Sample-preparation methods for direct and indirect analysis of natural estrogens

• Sample preparations of estrogen conjugates for direct and indirect analysis.
• Advantages and disadvantages of enzymatic hydrolysis and chemical deconjugation.
• Progress and future challenges in sample preparation.

Estrogen conjugates can be analyzed directly or indirectly using different sample-preparation methods. This review first summarizes the sample-preparation method for direct analysis; then indirect analysis by enzymatic-hydrolysis and chemical-deconjugation methods. We discuss progress and challenges for sample-preparation methods. Overall, sample preparation for direct analysis represents the latest development, while we describe the most widely employed method, enzymatic hydrolysis, as the limiting step in sample analysis due to long incubation times. Although the chemical deconjugation is currently the least used method, its potential is probably underestimated. A recently developed acid-solvolysis method may be widely applied to environmental samples for its high deconjugation efficiency and because it is more cost effective. This method can possibly extend to biological samples, although great efforts must be made to validate its feasibility.