Selective and sensitive detection

2 = estradiol 3 = estriol. Stationary phase, JXR temperature, 210°C. (Reproduced from ref. 1 by courtesy of W.J.A. VandenHeuvel and the publisher.) 

The use of mass spectrometry combined with GC does not necessitate derivatives other than those used in GC itself. In spite of this, particularly recently, special derivatives for GC-MS have been developed, some of which are mentioned in this book. Derivatives are considered that provide characteristic mass spectra and facilitate identification and quantitative evaluation. Various trialkylsilyl derivatives [7] are an example. They do not offer any particular improvement over conventional TMS derivatives from the viewpoint of the chromatographic analysis, but their highly characteristic fragments permit the sample compounds under analysis to be identified unambiguously in GC—MS. 

The study of structural differences of alcohols and phenols [8], using as a characteristic the ratio of the adjusted retention time of the alcohol or the phenol to that of its TMS derivative, can serve as an example of the first aspect. Alcohols, thiols and primary and secondary amines in food flavours were identified successfully after the preparation and GC analysis of different derivatives [9]. The third method is fairly frequent in the analysis of residues of pesticides (see p.177). 

Post-column identification reactions constitute a special kind of ancillary GC technique [10] that will briefly be discussed in Section 3.1.2, p.34. 

VandenHeuvel, in M.B. Lipsett (Editor), Gas Chromatography of Steroids in Biological Fluids, Plenum Press, New York, 1965, p. 277. 

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Some of the analytical methods utilize highly selective and sensitive detection techniques for specific functional groups of atoms in compounds, whereas others respond in a more universal manner, i.e., to the number of carbon atoms present in the organic molecule.- ... 

Babu, P., Sinha, S., and Surolia, A. (2007) Sugar-quantum dot conjugates for a selective and sensitive detection of lectins. Bioconjugate Chem. 18, 146-151. 

Whilst these methods are informative for the characterisation of synthetic mixtures, the information gained and the nature of these techniques precludes their use in routine quantitative analysis of environmental samples, which requires methods amenable to the direct introduction of aqueous samples and in particular selective and sensitive detection. Conventionally, online separation techniques coupled to mass spectrometric detection are used for this, namely gas (GC) and liquid chromatography (LC). As a technique for agrochemical and environmental analyses, high performance liquid chromatography (HPLC) coupled to atmospheric pressure ionisation-mass spectrometry (API-MS) is extremely attractive, with the ability to analyse relatively polar compounds and provide detection to very low levels. 

Guo W, Yuan J, Wang E (2009) Oligonucleotide-stabilized Ag nanoclusters as novel fluorescence probes for the highly selective and sensitive detection of the Hg2+ ion. Chem Commun 23 3395-3397... 

Lan G-Y, Fluang C-C, Chang FI-T (2010) Silver nanoclusters as fluorescent probes for selective and sensitive detection of copper ions. Chem Commun 46 1257-1259... 

A fraction collector and a post-column derivatization system were included (Figure 2.1) for a comprehensive and multi-purpose instrument. However, the fraction collector is needed only when collecting components from the effluent, and is generally not included in an analytical system. The post-column derivatization system is connected only when required for the selective and sensitive detection of specially targeted compounds. Usually, most compounds are directly detected by an on-line spectroscopic or other detector. 

LC/MS/MS techniques with selective and sensitive detection methods make it possible to quantitatively analyze samples from Caco-2 cell and PAMPA buffer matrices. A high-throughput permeability screen with robust LC/MS technology can quickly generate information about structure-permeability relationships that are extremely valuable in the lead optimization phase for the selection of pre-clinical candidates with favorable oral bioavailability properties. 

Fluorometric detection has been mainly employed for the determination of aminopenicillins such as amoxicillin and ampicillin in edible animal products because it confers the advantages of selectivity and sensitivity. Fluorometric detection of penicillins, however, necessitates their precolumn derivatization to produce the corresponding fluorescent derivatives. The most commonly used derivatizing reagents are formaldehyde (100, 117, 118), salicylaldehyde (83), and mercury dichloride (91). 4-Bromomethyl-7-methoxycoumarin has also been employed as a fluorescence label for the selective and sensitive detection of seven penicillins in milk (96). 

Although tetracyclines possess the inherent ability to fluoresce, few methods exploiting this property have been reported (300, 306, 309). Instead, fluorometric methods based on the reaction of tetracyclines with suitable derivatizing agents have been developed. The use, for example, of zirconyl chloride as a fluorescence label in the postcolumn derivation of tetracyclines, has allowed highly selective and sensitive detection of these antibiotics in animal tissues (294, 295). 

TMP Cattaneo, A Feroldi, PM Toppino, C Olieman. Sample preparation for selective and sensitive detection of soya proteins in dairy products with chromatographic and electrophoretic techniques. Neth Milk Dairy J 48 225 -234, 1994. 

Ma, L., Zhang, H., Humphreys, G., Sanders, M., and Zhu, M. (2005). Selective and sensitive detection of GSH adducts in complex biological samples using high resolution LC/FIMS with mass defect filtering. In Proceedings of the 53rd ASMS Conference on Mass Spectrometry and Allied Topics, San Antonio, TX. 

A. J. Bulterman, J. J. Vreuls, R. T. Ghijsen and U. A. Th Brinkman, Selective and sensitive detection of organic contaminants in water samples by on-line trace enrichment-gas chro-matography-mass spectrometry , 7. High Resolut. Chmmatogr. 16 397-403 (1993). 

The fact that CPL and FDCD instruments are not commercially available, and, therefore must be constructed in individual research laboratories, has certainly affected the quantity of applications of these chiroptical techniques. Nevertheless, both have been shown to provide interesting, important, and often unique information concerning the structure and dynamics of optically active molecules. As needs develop for more selective and sensitive detection or imaging of chiral molecules or chiral molecular systems, the range of applications of CPL and FDCD will certainly increase, and these spectroscopic techniques should become even more important tools of molecular stereochemistry. 

Esters for the selective and sensitive detection of carboxylic acids contain atoms or functional groups for which the detector is specific. As with substances of other types, preferably derivatives containing halogens are used for electron-capture detection. Karmen [151] suggested 2-chloroethyl esters for specific detection with the aid of the alkali FID. They are much less volatile than methyl or ethyl esters, and are therefore interesting for the analysis of short-chain fatty acids. GC separation was carried out on EGA polyester or SE-30. 

Derivatives other than esters and silyl compounds were mostly developed for particular groups of acids, if not for individual substances, containing in addition to carboxylic also other functional groups, in an attempt to obtain derivatives possessing convenient properties, e.g., derivatives of lower acids less volatile than methyl esters, or more stable, or derivatives suitable for selective and sensitive detection. 

The choice of detectors in LC is often a trade-off between wide scope and high sensitivity. For instance, the refractometer is readily available and easy to operate it can detect most compounds (wide scope), but it often has a lack of sensitivity for many compounds. A variable-wavelength UV detector offers a good choice for solutes that have some UV absorbance capability. Absorption at a specific wavelength results in a more selective and sensitive detection mode than a refractometer, but only for UV-absorbing compounds. In other specific cases the fluorescence or electrochemical detector can be used. These have a high sensitivity for individual compounds but are also limited in the number and type of compounds they can detect (narrow scope). 

Negative chemical ionization (NCI) mass spectrometry detects ABA with a high sensitivity since the negative molecular ion M of methyl ester of ABA is more stable than the positive molecular ion [M] + due to the high electrophilicity of ABA. The NCI mass spectrum shows [M]- at m/z 278 as a base peak, and other fragment ions at m/z 310, 260, 245, 141, and 152.601 The combination of SIM with NCI gives highly selective and sensitive detection of ABA the lowest detection limit is 0.3 pg, which is 200 times lower than that... 

Challenges for HPLC method development are for combination products where more than one active is present in the same formulation. Since degradation products are of big concern from a safety perspective, HPLC will continue to be utilized as a major separation technique during formulation development to develop the most safe and efficacious formulations to be used for human use. Many different types of bonded phases are currently available for routine HPLC analysis in addition, very selective and sensitive detection techniques can be integrated with HPLC to help an analytical chemist control the final quality of the drug product. Furthermore, the availability of this technique makes it the first and sometimes the only choice for the analysis of degradation products. 

Ryu K, Haes AJ, Park HY, Nah S, Kim H, Chung J, Yoon MY, Han SH (2010) Use of peptide for selective and sensitive detection of an anthrax biomarker via peptide recognition and surface-enhanced Raman scattering. J Raman Spectrosc 41 121-124... 

To undertake the analyses of drugs and metabolites in body fluids the analyst is faced with several major problems. Firstly, due to the complex nature of the body fluid, the drugs must be isolated by an extraction technique which ideally should provide a relatively clean extract, and the separation system must be capable of resolving the drugs of interest from any co-extractives. Secondly, both selective and sensitive detection techniques are essential because of the small sample sizes available (typically less than 500 pi), and the extremely small quantities of drug often present. 

The natural fluorescence of the opium alkaloids has been used to obtain a selective and sensitive detection method. In this way codeine has been detected (the detector operating with an excitation wavelength of 213 nm and a cut-off filter for the emission... 

Hao, C., Shepson, P. B., Drummond, J. W., and Muthuramu, K., Gas chromatographic detector for selective and sensitive detection of atmospheric organic nitrates. Anal. Chem., 66, yiyi- ilA i, 1994. 

Selective and sensitive detection and identification with numerous chromogenic, fluorogenic, and biological reagents... 

Although conventional extraction methods, such as Soxhlet, do provide accurate results, it can be assumed that the faster and less laborious extraction techniques such as PLE, sonication assisted extraction, and SFE will gain popularity. The relevant figures of merit of the techniques utilized in the extraction of FRs in environmental samples are summarized in Table 31.9. Gas chromatography is the obvious choice for the final separation of the analytes. Selective and sensitive detection, i.e., MS or ECD, is required, as the levels of the FRs tend to be very low and other compounds are typically present in much higher concentrations. 

In addition to laser fluorescence excitation, several other laser spectroscopic methods have been found to be useful for the state-selective and sensitive detection of products of reactive collisions resonance-enhanced multiphoton ionization [58], coherent anti-Stokes Raman scattering [M], bolometric detection with laser excitation [30], and direct infrared absorption [7]. Several additional laser techniques have been developed for use in spectroscopic studies or for diagnostics in reacting systems. Of these, four-wave mixing [ ] is applicable to studies of reaction dynamics although it does have a somewhat lower sensitivity than the techniques mentioned above. 

We wish to thank the editor, S. Ahuja, for the original invitation to contribute to this ACS Symposium Series publication, and to be able to enjoy describing some of our recent efforts in the areas of selective and sensitive detection for HPLC/FIA. Much of our own work described here was made possible by various grants and/or contracts to Northeastern University over the past several years. We wish to acknowledge firms who have made such commitments, including Instrumentation Laboratory (Allied Analytical Systems, Inc.), HNU Systems, Inc, Bloanalytlcal Systems, Inc., Pfizer, Inc., EH Science, Inc.,... 

The determination of the different forms (e. g. compounds or complexes) in which an element occurs (often referred to as the speciation of an element and speciation analysis, respectively [28]) is in most cases performed by hyphenated techniques. These are the combination of a high-performance separation technique such as gas or liquid chromatography, or electrophoresis, and an element- or compound-specific detector [29]. While the former provides the separation of the different elemental species prevalent in the sample, the latter brings selective and sensitive detection. In the case of AAS, only the hyphenation with gas and Hquid chromatography, respectively, has gained importance. The combination of atomic absorption spectrometry and electrophoresis has never proven successfiil, obviously due to the incompatibility of the extremely low flow rates of electrophoretic separations with the aspiration volumes of flame atomisers and the difficulties of interfacing the two techniques. 

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