Optical path cells

Expansion of the time scale to monitor phenomena in other time scales. Picosecond and femtosecond techniques are well established. Use of nLFP for very long time scales (10 ms to s) requires very stable light sources, and frequently, long optical path cells. 

IWo models of long optical path cells, shown in Figure 17-32, have been investigated. One is an adaptation of the White cell [185] for measuring hydrocarbons [173, 186]. The second, used in solutions for nuclear plants, was made specially with optical fibers for maximum improvement of the optical path/dead volume ratio of the cell [187]. The best performance ob-... 

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

We used the flash photolysis facilities of the Physical Chemistry Group, Pioneering Research Laboratory, U.S. Army NaticlL Laboratory. The apparatus has been described in Dogliotti and Hayon (4) and Zafiriou (6). and technical details of this study are given in Zafiriou and True (8). The salient features are that the system has sufficient shortwave UV intensity to excite simple anions, a ca 30 vs flash, and a 20 cm optical path cell surrounded by a solution-filter jacliet of l cm path. 

Because the cells can intermpt the optical path in random orientations, individual scattering intensities are not proportional to cell volume. However, because thousands of cells of each type pass through the flow cell, the effects of orientation can be averaged To a first approximation HCT and platelet crit (PCT), the percentage of blood sample volume occupied by platelets, is proportional to the sums of the scattering intensities of the ted cells and platelets, respectively. MCV can be computed from HCT and RBC, whereas MPV can be computed from PCT and PLT. The accuracy of MCV deterrnined by this method is tied to the RBC accuracy, as is the case for the manual MCV method. Ortho Instmments Corporation s ELT-8 uses these counting and sizing methods. 

Wave-front shearing interferometry has been applied to transparent molten salt systems by Gustafsson et al. " The optical path of a light beam traversing the cell at an arbitrary level x is expressed by... 

Fig. 6. Schematic diagram of the Nottingham apparatus for IR kinetic measurements on solutions. Solid lines represent the light path, broken lines the electrical connections. L = Line tunable CO laser, S = sample cell, F = flash lamp, P = photodiode, D = fast MCT IR detector, T = transient digitizer, O = oscilloscope, and M = microcomputer. Nonfocussing optics were used throughout, and the IR laser beam was heavily attenuated by a variable path cell V, filled with liquid methanol, placed immediately in front of the detector. [Reproduced with permission from Moore et al. (61).]...

Both the identification of a species and the determination of the kinetics of its formation or decay can be achieved with longer pathlength cells, such as that depicted in Figure 2.103. In kinetic experiments, however, there is the proviso that the experiment can be performed before natural convection currents interfere with the measurements i.e. the operator must be certain that the removal of a chromophore from the optical path is due to reaction and not due to convection currents. It should be noted that the strength of UV-visible spectroscopy does not lie primarily in the identification of unknown species as the information it provides is not of a molecularly specific nature. 

Resonant photoacoustic gas spectrometry was adapted to fiber optic sensor technology32 as early as in 1984. A Mach-Zehnder arrangement was combined with a resonant photoacoustic cell for gap analysis. The pollutant gas NO2 was detectable in a concentration of 0.5 ppm. In a smart optical fiber hydrogen sensor, the fiber is coated with palladium metal which expands on exposure to hydrogen. This changes the effective optical path length of the fiber, which is detected by interferometry33. 

This intensity is expressed by the molar absorption coefficient 8 which can be calculated from the (measured) absorbance A, (A = log Iq/I) via the well known equation of Lambert Beer (1.3), wherein c is the concentration (mole/1) and d is the optical path length of the cell (in cm). 

Other less definite yet important effects such as profile changes due to nonlinear refractive index alteration in spatially nonuniform high power beams must be carefully considered. As example, the use of nonidentical liquids and optical paths prior to and in, say, EFISH cells and the usual quartz calibration cells could cause potentially inaccurate x determinations. Obviously these types of considerations are important when precise experimentation to test fine models of molecular behavior are intended, but have not stood as obstacle to uncovering the important general trends in molecular nonlinearity enhancement. 

Gaseous samples require long path length cells to produce absorption bands of reasonable intensity up to several metres of optical path are obtainable from cells incorporating mirrors which produce multiple reflections. For GC-IR, light pipes provide the best sensitivity (p. 117). 

Infrared analysis is usually used as a qualitative method to identify substances. Liquids are usually analyzed as pure substances in cells with very small optical path lengths of 0.1-1.0 mm. Usable spectra can be obtained by placing a drop of relatively non-volatile sample between two sodium chloride plates, allowing them to be held together by capillary action. 

It is often necessary to determine the optical path length of salt cells since they are subject to wear and erosion from moisture. To determine the optical path length, b, a spectrum is obtained on the empty cell. Reflections from the internal walls of the cell create an interference pattern that looks like a series of waves in the spectrum. Using as many well-formed waves as possible, the start and ending frequencies (in cm-1) are determined along with the total number of waves. The optical path length is then calculated from the following relationship ... 

The optical path length of an infrared cell can be determined using the method shown in the text. Determine the optical path lengths for the following sodium chloride cells. 

Even if these problems with overlapping bands can be avoided, at high enough pressures the gas phase absorption will be so strong that the sensitivity will be detector noise limited. This problem can only be handled by keeping the optical path length in the pressure cell as small as possible. 

On some detectors, a holmium oxide filter or special holmium oxide cell is integrated into the detector these can then be moved into the optical path during calibration mode. The detector itself or associated control software may then be able to check for wavelength accuracy. Note If the filter or cell is made of glass, and not quartz, 241 nm may not be used. 

A Altered flow cell offers the advantage of removing particles or bubbles from the optical path. This makes reliable process measurements possible in difficult enviromnents where particles and/or bubbles abound. A fermenter is one example of such an environment. 

Particles also cause the scattering of light in a process sample. Filtering probes and cells are available, which can eliminate most particles and bubbles from the optical path and make reliable process measurements possible for difhcult sample matrices (see e.g. Section 4.7). If particle formation is inherent, like insoluble excipients in dissolution studies of tablets, an ATR UV probe may be considered. 

In practice, double-beam instruments are used where the absorption of a reference cell, containing only solvent, is subtracted from the absorption of the sample cell. Double beam instruments also cancel out absorption due to the atmosphere in the optical path as well as the solvent. 

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