Linear Retention

As in isocratic mode, the estimate of log P is indirect and based on the construction of a linear retention model between a retention property characteristic of the solute (logkw) and a training set with known logP ci values. To assess the most performing procedures, the three hydrophobicity indexes (( )o, CHI and logkw) were compared on the basis of the solvation equation [41]. These parameters were significantly inter-related with each other, but not identical. Each parameter was related to log P with values between 0.76 and 0.88 for the 55 tested compounds fitting quality associated with the compound nature. 

Mondello, L., Dugo, R, Basile, A., Dugo, G., Bartle, K.D. (1995) Interactive use of linear retention indices, on polar and apolar columns, with a ms-library for reliable identification of complex mixtures J. Microcolumn Sep. 7 581-591. 

Figured.1.4 An FID chromatogram of concentrated extract of the same Concord grape essence shown Figure G1.1.3, drawn to display the data on a linear retention index scale. By simply comparing the index of a peak with the data listed in the flavornet, the odorants that have similar retention indices can be determined. Notice how large the methyl anthranilate peak is, but still no convincing peak for p-damascenone, even though both compounds have the same odor activity (intensity).

With binary and ternary supercritical mixtures as chromatographic mobile phases, solute retention mechanisms are unclear. Polar modifiers produce a nonlinear relationship between the log of solute partition ratios (k ) and the percentage of modifier in the mobile phase. The only form of liquid chromatography (LC) that produces non-linear retention is liquid-solid adsorption chromatography (LSC) where the retention of solutes follows the adsorption isotherm of the polar modifier (6). Recent measurements confirm that extensive adsorption of both carbon dioxide (7,8) and methanol (8,9) occurs from supercritical methanol/carbon dioxide mixtures. Although extensive adsorption of mobile phase components clearly occurs, a classic adsorption mechanism does not appear to describe chromatographic behavior of polar solutes in packed column SFC. 

Mondello, L., A. Salvatore, P.Q. Tranchida, et al. 2008. Reliable identification of pesticides using linear retention indices as an active tool in gas chromatographic-mass spectrometric analysis. J. Chromatogr. A 1186 430-433. 

A Varian 3400 gas chromatograph equipped with a flame ionization detector and a nonpolar fused silica capillary column (60 m x 0.25 mm i.d. 0.25 pm thickness, SPB-1, Supelco, Inc.) was used to analyze the volatile compounds from the model systems. The injector temperature was 250°C, and the detector temperature was 260°C. The flow rate of the helium carrier gas was 1 mL/min and the split ratio was 50 1. The temperature program consisted of a 10 min isothermal period at 35°C, temperature increases of 2°C/min from 35°C to 120°C and of 4°C/min from 120°C to 235°C, and a 40 min. isothermal period at 235°C. The chromatograms were plotted and integrated on a Varian 4270 integrator. Linear retention indices for the volatile compounds were calculated using n-paraffin standards (C6-C25 Alltech Associates) as references according to the method of Majlat and co-workers (5). 

M. Kokko, Effect of variations in gas chromatographic conditions on the linear retention indices of selected chemical warfare agents, J. Chromatogr., 630, 231-249 (1993). 

Figure 3.15 Variation of the slope with the intercept for linear retention vs. composition curves in RPLC for 32 aromatic solutes on an ODS column using methanol water mixtures as the mobile phase. Parameters S and In fcg correspond to eqn. (3.45). Figure taken from ref. [322]. Reprinted with...

Parameters describing the linear relationship between the slope and the intercept of linear retention vs. composition curves in RPLC (eqn.3.46). Data taken from refs. [322] and [333] (1). 

Linear relationships are preferred, but not mandatory. For non-linear retention lines... 

Donnelly JR, Sovocool GW (1992), Chemosphere 25 1299-1304. Gas chromatographic elution order and elution shift modelling for isomer specific analysis of halogenated dioxins" Donnelly JR, Munslow WD, Grange AH Pettit TL, Simmons RD, Sovocool GW (1991a), J. Chromat. 540 293-310. Correlation of structure with linear retention index for bromo- and bromochlorodibenzo-p-dioxins and bromodibenzofurans"... 

Standardization of retention times. This is the first step. Retention times of our odor zones have to be normalized by those of n-alkanes used as standards. To do that, the mixture of alkanes, which can be directly obtained from different chromatographic suppliers, is injected in exactly the same conditions in which the GC-O experiment was carried out. As nearly all the GC-O experiments use temperature gradients, the Retention Index or Linear Retention Index - not Kovats Index according to the lUPAC (lUPAC 1997) - will be determined with the formula... 

At temperatures below the melting point of the polymer, in region CD, retention proceeds by bulk sorption but the polymer-solute interaction is restricted to the amorphous domains of the stationary phase. Upon meltir, in r on DF, the fraction of amorphous material increases, leading to an incre in retention volume. At temperatures above the melting point, segment FG, a linear retention diagram, corresponding to bulk sorption into the completely amorphous polymer, is obtained. By extrapolation of this line to lower temperatures (dashed line FE), the crystalline content of the stationary phase can be determined by comparison of the experimental retention volume with the extrapolated value. 

Stead of the expected reversal, a linear retention digram was recorded through Tg. These findings, which could severely undermine the reliability of the method, focused attention on the importance of experimental conditions used in these experiments. 

This is a reason to change the formula for the RI calculation (the set of reference compounds and their attributed RI values remain the same). This version of RIs developed especially for a linear temperature programming regime (linear retention indices) have been proposed by Van den Dool and Kratz in 1963 [2] ... 

Vassilaros DL, Kong RC, Later DW, Lee ML (1982) Linear retention index system for polycyclic aromatic compounds. Critical evaluation and additional indices. J Chrom 252, 1-20. 

LRIs (linear retention indices) depend on programming and flow conditions, but changes in their values are not very marked, particularly when the variation of RI with temperature is small [9] in this case, LRI and RI values for a compound are similar. 

Some standard one-dimensional GC methods use reference peaks to help recognize drift [23]. For more widely varying chromatographic conditions, retention times for targets can be related using a linear retenhon index (LRI) [24], in which retention times are referenced relative to the retention times of marker compounds. A common LRI scheme uses the n-alkanes as marker points with indices equal to 100 times the carbon number (foUowing the Kovats index [24]) then the indices for peaks between marker points are computed using piecewise hnear interpolation. If retention-time windows are defined relative to marker p>eaks that can be located, then any linear retention-time transformation observed in the marker jjeaks can be apphed to the windows used for chemical identification. 

At this point we can proceed to use either a permeation analysis or the linearized retention analysis. Both will be trial and error, but the easier retention analysis will be demonstrated. 

When a pure standard is not commercially available and its synthesis is not possible, identification may be carried by comparing retention indexes e.g. linear retention indexes, Kovats retention indexes) of compounds of interest with those of published in literature retention indexes are based on the values attributed, by definition, to a series of homologous compounds e.g. alkanes Cs = 800, C9 = 900) and its relation with its retention time. 

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