An important part of the Green Chemistry philosophy is the need to develop and adopt green analytical techniques and procedures. These include the reduction of reagent and solvent usage, the minimization of solid, liquid and gaseous materials produced by analytical processes, the replacement of reagents and solvents of high occupational or environmental toxicity with much more innocuous materials, and the reduction of energy use in analytical processes. One aspect that has received little attention in this context is the use of unrefined natural reagents derived from plant and animal tissues or microbial cells. Crude plant extracts may contain chemical compounds that enable their use as indicators in acid–base or redox titrations, or as chromogenic or fluorogenic agents. Enzymes extracted from plants may be used directly in soluble form, or incorporated in biosensors or solid phase reactors, for analytical measurements, or as a means of removing interferences or performing speciation studies. The use of natural reagents in conjunction with a flow injection system can confer a number of advantages. The enhanced kinetic control that flow analysis offers may assist in avoiding undesirable side reactions that would otherwise occur using unpurified reagents. The lifetime of natural reagents may be prolonged when used in a flow analysis system because their exposure to light or air can be controlled. Any changes in response that do occur may be readily corrected by regular standard checking and recalibration. This article reviews the use of natural reagents with an emphasis on flow-based analytical systems, and makes the case for further research in this latent area of green analytical chemistry.
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