Theoretical investigation of improved ion trapping in matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry: independence of ion initial velocity

Ion Behavior in an Electrically Compensated Ion Cyclotron Resonance Trap

We recently described a new electrically compensated trap in FT ion cyclotron resonance mass spectrometry and developed a means of tuning traps of this general design. Here, we describe a continuation of that research by comparing the ion transient lifetimes and the resulting mass resolving powers and signal-to-noise (S/N) ratios that are achievable in the compensated vs. uncompensated modes of this trap. Transient lifetimes are ten times longer under the same conditions of pressure, providing improved mass resolving power and S/N ratios. The mass resolving power as a function of m/z is linear (log-log plot) and nearly equal to the theoretical maximum. Importantly, the ion cyclotron frequency as a function of ion number decreases linearly in accord with theory, unlike its behavior in the uncompensated mode. This linearity should lead to better control in mass calibration and increased mass accuracy than achievable in the uncompensated mode.

An electrically compensated trap designed to eighth order for FT-ICR mass spectrometry

We present the design, guided by theory to eighth order, and the first evaluation of a Fourier transform ion cyclotron resonance (FT-ICR) compensated trap. The purpose of the new trap is to reduce effects of the nonlinear components of the trapping electric field; those nonliner components introduce variations in the cyclotron frequency of an ion depending on its spatial position (its cyclotron and trapping mode amplitudes). This frequency spread leads to decreased mass resolving power and signal-to-noise. The reduction of the spread of cyclotron frequencies, as explicitly modeled in theory, serves as the basis for our design. The compensated trap shows improved signal-to-noise and at least a threefold increase in mass resolving power compared to the uncompensated trap at the same trapping voltage. Resolving powers (FWHH) as high as 1.7 x 10(7) for the [M + H](+) of vasopressin at m/z 1084.5 in a 7.0-tesla induction can be obtained when using trap compensation.

Trapped-ion cell with improved DC potential harmonicity for FT-ICR MS

The trapped-ion cell is a key component critical for optimal performance in Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS). To extend the performance of FT-ICR MS, we have developed a new cell design that is capable of generating a DC trapping potential which closely approaches that of an ideal Penning trap, i.e., a 3D axial quadrupolar potential distribution. The new cell design was built upon an open cylindrical geometry, supplemented with two pairs of cylindrical compensation segments. Electric potential calculations for trial cell geometries were aimed at minimizing spatial variations of the radial electric field divided by radius. The resulting cell proportions and compensation voltages delivered practically constant effective ion cyclotron frequency that was independent of ion radial and axial positions. Our customized 12 tesla FT-ICR instrument was upgraded with the new cell, and the performance was characterized for a range of ion excitation power and ion populations. Operating the compensated cell at increased postexcitation radii, approximately 0.7 of the cell inner radius, resulted in improved mass measurement accuracy together with increased signal intensity. Under these same operating conditions the noncompensated open cell configuration exhibited peak splitting and reduced signal life time. Mass accuracy tests using 11 calibrants covering a wide m/z range reproducibly produced under 0.05 ppm RMS precision of the internal calibration for reduced ion populations and the optimal excitation radius. Conditions of increased ion population resulted in a twofold improvement in mass accuracy compared with the noncompensated cell, due to the larger achievable excitation radii and correspondingly lower space charge related perturbations of the calibration law.

Evaluation of different combinations of gated trapping, RF-only mode and trap compensation for in-field MALDI Fourier transform mass spectrometry

MALDI, while providing advantages such as the ability to do in-depth and repeated exploration of the sample, challenges the existing performance capabilities of Fourier transform mass spectrometry (FTMS). The challenge arises because MALDI-produced ions have high mass-to-charge ratios and uncertain kinetic-energy distributions. We demonstrate that a combination of a gated trapping event, a RF-only mode pressure focusing event, and an electrically compensated trap provides a compelling advantage in meeting these challenges. Removal of any of the above combination elements significantly degrades the detection performance of substance P from 850 K resolving power at 34.9 kHz and of melittin from 278 K resolving power at 16.5 kHz when using a 3-Tesla magnet-based spectrometer.

A gated trapping strategy with a two-time constant and a delay for catching in-field generated ions that range over three decades in mass-to-charge and two decades in velocity in Fourier-transform mass spectrometry

In-field, matrix-assisted laser desorption/ionization (MALDI) may provide a means to keep part of the original promise of Fourier-transform mass spectrometry (FTMS) to give high performance and versatile mass spectrometry from a mechanically simple instrument. Gated trapping has been employed as a means of catching MALDI-produced ions in the FTMS trap. This approach is important for both in-field and externally produced ions. Even with improvements, gated trapping has not yet been able to catch ions over wide ranges of mass-to-charge and velocity. A design of a "two-time constant with a delay" gated trapping strategy using "idealized" potentials in a normalized system is given as an example to establish that in principle gated trapping strategies can capture ions that range over three decades of m/z and two decades in velocity. A procedure for calculating a physical system from the normalized system is given. The design is tolerant of variations in the physical parameters used to define the physical system from the normalized system.

Dynamically assisted gated trapping for Fourier transform ion cyclotron mass spectrometry

An efficient approach for trapping ions and enhancing signal based on 'adiabatic amplitude reduction' for Fourier transform ion cyclotron resonance (FTICR) mass spectrometry is described and evaluated. This method is a modification to the widely used gated trapping technique in which the trapping potential is raised adiabatically rather than instantaneously (non-adiabatically). Compared with non-adiabatic gated trapping, the final amplitudes of ion axial oscillations and energies are lower in the proposed method. All performance aspects of the FTICR spectrum (e.g., peak intensities, mass resolution, and mass accuracy) improve significantly compared to the conventional gated trapping technique.

Deceleration of high-energy matrix-assisted laser desorption/ionization ions in an open cell for Fourier transform ion cyclotron resonance mass spectrometry

A new method of ion deceleration in a Fourier transform ion cyclotron resonance (FTICR) open cell is described that improves the performance of FTICR-MS instruments equipped with an internal source for laser desorption/ionization. Ion deceleration occurs in the front trapping cylinder of an open cylindrical cell. Decelerating voltages up to 100 V can be applied for 10-500 micros to the front cylinder during ion introduction. The deceleration field is uniformly distributed along the cylinder length giving a "smooth" deceleration, which means that the deceleration is effective over a large time interval and a large m/z range. This results in improved trapping efficiency of high-energy ions. We demonstrate efficient trapping of high (m/z 66 kDa) mass ions and the possibility to reduce the width of the kinetic energy distribution of MALDI ions with this arrangement.

Laser desorption in transmission geometry inside a Fourier-transform ion cyclotron resonance mass spectrometer

We report here the first application of laser desorption (LD) in transmission geometry (backside irradiation of the sample through a transparent support) inside a Fourier-transform ion cyclotron resonance mass spectrometer (FT-ICR). A probe-mounted fiber optic assembly was used to simplify the implementation of this LD technique. This setup requires little or no instrument modifications, has minimum maintenance requirements, and is relatively inexpensive to build. The performance of the probe was tested by determining the molecular weight of a commercial polystyrene standard from its matrix-assisted laser desorption/ionization (MALDI) spectrum. The measured average molecular weight is comparable to that obtained for the same sample by MALDI in the conventional top-illumination arrangement (reflection geometry) and by the manufacturer of the sample by gel permeation chromatography. The average velocities measured for ions evaporated by transmission mode LD of several neat samples are about half the velocity of those obtained by using the reflection geometry. Therefore, transmission mode irradiation of the sample holds promise to desorb ions that are easier to trap in an ICR cell. An oscillating capillary nebulizer was adapted for the deposition of analytes to improve sampling reproducibility.