Flat Disc-Shaped Sampling Probe and Online Re-extraction Apparatus for Mass Spectrometric Analysis of Skin Metabolites: A Proof of Concept.

Sweat analysis provides an alternative and noninvasive way of clinical diagnostics. However, sampling and transferring sweat-derived samples to analytical instruments is challenging. In this report, we demonstrate a method utilizing a flat disc-shaped sampling probe, and a compatible re-extraction apparatus coupled online with extractive electrospray ionization (EESI) mass spectrometry (MS). The probe enables sampling of metabolites from a skin area of ∼2.2 cm2. The subsequent online re-extraction and analysis by EESI-MS further mitigates matrix effects caused by sweat components, thus eliminating sample preparation steps. The total analysis time is only 6 min. We have optimized the key parameters of the system, including flow rate of the nebulizing gas in ESI, pressure of the nebulizing gas in pneumatic sample nebulizer, flow rate of the solvent in ESI, and composition of extractant. The standard solutions (0.1 mL) were supplemented with 0.04 M sodium chloride to mimic the matrix effect normally observed in sweat samples. The method has been characterized with four chemical standards (positive-ion mode of histidine, leucine, urocanic acid; negative-ion mode of lactic acid). The limits of detection range from 1.09 to 95.9 nmol. We have further demonstrated the suitability of the method for analysis of sweat. An attempt was made to identify some of the recorded signals by product-ion scan and accurate/exact mass matching.

Facilitating chemical and biochemical experiments with electronic microcontrollers and single-board computers.

Since the advent of modern science, researchers have had to rely on their technical skills or the support of specialized workshops to construct analytical instruments. The notion of the 'fourth industrial revolution' promotes construction of customized systems by individuals using widely available, inexpensive electronic modules. This protocol shows how chemists and biochemists can utilize a broad range of microcontroller boards (MCBs) and single-board computers (SBCs) to improve experimental designs and address scientific questions. We provide seven example procedures for laboratory routines that can be expedited by implementing this technology: (i) injection of microliter-volume liquid plugs into microscale capillaries for low-volume assays; (ii) transfer of liquid extract to a mass spectrometer; (iii) liquid-gas extraction of volatile organic compounds (called 'fizzy extraction'), followed by mass spectrometric detection; (iv) monitoring of experimental conditions over the Internet cloud in real time; (v) transfer of analytes to a mass spectrometer via a liquid microjunction interface, data acquisition, and data deposition into the Internet cloud; (vi) feedback control of a biochemical reaction; and (vii) optimization of sample flow rate in direct-infusion mass spectrometry. The protocol constitutes a primer for chemists and biochemists who would like to take advantage of MCBs and SBCs in daily experimentation. It is assumed that the readers have not attended any courses related to electronics or programming. Using the instructions provided in this protocol and the cited material, readers should be able to assemble simple systems to facilitate various procedures performed in chemical and biochemical laboratories in 1-2 d.