Modern qualitative analysis by miniaturized and microfluidic systems
Introduction
Analytical science is the scientific discipline dealing with the production of information (qualitative, quantitative and structural) about materials or target systems, in general. This information is mainly addressed to satisfy client information needs according to specific requirements (“fitness for purpose”). It is generated through the implementation of the analytical process. Simplification, automation and miniaturization are clear trends for laboratories, characterizing new analytical processes that are more efficient and have greater potential to provide information. These trends in laboratories are possible because of technological developments in the past few years, in parallel with an evolution towards simplification and the miniaturization affecting many other facets of our life (e.g., computers and telephones). Simplification, automation and miniaturization have produced a revolution in laboratories in every field of application.
Miniaturization is rapidly growing with novel ideas in recent years [1]. As in other fields, analytical systems have been affected by this tendency. Concretely, the capacity to carry out laboratory operations on a small scale using miniaturized devices is very appealing. Thus, micro-total analysis systems (µTAS), also called lab-on-a-chip (LOC), have renewed interest in the scaling laws of 20–25 years ago [2]. To this end, small scale reduces the required time to synthesize and to analyze a product, as greater control of molecular interactions is achieved at the microscale level. In addition, reagent cost and the amount of chemical waste can be very much reduced. Now, at the beginning of this century, it is clear that lab-on-a-chip approach is starting to be considered as a potential analytical tool in many application fields. However, some miniaturized analytical systems, such as capillary gas chromatography (CGC), micro-liquid chromatography (µGC) and micro-capillary electrophoresis (µCE), which can be considered as intermediate level of miniaturization (partial miniaturization), have been consolidated in routine laboratories for the analysis of complex samples. Miniaturized analytical systems really do constitute powerful tools in modern chemical analysis, although facing existing challenges [3].
One primary, important part of chemical analysis is qualitative analysis. As previously reported by the Valcárcel group, qualitative analysis cannot be considered a declining branch of analytical science [4]. Instrumental analysis and other recent “at home” alternatives, such as kit or spot tests (e.g., the popular pregnancy test), reinforce the role of qualitative analysis viewed in a modern way {i.e., not only for identification of compounds in the sample [5], but also classification of samples following screening strategies [6]}.
This modern view of qualitative analysis connects very well with the present needs of laboratories. This new view received practical support on the publication, in Europe, of the Decision of the European Commission 2002/657/EC about the analytical requirements of screening methods applied to the analysis of residues of organic pollutants for food-safety-control purposes. In this way, the support of the EC for the clear establishment of the principles of metrology in qualitative chemical analysis through the G6MA-CT-2000-01012 project (MEQUALAN) [7], [8], allowed reporting the implementation of quality principles in qualitative analysis.
From this background, the convergence of qualitative analysis and miniaturization opens up new approaches and opportunities to solve a good number of real-world analytical problems. This goal is the objective of this article.
Section snippets
Miniaturized analytical systems
Miniaturization means “small or very small scale”, and is therefore associated with the size of the systems. From this point of view, we can distinguish three different levels:
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mini-scale (several mm or µL);
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micro-scale (a few mm to 50 µm and processing samples between a few µL and 10 nL); and,
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nano-scale (below 50 µm – even 1 µm in some cases, and with sample size below 10 nL, or even at the pL or fL level).
But, in any case, the association with the reduction of size can be somewhat confusing,
Modern qualitative analysis
Qualitative analysis is the first step of chemical analysis, and is characterized by information not represented by numbers. Traditionally, it has been associated with the identification of compounds [5], [12], but today this is only a partial view of the qualitative analysis, as Fig. 2 shows. The classification of samples according to preset criteria is increasingly routine work in analytical laboratories. The analytical features of this type of qualitative analysis are not exactly the same as
Qualitative analysis using miniaturized analytical systems
The immediate approach to implement qualitative analysis using miniaturized analytical systems is coupling or integrating detectors with high qualitative capabilities. In a recent article, Yue et al. summarized very well the latest contributions on the integration of spectroscopic analyses into µTAS [14]. The common detection modes used in miniaturized systems are laser-induced fluorescence (LIF), electrochemical detection (ED), and mass spectrometry (MS). MS is recognized for the
Conclusions
Miniaturized analytical systems can be used for qualitative purposes, covering the different aspects of qualitative analysis: identification of compounds, classification of samples (screening methods), and, in a wider view, “diagnosis” testing for diseases (clinical) or fraud detection (food). Partial miniaturization of the analytical process can be of interest for solving or improving some specific analytical problems, but the main achievements and revolutionary approaches involve
Acknowledgements
Financial support from the Spanish Ministry of Economy and Competitiveness (CTQ2013-48411-P) and Junta Comunidades Castilla-La Mancha (Project PEIC-2014-001-P) are gratefully acknowledged. The support given through an “INCRECYT” research contract to M. Zougagh is also acknowledged.
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