Although many individual choices for sample preparation exist, sometimes the combination of two techniques yields a more desirable result. Within a drug discovery environment, however, throughput and cost are important factors that influence the choice of sample-preparation methods. The protein-precipitation procedure can quickly yield a sample that is ready for analysis, but its potential for carryover of matrix interferences is problematic. The isolated supernatant can be filtered or centrifuged before injection, but these procedures remove only particulates or proteinaceous material, not the materials that cause matrix interferences. Thus, protein precipitation is sometimes followed by LLE, SPE, or an on-line technique for a more selective cleanup before analysis. A particularly challenging sample-preparation requirement is the analysis of animal tissues, as is commonly performed in drug-discovery support laboratories that perform drug uptake studies. Here, a series of sequential sample-preparation steps are common .
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