Membrane introduction mass spectrometry

A Laser-Activated Membrane Introduction Mass Spectrometry Study of Proton Spillover Promoted Alkane Dehydrogenation

Operando high-throughput evaluation of heterogeneous catalysts by laser-activated membrane introduction mass spectrometry (LAMIMS) elucidates the Pt loading dependence of methylcyclohexane dehydrogenation on platinized γ-alumina beads. A CO₂ marking laser rapidly and sequentially heats catalyst beads positioned on a heat-dissipating carbon paper support that overlays a silicone membrane, separating the bead library reaction zone from a quadrupole mass analyzer. The toluene m/z peak varies logarithmically with Pt loading, suggesting that reactivity includes factors that are negatively correlated to Pt loading. These factors may include the Pt/γ-Al₂O₃ surface interfacial region as one component of a heterogeneous catalytically active surface area/mass. This work demonstrates LAMIMS as a broadly applicable high-throughput operando screening method for heterogeneous catalysts.

Toward high pressure miniature protein mass spectrometer: Theory and initial results

Current miniature mass spectrometers mainly focus on the analyses of organic and small biological molecules. In this study, we explored the possibility of developing high resolution miniature ion trap mass spectrometers for whole protein analysis. Theoretical derivation, GPU assisted ion trajectory simulation, and initial experiments on home-developed "brick" mass spectrometer were carried out. Results show that ion-neutral collisions have smaller damping effect on large protein ions, and a higher buffer gas pressure should be applied during ion trap operations for protein ions. As a result, higher pressure ion trap operation not only benefits instrument miniaturization, but also improves mass resolution of protein ions. Dynamic mass scan rate and generation of low charge state protein ions are also found to be helpful in terms of improving mass resolutions. Theory and conclusions found in this work are also applicable in the development of benchtop mass spectrometers.

Direct quantitation and characterization of fatty acids in salmon tissue by condensed phase membrane introduction mass spectrometry (CP-MIMS) using a modified donor phase

Existing mass spectrometric methods for the analysis of fatty acids often require derivatization, chromatographic separations, and/or extensive sample preparation. Direct mass spectrometry strategies can avoid these requirements, but may also suffer from poor quantitation and/or lack of sensitivity. Condensed phase-membrane introduction mass spectrometry (CP-MIMS) provides direct quantitative measurements of analytes in complex samples with little or no sample preparation. CP-MIMS uses a semipermeable membrane to transfer neutral, hydrophobic compounds from real-world samples to a mass spectrometer. The results presented utilize aqueous/organic sample solvent (donor) mixtures to allow for the sensitive (pptr) detection of a range of fatty acids. The relative sensitivity across a homologous series of fatty acids is observed to change, favoring short- or long-chain fatty acids, depending on the amount of miscible co-solvent added to the donor phase. Further, lithium acetate added online via the acceptor phase was used in tandem mass spectrometry experiments to determine the location of double bonds in polyunsaturated fatty acids (PUFAs). The method was applied to direct measurements and structural determinations for selected PUFAs in salmon tissue samples. Standard addition was employed to quantify the amount of PUFAs in a variety of salmon samples, yielding 0.27-0.42 and 0.40-0.84 w/w % for eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), respectively, for Sockeye and Chinook salmon, in good agreement with the literature. This work presents, to our knowledge, the first use of CP-MIMS for the direct analysis of fatty acids in oily foodstuff samples. Graphical abstract ᅟ.

Direct Analysis of Aerosolized Chemical Warfare Simulants Captured on a Modified Glass-Based Substrate by "Paper-Spray" Ionization

Paper spray ionization mass spectrometry offers a rapid alternative platform requiring no sample preparation. Aerosolized chemical warfare agent (CWA) simulants trimethyl phosphate, dimethyl methylphosphonate, and diisopropyl methylphosphonate were captured by passing air through a glass fiber filter disk within a disposable paper spray cartridge. CWA simulants were aerosolized at varying concentrations using an in-house built aerosol chamber. A custom 3D-printed holder was designed and built to facilitate the aerosol capture onto the paper spray cartridges. The air flow through each of the collection devices was maintained equally to ensure the same volume of air sampled across methods. Each approach yielded linear calibration curves with R² values between 0.98-0.99 for each compound and similar limits of detection in terms of disbursed aerosol concentration. While the glass fiber filter disk has a higher capture efficiency (≈40%), the paper spray method produces analogous results even with a lower capture efficiency (≈1%). Improvements were made to include glass fiber filters as the substrate within the paper spray cartridge consumable. Glass fiber filters were then treated with ammonium sulfate to decrease chemical interaction with the simulants. This allowed for improved direct aerosol capture efficiency (>40%). Ultimately, the limits of detection were reduced to levels comparable to current worker population limits of 1 × 10^-6 mg/m³.

Development of multi-membrane near-infrared diode mass spectrometer for field analysis of aromatic hydrocarbons

Membrane Inlet Mass Spectrometry (MIMS) is a technique that incorporates a semi-permeable membrane selective for differing organic molecules and chemistries. This eliminates the need for time-consuming sample preparation and facilitates near instantaneous analysis. This study will examine how the front end of MIMS incorporates three dual inlet ports, allowing for differing MIMS materials and selectivity for specific environments. Polydimethylsiloxane (PDMS) membranes have proven to be selective of benzene, toluene, and xylene (BTX) as well as aromatic hydrocarbons that are common in petroleum products while remaining selective against the aliphatic chains. PDMS has proven to be a successful choice of membrane with high permeability in atmospheric environments. In addition, polycyclic aromatic hydrocarbons (PAHs) such as acenaphthene, acenapthylene, naphthalene, and fluorene have recently been detected to the 5 ppb level in a nitrogen atmosphere with our current configuration. This preliminary work provides proof of concept using near-infrared laser diodes that act upon the membrane to increase its permeability and provide higher sensitivity of aromatic samples.

Detection of nitroxyl (HNO) by membrane inlet mass spectrometry

Membrane inlet (or introduction) mass spectrometry (MIMS) was used to detect nitroxyl (HNO) in aqueous solution for the first time. The common HNO donors Angeli's salt (AS) and Piloty's acid (PA), along with a newly developed donor, 2-bromo-N-hydroxybenzenesulfonamide (2-bromo-Piloty's acid, 2BrPA), were examined by this technique. MIMS experiments revealed that under physiological conditions 2BrPA is an essentially pure HNO donor, but AS produces a small amount of nitric oxide (NO). In addition, MIMS experiments also confirmed that PA is susceptible to oxidation and NO production, but that 2BrPA is not as prone to oxidation.

Natural and artificial markers of gasoline detected by membrane introduction mass spectrometry

A few hydrocarbons in gasoline display relatively high solubility in water and may function therefore as its characteristic set of natural markers. These markers are detected from an aqueous gasoline extract via membrane introduction mass spectrometry (MIMS) producing characteristic chemical profiles. MIMS adds a second selectivity criterion detecting only the water soluble hydrocarbons that most easily permeate through a silicone membrane. MIMS screening and the use of artificial markers for gasoline with similar chemical properties (high water solubility and membrane permeability) as those of its natural markers is proposed. MIMS provides a reliable screening method for natural and artificial markers in gasoline for its typification and to monitor adulteration and origin.

Use of Nafion as a membrane separator in membrane introduction of mass spectrometry

Nafion is a commercially available perfluorosulphonate cation exchange membrane commonly used as a perm-selective separator in chlor-alkali electrolysers and as the electrolyte in solid polymer fuel cells. In our experiments, a Nafion sheet membrane serves as the interface between the aqueous sample and the vacuum in membrane introduction to the mass spectrometer (MIMS). The penetration by volatile polar compounds (VOC-methanol, ethanol, 1-propanol), volatile non-polar compounds (VOC-benzene, toluene and p- xylene), semi-volatile low polar compounds (SVOC-fluorobenzene, chlorobenzene and bromobenzene) and non-volatile polar compounds (o-chlorophenol, m-chlorophenol and p-chlorophenol) in aqueous solution through the Nafion membrane to the mass spectrometer was studied. In all cases, a simple fragmentation pattern of the intact molecule was observed, typically with m/z = nominal mass + 1 as the most intensive ion current, which suggests that the ionisation process takes part in which water acts as the chemical ionisation reagent. No additional gases were needed for chemical ionisation. We also measured detection limits and linear dynamic ranges of all observed compounds with Nafion membrane MIMS. The observed detection limits were in the order of ppb for the alcohol and aromatic groups and for the halogenbenzene and monochlorophenol groups they were in the order of ppm. Linear dynamic ranges for all tested compounds were one order of magnitude.

Quantitation of trace phenolic compounds in water by trap-and-release membrane introduction mass spectrometry after acetylation

Trap-and-release membrane introduction mass spectrometry (T&R-MIMS) with a removable direct insertion membrane probe (DIMP) is used to quantitate a variety of trace phenolic compounds in water after acetylation. The procedure is simple, rapid and robust, producing linear and reproducible responses for phenolic compounds with varying polarities. Acetylation minimizes the polarity effects of ring substituents; hence, T&R-MIMS of the acetylated phenols provides lower and more uniform limits of detection (LODs) (2-15 microg L(-1)) than those obtained by direct T&R-MIMS analysis of the non-derivatized phenols.

Membrane introduction mass spectrometry for monitoring complexation equilibria of beta-cyclodextrin with substituted benzenes

Membrane introduction mass spectrometry (MIMS) was used to monitor complexation reactions between beta-cyclodextrin (CD) and a series of benzene derivatives in aqueous solution. The equilibrium constants for benzene, chlorobenzene, bromobenzene, iodobenzene, toluene, cyanobenzene and nitrobenzene were determined. The suitability of MIMS for monitoring complexation reactions of organic compounds with host molecules was demonstrated. Structure-activity relationship analysis shows that the inclusion phenomena are driven by a variety of chemical forces, of which hydrophobicity is predominant for non-polar compounds, but not the only factor for more polar ones.

Direct mass spectrometric measurement of gases in soil monoliths

An integrated approach to gas analysis in soil cores was conducted to provide a novel method for observing the gas dynamics associated with upland soil ecosystems. Depth profiles of the O(2), Ar, CO(2), CH(4), N(2) and NO(x) concentrations in intact soil monoliths were obtained simultaneously using membrane inlet mass spectrometry (MIMS). This technique enables the direct measurement of multiple gas species throughout the soil core with minimal disturbance. Depth profiles provided data on the vertical heterogeneity of gas concentrations, while horizontal heterogeneity was monitored by comparison between profiles. Detailed descriptions of the modifications to current MIMS methods for in situ environmental monitoring of terrestrial soils are provided. These included a thorough examination of calibration of the MIMS probe in gas phase, stirred and unstirred H(2)O, or between glass beads immersed in H(2)O. Calibration was also carried out in sterile (autoclaved) soil. The mean concentrations of CO(2) and CH(4) in the soil monoliths increased from 27 microM and undetectable levels respectively at the surface, to maximum values of 3.6 mM and 4.3 microM at 12-cm depth. These changes corresponded with decreases in mean O(2), Ar and N(2) concentration from 300, 20 and 720 microM respectively to 0-6, 10 and 574 microM at 12-cm depth. These data indicated the presence of a gradient within the core from an aerobic environment to an O(2)-depleted, but not in all cases a completely anaerobic, one. This transition corresponded, to some extent, with that between the upper and lower soil horizons. The increased methane and CO(2) concentrations observed at depth are indicative of anaerobic environments. General trends associated with the gradually changing vertical heterogeneity of these gas profiles and the transition towards anaerobiosis did not provide evidence for the existence of localised microsites. Some evidence for microsite-specific microbial communities was however, provided by observation of broad zones of accumulation of NO(x) species, but only at concentrations close to the limit of detection of the method. The ratio of each gas, to argon was calculated at each depth. This was done to correct for physical parameters, which influence inert and biologically active gases, equally. The amount of di-nitrogen as a ratio to Ar was seen to increase with depth. This could be evidence for denitrification in the lower horizon. An example of the dynamic 'online' data collection capabilities is provided for diurnal oscillations in subsurface (5 cm) soil gas concentrations.

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.

Selective trace level analysis of phenolic compounds in water by flow injection analysis--membrane introduction mass spectrometry

Flow injection analysis coupled with membrane introduction mass spectrometry (FIA-MIMS) with on-line derivatization is shown to allow fast, accurate, nearly interference-free, and sensitive (low microgram/L) quantitation of phenolic compounds in water. On-line FIA derivatization of the phenolic compounds is performed by acetic anhydride acetylation in a K2CO3-buffered alkaline medium. The phenol acetates so formed efficiently permeate a silicone membrane and are directly transferred to the mass spectrometer, in which they are analyzed with selectivity and high sensitivity via selected ion monitoring. FIA-MIMS analysis was performed for aqueous solutions of phenol, 2-methylphenol, 4-chlorophenol, 4-chloro-3-methylphenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol, and detection limits in the 0.5-20 micrograms/L (ppb) range were observed for an analytical frequency of six samples/h. FIA-MIMS for phenolic compound analysis is considerably less time-consuming and labor intensive than most chromatographic methods based on liquid-liquid extraction and preconcentration procedures and is therefore applicable for on-line and in-situ monitoring of phenols in wastewaters and in the environment. FIA-MIMS employing acetic anhydride derivatization is also virtually free of interferences since it combines chemical, membrane, and enhanced MS selectivity; hence quantitation of phenolic compounds can be performed in the presence of congeners.

Application of quantitative chemometric analysis techniques to direct sampling mass spectrometry

This paper explores the use of direct sampling mass spectrometry coupled with multivariate chemometric analysis techniques for the analysis of sample mixtures containing analytes with similar mass spectra. Water samples containing varying mixtures of toluene, ethyl benzene, and cumene were analyzed by purge-and-trap/direct sampling mass spectrometry. Multivariate calibration models were built using partial least-squares regression (PLS), trilinear partial least-squares regression (tri-PLS), and parallel factor analysis (PARAFAC), with the latter two methods taking advantage of the differences in the temporal profiles of the analytes. The prediction errors for each model were compared to those obtained with simple univariate regression. Multivariate quantitative methods were found to be superior to univariate regression when a unique ion for quantitation could not be found. For prediction samples that contained unmodeled, interfering compounds, PARAFAC outperformed the other analysis methods. The uniqueness of the PARAFAC model allows for estimation of the mass spectra of the interfering compounds, which can be subsequently identified via visual inspection or a library search.

On-line monitoring trihalomethanes in chlorinated water by membrane introduction-fast gas chromatography mass-spectrometry

An analytical method based on membrane introduction and fast gas chromatography-mass spectrometry (GC-MS) has been developed for the on-line monitoring of trihatomethanes (THMs) in chlorinated drinking water. The coupling of membrane introduction with fast GC-MS offers the advantage of membrane introduction as an on-line sampling device and fast GC-MS as a separation and identification method. While maintaining the on-line monitoring characteristic of traditional membrane introduction mass spectrometry (MIMS), the difficulty of distinguishing CHCl3 and CHBrCl2 in MIMS was overcome by rapid GC separation and MS analysis. Water permeated across the membrane affected the analysis of CHBr2Cl and CHBr3. A method based on controlling the injection temperature and injection time has been developed to overcome the moisture problem. This method is simple and less time consuming than the conventional moisture removing method. Under typical operating conditions, the sampling rate was about 20 samples h(-1) capable of on-line monitoring THMs in chlorinated drinking water. The detection limits of this system were found to be about 2 ppt, 4 ppt, 4 ppt, and 8 ppt for CHCl3 CHBrCl2, CHBr2Cl, and CHBr3, respectively.

On-line monitoring by membrane introduction mass spectrometry of chlorination of organics in water. Mechanistic and kinetic aspects of chloroform formation

Chloroform formation during the chlorination of simple organic molecules modeling humic substances, such as phenol and di- and trihydroxybenzenes, was studied by on-line membrane introduction mass spectrometry (MIMS). Under the reaction conditions employed, chloroform was rapidly formed from 1,3-dihydroxybenzene, 1, 4-dihydroxybenzene, phenol and 1,2,3-trihydroxybenzene with yields of 17, 13, 7 and 5%, respectively. With the exception of aniline, which afforded a 17% chloroform yield, non-phenolic compounds, such as nitrobenzene, chlorobenzene, toluene, benzene and cyclohexanol, furnished low yields. Mechanistic studies showed that phenol is chlorinated consecutively and produces initially chlorophenol. It is suggested that chloroform might be formed mainly from chlorinated 3, 5-cyclohexadienone-type intermediates. MIMS was also used to determine the reaction rates and to study the kinetics of the chlorination. A good Hammett linear correlation for an electrophilic substitution mechanism was found for the compounds C(6)H(5)X (X = NH(2), OH, CH(3), H, Cl and NO(2)).

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).

Membrane introduction mass spectrometry: trends and applications

Recent advances in membrane introduction mass spectrometry (MIMS) are reviewed. On-line monitoring is treated by focusing on critical variables, including the nature and dimensions of the membrane, and the analyte vapor pressure, diffusivity, and solubility in the membrane barrier. Sample introduction by MIMS is applied in (i) on-line monitoring of chemical and biological reactors, (ii) analysis of volatile organic compounds in environmental matrices, including air, water and soil, and (iii) in more fundamental studies, such as measurements of thermochemical properties, reaction mechanisms, and kinetics. New semipermeable membranes are discussed, including those consisting of thin polymers, low vapor pressure liquids, and zeolites. These membranes have been used to monitor polar compounds, selectively differentiate compounds through affinity-binding, and provide isomer differentiation based on molecular size. Measurements at high spatial resolution, for example, using silicone-capped hypodermic needle inlets, are also covered, as is electrically driven sampling through microporous membranes. Other variations on the basic MIMS experiment include analyte preconcentration through cryotrapping (CT-MIMS) or trapping in the membrane (trap-and-release), as well as differential thermal release methods and reverse phase (i.e., organic solvent) MIMS. Method limitations center on semivolatile compounds and complex mixture analysis, and novel solutions are discussed. Semivolatile compounds have been monitored with thermally assisted desorption, ultrathin membranes and derivatization techniques. Taking advantage of the differences in time of membrane permeation, mixtures of structurally similar compounds have been differentiated by using sample modulation techniques and by temperature-programmed desorption from a membrane interface. Selective ionization techniques that increase instrument sensitivity towards polar compounds are also described, and comparisons are made with other direct sampling (nonchromatographic) methods that are useful in mixture analysis.