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1.
Mass selective axial ion ejection from linear quadrupoles with added octopole fields 总被引:1,自引:1,他引:1
Mass selective axial ejection of ions from linear quadrupoles with added octopole fields is described. Quadrupoles with 2.0% and 2.6% added octopole fields have been tested and compared with a conventional quadrupole. The effects of trapping ions at different q values, excitation voltage, scan direction, balanced and unbalanced rf voltages on the rods, and dc applied between the rods have been investigated. The highest scan speeds and best resolution are obtained with resonant excitation and ejection at high q (q = 0.8). With axial ejection, the quadrupole with a 2.0% added octopole field provides mass resolution and ejection efficiencies similar to a conventional rod set. Quadrupole, dipole, and simultaneous dipole-dipole excitation between the x and y rod pairs were compared, and no advantage was found with quadrupole or dipole-dipole excitation. The effects of scan speed were investigated and a resolution at half height of about 1600 is possible at scans speed up to 5000 Th/s. 相似文献
2.
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, 'stretching' 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 RF to the electrodes. 相似文献
3.
XianZhen Zhao Zilan Xiao D. J. Douglas 《Journal of the American Society for Mass Spectrometry》2010,21(3):393-402
Mass analysis with islands of stability has been investigated with three linear quadrupole mass filters: two with 4% added
hexapole fields constructed with equal diameter (quadrupole 4A) and unequal diameter (quadrupole 4B) rods, and a conventional
round-rod quadrupole that has apparently been slightly damaged. Islands are formed by applying auxiliary quadrupole excitation.
With the Mathieu parameter, a < 0, mass analysis with both quadrupoles with hexapole fields operated normally, i.e., without islands, gives only low resolution.
A factor of 10 or more increase in resolution is possible with the use of stability islands. With a > 0, when quadrupole 4A is operated normally, peak shapes similar to that of a conventional quadrupole can be obtained at
resolutions higher than 850. At lower resolutions, peaks are split. When quadrupole 4B is operated without islands, resolution
up to 2000 is possible, but there are low mass tails and structure is formed on the peaks. With mass analysis with an island
of stability, both quadrupoles 4A and 4B show peaks free of structure and without tails. Ion transmission is also improved
with some operating conditions. With the conventional round-rod quadrupole, mass analysis with islands of stability increases
the limiting resolution from 2500 to 4360. At a resolution of 2500, the transmission is increased by about two orders of magnitude.
These results show that the use of islands of stability improves mass analysis with quadrupoles with distorted fields, and
may, in the future, allow use of quadrupoles constructed with at least some lower mechanical tolerances. 相似文献
4.
Linear quadrupoles with added hexapole fields 总被引:1,自引:0,他引:1
Konenkov N Londry F Ding C Douglas DJ 《Journal of the American Society for Mass Spectrometry》2006,17(8):1063-1073
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. 相似文献
5.
Quadrupole excitation of ions in linear quadrupole ion traps with added octopole fields 总被引:1,自引:1,他引:0
Zhao X Granot O Douglas DJ 《Journal of the American Society for Mass Spectrometry》2008,19(4):510-519
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. 相似文献
6.
Michaud AL Frank AJ Ding C Zhao X Douglas DJ 《Journal of the American Society for Mass Spectrometry》2005,16(6):835-849
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 (N(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. 相似文献
7.
Quadrupole mass filters with octopole fields 总被引:1,自引:0,他引:1
Ding C Konenkov NV Douglas DJ 《Rapid communications in mass spectrometry : RCM》2003,17(22):2495-2502
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. 相似文献
8.
Dipole excitation of ions in linear radio frequency quadrupole ion traps with added multipole fields
Ion motion with auxiliary dipole excitation and collisional damping in a linear radiofrequency quadrupole ion trap incorporating small amounts of even higher order multipoles is studied analytically. The ion motion is modeled in a pseudopotential that is mostly quadratic with small amounts of higher spatial harmonics. Ion motion along x and y axes is characterized by two uncoupled forced and damped anharmonic oscillator equations. A multiple time scales method is used to solve the equations of motion of ions with a first order perturbation correction. Analytical relations between the oscillation amplitudes at steady state (the stationary amplitudes) and excitation frequency are calculated. The frequency response curves show that in some cases bistable behavior might be obtained, i.e., there are two stable stationary amplitudes for a given excitation frequency. 相似文献
9.
Wilcox BE Hendrickson CL Marshall AG 《Journal of the American Society for Mass Spectrometry》2002,13(11):1304-1312
Externally generated ions are accumulated in a linear octopole ion trap before injection into our 9.4 T Fourier transform ion cyclotron resonance (FT-ICR) mass analyzer. Such instrumental configuration has previously been shown to provide improved sensitivity, scan rate, and duty cycle relative to accumulated trapping in the ICR cell. However, inefficient ion ejection from the octopole currently limits both detection limit and scan rate. SIMION 7.0 analysis predicts that a dc axial electric field inside the linear octopole ion trap expedites and synchronizes the efficient extraction of the octopole-accumulated ions. Further SIMION analysis optimizes the ion ejection properties of each of three electrode configurations designed to produce a near-linear axial potential gradient. More efficient extraction and transfer of accumulated ions spanning a wide m/z range promises to reduce detection limit and increase front-end sampling rate (e.g., to increase front-end resolution for separation techniques coupled with FT-ICR mass analysis). Addition of the axial field improves experimental signal-to-noise ratio by more than an order of magnitude. 相似文献
10.
When a fluid is displaced by a less viscous one in a porous medium, a hydrodynamic instability appears leading to the formation of some kind of fingers of the upstream fluid invading the downstream one, hence the name "viscous fingering" (VF) given to this instability. In a LC column, such an instability is likely to appear at that of the two interfaces between the sample and the eluent which exhibits an unfavorable viscosity contrast. It leads to distorted peak shapes and contributes to peak broadening. This phenomenon has been observed for long in SEC and more recently in RPLC on elution peak shapes as well as with various methods of in-column visualization. A simplistic LC column model is described to explain the origin of the VF instability and its characteristics. The general principles for analyzing hydrodynamic instabilities are described and the results of the linear stability analysis performed by Tan and Homsy [C.T. Tan, G.M. Homsy, Phys. Fluids 29 (1986) 3549 [1]], at the onset of the VF phenomenon for a step interface between two fluids are here applied to typical operating conditions encountered in analytical LC. The most probable growth rate and wavelength (linked to the finger width) of the instability are given in terms of particle size and solute diffusion coefficient, with particular emphasis on the role of the carrier velocity. Previously published qualitative observations about VF in chromatography are examined and interpreted at the light of this theory. The role of the column geometry on the development of the instability, the possible sources of noise or fluctuations triggering the instability, and the various experimental situations in which a significant viscosity contrast is encountered in LC are discussed. 相似文献
11.
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13.
Biochemistry has many examples of linear chain polymers, i.e., molecules formed from a sequence of units from a finite set
of possibilities; examples include proteins, RNA, single-stranded DNA, and paired DNA. In the field of mass spectrometry,
it is useful to consider the idea of weighted alphabets, with a word inheriting weight from its letters. We describe the distribution
of the mass of these words in terms of a simple recurrence relation, the general solution to that relation, and a canonical
form that explicitly describes both the exponential form of this distribution and its periodic features, thus explaining a
wave pattern that has been observed in protein mass databases. Further, we show that a pure exponential term dominates the
distribution and that there is exactly one such purely exponential term. Finally, we illustrate the use of this theorem by
describing a formula for the integer mass distribution of peptides and we compare our theoretical results with mass distributions
of human and yeast peptides. 相似文献
14.
The mobilities of various DNA fragments in two normally migrating molecular weight ladders were studied in polyacrylamide gels containing different concentrations of the crosslinker N,N'-methylenebisacrylamide (Bis). The acrylamide concentration ranged from 2.5-10.5%T (w/v); the Bis concentration ranged from 0.5-10%C (w/w), with respect to total acrylamide. Ferguson plots were constructed for each of the DNA fragments in gels of each composition. The Ferguson plots of the different multimers in each molecular weight ladder were nearly parallel in gels containing 0.5-3%C, converged close to a common intercept at zero gel concentration in gels containing 4%C, and crossed at approximately 1.5%T in gels containing 5 and 10%C. If the mobilities observed for the different DNA fragments at zero gel concentration were also extrapolated to zero DNA molecular weight, a common limiting mobility was observed in gels of all crosslinker concentrations. This limiting mobility was approximately equal to the free solution mobility of DNA. The effective pore radius of each gel was estimated from Ferguson plots based on relative mobilities, using the mobility of the smallest DNA fragment in each molecular weight ladder as the reference mobility. The calculated gel pore radii ranged from 142 nm to 19 nm, respectively, for gels containing 4.6%T, 1.5%C, and 10.5%T, 5 or 10%C. These pore radii are an order of magnitude larger than previously accepted values, but are consistent with scanning electron microscope measurements (Rüchel, R., et al., J. Chromatogr. 1978, 42, 77-90).(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
15.
Association of macromolecules in aqueous media through hydrogen bonding results in the formation of discrete interpolymer complexes (IPCs). In this work, the effect of added salt on the stability of IPCs consisting of polyacrylic acid and either a flexible polymer (polyethylene oxide or polyvinylpyrrolidone) or a semirigid polymer (hydroxy propylcellulose) is examined by a combination of spectrophotometry, viscometry and potentiometry. Addition of a neutral salt (e.g., NaCl) typically results in IPC aggregation. The response of the IPCs to salts that promote salting-in (i.e., LiSCN, NaSCN and GuSCN, where Gu denotes guanidine) is strongly cation-dependent, since Li+ cations induce an increase in solution pH, whereas Gu+ cations compete with polyacrylic acid for complexation sites. The degree of complexation (), calculated from potentiometric data, is greatest for IPCs in the absence of salt and is found to decrease in the order NaCl>NaSCN>GuSCN. Addition of salts that induce salting-in at polymer solution concentrations favoring complexation is found to enhance IPC solubility (and reduce ) until the IPC particles are either completely solubilized or dissociated. 相似文献
16.
Mathur D Hatamoto T Okunishi M Prümper G Lischke T Shimada K Ueda K 《The journal of physical chemistry. A》2007,111(38):9299-9306
We have experimentally probed the strong-field ionization dynamics of gas-phase linear alcohols, methanol, ethanol, and 1-propanol, by irradiating them with intense, femtosecond-duration laser pulses of 800 and 400 nm wavelength. Specifically, we make high resolution measurements of the energies of electrons that are ionized by the action of the optical field. Our electron spectroscopy measurements enable us to bifurcate the dynamics into multiphoton ionization and tunneling ionization regimes. In the case of 800 nm irradiation, such bifurcation into different ionization regimes is reasonably rationalized within the framework of the adiabaticity parameter based on the original Keldysh-Faisal-Reiss model of atomic ionization, without recourse to any structure-dependent modifications to the theory. In that sense, our 800 nm spectroscopy indicates that the linear alcohols exhibit atom-like properties as far as strong field ionization dynamics in the multiphoton ionization and tunneling regimes are concerned. We also explore the limitations of this atom-like picture by making measurements with 400 nm photons wherein the ponderomotive potential experienced by the ionized electrons is much less than the photon energy; effects that are purely molecular then appear to influence the strong field dynamics. 相似文献
17.
We used replica exchange molecular dynamics (REMD) simulations to evaluate four different AMBER force fields and three different implicit solvent models. Our aim was to determine if these physics-based models captured the correct secondary structures of two alpha-helical and two beta-peptides: the 14-mer EK helix of Baldwin and co-workers, the C-terminal helix of ribonuclease, the 16-mer C-terminal hairpin of protein G, and the trpzip2 miniprotein. The different models gave different results, but generally we found that AMBER ff96 plus the implicit solvent model of Onufriev, Bashford, and Case gave reasonable structures, and is fairly well-balanced between helix and sheet. We also observed differences in the strength of ion pairing in the solvent models, we but found that the native secondary structures were retained even when salt bridges were prevented in the conformational sampling. Overall, this work indicates that some of these all-atom physics-based force fields may be good starting points for protein folding and protein structure prediction. 相似文献
18.
E. V. Mamontov V. S. Gurov A. A. Dyagilev E. Yu. Grachev 《Journal of Analytical Chemistry》2012,67(13):981-986
The space-time focusing of ions in planar linear high-frequency electric fields was investigated. The possibility of the time-of-flight mass separation of ions in radiofrequency fields was demonstrated and the analytical parameters of this separation were evaluated. The results of the numerical simulation of a radiofrequency mass-reflectron with and without taking into account the space charge of ions were given. 相似文献
19.
Robert A. Bethem Robert K. Boyd 《Journal of the American Society for Mass Spectrometry》1998,9(6):643-648
This report is an opinion piece arising from the response to the 1996 ASMS Fall Workshop “Limits to Confirmation, Quantitation, and Detection.” The two subjects that generated the most heated discussion at the Workshop were the criteria for qualitative confirmation of target analytes and those for defining and measuring limits of detection and of quantitation. A reportorial account of the Workshop has been published previously (Baldwin, R.; et al. J. Am. Soc. Mass Spectrom. 1997, 8, 1180–1190). The purpose of the present work is to (1) attempt to reconcile the sometimes highly divergent views expressed by some of the invited Workshop speakers and (2) provide some impetus toward a consensus approach to the dual problems of analyte identification and operational definitions of limits of detection and quantitation for application to trace analysis using chromatography with mass spectrometric detection. In view of the wide range of analytical problems dealt with by modern mass spectrometrists and the varied contexts in which these activities are undertaken (including regulatory requirements and possible legal challenges), both issues are addressed using the concept of fitness for purpose. It is proposed that an appropriate goal is not to define a set of universally applicable criteria, but rather to recommend guidelines for establishing integrated analytical methods best suited to the particular purpose and context. 相似文献