A new microfluidics-based droplet dispenser for ICPMS.

In this work, a novel droplet microfluidic sample introduction system for inductively coupled plasma mass spectrometry (ICPMS) is proposed and characterized. The cheap and disposable microfluidic chip generates droplets of an aqueous sample in a stream of perfluorohexane (PFH), which is also used to eject them as a liquid jet. The aqueous droplets remain intact during the ejection and can be transported into the ICP with >50% efficiency. The transport is realized via a custom-built system, which includes a membrane desolvator necessary for the PFH vapor removal. The introduction system presented here can generate highly monodisperse droplets in the size range of 40-60 µm at frequencies from 90 to 300 Hz. These droplets produced very stable signals with a relative standard deviation (RSD) comparable to the one achieved with a commercial droplet dispenser. Using the current system, samples with a total volume of <1 µL can be analyzed. Moreover, the capabilities of the setup for introduction and quantitative elemental analysis of single cells were described using a test system of bovine red blood cells. In the future, other modules of the modern microfludics can be integrated in the chip, such as on-chip sample pretreatment or parallel introduction of different samples.

Interfacing droplet microfluidics with matrix-assisted laser desorption/ionization mass spectrometry: label-free content analysis of single droplets.

Droplet-based microfluidic systems have become a very powerful tool to miniaturize chemical and biological reactions. However, droplet content analysis remains challenging and relies almost exclusively on optical methods such as fluorescence spectroscopy. Hence, labeling of the analyte is typically required which impedes a more universal applicability of microdroplets. Here we present a novel interface coupling droplet microfluidics and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry for label-free content analysis of single droplets. Nanoliter aqueous droplets immersed in perfluorinated oil are created in a microfluidic T-junction, transferred into a capillary, and deposited on a high-density microarray MALDI plate mounted on a motorized xy-stage. The fully automated system is robust and reliable due to two unique features. First, a simple optical droplet detection system is used to synchronize stage movement and exit of droplets from the capillary. Second, the microarray plate contains an array of over 26,000 hydrophilic spots within a hydrophobic coating, each spot acting as a recipient to confine the droplets and to prevent cross-contamination. The MALDI matrix can also be applied using our system by spotting matrix droplets on the microarray in a separate run. To demonstrate the potential of our system, we studied the enzymatic cleavage of angiotensin I by angiotensin converting enzyme and monitored the increasing concentration of the product angiotensin II over time. The interface provides a robust and fully automated method for rapid label-free and information-rich content analysis of single droplets. With the high number of droplets per plate, this method is particularly suitable for high-throughput screening applications.

Detection of Infertility-related Neutralizing Antibodies with a Cell-free Microfluidic Method.

The unwanted emergence of neutralizing antibodies (nAbs) against an endogenous or a therapeutic protein can result in deficiency diseases or therapy failure. Here, we developed a cell-free microfluidic method for the sensitive detection and quantification of nAbs in human serum that are associated with infertility. We used cell-derived vesicles containing the luteinizing hormone (LH)/choriogonadotropin receptor (LHHCGR) to detect nAbs against LH. The method exploits the entire cellular signal amplification mechanism, and facilitates the detection of as little as 0.44 nM of LH-nAb (Kd 1.5 nM) in human serum matrix within only 15 minutes. In addition, dose-response curves can be generated in less than 2 hours to evaluate the nAB concentration and dissociation constant. The developed system is devoid of problems associated with cell-based assays and we believe that this simple effect-directed analysis can be used in clinical environments, and is adaptable to other hormones or cytokines and their respective nAbs.

A microfluidic chip for ICPMS sample introduction.

This protocol discusses the fabrication and usage of a disposable low cost microfluidic chip as sample introduction system for inductively coupled plasma mass spectrometry (ICPMS). The chip produces monodisperse aqueous sample droplets in perfluorohexane (PFH). Size and frequency of the aqueous droplets can be varied in the range of 40 to 60 µm and from 90 to 7,000 Hz, respectively. The droplets are ejected from the chip with a second flow of PFH and remain intact during the ejection. A custom-built desolvation system removes the PFH and transports the droplets into the ICPMS. Here, very stable signals with a narrow intensity distribution can be measured, showing the monodispersity of the droplets. We show that the introduction system can be used to quantitatively determine iron in single bovine red blood cells. In the future, the capabilities of the introduction device can easily be extended by the integration of additional microfluidic modules.