A Critical Look at the Evolution of Flow Analysis

The inception of flow analysis (FA) is closely related to the need for a better understanding of chemical reactions. At the beginning of the last century, these reactions were intensively investigated, and the main involved parameters were the reactant amounts and the available reaction time.¹ It was soon realized that solution mixing and detection were often time-dependent, so they were better accomplished on a fluidic basis.² This stimulated the development of simple flow-based instruments³ that allowed chemical reactions to be more efficiently investigated. The availability of commercial colorimetric detectors in the 1930s4 increased the demand for chemical analysis and, thus, the laboratory workload. This motivated the proposal of the AutoAnalyzer®, the first commercial air-segmented flow analyzer.5 It was intensively used for large-scale assays, persisted for several decades, and involved only a few manufacturing companies. As the implementation of official, recommended, or tentative methods was preferred, the AutoAnalyzer underwent a relatively slow evolution yet an amazing worldwide acceptance. Later on, flow analyzers without segmentation were proposed.6 An example is the flow injection analyzer, which involves precise sample insertion, controlled dispersion, and reproducible timing. Exploitation of these features improved the simplicity, versatility, and flexibility of the analyzer. Further evolution led to the emergence of the sequential injection analyzer and derived ones, which occasionally involved segmented and unsegmented streams. Nowadays, flow analysis is approaching maturity, as evidenced by the number of applications, books, book chapters, tutorials, thesis, academic courses, workshops, scientific articles, events, seminars, commercially available instruments (some of them specially designed for educational purposes), patents, etc. Intensive development of expert flow systems,7 usually exploiting manifold programming,8 is underway. Micro-flow analyzers are also being proposed, especially for large-scale assays under laboratory, in-situ and in-vivo conditions. Compliance with the principles of Green9 and White10 Analytical Chemistry has always been a positive factor for FA development. For a FA healthy evolution, inertia or setback should be avoided, and synergy rather than divergence should be ensured. To this end, flashback and consensus are relevant. Flashback permits the evaluation a particular situation in the past, whereas consensus is more related to future developments.