A simple nanoelectrospray arrangement with controllable flowrate for mass analysis of submicroliter protein samples

Challenges and Strategies of Chemical Analysis of Drugs of Abuse and Explosives by Mass Spectrometry

In analytical science, mass spectrometry (MS) is known as a "gold analytical tool" because of its unique character of providing the direct molecular structural information of the relevant analyte molecules. Therefore, MS technique has widely been used in all branches of chemistry along with in proteomics, metabolomics, genomics, lipidomics, environmental monitoring etc. Mass spectrometry-based methods are very much needed for fast and reliable detection and quantification of drugs of abuse and explosives in order to provide fingerprint information for criminal investigation as well as for public security and safety at public places, respectively. Most of the compounds exist as their neutral form in nature except proteins, peptides, nucleic acids that are in ionic forms intrinsically. In MS, ion source is the heart of the MS that is used for ionizing the electrically neutral molecules. Performance of MS in terms of sensitivity and selectivity depends mainly on the efficiency of the ionization source. Accordingly, much attention has been paid to develop efficient ion sources for a wide range of compounds. Unfortunately, none of the commercial ion sources can be used for ionization of different types of compounds. Moreover, in MS, analyte molecules must be released into the gaseous phase and then ionize by using a suitable ion source for detection/quantification. Under these circumstances, fabrication of new ambient ion source and ultrasonic cutter blade-based non-thermal and thermal desorption methods have been taken into account. In this paper, challenges and strategies of mass spectrometry analysis of the drugs of abuse and explosives through fabrication of ambient ionization sources and new desorption methods for non-volatile compounds have been described. We will focus the literature progress mostly in the last decade and present our views for the future study.

Atmospheric pressure ion sources

This review of atmospheric pressure ion sources discusses major developments that have occurred since 1991. Advances in the instrumentation and understanding of the key physical principles are the primary focus. Developments with electrospray and atmospheric pressure chemical ionization and variations encompassing adaptations for surface analysis, ambient air analysis, high throughput, and modification of the ionization mechanism are covered. An important and limiting consequence of atmospheric pressure chemical ionization, chemical noise, is discussed as is techniques being employed to ameliorate the problem. Ion transfer and transport from atmospheric pressure into deep vacuum is an area undergoing constant improvement and refinement so is given considerable consideration in this review.

Determination of protein aggregation with differential mobility analysis: application to IgG antibody

Here we describe the use of electrospray differential mobility analysis (ES-DMA), also known as gas-phase electrophoretic mobility molecular analysis (GEMMA), as a method for measuring low-order soluble aggregates of proteins in solution. We demonstrate proof of concept with IgG antibodies. In ES-DMA, aqueous solutions of the antibody protein are electrosprayed and the various aerosolized species are separated according to their electrophoretic mobility using a differential mobility analyzer. In this way, complete size distributions of protein species present from 3 to 250 nm can be obtained with the current set up, including distinct peaks for IgG monomers to pentamers. The sizes of the IgG and IgG aggregates measured by DMA were found to be in good agreement with those calculated from simple models, which take the structural dimensions of IgG from protein crystallographic data. The dependence of IgG aggregation on the solution concentration and ionic strength was also examined, and the portion of aggregates containing chemically crosslinked antibodies was quantified. These results indicate that ES-DMA holds potential as a measurement tool to study protein aggregation phenomena such as those associated with antibody reagent manufacturing and protein therapeutics.

New interface plate for microspray ionization mass spectrometry

A new interface plate was employed in microspray ionization mass spectrometry (microESI-MS) to improve ion transmission from the sprayer into the sampling nozzle of the mass spectrometer at atmospheric pressure. Using a time-of-flight mass spectrometer (TOFMS), a fivefold increase in ion intensity and a sevenfold reduction in method detection limit were observed. The interface plate attenuated the dependence of the ion intensity on the sprayer position. Even when the distance between the sprayer tip and sampling nozzle was 15.0 mm, ion signals were still stronger than when the sprayer tip was positioned 3.0 mm in front of the sampling nozzle with the original interface plate. This enhancement in the performance of microESI-MS was due to the improved shapes of the equipotential lines near the sprayer tip and the long desolvation distance between the sprayer and the sampling nozzle of the MS.

An open design microfabricated nib-like nanoelectrospray emitter tip on a conducting silicon substrate for the application of the ionization voltage

This paper describes a novel emitter tip having the shape of a nib and based on an open structure for nano-electrospray ionization mass spectrometry (nanoESI-MS). The nib structure is fabricated with standard lithography techniques using SU-8, an epoxy-based negative photoresist. The tip is comprised of a reservoir, a capillary slot and a point-like feature, and is fabricated on a silicon wafer. We present here a novel scheme for interfacing such nib tips to MS by applying the ionization voltage directly onto the semi-conductor support. The silicon support is in direct contact with the liquid to be analyzed at the reservoir and microchannel level, thus allowing easy use in ESI-MS. This scheme is especially advantageous for automated analysis as the manual step of positioning a metallic wire into the reservoir is avoided. In addition, the analysis performance was enhanced compared with the former scheme, as demonstrated by the tests of standard peptides (gramicidin S, Glu-fibrinopeptide B). The limit of detection was determined to be lower than 10(-2) microM. Due to their enhanced performance, these microfabricated sources might be of great interest for analysis requiring very high sensitivity, such as proteomics analysis using nanoESI-MS.

Investigation of electrospray ionization and electrostatic focusing devices using a three-dimensional electrospray current density profiler

A novel instrument for profiling the current density of nanoelectrospray ionization plumes in three dimensions has been developed. A hemispherically-shaped electrostatic lens at atmospheric pressure is found to be able to compress the space-charge in nano-ESI and increase the average current density in the plume to three times the nominal value. Ion transmission into a single-quadrupole mass spectrometer is found to roughly double using the electrostatic lens. Data also suggest that ion transmission into the first vacuum region for a skimmer-type mass spectrometer interface using nano-ESI may be typically 40% or better with no special focusing device used.

The development of a higher throughput reactive intermediate screening assay incorporating micro-bore liquid chromatography–micro-electrospray ionization–tandem mass spectrometry and glutathione ethyl ester as an in vitro conjugating agent

An in vitro reactive intermediate screening assay, incorporating the use of the close analog of glutathione, glutathione ethyl ester (GSH-EE) as a conjugating agent, was developed to identify compounds that form reactive intermediates in an in vitro metabolite generating system. The biological assay consisted of substrate [s] = 10 microM, human liver microsomes, an NADPH generating system and glutathione ethyl ester. Conjugates were extracted from the biological matrix using a combination of protein precipitation and a semi-automated 96-well plate solid phase extraction (SPE) procedure. A micro-bore liquid chromatography-micro-electrospray ionization-tandem mass spectrometry (microLC-microESI-MS/MS) method detected glutathione ethyl ester conjugates using selected reaction monitoring (SRM) to simultaneously monitor for multiple MH+ to [MH - 129]+ transitions, where the 129 mass unit (Da) represents the neutral loss of the pyroglutamate moiety from GSH-EE. The multiple MH+ to [MH - 129]+ transitions (SRM mass table) were generated for potential reactive intermediates of each compound. Glutathione (GSH) and GSH-EE conjugate standards were used to evaluate MS detection sensitivity. Based on direct comparison of standard curve data, an approximate 10-fold increase in sensitivity was observed for conjugates containing GSH-EE moiety versus GSH. In vitro experiments were conducted using literature substrates acetaminophen, rosiglitazone, clozapine, diclofenac and either GSH-EE or GSH as a reactive intermediate conjugating agent. An increase in detection sensitivity was observed for each GSH-EE conjugate and in the case of acetaminophen-GSH-EE the peak area increase was approximately 80-fold. Twelve drug compounds, each having known biotransformation mechanisms, were used to further test the detection capabilities of the assay and establish a concordance to literature data. When GSH was used in the assay, conjugates were detected for 4 out of the 12 test compounds (33%). When GSH-EE was used in the assay, conjugates were detected for 10 out of the 12 test compounds (83%).

Two-dimensional microfabricated sources for nanoelectrospray

The idea of a novel two-dimensional (2D) nanoelectrospray ionization emitter tip with the shape of a nib is explored here. This novel planar design is studied as an alternative to the needle-like standard emitter tips that suffer from a lack of reproducibility and robustness and from an inherent incompatibility with high-throughput analysis. The composition of the micro-nib sources is analogous to the working of a simple fountain pen, with a liquid reservoir linked to a micro-nib tip from which the sample is electrosprayed via a capillary slot. The micro-nib prototypes described here were fabricated using microtechnology techniques and using the epoxy-based negative photoresist SU-8. The resulting free-standing micro-nib structure was supported by a silicon wafer. We present here two series of such micro-nib sources, the latter series exhibiting improved characteristics such as a 8 micro m source width of the nib tip. They were tested in mass spectrometry experiments on an ion trap mass spectrometer (LCQ Deca XP+, Thermo Finnigan) using standard peptide samples having concentrations down to 1 micro M and with a high voltage (HV) supply around 1 kV for the second series of micro-nib sources. In addition to the stability of the spray, the obtained mass spectra showed the reliability of these sources for peptide analysis; the signal of the spectra was as intense and the signal-to-noise ratio (S/N) as high as that obtained with the use of standard emitter tips.

Thermostability of endo-1,4-beta-xylanase II from Trichoderma reesei studied by electrospray ionization Fourier-transform ion cyclotron resonance MS, hydrogen/deuterium-exchange reactions and dynamic light scattering

Endo-1,4-beta-xylanase II (XYNII) from Trichoderma reesei is a 21 kDa enzyme that catalyses the hydrolysis of xylan, the major plant hemicellulose. It has various applications in the paper, food and feed industries. Previous thermostability studies have revealed a significant decrease in enzymic activity of the protein at elevated temperatures in citrate buffer [Tenkanen, Puls and Poutanen (1992) Enzyme Microb. Technol. 14, 566-574]. Here, thermostability of XYNII was investigated using both conventional and nanoelectrospray ionization Fourier-transform ion cyclotron resonance MS and hydrogen/deuterium (H/D)-exchange reactions. In addition, dynamic light scattering (DLS) was used as a comparative method to observe possible changes in both tertiary and quaternary structures of the protein. We observed a significant irreversible conformational change and dimerization when the protein was exposed to heat. H/D exchange revealed two distinct monomeric protein populations in a narrow transition temperature region. The conformational change in both the water and buffered solutions occurred in the same temperature region where enzymic-activity loss had previously been observed. Approx. 10-30% of the protein was specifically dimerized when exposed to the heat treatment. However, adding methanol to the solution markedly lowered the transition temperature of conformational change as well as increased the dimerization up to 90%. DLS studies in water confirmed the change in conformation observed by electrospray ionization MS. We propose that the conformational change is responsible for the loss of enzymic activity at temperatures over 50 degrees C and that the functioning of the active site in the enzyme is unfeasible in a new, more labile solution conformation.

Identification of novel MAP kinase pathway signaling targets by functional proteomics and mass spectrometry

Functional proteomics provides a powerful method for monitoring global molecular responses following activation of signal transduction pathways, reporting altered protein posttranslational modification and expression. Here we combine functional proteomics with selective activation and inhibition of MKK1/2, in order to identify cellular targets regulated by the MKK/ERK cascade. Twenty-five targets of this signaling pathway were identified, of which only five were previously characterized as MKK/ERK effectors. The remaining targets suggest novel roles for this signaling cascade in cellular processes of nuclear transport, nucleotide excision repair, nucleosome assembly, membrane trafficking, and cytoskeletal regulation. This study represents an application of functional proteomics toward identifying regulated targets of a discrete signal transduction pathway and demonstrates the utility of this discovery-based strategy in elucidating novel MAP kinase pathway effectors.

Mass spectrometry: a tool for the identification of proteins separated by gels

Mass spectrometry (MS) has become the technique of choice to identify proteins. This has been largely accomplished by the combination of high-resolution two-dimensional (2-D) gel separation with robotic sample preparation, automated MS measurement, data analysis, and database query. Developments during the last five years in MS associated with protein gel separation are reviewed.