Flow long optical cells

The elongated rectangular cross-section of the cell provides a reasonably long optical path for recording the boundary positions, and at the same time permits efficient thermostatting. By working at around 0-4°C (at which aqueous solutions have maximum density and dp/dT is small), convectional disturbance of the boundaries due to the heating effect of the applied current can be minimised even further. The density difference at the boundaries is usually sufficient to prevent disturbance due to electro-osmotic flow at the cell walls. 

The analytical sensitivity is enhanced by increasing the b value (Eqs. 4.4, 4.9, 4.17) hence, flow cells can be designed to provide long optical path lengths. However, the b value in classical, Z-shaped or spiral flow cells cannot be increased indefinitely due to limitations in cell geometry and radiation beam collimation. 

FIGURE 6.17 Schematic representation of a long optical path flow cell. 1 = support 2 = coiled liquid core waveguide 3 = manifold tubing a and b = points for connecting optical fibres. For details, see text. 

Figure 17-32. Flow sensing in absorption with long optical path cells, (a) White cell [185] (b) CEA cell I187J.

Many optical studies have employed a quasi-static cell, through which the photolytic precursor of one of the reagents and the stable molecular reagent are slowly flowed. The reaction is then initiated by laser photolysis of the precursor, and the products are detected a short time after the photolysis event. To avoid collisional relaxation of the internal degrees of freedom of the product, the products must be detected in a shorter time when compared to the time between gas-kinetic collisions, that depends inversely upon the total pressure in the cell. In some cases, for example in case of the stable NO product from the H + NO2 reaction discussed in section B2.3.3.2. the products are not removed by collisions with the walls and may have long residence times in the apparatus. Study of such reactions are better carried out with pulsed introduction of the reagents into the cell or under crossed-beam conditions. 

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... 

Table 1 lists many of the short-lived species detected in the gas phase with Fourier transform infrared spectroscopy. Two prominent groups are those headed by Bernath, now at the University of Waterloo, and by Howard at the National Oceanic and Atmospheric Administration (NO AA). The former group has used IR emission to study unstable diatomics produced in discharge sources or furnaces. The molecules studied in this group tend to be of astrophysical interest. The research team at NOAA mainly studies short-lived molecules of atmospheric significance. They employ a long flow tube fitted with White cell optics and coupled to a Bomem DA3.002 spectrometer. They usually make the transient they are interested in by performing a carefully controlled series of chemical reactions. 

The STAC optical system is equipped with a Flow Though Cell and the reading chamber size is 5, 10 or 50 mm long depending on the measuring range (5 mm cell is mostly used for waste water and 10 mm to 50 mm cells are designed for natural water applications). 

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