Selectivity, of detection

Since only relatively few substances are capable of emitting fluorescent radiation, they can be particularly selectively detected. This means that the selectivity of the chromatographic separation, which is always aimed at, is meaningfully extended by the selectivity of detection. Accompanying substances that absorb radiation but do not emit light do not interfere when the analysis is made by the selective determination of fluorescence ... 

As is well known the difficulty of analysis of a sample increases as its complexity increases. Analysis usually commences with a rather nonspecific clean-up step and requires that the separation step that follows be highly selective and depends on a detection step that is as specific as possible. As the selectivity of detection increases there is also an increase in the reliability of the identification and it is possible to reduce the demands made on the selectivity of the preceding separation method. This is the case for radiometric and enzymatic methods and also explains the popularity of fluorescence measurements. The latter obtain their selectivity from the freedom to choose excitation and measurement wavelengths. 

It is often possible to increase the selectivity of detection by carrying out a sequence of reactions on one and the same chromatographic plate - a technique that is only possible in thin-layer chromatography. In principle it is possible to distinguish between two sorts of reagent sequence ( TYPE A and TYPE B ), which are discussed in this and the next volume. 

Carey et al. 1979 Kadoum 1968). Some problems still exist with sample preparation and separation, which affect the precision, accuracy, and specificity of analyses. Further studies to improve sample preparation and selectivity of detection might be beneficial in improving the reliability of existing methods. 

The selection of detection techniques capable of providing detection improvements has been a principal issue of research. A wide range of methods applied to meet detection limitations in CE have been taken mainly from liquid chromatographic techniques with only minor modifications, including ultraviolet (UV) absorption, fluorescence, mass spectrometry, conductivity, and electrochemistry principles. 

Fig. 4. Selectivity of detection of mechanical waves. The relative phase shift A4>/F (referred to as (4>(r, t, I/))/yG0M0cos F in Eq. (12)) is plotted versus the ratio a as v/vG for w= 1 (solid line), w = 4 (dashed line) and w=10 (dotted line) cycles of motion-sensitizing gradient. Note that the selectivity increases according to the number of periods of motion encoding. The open symbols are experimental relative intensities measured on wave images obtained with a SPAMM sequence with n = 1 (from Ref. 15, reprinted by permission of Wiley-Liss, Inc., a subsidiary of John Wiley Sons, Inc.).

The ultimate goal of an assay method is the separation and visualization of all components in a single chromatogram. Proper selection of detection wavelength is a critical part of method development. When choosing a detection wavelength, the following factors need to be taken into consideration ... 

The key to achieving chemical selectivity using microcantilevers is the ability to functionalize one surface of the microcantilever with receptive molecules so that explosive molecules will preferentially bind to the treated surface. Choosing receptor molecules that can provide highest affinity, therefore, can control the selectivity of detection. Another important requirement for a sensor system is fast regeneration (recovery), so that the sensor can be used repetitively. 

As a consequence, analytical techniques providing the highest selectivity of detection and adequate sensitivity are generally adopted in forensic science. It is likely owing to this reason that... 

Favretto et al. [33] proposed a procedure for the analysis of BUP and nor BUP based on RP-HPLC conpled with ion trap MS nsing an ESI ion source. The use of the ion trap allowed to obtain intense prodnct ions nnder MS-MS conditions, thus achieving better selectivity of detection with respect to LC-ESI-MS-MS methods with triple qnadrupoles where effective fragmentation in the collision cell was fonnd difficnlt to obtain. However, this method does not inclnde phase II metabolites of BUP, which can be only indirectly determined after enzymatic hydrolysis. 

Recently, Quintela et al. [58] have determined THC and the carboxy metabolite in oral fluid using HPLC coupled to a quadrupole-TOF mass spectrometer. Extreme selectivity of detection and LLOQs of 0.1 and 0.5ng/mL, respectively, were achieved through accurate mass measurement. None of the real positive samples examined was found to contain THC-COOH. 

In the past, PTRC screening was mainly based on gas chromatography-mass spectrometry (GC-MS) [116]. The choice of GC-MS was based on a number of good reasons (separation power of GC, selectivity of detection offered by MS, inherent simplicity of information contained in a mass spectrum, availability of a well established and standardized ionization technique, electron ionization, which allowed the construction of large databases of reference mass spectra, fast and reliable computer aided identification based on library search) that largely counterbalanced the pitfalls of GC separation, i.e., the need to isolate analytes from the aqueous substrate and to derivatize polar compounds [117]. 

The single most useful and versatile physicochemical detectors in drug residue analysis are probably those based on ultraviolet-visible (UV-Vis) spectrophotometry. These detectors allow a wide selection of detection wavelengths, thus offering high sensitivity for analytes that exhibit absorbance in either the ultraviolet or the visible region of the electromagnetic radiation. 

In order to increase the sensitivity and selectivity of detection, a number of UV-absorbing or fluorescent derivatives have been prepared. Several reasons can be given for the use of deriva-tization in chromatography to enhance solute volatility (GC), to enhance separation, and to enhance detectability. The selection of a derivative can often become much more difficult than the actual derivatization itself. In all areas of chromatography these effects have been succesfully applied to difficult or otherwise impossible separation. 

High resolving power also allows an increase in the selectivity of detection in analyses aimed at screening known target compounds. 

The wide variety of interstellar molecules detected so far in our Galaxy (see Table 6) are composed of the most abundant chemically reactive elements, i.e. H, C, N, 0, Si and S. The selection of detected molecules is influenced by molecular and observational considerations i) the molecules must be polar ii) they must have sufficient vapor pressure for their laboratory spectra to be known, iii) of the known spectra, only the most intense transitions can be expected to be observable in interstellar space, and iv) the frequencies of these transitions have to be located within the Earth s atmospheric windows Only molecules which satisfy these conditions are amenable to radio techniques. 

How is selectivity of detection achieved when using a fluorescence detector to detect analytes in an HPLC eluant ... 

In the gase phase, the infrared bands are broad (50 cm ), due to the rotational structure, overlapping vibrations, and hot transitions. In the solid state, the rotational motions are quenched, but due to intermolecular (hydrogen bond) and correlation field interactions, the band positions are shifted and the bands are even broader. The infrared absorptions of matrix-isolated molecules are close to the gas-phase frequencies and exhibit a sharp line-like character (half-widths 0.1 to 2 cm ). Hence the spectra of matrix-isolated molecules are less complicated, and, in comparison to gas phase or solid state spectra, the sensitivity and selectivity of detection increase by a factor of about 10 to 100. Closely spaced vibrations attributed to mixtures of similar molecules, such as conformers, rotamers, molecular complexes, or isotopic species, e.g., H C104 and H CI04, are easily distinguished. 

Either a gold or platinum electrode may be used in a PAD, although gold electrodes are more popular. One reason is that dissolved oxygen contributes a cathodic response at a platinum electrode over the entire useful range for anodic detection. Serious oxygen interference at the gold electrode can be avoided by careful selection of detection potential. 

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