Chromatography, general detection

For reliable identification of a residue, detailed information about the molecular structure of the analyte is essential. The total information about the molecular structure of the analyte is the sum of the information derived from each individual analytical step of tire method. Frequently used selective analytical steps based on chromatography or immunoaffinity, provide more or less general indirect information. For example, solid-phase extraction (SPE) cleanup followed by liquid chromatography/ultraviolet detection (LC/UV) has been suggested for screening and quantification of ivermectin residues in liver, but presumptive positive samples can be confirmed by derivatizing an aliquot of the SPE eluate and reanalyzing the fluorescent derivative of ivermectin in an LC-fluorescence system (17). 

Comparative assessments of purification protocols are generally difficult, and wide ranges of activities have been reported. ACV synthetases have been assayed by either employing a peptide adsorption resin (Porapak) [29,48] or high performance liquid chromatography (HPLC) detection of the derivatized tripeptide [26,27], The adsorption assay, employing radiolabeled amino acids, has been... 

The non-polar chlorinated hydrocarbon pesticides are routinely quantified using gas chromatography (GC) and electron capture(EC) detection. Alternate detectors include electrolytic conductivity and microcoulometric systems. Organophosphate pesticides which are amenable to GC are responsive to either the flame photometric detector (FPD) or the alkali flame detector (AFD). Sulfur containing compounds respond in the electrolytic conductivity or flame photometric detectors. Nitrogen containing pesticides or metabolites are generally detected with alkali flame or electrolytic conductivity detectors. 

Even the simplest bifunctional compounds of these classes being analyzed on nonpolar phases indicate broad nonsymmetrical peaks on chromatograms. This leads to poor detection limits and reproducibility of retention indices (the position of peaks maxima depends on the quantity of analytes) compared with nonpolar compounds. The general way to avoid these problems is based on the conversion of hydroxy compounds to thermally stable volatile derivatives. This task is a most important purpose of derivatization (see the entry Derivatization of Analytes in Chromatography, General Aspects). This chemical treatment may be used not only for nonvolatile compounds but also for volatile substances. The less polar products typically yield narrower chromatographic peaks that provide the better signal-to-noise ratio and, hence, lower detection limits. Nonpolar derivatives have much better interlaboratory reproducibility of retention indices compared with this parameter for initially polar compounds. 

Sims RPA, Larose JAG (1962) Use of iodine vapor as a general detecting agent in thin layer chromatography. J Am Oil Chem Soc 39 232... 

But selective detection is not always available. There have been several reports where general detection methods such as conductivity or post-column reaction photometric detection have been used. The chromatography is much more important in these cases. With general detection, the metal species must be the same ionic charge. Non-retained sample peaks that elute with the void volume generally contain other components that cause a detector response. This would interfere with quantification of the non-retained peak. 

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