Detection selectivity—109------------------, sensitivity enhancement

Fig. 2. Pulse scheme for the gradient-selected, sensitivity-enhanced X/Y se-HSQC experiment as employed for 31P/15N correlation spectroscopy in Ref. 25. 90° and 180° hard pulses are denoted by solid and open bars, respectively. 2 are delays of length 1/(4 /x,y)> and is a short delay of the same length as the gradient pulse (typically rj 1 ms). Pulse phases are x, unless specified. The ratio of gradient pulse strengths is set to G2/Gi = Yy/Yx, and quadrature detection in Fi is achieved by recording every transient twice and changing the sign of G2 in the second scan.

The first part of the book consists of a detailed treatment of the fundamentals of thin-layer chromatography, and of measurement techniques and apparatus for the qualitative and quantitative evaluation of thin-layer chromatograms. In situ prechromatographic derivatization techniques used to improve the selectivity of the separation, to increase the sensitivity of detection, and to enhance the precision of the subsequent quantitative analysis are summarized in numerous tables. 

Fluorescence detection was employed for quantitation of coumarins because of sensitivity and specificity. However, monitoring the fluorescence at one wavelength was not sufficient to observe all of the coumarin derivatives. A wide range of fluorescence maxima (314-346 nm for excitation and 420 -517 nm for emission) for coumarins are also utilized possibly to identify the individual components and selectively to enhance the fluorescence of coumarins in citrus oils (25). 

The common properties attributed to helper ligands in improving lanthanide-based detection techniques include enhanced stability, sensitivity, and selectivity. We will start with the effects of ancillary ligands on lanthanide photophysics and then explore the impacts of these ligands on sensor properties (Section II). Other factors influencing stability, such as sterics and oxophilicity, will be summarized in Section III. A brief discussion of future applications will follow (Section IV). 

From what has been learned from the near-UV studies, the selectivity and the sensitivity of CD detection are greatly enhanced if the CD-active absorption bands are shifted from the wavelength range where the matrix absorbances are highest. With a few exceptions, the range of least interference is the visible. Wavelengths are shifted by using selective color or fluorescence deri-vatization reactions on chiral analytes as they exist in the matrix. 

Enzyme DNA hybridization assays with electrochemical detection can offer enhanced sensitivity and reduced instrumentation costs in comparison with their optical counterparts. Efforts to prevent non-specific binding of the codissolved enzyme and to avoid fouling problems by selecting conditions suitable to amplify the electrode response have been reported by Heller and co-workers [107]. A disposable electrochemical sensor based on an ion-exchange film-coated screen-printed electrode was described by Limoges and co-workers for an enzyme nucleic acid hybridization assay using alkaline phosphatase [108] or horseradish peroxidase [109]. In another methodology to improve sensitivity, a carbon paste electrode with an immobilized nucleotide on the electrode surface and methylene blue as hybridization indicator was coupled, by Mascini and co-workers [110], with PGR amplification of DNA extracted from human blood for the electrochemical detection of virus. 

If no derivatization takes place, detection is preferably accomplished by UV at a low wavelength (200-210 nm) in order to enhance detection sensitivity. However, detection selectivity is sacrificed at such low wavelengths. Electrochemical detection, when applied to the analysis of free amino acids, offers higher selectivity but suffers from a small linearity range. Furthermore, most amino acids (with the exception of tryptophan, tyrosine, and cysteine) are not intrinsically electrochemically active within the current useful potential range [5]. Lately, the development of the evaporative light-scattering detector (ELSD) offers an attractive alternative for the determination of nonderivatized amino acids (see Fig. 1). 

Precolumn derivatization is the generally accepted approach for the determination of amino acids, because it offers significant advantages increased detection sensitivity, enhanced selectivity, enhanced resolution, and limited needs for sophisticated instrumentation (in contrast with postcolumn derivatization techniques). 

Particular improvements to the HSQC experiment has been the implementation of phase sensitive echo/antiecho-detection in combination with gradient coherence selection. Further variants include the sensitivity-enhanced HSQC experiment [5.194] and experiment developed for long-range coupling detection and J-scaled experiments (for references see sections 5.2.5 and 5.7.3). 

A further improvement is the gradient selected HSQC experiment that can be run like the gradient selected HMQC experiment with one scan per experiment. In the following Check it an experiment with decoupling during acquisition is simulated and compared with the other HSQC experiments which differ in the detection mode and the sensitivity enhancement. 

A fundamentally different approach to signal excitation is present in polarization transfer methods. These rely on the existence of a resolvable J coupling between two nuclei, one of which (normally the proton) serves as a polarization source for the other. The earliest of these type of experiments were the SPI (Selective Population Inversion) type (19>) in which low-power selective pulses are applied to a specific X-satellite in the proton spectrum for an X-H system. The resultant population inversion produces an enhanced multiplet in the X spectrum if detection follows the inversion. A basic improvement which removes the need for selective positioning of the proton frequency was the introduction of the INEPT (Insensitive Nucleus Excitation by Polarization Transfer) technique by Morris and Freeman (20). This technique uses strong non-selective pulses and gives general sensitivity enhancement. 

Much of the work in the past decade on IC development has been in the areas of selectivity improvement to allow more complex application (particularly relevant to environmental analysis), and improved detection methods for enhanced sensitivity and selectivity. 

In the early 1990s Amirav et al. introduced a new strategy for the operation of FPD based on a pulsed flame instead of a continuous flame for the generation of flame chemiluminescence. This pulsed flame photometric detector (PFPD) is characterized by the additional dimension of a light emission time and the ability to separate in time the emission of sulfur species from those of carbon and phosphorus, resulting in considerable enhancement of detection selectivity. In addition, detection sensitivity is markedly improved, thanks to ... 

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