Homologue distribution

While the alkyl chain distribution is determined on a non-polar RP8 and RP18, EO homologue distribution is determined using a polar phase. AEOs are not UV-absorbing species, so they cannot be directly determined by HPLC followed by standard optical detection systems (UV and FL), unless suitable derivatives are prepared [2], Because of this, methods based on liquid chromatography-mass spectrometry [77-79] are currently considered as the benchmark procedure that gives sufficiently high selectivity and sensitivity. 

Both the extent and the kinetics of LAS biodegradation depend on the characteristics of the compound (e.g. isomer and homologue distribution, and the initial concentration), the properties of the medium... 

Initial LAS concentration, and isomer and homologue distribution Tests conducted with concentrations found in the natural environment (1 ppm) present percentages of primary biodegradation in excess of 99%, although when the initial concentration tested is higher (10-20 ppm), the extent is less (see Table 5.3.1). The rate of degradation also decreases in line with the initial concentration due to the toxic effect on the microbiota [29]. In Table 5.3.1 it can be observed that the average life-times increase in line with the increase in the initial concentration of LAS. 

Fig. 6.3.4. Alkyl homologue distribution of LAS (sample batch from December 1999) (from Ref. [23]).

The ability of several methods to extract polycyclic aromatic hydrocarbons (PAHs) and other petroleum hydrocarbons from marine sediments was examined. Comparisons of soxh-let and methylene chloride reflux methods gave extraction efficiencies that showed no statistical difference in the return of PAHs however, the return when using ball-mill tumbling was significantly lower. The relative content of individual parent PAH compounds, parent compound distributions (PCDs), and alkyl homologue distributions (AHDs) of PAHs was calculated using capillary column GC-MS. The similarities of the distributions showed that any of the three methods could have been used to calculate these distributions. An examination of several extraction methods commonly used for the extraction of petroleum hydrocarbon material from sediment samples showed differences in results that depended on the extraction method employed. In addition, the methods varied in their ability to extract resolved versus unresolved material and to return aliphatic and aromatic hydrocarbon compounds. 

Several studies have examined the total PAH content, the relative content of individual parent PAH compounds, that is, parent compound distributions (PCDs), and the alkyl homologue distributions (AHDs) of PAHs from extracts of marine sediments (1-9). The analytical methods used in these studies were generally complex and lengthy, and consisted of an extraction, an isolation of the PAH material (by either complex formation or chromatographic separation), and an analysis. Based on comparisons of PCDs and AHDs of the sediment samples with those of samples from known origins (e.g., petroleum, combustion products), these studies discussed the origins of PAH compounds found in sediments. 

Soil removal from blend fabrics is a rather difficult task. It is shown that polyoxyethylene alcohols with a broader polymer homologue distribution are more effective soil removers [178]. The effect of molar mass distribution on the properties of nonionic surfactants is discussed above [13, 14]. 

FIGURE 16.3 Comparison of homologue distribution in OD and OMD for NaOH and calcium-based catalyst (DC-L70) (average degree of oxyethylation is equal to 4). 

FAB or LSIMS using a probe inlet does not readily lend itself to quantitative work. Firstly, it is not possible to know how much of the sample has been consumed in the analysis. Secondly, discrimination effects (see section 12.3.3) prevent the comparison of intensities between species of differing surface activity. Semiquantitative results may be readily obtained if discrimination effects are assumed to be constant for the species of interest, for example the determination of homologue distributions in a mixture. For accurate quantitation an internal standard of an isotopically enriched analogue of the analyte should be used. For example, in the determination of cationic surfactants in environmental samples [10], quantitation was achieved by using an internal standard of a trideuterated form of the analyte. In this way the standard will be subject to the same level of discrimination as the analyte. Discrimination effects between different cationic species may also be reduced by adding to the sample an excess of a highly surface-active anionic surfactant. The anionic species will dominate the matrix surface and attract cations into the surface monolayer [10]. 

Table 4 Homologue Distribution for Aroclor 5460 and Leromoll 141 Obtained by HRGC-EI-LRMS-SIM (Resolving Power 1,500), HRGC-EI-HRMS (Resolving Power 35,000) and HRGG-EI-MS-MS-MRM...

Four reductive pathways (R -R ) are presumed to give rise to different types of homologue distributions (see text). The wild type is WF-9. 

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