Chromatographic response peak width

The ELS detector was previously also referred to as a mass detector, pointing to the fact that the response is (mainly) determined by the mass of the sample rather than by its chemical structure. Van der Meeren et al., though, demonstrated that the ELSD calibration curves of phospholipid classes were also dependent on the fatty acid composition (52). The dependence on the fatty acid composition is, however, completely different in nature and much less pronounced than for UV detection. The reason for this behavior is to be found in the partial resolution of molecular species, even during normal-phase chromatography. Thus, the peak shape depends not only on the chromatographic system but also on the fatty acid composition and molecular species distribution of the PL sample (47). Because it was shown before, based on both theoretical considerations and practical experiments, that the ELS detector response is generally inversely proportional to peak width (62,104), it follows that the molecular species distribution of the PL standards used should be similar to the sample components to be quantified. It was shown that up to 20% error may be induced if an inappropriate standard is used (52). 

The most important parameters related to chromatographic response (retention time, peak width, resolution), well known by GC users, are also fundamental in the characterization of GCxGC peaks. However, some of them take in GCxGC a new meaning for instance, resolution or peak capacity must be considered in a different way. Other characteristics, such as orthogonality or structured chromatograms, are specific of GCxGC. 

The time that elapses between an analyte injection in a GC column and the elution of its peak maximum is the retention time (fj ). Although retention time is related to the geometry of the column and to its operation conditions, for a given column and conditions it depends only on the interaction between analyte and stationary phase, and hence on the analyte structure. Its value contains all the qualitative information that GC affords for a compound the coincidence of retention times between an analyte and a standard compound (analysed in the same column and conditions) is a requirement for a positive identification in GC. Differences in retention time for two anal)d es, together with peak width, are responsible for the chromatographic separation (resolution) between them. 

HPLC theory could be subdivided in two distinct aspects kinetic and thermodynamic. Kinetic aspect of chromatographic zone migration is responsible for the band broadening, and the thermodynamic aspect is responsible for the analyte retention in the column. From the analytical point of view, kinetic factors determine the width of chromatographic peak whereas the thermodynamic factors determine peak position on the chromatogram. Both aspects are equally important, and successful separation could be achieved either by optimization of band broadening (efficiency) or by variation of the peak positions on the chromatogram (selectivity). From the practical point of view, separation efficiency in HPLC is more related to instrument optimization, column... 

The response of a gas chromatographic detector to an analyte band eluting from the end of the column is the chromatographic peak. The three main characteristics that define a peak in ID GC are its size, width, and position in the chromatogram. 

The positional criteria can lead to reliable resolution optima using the retention data from a few mobile phases. However, as the shape and width of the chromatographic peaks are not considered, for largely overlapped asymmetric peaks, an unacceptable optimum can be obtained. If this is the case, only an increase in plate count will provide the desired separation, using for instance two identical columns in series [8]. Alternatively, a response surface related to a different criterion must be examined. 

Any fluctuation of an operating parameter of the chromatograph, including tho.se that control the retention times and band widths of peaks and those that determine response factors, constitutes a source of error in chromatographic analysis [58]. Such parameters include the column temperature and the mobile-phase flow rate, as well as various detector settings [67]. 

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