Sample Preparation and Choice of Internal Standard

Aspects of sample preparation have been reviewed. Phenols and aromatic amines, although amenable to EC oxidation detection, tend to be relatively water soluble and thus polar extraction solvents such as ethyl acetate may have to be used if liquid-liquid extraction (LLE) is to be employed. Solid-phase extraction (SPE), using disposable cartridges or plastic syringe barrels packed usually with chemically-modified silica materials (Table 4.2) analogous to those used in HPLC columns, but with greater porosity, provides an alternative and may be more effective in extracting relatively hydrophilic analytes. On-line sample preparation procedures such as prior to EC detection have also been described. 

Internal standards are commonly used in chromatographic analyses and some aspects of the rationale for, and use of, internal standardisation have been 

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