Automatic analytical methods for environmental monitoring and control

1, solution of MnSO₄ to inject 50 l; 2, water sample (as carrier); 3, mixture of KI and NaOH; 4, acid iodide wash solution; and 5, H₂SO₄. Flow-rates 1.4 ml min^-1.R₁, reactor 100 cm long; W, waste, I_v, injection valve; s-v, 3-way solenoid valve; D, detector; and P, peristaltic pump.

The amperometric determination of hydrogen peroxide in rainwater was performed in a flow assembly provided with aquarium pumps for producing the flow. The manifold is provided with a solid-phase enzymatic reactor filled with catalase immobilized on a resin type Amberlite and the detector is formed by gold electrodes modified by electro-deposition of platinum. The H₂O₂ is amperometrically monitored at +0.60 V versus the reference electrode Ag/AgCl and a stainless steel tube was used as auxiliary electrode.

The sample is inserted into a carrier of electrolyte; this channel splits to form two independent channels; one contains the solid-phase reactor to eliminate the hydrogen peroxide. Both channels (with and without reactor) merge in a single channel to lead the sample (treated and untreated) to the electrochemical cell. The outputs difference gives the hydrogen peroxide concentration.

The immobilization of the catalase is performed by the usual way of treating the resin with glutaraldehyde to be linked to the amino groups of the Amberlite (IRA – 743); then is added the enzyme and finally the reactor is washed.

The proposed flow-assembly is depicted in the following Figure III.2.28.

E: electrolyte; AP: aquarium air pump: AV: aquarium valve; Iv: injection valve; TR: tubular reactor; D: Potentiostat and electrochemical cell; W: waste.

A former article was also dealing with the amperometric determination of hydrogen peroxide and sulfur (IV) in rain, fog, snow and cryo-sampled atmospheric water vapor. The bisulfite is directly measured; and, the rest of the S(IV), present as hydroxyl methane sulfonate (HMS) or forming other carbonyl adducts, indirectly determined by releasing sulfite with an alkaline treatment.

The electrochemical micro-cell contains two homemade platinum electrodes (indicator and auxiliary electrodes). The reference electrode is Ag/AgCl and placed at the tip end of the tubular micro-cell. The working electrode is mounted on a Nafion tube, which is a cationic exchanger with the goal of separating hydrogen peroxide and bi-sulfite (it is only permeable to hydrogen peroxide). The selectivity among both compounds is implemented with a careful selection of the pH. At high pH (alkaline medium) the HO₂^- is firstly oxidized (0.30 V) and bisulfite do not interferes the output. At acidic pH and 0.65 V, SO₂ is the first to be oxidized with an almost null interference from hydrogen peroxide.

The process for the determination of HMS is based on two steps; first is measured the non protected S(IV); and second, all the SMS is converted in S(IV) by increasing pH to 11 -12. The assembly is depicted in Figure III.2.29.

Other compounds present in the matrix and presenting a low oxidation potential (versus Ag/AgCl) interferes the measurements. An auxiliary enzymatic process of destroying hydrogen peroxide (catalase) and sulfite (sulfite oxidase) avoids interferences from iron, formaldehyde, etc. Sample throughput 30 h^-1 and limit of detection 2 10^-8 mol L^-1.

ME: auxiliary flow, 0.024 to 0.06 mL min^-1, 0.1 mol L^-1 KOH or 0.1 mol L^-1 HClO₄ for H₂O₂ or HSO₃^- and HMS respectively; AE: main electrolyte flow, 0.075 to 0.18 mL min^-1, 0.4 mol L^-1 KOH or 0.4 mol L^-1 HClO₄ for H₂O₂ or HSO₃^- and HMS respectively; Water flow-rate, 0.24 to 0.54 ml min^-1; P: peristaltic pump; Iv injection valve (200 L sample loop); D: electrochemical cell and polarograph.

A multi-parametric SIA assembly designed for wastewater quality in-situ and real-time monitoring is depicted in Figure III.2.30. It is designed to determine total organic carbon (TOC), chemical oxygen demand (COD), biochemical oxygen demand (BOD), total suspended solids (TSS), nitrate, nitrite, ammonium, total nitrogen, phosphate and total phosphorous. Probably and according to many authors, SIA fits better than any other flow methodology with the multi-parametric analytical task. When a special diode array detector, is incorporated in one of the free ports of the second injection valve, additional parameters may also be evaluated in 3 min, like detergents.

In atmospheric monitoring the systems for sampling are formed of different parts; it requires a device for the sample collection; the trapping device to retain the pollutant and, the measuring assembly. The sampling device must be able of an accurate measure of the air volume sampled. The sampling methods frequently used are filtration (passive or active), sedimentation, electrostatic precipitation, centrifugation and impaction; filtration being the most common. The different parts involved in sampling should be constructed in a material not introducing pollution in the sample. Theses considerations should be added to the next step of the analysis; the sample storage; when analysis is not in-line with the sampling.

Figure III.2.30. Multi-parametric SIA assembly designed for wastewater quality in-situ and real-time monitoring.

Studies on air pollution require different sampling procedures according to the type of ambient to be monitored; a workplace analysis differs from the procedures to be applied for the study of the environment in an industrial area. The study of workplace atmosphere is special based on the level of pollutants to which the workers are exposed; e. g. sometimes a micro-sampling is recommended; the worker carries an adsorptive badge which is send to laboratory at the end of the day.

Continuous-flow methods have been connected with the other two parts (sampler and trap) to configure a complete in-line monitoring system.
Monitoring ethanol in ambient air

There are many anthropogenic actions which consequence is a change in atmospheric chemistry. New compounds were released to the atmosphere in increasing amounts from relatively recent time. The economic dependence of many countries on imported petrol derived to search for new alternatives to fossil fuels. A representative example is the extensive use of ethanol as vehicle fuel (very important in some countries like Brazil), which resulted in an increased concentration of atmospheric unburned alcohol from car exhausts. At the end it resulted in a clear change on the atmospheric composition, to a dramatic extent in urban areas.

To known the chemistry interactions at present in atmospheric gasses is required to develop analytical methods for monitoring the ethanol contents in ambient air. Unburned alcohol is an important ingredient in photo-oxidation or reactions with hydroxyl radicals. The wet chemistry of ethanol is well known contrary to the situation of the atmospheric samples.

The following method provides an enzymatic-fluorimetric procedure for monitoring the ethanol in ambient air. During air sampling the ethanol reacts with alcohol dehydrogenase, ADH, and nicotinamide adenine dinucleotide, NAD⁺, producing NADH; which is monitored by fluorescence.

The description of the scheme and operating mode has the following steps (see Figure III.2.31):

(a) Air is aspirated through a KI filter to eliminate the ozone influence

The flowing sample bubbles through the absorbing solution; a mixture of 1 mmol L^-1 NAD⁺ and 7.5 U mL^-1 ADH at pH 9 with a phosphate buffer.

(c) The product of the reaction, the NADH, is lead to the detector flow-cell where fluorescence is measured at 460 nm (excitation wavelength, 360 nm).

(d) A correction is required to avoid bubbles arriving to detector from the solvent volatility; the flow-rate in channel B is smaller than in A and the excess of solution is eliminated through channel C.