Steven C. Beu

Open trapped ion cell geometries for Fourier transform ion cyclotron resonance mass spectrometry

Fourier transform ion cyclotron resonance mass spectrometry is evaluated in trapped ion cells which substitute open rectangular trapping electrodes for trapping plates positioned perpendicular to the magnetic field. Compared with closed cells, the open cell geometry offers improved access to the cell interior with obvious benefits for external ion injection. The well depth along the z-axis centerline of the open cell is about 37% of the applied potential for cubic geometry but increases to 81% for an elongated cell with aspect ratio two. Isopotential field maps are created to compare axial and radial components of trapping and excitation fields for open and closed orthorhombic cells. Radial trapping electric fields are somewhat larger in the center of the open cells but axial excitation fields are a factor of two smaller compared with the closed cell. An open elongated cell with an aspect ratio of two is constructed and compared with a closed cell of similar aspect ratio. Experimentally, a −57 Hz V−1 shift in cyclotron frequency owing to the radial trapping field is obtained for the open cell compared with a −32 Hz V−1 shift for the closed cell. However, axial ejection is significantly reduced in the open cell compared with the closed cell. Suggestions are given for generalizing the open cell to a mutisegmented electrode assembly in which excitation, detection and trapping are performed interchangeably with the same electrodes. For example, capacitive coupling of the excitation potential to the trap electrodes is suggested as a simple means of eliminating axial ejection during excitation.

Automated broadband phase correction of Fourier transform ion cyclotron resonance mass spectra.

It has been known for 35 years that phase correction of FTICR data can in principle produce an absorption-mode spectrum with mass resolving power as much as a factor of 2 higher than conventional magnitude-mode display, an improvement otherwise requiring a (much more expensive) increase in magnetic field strength. However, temporally dispersed excitation followed by time-delayed detection results in steep quadratic variation of signal phase with frequency. Here, we present a robust, rapid, automated method to enable accurate broadband phase correction for all peaks in the mass spectrum. Low-pass digital filtering effectively eliminates the accompanying baseline roll. Experimental FTICR absorption-mode mass spectra exhibit at least 40% higher resolving power (and thus an increased number of resolved peaks) as well as higher mass accuracy relative to magnitude mode spectra, for more complete and more reliable elemental composition assignments for mixtures as complex as petroleum.

Ion trapping and manipulation in a tandem time-of-flight-Fourier transform mass spectrometer

Abstract An external source tandem time-of-flight-Fourier transform mass spectrometer which provides programmed ion kinetic energy manipulation and trapping control is described. Design considerations are discussed and a detailed description of the instrument is provided. Typical ion injection efficiencies of 80–90% are obtained over an injection energy range of 30–500 eV. The instrument is capable of sensitive time-of-flight analysis of ions which have been injected into, and subsequently discharged from, the magnetic field. This allows a diagnostic treatment of ion injection and trapping performance independent of the Fourier transform mass spectrometry (FTMS) experiment. A theoretical treatment of the propagation characteristics of the ions detected in this manner is presented along with supporting data. Also described are new external source FTMS trapping procedures for pulsed injection that provide full control over the mass range of injected ions. Preliminary results for one such procedure are presented. In this example, a 230 dalton mass range is acquired in the trapped-ion cell for pulsed ion injection parameters that would ordinarily limit the acquired mass range to less than 150 daltons.

Radial ion transport due to resistive-wall destabilization in Fourier transform mass spectrometry

Abstract Destabilization of coherent magnetron motion due to loss of ion energy in resistive external circuit elements is found to result in radial ion transport and ultimately ion loss from the trapped-ion cell of Fourier transform mass spectrometers. In contrast to random-walk diffusion and drift in radial electric fields, the resistive-wall destabilization described here does not involve collisions and is therefore independent of system pressure. Instead the mechanism prevails when large ion populations exhibit a significant degree of coherent magnetron motion. In this work a simple model to describe the transport effect is derived and is found to relate the rate of transport directly to ion number, external circuit resistance, and applied trapping potential. Experimental data for ions formed by electron ionization are acquired and found to support the model. Under certain conditions ion loss is shown to occur within a few hundred milliseconds which has important implications for quantitative Fourier transform mass spetrometry experiments that include lengthy collisional stabilization or reaction delays.

Two-dimensional coulomb-induced frequency modulation in Fourier transform ion cyclotron resonance: A mechanism for line broadening at high mass and for large ion populations

Fourier transform ion cyclotron resonance (FTICR) spectra generated for large ion populations exhibit frequency shifts and line broadening, apparently due to Coulomb forces between ions. Although previous two-dimensional (2D) models of Coulomb effects in FTICR accounted for frequency shifts, they did not account for spectral line broadening. In this article, a 2D model is proposed that predicts line broadening due to Coulomb-induced frequency modulation. The model considers the case of two different-mass ions orbiting at their respective cyclotron frequencies around a common guiding center. A mutual modulation of the cyclotron frequency occurs at the difference frequency between ions. If the modulation period is much shorter than the FTICR observation time, then sidebands spaced at intervals approximately equal to the modulation frequency are predicted. However, if the modulation period is similar in duration to the FTICR observation period, the sidebands can no longer be resolved, which results in spectral line broadening. This latter case is a necessary consequence for isotopic peaks in the high mass region around m/z 2000, where deterioration in FTICR performance has been observed. Computer simulations are used to confirm the mass dependence and to demonstrate other features of the model, including a strong dependence of the modulation on ion number. In support of the model, experimental FTICR spectra for large populations of methylnaphthalene ions at m/z 141 and 142 exhibit constant frequency sidebands corresponding to multiples of the difference frequency for the two ions extending from nominal values of m/z 136 to 147.

Initiation of coherent magnetron motion following ion injection into a Fourier transform ion cyclotron resonance trapped ion cell

Abstract The initiation and growth of coherent magnetron motion for ions injected into a Fourier transform ion cyclotron resonance trapped ion cell is investigated. The phenomenon is demonstrated for laser desorption/ionization (LDI) of metals and salts and for electrospray ionization (ESI) of multiply charged proteins. In all cases, the rate of growth of the magnetron orbit increases proportionally with trap potential, ion density, and external detection circuit resistance. The data support resistive damping of the magnetron motion as the primary relaxation mechanism leading to ion ejection from the cell within time periods ranging from a few hundred milliseconds to tens of seconds. Factors associated with ion injection that contribute to initiation and growth of the destabilizing motion are length of the injection period relative to the magnetron period and extent of off-axis injection. In general, if the injection period is smaller than one period of the magnetron motion, as is the case for LDI, and the injection point is severely off the centerline of the cell, coherent growth is observed instantaneously and ions are expelled from the cell within a few hundred milliseconds. If the LDI ions are introduced along the centerline, coherent motion leading to expulsion is still observed but initiation times increase ten-fold. For the case in which ESI ions focused along the centerline are injected into the trap for a time much longer than the period of a magnetron oscillation, initiation and growth of coherent magnetron is only observed at much longer times and with a larger trap potential.

Mechanism for an ion accumulation process in external source fourier transform mass spectrometers

Abstract An ion accumulation effect has been observed in the trapped ion cell of a Fourier transform mass spectrometer during the injection of externally generated ions. The accumulation is unexpected because, in the absence of deceleration in the cell, ions with kinetic energy greater than the trapping potential should pass through the cell or collide with the rear trap plate. Previous suggestions that the trapping mechanism involves collisional damping or radial magnetic field inhomogeneities could not be demonstrated under the conditions employed in this work. We propose, instead, an alternate mechanism for ion accumulation that involves charge exchange between injected ions and neutral analyte molecules present in the analyzer. It is shown that the observed accumulation rates are consistent with known rate constants for charge exchange and are directly proportional to the density of neutral analyte molecules in the analyzer. The energetics of charge reactions are shown to account for the presence of apparent dissociation effects and for the absence of an expected production ion in the spectrum of one analyte.

Debye-shielding mechanism for trapping ions formed by laser desorption Fourier transform ion cyclotron resonance mass spectrometry

Abstract The trapping of metal ions in laser desorption/ionization (LDI) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry is attributed to an electrostatic shielding mechanism promoted by the sustained quasi-neutral plasma behavior of the desorbed particle plume in strong magnetic fields. This shielding process allows low energy ions to penetrate the applied trapping potentials at the trapped-ion cell, thus resulting in the introduction of ions with energies less than the effective depth of the trapping potential well. Subsequent deshielding of these ions while in the cell exposes them to the trapping field and results in their retention. Data from time-of-flight (TOF) studies indicate that large spatially and temporally overlapped populations of high energy ions and low energy electrons are generated by LDI of a variety of metal targets when laser power density exceeds 107–108W cm−2. The charge density in the desorbed plasma is shown to increase during flight along converging magnetic field lines but to dissipate rapidly on exiting the field. Retarding potential studies in the magnetic field performed with both TOF and FT-ICR detection indicate that the Debye shielding exhibited by these quasi-neutral populations is sufficient in some cases to allow ions with energies on the order of 1 eV to penetrate retarding potentials as high as 500 V. Further indication that such effects are present in the LDI FT-ICR experiment is given by TOF kinetic energy analysis of ions acquired in the trapped-ion cell from LDI and then dumped to an external detector. This analysis indicates that the average kinetic energy of such ions is typically only 60% of the applied trapping potential.

Petroleomics: a test bed for ultra-high- resolution Fourier transform ion cyclotron resonance mass spectrometry

Within a relative abundance dynamic range of ~10,000: 1, the worlds most compositionally complex organic mixture is petroleum crude oil. As such, it provides the most challenging target for mass spectral resolution and identification of molecules below m/z 2000. The mass “splits” in petroleum include most of those that also appear in proteomics, metabolomics and other complex organic mixture analysis. Therefore, petroleum provides an excellent test bed for optimizing mass spectrometer performance in general. The presence of multiple elemental compositions spanning less than 1 Da in mass facilitates mapping and correction of rf phase variation across a Fourier transform ion cyclotron resonance mass spectrum, as well as exposing otherwise inaccessible systematic mass deviations, for additional improvement in mass resolving power and mass accuracy by a factor of up to 5. Internal mass calibration, combined with systematic peak assignment for successive homologous series, enables automated elemental composition assignment of tens of thousands of peaks in a single mass spectrum.

Excitation of Radial Ion Motion in an rf-Only Multipole Ion Guide Immersed in a Strong Magnetic Field Gradient

Radiofrequency (rf) multipole ion guides are widely used to transfer ions through the strong magnetic field gradient between source and analyzer regions of external source Fourier transform ion cyclotron resonance mass spectrometers. Although ion transfer as determined solely by the electric field in a multipole ion guide has been thoroughly studied, transfer influenced by immersion in a strong magnetic field gradient has not been as well characterized. Recent work has indicated that the added magnetic field can have profound effects on ion transfer, ultimately resulting in loss of ions initially contained within the multipole. Those losses result from radial ejection of ions due to transient cyclotron resonance that occurs when ions traverse a region in which the magnetic field results in an effective cyclotron frequency equal to the multipole rf drive frequency divided by the multipole order (multipole order is equal to one-half the number of poles). In this work, we describe the analytical basis for ion resonance in a rf multipole ion guide with superposed static magnetic field and compare with results of numerical trajectory simulations.