Broad-band fourier transform quadrupole ion trap mass spectrometry

Broad-band nondestructive ion detection is achieved in a quadrupole ion trap mass spectrometer by impulsive excitation of a collection of trapped ions of different masses and recording of ion image currents induced on a small detector electrode embedded in but isolated from the adjacent end cap electrode. The image currents are directly measured using a simple differential preamplifier, filter, and amplifier combination and then Fourier analyzed to obtain broad-band frequency domain spectra characteristic of the sample ions. The use of the detector electrode provides a significant reduction in capacitive coupling with the ring electrode. This minimizes coupling of the rf drive signal, which can saturate the front-end stage of the detection circuit and prevent measurement of the relatively weaker ion image currents. Although impulsive excitation is preferred due to its broad-band characteristics and simplicity of use, results are also given for narrow-band ac and broad-band SWIFT (stored wave-form inverse Fourier transform) excitation. Data using argon, acetophenone, and n-butylbenzene show that a resolution of better than 1000 is obtained with a detection bandwidth of 400 kHz. An advantage of nondestructive ion detection is the ability to measure a single-ion population multiple times. This is demonstrated using argon as the sample gas with an average remeasurement efficiency of >90%. Tandem mass spectrometry experiments using a population of acetophenone ions are also shown.     

A "screened" electrostatic ion trap for enhanced mass resolution, mass accuracy, reproducibility, and upper mass limit in Fourier transform ion cyclotron resonance mass spectrometry

Until now, it was thought that the optimal static electromagnetic ion trap for Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry should be designed to produce a quadrupolar electrical potential, for which the ion cyclotron frequency is independent of the ion's preexcitation location within the trap. However, a quadrupolar potential results in a transverse (to the magnetic field) electric field that increases linearly with distance from the center of the trap. That radially linear electric field shifts the observed ICR frequency, increases the ICR orbital radius, and ultimately limits the highest mass-to-charge ratio ion that can be contained within the trap. In this paper, we propose a new static electromagnetic ion "trap" in which grounded screens placed just inside the usual "trapping" plates produce a good approximation to a "particle-in-a-box" potential (rather than the quadrupolar "harmonic oscillator" potential). SIMION calculations confirm that the electric potential of the screened trap is near zero almost everywhere within the trap. For our screened orthorhombic (2.5 in. X 2 in. X 2 in.) trap, the experimental ICR frequency shift due to trapping voltage is reduced by a factor of approximately 100, and the experimental variation of ICR frequency with ICR radius is reduced by a factor of approximately 10 compared to a conventional (unscreened) 2-in. cubic ion trap.(ABSTRACT TRUNCATED AT 250 WORDS)     

Modeling and analysis of the induced signal detected in electrostatic ion beam trap for mass spectrometry

The induced image charge and image current acquired by a detector tube for mass analysis are simulated using a numerical electrostatic model in the context of the electrostatic ion beam trap (EIBT). With the simulation results, the principle of mass analysis using the induced signal is demonstrated and studied systematically. The results show that the intensity of the detected signal is significantly influenced by the size and configuration of the detector, and also impacted by ion velocity, the number of ions in the ion group, and the ion beam length. The simulation results could not only be used to optimize the size and configuration of the detector and thus to improve the detected signal, but also to support the signal analysis (such as FFT) at an EIBT for mass spectrometry.     

Automated gain control ion funnel trap for orthogonal time-of-flight mass spectrometry

Time-of-flight mass spectrometry (TOF MS) is increasingly used in proteomics research. Herein, we report on the development and characterization of a TOF MS instrument with improved sensitivity equipped with an electrodynamic ion funnel trap (IFT) that employs an automated gain control (AGC) capability. The IFT-TOF MS was coupled to a reversed-phase capillary liquid chromatography (RPLC) separation and evaluated in experiments with complex proteolytic digests. When applied to a global tryptic digest of Shewanella oneidensis proteins, an order-of-magnitude increase in sensitivity compared to that of the conventional continuous mode of operation was achieved due to efficient ion accumulation prior to TOF MS analysis. As a result of this sensitivity improvement and related improvement in mass measurement accuracy, the number of unique peptides identified in the AGC-IFT mode was 5-fold greater than that obtained in the continuous mode.     

C60 secondary ion Fourier transform ion cyclotron resonance mass spectrometry

Secondary ion mass spectrometry (SIMS) has seen increased application for high spatial resolution chemical imaging of complex biological surfaces. The advent and commercial availability of cluster and polyatomic primary ion sources (e.g., Au and Bi cluster and buckminsterfullerene (C(60))) provide improved secondary ion yield and decreased fragmentation of surface species, thus improving accessibility of intact molecular ions for SIMS analysis. However, full exploitation of the advantages of these new primary ion sources has been limited, due to the use of low mass resolution mass spectrometers without tandem MS to enable enhanced structural identification capabilities. Similarly, high mass resolution and high mass measurement accuracy would greatly improve the chemical specificity of SIMS. Here we combine, for the first time, the advantages of a C(60) primary ion source with the ultrahigh mass resolving power and high mass measurement accuracy of Fourier transform ion cyclotron resonance mass spectrometry. Mass resolving power in excess of 100?000 (m/Δm(50%)) is demonstrated, with a root-mean-square mass measurement accuracy below 1 part-per-million. Imaging of mouse brain tissue at 40 μm pixel size is shown. Tandem mass spectrometry of ions from biological tissue is demonstrated and molecular formulas were assigned for fragment ion identification.     

Fourier transform detection in a cylindrical quadrupole ion trap

Broad-band nondestructive ion detection based on induced image current measurement is performed in a quadrupole ion trap having cylindrical geometry. Spectra of krypton and acetophenone are shown to demonstrate the first use of nondestructive detection with a cylindrical ion trap.     

Planar multipole ion trap/time-of-flight mass spectrometer

We present a novel, hybrid ion trap/time-of-flight mass spectrometer that is based on a planar multipole design. Compared with Paul trap/time-of-flight instruments, this design possesses the principal advantages of higher injection efficiency and more homogeneous extraction fields. We demonstrate the viability of the concept and describe the characterization of a first prototype. Ions can be injected into the trap with little mass discrimination and stored for several minutes. A resolution of over 1300 is achieved in reflectron mode, and the influence of the RF amplitude and pressure on the resolution is analyzed. We suggest several applications in which this new instrument could offer advantages over existing technology.     

High-performance mass spectrometry: Fourier transform ion cyclotron resonance at 14.5 Tesla

We describe the design and current performance of a 14.5 T hybrid linear quadrupole ion trap Fourier transform ion cyclotron resonance mass spectrometer. Ion masses are routinely determined at 4-fold better mass accuracy and 2-fold higher resolving power than similar 7 T systems at the same scan rate. The combination of high magnetic field and strict control of the number of trapped ions results in external calibration broadband mass accuracy typically less than 300 ppb rms, and a resolving power of 200,000 (m/Delta m50% at m/z 400) is achieved at greater than 1 mass spectrum per second. Novel ion storage optics and methodology increase the maximum number of ions that can be delivered to the FTICR cell, thereby improving dynamic range for tandem mass spectrometry and complex mixture applications.     

Dynamically multiplexed ion mobility time-of-flight mass spectrometry

Ion mobility spectrometry-time-of-flight mass spectrometry (IMS-TOFMS) has been increasingly used in analysis of complex biological samples. A major challenge is to transform IMS-TOFMS to a high-sensitivity, high-throughput platform, for example, for proteomics applications. In this work, we have developed and integrated three advanced technologies, including efficient ion accumulation in an ion funnel trap prior to IMS separation, multiplexing (MP) of ion packet introduction into the IMS drift tube, and signal detection with an analog-to-digital converter, into the IMS-TOFMS system for the high-throughput analysis of highly complex proteolytic digests of, for example, blood plasma. To better address variable sample complexity, we have developed and rigorously evaluated a novel dynamic MP approach that ensures correlation of the analyzer performance with an ion source function and provides the improved dynamic range and sensitivity throughout the experiment. The MP IMS-TOFMS instrument has been shown to reliably detect peptides at a concentration of 1 nM in the presence of a highly complex matrix, as well as to provide a 3 orders of magnitude dynamic range and a mass measurement accuracy of better than 5 ppm. When matched against human blood plasma database, the detected IMS-TOF features were found to yield approximately 700 unique peptide identifications at a false discovery rate (FDR) of approximately 7.5%. Accounting for IMS information gave rise to a projected FDR of approximately 4%. Signal reproducibility was found to be greater than 80%, while the variations in the number of unique peptide identifications were <15%. A single sample analysis was completed in 15 min that constitutes almost 1 order of magnitude improvement compared to a more conventional LC-MS approach.     

Ion funnel trap interface for orthogonal time-of-flight mass spectrometry

A combined electrodynamic ion funnel and ion trap coupled to an orthogonal acceleration (oa)-time-of-flight mass spectrometer was developed and characterized. The ion trap was incorporated through the use of added terminal electrodynamic ion funnel electrodes enabling control over the axial dc gradient in the trap section. The ion trap operates efficiently at a pressure of approximately 1 Torr, and measurements indicate a maximum charge capacity of approximately 3 x 10(7) charges. An order of magnitude increase in sensitivity was observed in the analysis of low concentration peptides mixtures with orthogonal acceleration (oa)-time-of-flight mass spectrometry (oa-TOF MS) in the trapping mode as compared to the continuous regime. A signal increase in the trapping mode was accompanied by reduction in the chemical background, due to more efficient desolvation of, for example, solvent related clusters. Controlling the ion trap ejection time was found to result in efficient removal of singly charged species and improving signal-to-noise ratio (S/N) for the multiply charged analytes.     

Multiplexed ion mobility spectrometry-orthogonal time-of-flight mass spectrometry

Ion mobility spectrometry (IMS) coupled to orthogonal time-of-flight mass spectrometry (TOF) has shown significant promise for the characterization of complex biological mixtures. The enormous complexity of biological samples (e.g., from proteomics) and the need for both biological and technical analysis replicates imposes major challenges for multidimensional separation platforms with regard to both sensitivity and sample throughput. A major potential attraction of the IMS-TOF MS platform is separation speeds exceeding that of conventional condensed-phase separations by orders of magnitude. Known limitations of the IMS-TOF MS platforms that presently mitigate this attraction include the need for extensive signal averaging due to factors that include significant ion losses in the IMS-TOF interface and an ion utilization efficiency of less than approximately 1% with continuous ion sources (e.g., ESI). We have developed a new multiplexed ESI-IMS-TOF mass spectrometer that enables lossless ion transmission through the IMS-TOF as well as a utilization efficiency of >50% for ions from the ESI source. Initial results with a mixture of peptides show a approximately 10-fold increase in signal-to-noise ratio with the multiplexed approach compared to a signal averaging approach, with no reduction in either IMS or TOF MS resolution.     

Ion trap/ion mobility/quadrupole/time-of-flight mass spectrometry for peptide mixture analysis

An ion trap/ion mobility/quadrupole/time-of-flight mass spectrometer has been developed for the analysis of peptide mixtures. In this approach, a mixture of peptides is electrosprayed into the gas phase. The mixture of ions that is created is accumulated in an ion trap and periodically injected into a drift tube where ions separate according to differences in gas-phase ion mobilities. Upon exiting the drift tube, ions enter a quadrupole mass filter where a specific mass-to-charge (m/z) ratio can be selected prior to collisional activation in an octopole collision cell. Parent and fragment ions that exit the collision cell are analyzed using a reflectron geometry time-of-flight mass spectrometer. The overall configuration allows different species to be selected according to their mobilities and m/z ratios prior to collision-induced dissociation and final MS analysis. A key parameter in these studies is the pressure of the target gas in the collision cell. Above a critical pressure, the well-defined mobility separation degrades. The approach is demonstrated by examining a mixture of tryptic digest peptides of ubiquitin.     

Postacquisition mass resolution improvement in time-of-flight secondary ion mass spectrometry

Good mass resolution can be difficult to achieve in time-of-flight secondary ion mass spectrometry (TOF-SIMS) when the analysis area is large or when the surface being analyzed is rough. In most cases, a significant improvement in mass resolution can be achieved by postacquisition processing of raw data. Methods are presented in which spectra are extracted from smaller regions within the original analysis area, recalibrated, and selectively summed to produce spectra with higher mass resolution than the original. No hardware modifications or specialized instrument tuning are required. The methods can be extended to convert the original raw file into a new raw file containing high mass resolution data. To our knowledge, this is the first report of conversion of a low mass resolution raw file into a high mass resolution raw file using only the data contained within the low mass resolution raw file. These methods are applicable to any material but are expected to be particularly useful in analysis of difficult samples such as fibers, powders, and freeze-dried biological specimens.     

On-line capillary electrophoresis/microelectrospray ionization-tandem mass spectrometry using an ion trap storage/time-of-flight mass spectrometer with SWIFT technology.

The development of a system capable of the speed required for on-line capillary electrophoresis-tandem mass spectrometry (CE-MS/MS) of tryptic digests is described. The ion trap storage/reflectron time-of-flight (IT/reTOF) mass spectrometer is used as a nonscanning detector for rapid CE separation, where the peptides are ionized on-line using electrospray ionization (ESI). The ESI produced ions are stored in the ion trap and dc pulse injected into the reTOF-MS at a rate sufficient to maintain the separation achieved by CE. Using methodology generated by software and hardware developed in our lab, we can produce SWIFT (Stored Waveform Inverse Fourier Transform) ion isolation and TICKLE activation/fragmentation voltage waveforms to generate MS/MS at a rate as high as 10 Hz so that the MS/MS spectra can be optimized on even a 1-2 s eluting peak. In CE separations performed on tryptic digests of dogfish myelin basic protein (MBP) where eluting peaks 4-8 s wide are observed, it is demonstrated that an acquisition rate of 4 Hz provides > 20 spectra/peak and is more than sufficient to provide optimized MS/MS spectra of each of the eluting peaks in the electropherogram. The detailed structural analysis of dogfish MBP including several posttranslational modifications using CE-MS and CE-MS/MS is demonstrated using this method with < 10 fmol of material consumed.