Automatic analytical methods for environmental monitoring and control

III.1. INTRODUCTION

In the last decades can be observed the use on a larger and larger scale of automation in various domains of science and technique, and especially in the domain of environmental quality monitoring. This was possible due to the great progresses in the top technology fields, like micromechanics, microelectronics and especially computer construction.

The automation of some processes or technologies presents great advantages, which will not be mentioned here, allowing the achievement of great performances and low costs for the products obtained, which would otherwise be inaccessible.

Automation found a wide application domain even in the field of analytical chemistry, presently being harder and harder, and in some cases even impossible, to resolve the problems arising for a chemist without using automated analysis methods.

Generally, the automation of the analytical processes can be made in several ways. Thus, automated analyzers for discrete samples were created, together with flow analyzers and robotic analyzers. From these, of extreme importance are the analyzers based on the principle of flow analysis.

In the last decades special attention was paid to the flow analysis methods being applied for the solving of routine or research analytical problems. The common characteristic of these methods is the fact that the measurements are made in an analytical channel through the sample to be analyzed and the reagents circulate. The devices constructed for the applying of these analysis methods are characterized by simplicity from a mechanical point of view and operation safety. In addition, certain important characteristics of the flow analysis methods, like analysis quickness and reproducibility of the determinations, are very high.

The use of computers to command, control and diagnose the equipment used, and for the processing of the obtained experimental data, has increased even further the performances of the flow analyzers, many of them being able to operate in a fully automated regime.

The flow analyzers can be used for the ‘on-line’ or ‘off-line’ analytical determinations.

In the ‘on-line’ version, the flow analyzer is placed near the sampling location. Thus, the samples can be taken (generally automatically) from an industrial flow, a wastewater evacuation channel, etc. The analysis begins immediately after sampling, the result of the analysis being obtained in a short time from a few tens of seconds to at most a few minutes. Proceeding in this manner, the results of the analysis are obtained in ‘real time’ so that efficient actions can be taken, whenever necessary. The ‘on-line’ flow analyzers are generally analyzers dedicated to certain chemical species, for some concentration domains. These analyzers, generally, must have a robust construction and this is due to the location in which they operate and where vibrations, large temperature variations, etc. may be present.

In the ‘off-line’ version, the flow analyzer is located at a certain distance from the sampling location, in a laboratory. The samples, collected from various points, are brought to the laboratory for analysis. Between the sampling and the analysis a long time may pass (hours or even days) which is often inconvenient.

Different methods for conducting the flow determination ‘in-line’ also exist. In this case a parameter, or even a species to be analyzed from an industrial flow, is determined with the aid of a sensor introduced in the respective flow.

In the following pages a short presentation of the automated flow analysis methods will be made with applications in the environmental quality domain, together with other automated analysis methods with important applications in the mentioned domain.

Automatic flow methods of chemical analysis, unknown for more then a half a century ago are now widespread in most analytical laboratories. Since the original paper published by Skeggs in 1957 on multisegmented continuous flow analysis, many improvements and even simplifications have been made on this field.

The importance and interest of these continuous flow analyses regarding the study of water quality is reflected in numerous reviews which have been carried out within this field either with a general approach or in the case of their application to the determination of certain parameters. The majority of these papers are related to the application of flow injection analysis (FIA) and, in the last years, to sequential flow analysis (SIA). The explanation is the fact that either they are no longer in an widespread use (segmented flow analysis) or they have no fast development (multicommutation flow analysis and multisyringe flow analysis).
III.2.1.1. Continuous Flow Analysis
Continuous flow analysis (CFA) refers to any process in which the concentration of the analyte is measured uninterruptedly in a stream of liquid or gas. The basic principle of continuous flow analysis is to eliminate chemical analysis by hand-mixing of reagents in individual items of glassware and to substitute a continuously flowing stream of liquid reagents circulating through a closed system of tubing. Therefore, in CFA the sample is converted into a flowing stream by a pumping system and the necessary reagent additions are made by continuous pumping and merging of the sample and reagent streams. The mixing and the chemical reactions take place while the sample solution is on its way toward a low-through cuvette, where the analytical signal is continuously monitored and recorded. The principal difficulty to overcome is to prevent intermixing of successive samples during their passage through the analyzer, this intermixing causing overlap and loss of discrimination at the recording stage. In general, to minimize this so-called carry-over, the design of the timing sequence between samples was optimized by reducing the processing rate and by inserting a washing solution (e.g. water) between each sample. The higher the sample throughput the greater is the interaction between samples and this accounts for the restriction on sample processing rate in continuous analyzers.

The diagram of the simplest CFA system is presented in Figure III.2.1.a and it consists of:

• 
  a pump (P) that is used to propel the carrier stream through a thin tube;
  
• 
  a coil of tubing also named reaction coil (RC) in which the sample zone disperses and reacts with the components of the carrier, forming species that are detected by a flow-through detector;
  
• 
  a recorder/personal (PC) computer which registers the CFA typical signals.
  

The continuous flow approach is the most flexible way to carry out a number of operations necessary to perform a chemical assay. In addition to sample dilution, heating, mixing, and reagent insertion, operations executed in a discrete analyzer too, the continuous flow mode can perform dialysis, gaseous diffusion, distillation, solvent extraction and other types of chemical pretreatment directly on the sample, during its transport to the detector.

A greater variety of detectors may be applied to a flowing stream, the most common being the photometric ones.
Types of continuous flow methods

The continuous flow methods can be classified into three groups:

1. 
   Continuous mixing methods (Figure III.2.1.a) – that involve the sample insertion into the system, mixing it with the carrier or reagent, measuring the reaction plug as it passes through a suitable detector and either sending to the waste (open systems) or recirculating it (closed systems). Also, the sample can be inserted into the system in an intermittent mode, with washing solutions intercalated between samples in order to avoid carry-over.
   
2. 
   Stopped-flow continuous mixing methods (Figure III.2.1.b) – the flow is stopped at various stages during the process in order to prevent air from entering the system between sample aspiration and reagent aspiration or washing. A kinematically controlled probe aspirates a certain sample volume through a steel tube dipped into the sample solution, after which it is raised and the pump is stopped. The probe is then immersed in the reagent/washing solution reservoir and the pump is restarted. In kinetic methods, the flow is stopped into the detector flow-cell to monitor the evolution of the reaction.
   

(c)