Microextraction techniques in bioanalysis

Instrumental analysis has evolved exponentially in the last decades, giving rise to techniques with improved sensitivity and selectivity. This evolution has made a breakthrough in different application fields and it has been of paramount importance in bioanalysis taking into consideration its inherent characteristics. From the practical point of view, bioanalysis faces problems derived from the low concentration of the targets, the different possible states (e.g. free or bonded) of the analytes, the large number of potential interferents that are usually present at high concentrations, and the limited volume of sample. These aspects may be considered minor issues when compared with the complexity of obtaining proper conclusions from the final results. However, these shortcomings should be overcome to obtain final quality results. All of these aspects make bioanalysis both a challenging and an exciting discipline.

Although direct analysis of samples (e.g. direct infusion in MS) is possible in some cases, bioanalytical specimens are usually processed following a defined protocol before the analysis. Sample treatments may allow the preconcentration of the target analytes and their efficient isolation from the sample matrix, thus improving the sensitivity and selectivity of the final determination. In fact, the latter aspect is crucial even when LC-MS is employed in the final analysis, since it avoids or minimizes negative effects such as ion suppression. In addition, sample treatment is crucial if special samples are to be compatible with the final instrumental techniques. The binomial of dried blood spot/LC-MS is a clear example of this situation since at least a solid-liquid extraction is required.

Sample treatment has also evolved in the last few years following three main trends (automation, miniaturization and simplification) that have led to the development of a number of techniques that can be grouped under the term “microextraction techniques” (MEs). MEs have found the perfect field of application in bioanalysis since their characteristics match with bioanalytical requirements. In this sense, automation has resulted essentially in high throughput analysis where a large number of samples can be processed. Simplification allows the reduction of steps and is reflected in improved precision. Finally, miniaturization reduces the sample volume requirements. The gradual substitution of classical techniques, mainly liquid-liquid (LLE) or solid phase extractions (SPE), by the miniaturized alternatives is the proof of concept of this situation.

We can ponder what the main trends in MEs are. The question is not simple and it can be answered in different ways. In my humble opinion, the use of nanomaterials and the use of selectivity enhancement ’receptors’ should be considered among them. On the one hand, the potential of nanomaterials is beyond doubt as they become a reality in many extraction protocols. The great variety of nanoparticles, covering a wide range of chemical interactions, and their high superficial area, that dramatically increases their efficiency and their special properties (e.g. super-paramagnetism) may simplify the extraction processes. On the other hand, selective receptors that may interact only with the analytes or structure-related compounds are also remarkable. Aptamers, which are nucleic acids or peptides that can bind target molecules with high affinity and specificity, are promising materials in this context.

Click here to view the Micoextraction Tech blog, dedicated to sample treatment in analytical science, edited by Lucena.