Direct analysis of biological samples using extractive electrospray ionization mass spectrometry (EESI-MS)

Mass spectrometry (MS) is one of the most widely used techniques for the analysis of biological samples. In the past decade, a novel improvement in MS was the invention of ambient ionization which stands out owing to its unique capability of direct analysis of complex samples with no or minimal pretreatment. In this review, extractive electrospray ionization (EESI), a representative ambient ionization technique, is introduced focusing on its mechanism, instrumentation, and applications in biological analysis. EESI uses a traditional ESI channel to produce primary ions which subsequently ionize neutral chemicals from the sample introduction channel through an online extraction process. When analyzing biological samples, EESI has advantages of rapid analysis, high matrix tolerance, and the ability to perform in vivo analysis. According to previous studies, EESI is able to directly analyze various chemicals in complex biological specimens in liquid, gas, and solid states. EESI can provide a sensitive and selective measurement of biological samples for both qualitative and quantitative purposes. Therefore, it is anticipated that EESI will have promising applications, especially in fields which require the fast and/or in vivo analysis of biological samples with complicated matrixes.