The Retention Index

If the chromatographic conditions are kept constant, the retention times of the compounds remain the same. All identification concepts using classical detectors function on this basis. The retention times of compounds, however, can change through ageing of the column and more particularly through differing matrix effects between samples. 

The measurement of the retention times relative to a co-injected standard can help to overcome these difficulties. Fixed retention indices (RI) are assigned to these standards. An analyte is included in a RI system with the RI values of the standards eluting before and after it. It is assumed small variances in the retention times affect both the analyte and the standards so that the RI values calculated remain constant (Deutsche Forschungsgemeinschaft, 1982). 

The first RI system to become widely used was developed by Kovats (Zenkevich, 1993). In this system, a series of w-alkanes is used as the standard. Each -alkane is assigned the value of the number of carbon atoms multiplied by 100 as the retention index (pentane 500, hexane 600, heptane 700, etc.). For isothermal 

On working with linear temperature programs (Berngard et al., 1994 van den Dool and Kratz, 1963) a simplification is used which was introduced by van den Dool and Kratz, whereby direct retention times are used instead of the logarithmic terms used by Kovats (van den Dool and Kratz, 1963 Bianchi et al. 2007)  

The weakness of RI systems using alkanes lies in the fact that not all analytes are affected by variances in the measuring system to the same extent, or selective 

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Kovats, E. and A. Wehrli (1959), Gas-chromatography characterization of organic compounds. III. Calculation of the retention indexes of aliphatic, alicyclic and aromatic compounds . Helv. Chim. Acta, Vol. 42, p. 2709. 

Van Den Dool, H. and P.D. Kratz (1963), Generalization of the retention index system including linear temperature programmed gas-liquid partition chromatography . J. Chromatogr, Vol. 11, p. 463. 

The adjusted retention times for octane, toluene, and nonane on a particular GC column are 15.98 min, 17.73 min, and 20.42 min, respectively. What is the retention index for all three compounds ... 

What is the retention index for a compound whose adjusted retention time is 9.36 min ... 

Qualitative (identification) applications depend upon the comparison of the retention characteristics of the unknown with those of reference materials. In the case of gas chromatography, this characteristic is known as the retention index and, although collections of data on popular stationary phases exist, it is unlikely that any compound has a unique retention index and unequivocal identification can be effected. In liquid chromatography, the situation is more complex because there is a much larger number of combinations of stationary and mobile phases in use, and large collections of retention characteristics on any single system do not exist. In addition, HPLC is a less efficient separation... 

The retention index value is tenperature dependent and when eui index value is required at another tenperature it can be obtained by Interpolation using an Antoine-type hyperbolic ] function... 

The HcReynolds abroach, which was based on earlier theoretical considerations proposed by Rohrschneider, is formulated on the assumption that intermolecular forces are additive and their Individual contributions to retention can be evaluated from differences between the retention index values for a series of test solutes measured on the liquid phase to be characterized and squalane at a fixed temperature of 120 C. The test solutes. Table 2.12, were selected to express dominant Intermolecular interactions. HcReynolds suggested that ten solutes were needed for this purpose. This included the original five test solutes proposed by Rohrschneider or higher molecular weight homologs of those test solutes to improve the accuracy of the retention index measurements. The number of test solutes required to adequately characterize the solvent properties of a stationary phase has remained controversial but in conventional practice the first five solutes in Table 2.12, identified by symbols x through s have been the most widely used [6). It was further assumed that for each type of intermolecular interaction, the interaction energy is proportional to a value a, b, c, d, or e, etc., characteristic of each test solute and proportional to its susceptibility for a particular interaction, and to a value x, X, Z, U, s, etc., characteristic of the capacity of the liquid phase... 

Column temperature. Normally between 100 and 300 °C. As an approximate guide, the temperature to be used is the retention index -t-10. 

Only one reproductive female-specific compound with the retention index of 2348 was detected. Non-reproductive female-specific compounds and boar-specific compounds were not detected in the extracts. 

The studies with barbiturates revealed that the logarithm of the retention time is linearly related to the octanol/water partition coefficients [66,67]. It has been observed that the retention index of the drug is linearly related to the octanol/water partition coefficient (logP), and that results are very close to that of the 2-keto alkane standard (solid line in Fig. 15.9). 

McReynolds used the retention index of certain solutes to compare different stationary phases and to assess their selectivity compared with a reference liquid phase, squalane. Squalane is considered to be non-polar and any increase in the retention index of the selected solute on the test column compared to squalane may be considered to be due to the greater polarity of that solvent. McReynolds constants have been determined for all stationary phases using a range of solutes of varying polarity (Table 3.8) and may be used to assist in selecting an appropriate stationary phase. 

The phenanthrene/anthracene series is shown in Fig. 7 and the major peak in the m/z 234 trace has the retention index of retene which is generally derived from conifer wood burning. Sometimes there is a triplet of peaks in the same C4 plot due to benzonaphthothiophenes (C10H16S) which are components of some... 

The retention time of the measured compound with that of a standard compound (usually an n-hydrocarbon) under the same conditions, the retention index /, was calculated as ... 

PGHM Muijselaar, HA Claessens, CA Cramers. Application of the retention index concept in micellar electrokinetic capillary chromatography. Anal Chem 66 635-644, 1994. 

Figure 11.5 shows the structures of some of the major components in peppermint oil. The use of the retention index system is illustrated in Figures 11.6 and 11.7 for peppermint oil run in comparison with n-alkane standards on both a weakly polar OV-5-type column and a polar carbowax column. 

The value of each phase constant (i.e., X, Y, Z, U, and S ) is determined by subtracting the retention index of the probe on a squalane stationary phase (Isq) from the retention index of the probe on the stationary phase being characterized (Ijp). For example, the phase constant of benzene (X ) would be calculated as shown in Equation 4.2. 

Fig. 15.17 Simultaneous EI-MS (top trace) and PTR-MS (bottom trace) total ion coimt analysis of coffee headspace. Identification was based on MS spectra obtained at 70 eV and the retention index of the reference compounds. (Adapted from [199])...

A great number of stationary phases are listed in catalogues and it is sometimes difficult to choose the best column for a particular analysis. The chemical nature of the phases and their polarities do not always allow one to predict which column will be optimal for a given separation. Therefore, a technique called the retention index system has been developed with the use of reference compounds whose retention factors differ with different stationary phases. Using retention indices obtained on columns of different stationary phases, it is possible to characterise a compound and facilitate its identification. 

The retention index of a compound obtained on a given stationary phase under given experimental conditions constitutes worthwhile information. However, if several indices of the same compound obtained on different stationary phases are available, better identification of this compound can be made. Because of the excellent reproducibility of retention times on modern chromatographs, this method is reliable for known control compounds. While obtaining retention indices does not constitute absolute identification of a compound, this method can be quite useful to identify unknowns if the proper retention indices tables are available on the most common stationary phases (Squalane, Apiezon, SE30, Carbowax 20M). However, the use of retention indices is now of lesser interest because of capillary columns that involve new stationary phases. This limits the information that can be obtained from the retention indices. Hyphenated techniques are currently more popular. They represent excellent methods for compound identification but depend on instruments that are more complex and more expensive. 

If the retention index for pyridine on squalane is 695, what is the McReynolds constant of this compound on the column studied, if it is known that under the conditions of the experiment, the retention time is 346 s ... 

The retention index of 657 for benzene on poly(dimethylsiloxane) in Table 24-3 means that benzene is eluted between hexane (7 = 600) and heptane (7 = 700) from this nonpolar station-aiy phase. Nitropropane is eluted just after heptane on the same column. As we go down the table, the stationary phases become more polar. For (biscyanopropyl)09(cyanopropylphenyl)0l-polysiloxane at the bottom of the table, benzene is eluted after decane, and nitropropane is eluted after -Cl4H30. 

An unknown compound was co-chromatographed with heptane and decane. The adjusted retention times were heptane, 12.6 min decane. 22.9 min unknown, 20.0 min. The retention indexes for heptane and decane are 700 and 1 000, respectively. Find the retention index for the unknown. 

The suitability of a stationary phase for a specific separation depends upon the selectivity of the phase. This is a measure of the degree to which polar compounds are retarded relative to their elution on a nonpolar phase. A systematic method for expressing the retention data is based on retention indices. For this sytem, the retention indices of the n-paraffins are by definition equal to 100 times the number of carbon atoms in the molecule. For example, the retention index for n-hexane is 600 and for n-octane 800. These values are defined and apply regardless of the column used and regardless of the temperature. 

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