Restrained ion population transfer: a novel ion transfer method for mass spectrometry

A planar quadrupole device for transmitting and trapping ions in high vacuum

RATIONALE

Hybrid mass spectrometers combine multiple mass analyzers to achieve optimal performance in terms of tandem mass spectrometry, high mass resolving power, and mass measurement accuracy for studying highly complex samples. As a result, the need for transport, trapping, and control of ion kinetic energies is critical for the successful integration of multiple mass analyzers and hybrid instrument operation. In addition, transportation of ion populations between two physically distinct locations can result in time-of-flight (TOF) discrimination against ions with widely disparate m/z values, compromising full mass spectral performance. In this work, we demonstrated a new ion guide, referred to as a planar quadrupole (PQ) ion guide, composed of two parallel printed circuit boards (PCB) that allow radiofrequency (RF) and direct current (DC) voltages to be combined to enable both axial transport and trapping of ion populations in the ultrahigh vacuum region of the mass spectrometer. As compared with a conventional multipole ion guide, the PQ ion guide showed comparable performance in ion m/z values, signal-to-noise, and intensity and effectively reduced mass discrimination caused by TOF effects.

METHODS

A PQ device was developed with two PCBs and simulated with SIMION 8.1. Electrospray ionization and Fourier transform ion cyclotron resonance mass spectrometry instrumentation were used for the testing of PQ performance.

RESULTS

.In this work, we demonstrated a planar quadrupole (PQ) ion guide composed of two parallel PCB plates. The PQ enables both axial ion transport and trapping of ion populations throughout the ion transfer process from a LTQ to an ICR cell. As compared with a conventional multipole ion guide, the PQ showed comparable ion transmission efficiency and effectively reduced mass discrimination caused by TOF effects.

CONCLUSIONS

The PQ is a simple design that can be implemented for ion transmission and trapping on virtually any mass spectrometer.

Developments in FTICR-MS and Its Potential for Body Fluid Signatures

Fourier transform mass spectrometry (FTMS) is the method of choice for measurements that require ultra-high resolution. The establishment of Fourier transform ion cyclotron resonance (FTICR) MS, the availability of biomolecular ionization techniques and the introduction of the Orbitrap™ mass spectrometer have widened the number of FTMS-applications enormously. One recent example involves clinical proteomics using FTICR-MS to discover and validate protein biomarker signatures in body fluids such as serum or plasma. These biological samples are highly complex in terms of the type and number of components, their concentration range, and the structural identity of each species, and thus require extensive sample cleanup and chromatographic separation procedures. Clearly, such an elaborate and multi-step sample preparation process hampers high-throughput analysis of large clinical cohorts. A final MS read-out at ultra-high resolution enables the analysis of a more complex sample and can thus simplify upfront fractionations. To this end, FTICR-MS offers superior ultra-high resolving power with accurate and precise mass-to-charge ratio (m/z) measurement of a high number of peptides and small proteins (up to 20 kDa) at isotopic resolution over a wide mass range, and furthermore includes a wide variety of fragmentation strategies to characterize protein sequence and structure, including post-translational modifications (PTMs). In our laboratory, we have successfully applied FTICR “next-generation” peptide profiles with the purpose of cancer disease classifications. Here we will review a number of developments and innovations in FTICR-MS that have resulted in robust and routine procedures aiming for ultra-high resolution signatures of clinical samples, exemplified with state-of-the-art examples for serum and saliva.

Controlled ion ejection from an external trap for extended m/z range in FT-ICR mass spectrometry

An auxiliary rf waveform of the same amplitude and phase applied to all the rods of an ion accumulation multipole creates an m/z-dependent axial pseudo potential. Controlled decrease of the auxiliary rf amplitude releases ions from the accumulation multipole sequentially from high to low m/z. The slope of the auxiliary rf voltage ramp is adjusted so that ions of different m/z reach the center of the ICR cell at the same time point, which mitigates the typical time dispersion observed in external source FT-ICR and extends the observable mass range for a single data acquisition by 2- to 3-fold. For complex mixture analysis, twice the number of elemental compositions are assigned when the auxiliary rf ejection is applied compared with the standard gated trapping.

A novel 9.4 tesla FTICR mass spectrometer with improved sensitivity, mass resolution, and mass range

Fourier transform ion cyclotron resonance (FTICR) mass spectrometry provides unparalleled mass measurement accuracy and resolving power. However, propagation of the technique into new analytical fields requires continued advances in instrument speed and sensitivity. Here, we describe a substantial redesign of our custom-built 9.4 tesla FTICR mass spectrometer that improves sensitivity, acquisition speed, and provides an optimized platform for future instrumentation development. The instrument was designed around custom vacuum chambers for improved ion optical alignment, minimized distance from the external ion trap to magnetic field center, and high conductance for effective differential pumping. The length of the transfer optics is 30% shorter than the prior system, for reduced time-of-flight mass discrimination and increased ion transmission and trapping efficiency at the ICR cell. The ICR cell, electrical vacuum feedthroughs, and cabling have been improved to reduce the detection circuit capacitance (and improve detection sensitivity) 2-fold. The design simplifies access to the ICR cell, and the modular vacuum flange accommodates new ICR cell technology, including linearized excitation, high surface area detection, and tunable electrostatic trapping potential.

Excite-coupled trapping ring electrode cell (eTREC): radial trapping field control, linearized excitation, and improved detection

A novel excite-coupled Trapping Ring Electrode Cell (eTREC) was designed and developed. eTREC technology provides greater linearity in the excitation electric field along with minimized variation in radial trapping field during detection. The variation in the radial trapping electric field is reduced through postexcitation modulation of the trapping potentials applied to the Trapping Ring Electrode Cell (TREC). Linearization of the electric field generated during radio frequency (RF) excitation is accomplished by coupling the RF excitation to a novel electrode arrangement superimposed onto the trapping rings of a TREC. The coupling of RF excitation to the trap plates effectively reduces z-axis ejection and allows for a more uniform postexcitation radius for the entire ion population. Using this technology, sensitivity was increased by >50%, resolution of (13)C(2) and (34)S fine structure peaks was achieved with the peptide MMMMG (approximately 330,000 RP) on a 3 T system, and the limit of detection was significantly reduced.