@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 {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 {4759,
	title = {High-resolution mass spectrometry with a quadrupole operated in the fourth stability region},
	journal = {Analytical Chemistry},
	volume = {72},
	number = {3},
	year = {2000},
	note = {ISI Document Delivery No.: 281NVTimes Cited: 7Cited Reference Count: 15},
	month = {Feb},
	pages = {540-545},
	type = {Article},
	abstract = {The properties of a quadrupole mass filter operated in the fourth stability region with Mathieu parameters (a,q) = (2 x 10(-3),21.3) have been investigated experimentally and theoretically. A resolution at half-height (R-1/2) of 13 900 has been obtained for 40 eV K-39(+) ions formed by thermal ionization, With ion energies of 750 eV or more, a resolution at half-height of up to 5000 can be obtained. Extrapolation of resolution data obtained with ions of 40 eV-4800 eV energy shows that unit resolution should be possible for ions with energies of similar to 10 keV, For low-energy ions (e.g., 75 eV), the transmission of the quadrupole operated in the fourth region is many orders of magnitude less than that of a quadrupole operated in the first or second stability regions. Acceptance calculations show this is a result of the combination of a much lower acceptance and strongly defocusing hinging fields. However, for higher ion energies, the transmission with operation in the fourth region is found to be comparable to that of a quadrupole operated at the same resolution in the second region. The implications for inductively coupled plasma mass spectrometry are briefly discussed.},
	keywords = {FILTER},
	isbn = {0003-2700},
	url = {<Go to ISI>://000085168200024},
	author = {Chen, W. and Collings, B. A. and Douglas, D. J.}
}
@article {4526,
	title = {A novel tandem quadrupole mass analyzer},
	journal = {Journal of the American Society for Mass Spectrometry},
	volume = {10},
	number = {11},
	year = {1999},
	note = {ISI Document Delivery No.: 245ZBTimes Cited: 6Cited Reference Count: 18},
	month = {Nov},
	pages = {1053-1066},
	type = {Article},
	abstract = {A new "tandem mass analyzer" is described. Two quadrupole mass filters are operated in series. Each is operated at low resolution and a small mass offset is introduced between the two quadrupoles so that the pair operate together to give higher resolution. The resolution of the tandem analyzer can be changed by changing the mass offset. The transmission is highest when the quadrupoles are operated as close together as possible with the poles aligned, with no intervening ion lens, and with the radio frequency (rf) voltages phase locked. A phase shift between the rf voltages applied to the quadrupoles also improves the ion transmission. Far a given resolution the tandem analyzer has transmission comparable to that of a single quadrupole. Results obtained with operation of the quadrupoles in the first, second, and third stability regions are described. Operation in the third stability region is particularly advantageous because the tandem analyzer exhibits good abundance sensitivity on the low and high mass sides under conditions where a single quadrupole produces a long peak tail on at least one side. It is also shown that scattering losses in the tandem analyzer are about half of those of a conventional quadrupole. The results suggest that it may be possible to build a low cost tandem analyzer that has relatively poor mechanical precision and yet that produces satisfactory peak shape and resolution. (J Am Soc Mass Spectrom 1999, 10, 1053-1066) (C) 1999 American Society for Mass Spectrometry.},
	keywords = {2ND, FILTER, INTERMEDIATE STABILITY REGIONS, RESOLUTION},
	isbn = {1044-0305},
	url = {<Go to ISI>://000083137500001},
	author = {Du, Z. H. and Douglas, D. J.}
}
