Flow-based method

During the fast passage of the sample through the analytical path, some processes inherent to the specific analytical method may not reach completion, and this feature makes it easier to exploit partial yet reproducible development of chemical reactions, monitor unstable chemical species, and implement catalytic methods of analysis. The favourable characteristics of flow-based methods that do not require chemical equilibrium to be reached have often been emphasised, and can be exploited to expand the potential and application range of analytical procedures. 

Flow-based analytical procedures are already used in many fields, e.g., environment, food and health. In tandem with the developments described above, flow analysers will also be used in new and emerging fields of application such as the "omics" (metallomics, genomics and proteomics), biotechnology (enzymatic assays, platforms for bio-sensors and flow immunoassays) and quality-control applications in general. Economic resources will also be a driver for more multi-parametric flow-based methods, particularly with spectrophotometric detection. 

Although contact-based methods have proved useful for biological studies, there is a strong need for contact-free, flow-based methods for precise confinement and manipulation of biomolecules and cells. To this end, a number of contact-free fluid flow-based trapping and manipulation methods have been developed. These methods rely on microvortices, microeddies, stagnation point flows, or stagnant regions within a fluid stream to confine particles for further analysis. 

In this context, solid-phase extraction (SPE) appears a suitable preparative technique (see Key Facts), fostering elimination of interferences from the matrix, selective extraction of analyte and even analyte pre-concentration. This chapter addresses some of the methods that adopt the use of SPE prior to chromatographic determination by high-performance liquid chromatography (HPLC), with particular emphasis on automated flow-based methods. 

Obviously, cyanide cannot be directly measured by flame atomic absorption spectrometry (FAAS), but an indirect approach, as that schematically depicted in Figure 7.16, allows this possibility to be implemented, improving detection limits with regard to those reported previously for flow-based methods. The FIA manifold relies on the formation of soluble metal-cyanide complexes as the sample passes through a small column packed with soUd-phase reagent (SPR). Different SPR have been tested for indirect determination of cyanide using FIA. In all cases the eluted complex is measured by FAAS. Detection limits close to 0.05 mg/1 cyanide have been reported [28]. 

This chapter reviews the automated sample processing methods and separation and detection methods used with flow injection analysis in the determination of parabens in various consumer products. The advantages and limitations of the analytical methods are also highlighted. In addition, the applications of the different flow-based methods in the determination of parabens are highlighted in Table 10.1. 

TABLE 10.1 Applications of Flow-Based Methods in Analysis of Consumer Products... 

While the benefits of flow-based methods such as the reduction of the analysis time and higher sample throughput are greatly appreciated in the analytical sciences, their applications in the determination of parabens are greatly limited. For comparison, flow-based techniques are widely applied in the analysis of other preservatives such as sulfites and nitrites [30,31]. Thus, more investigations and publications on this subject are called for. The applications of this technique in the analysis of foodstuffs are limited to simple techniques such as flow injection and sequential injection analysis. However, the use of novel hyphenated flow-based techniques would bring about a breakthrough in the determination of parabens in various consumer products. Moreover, there should be a shift to the use of mass spectrometric detectors from the selective detectors such as chemiluminescence and UV-Vis. 

Christodouleas, D., C. Fotakis, A. Economou, K. Papadopoulos, M. Timotheou-Potamia, and A. Calokerinos. 2011. Flow-based methods with chemiluminescence detection for food and environmental analysis A review. Anal. Lett. 44(1-3) 176-215. 

Flow-Based Methods Coupled to Spectrophotometric Detection 288... 

Flow-Based Method Coupled to Fluorometric Detection 292... 

FLOW-BASED METHODS COUPLED TO SPECTROPHOTOMETRIC DETECTION... 

TABLE 17.1 Application of Flow-Based Methods for the Determination of Tartaric Acid... 

FLOW-BASED METHOD COUPLED TO FLUOROMETRIC DETECTION... 

Table 19.4 reports comparison of the analytical characteristics of flow-based methods with CL and spectrophotometric detections for the determination of vitamin E (a-tocopherol). 

Flow-Based Methods for Determination of Antioxidant Activity 644... 

Contributions of Flow-Based Methods and Microplate Format Protocols for... 

FLOW-BASED METHODS FOR DETERMINATION OF ANTIOXIDANT ACTIVITY... 

TABLE 33.1 Determination of Antioxidant Activity by Flow-Based Methods... 

This analytical feature of flow-based methods meets the requirement that more than one method for antioxidant activity characterization should be applied due to the complexity of samples and the absence of a standard antioxidant assay (Huang et ah, 2005). Following the same challenge, an automatic flow procedure based on MSFIA was proposed for the sequential determination of ABTS -scavenging capacity and Folin-Ciocalteu... 

CONTRIBUTIONS OF FLOW-BASED METHODS AND MICROPLATE EORMAT PROTOCOLS FOR ANTIOXIDANT ASSESSMENT... 

The analytical methods reported in the literature were mainly applied to pure compounds, to beverages, and to extracts of solid food products. In our opinion, the future challenge of both microplate- and flow-based methods is to determine the antioxidant activity of solid food samples simultaneous with the extraction procedure, with the aim of reducing both the laborious extraction protocols and the high variability between experiments. This is a critical point when digestive gastrointestinal fluids are used as extractor solvents to assess the fraction of antioxidant capacity that is really bioaccessible. 

---