Automatic analytical methods for environmental monitoring and control

3. 
   Continuous-flow titrations – the sample is continuously inserted into the system either by keeping its speed constant and changing that of the titrant (Figure III.2.1.c), or both sample and titrant can be kept at a continuous speed and measurements can be made as a function of the analytical signal obtained.
   

The continuous monitoring of cyanide anion, which is a highly toxic ion, has been carried out by using a CFA system (Figure III.2.2) and a classical analytical method with spectrophotometric detection (barbituric acid and chloramines T). As Figure III.2.2 illustrates, the stream of chloramines T is mixed with the sample then merged with a pyridine/barbituric acid stream after the corresponding reaction coil. A second reaction coil allows the colored reaction product to form and be measured at 578 nm. This method, also realized by normal FIA and reverse FIA, permits a comparison between these two modes and CFA.

Several international corporations, such as Technicon, Skalar, Burkard, etc. have contributed to the development and commercialization of analyzers and assemblies based on Skeggs’s idea. For many years, these were the only alternative available for the automation of high-throughput control laboratories.

SFA is characterized by the use of one or several liquid streams where the samples are introduced by sequential aspiration and separated by means of air bubbles aimed at avoiding the carry-over. Therefore, the final liquid stream is segmented into small discrete liquid slugs by bubbles of air or other gas that entirely fills the stream tubing bore.

The sample and reagents are mixed by passing through glass coils and through a temperature controlled heating coil if heat is required to speed the development of the reaction product before detection. In the initial work, it was found that in some analyses the high molecular weight components contained in samples were interfering with the chemical reactions. This problem was ingeniously overcome by the use of a cellophane dialysis membrane to remove them. Today, the technique has become one of the most reliable and widely used methods for automatic chemical analysis in routine and research analytical laboratories. This technique has the advantage of for measuring large batches of samples for up to 16 analytes simultaneously at speeds of up to 120 samples.

A SFA system is presented in Figure III.2.3 and it comprises a series o modules each performing a specific function, e.g. sampling, propelling device, sample transport, heating, separation unit, detector equipped with a flow-cell, data recording and analysis module, all of them coupled on-line to one another.