An improved model of the fringing fields of a quadrupole mass filter

The effect of fringing fields on the divergence of the ion beam exiting an RF quadrupole ion guide was studied using a computer simulation. It was shown that reducing the strength of the RF field towards the ion guide exit reduces ion beam divergence. Further improvement was demonstrated when creating a DC gradient towards the exit. The results of the numerical simulation were verified experimentally using a time-of-flight (TOF) mass analyzer with orthogonal acceleration. Decreasing the ion beam divergence resulted in considerably improved mass resolution of the instrument. T he use of radiofrequency (RF) multipole ion guides for the efficient transport of ions produced at the atmospheric pressure into a mass analyzer operating under high vacuum is a common practice in mass spectrometry. A detailed description of the operation of RF-only ion guides was presented by Gerlich [1]. The ion guides employed in mass spectrometry are generally quadrupoles, hexapoles, or octopoles, and have found particular utility in regions where neutral gas pressure is high enough (10 Ϫ3 -10 Ϫ2 torr) that multiple collisions take place with gas molecules. Radially confining effective potential well, generated in RF-only multipoles, significantly reduces losses and also cools ions translationally [2]. The benefits of using ion guides for efficient transmission of ions, generated by electrospray ionization (ESI), into a quadrupole ion trap were demonstrated by Stafford et al. [3]. The cooling of ions is especially beneficial in the case of a time-of-flight (TOF) mass analyzer with orthogonal acceleration (oa), where the energy spread of the ion beam exiting an RF ion guide irreversibly affects the resolution and sensitivity of TOF mass spectrometer [4].In oa-TOF instruments, ions after leaving the RF ion guide are reaccelerated in the axial direction to the necessary energy and focused by electrostatic optics to make the beam divergence properties close to a parallel beam. The resulting ion beam is accelerated in the orthogonal (y-) direction by applying an extraction voltage pulse to the parallel acceleration plates. An electrostatic mirror can correct the energy spread resulting from ion initial spatial spread by compressing the ion packet when it approaches the detector [5]. It was pointed out [6] that the effect of the initial velocity spread in the y-direction on resolution of oa-TOF mass spectrometers is probably the largest. The ion moving opposite to the time-of-flight direction will turn around after applying the extraction pulse and will get to the same starting plane with the velocity directed in the time-of-flight direction with some delay referred to as a turn-around time. The ion that had originally the same velocity directed in the time-of-flight direction will be ahead in time scale. The contribution in the peak width ⌬t due to this effect is determined by [7]:where v y is the velocity spread in y direction, m is the mass, ze is the ion charge, and E is the strength of the electric field during the extraction. It is well...

Section: Resultsmentioning

confidence: 99%

Improving the quality of the ion beam exiting a quadrupole ion guide

Berkout

Doroshenko

2006

“…Hunter and McIntosh [13] obtained an analytic expression for f(z) by fitting an exponential function to a numerical solution of the Laplace equation. Adapting their expression to an exit fringing field, which terminates in the plane of the exit lens at z ϭ z 1 ,…”

Section: Theorymentioning

confidence: 99%

“…The positive constants, a and b, depend on the rod-lens separation and were tabulated for specific values in reference [13]. Differentiating Eq 21 with respect to z one obtains…”

Section: Theorymentioning

confidence: 99%

Mass selective axial ion ejection from a linear quadrupole ion trap

Londry¹,

Hager²

2003

193

218

The electric fields responsible for mass-selective axial ejection (MSAE) of ions trapped in a linear quadrupole ion trap have been studied using a combination of analytic theory and computer modeling. Axial ejection occurs as a consequence of the trapped ions' radial motion, which is characterized by extrema that are phase-synchronous with the local RF potential. As a result, the net axial electric field experienced by ions in the fringe region, over one RF cycle, is positive. This axial field depends strongly on both the axial and radial ion coordinates. The superposition of a repulsive potential applied to an exit lens with the diminishing quadrupole potential in the fringing region near the end of a quadrupole rod array can give rise to an approximately conical surface on which the net axial force experienced by an ion, averaged over one RF cycle, is zero. This conical surface has been named the cone of reflection because it divides the regions of ion reflection and ion ejection. Once an ion penetrates this surface, it feels a strong net positive axial force and is accelerated toward the exit lens. As a consequence of the strong dependence of the axial field on radial displacement, trapped thermalized ions can be ejected axially from a linear ion trap in a mass-selective way when their radial amplitude is increased through a resonant response to an auxiliary signal. . Both approaches yield high quality mass spectra and offer advantages over conventional three-dimensional ion traps, such as greater ion capacity, higher trapping efficiencies, less mass discrimination, and reduced effects of space charge [1,3]. In certain cases, linear ion trap mass spectrometers can be incorporated into the ion path of triple quadrupoles yielding an instrument that combines the strengths of both platforms [1,4]. Ions are trapped radially in these devices by the RF quadrupole field and axially by static DC potentials at the ends of the quadrupole rod array, in contrast to conventional Paul traps in which ions are trapped by a three-dimensional RF quadrupole field. Radial mass-selective ion ejection occurs when the RF voltage is ramped in the presence of a sufficiently intense auxiliary AC voltage. The auxiliary AC resonance-ejection voltage is applied radially and the ions emerge from the linear ion trap through slots cut in the quadrupole rods [3]. Radial ejection requires that the RF field be of high quality over the entire length of the ion trap [3] in order to preserve mass spectral resolution, since resolution depends on the fidelity of the secular frequency of the trapped ions. Thus, very high mechanical precision is required in fabrication of the quadrupole rods in order to maintain the same secular frequency over the length of the device. Of course, the greater the length of the linear ion trap, the more difficult it is to maintain the high degree of mechanical precision. In addition, considerable care must be taken to ensure that the ions that are intended to be ejected radially from the linear ion trap are isolated as ...

“…Other computations and investigations [4,5,7,[14][15][16][17][18][19][20][21][22] have attempted to determine the effects on filter behavior arising because electrodes have finite length and because there are end plates, usually at zero potential, to hold the ion source at one end and the detector system at the other. The fields in regions extending from the end plates for some distance into the filter are fringe fields; they are complex and do not match the ideal fields required.…”

Section: Introductionmentioning

confidence: 99%

“…Most previous investigations represent the fringe fields using some approximation; early work used a simple linear variation from the ideal field some distance inside the electrodes to zero at the end plates. Later, Hunter and McIntosh [19,20] determined the fringe fields using a relaxation technique and fitted an exponential expression to these fields.…”

Section: Introductionmentioning

confidence: 99%

A Method of Computing Accurate 3D Fields of a Quadrupole Mass Filter and Their Use for Prediction of Filter Behavior

Gibson

Evans

Syed

et al. 2012

A method is described that enables the three-dimensional fields of a simple quadrupole mass filter (QMF) to be determined to a high accuracy. The technique produces accurate field values in the fringe field region as well as in the center of the filter. Using fields obtained typical filter performance is determined and shown to differ from that predicted when fringe fields are ignored. The computed performance shows features obtained experimentally and displays more complex variation with ion mass and other parameters than when fringe fields are ignored.