Gas chromatography-microchip atmospheric pressure chemical ionization-mass spectrometry

An atmospheric pressure chemical ionization (APCI) microchip is presented for combining a gas chromatograph (GC) to a mass spectrometer (MS). The chip includes capillary insertion channel, stopper, vaporizer channel, nozzle and nebulizer gas inlet fabricated on the silicon wafer, and a platinum heater sputtered on a glass wafer. These two wafers are joined by anodic bonding creating a two-dimensional version of an APCI microchip. The sample from GC is directed via heated transfer line capillary to the vaporizer channel of the APCI chip. The etched nozzle forms narrow sample plume, which is ionized by an external corona discharge needle, and the ions are analyzed by a mass spectrometer. The GC-microchip APCI-MS combination provides an efficient method for qualitative and quantitative analysis. The spectra produced by microchip APCI show intensive protonated molecule and some fragmentation products as in classical chemical ionization for structure elucidation. In quantitative analysis the GC-microchip APCI-MS showed good linearity (r(2) = 0.9989) and repeatability (relative standard deviation 4.4%). The limits of detection with signal-to-noise ratio of three were between 0.5 and 2 micromol/L with MS mode using selected ion monitoring and 0.05 micromol/L with MS/MS using multiple reaction monitoring.     

Gas chromatography connected to multiple channel electrospray ionization mass spectrometry for the detection of volatile organic compounds

This work successfully connected gas chromatography (GC) to seven-channel electrospray ionization (ESI) mass spectrometry to separate and detect a mixture of volatile organic compounds. Gaseous analyte was eluted separately from a GC column and directed into the central channel of the ESI source. The analyte was protonated by ion-molecule reactions between the analyte and the ions which were generated by electrospraying the acidic solution through the outside six channels surrounding the central channel. Real-time analysis of the organic reaction involving volatile and thermally unstable compounds (dimethylhydrazine ? azomethane + H(2)) was also achieved by continuously purging the air in the reaction vessel to the seven-channel ESI source.     

Determination of lysergic acid diethylamide (LSD), iso-LSD, and N-demethyl-LSD in body fluids by gas chromatography/tandem mass spectrometry.

Procedures for detection and quantitation of lysergic acid diethylamide (LSD), iso-LSD, and N-demethyl-LSD by capillary chromatography/tandem mass spectrometry (GC/MS/MS) are presented. Several methods for derivatization, sample introduction, and ionization, in combination with mass spectrometry/mass spectrometry (MS/MS), have been evaluated for overall ionization efficiency and product-ion sensitivity and specificity. Fragmentation pathways derived from low-energy collision-induced dissociation (CID) spectra of protonated LSD, and the protonated trimethylsllyl derivatives of LSD (LSD-TMS) and deuterium-labeled analogs of LSD, have been proposed. Principal dissociations primarily involve the amide and piperidine-ring moieties in which losses of CH3 radical, CH3NH2, CH3NCH2, diethylamine, diethylformamide, and N,N-diethylpropenamide from MH+ are observed. Positive-ion ammonia chemical ionization and subsequent MS/MS analysis of the protonated molecules (MH+) of the trimethylsilyl (TMS) derivatives of LSD, iso-LSD, and N-demethyl-LSD provide a high degree of specificity for identification of these compounds in urine or blood at low-pg/mL concentrations. Negative-ion chemical ionization and GC/MS/MS analysis of the molecular anion (M-) of the trifluoroacetyl (TFA) derivative is well suited for trace-level identification of N-demethyl-LSD, a metabolite of LSD.     

Analysis of endocrine disruptors, pharmaceuticals, and personal care products in water using liquid chromatography/tandem mass spectrometry

A method has been developed for the trace analysis of 27 compounds from a diverse group of pharmaceuticals, steroids, pesticides, and personal care products. The method employs solid-phase extraction (SPE) and liquid chromatography/tandem mass spectrometry (LC/MS/MS), using electrospray ionization (ESI) in both positive and negative modes and atmospheric pressure chemical ionization in positive mode. Unlike many previous methods, a single SPE procedure using 1 L of water coupled to a simple LC method is used for all ionization modes. Instrument detection limits for most compounds were below 1.0 pg on column with reporting limits of 1.0 ng/L in water. Recoveries for most compounds in deionized water were greater than 80%. Sulfuric acid was found to be the preferred sample preservative, and structures of all MS/MS product ions are proposed. Matrix effects from waters with a high content of treated municipal effluent were observed in both ESI modes and are discussed in the paper.     

Comprehensive gas chromatography-time-of-flight mass spectrometry using soft and selective photoionization techniques

The hyphenation of gas chromatography and mass spectrometry (GC/MS) revolutionized organic analysis. In GC/MS coupling, usually electron impact ionization is applied, and molecules are identified by their fragment pattern. Although mass spectrometry in principle is a separation method, it is used predominantly as a spectrometric technique. However, if soft (i.e., fragmentation-free) ionization techniques are applied, the inherent separation character of MS is emphasized, which has similarities to a GC boiling point separation. By combining polar column GC separation and fast soft ionization time-of-flight mass spectrometry technology, a comprehensive separation of complex petrochemical samples can be obtained (GC x MS approach). Compounds of comparable physical-chemical properties are characteristically grouped together in a two-dimensional retention time-m/z representation. This resembles the separation characteristics of comprehensive two-dimensional gas chromatography (GC x GC) and, thus, represents a novel multidimensional separation approach. In this work, a gas chromatograph equipped with a polar separation column was coupled to a home-built laser ionization time-of-flight mass spectrometer. Laser-based, single-photon ionization was used for universal soft ionization and resonance-enhanced multiphoton ionization for selective ionization of aromatic compounds. A novel capillary-jet inlet system was used for the coupling. Multidimensional comprehensive analysis of complex petrochemical hydrocarbon samples using gas chromatography coupled to mass spectrometry with soft and selective photo ionization sources is first demonstrated.     

Study of the fragmentation mechanism of protonated 6-hydroxychlorzoxazone: application in simultaneous analysis of CYP2E1 activity with major human cytochrome P450s

The application of liquid chromatography tandem mass spectrometry for simultaneous analysis of major human cytochrome P450 activities via a single atmospheric pressure ionization (API) LC/MS/MS method has been hampered by the preferred detection of 6-hydroxychlorzoxazone (HCZ), the metabolite of the CYP2E1 probe, chlorzoxazone, under negative API. An initial simulation of the dissociation constants suggested the potential ionization of the enol form of HCZ at low pH, and the accurate mass measurements confirmed the presence of the protonated HCZ signal under (+) ESI at pH 3. However, the CID spectrum of the protonated HCZ resulted in a few intense, but uncommon, fragment ions that could be utilized for specific selected reaction monitoring (SRM) transitions. The deduced elemental compositions of these fragment ions indicated possible aromatic ring opening for the first two intense product ions at m/z 130 and 115, as well as chlorine radical loss for the third ion at m/z 151. Further precursor and product ion scan studies, along with the deuterium ion exchange in solution, revealed the involvement of three distinct pathways of fragmentation. The m/z 186-->130 transition, which was shown to be specific in human plasma and rat hepatic microsomes, was further combined with the SRM transition of reserpine (internal standard) and eight probe substrates for human cytochrome P450 isoforms. This led to the development of a full LC/MS/MS method capable of analyzing a total of nine human P450 activities within 3 min, including CYP2E1, using a single assay in the (+) ESI mode. The HCZ assay showed excellent linearity with a coefficient of determination (R2) greater than 0.98 at dynamic range of 0.05 (LOQ) to 40 microM. Preliminary data from the three-day validation of the HCZ assay indicated that the accuracy and precision for quality control samples was within +/- 15% of the spiked concentration at all levels.     

Molecular resolution and fragmentation of fulvic acid by electrospray ionization/multistage tandem mass spectrometry

Molecular weight distributions of fulvic acid from the Suwannee River, Georgia, were investigated by electrospray ionization/quadrupole mass spectrometry (ESI/ QMS), and fragmentation pathways of specific fulvic acid masses were investigated by electrospray ionization/ion trap multistage tandem mass spectrometry (ESI/MST/ MS). ESI/QMS studies of the free acid form of low molecular weight poly(carboxylic acid) standards in 75% methanol/25% water mobile phase found that negative ion detection gave the optimum generation of parent ions that can be used for molecular weight determinations. However, experiments with poly(acrylic acid) mixtures and specific high molecular weight standards found multiply charged negative ions that gave a low bias to molecular mass distributions. The number of negative charges on a molecule is dependent on the distance between charges. ESI/MST/MS of model compounds found characteristic water loss from alcohol dehydration and anhydride formation, as well as CO2 loss from decarboxylation, and CO loss from ester structures. Application of these fragmentation pathways to specific masses of fulvic acid isolated and fragmented by ESI/MST/MS is indicative of specific structures that can serve as a basis for future structural confirmation after these hypothesized structures are synthesized.     

Development of LC/MS/MS methods for cocktail dosed Caco-2 samples using atmospheric pressure photoionization and electrospray ionization

Good reliability of Caco-2 permeability studies requires competent sampling and analytical methods to ensure the comparability of day-to-day experiments. In this work, two n-in-one LC/MS/MS methods based on two different ionization techniques were developed and validated for a group of reference compounds; eight of them are recommended by the Food and Drug Administration (FDA) for the evaluation of oral drug permeability. The performance of a new ionization technique, atmospheric pressure photoionization (APPI), as an interface for quantitative LC/MS analysis was evaluated in comparison to the electrospray ionization (ESI). Generally, the validation parameters, including sensitivity, accuracy, and repeatability, were comparable for the APPI and ESI methods. The main difference was that the linear quantitative range of APPI was 3-4 orders of magnitude (r(2) >/= 0.998) whereas in ESI it was typically 2-3 orders of magnitude (r(2) >/= 0.990). By the APPI and ESI methods, the simultaneous analysis of nine highly heterogeneous compounds was achieved within 5.5-7 min, which leads to significant savings in time and cost of the analyses. The successful validation data indicate the usefulness of both the methods for the rapid and sensitive (LOD values typically 相似文献   

Comparison of high-field asymmetric waveform ion mobility spectrometry with GC methods in analysis of haloacetic acids in drinking water

Haloacetic acids (HAAs) are major byproducts of chlorination of drinking water. Electrospray ionization high-field asymmetric waveform ion mobility spectrometry mass spectrometry (ESI-FAIMS-MS) provides a tool for direct monitoring of these compounds. However, treated drinking water samples can be challenging to analyze due to the large number of chemicals present and due to matrix effects that can hinder quantitation of analytes. We developed a standard addition ESI-FAIMS-MS method that permits submicrogram per liter detection of haloacetic acids and overcomes matrix effects. An advantage of FAIMS is increased selectivity through a significant reduction in the chemical background from ESI. Moreover, detection limits with this method are much lower than with previously existing GC and GC/MS methods, and quantitation results compare favorably with other existing methods. This new method does not require sample preparation or chromatographic separation and provides a fast, simple, sensitive, and selective method for monitoring HAAs.     

Electron-transfer reagent anion formation via electrospray ionization and collision-induced dissociation

A strategy is described and demonstrated for the formation of reagent anions via electrospray ionization (ESI) for electron-transfer dissociation (ETD). To circumvent difficulties associated with formation of high mass-to-charge ratio (m/z) reagent anions, it is desirable to form ETD reagents via means other than those that require reagent molecule vaporization. ESI is a candidate method, but anions that are generally generated efficiently by ESI tend to react with multiply protonated polypeptides via proton transfer. The strategy described herein involves the use of a precursor reagent molecule that ionizes efficiently via electrospray ionization and that can subsequently be converted to an ETD reagent via gas-phase dissociation. The approach is demonstrated with arenecarboxylic acids that yield strong signals associated with the deprotonated molecule and that subsequently undergo collision-induced dissociation (CID) by loss of CO(2). In the present work, triply protonated KGAILKGAILR served as a test substrate for the CID product ions to give rise to ETD. Several precursor molecules were shown to be capable of generating ETD reagents via ESI followed by CID. These included 9-anthracenecarboxylic acid, 2-fluoro-5-iodobenzoic acid, and 2-(fluoranthene-8-carbonyl)benzoic acid. The latter molecule has the most attractive set of characteristics as a precursor for a relatively high m/z ratio ETD reagent.     

Molecular weight distributions of heavy aromatic petroleum fractions by Ag+ electrospray ionization mass spectrometry

The ability of electrospray ionization mass spectrometry (ESI MS) to analyze heavy aromatic petroleum fractions using silver nitrate as a reagent compound to form characteristic adduct ions has been examined. The complexation of aromatic compounds containing long alkyl substituents with the silver ion leads to the formation of abundant adduct ions such as [M + Ag]+ and [2M + Ag]+. The concentration of the [2M + Ag]+ ions can be reduced by increasing the sampling cone voltage. Molecular ions and other adduct ions may also be formed depending on the structure of the aromatic molecule. Results obtained from the analysis of representative heavy petroleum fractions and vacuum residues by the Ag+ ESI MS method and conventional ionization methods were in good agreement. The current method extends the applicability of electrospray ionization to the analysis of neutral hydrocarbons in heavy aromatic petroleum fractions. It is simple and compatible with widely available LC/MS instrumentation. The extreme complexity of the Ag     

GC/MS with post-column switching for large volume injection of headspace samples: sensitive determination of volatile organic compounds in human whole blood and urine

When volatile or semivolatile compounds are measured by headspace (HS) gas chromatography (GC)/mass spectrometry (MS), the maximum gas volume to be injected is usually 0.5-1.0 mL; over the volume, the MS detector automatically shuts down due to impairment of the vacuum rate of the MS ionization chamber. To overcome the problem, we modified the gas flow routes of a new type of GC/MS instrument to create a postcolumn switching system, which can eliminate the large volume of gas before introduction of target compounds into the MS ionization chamber. Our HS-GC/MS system enabled injection of as large as 5 mL of HS gas without any disturbance. As the first example analysis, we tried to establish the analysis of naphthalene and p-dichlorobenzene in human whole blood and urine by this method with large volume injection. The limits of detection for both compounds in whole blood and urine were as low as about 10 and 5 pg/mL, respectively. The validation data and actual measurements were also demonstrated. The new GC/MS system has great potential to analyze any type of volatile or semivolatile organic compounds in biological matrixes with very high sensitivity and full automation.     

Identification of weak and strong organic acids in atmospheric aerosols by capillary electrophoresis/mass spectrometry and ultra-high-resolution fourier transform ion cyclotron resonance mass spectrometry

A novel approach using a combination of capillary electrophoresis/mass spectrometry (CE/MS) and off-line Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) revealed the structural details of acidic constituents of atmospheric organic aerosol. Both techniques utilized electrospray ionization (ESI), a soft ionization method, to facilitate the analysis of complex mixtures of organic compounds. CE/ESI-MS using an UltraTrol LN-precoated capillary and acidic background electrolytes at different pH values (2.5 and 4.7) was used to differentiate between weak (carboxylic) and strong (sulfonic) organic acids. On the basis of the electrophoretic mobility, m/z constraints from CE/ESI(-)-MS, and elemental composition information retrieved from off-line FTICR-MS, a variety of aliphatic and aromatic carboxylic acids (CHO-bearing molecules), nitrogen-containing carboxylic acids (CHON-bearing molecules), organosulfates (CHOS-bearing molecules), and (nitrooxy)organosulfates (CHONS-bearing molecules) were tentatively identified in the Oasis-HLB-extracted urban PM(2.5) (particulate matter with an aerodynamic diameter of <2.5 μm). The chemical known/unknown structures of detected compounds were confirmed by the semiempirical Offord model (effective mobility linearly correlated to Z/M(2/3)). The majorities of the identified compounds are products of atmospheric reactions and are known contributors to secondary organic aerosols.     

Development of a proton-transfer reaction-linear ion trap mass spectrometer for quantitative determination of volatile organic compounds

Currently, proton-transfer reaction mass spectrometry (PTR-MS) allows for quantitative determination of volatile organic compounds in real time at concentrations in the low ppt range, but cannot differentiate isomers or isobaric molecules, using the conventional quadrupole mass filter. Here we pursue the application of linear quadrupole ion trap (LIT) mass spectrometry in combination with proton-transfer reaction chemical ionization to provide the advantages of specificity from MS/MS. A commercial PTR-MS platform composed of a quadrupole mass filter with the addition of end cap electrodes enabled the mass filter to operate as a linear ion trap. The rf drive electronics were adapted to enable the application of dipolar excitation to opposing rods, for collision-induced dissociation (CID) of trapped ions. This adaptation enabled ion isolation, ion activation, and mass analysis. The utility of the PTR-LIT was demonstrated by distinguishing between the isomeric isoprene oxidation pair, methyl vinyl ketone (MVK) and methacrolein (MACR). The CID voltage was adjusted to maximize the m/ z 41 to 43 fragment ratio of MACR while still maintaining adequate sensitivity. Linear calibration curves for MVK and MACR fragments at m/ z 41 and 43 were obtained with limits of detection of approximately 100 ppt, which should enable ambient measurements. Finally, the PTR-LIT method was compared to an established GC/MS method by quantifying MVK and MACR production during a smog chamber isoprene-NO x irradiation experiment.     

Qualitative and quantitative analyses of ginkgo terpene trilactones by liquid chromatography/sonic spray ionization ion trap mass spectrometry

A liquid chromatography/sonic spray ionization mass spectrometry method (LC/SSI-MS) was developed for qualitative and quantitative analyses of ginkgo terpene trilactones. Five ginkgo terpene trilactones were successfully protonated for qualitative and quantitative analyses under the study conditions. The typical ion adducts were identified as (M + H)+, (M + NH4)+, and (M + Na)+. The limits of detection were achieved between 2.5 and 10 ng with RSD of 0.173-4.82% and a linear range of 10-80 ng with R2 = 0.991-0.999. This method was used to identify and quantify ginkgo terpene trilactones in extractions of ginkgo biloba leaves obtained from three different extraction methods. This is the first completely validated LC/MS method for quantification of ginkgo terpene trilactones. The factors that contributed to reduce the errors of identification and quantification of ginkgo terpene trilactones are systematically reported, and the advantages and disadvantages of LC/MS method in quantitative analysis are also discussed.     

Analysis of human skin emanations by gas chromatography/mass spectrometry. 2. Identification of volatile compounds that are candidate attractants for the yellow fever mosquito (Aedes aegypti)

Volatile compounds emanated from human skin were studied by gas chromatography/mass spectrometry (GC/MS). The purpose of this study was to identify compounds that may be human-produced kairomones which are used for host location by the mosquito, Aedes aegypti (L.). The procedure used to collect volatiles was chosen because of prior knowledge that attractive substances can be transferred from skin to glass by handling. Laboratory bioassays have shown that the residuum on the glass remains attractive to mosquitoes until the compounds of importance evaporate. The sampling and analytical procedures modeled the above-cited process as closely as possible except that the evaporation of compounds from the glass surface was accomplished by thermal desorption from glass beads in a heated GC injection port. This made possible the solventless injection of volatiles onto the column. The compounds were cryofocused on the head of the column with liquid nitrogen prior to GC separation. A single stage of mass spectrometry on a triple quadrupole instrument was used for mass analysis. A combination of electron ionization and pulsed positive ion/negative ion chemical ionization modes on two different GC columns (one polar, one relatively nonpolar) was used to identify most of the 346 compound peaks detected by this technique.     

Using electrospray-assisted laser desorption/ionization mass spectrometry to characterize organic compounds separated on thin-layer chromatography plates

Electrospray-assisted laser desorption/ionization (ELDI), an ionization method that combines laser desorption and electrospray ionization (ESI), can be used under ambient conditions to characterize organic compounds (including FD&C dyes, amines, extracts of a drug tablet) separated in the central track on a thin-layer chromatography (TLC) plate coated with either reversed-phase C18 particles or normal-phase silica gel. After drying, the TLC plate was placed on an acrylic sample holder set in front of the sampling skimmer of an ion trap mass analyzer. The chemicals at the center of the TLC plate were analyzed by pushing the sample holder into the path of a laser beam with a syringe pump. The molecules in the sample spot were desorbed by continuously irradiating the surface of the TLC plate with a pulsed nitrogen laser. Then, the desorbed sample molecules entered an ESI plume where they were ionized through the reactions with the charged species (including protons, hydronium ions and their cluster ions, solvent ions, and charged droplets) generated by electrospraying a methanol/water solution. MS/MS analyses were also performed to further characterize the analytes. The detection limit of TLC/ELDI/MS is approximately 10(-6) M. This was evaluated by using FD&C red dye as the standard. A linear relationship was found for the calibration curve with the concentration of FD&C red dye ranged from 10(-3) to 10(-6) M.     

Comparison of MALDI-TOF-MS and HPLC-ESI-MS/MS for Endopeptidase Activity-Based Quantification of Anthrax Lethal Factor in Serum

Diagnosing and treating anthrax at the earliest stage of disease is critical. We developed a method to diagnose anthrax at early stages of infection by detecting anthrax lethal factor (LF) at the attomol/mL level in plasma or serum. This method uses antibody capture and quantification of LF endoproteinase activity by isotope dilution matrix-assisted laser-desorption ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS). Many public health laboratories do not use MALDI-TOF-MS; thus, we have adapted the LF method for detection by electrospray ionization (ESI) tandem MS (MS/MS), which allowed comparison of both MS platforms for LF quantification. Calibration curves were linear from 0.05-2.5 ng/mL when measured after 2 h and from 0.005-1.0 ng/mL after 18 h incubation time. The limit of detection was 0.005 ng/mL using a 200 μL sample. The coefficient of variation for quality control samples was 6-12% for both MS platforms. Samples used to perform cross-validation included 158 serum samples from a study in rabbits exposed to anthrax spores by inhalation. Some were treated with anthrax immune globulin before exposure. Concentrations measured by ESI-MS/MS matched those by MALDI-TOF-MS with p = 0.99 (r(2) = 0.997) and -0.25% mean relative difference (±9% standard deviation). This study shows that isotope dilution MALDI-TOF-MS is a robust and precise quantitative MS platform.     

Design and characterization of a multisource hand-held tandem mass spectrometer

A wireless-controlled miniature rectilinear ion trap mass spectrometer system, total weight with batteries 5.0 kg, consuming less than 35 W of power, and having dimensions of 22 cm in length by 12 cm in width by 18 cm in height, is characterized. The design and construction of the mass spectrometer including mass analyzer, vacuum system, electronics system, and data acquisition and processing systems, is detailed. The mass spectrometer is compatible with various types of ionization sources including a glow discharge electron impact ionization source used in the internal ionization mode, and various atmospheric pressure ionization sources, including electrospray ionization, atmospheric pressure chemical ionization, and desorption electrospray ionization, which are employed for external, atmospheric pressure ionization. These external sources are coupled to the miniature mass spectrometer via a capillary interface that is operated in a discontinuous fashion (discontinuous atmospheric pressure interface) to maximize ion transport. The performance of the mass spectrometer for large and small molecules is characterized. Limits of detection in the parts-per-billion range were obtained for selected compounds examined using both the internal ionization and external ionization modes. Tandem mass spectrometry and fast in situ analysis capabilities are also demonstrated using a variety of compounds and ionization sources. Protein molecules are analyzed as the multiply protonated molecules with mass/charge ratios up to 1500 Da/charge.