Identification retention behaviour

Identification methods based on the retention behaviour of compounds. 27... 

Prospects of performing identification by virtue of retention behaviour only. 39... 

The FID response is roughly dependent on the number of carbon atoms in a molecule and is quite predictable. However, the BCD response varies widely for different compounds, is dependent on the electron-deficient part of the compound, and is difficult to predict. The AFID response of a compound depends to some extent on the number of phosphorus or nitrogen atoms in a molecule, but it also depends on their environment. Thus, by using the FID as a reference and measuring the BCD or AFID response relative to it, another characteristic for identification is obtained in addition to retention behaviour. 

ECL chromatographic retention behaviour of fatty acid methyl esters relative to the saturated homologues, which exhibit a straight line when the log of the retention times are plotted against the number of acyl carbons atoms. Used in the tentative identification of fatty acids by gas chromatography. 

HPLC units have been interfaced with a wide range of detection techniques (e.g. spectrophotometry, fluorimetry, refractive index measurement, voltammetry and conductance) but most of them only provide elution rate information. As with other forms of chromatography, for component identification, the retention parameters have to be compared with the behaviour of known chemical species. For organo-metallic species element-specific detectors (such as spectrometers which measure atomic absorption, atomic emission and atomic fluorescence) have proved quite useful. The state-of-the-art HPLC detection system is an inductively coupled plasma/MS unit. HPLC applications (in speciation studies) include determination of metal alkyls and aryls in oils, separation of soluble species of higher molecular weight, and separation of As111, Asv, mono-, di- and trimethyl arsonic acids. There are also procedures for separating mixtures of oxyanions of N, S or P. 

De Beer et al. [501] reported an extensive comparative study of the chromatographic behaviour of methyl esters and pentafluorobenzyl esters of these substances. They correlated retention data (Kovats retention indices) on nine stationary phases with the structure of the derivatives and with the polarity of the stationary phases with the aim of utilizing these dependences for identification purposes. However, much more significant is the better separation obtained with pentafluorobenzyl esters and the possibility of increasing the sensitivity of the analysis. 

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