LC-NMR in Environmental Analysis

Fraunhofer Institut fur Toxikologie und Aerosolforschung, Hannover, Germany and 

The introduction of chemicals into the environment is considerable. Large amounts of organic compounds are released into the environment every year by industrial and agricultural processes, traffic, urban waste disposal and ecological disasters. Once present in the environment, they are subjected on the one hand to transport processes in air, water and soil and, on the other hand, they are subjected to the influence of the reactor environment , i.e. transformation products may be formed by chemical, photochemical and microbiological transformation processes. Chemical reactions with other pollutants present in the environment can also take place. As a result of these processes, a variety of new and unexpected compounds can be formed from the originally released pollutants and, as a rule, they are more polar than the parent compounds. 

It is interesting that, in analytical chemistry, besides the efforts to increase the sample throughput and to decrease the detection limits, another trend can be observed which is directed to the analysis of more and more complex mixtures without laborious sample preparation and separation steps. This development was triggered by the requirements of bio- and environmental analysis and is closely connected to the development of multidimensional analytical methods, as well as hyphenated techniques which provide much more selectivity than one-dimensional analytical methods. Among the range of hyphenated techniques, those which combine a high separation efficiency with a maximum of structural information are of particular importance. These are hyphenated techniques such as GC-MS, LC-MS, LC-NMR and LC-NMR-MS. 

In the case of degradation products and metabolites, the situation is different. Generally, these compounds were not analysed because in most cases they are not regulated and no effective analytical methods exist for their determination. This means that a correct diagnosis of the environmental situation cannot be made and, as a consequence, no appropriate action can be taken. Therefore, in order to improve the risk assessment of a hazardous waste site for example, as many compounds as possible should be analysed at the beginning of the investigations (non-target analysis). 

This is especially true in cases where the pollutants are non-persistent, and degradation products, as well as metabolites, can be expected. After the identification of the pollutants, suitable leading components should then be selected for further monitoring. 

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At present, applications of LC-NMR in environmental analysis are still limited by the relatively low sensitivity of the NMR detector. Further progress is expected when cryoprobes and new capillary flow cells hopefully become available in the near future. 

Different applications of LC/NMR in environmental problems have also demonstrated the interest of the approach for nontarget analysis of organic compounds in the samples. On the basis of the LC/NMR and LC-MS, a screening of various pollutants can be performed and target analysis of specific pollutants can be efficiently developed in a second step with more sensitive methods for a definitive identification. 

Two independent analytical methods—LC-MS-MS and 19F-NMR— for the determination of perfluorinated anionic surfactants in environmental water samples were presented. Perfluorinated alkanesulfonates and perfluorocarboxylates were determined qualitatively and quantitatively because of an accidental release of perfluorosurfactant contaminated fire-fighting foam [55]. Ci8-SPE was applied for concentration of the compounds from water samples. Methanol was used for elution prior to ESI-LC-MS(—) analysis. The negatively recorded LC-MS-MS TIC for the determination of PFOS, PFHxS, PFOA, perfluor-oheptanoic acid (PFHpA), perfluorododecanoic acid (PFDoA internal standard) in water samples was presented [55]. 

As liquid chromatography plays a dominant role in chemical separations, advancements in the field of LC-NMR and the availability of commercial LC-NMR instrumentation in several formats has contributed to the widespread acceptance of hyphenated NMR techniques. The different methods for sampling and data acquisition, as well as selected applications will be discussed in this section. LC-NMR has found a wide range of applications including structure elucidation of natural products, studies of drug metabolism, transformation of environmental contaminants, structure determination of pharmaceutical impurities, and analysis of biofiuids such as urine and blood plasma. Readers interested in an in-depth treatment of this topic are referred to the recent book on this subject [25]. 

The hyphenation of CE and NMR combines a powerful separation technique with an information-rich detection method. Although compared with LC-NMR, CE-NMR is still in its infancy it has the potential to impact a variety of applications in pharmaceutical, food chemistry, forensics, environmental, and natural products analysis because of the high information content and low sample requirements of this method [82-84]. In addition to standard capillary electrophoresis separations, two CE variants have become increasingly important in CE-NMR, capillary electrochromatography and capillary isotachophoresis, both of which will be described later in this section. 

From the selectivity point of view, LC-NMR coupling is especially suited to the analysis of compound classes such as nitroaromatics, phenols, aromatic amines, aromatic carboxylic acids, polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and azo- and anthraquinone dyes. Another advantage of LC-NMR coupling for the investigation of aromatic compounds in environmental samples is that the position of substituents on the aromatic ring, e.g. in unknown metabolites or degradation products, can best be determined by NMR spectroscopy. 

For aliphatic compounds with longer alkyl chains, such as surfactants, the NMR detector can contribute little to an increased selectivity of the LC-NMR coupling since, in the range of aliphatic protons, the spectra are often complex. Moreover, analyte signals around 2 ppm can be suppressed or influenced by the solvent suppression when acetonitrile is used as the organic component of the eluent. Since surfactants are present in many environmental samples, they pose problems for non-target analysis, not only because of their complex spectra but also because they can influence the separating properties of the analytical column by their surface activity [2]. 

On-line H P LC- N M R using analytical columns has become established as a routine method. Several problems in pharmaceutical, biomedical, and environmental analysis have been solved by simply hyphenating LC with NMR. For sparingly soluble compounds, the existing probes with detection volumes in the range 40 to 120 pL are the best solution. 

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