Valved sampling cell for membrane introduction mass spectrometry

Membrane extractors comprising a membrane house inside of a valve have been developed to separate compounds of interest from a sample matrix and introduce these compounds into a mass spectrometer. Experimental control over parameters that affect permeability or that may damage the membrane, such as the membrane temperature, is provided with the valve. The valve was tested for response and response times with the valve separated from the mass spectrometer by various interface tube lengths. Data for steady state response measurements showed no significant change with the valve at different distances from the ion source. Polar compounds show a strong response time dependency on the interface tube length. This adsorption phenomenon is minimized by simply heating the interface tube. Other factors affecting the performance of the device are discussed.     

Experimental upgrades of membrane introduction mass spectrometry for water and air analysis

Some improvements to the membrane introduction mass spectrometry (MIMS) technique, resulting in low-ppt detection limits for volatile organohalogen compounds (CX) in water (namely, chloroform, bromoform, bromodichloromethane, chlorodibromomethane, tetrachloroethylene, trichloroethylene, 1,1,1-trichloroethane, and carbon tetrachloride) and low-microgram per cubic meter detection limits for benzene, toluene, ethylbenzene, and xylenes (BTEX) in gaseous samples, are shown. A static MIMS configuration was compared to a dynamic one, the former requiring longer time to obtain the analytical response. A cryotrapping preconcentration step is introduced and linearity of response, mixture effects, and detection limits are presented. The instrumental setup consists of a hollow fiber silicone membrane, a water or air container, a cryofocusing trap based on Tenax adsorbent, a Peltier cell, and a Varian ion trap benchtop mass spectrometer is described. This instrumental setup, which we named membrane extraction trap focusing mass spectrometry, allowed the detection of CX in water at a concentration as low as 8 ppt and of benzene in air at 0.1 microg/m3. The whole assembly shows great potential for on-site routine monitoring of drinking water resources and urban and indoor air under current EU and Italian regulations.     

Laser desorption-membrane introduction mass spectrometry

In this paper we describe the first use of laser desorption in conjunction with membrane introduction mass spectrometry (MIMS). In this technique, a low-powered carbon dioxide laser is used to irradiate the low-pressure (vacuum) side of a silicone membrane during a typical MIMS analysis of an aqueous solution. The absorption of laser energy results in direct membrane heating and rapid desorption of permeate molecules. This improves both the sensitivity and response times of MIMS when analyzing compounds having high molecular weight and low volatility. Two simple interfaces are described for performing laser desorption inside and outside the vacuum manifold of a GCQ ion trap mass spectrometer. Together with flow injection (FI) sample introduction, we demonstrate direct on-line monitoring of aqueous solutions of high boiling point (200-530 °C) polynuclear aromatic hydrocarbons such as naphthalene, anthracene, pyrene, benzo[b]fluoranthene, and indeno[123-cd]pyrene.     

Accurate mass multiplexed tandem mass spectrometry for high-throughput polypeptide identification from mixtures

We report a new tandem mass spectrometric approach for the improved identification of polypeptides from mixtures (e.g., using genomic databases). The approach involves the dissociation of several species simultaneously in a single experiment and provides both increased speed and sensitivity. The data analysis makes use of the known fragmentation pathways for polypeptides and highly accurate mass measurements for both the set of parent polypeptides and their fragments. The accurate mass information makes it possible to attribute most fragments to a specific parent species. We provide an initial demonstration of this multiplexed tandem MS approach using an FTICR mass spectrometer with a mixture of seven polypeptides dissociated using infrared irradiation from a CO2 laser. The peptides were added to, and then successfully identified from, the largest genomic database yet available (C. elegans), which is equivalent in complexity to that for a specific differentiated mammalian cell type. Additionally, since only a few enzymatic fragments are necessary to unambiguously identify a protein from an appropriate database, it is anticipated that the multiplexed MS/MS method will allow the more rapid identification of complex protein mixtures with on-line separation of their enzymatically produced polypeptides.     

Analysis of polar organic compounds using charge exchange ionization and membrane introduction mass spectrometry

Charge exchange ionization in conjunction with membrane introduction mass spectrometry provides a sensitive method for the detection of polar volatile organic compounds and semivolatile compounds in air. Sample introduction into an ion trap mass spectrometer was accomplished with a hollow fiber silicone membrane assembly. Atmospheric oxygen, which diffuses through the membrane, was used as the charge exchange reagent. Chemical ionization parameters were optimized using methyl ethyl ketone (2-butanone) standards in air. Several other oxygen-containing compounds, including acetone (2-propanone), methyl isobutyl ketone (4-methyl-2-pentanone), propanal, isopropyl alcohol (2-propanol), cyclohexanol, dimethyl sulfoxide (sulfinylbismethane), 2-(diethylamino)ethanol, and dimethyl methylphosphonate were analyzed with this technique. This method was used to obtain mass spectra for a variety of classes of compounds and produced a 4-20-fold improvement in response for all of the polar compounds we examined when compared to signal obtained from electron ionization.     

Real-time analysis of methanol in air and water by membrane introduction mass spectrometry.

We present results for the near-real-time, on-line detection of methanol in both air and water using membrane introduction mass spectrometry (MIMS). In these experiments, we compare the sensitivity of a poly(dimethylsiloxane) (PDMS) membrane and an allyl alcohol (AA) membrane to the detection of methanol. In MIMS, the membrane serves as the interface between the sample and the vacuum of the mass spectrometer. Membrane-diffused water was used as the reagent ion (H3O+) for chemical ionization of methanol in an ion trap mass spectrometer. Linear calibration curves have been obtained for methanol using both PDMS and AA membranes. For PDMS, detection limits of methanol are 14 ppmv and 5 ppm in air and water, respectively. For AA, detection limits are 3.3 ppmv and 2 ppm in air and water, respectively. We demonstrate that the sensitivity of the analysis can be altered by the chemistry of the membrane. When the AA membrane is used, the sensitivity of MIMS is enhanced over that of PDMS by a factor of 8.5 for methanol in air and by a factor of 23.4 for methanol in water.     

Packed column supercritical fluid chromatography/mass spectrometry for high-throughput analysis

A supercritical fluid chromatograph was interfaced to a mass spectrometer, and the system was evaluated for applications requiring high sample throughput. Experiments presented demonstrate the high-speed separation capability of supercritical fluid chromatography (SFC) and the effectiveness of supercritical fluid chromatography/mass spectrometry (SFC/MS) for fast, accurate determinations of multicomponent mixtures. A high-throughput liquid chromatography/mass spectrometry (LC/MS) analysis cycle time is reduced 3-fold using our general SFC/MS high-throughput method, resulting in substantial time saving for large numbers of samples. Unknown mixture characterization is improved due to the increased selectivity of SFC/MS compared to LC/MS. This was demonstrated with sample mixtures from a 96-well combinatorial library plate. In this paper, we report a negative mode atmospheric pressure chemical ionization (APCI) method for SFC/MS suitable for most of the components in library production mixtures. Flow injection analysis (FIA) also benefits from this SFC/MS system. A broader range of solvents is amenable to the SFC mobile phase compared with standard LC/MS solvents, and solutes elute more rapidly from the SFC/MS system, reducing sample carryover and cycle time. Finally, our instrumental setup allows for facile conversion between LC/MS and SFC/MS modes of operation.     

Versatile platform employing desorption electrospray ionization mass spectrometry for high-throughput analysis

A simple and easy-to-build high-throughput analysis system was constructed. The system consisted of three major components: (1) a multichannel device with 16 parallel capillaries, (2) a desorption electrospray ionization (DESI) source, and (3) a linear ion trap mass spectrometer. When analyses were performed, the multichannel device was moved horizontally on a translation stage controlled by a step motor. Our design expands the functions of DESI, in which the liquid sample in capillary was driven out by the nebulizing gas, ionized, and then transferred to a mass spectrometer. To assess the high-throughput performance of the system, 5 mg/L 1,3-diethyl-1,3-diphenylurea (DDU) solution and 10 mg/L angiotensin I solution were alternatively loaded into the reservoirs and capillaries in the multichannel device. Results indicated that analyses of the all the samples in 16 capillaries were completed within 1.6 min, which means a throughput of 600 samples/h. Reactive DESI experiment was also successfully performed with this system to show the feasibility of online derivatization. The relative standard deviations for a single capillary and five identical capillaries were 7.6 (n = 16) and 12.3%, respectively. Linear relative abundance response was achieved for DDU (r = 0.9971).     

Headspace membrane introduction mass spectrometry for trace level analysis of VOCs in soil and other solid matrixes

A new MIMS-derived technique, headspace membrane introduction mass spectrometry (HS-MIMS), is described for direct trace level analysis of volatile organic compounds (VOCs) in soil and other dry or wet solid matrixes. A silicone membrane interface is placed about 15 cm from the ion source, and a closed airspace (headspace) is created by connecting a toggle valve to the 1/4 in. tubing that connects the membrane interface to the ion source. For the VOC analysis, the headspace is evacuated and the solid sample vessel is heated to 90 degrees C. The VOCs are rapidly desorbed from the sample, pervaporated through the membrane, and preconcentrated for 4 min in the evacuated headspace. Then, the toggle valve is opened and the trapped VOCs are released into the ion source region of a quadrupole mass spectrometer. By electron ionization and selected-ion monitoring, a relatively sharp and intense peak is obtained and used for quantification. The HS-MIMS analysis shows excellent linearity and reproducibility and detection limits for many VOCs typically of 50-100 ng/kg (ppt).     

Fiber introduction mass spectrometry: fully direct coupling of solid-phase microextraction with mass spectrometry

This work describes the first fully direct coupling of solid-phase microextraction (SPME) with mass spectrometry. An inlet system using a septum as the only interface between the ambient and the high-vacuum mass spectrometer was constructed to allow the introduction of the SPME needle directly into the ionization region of a mass spectrometer. The PDMS-coated fiber was then placed and exposed exactly between the two ionization filaments. Uniform heating of the fiber, efficient thermal desorption, and electron ionization of the analytes were achieved. Using this new analytical technique, here termed fiber introduction mass spectrometry (FIMS), we have been able to detect and quantitate several volatile (VOC) and semivolatile (SVOC) organic chemicals (carbon tetrachloride, benzene, toluene, xylenes, gamma-terpinene, diisoamyl ether, chlorobenzene, and many PAHs) and two herbicides (Sylvex and its methyl ether) from aqueous solutions at low-ppb to ppt levels using either SPME headspace or solution extraction. FIMS shows high sensitivity (ng/L), good reproducibility, and accuracy, providing therefore a simple and effective approach to rapid analysis of VOC and SVOC in various matrixes.     

Radio frequency glow discharge mass spectrometry for the characterization of bulk polymers

A radio frequency (rf)-powered glow discharge (GD) atomization/ionization source is utilized to determine the applicability of the technique for direct polymer analysis. A series of PTFE-based polymers are studied to assess their fingerprint mass spectra and to distinguish each sample by its differing base peaks and relative peak intensities. A parametric study with respect to discharge gas pressure and rf power is conducted to evaluate their respective roles in the sputtering process as well as possible ionization mechanisms. The results of the GD sputtering processes are examined by scanning electron micrographs of a sputtered PTFE surface. Excellent discharge stabilization characteristics (<3 min) were observed in temporal response curves. Internal stability with respect to signal intensity is observed to be <5% RSD for samples of different thicknesses. Finally, the ability to obtain depth profiles of layered samples was demonstrated for the case of a Cu layer deposited on a PTFE substrate.     

Development of multichannel devices with an array of electrospray tips for high-throughput mass spectrometry

The basic principles of multichannel devices with an array of electrospray tips for high-throughput infusion electrospray ionization mass spectrometry (ESI-MS) have been developed. The prototype plastic devices were fabricated by casting from a solvent-resistant resin. The sample wells on the device were arranged in the format of the standard 96-microtiter well plate, with each sample well connected to an independent electrospray exit port via a microchannel with imbedded electrode. A second plastic plate with distribution microchannels was employed as a cover plate and pressure distributor. Nitrogen gas was used to pressurize individual wells for transport of sample into the electrospray exit port. The design of independent microchannels and electrospray exit ports allowed very high throughput and duty cycle, as well as elimination of any potential sample carryover. The device was placed on a computer-controlled translation stage for precise positioning of the electrospray exit ports in front of the mass spectrometer sampling orifice. High-throughput ESI-MS was demonstrated by analyzing 96 peptide samples in 480 s, corresponding to a potential throughput of 720 samples/h. As a model application, the device was used for the MS determination of inhibition constants of several inhibitors of HIV-1 protease.     

Sample prefractionation for mass spectrometry quantification of low-abundance membrane proteins

Use of stable isotope-labeled full-length proteins as an internal standard prior to multiple reaction monitoring (MRM) analysis enables prefractionation of the target proteins and quantification of those low-abundance proteins, which cannot be reached without biological sample enrichment. In terms of membrane proteins, this benefit can be used if a sample processing workflow allows entire solubilization of membrane proteins. We have developed a universal workflow for sample processing and enrichment by optimizing washing and solubilization conditions and implementing sample fractionation by Whole Gel Eluter. The optimized protocol was applied to various membrane-bound cytochromes P450 (CYPs) and their electron transferring protein partners, cytochrome P450 reductase (CPR), ferredoxin reductase (FdR), and ferredoxin (Fdx), all important proteins for cholesterol elimination from different organs. Both, weakly associated (CPR and FdR) and tightly associated (CYP7B1, CYP11A1, CYP27A1, and CYP46A1) membrane proteins were quantified. Measurements were performed on three human tissues (temporal lobe of the brain, retina, and retinal pigment epithelium) obtained from multiple donors. The biological implications of our quantitative measurements are also discussed.     

Oil-in-water monitoring using membrane inlet mass spectrometry

A membrane inlet mass spectrometry (MIMS) system has been used for detection and analysis of two types of North Sea crude oil. The system was installed on-field on the Flotta Oil Terminal (Orkney, UK). It consisted of a quadrupole mass spectrometer (QMS) connected to the capillary probe with a silicone-based membrane. The produced mass spectra and calibration plots from the MIMS instrument showed the capability to measure levels of individual hydrocarbons within crude oil in seawater. The generated mass spectra from the field tests also showed the ability to distinguish between different types of oil and to determine concentrations of toxic hydrocarbons in oil (e.g., benzene, toluene, and xylene (BTX)). The performance of the instrument at different temperatures of seawater and oil droplet sizes was also investigated. The results showed that the QMS-based MIMS system has a potential to complement existing oil-in-water (OiW) monitors by being able to detect different oil types and specific hydrocarbon concentrations with high accuracy, which are currently not supported in commercially available OiW monitors.     

Packed column supercritical fluid chromatography/mass spectrometry for high-throughput analysis. Part 2

A supercritical fluid chromatograph was previously interfaced to a mass spectrometer (SFC/MS) and the system evaluated for applications requiring high sample throughput using negative-mode atmospheric-pressure chemical ionization (APCI) (Ventura et al. Anal. Chem. 1999, 71, 2410-2416). This report extends the previous work demonstrating the effectiveness of SFC/MS, using positive ion APCI for the analysis of compounds with a wide range of polarities. Substituting SFC/MS for LC/MS results in substantial time saving, increased chromatographic efficiency, and more precise quantitation of sample mixtures. Flow injection analysis (FIA) also benefits from our SFC/MS system. A broader range of solvents is compatible with the SFC mobile phase compared with LC/MS, and solutes elute more rapidly from the SFC/MS system, reducing sample carryover and cycle time. Our instrumental setup also allows for facile conversion between LC/MS and SFC/MS modes of operation.     

Desorption electrospray ionization mass spectrometry for high-throughput analysis of pharmaceutical samples in the ambient environment

Desorption electrospray ionization (DESI) allows mass spectrometry to be used for on-line high-throughput monitoring of pharmaceutical samples in the ambient environment, without prior sample preparation. Positive and negative ion DESI are used to characterize the active ingredients in pharmaceutical samples formulated as tablets, ointments, and liquids. Compounds of a wide variety of chemical types are detected in these complex matrices. The effects on analytical performance of operating parameters, including the electrospray high voltage, heated capillary temperature, solvent infusion rate, and solvent composition, are evaluated and optimized. In addition to experiments in which a simple solvent is sprayed onto the solid analyte samples, reactive desorption is performed by adding reagents to the solvent spray to generate particularly stable or characteristic ions with the analytes of interest. A variable-speed moving belt was built for high-throughput sampling and used to provide rapid qualitative and semiquantitative information on drug constituents in tablets. Sampling rates as high as 3 samples/s are achieved in the ambient environment. Relative standard deviations of the relative ion abundances for major components in the mass spectra are in the range of 2-8%. Impurities and components present at levels as low as approximately 0.1% are identified and carryover effects are minimized in high-throughput on-line analysis of pharmaceutical samples.     

MALDI-TOF mass spectrometry analysis of amphipol-trapped membrane proteins

Amphipols (APols) are amphipathic polymers with the ability to substitute detergents to keep membrane proteins (MPs) soluble and functional in aqueous solutions. APols also protect MPs against denaturation. Here, we have examined the ability of APol-trapped MPs to be analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). For that purpose, we have used ionic and nonionic APols and as model proteins (i) the transmembrane domain of Escherichia coli outer membrane protein A, a β-barrel, eubacterial MP, (ii) Halobacterium salinarum bacteriorhodopsin, an α-helical archaebacterial MP with a single cofactor, and (iii, iv) two eukaryotic MP complexes comprising multiple subunits and many cofactors, cytochrome b(6)f from the chloroplast of the green alga Chlamydomonas reinhardtii and cytochrome bc(1) from beef heart mitochondria. We show that these MP/APol complexes can be readily analyzed by MALDI-TOF-MS; most of the subunits and some lipids and cofactors were identified. APols alone, even ionic ones, had no deleterious effects on MS signals and were not detected in mass spectra. Thus, the combination of MP stabilization by APols and MS analyses provides an interesting new approach to investigating supramolecular interactions in biological membranes.     

Interfacial engineering of heterogeneous catalysts for electrocatalysis

Electrocatalysis is recognized as a practical process for chemical energy conversion, which has attracted considerable research efforts in the design and development of high-performing electrocatalysts in recent decades. The interface formed between two or more components in heterogeneous catalyst plays a critical role for electrocatalysis as it can tune the electron structure to enhance the catalytic performance. Although there are some reviews focusing on the progress about the interfacial engineering for electrocatalysis, a comprehensive discussion on the working mechanism of interface structures from design and identification is still lacking in this field. In this review, the recent advancement of interfacial engineering in heterogeneous catalyst for electrocatalytic application is reviewed. We start with introducing theoretical basics relay on electronic information of the interface structure and focusing on the interaction between the interface structure and reactant. Then, the most widely employed strategies for interface structure construction are summarized. Subsequently, the latest advanced techniques, involving ex situ and in situ approaches, for interface structure characterization and identification in electrocatalysis applications are discussed. Finally, some perspectives and challenges on materials design and research about interfacial engineering for electrocatalysis are represented.     

Evaluation of a high-throughput online solid phase extraction-tandem mass spectrometry system for in vivo bioanalytical studies

High throughput-solid phase extraction tandem mass spectrometry (HT-SPE/MS) is a fully automated system that integrates sample preparation using ultrafast online solid phase extraction (SPE) with mass spectrometry detection. HT-SPE/MS is capable of conducting analysis at a speed of 5-10 s per sample, which is several fold faster than chromatographically based liquid chromatography-mass spectrometry (LC-MS). Its existing applications mostly involve in vitro studies such as high-throughput therapeutic target screening, CYP450 inhibition, and transporter evaluations. In the current work, the feasibility of utilizing HT-SPE/MS for analysis of in vivo preclinical and clinical samples was evaluated for the first time. Critical bioanalytical parameters, such as ionization suppression and carry-over, were systematically investigated for structurally diverse compounds using generic SPE operating conditions. Quantitation data obtained from HT-SPE/MS was compared with those from LC-MS analysis to evaluate its performance. Ionization suppression was prevalent for the test compounds, but it could be effectively managed by using a stable isotope labeled internal standard (IS). A structural analogue IS also generated data comparable to the LC-MS system for a test compound, indicating matrix effects were also compensated for to some extent. Carry-over was found to be minimal for some compounds and variable for others and could generally be overcome by inserting matrix blanks without sacrificing assay efficiency due to the ultrafast analysis speed. Quantitation data for test compounds obtained from HT-SPE/MS were found to correlate well with those from conventional LC-MS. Comparable accuracy, precision, linearity, and sensitivity were achieved with analysis speeds 20-30-fold higher. The presence of a stable metabolite in the samples showed no impact on parent quantitation for a test compound. In comparison, labile metabolites could potentially cause overestimation of the parent concentration if the ion source conditions are not optimized to minimize in-source breakdown. However, with the use of conditions that minimized in-source conversion, accurate measurement of the parent was achieved. Overall, HT-SPE/MS exhibited significant potential for high-throughput in vivo bioanalysis.     

Preliminary investigation of electrothermal vaporization sample introduction for inductively coupled plasma time-of-flight mass spectrometry

The coupling of an electrothermal vaporization (ETV) apparatus to an inductively coupled plasma time-of-flight mass spectrometer (ICP-TOFMS) is described. The ability of the ICP-TOFMS to produce complete elemental mass spectra at high repetition rates is experimentally demonstrated. A signal-averaging data acquisition board is employed to rapidly record complete elemental spectra throughout the vaporization stage of the ETV temperature cycle; a solution containing 34 elements is analyzed. The reduction of both molecular and atomic isobaric interferences through the temperature program of the furnace is demonstrated. Isobaric overlaps among the isotopes of cadmium, tin, and indium are resolved by exploiting differences in the vaporization characteristics of the elements. Figures of merit for the system are defined with several different data acquisition schemes capable of operating at the high repetition rate of the TOF instrument. With the use of both ion counting and a boxcar averager, the dynamic range is shown to be linear over a range of at least 6 orders of magnitude. A pair of boxcar averagers are used to measure the isotope ratio for silver with a precision of 1.9% RSD, despite a cycle-to-cycle precision of 19% RSD. Detection limits of 10-80 fg are calculated for seven elements, based upon a 10-microL injection.