gives a clear result; FIA is the preferred and by far, the most used methodology. Different extensive reviews can be found in analytical literature dealing with FIA-water monitoring. And the detector “married” with the FIA is also by far, the UV-VIS absorption spectrophotometer. A different problem is the soil monitoring (nutrients and pollutants) due to non-fluid nature of the matrix, a problem of special interest in regions with intensive agriculture. Most of the problems from polluted soils are quickly passed to the water quality.

When thinking about environmental monitoring and control, one should think on the commercially availability of robust, compact, portable (even submersibles) and multi-parametric systems. Most of flow assemblies were originally designed for one or few parameters and for off-line analysis. In addition, the miniaturization of the flow manifolds will result in lesser sample and reagents consumption. The present requirements to be pointed out are the design of miniaturized, multi-parametric systems to fulfill the needs of the on-line complete analysis.

The environmental monitoring requires separating water and atmospheric samples; both of them comprising many different matrixes (like drinking water, marine water, wastewater, deep ground waters, etc.) and each matrix includes a large number of analytical parameters; like common sample constituents and pollutants from an external source. A simple classification will divide constituents and pollutants in water samples in anions (mostly of nutrients are included into this group), cations and organic compounds.

On the other hand and as above reported, there are different flow methodologies. An overall vision of the complete problem obliges necessarily to select examples to illustrate the present possibilities.

A selection of some examples based on different flow-methodologies and devoted to one kind of the following areas:

(a) Water monitoring and control: coastal sea waters, FIA nitrites and nitrates; aqueous sediments, dissolved oxygen; rain water, fog and snow, hydrogen peroxide; and sulfur(IV); and wastewater, SIA multi-parametric.

(c) Soil pollutants, pesticides.

The adaptation to flow systems from classical batch procedures means in many occasions to change some chemical steps. The determination of the total nitrogen is performed through nitrate or ammonium, after a digestion step by the traditional Kjeldhal; a suitable alternative for a flowing stream method is the on-line photo-degradation with the aid of chemical oxidants.

On the other hand, a plethora of flow spectrophotometric procedures for nitrite determination have been published; most of them rely on the modified Griess or Shinn procedures. The nitrate determination is performed by the same nitrite reaction after a prior redox process to convert nitrate into nitrite, being the solid-phase reactor filled with copperized cadmium is the preferred for most of authors. Other alternative fitting best for flow methods are the homogeneous reduction with hydrazine; the photo-reduction by irradiation with a low pressure Hg-lamp; or enzymatic processes. A certain number of FIA assemblies integrated nitrate, nitrite, ammonium and total nitrogen with the aid of a spectrophotometric detector. For more information about the chemistry of these procedures see Section III.3 “Automation of the Spectrophotometric methods”.

The speciation of nitrogen, sequential determination of nitrite (Sinn reaction), nitrate (with the copperized-cadmium reactor) and total nitrogen (UV photo-degradation) can be easily integrate in a compact flow-manifold as depicted in figure III.2.25.

The main point to support the in-situ monitoring is the changes suffered by samples during collecting and storage: this analytical requirement obliges to design and prepare field-portable laboratories able to perform in different natural environments and conditions. This has been solved in some less difficult situations like workplace air; rivers, lakes, etc.; in more complicated situations the number of available multi-parametric solutions is certainly rare. Some situations are not clearly solved like sampling deep water in a lake, sea or estuarine; transporting the sample from the original place to the ship board-laboratory increases the solved air (among other inconveniences) with the inherent disadvantages.

A recent and maybe representative example of the efforts to solve this problem is the manifold nitrate determination in estuarine and coastal water samples by a “submersible flow injection analyzer” in which size, weight and low buoyancy [the difference between the upward and downward forces acting on the bottom and the top of the cube, respectively, is called buoyancy] and easy use of the set-up were carefully studied. According to the moment requirements, the instrument can be operated in manual (diagnostic) or automatic (long term monitoring) mode and operated as a bench-top, shipboard or submersible instrument. The instrument has been proved as a shipboard mode for mapping nitrate concentration in North Sea and submersible mode for the Tamar estuary transect.

The chemical procedure is the well known modified Griess procedure for nitrite with the prior reduction of nitrate by the cadmium-copperized solid-phase reactor as depicted in the Figure III.2.25. A sample on-line filtration unit is also included.

F
igure III.2.25.

Figure III.2.26.

The Winkler titration is the traditional robust method for the determination of water-dissolved oxygen, DO. In a first step, the oxygen oxidizes the Mn(II) hydroxide block. Addition of sulfuric acid in the presence of an excess potassium iodide dissolves the oxidized manganese hydroxide producing tri-iodide, which is in a final step, titrated by thiosulfate with the aid of starch for clear end-point. The classical procedure seems not suitable for purposes of accurate respirometric measurements. Several suggestions appeared in the analytical literature for automating the DO measurements, some of them into the FIA field. The selected example is based on the spectrophotometric monitoring of released tri-iodide; the assembly is of type of r-FIA (reverse FIA, where the sample is the carrier and reagents are inserted instead of the sample) to avoid blockage of tubing when Mn (II) ion solution merges continuously with the alkaline (sodium hydroxide) stream. The water sample is the carrier where the Mn (II) is inserted. The resulting stream merges with an alkaline-KI mixture, and the reaction product (oxidized manganese hydroxide) is solved by merging with the sulfuric acid solution. Tri-iodide is spectrophotometrically monitored at 440 nm.