Sensors flow-cell-based

Fig. 4 Different optochemical configurations for flow-cell-based sensors. Notice that flow cells are expendable. S source, D detector, Of optical fiber, a Non-guided sensors the transmission of a non-guided collimated radiation beam is measured, b Radiation is guided though optical fibers from the source to the recognition element and from this position to the detector...

The laboratory-based eiqieriments for sensor development placed the sensor flow cell within die detection zone of a commercial flow dirough liquid scintillation counter, using the photomultiplier tubes of this instrument to detect... 

This flow system is based on SIA technique and also uses injection valves. The sensor column flow cell was fabricated from a poly(methyl methacrylate) (PMMA) block, where the central column channel was 3 cm long and 3 mm internal diameter (i.d.), with a volume of 212 p.1. A frit disk was placed directly into the threaded outlet of the column bed channel in order to retain scintillating sorbent beads within the sensor flow cell. The detection was performed using an online flow-through scintillation detector, in which the sensor flow cell assembly was positioned in the place of a conventional flow cell. To prevent... 

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. 

Freeman and Seitz [6] developed one of the first enzyme-based CL sensors with convincing performance. They immobilized horseradish peroxidase (HRP) at the end of an optical fiber and achieved a detection limit of 2 X 10 4 6 mol/L H202. Preuschoff et al. [23] developed a fiberoptic flow cell for H202 detection with long-term stability, suitable for fast FTA. Different peroxidases were covalently... 

Different concentrations of BG bacterial spores (103 107 spores per mL) were introduced into the electric field sensor chip-based flow cell and an appropriate... 

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. 

One other difference lies in the type of detection technique used, which dictates the flow-cell design. Thus, a distinction can be made in this respect between optical (absorptiometric, luminemetric) sensors, which make measurements of the bulk solution where the flow-cell is immersed, and electroanalytical (amperometric, potentiometric) sensors, where measurements are based on phenomena occurring at the electrode-solution interface. 

Reflectance measurements provided an excellent means for building an ammonium ion sensor involving immobilization of a colorimetric acid-base indicator in the flow-cell depicted schematically in Fig. 3.38.C. The cell was furnished with a microporous PTFE membrane supported on the inner surface of the light window. The detection limit achieved was found to depend on the constant of the immobilized acid-base indicator used it was lO M for /7-Xylenol Blue (pAT, = 2.0). The response time was related to the ammonium ion concentration and ranged from 1 to 60 min. The sensor remained stable for over 6 months and was used to determine the analyte in real samples consisting of purified waste water, which was taken from a tank where the water was collected for release into the mimicipal waste water treatment plant. Since no significant interference fi-om acid compounds such as carbon dioxide or acetic acid was encountered, the sensor proved to be applicable to real samples after pH adjustment. The ammonium concentrations provided by the sensor were consistent with those obtained by ion chromatography, a spectrophotometric assay and an ammonia-selective electrode [269]. 

The cyanide sensor developed by the authors group is based on the formation of an addition product between cyanide ion and pyridoxal-5-phosphate, and its subsequent retention in the sensor (a fluorimetric flow-cell packed with QAE-Sephadex resin). The eluent is not injected, but merged with a stream of 0.05 M HCl after the reactor that is used both to acidify the complex and elute it after measurement. The calibration graph for the target analyte was linear from 50 ng/mL to 3.0 pg/mL, and the relative standard deviation and sample throughput were 1.4% (for 2 pg CN7mL) and... 

Notwithstanding the excellent analytical features inherent in molecular phosphorimetric measurements, their use has been impeded by the need for cumbersome cryogenic temperature techniques. The ability to stabilize the "triplet state" at room temperature by immobilization of the phosphor on a solid support [69,70] or in a liquid solution using an "ordered medium" [71] has opened new avenues for phosphorescence studies and analytical phosphorimetry. Room-temperature phosphorescence (RTF) has so far been used for the determination of trace amounts of many organic compounds of biochemical interest [69,72]. Retention of the phosphorescent species on a solid support housed in a flow-cell is an excellent way of "anchoring" it in order to avoid radiationless deactivation. A configuration such as that shown in Fig. 2.13.4 was used to implement a sensor based on this principle in order to determine aluminium in clinical samples (dialysis fluids and concen-... 

A photometric flow-through sensor for the determination of carbamate pesticides (carbofuran, propoxur and carbaryl) based on similar principles as regards the detector and sensor used (a diode array spectrophotometer and a flow-cell packed with C,g resin, respectively) was employed to monitor the formation of the products resulting from hydrolysis of the analytes and online coupling of the respective phenols with diazotized sulphanilic acid. This... 

Several classical ion-selective electrodes (some of which are commercially available) have been incorporated into continuous systems via suitable flow-cells. In fact, Lima et al. [112] used a tubular homogeneous crystal-membrane (AgjS or AgCl) sensor for the determination of sulphide and chloride in natural and waste waters. However, the search for new active materials providing higher selectivity and/or lower detection limits continues. Thus, Smyth et al [113] tested the suitability of a potentiometric sensor based on calix[4]arene compounds for use in flow injection systems. They found two neutral carriers, viz. methyl-j3-rerr-butylcalix[4]aryl acetate and... 

Figure 5.16 — Flow-through photometric sensor for the determination of traces of copper based on the immobilization of a chromogenic ligand (PAN) in a special flow-cell coupled on-line with a flow injection (A) or continuous-flow (B) configuration. IV injection valve SV switching valve W waste TGA thioglycollic acid. For details, see text. (Adapted from [42] with permission of Elsevier Science Publishers).

Figure 5.22 — Reversible flow-through fluorimetric sensor for the determination of potassium in human blood plasma based on the mechanism shown in Fig. 5.21.3. (A) Flow-cell containing the lipophilic membrane. (B) Flow injection conflguration. P pump IV injection valve W waste. For details, see text. (Reproduced from [86] with permission of Elsevier Science Publishers).

Figure 5.23 — Flow-through ionophore-based sensor for the determination of lithium in serum. (A) Mechanism involved in the sensor response (symbol meanings as in Fig. 5.20). (B) Diffuse reflectance flow-cell (a) upper stainless steel cell body (A) silicon rubber packing (c) quartz glass window (d) Teflon spacer (0.05 mm thickness) (e) hydrophobic surface mirror (/) lower stainless steel cell body. For details, see text. (Reproduced from [90] with permission of the American Chemical Society).

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