Sensitivity and Selectivity

The pump-probe concept can be extended, of course, to other methods for detection. Zewail and co-workers [16,18, 19 and 2Q, 93] have used the probe pulse to drive population from a reactive state to a state that emits fluorescence [94, 95, 96, 97 and 98] or photodissociates, the latter situation allowing the use of mass spectrometry as a sensitive and selective detection method [99, 100]. 

This example demonstrates the most challenging problem of flavor chemistry, ie, each flavor problem may require its own analytical approach however, a sensory analysis is always required. The remaining unknown odorants demand the most sensitive and selective techniques, and methods of concentration and isolation that preserve the sensory properties of complex and often dehcate flavors. Furthermore, some of the subtle odors in one system will be first identified in very different systems, like o-amino acetophenone in weasels and fox grapes. 

Tandem mass spectrometry or ms/ms was first introduced in the 1970s and gained rapid acceptance in the analytical community. The technique has been used for stmcture elucidation of unknowns (26) and has the abiUty to provide sensitive and selective analysis of complex mixtures with minimal sample clean-up (27). Developments in the mid-1980s advancing the popularity of ms/ms included the availabiUty of powerhil data systems capable of controlling the ms/ms experiment and the viabiUty of soft ionisation techniques which essentially yield only molecular ion species. 

The main advantages of the ms/ms systems are related to the sensitivity and selectivity they provide. Two mass analyzers in tandem significantly enhance selectivity. Thus samples in very complex matrices can be characterized quickly with Htde or no sample clean-up. Direct introduction of samples such as coca leaves or urine into an ms or even a gc/lc/ms system requires a clean-up step that is not needed in tandem mass spectrometry (28,29). Adding the sensitivity of the electron multiplier to this type of selectivity makes ms/ms a powerhil analytical tool, indeed. It should be noted that introduction of very complex materials increases the frequency of ion source cleaning compared to single-stage instmments where sample clean-up is done first. 

Colorimetric procedures are often used to determine copper in trace amounts. Extraction of copper using diethyldithiocarbamate can be quite selective (60,62), but the method using dithhone is preferred because of its greater sensitivity and selectivity (50—52). Atomic absorption spectroscopy, atomic emission spectroscopy, x-ray fluorescence, and polargraphy are specific and sensitive methods for the deterrnination of trace level copper. 

The methods of investigation of metal species in natural waters must possess by well dividing ability and high sensitivity and selectivity to determination of several metal forms. The catalytic including chemiluminescent (CL) techniques and anodic stripping voltammetry (ASV) are the most useful to determination of trace metals and their forms. The methods considered ai e characterized by a low detection limits. Moreover, they allow detection of the most toxic form of metals, that is, metal free ions and labile complexes. 

I have carried out widespread studies on the application of a sensitive and selective preconcentration method for the determination of trace a mounts of nickel by atomic absorption spectrometry. The method is based on soi ption of Cu(II) ions on natural Analcime Zeolit column modified with a new Schiff base 5-((4-hexaoxyphenylazo)-N-(n-hexyl-aminophenyl)) Salicylaldimine and then eluted with O.IM EDTA and determination by EAAS. Various parameters such as the effect of pH, flow rate, type and minimum amount of stripping and the effects of various cationic interferences on the recovery of ions were studied in the present work. 

Realization of many photometric reactions in water-organics mediums often leads to substantial increasing their sensitivity and selectivity. However, the description of extraction of ion-associates (lA) of basic dyes from water-organic mediums practically is absent in scientific literature. 

Linear dependence of current of additional peak 1 on concentration of Zr(IV) can be used for elaboration sensitive and selective determination of zirconium with detection limit of 1.7x10 mol/1. 

The voltammograms of complex compounds of iridium with azodye appears considerably more clear separate than in the case of tritane dyes, but a sensitivity and selectivity of this method is considerably less. 

The immobilization of the enzymes on various solid carriers gives perfect possibilities to develop not only sensitive and selective, but also rapid and simple test procedures for different compounds determination with visual detection of the analytical signal. 

Biosensors ai e widely used to the detection of hazardous contaminants in foodstuffs, soil and fresh waters. Due to high sensitivity, simple design, low cost and real-time measurement mode biosensors ai e considered as an alternative to conventional analytical techniques, e.g. GC or HPLC. Although the sensitivity and selectivity of contaminant detection is mainly determined by a biological component, i.e. enzyme or antibodies, the biosensor performance can be efficiently controlled by the optimization of its assembly and working conditions. In this report, the prospects to the improvement of pesticide detection with cholinesterase sensors based on modified screen-printed electrodes are summarized. The following opportunities for the controlled improvement of analytical characteristics of anticholinesterase pesticides ai e discussed ... 

The sensitive and selective soi ption-spectroscopic and visual test-methods for detenuination of Pb(II), Zn(II), 0,0 and F" in water and food-stuffs were proposed in comparative with known DRS and VT methods. 

Authors are designed row sensitive and selective test-systems for analysis of heavy metals, active chlorine, phenols, nitrates, nitrites, phosphate etc. for analysis of objects of an environment and for control of ions Ee contents in the technological solutions of KH PO, as well as for testing some of pharmacological psychotropic daigs alkaloids (including opiates), cannabis as well as pharmaceutical preparations of phenothiazines, barbiturates and 1,4-benzodiazepines series too. 

For modern analysis, high sensitivity and selectivity of methods for the determination of these elements are important parameters. Catalymetric methods of analysis possess such characteristics. Using of heterogeneous catalytic reactions on the interface also penults to reduce determination time. 

The high sensitivity and selectivity of the EPR response enables diamagnetic systems to be doped with very low concentrations of paramagnetic ions, the fate of which can be followed during the progress of a reaction. The criteria [347] for the use of such tracer ions are that they should give a distinct EPR spectrum, occupy a single coordination site and have the same valency as, and a similar diffusion coefficient to, the host matrix ion. Kinetic data are usually obtained by comparison with standard materials. 

Quantitative methodology employing mass spectrometry usually involves selected-ion monitoring (see Section 3.5.2.1) or selected-decomposition monitoring (see Section 3.4.2.4) in which a small number of ions or decompositions of ions specific to the compound(s) of interest are monitored. It is the role of the analyst to choose these ions/decompositions, in association with chromatographic performance, to provide sensitivity and selectivity such that when incorporated into a method the required analyses may be carried out with adequate precision and accuracy. 

The need for choosing appropriate ions/decompositions to provide adequate sensitivity and selectivity has been stressed and the selectivity provided by mass... 

Current analytical methods have difficulty detecting picogram levels of nucleic acids, particularly when high levels of other biopolymers (e.g., proteins) are present. The most widely used assay method employed by the pharmaceutical industry involves a nick translation DNA hybridization method (1). This assay offers high sensitivity and selectivity but has a number of drawbacks. 

Solid-surface room-temperature phosphorescence (RTF) is a relatively new technique which has been used for organic trace analysis in several fields. However, the fundamental interactions needed for RTF are only partly understood. To clarify some of the interactions required for strong RTF, organic compounds adsorbed on several surfaces are being studied. Fluorescence quantum yield values, phosphorescence quantum yield values, and phosphorescence lifetime values were obtained for model compounds adsorbed on sodiiun acetate-sodium chloride mixtures and on a-cyclodextrin-sodium chloride mixtures. With the data obtained, the triplet formation efficiency and some of the rate constants related to the luminescence processes were calculated. This information clarified several of the interactions responsible for RTF from organic compounds adsorbed on sodium acetate-sodium chloride and a-cyclodextrin-sodium chloride mixtures. Work with silica gel chromatoplates has involved studying the effects of moisture, gases, and various solvents on the fluorescence and phosphorescence intensities. The net result of the study has been to improve the experimental conditions for enhanced sensitivity and selectivity in solid-surface luminescence analysis. 

Solid-surface luminescence analysis is a useful approach for organic trace analysis because of its simplicity, sensitivity, and selectivity. It will continue to be used in environmental analysis and other areas not only for the reasons mentioned above but also because it is readily adaptable to field work. By developing a fundamental understanding of the interactions responsible for strong RTF and RTF signals, the advantages and disadvantages of the luminescence approach will be more specifically defined in the future. 

Vries, G., Brinkman, U.A.T. A Sensitive and Selective Reaction for NitntoAppIication in Tlun-Laiier Chromatography , Mikrochim. Aeta (Vienna 19M, 47-52. 

The simultaneous analysis of orthophosphate, glycerol phosphates, and inositol phosphates has been achieved by spectrophotometric analysis of the molybdovanadate complexes. Also, a sensitive and selective chemiluminescent molecular emission method for the estimation of phosphorus and sulphur is described, which is based on passing solutions into a cool, reducing, nitrogen-hydrogen diffusion flame. For organic compounds it was usually necessary to prepare test solutions by an oxygen-flask combustion technique. 

Spectrofluorometry presents sensitivity and selectivity greater than the absorbance spectroscopy, being more suitable for chlorophyll estimates in the nmol range and for residual amounts of derivatives in food products. Absorbance spectroscopy is satisfactory for concentrations > 1 xMP Spectrofluorometry is also more accurate for a wide range of chlorophyll a-to-chlorophyll b ratios, but it is less accurate when applied to complex sample matrices because of unpredictable quenching effects. 

In this zeolitic material a very low percentage of Ti(IV), dispersed in a pure siliceous microporous matrix (with the MFI framework, the same as that of the ZSM-5 zeolite), is able to oxidize in mild conditions many substrate with extremely high activity and selectivity (see Sect. 2). However, after more than three decades, a complete picture of reaction mechanisms is still missing. Major problems related to characterization are due to the extremely high dilution of Ti(IV) in the zeolitic matrix and the presence of high amounts of water in the reaction media. The first point requires characterization techniques very sensitive and selective towards Ti(IV). For instance, XRD measurements have been able to recognize the presence of Ti(IV) in the framework only indirectly, via the measured unit cell volume increase [21,22], but attempts to... 

The sensitivity and selectivity can be raised when recording as a function of potential not the current but its derivative with respect to potential. In this case a curve with maximum is obtained (Fig. 23.4) instead of the polarographic wave. The potential of the maximum corresponds to the half-wave potential in an ordinary polarographic curve, and the height of the maximum is proportional to the concentration of the substance being examined. A signal proportional to the derivative can be formed in polarographs with the aid of relatively simple electric circuitry. 

---