Electrophoretic Extraction of Low Molecular Weight Cationic Analytes from Sodium Dodecyl Sulfate Containing Sample Matrices for Their Direct Electrospray Ionization Mass Spectrometry

Abstract

While the use of sodium dodecyl sulfate (SDS) in separation buffers allows efficient analysis of complex mixtures, its presence in the sample matrix is known to severely interfere with the mass-spectrometric characterization of analyte molecules. In this article, we report a microfluidic device that addresses this analytical challenge by enabling inline electrospray ionization mass spectrometry (ESI-MS) of low molecular weight cationic samples prepared in SDS containing matrices. The functionality of this device relies on the continuous extraction of analyte molecules into an SDS-free solvent stream based on the free-flow zone electrophoresis (FFZE) technique prior to their ESI-MS analysis. The reported extraction was accomplished in our current work in a glass channel with microelectrodes fabricated along its sidewalls to realize the desired electric field. Our experiments show that a key challenge to successfully operating such a device is to suppress the electroosmotically driven fluid circulations generated in its extraction channel that otherwise tend to vigorously mix the liquid streams flowing through this duct. A new coating medium, N-(2-triethoxysilylpropyl) formamide, recently demonstrated by our laboratory to nearly eliminate electroosmotic flow in glass microchannels was employed to address this issue. Applying this surface modifier, we were able to efficiently extract two different peptides, human angiotensin I and MRFA, individually from an SDS containing matrix using the FFZE method and detect them at concentrations down to 3.7 and 6.3 µg/mL, respectively, in samples containing as much as 10 mM SDS. Notice that in addition to greatly reducing the amount of SDS entering the MS instrument, the reported approach allows rapid solvent exchange for facilitating efficient analyte ionization desired in ESI-MS analysis.

Enhancement in MS‐based peptide detection by microfluidic free‐flow zone electrophoresis

“…To prepare our microchips for an experiment, the entire fluidic network was treated with 1 M NaOH for 1 h followed by sequential rinses with water, methanol, and acetone for 10 min each. The glass surface within the microchannels was then derivatized with a solution of N ‐(2‐triethoxysilylpropyl) formamide for 2 h at room temperature to suppress EOF in the system [33, 41]. This coating material was prepared by mixing stoichiometric amounts of commercially available 3‐aminopropyltriethoxysilane with ethyl formate and reacting the two chemicals under ambient conditions for 48 h [33].…”

“…To address this challenge, our group has reported the use of N ‐(2‐triethoxysilylpropyl) formamide as a coating medium [39, 40] that was shown to diminish EOF in glass microchannels by over five orders of magnitude practically eliminating the unwanted mixing observed in narrow FFZE conduits. These devices were later applied to efficient peptide separations [33] as well as peptide extraction from SDS‐containing sample matrices [41] coupled to analyte quantitation based on the ESI‐MS method. Continuing this effort, the present article describes the application of the noted FFZE device toward reducing matrix effects in the quantitation of low molecular weight species suspended in a complex biofluid.…”

In‐capillary approach to eliminate SDS interferences in antibody analysis by capillary electrophoresis coupled to mass spectrometry

“…Therefore, methodologies for in‐line removal of SDS prior to MS analysis are desirable. Recently, some interesting in‐line approaches with microfluidic devices were based on free‐flow zone electrophoresis , on semipermeable membranes in a cross‐flow filtration process , or on membranes to collect CGE resolved peaks . The latter approach made use of additional membrane washing steps for SDS removal.…”

“…Subsequently, it allowed direct on‐membrane MALDI‐TOF‐MS protein analysis. In the case of MEKC‐MS where SDS is included in the BGE, partial filling strategies or anodically migrating micelles were the most employed strategies to avoid SDS entering the MS source .…”

Membrane-Free Electrokinetic Device Integrated to Electrospray-Ionization Mass Spectrometry for the Simultaneous Removal of Sodium Dodecyl Sulfate and Enrichment of Peptides

“…SDS-removal systems integrated with ESI-MS (integrated SDS removal–ESI-MS) should increase sample throughput and minimize sample loss and thus are desired in proteomic workflows. Despite these advantages, integrated systems for SDS removal are scarce with only one paper reporting on them . Efficient removal of SDS is the basic requirement in integrated SDS removal–ESI-MS, because even small concentrations of SDS (i.e., >0.3 mM) are detrimental to ESI-MS .…”