Ultra-low flow nanospray for the normalization of conventional liquid chromatography/mass spectrometry through equimolar response: standard-free quantitative estimation of metabolite levels in drug discovery

Nanotechnology in proteomics

In genomics, the ability to amplify rare transcripts has enabled rapid advances in the understanding of gene expression patterns in human disease. The inability to increase the copy number and to detect the signal of rare proteins as unique species in biological samples has hindered the ability of proteomics to dissect human disease with the same complexity as genomic analyses. Advances in nanotechnology have begun to allow researchers to identify low-abundance proteins in samples through techniques that rely upon both nanoparticles and nanoscale devices. Coupled with rapid advances made in protein identification and isolation over the past decade, currently available technology enables more effective multiplexing and improved signal-to-noise, which enhances detection of low-abundance proteins in cellular and tissue lysates significantly. Techniques, including nanowires, nanocantilevers, bio-barcoding and surface-enhanced Raman spectroscopy, permit the detection of proteins into the low attomolar range, where many biologically important cellular processes occur. In this review, we summarize several such techniques, highlight their implementation in current protein research and comment on their potential role in future proteomic investigations and biomedical applications.

Response normalized liquid chromatography nanospray ionization mass spectrometry

The widely different LC-MS response observed for many structurally different compounds limits the use of LC-MS in full scan detection mode for quantitative determination of drugs and metabolites without using reference standard. The recently introduced nanospray ionization (NSI) technique shows comparable MS response for some compounds under non-LC-MS conditions. However, in the presence of numerous endogenous compounds commonly associated with biological samples such as urine, plasma, and bile, LC-MS is required to separate, detect, identify, and measure individual analytes. An LC-NSI-MS system was devised and the MS response obtained in this system for a variety of pharmaceutical drugs and their metabolites. The set-up involves two high-performance liquid chromatography (HPLC) systems, a chip-based NSI source and a quadrupole-time-of-flight (Q-TOF) mass spectrometer. Herein this is referred to as the response normalized-liquid chromatography NSI-MS (RNLC-NSI-MS) system. One HPLC unit performs the analytical separation, while the other unit adds solvent post-column with an exact reverse of the mobile phase composition such that the final composition entering the NSI source is isocratic throughout the entire HPLC run. The data obtained from four different structural classes of compounds [vicriviroc (VCV), desloratadine (DL), tolbutamide, and cocaine] and their metabolites indicate that by maintaining the solvent composition unchanged across the HPLC run, the influence of the solvent environment on the ionization efficiency is minimized. In comparison to responses obtained from radiochromatograms, responses from conventional LC-ESI-MS overestimated the VCV and DL responses, respectively, by 6- and 20-fold. Although VCV and DL responses obtained using LC-NSI-MS are within 2- to 6-fold from the respective radiochromatographic responses, the response normalization modification results in nearly uniform LC-NSI-MS response for all compounds evaluated.

Array of Chemically Etched Fused-Silica Emitters for Improving the Sensitivity and Quantitation of Electrospray Ionization Mass Spectrometry

An array of emitters has been developed for increasing the sensitivity of electrospray ionization mass spectrometry (ESI-MS). The linear array consists of 19 chemically etched fused-silica capillaries arranged with 500 microm (center-to-center) spacing. The multiemitter device has a low dead volume to facilitate coupling to capillary liquid chromatography (LC) separations. The high aspect ratio of the emitters enables operation at flow rates as low as 20 nL/min/emitter, effectively extending the benefits of nanoelectrospray to higher flow rate analyses. To accommodate the larger ion current produced by the emitter array, a multicapillary inlet to the mass spectrometer was also constructed. The inlet, which matched the dimensions of the emitter array, preserved ion transmission efficiency. Standard reserpine solutions of varying concentration were electrosprayed at 1 microL/min using the multiemitter/multi-inlet combination, and the results were compared to those from a standard, single-emitter configuration. A 9-fold sensitivity enhancement was observed for the multiemitter relative to the single emitter. A bovine serum albumin tryptic digest was also analyzed, and a sensitivity increase ranging from 2.4- to 12.3-fold for the detected tryptic peptides resulted; the varying response was attributed to reduced ion suppression under the nanoESI conditions afforded by the emitter array. An equimolar mixture of leucine enkephalin and maltopentaose was studied to verify that ion suppression is indeed reduced for the multiplexed ESI (multi-ESI) array relative to a single emitter over a range of flow rates.

Bioanalytical procedures for determination of drugs of abuse in blood

Determination of drugs of abuse in blood is of great importance in clinical and forensic toxicology. This review describes procedures for detection of the following drugs of abuse and their metabolites in whole blood, plasma or serum: Delta9-tetrahydrocannabinol, 11-hydroxy-Delta9-tetrahydrocannabinol, 11-nor-9-carboxy-Delta9-tetrahydrocannabinol, 11-nor-9-carboxy-Delta9-tetrahydrocannabinol glucuronide, heroin, 6-monoacetylmorphine, morphine, morphine-6-glucuronide, morphine-3-glucuronide, codeine, amphetamine, methamphetamine, 3,4-methylenedioxymethamphetamine, N-ethyl-3,4-methylenedioxyamphetamine, 3,4-methylenedioxyamphetamine, cocaine, benzoylecgonine, ecgonine methyl ester, cocaethylene, other cocaine metabolites or pyrolysis products (norcocaine, norcocaethylene, norbenzoylecgonine, m-hydroxycocaine, p-hydroxycocaine, m-hydroxybenzoylecgonine, p-hydroxybenzoylecgonine, ethyl ecgonine, ecgonine, anhydroecgonine methyl ester, anhydroecgonine ethyl ester, anhydroecgonine, noranhydroecgonine, N-hydroxynorcocaine, cocaine N-oxide, anhydroecgonine methyl ester N-oxide). Metabolites and degradation products which are recommended to be monitored for assessment in clinical or forensic toxicology are mentioned. Papers written in English between 2002 and the beginning of 2007 are reviewed. Analytical methods are assessed for their suitability in forensic toxicology, where special requirements have to be met. For many of the analytes sensitive immunological methods for screening are available. Screening and confirmation is mostly done by gas chromatography (GC)-mass spectrometry (MS) or liquid chromatography (LC)-MS(/MS) procedures. Basic information about the biosample assayed, internal standard, workup, GC or LC column and mobile phase, detection mode, and validation data for each procedure is summarized in two tables to facilitate the selection of a method suitable for a specific analytic problem.

Recent applications of liquid chromatography-mass spectrometry in natural products bioanalysis

Natural flavonoids, alkaloids, saponins and sesquiterpenoids have been extensively investigated because of their biological and physiological significances, as well as their promising clinical uses. It is necessary to monitor them or their metabolites in biological fluids for both pre-clinical studies and routine clinical uses. The successful hyphenation of LC and MS, which was thought as "the bird wants to marry with fish", has been conducted widely in biological samples analysis. This present paper reviewed the feasibility of LC-MS techniques in the identification and quantification of natural products (flavonoids, alkaloids, saponins and sesquiterpenoids) in biological fluids, dealing with sample preparation, LC techniques, suitability of different MS techniques. Perspective of LC-MS was also discussed to show the potential of this technology. The citations cover the period 2002-2006. We conclude that LC-MS is an extremely powerful tool for the analysis of natural products in biological samples.

Evaluation of high-field asymmetric waveform ion mobility spectrometry coupled to nanoelectrospray ionization for bioanalysis in drug discovery

The potential of high-field asymmetric waveform ion mobility spectrometry (FAIMS) coupled to nanoelectrospray ionization (nanoESI) as a method to improve sample throughput for bioanalysis in a discovery pharmaceutical setting was explored in this work. The ability of FAIMS to separate gas-phase ions in the millisecond timescale was exploited to eliminate the need for liquid chromatography. Samples were introduced into the FAIMS electrodes/mass spectrometer using offline nanoESI at 20 nL/min and 1.5 kV. Signals were averaged for 30 s after which the next sample could be analyzed. The separation of simple mixtures, e.g., the removal of metabolite and endogenous interferences from parent drug, was demonstrated. Moreover, the application of nanoESI attenuated the ion suppression effects that normally plague conventional electrospray. On average, approximately two-thirds of the neat sample signal intensity was preserved in extracted plasma samples. Standard curves were prepared for several compounds and linearity was obtained over approximately two to three orders of magnitude. This methodology was further tested with the analysis of plasma samples from a mouse pharmacokinetic study. Concentration values determined using nanoESI-FAIMS were comparable to those determined using conventional LC/MS as demonstrated by percent differences of less than 30%. This work demonstrated the proof of concept that the combination of FAIMS and nanospray ionization can be a potentially useful tool to improve the throughput of discovery bioanalysis.

Chemically etched open tubular and monolithic emitters for nanoelectrospray ionization mass spectrometry

We have developed a new procedure for fabricating fused-silica emitters for electrospray ionization-mass spectrometry (ESI-MS) in which the end of a bare fused-silica capillary is immersed into aqueous hydrofluoric acid, and water is pumped through the capillary to prevent etching of the interior. Surface tension causes the etchant to climb the capillary exterior, and the etch rate in the resulting meniscus decreases as a function of distance from the bulk solution. Etching continues until the silica touching the hydrofluoric acid reservoir is completely removed, essentially stopping the etch process. The resulting emitters have no internal taper, making them much less prone to clogging compared to, e.g., pulled emitters. The high aspect ratios and extremely thin walls at the orifice facilitate very low flow rate operation; stable ESI-MS signals were obtained for model analytes from 5-microm-diameter emitters at a flow rate of 5 nL/min with a high degree of interemitter reproducibility. In extensive evaluation, the etched emitters were found to enable approximately four times as many LC-MS analyses of proteomic samples before failing compared with conventional pulled emitters. The fabrication procedure was also employed to taper the ends of polymer monolith-containing silica capillaries for use as ESI emitters. In contrast to previous work, the monolithic material protrudes beyond the fused-silica capillaries, improving the monolith-assisted electrospray process.

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.