Path Length Enhancement

As discussed in Section 6.3.2, the sampling volume and Raman signal magnitude depend upon the optical properties of the sample, laser focus, and collection optics. Several cases were considered in Tables 6.2 and 6.3, but the situation of relevance here is the thick, optically transparent sample such as a clear liquid or solution. For a thick sample such as a large cell or other container, where the depth of field is much less than the sample thickness, the sampling depth is determined by the collection optics. One cannot arbitrarily increase a, since it is a function of //, slit width, and the like and is limited in practice to at most a few millimeters. 

---

Figure 6.21, Path length enhancement for a transparent sartiple by reflection of laser and scattered light inside a small-diameter tube.

Figure 6.22. (A) Path length enhancement for a clear sample inside an integrating sphere. (B) Additional mirrors to provide a double laser pass through the sample and increased collection efficiency.

Here F is the cavity finesse and the factor F/2n manifests the optical path length enhancement effect with respect to the cavity physical length at the resonance wavelength. Similar to waveguide evanescent sensors, the sensitivity of resonator sensors is also ultimately bounded by intrinsic propagation loss in the device. 

Sensitivity can be improved by factors of 10 using intracavity absorption, placing an absorber inside a laser resonator cavity and detecting dips in the laser emission spectmm. The enhancement results from both the increased effective path length, and selective quenching of laser modes that suffer losses by being in resonance with an absorption feature. 

Electrodriven separation techniques are destined to be included in many future multidimensional systems, as CE is increasingly accepted in the analytical laboratory. The combination of LC and CE should become easier as vendors work towards providing enhanced microscale pumps, injectors, and detectors (18). Detection is often a problem in capillary techniques due to the short path length that is inherent in the capillary. The work by Jorgenson s group mainly involved fluorescence detection to overcome this limit in the sensitivity of detection, although UV-VIS would be less restrictive in the types of analytes detected. Increasingly sensitive detectors of many types will make the use of all kinds of capillary electrophoretic techniques more popular. 

In the substrate configuration the stainless steel carrier is coated with a Ag-ZnO bilayer in order to enhance the back reflection of the back contact see Figure 73 [11]. An increase in 7sc of about 50% was achieved by Banerjee and Guha [589] by using a textured Ag-ZnO bilayer, which further enhances the optical path length and consequently the absorption. As at this stage no [Pg.172]

VCD and FTIR spectra should always be obtained on the same samples, the FTIR at higher resolution and optimized S/N to permit computation of deconvolved (resolution-enhanced) spectra (Kauppinen etal., 1981). VCD spectra of biomolecules are often normalized to the absorbance, since concentration and path lengths are rarely known to good accuracy. Because the absorbance coefficients for different molecules will vary, this is only an approximate correction for concentration variation. 

Owing to the short path length, the sensitivity of photometric detection in CE is limited. For this reason, procedures are necessary to enhance the sensitivity. This can be achieved by sample pre-concentration, improvement of the optical design and/or alternative capillary geometries [66]. Sample pre-concentration can be done off- or on-column. The off-column procedures are well described in the literature and have been applied extensively in chromatography [67]. The on-column procedures are more specific to CE and are therefore briefly discussed. 

With capillary electrophoresis (CE), another modern primarily analytically oriented separation methodology has recently found its way into routine and research laboratories of the pharmaceutical industries. As the most beneficial characteristics over HPLC separations the extremely high efficiency leading to enhanced peak capacities and often better detectability of minor impurities, complementary selectivity profiles to HPLC due to a different separation mechanism as well as the capability to perform separations faster than by HPLC are frequently encountered as the most prominent advantages. On the negative side, there have to be mentioned detection sensitivity limitations due to the short path length of on-capillary UV detection, less robust methods, and occasionally problems with run-to-run repeatability. Nevertheless, CE assays have now been adopted by industrial labs as well and this holds in particular for enantiomer separations of chiral pharmaceuticals. While native cyclodextrins and their derivatives, respectively, are commonly employed as chiral additives to the BGEs to create mobility differences for the distinct enantiomers in the electric field, it could be demonstrated that cinchona alkaloids [128-130] and in particular their derivatives are applicable selectors for CE enantiomer separation of chiral acids [19,66,119,131-136]. 

Matson, M. T., Ramstad, T., and Dunn, M. J. (2005). Purity determination of alprostadil by micellar electrokinetic chromatography with signal enhancement involving field-amplified sample stacking and extended path length detection. /. Liq. Chromatogr. Relat. Technol. 28, 3181—3203. 

Whereas current-producing reactions occur at the electrode surface, they also occur at considerable depth below the surface in porous electrodes. Porous electrodes offer enhanced performance through increased surface area for the electrode reacdon and through increased mass-transfer rates from shorter diffusion path lengths. The key parameters in determining the reaction distribution include the ratio of the volume conductivity of the electrolyte to the volume conductivity of the electrode matrix, the exchange current, the diffusion characteristics of reactants and products, and the total current flow. The porosity, pore size, and tortuosity of the electrode all play a role. 

Length of Liquid Flow Path Longer liquid flow paths enhance the liquid-vapor contact time, the significance of liquid plug flow, and therefore raise efficiency. Typically, doubling the flow path length... 

---