Integrated retention and detection

Most flow-through sensors integrating retention and detection involve placement of an inert support in the flow-cell of a non-destructive spectroscopic detector where the analytes or their retention products are retained temporarily for sensing, and then eluted. Rendering these sensors reusable entails including a regeneration step suited to the way retention is performed. 

In the best of cases, the carrier itself acts as regenerator otherwise, a continuous configuration (usually an FI manifold) is the most convenient choice for this purpose. 

The equipment required to develop this type of sensor is very simple and resembles closely that used to implement ordinary liquid-solid separations in FI manifolds. The only difference lies in the replacement of the packed reactor located in the transport-reaction zone with a packed (usually photometric or fluorimetric) flow-cell accommodated in the detector. Whether the packing material is inert or active, it should meet the following requirements (a) its particle diameter should be large enough ( 80-100 fim) to avoid overpressure (b) it should be made of a material compatible with the nature of the integrated detection system e.g. almost transparent for absorbance measurements) and, (c) the retention/elution process should be fast enough to avoid kinetic problems. 

Although somewhat specialized, integrated flow-cells are normally commercially available. Ideally, they should be short (0.2- 1.5 mm) and narrow-bore in order to avoid problems arising from inadequate detector capacity and sensitivity, respectively. Ideally, they should also have small inner volumes in order to boost sensitivity and sample throughput. 

A logical classification of the sensors dealt with in this section is based on which species (the analyte or a reaction product) is retained and monitored. However, because sensors based on retention of the analyte and measurement of some intrinsic property are few and far between, adopting such a classification here would be rather impractical. On the other hand, because most of these sensors — those based on mass measurements 

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FLOW-THROUGH SENSORS FOR MULTIDETERMINATIONS BASED ON INTEGRATED RETENTION AND DETECTION... 

Multi-sensors based on separation followed by integrated retention and detection... 

The above exceptions leave relatively few sensors based on integrated separation and detection, particularly of the types involving gas-liquid and liquid-liquid interfaces, which require the detector to be responsive to the gas or ion (molecule) transferred across the membrane. The scope of liquid-solid interactions is somewhat broader as it enables not only retention of the analyte and monitoring of some intrinsic property, but also to retain a product of a previous reaction, thereby substantially expanding the possibilities. 

Sensors based on integrated retention and luminescence detection... 

Fig. 4.21. Different ways of improving pervaporation efficiency. (A) By halting the flow in the acceptor chamber without disrupting the overall dynamic system. (B) By on-line retention of transferred volatile species and elution in the opposite direction. (C) By use of a packed flow-cell to integrate reaction and detection. E eluent, lEC ion-exchange column, PR preconcentration, EL elution. (For other abbreviations, see previous figures.) (Reproduced with permission of Wiley Sons.)...

In the detection system, the main aim being integration of the reaction (retention) and detection in order to exploit some of the typical advantages of these approaches enhanced sensitivity, selectivity, sampling rate, miniaturization, etc. 

The main important details of these procedures for AA determination are contained in Table 18.8. As can be seen in Table 18.8, among these flow analysis techniques, stopped-flow procedures are the most applied to AA determination, being mostly kinetic enzymatic determinations. BI methodologies for AA determination use spectrophotometric detection and a commercial flow cell, which is filled with appropriate solid beads, works as a flow-through chemical sensor integrating online reaction, retention, and detection on the solid-phase disposable beads. 

MT Tena, MD Luque de Castro, M Valcarcel. HPLC-postcolumn derivatizing-integrated retention-detection system for the determination of carbaryl and its hydrolysis product. J Chromatogr Sci 30 276-279, 1992. 

Software allows qualitative comparison of the shape of the DTG curve with integration over all detected mass numbers (total-ion curve) [338]. This can help to assure that the selected mass range for measurement is sufficient to describe all weight loss effects by corresponding MS signals (i.e. DTG and total-ion curve show parallel shape), and can be used to observe insufficient mass range, retention and condensation effects i.e. non-paraUel profiles of DTG and total-ion curve) (Fig. 2.22). 

Valencia et al. developed an integrated solid-phase spectrophotometry-FIA method for the determination of Sunset Yellow in drinks in the presence of its unsulfonated derivative Sudan I [43]. The procedure is based on the retention and the preconcentration of the low level Sudan I in the upper zone of a silica gel packed cell, while Sunset Yellow is not retained and its optical signal at 487 nm is read directly over the packed cell. The applicable concentration range and detection limit for Sunset Yellow were 0.5-20.0 mg/L and 0.2 mg/L, respectively. 

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