Zhouxin Shen

Desorption/ionization on silicon (DIOS) mass spectrometry: background and applications

Abstract Desorption/ionization on silicon mass spectrometry (DIOS-MS) is a matrix-less laser vaporization method for generating gas-phase ions. The physical properties of the silicon surfaces are crucial to DIOS-MS performance and are controlled by the selection of silicon type and the silicon etching conditions. DIOS-MS has been examined for its applicability to small-molecule analysis, quantitative studies, reaction monitoring, chromatography, protein identification, and protein functional characterization. In organic chemistry, DIOS has been applied to the analysis of reactions directed toward development of new catalysts and transformations. Because DIOS offers a chip-based format, it is capable of being used to raster the silicon surface for biological and chemical screening applications, such as enzymatic activity assays. DIOS-MS extends the mass analysis capabilities of laser desorption to small biomolecules and thus offers a platform on which multiple experiments can be performed on a wide variety of molecules.

Desorption/ionization on silicon (DIOS): A diverse mass spectrometry platform for protein characterization

Since the advent of matrix-assisted laser desorption/ionization and electrospray ionization, mass spectrometry has played an increasingly important role in protein functional characterization, identification, and structural analysis. Expanding this role, desorption/ionization on silicon (DIOS) is a new approach that allows for the analysis of proteins and related small molecules. Despite the absence of matrix, DIOS-MS yields little or no fragmentation and is relatively tolerant of moderate amounts of contaminants commonly found in biological samples. Here, functional assays were performed on an esterase, a glycosidase, a lipase, as well as exo- and endoproteases by using enzyme-specific substrates. Enzyme activity also was monitored in the presence of inhibitors, successfully demonstrating the ability of DIOS to be used as an inhibitor screen. Because DIOS is a matrix-free desorption technique, it also can be used as a platform for multiple analyses to be performed on the same protein. This unique advantage was demonstrated with acetylcholine esterase for qualitative and quantitative characterization and also by its subsequent identification directly from the DIOS platform.

Desorption–ionization on silicon mass spectrometry: an application in forensics

Abstract Desorption–ionization on silicon (DIOS) is a new, matrix-free laser desorption mass spectrometry approach that allows for the direct identification of low molecular weight compounds in the presence of potentially interfering compounds. The porous silicon surfaces provide a scaffold for trapping analyte molecules, and are readily adaptable to commercial time-of-flight instruments. As an example of its utility in forensic cases, DIOS mass spectrometry was used to distinguish between similar synthetic polymers and identify specific polymers from complex biological media. Despite the absence of matrix, specific low molecular weight polymers were rapidly identified without any fragmentation. This method was applied to the rapid identification of ethoxylate polymers during a criminal investigation.

A mass spectrometry plate reader: monitoring enzyme activity and inhibition with a Desorption/Ionization on Silicon (DIOS) platform.

A surface‐based laser desorption/ionization mass spectrometry assay that makes use of Desorption/Ionization on Silicon Mass Spectrometry (DIOS‐MS) has been developed to monitor enzyme activity and enzyme inhibition. DIOS‐MS has been used to characterize inhibitors from a library and then to monitor their activity against selected enzyme targets, including proteases, glycotransferase, and acetylcholinesterase. An automated DIOS‐MS system was also used as a high‐throughput screen for the activity of novel enzymes and enzyme inhibitors. On two different commercially available instruments, a sampling rate of up to 38 inhibitors per minute was accomplished, with thousands of inhibitors being monitored. The ease of applying mass spectrometry toward developing enzyme assays and the speed of surface‐based assays such as DIOS for monitoring inhibitor effectiveness and enzyme activity makes it attractive for a broad range of screening applications.

A Case Study on Forensic Polymer Analysis by DIOS-MS: The Suspect Who Gave Us the SLIP

New technology was used to identify traces of a commercial barrier/spermicide in evidence from a case of a man accused of rape of a minor. Examination of vaginal swabs performed by another laboratory had been negative for seminal fluid or other sources of DNA from the suspect and we were asked to examine the remaining swabs for any traces that might have originated from the commercial product. Encare consists of vaginal inserts having a suppository-like shape. They contain the spermicide, nonoxynol-9, in a matrix consisting of approximately two parts polyethylene glycol (PEG) 1000 to one part PEG 1450, plus minor inorganic components added to produce foaming. Portions of the cotton from vaginal swabs from the victim and penile swabs from the suspect were extracted with methanol and subsequently examined by desorption ionization on silicon time-of-flight mass spectrometry (DIOS TOF MS). Low levels of PEG in the same mass range as Encare were found on two separate vaginal swabs from the victim and one penile swab from the suspect. Subsequent to these findings, the suspect (through his attorneys) provided us with a sample of SLIP Plus, a commercial sexual lubricant that also contains nonoxynol-9. Traces of PEG in the same mass range as Encare were found in this sample, while no PEG was found in a sealed sample of SLIP Plus provided by the manufacturer. At trial the suspects attorneys stipulated that their client had added some Encare to the SLIP Plus sample he had provided.

DIOS-MSEED: A chip-based method for measurement of enantiomeric excess by kinetic resolution/mass spectrometry

The determination of enantiomeric excess by kinetic resolution mass spectrometry has been implemented with the Desorption/Ionization On Silicon (DIOS) MS technique. Measurements can thereby be made much more rapidly than was previously possible, bringing this general methodology for screening asymmetric catalysts closer to true high-throughput status.

Protein characterization using Liquid Chromatography Desorption Ionization on Silicon Mass Spectrometry (LC-DIOS-MS)

This paper presents a novel combination of off-line liquid chromatography (LC) separation with desorption ionization on silicon mass spectrometry (DIOS-MS) as an alternative to the traditional on-line coupling of LC and electrospray ionization (ESI). In this work, electrospray deposition is used to generate a spatially preserved linear track of the separated sample on a specially designed DIOS chip. The total sequence coverage analysis of two model protein systems was evaluated by both LC-DIOS-MS and nanoLC ESI tandem MS (MS/MS). LC-DIOS-MS yielded improved sequence coverage for both of the model systems (between 99.5 and 100%) compared with traditional LC-ESI-MS/MS analysis (between 82 and 87.6%). In addition to improved sequence coverage determination, LC-DIOS-MS also offers the potential for high-throughput protein characterization.