Online sensors selection

For gas-phase sensors, both remarkable selectivity and very low LOD are important. Sensors featuring MIP recognition combined with SAW transduction can meet these requirements. The MIP-PZ chemosensors operating in gases are devised for two main applications, namely for indoor gas inspection and online monitoring of volatile organic compounds. The latter is essential to protect humans from threats of environmental atmospheric pollutants. 

Although new techniques of analyzing VOCs like sensor systems and online mass spectrometers have been introduced in the last decade, the use of solid sorbents in combination with thermal desorption and gas chromatographic separation still seems to be the preferred method due to the sensitivity, selectivity, convenience of use and the reliability that has been proven over the years. 

During the past 40 years there have been numerous exciting extensions of electrochemistry to the field of analytical chemistry. A series of selective-ion potentiometric electrodes have been developed, such that most of the common ionic species can be quantitatively monitored in aqueous solution. A highly effective electrolytic moisture analyzer provides continuous online assays for water in gases. Another practical development has been the voltammetric membrane electrode for dioxygen (02), which responds linearly to the partial pressure of 02, either in the gas phase or in solution. The use of an immobilized enzyme (glucose oxidase) on an electrode sensor to assay glucose in blood is another extension of electrochemistry to practical analysis. 

Continuous, or very frequent, automated measurement of individual or groups of toxic compounds or general physico chemical properties, can provide a rapid warning of changed environmental conditions. A few chemical methods (such as the LC SAMOS for polar compounds (Slobodnik et al., 1992)) may even detect the occurrence of high concentrations of specific contaminants of concern at the test site. In situ, online and on-site approaches are described in other chapters of this volume, but as yet, application is restricted by the limited availability of suitable, sufficiently sensitive and selective (bio)sensors and monitors for the analysis of (priority) pollutants that need to be monitored. Once available, they may provide (semi)-qualitative and quantitative information on the occurrence of compounds in the waters. 

In general, it is difficult to make online measurement of product quality it is often an inferred entity based on experience. The moisture content can be a measured variable if a suitable moisture sensor is used in the process. Analyzers are available for moisture analysis, which can be adapted for automatic, closed-loop dryer control. The most successful of these units rely on infrared, microwave, or capacitance detection [1]. Care must be exercised in the selection to allow for changes in bulk density or for void spaces, which will... 

The coupling of sensors with flow injection analysis (FIA) is already a very popular option. The flow regime offers important advantages over discrete manual measurements that include (1) Sample preparation processes such as reagent mixing, selectivity enhancement (e.g., removal of large molecular mass interferents such as protein by dialysis in clinical assays), and solvent extraction can all be carried out online. The improved sample preparation and more reproducible sample delivery result in improved measurement precision and accuracy. Drift is less of a problem as measurements are made of peak heights relative to a baseline. (2) Improved sensor lifetime in flow analysis, the sensor may be exposed to the sample for only a short period of time, and maintained in a friendlier matrix between measurements that can help counteract or delay the deleterious effects of the sample. (3) Automation the entire analysis can be... 

The electrochemical sensors are compact, portable, and simple to handle instruments, able to provide analytical information in a real time, without or with a minimum sample pieparatimi. These performances, in cmicert with their sensitivity, selectivity, and low cost, make them suitable for infield aaA online environmental analysis [4—7] and an excellent complement to the expensive and time-cOTisuming offsite chromatographic and adsorption or emissimi spectrometric methods, currently applied in envirmimental monitoring [8]. 

A unique stopped-FIA system for simultaneous synchronous spectrofluorometric online dissolution monitoring of multicomponent solid preparations that makes use of a fiberoptic sensor was presented by Li et al. [3]. A new means of in vitro therapeutic drug monitoring was developed by hyphenating the fiberoptic sensor technique and a chemometric method. An artificial neural network method was applied to construct the mathematical model for the simultaneous analysis of the mixture of vitamins Bj, B2, and Bg by synchronous spectrofluorometry. The selection of the wavelength interval, the pFI of the carrier solution, and other experimental conditions were evaluated. The proposed method has been applied to the dissolution monitoring of vitamin B tablets with satisfactory results. The recovery was 97.8%-105%, and the relative standard deviation (RSD) was 1.1-7.5. 

Barnett et al. and Lines et al. introduced an online monitor system which comprised a sampling device, a control unit and a printer, to be connected to a conventional COULTER COUNTER instrument. It was used for continually monitoring the particulate contamination levels within an electrolyte solution, such as normal saline. The sampling device included three apertures, any one of which was used at any one time while the others were held in reserve in case of blockage. At pre-selected time intervals, samples of solution were passed through the active sensor and particle counts determined at various pre-set size levels. 

---