@article {2664,
	title = {Gas-Phase H/D Exchange and Collision Cross Sections of Hemoglobin Monomers, Dimers, and Tetramers},
	journal = {Journal of the American Society for Mass Spectrometry},
	volume = {20},
	number = {3},
	year = {2009},
	note = {ISI Document Delivery No.: 417NGTimes Cited: 1Cited Reference Count: 71Wright, P. John Douglas, D. J.},
	month = {Mar},
	pages = {484-495},
	type = {Article},
	abstract = {The conformations of gas-phase ions of hemoglobin, and its dimer and monomer subunits have been studied with H/D exchange and cross section measurements. During the H/D exchange measurements, tetramers undergo slow dissociation to dimers, and dimers to monomers, but this did not prevent drawing conclusions about the relative exchange levels of monomers, dimers, and tetramers. Assembly of the monomers into tetramers, hexamers, and octamers causes the monomers to exchange a greater fraction of their hydrogens. Dimer ions, however, exchange a lower fraction of their hydrogens than monomers or tetramers. Solvation of tetramers affects the exchange kinetics. Solvation molecules do not appear to exchange, and solvation lowers the overall exchange level of the tetramers. Cross section measurements show that monomer ions in low charge states, and tetramer ions have compact structures, comparable in size to the native conformations in solution. Dimers have remarkably compact structures, considerably smaller than the native conformation in solution and smaller than might be expected from the monomer or tetramer cross sections. This is consistent with the relatively low level of exchange of the dimers. (J Am Soc Mass Spectrom 2009, 20, 484-495) (C) 2009 Published by Elsevier Inc. on behalf of American Society for Mass Spectrometry},
	keywords = {ACID-INDUCED DENATURATION, CYTOCHROME-C IONS, DIFFERENT SOLUTION CONFORMATIONS, ELECTROSPRAY MASS-SPECTROMETRY, EXCHANGE, GLOBIN CHAINS, HYDROGEN/DEUTERIUM, ION-TRAP, MYOGLOBIN IONS, noncovalent protein complexes, OF-FLIGHT SYSTEM, QUADRUPOLE},
	isbn = {1044-0305},
	url = {<Go to ISI>://000264082200018},
	author = {Wright, P. J. and Douglas, D. J.}
}
@article {2432,
	title = {A new broadly tunable (7.4-10.2 eV) laser based VUV light source and its first application to aerosol mass spectrometry},
	journal = {International Journal of Mass Spectrometry},
	volume = {279},
	number = {2-3},
	year = {2009},
	note = {ISI Document Delivery No.: 399TJTimes Cited: 10Cited Reference Count: 78Hanna, S. J. Campuzano-Jost, P. Simpson, E. A. Robb, D. B. Burak, I. Blades, M. W. Hepburn, J. W. Bertram, A. K.},
	month = {Jan},
	pages = {134-146},
	type = {Article},
	abstract = {A laser based vacuum ultraviolet (VUV) light source using resonance enhanced four wave difference mixing in xenon gas was developed for near threshold ionization of organics in atmospheric aerosol particles. The source delivers high intensity pulses of VUV light (in the range of 10(10) to 10(13) photons/pulse depending on wavelength, 5 ns FWHM) with a continuously tunable wavelength from 122 nm (10.2 eV) to 168 nm (7.4 eV). The setup allows for tight (< 1 mm(2)) and precise focusing (mu rad pointing angle adjustability), attributes required for single particle detection. The generated VUV is separated from the pump wavelengths by a custom monochromator which ensures high spectral purity and minimizes absorptive losses. The performance of the source was characterized using organic molecules in the gas phase and optimal working conditions are reported. In the gas phase measurements, photoionization efficiency (PIE) curves were collected for seven different organic species with ionization energies spanning the full wavelength range of the VUV source. The measured appearance energies are very close to the literature values of the ionization energies for all seven species. The effectiveness of the source for single particle studies was demonstrated by analysis of individual caffeine aerosols vaporized by a pulsed CO2 laser in an ion trap mass spectrometer. Mass spectra from single particles down to 300 nm in diameter were collected. Excellent signal to noise characteristics for these small particles give a caffeine detection limit of 8 x 10(5) molecules which is equivalent to a single 75 nm aerosol, or approximately 1.5\% of a 300 nm particle. The appearance energy of caffeine originating from the aerosol was also measured and found to be 7.91 +/- 0.05 eV, in good agreement with literature values. (C) 2008 Elsevier B.V. All rights reserved.},
	keywords = {Aerosol mass spectrometry, analysis, CROSS-SECTIONS, EFFICIENCY, FUEL-RICH FLAMES, GENERATING PARTICLE BEAMS, ION-TRAP, ONLINE, ORGANIC-COMPOUNDS, PARTICULATE MATTER, PHOTOIONIZATION, SINGLE-PHOTON IONIZATION, SURFACE-ANALYSIS, Vacuum ultraviolet light, VACUUM-ULTRAVIOLET PHOTOIONIZATION},
	isbn = {1387-3806},
	url = {<Go to ISI>://000262821900012},
	author = {Hanna, S. J. and Campuzano-Jost, P. and Simpson, E. A. and Robb, D. B. and Burak, I. and Blades, M. W. and Hepburn, J. W. and Bertram, A. K.}
}
@article {1441,
	title = {Linear quadrupoles with added hexapole fields},
	journal = {Journal of the American Society for Mass Spectrometry},
	volume = {17},
	number = {8},
	year = {2006},
	note = {ISI Document Delivery No.: 071YITimes Cited: 14Cited Reference Count: 26Konenkov, Nikolai Londry, Frank Ding, Chuanfan Douglas, D. J.},
	month = {Aug},
	pages = {1063-1073},
	type = {Article},
	abstract = {Linear quadrupoles with added hexapole fields are described. The shifts in ion oscillation frequency caused by the addition of a hexapole field are calculated within the effective potential model. Methods to construct linear quadrupoles with added hexapole fields with exact electrode geometries and with round rods are discussed. A quadrupole with added hexapole field can be constructed with round rods by rotating two rods (say the y rods) towards an x rod. Computer simulations are used to investigate the possibility of mass analysis with quadrupoles with added hexapole fields. We find that a quadrupole with an added hexapole field in the range 2-12\% can provide mass analysis provided the dc is applied with the correct polarity and value. When a rod set is constructed with round rods, other multipoles in the potential degrade the peak shape, resolution and transmission. The largest of these after the quadrupole and hexapole are a dipole and octopole term. With round rod sets, the peak shape can be improved by using different diameters for the x and y rod pairs to minimize the octopole term in the potential and by injecting ions at the field center where the dipole term is zero. Calculations of the boundaries of the stability diagram for this case show the boundaries move out, relative to those of a pure quadrupole field, but remain sharp.},
	keywords = {EXCITATION, ION-TRAP, MASS-SPECTROMETRY, OCTOPOLE FIELDS},
	isbn = {1044-0305},
	url = {<Go to ISI>://000239639400003},
	author = {Konenkov, N. and Londry, F. and Ding, C. F. and Douglas, D. J.}
}
@article {366,
	title = {Influence of the 6th and 10th spatial harmonics on the peak shape of a quadrupole mass filter with round rods},
	journal = {Rapid Communications in Mass Spectrometry},
	volume = {16},
	number = {15},
	year = {2002},
	note = {ISI Document Delivery No.: 578QXTimes Cited: 32Cited Reference Count: 32},
	pages = {1425-1431},
	type = {Article},
	abstract = {The influence of the ratio of the rod radius, r, to field radius, r(0), on the peak shape for a linear quadrupole mass filter constructed with round rods has been investigated. The expansion of the potential in multipoles, phi(N),Phi(x,y)= Sigma(N=0)(infinity)A(N)phi(N)/r(0)(N), has been considered, and the peak shape and resolution have been determined by numerical calculation of ion trajectories in quadrupoles with different ratios, r/r(0). Geometries that make the dodecapole term (A(6)) zero (r/(0) = 1.14511) do not give the best performance because the contribution of the 20-pole term, A(10), must be considered. The optimum ratio is r/r(0) approximate to 1.13. With this ratio the dodecapole term (A(6)) is ca. 1 x 10(-3), but its effects are partially compensated by the A(10) term which has similar magnitude, but opposite sign. Copyright (C) 2002 John Wiley Sons, Ltd.},
	keywords = {2ND, ION-TRAP, REGION, SIMULATION},
	isbn = {0951-4198},
	url = {<Go to ISI>://000177131400003},
	author = {Douglas, D. J. and Konenkov, N. V.}
}
@article {422,
	title = {Matrix methods for the calculation of stability diagrams in quadrupole mass spectrometry},
	journal = {Journal of the American Society for Mass Spectrometry},
	volume = {13},
	number = {6},
	year = {2002},
	note = {ISI Document Delivery No.: 557DZTimes Cited: 24Cited Reference Count: 47},
	month = {Jun},
	pages = {597-613},
	type = {Article},
	abstract = {The theory of the computer calculation of the stability of ion motion in periodic quadrupole fields is considered. A matrix approach for the numerical solution of the Hill equation and examples of calculations of stability diagrams are described. The advantage of this method is that it can be used for any periodic waveform. The stability diagrams with periodic rectangular waveform voltages are calculated with this approach. Calculations of the conventional stability diagram of the 3-D ion trap and the first six regions of stability of a mass filter with this method are presented. The stability of the ion motion for the case of a trapping voltage with two or more frequencies is also discussed. It is shown that quadrupole excitation with the rational angular frequency omega = NOmega/P (where N, P are integers and Omega is the angular frequency of the trapping field) leads to splitting of the stability diagram along iso-P lines. Each stable region of the unperturbed diagram splits into P stable bands. The widths of the unstable resonance lines depend on the amplitude of the auxiliary voltage and the frequency. With a low auxiliary frequency splitting of the stability diagram is greater near the boundaries of the unperturbed diagram. It is also shown that amplitude modulation of the trapping RF voltage by an auxiliary signal is equivalent to quadrupole excitation with three frequencies. The effect of modulation by a rational frequency is similar to the case of quadrupole excitation, although splitting of the stability diagram differs to some extent. The methods and results of these calculations will be useful for studies of higher stability regions, resonant excitation, and non-sinusoidal trapping voltages. (J Am Soc Mass Spectrom 2002, 13, 597-613) (C) 2002 American Society for Mass Spectrometry.},
	keywords = {CLOUD, EXCITATION, FILTER, FREQUENCY, HIGH-RESOLUTION, ION-TRAP, OPERATION, PARAMETRIC RESONANCES, PAUL TRAP, REGION},
	isbn = {1044-0305},
	url = {<Go to ISI>://000175893800003},
	author = {Konenkov, N. V. and Sudakov, M. and Douglas, D. J.}
}
@article {442,
	title = {Tandem mass spectrometry of protein-protein complexes: Cytochrome c-cytochrome b(5)},
	journal = {Journal of the American Society for Mass Spectrometry},
	volume = {13},
	number = {1},
	year = {2002},
	note = {ISI Document Delivery No.: 504QXTimes Cited: 31Cited Reference Count: 53},
	month = {Jan},
	pages = {59-71},
	type = {Article},
	abstract = {An improved method to interpret triple quadrupole MS/MS experiments of complexes of large ions is presented and applied to a study of the complex formed by the proteins cytochrome c and cytochrome b(5). Modeling of the activation and dissociation process shows that most of the reaction occurs near the collision cell exit where ions have the highest internal energies. Experiments at different collision cell pressures or with different collision gases (Ne, Ar, Kr) are interpreted with a previously proposed collision model (Chen et al., Rapid Commun. Mass Spectrom. 1998, 12, 1003-1010) to calculate the internal energy added to ions to cause dissociation. Small but systematic differences under different experimental conditions are attributed to different times available for reaction. A method to correct for this is presented. Ne, Ar, and Kr are found to have similar energy transfer efficiencies. Complexes of cytochrome c and cytochrome b5 are detected in ESI mass spectra but with abundances less than expected from the solution equilibrium. Dissociation of the cytochrome c-cytochrome b(5) complexes with charge k gives as the most abundant fragments, cytochrome b(5)(+3) and cytochrome c(+(k+3)). Adding charges to the complex destabilizes it. A series of cytochrome c variants with Lys residues thought to be involved in solution binding replaced by Ala showed no differences in the energy required to induce dissociation of the gas phase complex. The implications for the binding of the gas phase ions are inconclusive. (J Am Soc Mass Spectrom 2002, 13, 59-71) ((C)) 2002 American Society for Mass Spectrometry.},
	keywords = {ACTIVATION, BINDING, COLLISION-INDUCED DISSOCIATION, CROSS-SECTIONS, ELECTROSPRAY-IONIZATION, GAS-PHASE, INTERNAL ENERGY, ION-TRAP, NONCOVALENT COMPLEXES, YEAST ISO-1-FERRICYTOCHROME-C},
	isbn = {1044-0305},
	url = {<Go to ISI>://000172868300007},
	author = {Mauk, M. R. and Mauk, A. G. and Chen, Y. L. and Douglas, D. J.}
}
@article {4920,
	title = {Electrospray ionization of alkali and alkaline earth metal species. Electrochemical oxidation and pH effects},
	journal = {Journal of Mass Spectrometry},
	volume = {35},
	number = {8},
	year = {2000},
	note = {ISI Document Delivery No.: 347MUTimes Cited: 7Cited Reference Count: 49},
	month = {Aug},
	pages = {981-989},
	type = {Article},
	abstract = {The utility of electrospray ionization mass spectrometry (ESI-MS) for characterizing dissolved metal species has generated considerable interest in the use of this technique for metal speciation, However, the development of accurate speciation methods based on ESI-R;IS requires a detailed understanding of the mechanisms by which dissolved metal species are ionized during electrospray. We report how the analysis of alkali and alkaline earth metal species pro,ides new information about some of the processes that affect electrospray ion yield. Selected metal ions and organic ligands were combined in 50:50 water-acetonitrile buffered with acetic acid or ammonium acetate and analyzed by flow injection ESI-MS using mild electrospray conditions. Species formed by alkali metal ions with thiol and oxygen-donating ligands were detected in acidic and neutral pH solutions. Electrochemical oxidation of N,N-diethyldithiocarbamate and glutathione during electrospray was indicated by detection of the corresponding disulfides as protonated or alkali metal species, The extent of ligand oxidation depended on solution pH and the dissociation constant of the thiol group, Tandem mass spectrometric experiments suggested that radical cations such as [NaL](+.) (where L = N,N -diethyldithiocarbamate) can be generated by in-source fragmentation of disulfide species. Greater complexation of alkali metals at neutral pH tvas indicated by a corresponding decrease in the relative abundance of the free metal ion, The number of alkali metal ions bound by glutathione and phthalic acid also increased with increasing pH, in accordance with thermodynamic equilibrium theory. Alkaline earth metal species were detected only in acidic solutions, the absence of 8-hydroxyquinoline complexes being attributed to their relative instability and subsequent dissociation during electrospray, Hence, accurate speciation by ESI-MS depends on experimental conditions and the intrinsic properties of each analyte, Copyright (C) 2000 John Wiley \&; Sons, Ltd.},
	keywords = {cell, COMPLEXES, electrochemical oxidation, ELECTROPHORESIS, electrospray ionization, ION-TRAP, MASS-SPECTROMETRY, metal speciation, OLIGONUCLEOTIDES, PEPTIDES, pH effects, PROTEINS, STOICHIOMETRY, thiol ligands, WATER},
	isbn = {1076-5174},
	url = {<Go to ISI>://000088934400006},
	author = {Ross, A. R. S. and Ikonomou, M. G. and Orians, K. J.}
}
@article {4787,
	title = {Peak structure with a quadrupole mass filter operated in the third stability region},
	journal = {International Journal of Mass Spectrometry},
	volume = {197},
	year = {2000},
	note = {ISI Document Delivery No.: 291DFTimes Cited: 6Cited Reference Count: 11Sp. Iss. SI},
	month = {Feb},
	pages = {113-121},
	type = {Article},
	abstract = {Peak structure for a quadrupole operated in the third stability region with Mathieu parameters (a, q) approximate to (3, 3) has been studied experimentally and modeled theoretically. It is shown that the structure is due to the imaging properties of the quadrupole field which are caused by the wavelike properties of the ion trajectories. Ions enter the quadrupole through a small inlet aperture on axis. When ions are focused on the center of the exit aperture the transmission is a maximum. Conversely when ions have trajectories that place them near the rods at the exit aperture the transmission is a minimum and a dip appears on a peak. The positions of dips on a peak can be assigned to lines in the stability diagram. These lines follow iso-beta lines, where beta is the parameter that determines the frequencies of ion motion. The positions of the lines are controlled by the number of rf cycles, which ions spend in the quadrupole field. At high resolution (>300) and low ion axial energy (<20 eV) the peak splitting is minimal or absent. (C) 2000 Elsevier Science B.V.},
	keywords = {FREQUENCIES, ION-TRAP, peak, peak shape, quadrupole mass filter, STRUCTURE, third stability region},
	isbn = {1387-3806},
	url = {<Go to ISI>://000085718300007},
	author = {Du, Z. H. and Douglas, D. J. and Glebova, T. and Konenkov, N. V.}
}
@article {4527,
	title = {Peak splitting with a quadrupole mass filter operated in the second stability region},
	journal = {Journal of the American Society for Mass Spectrometry},
	volume = {10},
	number = {12},
	year = {1999},
	note = {ISI Document Delivery No.: 257JLTimes Cited: 3Cited Reference Count: 16},
	month = {Dec},
	pages = {1263-1270},
	type = {Article},
	abstract = {Peak splitting or structure has been studied for a quadrupole mass filter operated in the second stability region with Mathieu parameters (a,q) approximate to (0.02.7.55). Two sources of peak splitting are considered and modeled theoretically: nonlinear resonances caused by field imperfections and ion collection effects caused by the periodic properties of ion motion in the quadrupole field. The conditions for the appearance of the nonlinear resonances and ion collection effects are derived and presented in terms of the beta variables which determine the frequencies of ion oscillation. Comparisons of calculated peak structure to experimental peak shapes show that ion collection effects dominate, at least for the experimental conditions reported here. It is also shown that neither nonlinear resonances nor ion collection effects can distort the peak at high resolution. (C) 1999 American Society for Mass Spectrometry.},
	keywords = {ION-TRAP},
	isbn = {1044-0305},
	url = {<Go to ISI>://000083778700005},
	author = {Du, Z. H. and Douglas, D. J. and Konenkov, N.}
}
