Sensors flow-through

In conclusion, synthetic dyes can be determined in solid foods and in nonalcoholic beverages and from their concentrated formulas by spectrometric methods or by several separation techniques such as TEC, HPLC, HPLC coupled with diode array or UV-Vis spectrometry, MECK, MEECK, voltammetry, and CE. ° Many analytical approaches have been used for simultaneous determinations of synthetic food additives thin layer chromatography, " " derivative spectrophotometry, adsorptive voltammetry, differential pulse polarography, and flow-through sensors for the specific determination of Sunset Yellow and its Sudan 1 subsidiary in food, " but they are generally suitable only for analyzing few-component mixtures. 

Qin, W., Zhang, Z., and Liu, H., Chemiluminescence flow-through sensor for copper based on anodic stripping voltammetric flow cell and ion-exchange column with immobilized reagents., Anal. Chem., 70, 3579, 1998. 

Bio)chemical sensors can be used in both the batch and the continuous mode. While this is also true of probe-type sensors, flow-through sensors can only be used in a continuous regime coupled on-line to a continuous-flow configuration. 

Figure 1.14 — Classification of flow-through sensors according to external shape. SMZ sensing microzone D detector W waste. For details, see text.

Compatibility between sensors and automatic and automated analytical systems is crucial as it allows two Analytical Chemistry trends to be combined (see Fig. 1.1). Probe-type and planar sensors can be used in automated batch systems including robot stations, as well as in continuous (mixed in-line/on-line) systems. On the other hand, flow-through sensors are only compatible with continuous configurations. 

The most salient feature of flow-through sensors is the way in which the sample is brought into contact with the sensitive microzone (see Fig. 1.14), which distinguishes them from probe and drop-planar sensors. In fact, the liquid (or gaseous) sample is passed over the microzone rather than dropped onto it or used to immerse the probe [1]. 

In broad terms, a flow-through sensor is an analytical device consisting of an active microzone where one or more chemical or biochemical reactions, in addition to a separation process, can take place. The microzone is connected to or incorporated into an optical, electric, thermal or mass transducer and must respond in a direct, reversible, continuous, expeditious and accurate manner to changes in the concentrations of chemical or biochemical species in the liquid or gaseous sample that is passed over it, whether forcefully (by aspiration or injection) or otherwise (gases). 

At this point it is worth setting a clear distinction between continuous-flow analytical systems (occasionally referred to as "sensor systems") and flow-through sensors, two terms that are often confused in the analytical literature. The primary difference between them lies in whether or not detection is performed simultaneously with other steps in the continuous system. Figures 2.1 and 2.2 illustrate the differential features of the two types of system. 

Figure 2.3 — Different uses of the word "sensor" (1,2) to refer to probe (A) and flow-through sensors (B) according to various criteria.

The classifications of sensors established in Chapter 1 can be used as guidelines to define various technical categories of flow-through sensors (Fig. 2.4). 

One possible classification is based on the type of physico-chemical phenomena that may occur in the sensor. Based on this criterion, there are passive flow-through sensors, which posses no reactive microzone and are... 

One other, very descriptive classification of flow-through sensors is based on the location of the active microzone and its relationship to the detector. Thus, the microzone can be connected (Figs 2.6. A and 2.6.B) or integrated (Fig. 2.6.C) with the measuring instrument. Sensors of the former type use optical or electric connections and are in fact probe sensors incorporated into flow-cells of continuous analytical systems they can be of two types depending on whether the active microzone is located at the probe end (e.g. see [17]) or is built into the flow-cell (e.g. see [18]) — in this latter case. 

Figure 2.6 — Classification of flow-through sensors according to the location of the active microzone relative to the measuring instrument (A,B) connected (C) built-in. (Reproduced from [1] with permission of the Royal Society of Chemistry).

Depending on their sensing ability, flow-through sensors can be classified into single-parameter and multi-parameter according to whether they can sense one or more analytes on passage of each sample. Multideterminations usually rely on the use of an instrument e.g. a diode array spectrophotometer) for virtually simultaneous multidetection e.g. [20]). 

Other possible classifications of flow-through sensors have been excluded from Fig. 2.4 because they are either of little consequence or dealt with in other sections below. Such is the case with the classification based on whether one or more of the active reaction ingredients (analyte, reagent, catalyst, reaction product) is immobilized temporarily or permanently on the active microzone. In addition, the immobilization process may involve one or several active components. 

Figure 2.7 — Types of species retained and immobilization at the active microzone of a flow-through sensor.

The most frequently used electrochemical flow-through sensors are depicted schematically in Fig. 2.9. A the electrode is brought into contact with the sensitive microzone, which is accommodated in the flow-cell... 

Thermal and mass flow-through sensors rely on differential measurements owing to the low selectivity of these types of detection. They use two flow-cells arranged in series (Fig. 2.9.B) or parallel (Fig. 2.9.C), each containing a sensitive microelement (a piezoelectric crystal or a thermistor). One of the cells houses the sensitive microzone, whereas the other is empty or accommodates an inert support containing no immobilized reagent (e.g. see [35]). 

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