Specific Detection Systems

Aqueous distillates are extracted, usually with dichloro-methane (DCM), concentrated to small volumes, generally in a Kuderna-Danish evaporator, and examined by gas chromatography (GC) using a specific detection system. Additional chromatographic cleanup may be required, depending on the complexity of the sample and specificity of the chromatographic detector. 

A specific approach for the measurement of base damage to DNA involves the hydrolysis of DNA into monomeric units. Acidic hydrolysis leads to the release of bases while enzymatic treatment yields nucleosides. The resulting mixture of lesions together with the overwhelming presence of normal bases or nucleosides is resolved by chromatography. The targeted damage is then quantified by use of specific detection systems. 

The following sections provide information on how these techniques have been employed to set up species-specific detection systems for potentially OTA-producing fungi. 

When a more specific detection system is used instead, a rigorous sample cleanup may not be necessary. This is actually the case with most of the microbiological and immunochemical detection systems applied in residue analysis. Owing to the selectivity and sensitivity of their detection principle, homogenization with an aqueous buffer is often tire only treatment required prior to analysis. Moreover, these detection systems are usually independent of the sample size as, in many cases, a single drop of milk or tissue fluid is sufficient to carry out a successful analysis. 

Since cyclamate has poor UV absorbing characteristics, HPLC methods for the analysis of this sweetener require specific detection systems, such as indirect photometry or conductivity. Herrmann et al. (24) used indirect photometry for the detection of cyclamate at 267 nm against a UV-absorbing mobile-phase component, p-toluenesulphonate. Biemer (17) and Wu et al. (47) used a conductivity detector for the determination of cyclamate. According to Biemer (17) the use of this detector offers distinct advantages, since compounds coeluting with cyclamate may not exhibit an electrochemical response and, hence, not appear in the chromatogram. 

RR Laski, RR Watts. Gas chromatography of organonitrogen pesticides, using a nitrogen-specific detection system. J Assoc Off Anal Chem 56 328-332, 1973. 

The application of further element-specific detection systems like ICP-MS (Inductively Coupled Plasma-Mass Spectrometry) can result in the element distribution in the SEC fractions (Schmitt et al., 2003). 

De Jonghe, W., Chakraborti, D. and Adams, F. (1980) Graphite furnace atomic absorption as a metal specific detection system for tetraalkyllead compounds separated by gas-liquid chromatography. Anal. Chim. Acta, 115, 89-105. 

Where it is possible to predict likely metabolites and a full interpretation of the parent drug spectrum has been made, the mass spectrometer can be used as a specific detection system in selected ion monitoring (SIM) mode (see Selected ion monitoring, p. 25). The early impetus provided by the development of mass fragmentography in the identification of chlorpromazine metabolites [64] has led to many further applications [65,66,68]. The extension of SIM to quantitative drug measurement with the incorporation of internal standards e.g. [88] has again opened up new possibilities in pharmacokinetic studies in clinical pharmacology, a topic discussed separately below. 

The parent compounds and the metabolites have very low aqueous, solubility. They have proven difficult to extract from some matrices the current method for ivermectin involves some 41 concentration and clean-up steps preceding HPLC (8). A Merck method for recovering abamectin residues from strawberries and formation of fluorescent derivatives for HPLC analysis has 18 separate steps (9). A recently published two-step solid-phase recovery procedure for ivermectin from serum indicates that it is possible to combine an abbreviated concentration and cleanup method with a sensitive and specific detection system in this case, liquid chromatography (10). An immunoassay for avermectins that could be interfaced with simplified residue recovery protocols is a promising solution to the intensifying demands on regulatory agencies to monitor these compounds. 

The ideal detector is universal yet selective, sensitive and structurally informative. Mass spectrometry (MS) currently provides the closest approach to this ideal. The combination of multi-dimensional gas chromatography with high resolution MS or mass-selective detectors in the single ion monitoring (SIM)-mode is currently the most potent analytical tool in enantioselective analysis of chiral compounds in complex mixtures [29]. Nevertheless, it must be pointed out that the application of structure specific detection systems like MS [51] or Fourier transform infrared (FT-IR) [52] cannot save the fundamental challenges to optimum (chiral) resolutions and effective sample clean-up [53]. 

When DMSO is present in excess of DMPO as in the study of Britigan et al. (1988), HO production is manifested primarily as DMPO-CH3, providing a more specific detecting system for HO. ... 

Liskevych, U., and David, J. M. Determination of Butaperazine in Biological Fluids by Gas Chromatography Using Nitrogen Specific Detection System... 

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