4.2. Unstable Reagents and Products
Another favorable aspect, inherent to the reproducible and controlled timing and reaction conditions, is the reproducible generation of unstable species in flow systems, paving the way for the exploitation of novel reagents and products [52]. Strong oxidizing, e.g., Ag(II), Co(III), and Mn(III), or reducing, e.g., Cr(II), U(III), and V(II) agents, can be in-line generated from stable species by exploiting e.g. electrochemical processes [53] or a mini-column filled with the Jones reductor [54]. Unstable reagents can be also chemically produced in solution, as exemplified by in-line generation of bromine from bromide and bromate [55]. Applications involving unstable reagents benefit from their generation in a closed system, without contact with the atmosphere. The ability to detect unstable yet reproductively formed species allows the proposals of novel analytical methods and represents a clear advantage of flow analysis [56][57]. Moreover, the instability of the measured species can be exploited to improve selectivity, as demonstrated in the ingenious determination of ascorbic acid relying on its intrinsic UV absorption and further decomposition [58].
4.3. Optosensing
A particular application of solid reagents/sorbents in flow analysis refers to solid-phase spectrophotometry or optosensing
[59], involving a suitable sorbent placed placed in the flow-through detector. The reagent is immobilized on the solid support, where chemical derivatization occurs directly, or the derivative is formed in solution and further retained on the support. Both approaches allow measurements to be carried out simultaneously with the analyte/derivative retention. The approach is worthy for improving sensitivity as the reaction product is accumulated at the support, and the dilution inherent to analyte elution before measurement is avoided. Selectivity may also be improved due to changes in reactivity of the immobilized reagent or kinetic discrimination
[60]. Reversible retention of the analyte and exploitation of the same immobilized reagent for successive measurement cycles is also feasible
[61], yielding more environmental friendly procedures.
4.4. Simultaneous Determinations and Chemical Speciation
Chemical derivatization is also involved in most simultaneous determinations in flow analysis, either by adjusting the reaction conditions or by adding selective reagents. To this aim, flow and manifold programming
[4] have been often exploited. Other strategies involve kinetic discrimination, multi-site detection
[49], multi-purpose flow systems
[62], asynchronous merging zones
[63], sandwich techniques
[64], and reverse flow analysis
[32].
A noteworthy application relies on chemical speciation, exploiting the differences in reactivity of the species resulting from the chemical derivatization. Selective determination of a species followed by total determination after sample pretreatment becomes then feasible. Classical examples are the determinations of Fe(II)/Fe(III), Cr(III)/Cr(VI), and NO
3−/NO
2− [65]. The determination of clinical iron parameters (serum iron, unsaturated iron binding capacity, and total iron binding capacity) in human serum, by exploiting sample processing under different acidities and complexation with ferrozine exemplifies a more recent application
[66]. Overall, the approach is usually more simple, cost-effective, and faster than the chromatographic ones, although the scope is limited by the number of analytes determined, typically restricted to 2–3 species.
4.5. Green Analytical Methods
While in GAC there is a premise to avoid chemical derivatization whenever possible
[67], several approaches have been proposed to minimize its impact in the environmental friendliness, relying mainly on the use of less hazardous chemicals and solvents, and more effective energy sources
[68]. In this sense, flow analysis is a powerful tool especially because of its potential to minimize reagent consumption and waste generation. This is inherent to modalities involving the intermittent addition of reagents in the chemical derivatizations, such as sequential injection, multipumping flow, and multisyringe flow analysis, as well as those involving solid-phase reagents. This goal is also successfully attained by miniaturization, including µ-FIA and lab-on-valve. However, the potential of flow analysis to GAC is significantly wider, involvin
g e.g. reagentless procedures
[69][70], replacement of toxic reagents
[71][72], reuse of reagents
[61][73], vegetable natural extracts as source of reagents
[74] and enzymes
[75], as well as in-line waste treatment
[72][76].
4.6. Expert Systems
The potential of chemical derivatizations in flow analysis is significantly expanded in expert flow systems
[2]. This encompasses e.g. in-line optimization of the reaction conditions, in-line adjustment of the medium and flow/manifold programming, aiming to avoid matrix effects, and to perform multi-analyte determinations. Accuracy assessment relying on analyte determination by different analytical methods, typically involving different approaches for chemical derivatization, is also feasible
[77].
5. Final Remarks
Chemical derivatization benefits itself from the favorable characteristics of flow analysis, allowing a better exploitation of chemical reactions without attaining equilibrium, the possibility of kinetic discrimination, the exploitation of unstable reagents and products, and the compliance with the GAC principles.
Most flow-based applications involve a simple mechanization of well-established analytical methods. Nevertheless, the advantages are increased when chemical derivatization exploits the characteristics inherent to flow analysis. This is a fertile field for further development, also when the flow analyzer is a front-of-end for chromatography.
The term “derivatization” is typically associated to chromatography, mass spectrometry, UV-vis spectrophotometry, and luminescence. However, its relevance to flow analysis has been increasingly emphasized. This also holds for µFIA and microfluidic devices.
In spite of the outstanding current development of flow analysis, an intense human labor is still required to minimize systematic errors and to ensure reliable analytical results and safer working conditions to the analysts. In this context, the development of expert systems plays an important role.
Several environmentally friendly innovations have been proposed for chemical derivatizations in flow analysis and the flow-based systems paved the way for making chemical derivatization a useful strategy for GAC. This is a counterpoint to the less realistic statement of GAC principle that chemical derivatization should be avoided [67].