Flow-through cell detection

Even the introduction of the FTIR technique did not increase the analytical value, but since the size of the sample for detection was reduced from 1 mg to 1 ng, the FTIR method was adapted for the detection of alkanes produced and analysed by other analytical methods, i.e. pyrolysis-FTIR" (see also Sections IV and V). Specific gas alkane molecules such as CH4, C2H6 and C3H8 are individually identified, therefore the use of FTIR gas cell permits continuous monitoring of these, in flight. This system, although designed for flow through cell detection, can be employed for remote sensors. 

A simple electrochemical flow-through cell with powder carbon as cathodic material was used and optimized. The influence of the generation current, concentration of the catholyte, carrier stream, flow rate of the sample and interferences by other metals on the generation of hydrogen arsenide were studied. This system requires only a small sample volume and is very easily automatized. The electrochemical HG technique combined with AAS is a well-established method for achieving the required high sensitivity and low detection limits. 

The alternative method is continuous-flow , in which the reactants flow through the detection coil during data acquisition. Continuous-flow NMR techniques have been used for the direct observation of short-lived species in chemical reactions [4—6]. The main difference between stopped- and continuous-flow NMR is that in the latter the sample remains inside the detection coil only for a short time period, termed the residence time, x [7], which is determined by the volume of the detection cell and the flow rate. The residence time alters the effective relaxation times according to the relationship in Eq. (2.5.1) ... 

Immobilization techniques have been applied in the preparation of immobilized CL reagents, with specific advantages such as reusability, improved stability, and increased efficiency. These strategies have been applied in the development of CL sensors, which today constitute the most important tools in analytical chemistry because of the high sensitivity offered. Optical fibers have been used to transfer light in order to improve the quality of detection, and new types of flow-through cells have been introduced in the construction of CL sensors. Also, selectivity has been considerably improved by the utilization of enzymatic or antigen-antibody reactions. 

When a fast LC system is connected to a detector, care must be taken to ensure that the detector is well suited for the expected flow ranges and peak widths. Most manufacturers, especially those offering dedicated systems for sub-2-micron particle columns, offer efficient UV detectors. Flow rate is usually not an issue for UV and other flow-through cell-based detection systems. However, flow rate can become limiting for dead-end detectors that alter the column effluent, mainly by eliminating mobile phases such as ELSD, CAD, CLND, and mass spectrometers. 

Voltammetry has been adapted to HPLC (when the mobile phase is conducting) and capillary electrophoresis (CE) as a detection technique for electroactive compounds. In this usage, the voltammetric cell has to be miniaturised (to about 1 pi) in order not to dilute the analytes after separation. A metal or carbon microelectrode has a defined potential (vs the reference electrode) depending on the substances to be detected (ions or molecules) and the mobile phase flows through the detection cell (Fig. 19.5). This method of amperometric detection in the pulsed mode is very... 

Although multistep affinity assays with redox-labeled targets have been described (Wang et al. [117]), most of the assays use enzyme-labeled species in conventional indirect formats (competitive, non-competitive). Direct EILAs based on multistep electrochemical affinity assays have also been developed with excellent results. In all these cases the MIP is used to extract the analyte from the sample and, after elution, the analyte is carried on to the electrochemical flow-through cell for being detected. 

Here, the outlet of the chromatographic system is connected to an NMR detection cell. The NMR spectra are acquired continuously while the sample is flowing through the detection cell. The result is a set of one-dimensional (ID) NMR spectra which cover the whole chromatogram and are typically displayed as a two-dimensional (2D) matrix showing NMR spectrum against retention time, similar to an LC-diode array detection (DAD) plot. 

From the fact that the dehydrogenases depend on a soluble cofactor follows that the enzyme need not be immobilized in close proximity to the detection device. There are several examples in flow systems (FLA and LC) where the enzymatic reaction and NADH production occur in an immobilized enzyme reactor (18,66, 104-106,118,119). The NADH is then transported downstream to the electrode situated in an amperometric flow through cell. As we have reported, the enzyme... 

Figure 5.45 (a) Molecular structures of the thiol-terminated phthalocyanine compounds reported by Russell and co-workers [77]. (b) Schematic of the experimental configuration used for evanescent wave excitation and emission detection at Pc SAMs, including a gas flow-through cell. Reprinted with permission from T. R. E. Simpson, D. J. Revell, M. J. Cook and D. A. Russell, Langmuir, 13, 460 (1997). Copyright (1997) American Chemical Society... 

Detection can be carried out either with an on-line detector coupled to the eluent flow or by the collection and subsequent analysis of discrete fractions. For collected fractions, a range of analytical methods can be used, both quantitative (e.g., radiotracer and metal analysis) and more qualitative (e.g., microscopic techniques). On-line detectors suitable for coupling to the FFF channels include both nondestructive flow through cell systems and destructive analysis systems. It is often desirable to use on-line detection if possible because the total analysis time is much less than for discrete fraction analysis. Regardless of detector type, the dead volumes and flows in the system between the FFF channel and detector or fraction collector must be accurately determined and corrected for. 

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