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Volatile organic compounds retention times

As discussed in Chapter 7, gas chromatography (GC) is used to separate complex mixtures of volatile organic compounds. However, unless pure authentic standards are also analyzed to compare retention times, it is not possible to identify the components by GC alone. However, by connecting the output of a GC to a mass spectrometer, and by removing the carrier gas to maintain the low pressures required, it is possible to both separate and identify these complex mixtures. This method is the gold standard for the identification of organic samples, if they are sufficiently volatile. [Pg.174]

Jalali-Heravi, M. and Kyani, A. (2004) Use of computer-assisted methods for the modeling of the retention time of a variety of volatile organic compounds a PCA-MLR-ANN approach./. Chem. Inf. Comput. Sci., 44, 1328—1335. [Pg.1078]

The strategy of Seeley and Seeley [29] only requires as starting data ID GC retention times from temperature-programmed runs. After calculation of retention indices in both and columns, a transformation of these values was used to construct a plot that tries to reproduce the 2D experimental retention. The method was applied to 139 volatile organic compounds with good results in the reproduction of general patterns, although prediction of D retention shows some errors, especially for compounds whose RI values depend markedly on temperature. [Pg.61]

Tables 1 and 2 provide one example of GC-MS results from analysis of the same urine sample by ECC/CT and SPME, respectively. Clearly, ECC/CT facilitates the observation of the more volatile organic compounds. Tables 3 and 4 illustrate typical results from automated SPME/GC-MS analysis of a urine sample at physiological pH 8 versus pH 3. It is noteworthy that the compounds listed in Table 4 (pH 3) at retention times of 64.49, 65.40, 65.69, 66.24, 66.49, 67.24, and 67.89 minutes constitute a total of 62.48 area percent of the total products observed, while none of these compounds appear at all in the pH 8 sample. The compound at 64.49 minutes is a known synthetic spirocycle (1) that has not been observed previously as a natural product (Ehrenfreund et al., 1974 Renold et al., 1975 Schulte-Elte, et al., 1978). This is not the first time that we have observed unique natural products in African elephant secretions and excretions (Goodwin et al, 1999, 2002). Tables 1 and 2 provide one example of GC-MS results from analysis of the same urine sample by ECC/CT and SPME, respectively. Clearly, ECC/CT facilitates the observation of the more volatile organic compounds. Tables 3 and 4 illustrate typical results from automated SPME/GC-MS analysis of a urine sample at physiological pH 8 versus pH 3. It is noteworthy that the compounds listed in Table 4 (pH 3) at retention times of 64.49, 65.40, 65.69, 66.24, 66.49, 67.24, and 67.89 minutes constitute a total of 62.48 area percent of the total products observed, while none of these compounds appear at all in the pH 8 sample. The compound at 64.49 minutes is a known synthetic spirocycle (1) that has not been observed previously as a natural product (Ehrenfreund et al., 1974 Renold et al., 1975 Schulte-Elte, et al., 1978). This is not the first time that we have observed unique natural products in African elephant secretions and excretions (Goodwin et al, 1999, 2002).
This technique offers the possibility to gain additional information by mass spectra [44], However, identification of volatile organic compounds based only on mass spectra does not completely affordable. Molecular rearrangement and isomerisation processes of unsaturated hydrocarbons result in very similar mass spectra lacking characteristic ftagmentation patterns. Combined data of retention times, Kovats or Sadler retention indices [55] and mass spectral data offer the possibility of an unambiguous identification of volatile organic constituents. [Pg.185]

Table 4.61 Volatile organic compounds of the BTEX test with retention times and quantitative precision. Table 4.61 Volatile organic compounds of the BTEX test with retention times and quantitative precision.
The first two points are best dealt with as part of the process for developing vahdated analytical methods. Vafidation should include testing the robustness of a method in repeated use over a period of time determining the precision and accuracy and study of potential interferences. As an example, it would be expected that in the capillary GC—TEA method for organic explosives, a peak should be at least three times the basefine noise to be counted as a real signal, and that the relative retention time should be within 1.0% of the standard for volatile compounds and within 0.5% for the rest. The relative retention time is simply the ratio of the analyte s retention time compared with that of an internal standard. Use of relative retention times significantly improves the repeatabdity of GC analysis... [Pg.237]


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