Analytical multiplex

Demand of analytical information is increasing continuously as well as improvements in quality and quantity. Characteristics that have to be kept in mind are depicted in Figure 9.20. They are at the same time achieved goals and never-end objectives. Determining selective and sensitively, in a precise and accurate way, a number of analytes (multiplexed) as fast as possible with low-cost, simple, and stable miniaturized devices that are easy-to-use, with low-energy consumption requires continuous research. 

The chemical world is often divided into measurers and makers of molecules. This division has deep historic roots, but it artificially impedes taking advantage of both aspects of the chemical sciences. Of key importance to all forms of chemistry are instruments and techniques that allow examination, in space and in time, of the composition and characterization of a chemical system under study. To achieve this end in a practical manner, these instruments will need to multiplex several analytical methods. They will need to meet one or more of the requirements for characterization of the products of combinatorial chemical synthesis, correlation of molecular structure with dynamic processes, high-resolution definition of three-dimensional structures and the dynamics of then-formation, and remote detection and telemetry. 

In recent years, rapid advancements in photonic technologies have significantly enhanced the photonic bio/chemical sensor performance, especially in the areas of (1) interaction between the light and analyte, (2) device miniaturization and multiplexing, and (3) fluidic design and integration. This has led to drastic improvements in sensor sensitivity, enhanced detection limit, advanced fluidic handling capability, lower sample consumption, faster detection time, and lower overall detection cost per measurement. 

Current security and health concerns require robust, cost-effective, and efficient tools and strategies for the simultaneous analysis, detection, and often even quantification of multiple analytes or events in parallel. The ability to screen for and quantify multiple targets in a single assay or measurement is termed multiplexing. 

Among optical sensors, those based on fluorescence are of major interest because of their ability to use spectral and temporal information multi-wavelength measurements allow simultaneous detection of two or more analytes, and discrimination between analytes is possible by time-resolved measurements. Multiplex capabilities represent the main advantage of such sensors compared to electrochemical devices. 

The potential for using multiplexed single-photon counting to acquire fluorescence lifetime data from a distributed array of optical fiber sensors, each sensing a different analyte, has recently been demonstrated by Birch etal.(39,47) Figure 12.9 illustrates such a network to be used in conjunction with the arrangement shown in Figure 12.6. 

This assay can be a stand-alone approach, or can be coupled with different chromatographic or detection techniques such as column switching, parallel analytical columns, or multiplex electrospray sources. 

Figure 6.22 PWG assay dose precision profiles of multiplexed three-analyte immunoassay for two sets of experiments. Dose precisions correspond to standard deviations of analyte concentrations that were back-calculated using corresponding dose response curves. (From Pawlak, M. et al., Proteomics, 2, 383-393, 2002. With permission.)...

In reality, these forays represent miniaturization of the standard sandwich ELISA to attain higher throughput assays by multiplexing a limited number (<50) of analytes, e.g., cytokine panels. Even at these low densities, quantification problems arise in part due to a lack of robustness in the printing process and also in the selection and stability of monoclonal antibodies that must be highly specific and of high binding affinity to be useful for microarrays. 

S.2.2.2 ICLS Example 2 This example discusses the determination of sodium hydroxide (caustic) concentration in an aqueous sample containing sodium hydroxide and a salt using NIR spearoscopy. An example of this problem in a chemical process occurs in process scrubbers where CO, is converted to Na,CO and H,S is converted to Na,S in the presence of caustic. Although caustic and salts have no distinct bands in the NIR, it has been demonstrated that they perturb the shape of the water bands (Watson and Baughman, 1984 Phelan et al., 1989)-Near-infrared spectroscopy is therefore a viable measurement technique. This method also has ad tages as an analytical technique for process analysis because of the stability of the instrumentation and the ability to use fiber-optic probes to multiplex tlie interferometers and Icx ate them rcm< >tely from the processes. 

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