Ultramicroextraction as a miniaturization of the already miniaturized. A step toward nanoextraction and beyond

Miniaturization of the analytical process has been a widespread trend, and the sample preparation stage is not exempted from this downscaling. Since the introduction of microextraction techniques as miniaturization of classical extraction techniques, they have become one of the strengths in this field. However, some of the original approaches to these techniques did not fully cover all the current principles of Green Analytical Chemistry. For this reason, during the last years, much emphasis has been placed on reducing/eliminating toxic reagents, reducing the amount of the extraction phase, and searching for new greener, and more selective extractant materials. On the other hand, even though high accomplishments have been achieved, the same attention has not always been paid to reducing the amount of sample, which is essential when treating low-availability samples such as biological samples, or in developing portable devices. In this review, we intend to give the readership an overview of the advances toward further miniaturization of microextraction techniques. Finally, a brief reflection is made on the terminology used, or that should, in our opinion, be used to term these new generation of miniaturized microextraction approaches. To this regard, the term, 'ultramicroextraction' is proposed to refer to those approaches beyond microextraction.

Automated summary

Miniaturization of the analytical process is a common trend, including the sample preparation stage. Microextraction techniques have become a strong approach in this field, but some original approaches did not fully adhere to the principles of Green Analytical Chemistry. Recent advancements have focused on reducing toxic reagents, the extraction phase, and finding greener and more selective extractant materials. However, there hasn't been sufficient attention given to reducing sample size, which is important for low-availability samples and portable devices. This review provides an overview of the progress in miniaturization of microextraction techniques and proposes the term "ultramicroextraction" for approaches beyond microextraction.