Chromatographic retention time

Comparison of the mass spectrum from a target compound (top), with the three best fits from the library of standard spectra (lower three traces). The closeness of fit of the mass spectra and the chromatographic retention time lead to a positive identification of 2, 6-dimethylheptane. 

The purity of the product is greater than 99% as determined by gas chromatographic analysis using a 6-m. column of 30% Carbowax 20M on 60-80 Chromosorb W. The major impurity (<1%) was shown to be 3-heptanol by comparison of gas chromatographic retention times and mass spectral fragmentation patterns with those of an authentic sample. 

Chlorambucil - there is no problem with the quantitation ion (at m/z 254), although the second ion proves to be a little difficult. While the ion at m/z 303 is the obvious choice, this is not very intense and therefore for samples containing small amounts of analyte the precision of measurement of this ion will be reduced and it may not be detectable at all levels at which the quantitation ion is observed. We could possibly consider the (M- -2) ion, as the combination o/m/z 254 (high mass, and therefore reasonable specificity), the presence of one chlorine, and the chromatographic retention time could be considered sufficient for definitive identification in those cases in which the intensity o/m/z 303 is insufficient. 

S. W., Melton, C. M. Estimation of gas-liquid chromatographic retention times from molecular structure.. Chromatogr. A 1994, 662, 269-280. 

By way of graphical example of the various algebraic and geometrical concepts that are introduced in this chapter, we will make use of a measurement table adapted from Walczak etal.[ ]. Table 31.2 describes 23 substituted chalcones in terms of eight chromatographic retention times. Chalcone molecules are constituted of two phenyl rings joined by a chain of three-carbon atoms which carries a double bond and a ketone function. Substitutions have been made on each of the phenyl rings at the para-positions with respect to the chain. The substituents are CFj, F, H, methyl, ethyl, i-propyl, t-butyl, methoxy, dimethylamine, phenyl and NO2. Not all combinations two-by-two of these substituents are represented in the... 

Chromatographic retention times of 23 doubly substituted chalcones, as determined by 8 HPLC chromatographic methods using heptane as the mobile phase. The methods differ by addition of 0.5 percent of a chemical modifier. 

Fig. 31.6. Biplot of chromatographic retention times in Table 31.2, after column-centering of the data. Two unipolar axes and one bipolar axis have been drawn through the representations of the methods DMSO and methylenedichloride (CH2CI2). The projections of three selected compounds are indicated by dashed lines. TTie values read off from the unipolar axes reproduce the retention times in the corresponding columns. The values on the bipolar axis reproduce the differences between retention times.

Fig. 31.8. Biplot of chromatographic retention times in Table 31.2, after log column-centering of the data. The values on the bipolar axis reproduce the (log) ratios between retention times in the two corresponding columns.

Fig. 31.10. Same biplot of chromatographic retention times as in Fig. 31.9. The line segments connect compounds that share a common substituent. The horizontal contrast reflects the presence or absence of a NO2 substituent. The vertical contrast expresses the electronegativity of the substituents.

The rank of the transformed table of chromatographic retention times Z is equal to seven. 

It is assumed that the structural eigenvectors explain successively less variance in the data. The error eigenvalues, however, when they account for random errors in the data, should be equal. In practice, one expects that the curve on the Scree-plot levels off at a point r when the structural information in the data is nearly exhausted. This point determines the number of structural eigenvectors. In Fig. 31.15 we present the Scree-plot for the 23x8 table of transformed chromatographic retention times. From the plot we observe that the residual variance levels off after the second eigenvector. Hence, we conclude from this evidence that the structural pattern in the data is two-dimensional and that the five residual dimensions contribute mostly noise. 

Cech, N. B. Krone, J. R. Enke, C. G. Predicting electrospray response from chromatographic retention time. Anal. Chem. 2001,73,208-213. 

Measurements of gas chromatographic retention time are often used as a fast and easy method of estimating vapor pressure. These estimated pressures are related to the gas/substrate partition coefficient, which can be regarded as a ratio of solubility of the substance in the gas to that in the substrate, both solubilities being of the substance in the liquid state. As a result the estimated vapor pressures are of the liquid state. To obtain the solid vapor pressure requires multiplication by the fugacity ratio. It is important to establish if the estimated and reported property is of the vapor or liquid. 

In principle, the determination of vapor pressure involves the measurement of the saturation concentration or pressure of the solute in a gas phase. The most reliable methods involve direct determination of these concentrations, but convenient indirect methods are also available based on evaporation rate measurements or chromatographic retention times. Some methods and approaches are listed below. 

Su,Y., Lei, D.L. Daly, G.,Wania, F. (2002) Determination of octanol-air partition coefficient (KqA) values for chlorobenzenes and polychlorinated naphthalenes from gas chromatographic retention times. J. Chem. Eng. Data, 47, 449 455. 

Wania, F., Lei, Y.D., Hamer, T. (2002) Estimating octanol-air partition coefficients of nonpolar semivolatile organic compounds form gas chromatographic retention times. Anal. Chem. 74, 3478-3483. 

A large number of primary metabolites can be readily identified because most of these compounds are commercially available. Standard compounds are derivatized, co-chromatographed, and the data are deposited into databases. Unknown metabolites are identified by matching chromatographic retention times... 

Enantiomers have identical chemical and physical properties in the absence of an external chiral influence. This means that 2 and 3 have the same melting point, solubility, chromatographic retention time, infrared spectroscopy (IR), and nuclear magnetic resonance (NMR) spectra. However, there is one property in which chiral compounds differ from achiral compounds and in which enantiomers differ from each other. This property is the direction in which they rotate plane-polarized light, and this is called optical activity or optical rotation. Optical rotation can be interpreted as the outcome of interaction between an enantiomeric compound and polarized light. Thus, enantiomer 3, which rotates plane-polarized light in a clockwise direction, is described as (+)-lactic acid, while enantiomer 2, which has an equal and opposite rotation under the same conditions, is described as (—)-lactic acid. 

Identification of compounds in volatiles collected from hunting M. cornigera revealed three common components of moth sex pheromone blends (Z)-9-tetradecenal, (Z)-9-tetradecenyl acetate, and (Z)-ll-hexade-cenal [while there was insufficient material for mass spectrometry, gas chromatographic retention time evidence suggests that (Z)-ll-hexadece-... 

Figure 2.2 shows the total ion current trace and a number of appropriate mass chromatograms obtained from the pyrolysis gas chromatography-mass spectrometry analysis of the polluted soil sample. The upper trace represents a part of the total ion current magnified eight times. The peak numbers correspond with the numbers mentioned in Table 2.1 and refer to the identified compounds. The identification was based on manual comparison of mass spectra and relative gas chromatographic retention times with literature data [34, 35] and with data of standards available. In some cases unknown compounds were tentatively identified on the basis of a priori interpretation of their mass spectra (labelled tentative in Table 2.1). 

Tributyltin carboxylates undergo rapid chemical exchange, as evidenced by NMR. As a consequence, even the interfacial reaction between tributyltin carboxylate and chloride is fast. IR, mass spectra, gas chromatographic retention time and chloride assay show that the product of the reaction is tributyltin chloride. 

Controlled release epoxy formulations in which tin is chemically anchored as tributyltin carboxylate to the polymer chain are discussed. NMR evidence is presented to establish that rapid exchange exists in tributyltin carboxylates. Consequently, even the interfacial reaction between tributyltin carboxylates and chloride is very fast equilibrium constants are reported for the reaction between tributyltin acrylate in hexane and sodium chloride in water. IR spectra, gas chromatographic retention time, chloride assay, and the complex intensity pattern of the molecular ion peaks in the mass spectrum show that the product of the reaction is tributyltin chloride, suggesting that it is the chemical species responsible for antifouling activity in marine environment. 

Lucic, B., Trinajstic, N., Sild, S., Karelson, M., Katritzky, A. R. J. Chem. Inf. Comput. Sci. 39, 1999, 610-621. A new efficient approach for variable selection based on multiregression Prediction of gas chromatographic retention times and response factors. 

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