A fully integrated monolithic microchip electrospray device for mass spectrometry

Collision-induced dissociation pathways of yeast sphingolipids and their molecular profiling in total lipid extracts: a study by quadrupole TOF and linear ion trap-orbitrap mass spectrometry

The yeast Saccharomyces cerevisiae synthesizes three classes of sphingolipids: inositolphosphoceramides (IPCs), mannosyl-inositolphosphoceramides (MIPCs), and mannosyl-diinositolphosphoceramides (M(IP)2C). Tandem mass spectrometry of their molecular anions on a hybrid quadrupole time-of-flight (QqTOF) instrument produced fragments of inositol-containing head groups, which were specific for each lipid class. MS(n) analysis performed on a hybrid linear ion trap-orbitrap (LTQ Orbitrap) mass spectrometer with better than 3 ppm mass accuracy identified fragment ions specific for the amide-linked fatty acid and the long chain base moieties in individual molecular species. By selecting m/z of class-specific fragment ions for multiple precursor ion scanning, we profiled yeast sphingolipids in total lipid extracts on a QqTOF mass spectrometer. Thus, a combination of QqTOF and LTQ Orbitrap mass spectrometry lends itself to rapid, comprehensive and structure-specific profiling of the molecular composition of sphingolipids and glycerophospholipids in important model organisms, such as fungi and plants.

Monitoring the zinc affinity of the metallo-beta-lactamase CphA by automated nanoESI-MS

Metallo-beta-lactamases are zinc containing enzymes that are able to hydrolyze and inactivate beta-lactam antibiotics. The subclass B2 enzyme CphA of Aeromonas hydrophila is a unique metallo-beta-lactamase because it degrades only carbapenems efficiently and is only active when it has one zinc ion bound. A zinc titration experiment was used to study the zinc affinity of the wild-type and of several mutant CphA enzymes. It shows that a second Zn(2+) is also bound at high ion concentrations. All samples were analyzed using mass spectrometry in combination with an automated nanoESI source. The metal-free enzyme has a bimodal charge distribution indicative of two conformational states. A completely folded enzyme is detected when the apo-enzyme has bound the first zinc. Intensity ratios of the different enzyme forms were used to deduce the zinc affinities. CphA enzymes mutated in metal ligands show decreased zinc affinity compared to wild-type, especially D120 mutants.

Fabrication of Porous Polymer Monoliths in Polymeric Microfluidic Chips as an Electrospray Emitter for Direct Coupling to Mass Spectrometry

Coupling of polymeric microfluidic devices to mass spectrometry is reported using porous polymer monoliths (PPM) as nanoelectrospray emitters. Lauryl acrylate-co-ethylene dimethacrylate porous polymer monolith was photopatterned for 5 mm at the end of the channel of microfluidic devices fabricated from three different polymeric substrate materials, including the following: poly(dimethylsiloxane) (PDMS), poly(methyl methacrylate) (PMMA), and cyclic olefin copolymer (COC). Spraying directly from the end of the chip removes any dead volume associated with inserted emitters or transfer lines, and the presence of multiple pathways in the PPM prevents the clogging of the channels, which is a common problem in conventional nanospray emitters. Spraying from a microfluidic channel having a PPM emitter produced a substantial increase in TIC stability and increased sensitivity by as much as 70x compared to spraying from an open end chip with no PPM. The performance of PPM emitter in COC, PMMA, and PDMS chips was compared in terms of stability and reproducibility of the electrospray. COC chips showed the highest reproducibility in terms of chip-to-chip performance, which can be attributed to the ease and reproducibility of the PPM formation due to the favorable optical and chemical properties of COC. We have further tested the performance of the COC chips by constant infusion of poly(propylene glycol) solution at organic content ranging from 10 to 90% methanol and at flow rates ranging from 50 to 1000 nL/min, showing optimum spraying conditions (RSD < 5%) at 50-70% organic content and at flow rates from 100 to 500 nL/min. The PPM sprayer was also used for protein preconcentration and desalting prior to mass spectrometric detection, and results were comparable with a chip spraying from an electrospray tip.

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.

Letter: radical ion and protonated molecule formation with retinal in electrospray and nanospray

This paper reports a preliminary study of the nanospray ionisation mass spectrometry analyses of retinoic acid and retinal. The results are compared and contrasted to the results normally observed with electrospray ionisation. A significant difference in behaviour was observed for the balance between radical ion formation and protonated molecule formation for retinal. The results suggest that the influence of the very different ionisation conditions present in nanospray is very important in determining how ionisation is achieved and has potentially wide ranging applications in the fields of mass spectral analysis of biological and medical extracts.

Miniaturization: Chip-based liquid chromatography and proteomics

Proteomic research is linked with significant technological challenges such as the high dynamic range and low abundance of biologically interesting proteins. Moreover, there is an increasing need for high-throughput and robustness of routinely performed analyses. Solving these difficulties requires refinements in the capability to fractionate and prepare biological samples as well as improvements in speed, automation, separation power and overall analytical sensitivity.Recent innovations in microfluidic devices with integrated on-chip sample enrichment, liquid chromatography and electrospray emitters and their applicability for specific proteomic applications are presented in this review.:

Ion sampling effects under conditions of total solvent consumption

The motivation of this work was to study some of the properties of nanoelectrospray operation under conditions where the entire sprayed liquid is vaporized and inhaled into the vacuum system. Under these conditions the desolvation requirements, sampling efficiency, concentration versus mass sensitivity, and molar response characteristics of various compounds were studied. The combined efficiency of ion production from solution and transfer into the vacuum system, referred to as sampling efficiency, is presented under various inlet conditions including different flow rates, solution compositions, and compound types. Under ideal solvent conditions the results for favorable compounds show sampling efficiencies of 70-85% at flows in the range of 50-500 nL/min. Efficiencies were lower for aqueous samples and compounds of different structures gave different molar response factors under these high sampling efficiency conditions. The relative molar response factors are presented in terms of those observed with higher flow rate sources which operate at significantly lower sampling efficiencies. In all cases, operating in this flow regime, the ion count rate was directly proportional to the absolute mass of analyte molecules entering the source. The experimental source used to carry out these studies included gas nebulization to stabilize the electrospray process, a heated laminar flow chamber to enhance desolvation and ion production, and various atmosphere-to-vacuum aperture diameters to maximize ion transfer.

Electrospray ionization from a gap with adjustable width

In this paper, we present a new concept for electrospray ionization mass spectrometry, where the sample is applied in a gap which is formed between the edges of two triangular-shaped tips. The size of the spray orifice can be changed by varying the gap width. The tips were fabricated from polyethylene terephthalate film with a thickness of 36 microm. To improve the wetting of the gap and sample confinement, the edges of the tips forming the gap were hydrophilized by means of silicon dioxide deposition. Electrospray was performed with gap widths between 1 and 36 microm and flow rates down to 75 nL/min. The gap width could be adjusted in situ during the mass spectrometry experiments and nozzle clogging could be managed by simply widening the gap. Using angiotensin I as analyte, the signal-to-noise ratio increased as the gap width was decreased, and a shift towards higher charge states was observed. The detection limit for angiotensin I was in the low nM range.

Receptor–ligand binding assays: Technologies and Applications

Receptor-ligand interactions play a crucial role in biological systems and their measurement forms an important part of modern pharmaceutical development. Numerous assay formats are available that can be used to screen and quantify receptor ligands. In this review, we give an overview over both radioactive and non-radioactive assay technologies with emphasis on the latter. While radioreceptor assays are fast, easy to use and reproducible, their major disadvantage is that they are hazardous to human health, produce radioactive waste, require special laboratory conditions and are thus rather expensive on a large scale. This has led to the development of non-radioactive assays based on optical methods like fluorescence polarization, fluorescence resonance energy transfer or surface plasmon resonance. In light of their application in high-throughput screening environments, there has been an emphasis on so called "mix-and-measure" assays that do not require separation of bound from free ligand. The advent of recombinant production of receptors has contributed to the increased availability of specific assays and some aspects of the expression of recombinant receptors will be reviewed. Applications of receptor-ligand binding assays described in this review will relate to screening and the quantification of pharmaceuticals in biological matrices.

A novel high-throughput automated chip-based nanoelectrospray tandem mass spectrometric method for PAMPA sample analysis

Parallel artificial membrane permeability assay (PAMPA) has recently gained popularity as a novel, high-throughput assay capable of rapidly screening compounds for their permeability characteristics in early drug discovery. The analytical techniques typically used for PAMPA sample analysis are HPLC-UV, LC/MS or more recently UV-plate reader. The LC techniques, though sturdy and accurate, are often labor and time intensive and are not ideal for high-throughput. On the other hand, UV-plate reader technique is amenable to high-throughput but is not sensitive enough to detect the lower concentrations that are often encountered in early drug discovery work. This article investigates a novel analytical method, a chip-based automated nanoelectrospray mass spectrometric method for its ability to rapidly analyze PAMPA permeability samples. The utility and advantages of this novel analytical method is demonstrated by comparing PAMPA permeability values obtained from nanoelectrospray to those from conventional analytical methods. Ten marketed drugs having a broad range of structural space, physico-chemical properties and extent of intestinal absorption were selected as test compounds for this investigation. PAMPA permeability and recovery experiments were conducted with model compounds followed by analysis by UV-plate reader, UV-HPLC as well as the automated nanoelectrospray technique (nanoESI-MS/MS). There was a very good correlation (r(2) > 0.9) between the results obtained using nanoelectrospray and the other analytical techniques tested. Moreover, the nanoelectrospray approach presented several advantages over the standard techniques such as higher sensitivity and ability to detect individual compounds in cassette studies, making it an attractive high-throughput analytical technique. Thus, it has been demonstrated that nanoelectrospray analysis provides a highly efficient and accurate analytical methodology to analyze PAMPA samples generated in early drug discovery.

Controlled Protein Precipitation in Combination with Chip-Based Nanospray Infusion Mass Spectrometry. An Approach for Metabolomics Profiling of Plasma

Liquid chromatography-mass spectrometry (LC-MS) is a common method for profiling biological samples in metabolomics. However, LC-MS data of metabolomic studies are often affected by high noise levels, retention time shifts, and high variability in signal intensities. With a new chip-based nanoelectrospray source it becomes possible to directly infuse complex biological samples such as plasma without any chromatographic separation beforehand. In combination with highly diluted samples and long data acquisition times, the parallel analysis of hundreds of compounds is now possible. In a proof-of-concept study, 10 human plasma samples from females and males were analyzed with the intention to separate the two groups by their different metabolomes. The reproducibility was so high that statistical analysis of the data could be performed without prior normalization. Two groups of female and male samples were separated by a supervised machine learning algorithm, principal component analysis, and hierarchical clustering. Peaks contributing to the group separation were characterized by accurate mass measurement and MS-MS fragmentation and by spiking experiments. The feasibility of direct sample infusion using the new chip-based nanoelectrospray device opens a new dimension for the rapid parallel analysis of complex biological mixtures.

Quantification of Relative Gene Dosage by Single-Base Extension and High-Performance Liquid Chromatography: Application to the SMN1/SMN2 Gene

One of the most commonly used techniques for genotyping of single-nucleotide polymorphism (SNP) is detection of single-base extensions (SBEs). We present a new, rapid, simple, and highly reliable method for accurate quantification of SNP variants in a single reaction. Our approach is based on SBE detection coupled with high-performance liquid chromatography (HPLC) quantification. To demonstrate the utility of our approach, we report data to determine the gene dosage for relative amounts of alleles in a homologous gene, allowing detection of mutation causing exon skipping in human SMN genes to determine the ratio between the copy numbers of the SMN1/SMN2 gene. We successfully determined the relative ratio of the SMN1 and SMN2 genes and showed assay characteristics using the SBE reaction coupled with HPLC. This assay approach readily scaled to high parallelization with multiplex SBE reactions in a single sample screened in one analysis. By screening for particular SNP genotypes, this assay can be used to determine the relative gene dosage that correlates highly with the patient's disease state. The next challenge is to apply this novel methodology in a clinical screening and quantification setting for special gene regions within highly homologous genes.

Electrospray interfacing of polymer microfluidics to MALDI-MS

The off-line coupling of polymer microfluidics to MALDI-MS is presented using electrospray deposition. Using polycarbonate microfluidic chips with integrated hydrophobic membrane electrospray tips, peptides and proteins are deposited onto a stainless steel target followed by MALDI-MS analysis. Microchip electrospray deposition is found to yield excellent spatial control and homogeneity of deposited peptide spots, and significantly improved MALDI-MS spectral reproducibility compared to traditional target preparation methods. A detection limit of 3.5 fmol is demonstrated for angiotensin. Furthermore, multiple electrospray tips on a single chip provide the ability to simultaneously elute parallel sample streams onto a MALDI target for high-throughput multiplexed analysis. Using a three-element electrospray tip array with 150 microm spacing, the simultaneous deposition of bradykinin, fibrinopeptide, and angiotensin is achieved with no cross talk between deposited samples. In addition, in-line proteolytic digestion of intact proteins is successfully achieved during the electrospray process by binding trypsin within the electrospray membrane, eliminating the need for on-probe digestion prior to MALDI-MS. The technology offers promise for a range of microfluidic platforms designed for high-throughput multiplexed proteomic analyses in which simultaneous on-chip separations require an effective interface to MS.

Chip electrospray mass spectrometry for carbohydrate analysis

Currently two types of chip systems are used in conjunction with MS: out-of-plane devices, where hundreds of nozzles, nanospray emitters are integrated onto a single silicon substrate from which electrospray is established perpendicular to the substrate, and planar microchips, embedding a microchannel at the end of which electrospray is generated in-plane, on the edge of the microchip. In the last two years, carbohydrate research greatly benefited from the introduction and implementation of the chip-based MS. In two laboratories the advantages of the chip electrospray in terms of ionization efficiency, sensitivity, reproducibility, quality of data in combination with high mass accuracy, and resolution of detection were systematically explored for several carbohydrate classes: O- and N-glycopeptides, oligosaccharides, gangliosides and glycoprotein-derived O- and N-glycans, and glycopeptides. The current state-of-the-art in interfacing the chip electrospray devices to high-performance MS for carbohydrate analysis, and the particular requirements for method optimization in both positive and negative ion modes are reviewed here. The recent applications of these miniaturized devices and their general potential for glycomic-based surveys are highlighted.

Integrated microfabricated systems including a purification module and an on-chip nano electrospray ionization interface for biological analysis

We report here on an integrated microfabricated device dedicated to the preparation of biological samples prior to their on-line analysis by electrospray ionization-mass spectrometry (ESI-MS). This microfluidic device is fabricated using the negative photoresist SU-8 by microtechnology techniques. The device includes a chromatographic module plus an ESI interface for MS. The chromatographic module is dedicated to sample purification and is based on a polymer monolithic phase which includes hydrophobic moieties. The ESI interface is integrated onto the chip and is based on a capillary slot. We present here the integration of these different modules onto a single system that is fabricated via a SU-8-based microtechnology route. We present also their testing for the purification of peptide samples. This started with a partial integration step with the combination of at least two of the modules (microsystem + monolith; microsystem + nib) and their test before the fabrication and testing of fully integrated microsystems.

Chip-based microfluidic devices coupled with electrospray ionization-mass spectrometry

We present the current status of the development of microfluidic devices fabricated on different substrates for coupling with electrospray ionization-mass spectrometry (ESI-MS). Until now, much success has been gained in fabricating the ESI chips, which show better performances due to miniaturization when compared with traditional methods. Integration of multiple steps for sample preparation and ESI sample introduction, however, remains a great challenge. This review covers the main technical development of electrospray device that were published from 1997 to 2004. This article does not attempt to be exclusive. Instead, it focuses on the publications that illustrated the breath of the development and applications of microchip devices for MS-based analysis.

Isoelectric focusing in cyclic olefin copolymer microfluidic channels coated by polyacrylamide using a UV photografting method

As an alternative material to glass or silicon, microfluidic devices made from a cyclic olefin copolymer (COC) were fabricated. This material is of interest because of the relative ease of fabrication, low costs, and solvent resistance. However, as a result of the strong hydrophobic interactions normally present, COC surfaces are not suitable for protein separations. To reduce the protein adsorption and make COC suitable for protein separations, UV-initiated grafting of polyacrylamide was used to coat the surface of COC devices. The change in surface properties caused by different graft times was studied. The surface hydrophilicity and electroosmotic mobility were characterized by contact angle and electroosmosis measurements. Isoelectric focusing was performed to test protein separations in polyacrylamide-coated COC microchannels. A single protein, carbonic anhydrase, was used to analyze the focusing effects and peak capacities in uncoated and polyacrylamide-coated COC devices. Peak capacities ranging from 75 to 190 were achieved with a polyacrylamide-coated surface. A mixture of two proteins, conalbumin labeled with Alexa Fluor 488 and beta-lactoglobulin A labeled with Alexa Fluor 546, was used to test protein separations. Linear and rapid separation of proteins was achieved in the polyacrylamide-coated COC microfluidic device.

Glass microfluidic devices with thin membrane voltage junctions for electrospray mass spectrometry

In this study a novel glass membrane was prepared for conducting high voltage (HV) to solution in the channel of a microfabricated device for generation of liquid electrospray. Taylor cone formation and mass spectra obtained from this microdevice confirmed the utility of the glass membrane, but voltage conduction through the membrane could not be successfully explained based solely on the conductivity of the glass itself. This novel method for developing a high-voltage interface for microdevices avoids direct metal/liquid contact eliminating bubble formation in the channel due to water hydrolysis on the surface of the metal. Further, this arrangement produces no dead volume as is often found with traditional liquid junctions. At the same time, preliminary investigations into the outlet design of glass microdevices for interfacing with electrospray mass spectrometry, was explored. Both the exit shape and the use of hydrophobic coatings at the channel exit of the microdevice electrospray interface were evaluated using standard proteins with results indicating the utility of this type of design after further optimization.

Automated nanospray using chip-based emitters for the quantitative analysis of pharmaceutical compounds

An automated nanospray system based on chip technology (the NanoMate) was successfully interfaced to a modified Particle Discriminator Interface on a triple quadrupole mass spectrometer. A number of the interface parameters were optimized to improve the sampling efficiency for ions from the chip-based system. Analytical performance was assessed using a number of biochemicals as well as via a methodology for a pharmaceutical that passed validation as required by Good Laboratory Practices. Infusion analyses in flow rates <1 microL/min provided advantages in terms of throughput and sample consumption when compared to other methodologies based on liquid chromatography.

Uniformity of ionization response of structurally diverse analytes using a chip-based nanoelectrospray ionization source

The major drawback of liquid chromatography/mass spectrometry (LC/MS) for the analysis of mixtures is the non-quantitative nature of these studies. The ionization efficiency of the various components in the mixture (e.g., a compound and its metabolites) can vary greatly and, therefore, relative intensities of signals cannot be related to relative abundance. A chip-based nanoelectrospray ionization source was used to compare the ionization efficiencies of compounds with different physical-chemical characteristics. The data indicate that the ionization efficiencies vary much less with the chip-based device than by LC/MS. This was ascribed to the generation of a much higher electric field around the nozzles, which supplies a large excess of protons to the small droplets and reduces/eliminates the differences in the ionization efficiency for the analytes.

The combination of liquid chromatography/tandem mass spectrometry and chip-based infusion for improved screening and characterization of drug metabolites

An approach has been developed for drug metabolism studies of non-radiolabeled compounds using on-line liquid chromatography/tandem mass spectrometry (LC/MS/MS) combined with chip-based infusion following fraction collection. The potential of this approach, which improves the data quality compared with only LC/MS analysis, has been investigated for the analysis of in vitro metabolites of tolcapone and talinolol, two compounds with well-characterized metabolism. The information-dependent LC/MS/MS analysis enables the characterization of the major metabolites while the chip-based infusion is used to obtain good product ion spectra for lower level metabolites, to generate complementary MS information on potential metabolites detected in the LC/MS trace, or to screen for unexpected metabolites. Fractions from the chromatographic analysis are collected in 20 second steps, into a 96-well plate. The fractions of interest can be re-analyzed with chip-based infusion on a variety of mass spectrometers including triple quadrupole linear ion trap (QqLIT or Q TRAP) and QqTOF systems. Acquiring data for several minutes using multi-channel acquisition (MCA), or signal averaging while infusing the fractions at approximately 200 nL/min, permits about a 50 times gain in sensitivity (signal-to-noise) in MS/MS mode. A 5-10 microL sample fraction can be infused for more than 30 min allowing the time to perform various MS experiments such as MS(n), precursor ion or neutral loss scans and accurate mass measurement, all in either positive or negative mode. Through fraction collection and infusion, a significant gain in data quality is obtained along with a time-saving benefit, because the original sample needs neither to be re-analyzed by re-injection nor to be pre-concentrated. Therefore, a novel hydroxylated talinolol metabolite could be characterized with only one injection.

Validating regulatory-compliant wide dynamic range bioanalytical assays using chip-based nanoelectrospray tandem mass spectrometry

Automated chip-based infusion nanoelectrospray ionization coupled to tandem mass spectrometry (nanoESI-MS/MS) was used to validate a bioanalytical assay conforming to United States Food and Drug Administration (FDA) regulatory guidelines and Good Laboratory Practices (GLP). Reboxetine was used as the analyte fortified in dog plasma along with an analog internal standard (IS). The best nanoESI response for reboxetine was observed with 90% acetonitrile (ACN)/water without any mobile phase modifiers. The analyte and IS were extracted from dog plasma samples by liquid-liquid extraction (LLE). The supernatant was concentrated to dryness and redissolved in 90% ACN/water for nanoESI. Selected reaction monitoring (SRM) data were collected for all samples to generate ion current profiles with a base width of approximately 20 s. Selectivity experiments showed no interferences in blank plasma samples. Interferences as a result of in-source collision-induced dissociation of metabolites were not an issue due to the previously documented metabolism of reboxetine. Matrix suppression was evaluated across multiple lots of dog plasma as well as over different animal species (rabbit, rat, mouse) and different anticoagulants (heparin, EDTA). Matrix suppression ranged from approximately 30-60% across the different lots, species etc.; however, in all instances, the analyte and the IS were suppressed by similar amounts, suggesting the similarity in ionization properties between the two. A three-batch validation was performed (each batch consisting of four different concentrations, six replicates of each concentration) and demonstrated inter-assay accuracy (% relative error; RE) of less than +/-8% and an inter-assay precision (% relative standard deviation; RSD) of less than 7%, thus meeting regulatory guidelines. A comparison of analyses by nanoESI-MS/MS and liquid chromatography coupled to tandem mass spectrometry (LC/MS/MS) showed that nanoESI-MS/MS had a greater slope for the calibration standard curve compared to LC/MS/MS, indicating greater sensitivity for the former technique. It is also noteworthy that the amount of sample infused during nanoESI-MS/MS was approximately 80-fold less compared to the amount of sample injected during LC/MS/MS. The absence of carryover (attributed to the lack of a common fluid path) in the nanoESI technique enabled the extension of the assay linear dynamic range to 500,000-fold, and the possibility of analyzing samples in a single batch without the need for re-analysis of samples with high concentrations. This technology offers the possibility for increased throughput for studies supporting drug development by providing fast data turnaround for assays conforming to regulatory guidelines and GLPs.

High-Throughput Mass Spectrometer Using Atmospheric Pressure Ionization and a Cylindrical Ion Trap Array

The analytical performance of an atmospheric pressure sampling, multiple-channel, high-throughput mass spectrometer was investigated using samples of a variety of types. The instrument, based on an array of cylindrical ion traps, was built with four independent channels and here is operated using two fully multiplexed channels (sources, ion optics, ion traps, detectors) capable of analyzing different samples simultaneously. Both channels of the instrument were incorporated within the same vacuum system and operated using a common set of control electronics. A multichannel electrospray ionization source was assembled and used to introduce samples including solutions of organic compounds, peptides, and proteins simultaneously into the instrument in a high-throughput fashion. Cross-talk between the channels of the instrument occurred in the detection system and could be minimized to 1-2% using shielding between detector channels. In this initial implementation of the instrumentation, an upper mass/charge limit of approximately 1300 Th was observed (+13 charge state of myoglobin) and unit mass/charge resolution was achieved to approximately 800 Th. The rather limited dynamic range (2-3 orders of magnitude for low-concentration analytes) is due to cross-talk contributions from more concentrated species introduced into a different channel. Analysis of mixtures of alkylamines and peptides is demonstrated, but analysis of mixtures with a wide spread in mass/charge ratios was not possible due to mass discrimination in the ion optics. Further refinement of the vacuum system and ion optics will allow the addition of more channels of parallel mass analysis and facilitate applications in fields such as proteomics and metabolomics.

Generation of mass tags by the inherent electrochemistry of electrospray for protein mass spectrometry

We present herein a review of our work on the on-line electrochemical generation of mass tags toward cysteine residues in peptides and proteins. Taking advantage of the inherent electrochemical nature of electrospray generated from a microfabricated microspray emitter, selective probes for cysteine were developed and tested for on-line nonquantitative mass tagging of peptides and proteins. The nonquantitative aspect of the covalent tagging thus allows direct counting of free cysteines in the mass spectrum of a biomolecule through additional adduct peaks. Several substituted hydroquinones were investigated in terms of electrochemical properties, and their usefulness for on-line mass tagging during microspray experiments were assessed with L-cysteine, peptides, and intact proteins. Complementarily, numerical simulations were performed to properly understand the respective roles of mass transport, kinetics of electrochemical-chemical reactions, and design of the microspray emitter in the mass tagging overall efficiency. Finally, the on-line electrochemical tagging of cysteine residues was applied to the analysis of tryptic peptides of purified model proteins for protein identification through peptide mass fingerprinting.

Fully-automated chip-based nanoelectrospray tandem mass spectrometry of gangliosides from human cerebellum

The introduction of chip-based electrospray (ESI) ion sources into biological mass spectrometry (MS) addressed the fundamental issue of how to analyze minute amounts of complex biological systems. The automation of sample delivery into the MS combined with the chip-based ESI allows for high quality bioanalysis in a high-throughput fashion. These advantages have already been demonstrated in proteomics, direct screening of drugs and drug discovery. As part of our continuing effort to implement automated chip-based mass spectrometry into the field of complex carbohydrate analysis, we hereby report the development of a chipESI MS and MS/MS methodology for the screening of gangliosides. A strategy to characterize a complex ganglioside mixture from human cerebellar tissue, by automated ESIchip-quadrupole time-of-flight (QTOF) MS and MS/MS is presented here. The feasibility of this method, and the general experimental requirements for automated chipESI MS analysis of these carbohydrate species is described.

Microchip Atmospheric Pressure Chemical Ionization Source for Mass Spectrometry

A novel microchip heated nebulizer for atmospheric pressure chemical ionization mass spectrometry is presented. Anisotropic wet etching is used to fabricate the flow channels, inlet, and nozzle on a silicon wafer. An integrated heater of aluminum is sputtered on a glass wafer. The two wafers are jointed by anodic bonding, creating a two-dimensional version of an APCI source with a sample channel in the middle and gas channels symmetrically on both sides. The ionization is initiated with an external corona-discharge needle positioned 2 mm in front of the microchip heated nebulizer. The microchip APCI source provides flow rates down to 50 nL/min, stable long-term analysis with chip lifetime of weeks, good quantitative repeatability (RSD < 10%) and linearity (r(2) > 0.995) with linear dynamic rage of at least 4 orders of magnitude, and cost-efficient manufacturing. The limit of detection (LOD) for acridine measured with microchip APCI at flow rate of 6.2 muL/min was 5 nM, corresponding to a mass flow of 0.52 fmol/s. The LOD with commercial macro-APCI at a flow rate of 1 mL/min for acridine was the same, 5 nM, corresponding to a significantly worse mass flow sensitivity (83 fmol/s) than measured with microchip APCI. The advantages of microchip APCI makes it a very attractive new microfluidic detector.

Atmospheric Pressure Photoionization-Mass Spectrometry with a Microchip Heated Nebulizer

A novel, microfabricated heated nebulizer chip for atmospheric pressure photoionization-mass spectrometry (APPI-MS) is presented. The chip consists of fluidic and gas inlets, a mixer, and a nozzle etched onto silicon wafer that is anodically bonded to a Pyrex glass wafer, on which an aluminum heater is sputtered. A krypton discharge lamp is used as the source for 10-eV photons to initiate the photoionization process. Dopant, delivered as part of the sample solution, is used to achieve efficient ionization. The use of the microfabricated heated nebulizer with APPI in the analysis of four analytes is demonstrated, and the spectra are compared to those obtained with a conventional APPI source. Ionization in positive and negative ion modes was successfully achieved and the spectra were mainly similar to those obtained with conventional APPI, indicating that the ionization in microfabricated and conventional APPI sources takes place by the same mechanisms. The flow rates with conventional APPI are approximately 100 muL/min, whereas the microchip heated nebulizer allows the use of flow rates 0.05-5 muL/min, thus being compatible with microfluidic separation systems or micro- and nano-LC. A stable signal was demonstrated throughout a 5-h measurement, which proved the excellent stability of the micro-APPI. The same heated nebulizer chip can be used for weeks.

Why the move to microfluidics for protein analysis?

There has been a recent trend towards the miniaturization of analytical tools, but what are the advantages of microfluidic devices and when is their use appropriate? Recent advances in the field of micro-analytical systems can be classified according to instrument performance (which refers here to the desired property of the analytical tool of interest) and two important features specifically related to miniaturisation, namely reduction of the sample volume and the time-to-result. Here we discuss the contribution of these different parameters and aim to highlight the factors of choice in the development and use of microfluidic devices dedicated to protein analysis.

Electrospray emission from nonwetting flat dielectric surfaces

Electrosprays are devices in which nanometer sized droplets and/or solvated ions are electrically extracted from a liquid surface and accelerated to high velocities. They are usually constructed from conductive capillaries with one of the ends tapered down to a sharp tip with the purpose of enhancing the local electric field that produces the instability that develops into the Taylor cone structure from which emission occurs. In an alternative configuration, the conductive needles are replaced by small holes through a dielectric, nonwetting flat block. An electrostatic model shows that if the emitter material has low dielectric constant then the local electric field near the emission site is enhanced in a very similar way as with sharp metallic needles. Furthermore, the combination of the nonwetting property of the material and the sharp corner formed in the hole-surface interface effectively anchors the Taylor cone to the edge of the hole, thus simplifying the process of manufacturing. The possible microfabrication of this configuration makes it especially attractive for producing arrays of large numbers of individual emitters. Such arrays may find use as space propulsion thrusters and in the analytical industry to improve the characteristics of mass-spectrometric analyses.

State-of-the-art of the hyphenation of capillary electrochromatography with mass spectrometry

The high separation efficiency and loading capacity of capillary electrochromatography (CEC) make it an attractive separation mode for coupling with mass spectrometry (MS), which has the ability to unambiguously identify analytes with high selectivity and sensitivity. We present an overview of recent advances on both instrumentation and separation columns employed in CEC-MS systems. In particular, the main characteristics of the stationary phases, as well as the configurations of the column outlet that are related with the coupling arrangements of the MS ionization sources, are reported. At present, packed columns and conventional electrospray ionization (ESI) sources are mainly employed in CEC-MS. Nevertheless, the use of monolithic capillary columns and nanoelectrospray sources has the potential for wide acceptance in the next future. Moreover, the main features of several mass analyzers including ion trap, quadrupole, time-of-flight, magnetic sector, and Fourier transform-ion cyclotron resonance are examined. Finally, current applications of this technology, mainly in the pharmaceutical field and proteomics, are reviewed.

A new on-chip ESI nozzle for coupling of MS with microfluidic devices

This paper presents a new on-chip electrospray ionisation (ESI) nozzle, which can be used as an interface for coupling microfluidic devices with mass spectrometric (MS) detection. The nozzle was micromilled in a polymer foil (polymethylmethacrylate (PMMA) 750 microm thick), normally used as a cover for microfluidic chips. The performance of this device was examined in the ESI-MS analysis of the tetrapeptide MRFA (methionine-argenine-phenylalanine-alanine). The spray quality is basically dependent on the inner diameter of the nozzle, beside the part of the organic modifier in the solution to be sprayed. Three different inner nozzle diameters (30, 50, 100 microm) and two different apex angles were investigated. Stable electrospray conditions can be generated with a relative standard deviation less than 10% of the total ion current, and down to a concentration of 0.01 micromol L(-1). The production of this ESI interface is relatively simple for the purpose of a low-cost batch fabrication of miniaturized analytical instruments.

Quantitative Mass Spectrometric Determination of Methylphenidate Concentration in Urine Using an Electrospray Ionization Source Integrated with a Polymer Microchip

We have demonstrated the use of a simple microfabricated electrospray ionization source for coupling microfluidic chips to mass spectrometry (MS). A polymer-based microchip, coupled to a triple quadrupole mass spectrometer, has been employed for direct infusion quantitative bioanalysis of methylphenidate (Ritalin) extracted from human urine samples. The approach used a microfabricated polymer electrospray emitter to couple a microfluidic channel to a stable electrospray ionization source. The microchip was fabricated from cycloolefin plastic plate by hot embossing and thermal bonding. This microfluidic chip contained two independent microfluidic channels, integrated with two corresponding electrospray emitters and an internal gold electrode. Liquid-liquid extraction was used to prepare urine samples, spiked with methylphenidate. A trideuterated analogue of methylphenidate (methylphenidate-d(3)) was used as the internal standard for the analysis. The system showed good electrospray stability and reproducibility with different spray tips. Four different electrospray tips were used to analyze the same sample, and the results showed very small variation with a relative standard deviation of 1.4%. A standard curve prepared for methylphenidate in urine (R(2) = 0.999) was linear over the range of 0.4-800 ng/mL. The precision of the quality control samples for three different concentrations ranged from 19.1% at 20 ng/mL, 3.2% at 200 ng/mL, to 3.5% at 667 ng/mL while the accuracy was 96.3% at 20 ng/mL, 101.2% at 200 ng/mL, and 101.6% at 667 ng/mL. No system carryover was detected even when the same device was used for sequential analysis. These results suggest the potential of this microdevice for MS-based quantitative analysis in drug discovery and development.

Fully Automated Chip-Based Mass Spectrometry for Complex Carbohydrate System Analysis

Carbohydrates represent a major class of biopolymers, which occur in nature either as oligosaccharides or glycoconjugates, in which the sugar moiety is linked to proteins or lipids. The significance of mass spectrometry for highly sensitive analysis of complex carbohydrates increased after the introduction of the electrospray ionization and matrix assisted laser desorption/ionization methods and the possibility of tandem MS for sequencing of single molecular species in complex mixtures. Rapid and sensitive characterization of carbohydrates in biological systems by automated nanoscale liquid delivery and chip-based electrospray interface techniques have not been developed so far. In this contribution, the implementation and optimization of a fully automated chip-based nanoelectrospray assembly (NanoMate system), operating in the negative ion mode, in combination with QTOF-tandem MS for mapping/sequencing and computer-assisted structure assignment for carbohydrate components in complex mixtures is presented.

A novel nib-like design for microfabricated nanospray tips

We present here novel tips for nanoelectrosray ionization-mass spectrometry (ESI-MS) applications. These ionization sources have a planar geometry in the shape of a nib. Their functioning is based on a principle much akin to that of a fountain pen in that fluids are actuated by capillarity. Once a voltage is applied, an electrospray is formed at the nib tip. The nib fabrication relies on micromachining techniques using the epoxy-based negative photoresist SU-8 and a double exposure photolithographic process. Two types of nib-like sources were fabricated; they were made either conductive by metallization with a nickel layer or non-conductive but hydrophilic by covering them with a SiO(2) layer. In the latter case, the HV was applied via a Pt wire inserted into the reservoir feature of the nib. The nib-like sources were tested on an ion trap mass spectrometer using Gramicidin S samples at concentrations as low as 1 microM and ionization voltages as low as 1.2 kV. We have observed a good overall stability of the spray during the tests with no marked decrease in the signal intensity even under extreme conditions.

A thin chip microsprayer system coupled to Fourier transform ion cyclotron resonance mass spectrometry for glycopeptide screening

A thin polymer microchip was coupled with a Fourier transform ion cyclotron resonance (FTICR) 9.4 T mass spectrometer and the method was optimized in negative ion mode for glycopeptide screening. The interface between the polymer microchip and FTICR mass spectrometer consists of an in-laboratory conceived and designed mounting system that exhibits robust and controllable alignment of the chip toward the inlet of the mass spectrometer. The particular attribute of the polymer chip coupled to the FTICR mass spectrometer, to achieve an increase in ionization efficiency and sensitivity under the premise of high mass accuracy of detection, is highlighted by the large number of major and minor glycopeptide structures detected and identified in highly heterogeneous mixtures obtained from urine matrices. Glycoforms expressing various saccharide chain lengths ranging from tri- to dodecasaccharide, bearing up to three sialic acid moieties, could be detected and assigned based on the accuracy of the mass measurement (average mass deviation below 6 ppm) of their molecular ions. -Thin chipESI-FTICRMS is a potent novel system for glycomic screening of complex mixtures, as demonstrated for identification of singly sialylated O-glycosylated amino acids and peptides from urine matrices, and could be considered for general applicability in the glycoanalytical field.

Evaluation of automated nano-electrospray mass spectrometry in the determination of non-covalent protein-ligand complexes

The use of electrospray ionization mass spectrometry (ESI-MS) for studying non-covalent interactions between macromolecules and ligands is well established. ESI-MS can be a useful tool for the determination of dissociation constants between molecules in the gas phase. We validate this method by studying the binding of the catalytic domain of cellobiohydrolase I (CBH I) from Trichoderma reesei to the disaccharide inhibitor cellobiose. The method was further applied to study two newly synthesized cellobiose derivatives (m-iodobenzyl 2-deoxy-2-azido-beta-cellobioside and p-benzyloxybenzyl beta-cellobioside). In a titration experiment, peak areas of different charge states of the free enzyme and the complex were summed in order to determine the dissociation constant. For cellobiose and m-iodobenzyl 2-deoxy-2-azido-beta-cellobioside, the calculated values are in good agreement with those reported from either displacement titration or equilibrium binding experiments in solution. Due to non-specific binding, the dissociation constant of p-benzyloxybenzyl beta-cellobioside does not correspond with the solution-based value. Our results indicate the need for careful interpretation of data sets when using nanoESI to study non-covalent interactions.

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.

Chip-Based Solid-Phase Extraction Pretreatment for Direct Electrospray Mass Spectrometry Analysis Using an Array of Monolithic Columns in a Polymeric Substrate

An array of eight porous monolithic columns, prepared in a Zeonor polymeric chip by UV-initiated polymerization of butyl methacrylate and ethylene dimethacrylate, was tested for solid-phase extraction (SPE) cleanup of biological samples prior to directly coupled electrospray mass spectrometry (ESI-MS). The chip, fabricated by hot embossing and thermal bonding, consists of eight parallel channels (10 mm long, 360 microm i.d.) connected via external fused-silica capillaries. The monomer mixture was aspirated simultaneously into the eight channels using a homemade vacuum manifold device and polymerized in parallel for 20 min under UV irradiation. The porous monolithic columns were then characterized by scanning electron microscopy and evaluated by ESI-MS applications with respect to sample capacity, recovery, reproducibility of peak area or peak height ratios, and linearity between peak height ratio and concentration using imipramine as a pharmaceutical test compound. The average sample capacity was estimated to be 0.30 microg with a relative standard deviation (RSD) of 26.5% for the eight monolithic columns on the same polymeric chip. For two chips prepared using the same monomer mixture, the difference in average sample capacity was 7.0%. The average recovery for the eight monolithic SPE columns on the same chip was 79.1% with an RSD of 7.9%. Using imipramine-d3 as an internal standard, the RSD of peak height ratios for the eight different columns was 2.0% for a standard solution containing 1 microg/mL imipramine. A linear calibration curve (R2 = 0.9995) was obtained for standard aqueous solutions of imipramine in the range from 0.025 to 10 microg/mL. To demonstrate the analytical potential of the chip-based SPE system, two different types of real-world samples including human urine sample and P450 drug metabolism incubation mixture were tested. Similar to standard aqueous solution, a linear correlation (R2 = 0.9995) was also found for human urine sample spiked with imipramine in the range of 0.025-10 microg/ mL. When aliquots of a human urine sample spiked with 1 microg/mL imipramine were loaded onto eight different monolithic columns, the RSD of peak height ratios was 3.8%. For a P450-imipramine incubation mixture, the formation of the N-demethylated metabolite (m/z 267.2) and the monohydroxylated metabolite (m/z 297.2) of imipramine was observed following chip-based monolithic SPE sample cleanup and preconcentration.

Introduction to micro-analytical systems: bioanalytical and pharmaceutical applications

This review presents a brief overview of recent developments in miniaturization of analytical instruments utilizing microfabrication technology. The concept 'Micro-Total Analysis Systems micro-TAS)', also termed 'Lab-on-a-chip', and the latest progresses in the development of microfabricated separation devices and on-chip detection techniques are discussed. Applications of micro-analytical methods to bioanalytical and pharmaceutical studies are also described, including chemical reactions, assays, and analytical separations of biomolecules in micro-scale.

Use of a Microchip Device Coupled with Mass Spectrometry for Ligand Screening of a Multi-Protein Target

Nanoflow electrospray mass spectrometry has been applied previously to investigate noncovalent protein-protein and protein-ligand interactions. Here we evaluate a commercial microchip device for this application. We show that the microchip can be used to obtain mass spectra of the noncovalent tetramer transthyretin. The device showed a 10-fold increase in signal stability compared with a nanoflow capillary and a high level of nozzle-to-nozzle reproducibility. Binding of the natural ligand thyroxine was clearly observed, and a range of small molecules proposed as inhibitors of transthyretin amyloidosis were shown to be effective in stabilizing the tetramer. We propose that measuring the ability of small molecules to stabilize protein complexes using this automated microchip technology will enable high-throughput screening of multi-protein complexes by mass spectrometry.

Quantitative determination of noncovalent binding interactions using automated nanoelectrospray mass spectrometry

Electrospray ionization mass spectrometry (ESI-MS) has proven to be an extremely powerful tool for studying biomolecular structures and noncovalent interactions. Here we report a method using a fully automated, chip-based nanoESI-MS system to determine the dissociation constants (Kd) for the complexes of two different proteins with their ligands. The automated nanoelectrospray system, consisting of the NanoMate and ESI chip, serves functionally as a combination of autosampler and nanoelectrospray ionization source. This system provides all the advantages of conventional nanoelectrospray plus automated, high-throughput analyses without carryover. The automated nanoESI system was used to investigate quantitative noncovalent interactions between ribonuclease A (RNase A) and cytidylic acid ligands (2'-CMP, CTP), a well-characterized model protein-ligand complex, and between an inactive endocellulase mutant (Thermobifida fusca Cel6A D117Acd) and four oligosaccharide ligands (cellotriose, cellotetraose, cellopentaose, cellohexaose). Both titration and competitive binding approaches were performed prior to automated nanoESI-MS analysis with a Q-TOF mass spectrometer. Dissociation constants for each complex were calculated from the sum of ion peak areas of free and complexed proteins during the titration and competition experiments. The measured Kd values for the RNase A-CMP and Cel6A D117Acd-G3 complexes were found to be in excellent agreement with the available published values obtained by standard spectroscopic titration techniques. To our knowledge, this is the first report of using an ESI-MS approach to study the interactions between a cellulase and oligosaccharides. The results provide new insights for understanding the nature of cellulase-cellulose interactions.

On-chip protein sample desalting and preparation for direct coupling with electrospray ionization mass spectrometry

A membrane-based desalting step integrated in a MS microchip is presented: drugs, peptides and proteins are adsorbed on a hydrophobic poly(vinylidene difluoride) membrane, which allows the washing out of salts. The integration with microfluidics permits a controlled elution of analytes from the membrane and their direct mass spectrometric analysis by electrospray ionisation MS. The desalting process is demonstrated with picomole amounts of propanolol, insulin and cytochrome c. Moreover, this stop-and-go desalting process is tolerant to high concentrations of urea, and to the presence of reductants such as dithiothreitol. This particular feature allowed the chemical tagging of cysteines in beta-lactoglobulin A with iodoacetamide. Finally, the integration of chemical tagging, on-chip desalting and MS microchip paves the way for the development of high-throughput analytical procedure for structural proteomics.

Quantitative high-throughput analysis of drugs in biological matrices by mass spectrometry

To support pharmacokinetic and drug metabolism studies, LC-MS/MS plays more and more an essential role for the quantitation of drugs and their metabolites in biological matrices. With the new challenges encountered in drug discovery and drug development, new strategies are put in place to achieve high-throughput analysis, using serial and parallel approaches. To speed-up method development and validation, generic approaches with the direct injection of biological fluids is highly desirable. Column-switching, using various packing materials for the extraction columns, is widely applied. Improvement of mass spectrometers performance, and in particular triple quadrupoles, also strongly influences sample preparation strategies, which remain a key element in the bioanalytical process.

Poly(dimethylsiloxane) electrospray devices fabricated with diamond-like carbon-poly(dimethylsiloxane) coated SU-8 masters

This study presents coupling of a poly(dimethylsiloxane) (PDMS) micro-chip with electrospray ionization-mass spectrometry (ESI-MS). Stable electrospray is generated directly from a PDMS micro-channel without pressure assistance. Hydrophobic PDMS aids the formation of a small Taylor cone in the ESI process and facilitates straightforward and low-cost batch production of the ESI-MS chips. PDMS chips were replicated with masters fabricated from SU-8 negative photoresist. A novel coating, an amorphous diamond-like carbon-poly(dimethylsiloxane) hybrid, deposited on the masters by the filtered pulsed plasma arc discharge technique, improved significantly the lifetime of the masters in PDMS replications. PDMS chip fabrication conditions were observed to affect the amount of background peaks in the MS spectra. With an optimized fabrication process (PDMS curing agent/silicone elastomer base ratio of 1/8 (w/w), curing at 70 degree C for 48 h) low background spectra were recorded for the analytes. The performance of PDMS devices was examined in the ESI-MS analysis of some pharmaceutical compounds and amino acids.

Lab-on-a-chip for drug development

Significant advances have been made in the development of micro-scale technologies for biomedical and drug discovery applications. The first generation of microfluidics-based analytical devices have been designed and are already functional. Microfluidic devices offer unique advantages in sample handling, reagent mixing, separation, and detection. We introduce and review microfluidic concepts, microconstruction techniques, and methods such as flow-injection analysis, electrokinesis, and cell manipulation. Advances in micro-device technology for proteomics, sample preconditioning, immunoassays, electrospray ionization mass spectrometry, and polymerase chain reaction are also reviewed.

Demonstration of direct bioanalysis of drugs in plasma using nanoelectrospray infusion from a silicon chip coupled with tandem mass spectrometry

Quantitative bioanalysis by direct nanoelectrospray infusion coupled to tandem mass spectrometry has been achieved using an automated liquid sampler integrated with an array of microfabricated electrospray nozzles allowing rapid, serial sample introduction (1 min/ sample). Standard curves prepared in human plasma for verapamil (r2 = 0.999) and its metabolite norverapamil (r2 = 0.998) were linear over a range of 2.5-500 ng/ mL. Based on the observed precision and accuracy, a lower limit of quantitation of 5 ng/mL was assigned for both analytes. Sample preparation consisted of protein precipitation with an organic solvent containing the structural analogue gallopamil as an internal standard. Protein precipitation was selected both to maximize throughput and to test the robustness of direct nanoelectrospray infusion. Aliquots of supernatant (10 pL) were transferred to the back plane of the chip using disposable, conductive pipet tips for direct infusion at a flow rate of 300 nL/min. Electrospray ionization occurred from the etched nozzles (30-microm o.d.) on the front of the chip, initiated by a voltage applied to the liquid through the pipet tip. The chip was positioned near the API sampling orifice of a triple quadrupole mass spectrometer, which was operated in selected reaction monitoring mode. Results are presented that document the complete elimination of system carry-over, attributed to lack of a redundant fluid path. This technology offers potential advantages for MS-based screening applications in drug discovery by reducing the time for methods development and sample analysis.

Characterization of micromachined hollow tips for two-dimensional nanoelectrospray mass spectrometry

In this work an improved design of chip-based nanoelectrospray nozzles is reported. Two-dimensional matrices of out-of-plane 10 microm i.d. silicon dioxide tips with a tapered shape were manufactured using deep reactive ion etching technology. Using a peptide sample, six micromachined tips and six commercially pulled silica capillary tips were compared employing an ion trap mass spectrometer. At a flow rate of 100 nL/min, the detectability obtained was approximately the same for the two types of tips. The relative standard deviation of the signal-to-noise ratio for the peptides between six different tips was on average 22% for the micromachined tips and 45% for the pulled capillary tips. The usefulness of the micromachined tips for analysis of non-covalent protein-ligand complexes was demonstrated by the analysis of a sample of RNase A and cytidine 2'-monophosphate. In another test, analyzing a tryptic digest of 1 pmol/microL cytochrome C, 18 peptides corresponding to a 82% sequence coverage were detected. Using MS/MS, the whole sequence of an 11 amino acid cytochrome C fragment was obtained. Computer simulations were performed on the shape and magnitude of the electrical field around micromachined and pulled capillary tips. To reach the threshold electric field density at the tip apex required to initiate an electrospray, a higher electrospray voltage was needed for the chip-based tips compared with pulled capillary tips. This is due to the influence of the chip base.

A fully automated nanoelectrospray tandem mass spectrometric method for analysis of Caco-2 samples

Caco-2 cells offer a means to rapidly screen permeability of drug candidates, allowing pharmaceutical companies to eliminate candidates unable to cross the intestinal barrier early in the discovery process. This screening process is typically performed by conventional liquid chromatography/tandem mass spectrometry (LC/MS/MS), which can require time-consuming method development. An alternative to LC/MS/MS, automated nanoelectrospray tandem mass spectrometry (nanoESI-MS/MS), is introduced. This novel approach requires an off-line ZipTip desalting step followed by automated nanoESI-MS/MS, using the NanoMate 100 and ESI Chip. In addition to reduced method development time, automated nanoESI-MS/MS also offers no carry-over between samples, low sample consumption, and ease-of-use as compared with conventional pulled-capillary nanoelectrospray. Furthermore, the infusion system described has the potential to be high-throughput. A comparison of Caco-2 samples analyzed both by LC/MS/MS and by automated nanoESI-MS/MS is presented. The permeability and recovery data of the two compounds analyzed in this study obtained from conventional LC/MS/MS and by automated nanoESI-MS/MS were in excellent agreement.