Sensitivity enhancement strategies

The absorption-based platforms described previously employed evanescent wave interrogation of a thin sensing layer coated onto a planar waveguide. A sensitivity enhancement strategy for optical absorption-based sensors based on planar, multimode waveguides was developed recently by us18. The objective was to apply this theory to the development of low-cost, robust and potentially mass-producible sensor platforms and the following section outlines the assumptions and predictions of this theoretical model. 

McLaughlin, G. M., Weston, A., and Hauffe, K. D., Capillary electrophoresis methods development and sensitivity enhancement strategies for the separation of industrial and environmental chemicals,... 

Sensitivity impacts upon the limit of detection and resolution of the device, making it a key performance parameter. Recently, several strategies have been developed in order to provide sensitivity enhancements for optical sensor platforms based on both optical absorption and fluorescence phenomena. These strategies are the result of rigorous theoretical analyses of the relevant systems and, combined with polymer processing technology and planar fabrication protocols, provide a viable route for the development of low-cost, efficient optical sensor platforms. 

The second (sine) term produced by the evolution delay has all the information we need—it is antiphase 13C coherence labeled with the 13C chemical shift in t —but it is lost because its phase (S ) causes it to be unaffected by the 90° 13C pulse at the end of t. Effectively we are throwing away half of our signal at this point. A new method was developed to save this wasted signal and boost the sensitivity of HSQC and many other experiments. These modified pulse sequences are called sensitivity enhanced or sensitivity improved (Bruker adds si to the pulse sequence name) and the strategy is called preservation of equivalent pathways (PEP) because the two terms are equivalent except for their phase. 

Other strategies that show great promise in reducing NMR acquisition time utilise methods to obtain multiple sets of data from one experiment through a concept known as time-shared evolution. An example of this process that should find utility in natural products elucidation was demonstrated by a pulse sequence called CN-HMBC.93 Traditionally, a separate 13C-HMBC and 15N-HMBC were acquired independently. However, the CN-HMBC allows both 13C- and 15N-HMBC spectra to be obtained simultaneously. By acquiring both data sets simultaneously, an effective 50% time reduction can be achieved.93 This approach has also been demonstrated for a sensitivity-enhanced 2D HSQC-TOCSY (heteronuclear multiple bond correlation total correlation spectroscopy) and HSQMBC (heteronuclear single quantum... 

Spectroscopic sensing requires that a chromophore must be available and be influenced by the rebinding of the imprinted analyte. This can be accomplished in a variety of ways, the simplest case being when the analyte is itself a chromophore. Sensors based on intrinsic analyte chromophores can benefit from molecular imprinting by both selectivity and sensitivity enhancement. In terms of selectivity, molecules similar to the analyte are likely to have similar spectroscopic parameters that could be the source of interference in a conventional sensing strategy. The inability of an interferent to bind to the imprinted polymer allows discrimination. 

Following are some aspects that limit the broad application of the depletion kits. The potential issue of employing this albumin depletion strategy is the limited sample size, typically less than 50 pL, which can be treated by the depletion kits. This sample size prevents the use of a large sample volume to obtain assay sensitivity enhancement. The depletion kits are relatively expensive. Perhaps the most significant drawback of the immunodepletion methods is nonspecific removal of plasma proteins of interest that may be bound to the removed proteins (Anderson and Hunter, 2006 Liu et al., 2006). 

AOS responsiveness to hormonal influences is shown in the action of sodefrin on the lateral nasal sinus of newts (Cynops). The receptors in the accessory pocket are differentially affected by pituitary and ovarian hormones (Toyoda et al., 2000). The local EOG response to the pheromone (Fig. 5.1) was enhanced by the presence of prolactin or of estrogen alone. Receptor sensitivity increase is perhaps an alternate strategy to AOS receptor density increase several alternate routes of signal receptor adaptation (Fig. 7.1) have been hypothesised (Sorenson and Stacey, 1998). 

Principles and Characteristics A sample can contain a great number of compounds, but analysts are usually interested only in the qualitative presence (and the quantitative amount) of a small number of the total compounds. Selectivity is an important parameter in analytical separations. The total analytical process clearly benefits from selectivity enhancement arising from appropriate sample preparation strategies. Selective separation of groups or compound classes can simplify a mixture of analytes before analysis, which in turn enhances analytical precision and sensitivity. Selective fractionation, in some cases, allows easier resolution of the compounds of interest, so analysts can avoid the extreme conditions of high-resolution columns. 

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