Relative retention data

Prinsloo SM, De Beer P143R. 1985. Gas chromatographic relative retention data for pesticides on nine packed columns I. Organophosphorus pesticides, using flame photometric detection. J Assoc Off Anal Chem 68 1100-1108. 

In cases where a mixture has a large number of components, or pure standards are not available, published retention data must be consulted. The uncorrected retention time, tR (p. 86), is not suitable for this purpose because it cannot be compared with data from different columns and instruments. Valid comparisons can be made using relative retention data which are dependent only on column temperature and type of stationary phase. An adjusted retention time, / R, is first obtained by subtracting from tR the time required to elute a non-retained substance such as air (Figure 4.26)... 

An attempt to compare retention times on two different columns of the same type can be difficult, at best. Differences in packing density, liquid loading, activity of the support, age and previous use of the packing, and variations in the composition of the column wall can lead to large differences in retention measurements between the two columns. If one must use two separate columns of the same type, then relative retention data is preferred since this measurement is reasonably constant for columns of the same type, it is not as subject to temperature and flow changes, and it is easy to obtain. 

Table 7.3 Comparison of SFC and HPLC Relative Retention Data of Chlordiazepoxide Mixtures...

The main problem with a is that there is no single standard to which data have been ratioed, and consequently there are no tabulations of relative retention data in the literature. Kovats suggested that a series of standards be used, and he proposed the n-paraffins. Before discussing his proposal further, we need to examine the relationship between retention volume and the members of a homologous series such as the paraffins. 

The introduction of GC as an analytical technique has had a profound impact on both qualitative and quantitative analysis of organic compounds. Identification of compounds by GC can be accomplished by their retention times on the column as compared to known reference standards, by inference from sample treatment prior to chromatography, " or by the concept of retention index. " The latter method and tables of retention indices " with associated conditions have been reported. " Although qualitative data and analytical techniques for identification of compounds are well-established " and relative retention data for over 600 substances also have been published, " the main utility of GC undoubtedly lies in its powerful combination of separation and quantitative capabilities. Use in quantitative analysis involves the implementation of two techniques being performed concurrently, i.e., separation of components and subsequent quantitative measurement. 

PCB materials in field-collected bald eagle specimens have been identified utilizing combined gas chromatography-mass spectrometry techniques in conjunction with thin-layer chromatographic separations of standard materials as well as tissue extracts (26). A total of 19 PCB compounds were detected in the field samples. Analyses were performed employing a spiral glass gas chromatographic column, 9 ft X 0.25 inch o.d., packed with 1% SE-30 on 100/120 mesh Gas Chrom Q. Mass spectra of the major components of Aroclor 1254 PCB standard were presented in this report as well as relative retention data for the individual components. 

To extract the additives the polymer was refluxed with toluene for 20 minutes then the polymer was reprecipitated with ethanol. Relative retention data is given in Table 4.1. 

Krishen [78] pointed out that one of the major drawbacks of previously published gas chromatographic techniques for the identification and/or determination of ester plasticisers of the type used in PVC is that, commonly, isothermal column conditions had previously been used, since relative retention data were available for these conditions. Only limited use had been made of temperature programming by earlier workers. 

Table 4.8 Relative retention data for hydrolysis products ...

It is easy to relate retention times to the thermodynamics of phase equilibria. In practice, however, it is difficult to measure many of the relevant thermodynamic parameters with sufficient accuracy, while retention times can be measured with great precision. Furthermore, it should be emphasized at the outset that even though very precise measurements can indeed be made in chromatography [38], the available stationary phases are often so poorly reproducible that such precision is unwarranted. Column-to-column reproducibility for silica-based phases is reasonable only for columns packed with material from a given supplier. For this reason, chromatographers prefer whenever possible to use relative retention data. 

Because absolute retention data depend on so many controlled (temperature, pressure) and uncontrollable (activity of the stationary phase) factors. and because their quantification requires independent determinations that are time-consuming (e.g density of the stationary liquid phase in the case of GC) or not very accurate (flow-rate measurements), and also because chromato-graphers are more often concerned with practical separation problems than with phase thermodynamics, it is more convenient to work with relative retention data. The parameter of choice is the relative retention a, which is also the separation factor ... 

Relative retention data are more convenient than absolute data, but the choice of an appropriate reference may prove difficult. An ideal reference compound... 

Should elute near the middle of the chromatogram so the relative retention data will be neither too large nor too small... 

The accuracy of retention data is still more questionable than their reproducibility. Poor batch-to-batch reproducibility for silica-based phases has plagued LC for a generation. For this reason, retention data cannot be compiled in the same way as spectra. Such data collections have limited usefulness, providing orders of magnitude for relative retention rather than accurate information on which an analyst can rely for the identification of unknowns. Precision and accuracy with respect to absolute and relative retention data have been discussed in great detail [58]. 

Table 4.10 Peak identification and relative retention data (tricresylphosphate column at 35 ...

Denning and Marshall devised a method in which toluene extracts of polyethylene are examined for ultraviolet absorbers (and antioxidants) at two column temperatures in order to overcome the problem that whereas some compounds have a relatively low retention time, others of higher molecular weight have very long retention time at 250 C. Relative retention data obtained for ultraviolet absorbers, are given in Table 1.5. 

Table 7.45 - Peak Identification and Relative Retention Data, (tricresylphosphate column at35°C)...

In this Curie point filaiment pyrolysis technique the polymer is pyrolysed at a controlled temperature and the pyrolysis products passed into a gas clvomatograph. Relative retention data and peak height ratio data obtained when compared with tabulations of data obtained from known polymers provide a means of identifying the unknown polymer. 

Relative Retention Data<> for Vitamins D, Related Sterols, and Their Derivatives on SE52/949 - ... 

Relative Retention Data for Cholesterol, Ergocalciferol, Cholecalciferol, and Their Derivatives on SE52/Modified X 60 ><... 

---