@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 {2334,
	title = {Quadrupole excitation of ions in linear quadrupole ion traps with added octopole fields},
	journal = {Journal of the American Society for Mass Spectrometry},
	volume = {19},
	number = {4},
	year = {2008},
	note = {ISI Document Delivery No.: 289ZDTimes Cited: 2Cited Reference Count: 24Zhao, XianZhen Granot, Ori Douglas, D. J.},
	month = {Apr},
	pages = {510-519},
	type = {Article},
	abstract = {Modeling and experimental studies of quadrupole excitation of ions in linear quadrupole traps with added octopole fields are described. An approximate solution to the equations of motion of ions trapped in a quadrupole with added octopole and dodecapole fields, with quadrupole excitation and damping is given. The solutions give the steady-state or stationary amplitudes of oscillation with different excitation frequencies. Trajectory calculations of the oscillation amplitudes are also presented. The calculations show that there can be large changes in the amplitude of ion oscillation with small changes in excitation frequency, on both the low and high-frequency sides of a resonance. Results of experiments with quadrupole excitation of reserpine ions in linear quadrupole traps with 2.0\%, 2.6\%, and 4.0\% added octopole fields are given. It is found that as the excitation frequency is changed, two resonances are generally observed, which are attributed to the motion in the x and y directions. The two resonances can have quite different intensities. Sudden jumps or sharp sided resonances are not observed, although in some cases asymmetric resonances are seen. The calculated frequency differences between the two resonances are in approximate agreement with the experiments.},
	keywords = {CLOUD, fragmentation, FREQUENCIES, MASS-SPECTROMETRY, OF-FLIGHT SYSTEM, PAUL TRAP, PENNING TRAP, RESONANCES},
	isbn = {1044-0305},
	url = {<Go to ISI>://000255091500006},
	author = {Zhao, X. Z. and Granot, O. and Douglas, D. J.}
}
@article {1200,
	title = {Ion excitation in a linear quadrupole ion trap with an added octopole field},
	journal = {Journal of the American Society for Mass Spectrometry},
	volume = {16},
	number = {6},
	year = {2005},
	note = {ISI Document Delivery No.: 928AZTimes Cited: 17Cited Reference Count: 32},
	month = {Jun},
	pages = {835-849},
	type = {Article},
	abstract = {Modeling of ion motion and experimental investigations of ion excitation in a linear quadrupole trap with a 4 \% added octopole field are described. The results are compared with those obtained with a conventional round rod set. Motion in the effective potential of the rod set can explain many of the observed phenomena. The frequencies of ion oscillation in the x and y directions shift with amplitude in opposite directions as the amplitudes of oscillation increase. Excitation profiles for ion fragmentation become asymmetric and in some cases show bistable behavior where the amplitude of oscillation suddenly jumps between high and low values with very small changes in excitation frequency. Experiments show these effects. Ions are injected into a linear trap, stored, isolated, excited for MS/MS, and then mass analyzed in a time-of-flight mass analyzer. Frequency shifts between the x and y motions are observed, and in some cases asymmetric excitation profiles and bistable behavior are observed. Higher MS/MS efficiencies are expected when an octopole field is added. MS/MS efficiencies ((2) collision gas) have been measured for a conventional quadrupole rod set and a linear ion trap with a 4 \% added octopole field. Efficiencies are chemical compound dependent, but when an octopole field is added, efficiencies can be substantially higher than with a conventional rod set, particularly at pressures of 1.4 X 10(-4) torr or less. (c) 2005 American Society for Mass Spectrometry.},
	keywords = {EJECTION, MASS-SPECTROMETER, OF-FLIGHT SYSTEM, PAUL TRAP, RESONANT EXCITATION, SCAN, SIMULATION},
	isbn = {1044-0305},
	url = {<Go to ISI>://000229244800004},
	author = {Michaud, A. L. and Frank, A. J. and Ding, C. and Zhao, X. Z. and Douglas, D. J.}
}
@article {681,
	title = {Conformations of gas-phase lysozyme ions produced from two different solution conformations},
	journal = {Analytical Chemistry},
	volume = {75},
	number = {6},
	year = {2003},
	note = {ISI Document Delivery No.: 657PQTimes Cited: 15Cited Reference Count: 41},
	month = {Mar},
	pages = {1325-1330},
	type = {Article},
	abstract = {Near pH 2.0, lysozyme in water is in its native conformation, and in water/methanol (2/8) it adopts a helical denatured conformation (Kamatari et al. Protein Sci. 1998, 7, 681-688). Hydrogen/deuterium (H/D) exchange of lysozyme in solution confirms that it is partially unfolded at pH 2.0 in water/methanol (v/v = 2/8). With electrospray ionization (ESI) mass spectrometry (MS), lysozyme in water produces ions with charges +7 to +12, with the greatest intensity at +10, whereas lysozyme in water/methanol (2/8) produces ions with charges +6 to +12 with the greatest intensity at +7. Thus, lysozyme is an exception to the rule that a protein denatured in solution forms higher charge states than the same protein in its folded native conformations in solution. Because the same charge states are produced from these two solution conformations, a direct comparison of the properties of the gas-phase ions produced from two very different solution conformations is possible. The conformations of lysozyme ions in the gas phase were studied using cross section measurements and gas-phase H/D exchange. Similar cross sections and H/D exchange levels were observed for same-charge states of lysozyme ions formed from the native and helical denatured conformations in solution. Cross sections show that the ions have compact structures. Thus, disulfide-intact gaseous lysozyme ions generated from the denatured state in water/methanol (2/8) refold into compact structures in the gas phase on a time scale of milliseconds or less.},
	keywords = {COLLISION, CROSS-SECTIONS, CYTOCHROME-C, EGG-WHITE LYSOZYME, H/D EXCHANGE, HYDROGEN-DEUTERIUM EXCHANGE, HYDROGEN/DEUTERIUM EXCHANGE, IONIZATION MASS-SPECTROMETRY, IONS, OF-FLIGHT SYSTEM, PROTEIN, UNFOLDING DYNAMICS},
	isbn = {0003-2700},
	url = {<Go to ISI>://000181675900014},
	author = {Mao, D. M. and Babu, K. R. and Chen, Y. L. and Douglas, D. J.}
}
@article {682,
	title = {H/D exchange of gas phase bradykinin ions in a linear quadrupole ion trap},
	journal = {Journal of the American Society for Mass Spectrometry},
	volume = {14},
	number = {2},
	year = {2003},
	note = {ISI Document Delivery No.: 643ZYTimes Cited: 14Cited Reference Count: 51},
	month = {Feb},
	pages = {85-94},
	type = {Article},
	abstract = {The gas phase H/D exchange reaction of bradykinin ions, as well as fragment ions of bradykinin generated through collisions in an orifice skimmer region, have been studied with a linear quadrupole ion trap (LIT) reflectron time-of-flight (rTOF) mass spectrometer system. The reaction in the trap takes only tens of seconds at a pressure of few mTorr of D2O or CD3OD. The exchange rate and hydrogen exchange level are not sensitive to the trapping q value over a broad range, provided q is not close to the stability boundary (q = 0.908). The relative rates and hydrogen exchange levels of protonated and sodiated +1 and +2 ions are similar to those observed previously by others with a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer system. The doubly and triply protonated ions show multimodal isotopic distributions, suggesting the presence of several different conformations. The y fragment ions show greater exchange rates and levels than a or b ions, and when water or ammonia is lost from the fragment ions, no exchange is observed. (J Am Soc Mass Spectrom 2003, 14, 85-94) (C) 2003 American Society for Mass Spectrometry.},
	keywords = {ACTIVATED DISSOCIATION, COLLISIONAL ACTIVATION, CYTOCHROME-C, HYDROGEN-DEUTERIUM EXCHANGE, HYDROGEN/DEUTERIUM EXCHANGE, INFRARED, IONIZATION MASS-SPECTROMETRY, OF-FLIGHT SYSTEM, PEPTIDE IONS, PROTEIN IONS, RADIATIVE DISSOCIATION},
	isbn = {1044-0305},
	url = {<Go to ISI>://000180894500001},
	author = {Mao, D. M. and Douglas, D. J.}
}
@article {747,
	title = {Linear quadrupoles with added octopole fields},
	journal = {Rapid Communications in Mass Spectrometry},
	volume = {17},
	number = {20},
	year = {2003},
	note = {ISI Document Delivery No.: 731CTTimes Cited: 21Cited Reference Count: 29},
	pages = {2290-2294},
	type = {Article},
	abstract = {Two methods of adding relatively small octopole fields to the main quadrupole field of quadrupoles and linear ion traps with cylindrical rods are investigated. The first, {\textquoteright}stretching{\textquoteright} the quadrupole by moving two rods out from the axis, produces a combination of higher order fields with similar magnitudes in which the octopole field is not necessarily the greatest. The quadrupole field strength is changed significantly and a large potential appears on the axis. The second method uses rod pairs of different diameters. It adds octopole components of up to several percent while all other higher order fields remain small. An axis potential is also added, but it is only a few percent of the radio-frequency (RF) voltage and approximately equal to the strength of the octopole field. The axis potential can be removed by moving the larger rod pair out from the axis or applying unbalanced RIF to the electrodes. Copyright (C) 2003 John Wiley Sons, Ltd.},
	keywords = {EJECTION, FILTER, HARMONICS, MASS-SPECTROMETER, NONLINEAR ION-TRAP, OF-FLIGHT SYSTEM, RESONANCE, SPATIAL, STORAGE-RING},
	isbn = {0951-4198},
	url = {<Go to ISI>://000185867500008},
	author = {Sudakov, M. and Douglas, D. J.}
}
@article {604,
	title = {Quadrupole mass filters with octopole fields},
	journal = {Rapid Communications in Mass Spectrometry},
	volume = {17},
	number = {22},
	year = {2003},
	note = {ISI Document Delivery No.: 742BLTimes Cited: 16Cited Reference Count: 23},
	pages = {2495-2502},
	type = {Article},
	abstract = {The performance of quadrupole mass filters with added octopole fields in the range 2.0-4.0\% has been investigated. The added fields are much greater than those normally added to conventional rod sets by mechanical tolerances or construction errors. Quadrupole rod sets with added octopole fields were constructed with round rods by making one pair of rods greater in diameter than the other pair. For positive ions, resolution at half height of only about 200 is possible if the negative direct current (dc) output of the quadrupole power supply is connected to the smaller rods. If the positive dc output of the quadrupole power supply is connected to the smaller rods, the resolution improves dramatically; a resolution at half height of 5800 has been observed with a rod set with 2.6\% added octopole field. For negative ions the best resolution is obtained with the polarity of the dc reversed, i.e. with the negative dc applied to the smaller rods. These findings are unexpected in view of the literature that argues that to obtain high mass resolution with quadrupole mass filters, higher order multipoles must be kept as small as possible. Numerical simulations of peak shapes agree qualitatively with experiments. Simulation of the boundaries of the first stability region for positive ions shows that when the positive dc is applied to the smaller rods, the addition of a 2.0\% octopole field causes the boundaries to shift slightly but the boundaries are well defined, and the tip of the stability region remains sharp. When the positive dc is applied to the larger rods, the boundaries of the stability region move out and become diffuse. For instruments that require a rod set that can be used both as a linear trap and a mass filter, these rod sets may offer improved trap performance while still being capable of providing conventional mass analysis. Copyright (C) 2003 John Wiley Sons, Ltd.},
	keywords = {ELECTRODES, LINEAR ION-TRAP, OF-FLIGHT SYSTEM, PERFORMANCE, SPATIAL HARMONICS, SPECTROMETER},
	isbn = {0951-4198},
	url = {<Go to ISI>://000186495300007},
	author = {Ding, C. F. and Konenkov, N. V. and Douglas, D. J.}
}
