Evaluation of small mass spectrometer systems for permanent gas analysis

This work is aimed at understanding the aspects of designing a miniature mass spectrometer (MS) system. Several types of small MS systems are evaluated and discussed, including linear quadrupole, quadrupole ion trap, time of flight, and sector. Analysis of hydrogen, helium, oxygen, and argon in a nitrogen background with the concentrations of the components of interest ranging from 0 to 5000 parts per million (ppm). The performance of each system in terms of accuracy, precision, limits of detection, response time, recovery time, scan rate, size, and weight is assessed. The relative accuracies of the systems varied from <1% to approximately 40% with an average below 10%. Relative precisions varied from 1% to 20%, with an average below 5%. The detection limits had a large distribution, ranging from 0.2 to 170 ppm. The systems had a diverse response time ranging from 4 to 210 s, as did the recovery time with a 6-to-210-s distribution. Most instruments had scan times near 1 s; however, one instrument exceeded 13 s. System weights varied from 9 to 52 kg and sizes ranged from 15 x 10(3) cm3 to 110 x 10(3) cm3. A performance scale is set up to rank each system, and an overall performance score is given to each system.     

丙烯酸-淀粉-硅藻土复合耐盐性高吸水树脂的紫外光引发合成及其性能

以丙烯酸、淀粉和硅藻土为原料,使用自制的紫外光聚合装置合成了复合耐盐性高吸水树脂。研究了丙烯酸中和度、硅藻土和淀粉用量、辐照时间等因素对树脂的吸液性能的影响,及pH、盐溶液浓度对树脂吸水率的影响,并用红外光谱和扫描电镜对产物进行了表征。在最佳试验条件下合成的高吸水树脂的吸去离子水率为3 665 g/g,吸生理盐水率(w=0.9%的NaCl水溶液)为280 g/g。     

Photon and positron generation by ultrahigh intensity laser interaction with electron beams

This study investigates the generation of high energy photons and positrons using focused ultrahigh intensity femtosecond laser pulses on a relativistic electron beam with a set of two-dimensional particle-in-cell simulations. We consider circularly and linearly polarized, single and spatially separated double laser pulses. We model both 500 MeV and 1 GeV electron beams. Higher positron production is obtained using circularly polarized laser pulses. Using double pulses, the focusing effect of the ponderomotive force confines the electrons to a small volume, generating additional energetic photons and positrons. The positron spectral distributions are effectively modified by these variations. When the electron beam energy is doubled, the number of positrons increased, while the cutoff energy remained nearly constant.     

新型二氧代氮杂茂并[3′,4′-d]异噁唑衍生物的合成

在三乙胺作用下,N-取代苯基马来酰亚胺与α-氯代对甲氧基苯甲醛肟发生1,3-偶极环加成反应,合成了6个新型3-(对甲氧基苯基)-5-芳基-3a,6a-二氢-4,6-二氧代氮杂茂并[3′,4′-d]异噁唑衍生物,其结构经1HNMR,IR和元素分析确证。     

新型二氧代氮杂茂并[3'''',4''''-d]异噁唑衍生物的合成

在三乙胺作用下,N-取代苯基马来酰亚胺与α-氯代对甲氧基苯甲醛肟发生1,3-偶极环加成反应,合成了6个新型3-(对甲氧基苯基)-5-芳基-3a,6a-二氢-4,6-二氧代氮杂茂并[3',4'-d]异噁唑衍生物,其结构经1H NMR, IR和元素分析确证.     

Microfluidic fluorescence in situ hybridization and flow cytometry (μFlowFISH)

We describe an integrated microfluidic device (μFlowFISH) capable of performing 16S rRNA fluorescence in situ hybridization (FISH) followed by flow cytometric detection for identifying bacteria in natural microbial communities. The device was used for detection of species involved in bioremediation of Cr(vi) and other metals in groundwater samples from a highly-contaminated environmental site (Hanford, WA, USA). The μFlowFISH seamlessly integrates two components: a hybridization chamber formed between two photopolymerized membranes, where cells and probes are electrophoretically loaded, incubated and washed, and a downstream cross structure for electrokinetically focusing cells into a single-file flow for flow cytometry analysis. The device is capable of analyzing a wide variety of bacteria including aerobic, facultative and anaerobic bacteria and was initially tested and validated using cultured microbes, including Escherichia coli, as well as two strains isolated from Hanford site: Desulfovibrio vulgaris strain RCH1, and Pseudomonas sp.strain RCH2 that are involved in Cr(vi) reduction and immobilization. Combined labeling and detection efficiencies of 74-97% were observed in experiments with simple mixtures of cultured cells, confirming specific labeling. Results obtained were in excellent agreement with those obtained by conventional flow cytometry confirming the accuracy of μFlowFISH. Finally, the device was used for analyzing water samples collected on different dates from the Hanford site. We were able to monitor the numbers of Pseudomonas sp. with only 100-200 cells loaded into the microchip. The μFlowFISH approach provides an automated platform for quantitative detection of microbial cells from complex samples, and is ideally suited for analysis of precious samples with low cell numbers such as those found at extreme environmental niches, bioremediation sites, and the human microbiome.     

Structural disorder of the CD3zeta transmembrane domain studied with 2D IR spectroscopy and molecular dynamics simulations

In a recently reported study [Mukherjee, et al. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 3528] we used 2D IR spectroscopy and 1-(13)C=(18)O isotope labeling to measure the vibrational dynamics of 11 amide I modes in the CD3zeta transmembrane domain. We found that the homogeneous line widths and population relaxation times were all nearly identical, but that the amount of inhomogeneous broadening correlated with the position of the amide group inside the membrane. In this study, we use molecular dynamics simulations to investigate the structural and dynamical origins of these experimental observations. We use two models to convert the simulations to frequency trajectories from which the mean frequencies, standard deviations, frequency correlation functions, and 2D IR spectra are calculated. Model 1 correlates the hydrogen-bond length to the amide I frequency, whereas model 2 uses an ab initio-based electrostatic model. We find that the structural distributions of the peptidic groups and their environment are reflected in the vibrational dynamics of the amide I modes. Environmental forces from the water and lipid headgroups partially denature the helices, shifting the infrared frequencies and creating larger inhomogeneous distributions for residues near the ends. The least inhomogeneously broadened residues are those located in the middle of the membrane where environmental electrostatic forces are weakest and the helices are most ordered. Comparison of the simulations to experiment confirms that the amide I modes near the C-terminal are larger than at the N-terminal because of the asymmetric structure of the peptide bundle in the membrane. The comparison also reveals that residues at a kink in the alpha-helices have broader line widths than more helical parts of the peptide because the peptide backbone at the kink exhibits a larger amount of structural disorder. Taken together, the simulations and experiments reveal that infrared line shapes are sensitive probes of membrane protein structural and environmental heterogeneity.     

Electron donor-acceptor interactions with flanking purines influence the efficiency of thymine photodimerization

Quantum yields for thymine photodimerization (Φ(TT)) have been determined for a series of short DNA single-strand and base-paired hairpin structures possessing a single thymine-thymine step with flanking purines. Values of Φ(TT) are strongly dependent upon the oxidation potential of the flanking purine, decreasing in the order: inosine > adenine > guanine > deazaguanine. The dependence of Φ(TT) on the ionization potential of the flanking purine is more pronounced when the purine of lower oxidation potential is located at the 5'- versus 3'-position in either a single strand or a hairpin. Molecular dynamics simulations for hairpin structures indicate that the TT step is π-stacked with both the 5' and 3' purine, but that there is little π-stacking with either purine in single-strand structures. The observation of moderately intense long-wavelength UV absorption features for hairpins having 5'-Z or G flanking purines suggests that excitation of ground state donor-acceptor complexes may account for more extensive quenching of dimerization by 5'- versus 3'-purines. The "purine effect" on Φ(TT) is attributed to a combination of ground state conformation, ground state electron donor-acceptor interactions, and excited state exciplex formation.     

Magnetic field focused ion accumulation for an internal bore liquid chromatography electrospray ionization Fourier transform ion cyclotron resonance mass spectrometer using a central trapping electrode

Presented is the application and evaluation of a magnetic field focusing central trapping electrode ion accumulation cell for a capillary liquid chromatography electrospray Fourier transform ion cyclotron (LC-ESI/FTICR) mass spectrometer. The ESI source and accumulation cell are located within the magnetic field to confine the radial motion of the ions, eliminating the need for elaborate focusing optics to transport the ions to the low-pressure analyzer cell for analysis. The central trapping electrode accumulation cell increases sensitivity by providing the necessary potential well in a confined volume to capture ions currently lost during the detection event of LC/FTICR experiments. With this electrode geometry the time needed to gate the ions into the analyzer cell is reduced and pump down delays are minimized. The decreased scan time improves LC resolution and increases the number of mass spectral scans per eluted component while maintaining appropriate base pressures for high performance ESI/FTICR. Results achieved with the central trapping electrode accumulation cell include an effective duty cycle increase from 10% to 40%, a S/N increase by a factor of 30, and a mass resolution increase of 80%.     

Central ring electrode for trapping and excitation/detection in Fourier transform ion cyclotron resonance mass spectrometry

The use of a central trapping ring electrode for Fourier transform ion cyclotron resonance (FTICR) mass spectrometry is demonstrated. Ions are trapped with an oppositely biased static potential superimposed on both the excite and detect electrodes and maintained throughout the experiment, including the application of a dipolar rf excite waveform and the image current ion detection event. The use of a central trapping electrode for FTICR coupled with an open cell design retains the advantages of high ion throughput and gas conductance, while simplifying the electrode geometry and reducing the overall dimensions of the cell. This allows the central trapping electrode to be of utility in volume-limited vacuum chambers including FTICR instrument miniaturization. Presented here are the preliminary experimental results using the central trapping electrode as an FTICR cell in which the excitation and detection electrodes also create a trapping depression to constrain the z-axis motion of the ions. The cell overcomes the principle limitation of an earlier single trapping electrode design by producing a 91% effective potential well depth compared to 19% for the single trapping electrode and 33% for standard open cells. This allows the central trapping electrode configuration to achieve an order of magnitude improvement in ion capacity compared to more conventional open cell designs.