Guan, Ziqiang; Campbell, Victoria L.; Drader, Jared J.; Hendrickson, Chistopher L.; Laude, David A., Jr.
High performance detection of biomolecules using a high magnetic field electrospray ionization source/Fourier transform ion cyclotron resonance mass spectrometer
Rev. Sci. Instrum., 66 (9), 1995, pp. 4507-15
An improved, high-performance version of the concentric vacuum chamber design is shown for forming ions at high pressure in a strong magnetic field and detecting them in an adjacent Fourier transform ion cyclotron resonance mass spectrometry (FTICR) trapped ion cell. Improvements in system design, including primarily the addition of a mechanical shutter to halt the flow of neutrals to the trapped ion cell during FTICR detection, allow a more than 100-fold improvement in pressure drop between the source and analyzer chamber to be realized. Within a 20 cm distance, ions formed in an electrospray ion source at atm. are transported across five concentric tube conductance limits to a trapped ion cell at a shuttered pressure below 2 x 10-9 Torr. High resoln. detection of electrosprayed proteins is demonstrated and, for example, mass resolns. of 1 .times. 105 for the +14 charge state of horse heart myoglobin (at m/z 1211) and 2 .times. 105 for +5 charge state of bovine insulin (at m/z 1147) are obtained. The original advantages of the concentric tube vacuum chamber are retained. Forming the ions within the magnetic field permits a 40-fold enhancement in sensitivity to be obtained. Narrow kinetic energy distributions are achieved because magnetic field confinement eliminates the need for complex elec. focusing assemblies that exhibit mass discrimination and broaden the kinetic energy distribution. Finally, the shutter is demonstrated to serve effectively as an alternative to pulsed valve assemblies for the transient introduction of a collision gas to the trapped ion cell.

Hendrickson, C. L.; Laude, D. A., Jr.
Quadrupolar Axialization for Improved Control of Electrosprayed Proteins in FTICR Mass Spectrometry
Anal. Chem., 67 (10), 1995, pp. 1717-21
Quadrupolar axialization is implemented on a high-field electrospray ionization Fourier transform ion cyclotron resonance mass spectrometer. The method is utilized to perform highly efficient remeasurement of small proteins which were not previously amenable to the remeasurement process. For example, 100 consecutive remeasurements of the same population of melittin ions (MW 2845) yield the theor. 10-fold improvement in signal-to-noise ratio compared to the first measurement. The efficiency of the cooling process is evaluated as a function of excitation radius and ion mass, and the results are compared to instrumental performance prior to the implementation of quadrupolar axialization. The max. achievable excitation radius for efficient remeasurement is increased from 18% to 52% of the trapped ion cell radius. The application of quadrupolar axialization during the electrospray ion accumulation event results in a factor of 3 improvement in sensitivity when acquiring bovine insulin (MW 5734) spectra under high-resoln. conditions. The mass selectivity of the technique is used to isolate a single charge state of horse myoglobin (MW 16 951).

Guan, Ziqiang; Drader, Jared J.; Campbell, Victoria L.; Laude, David A.
Real-Time Monitoring of the Gas Phase Reactions of a Single Ion Population Using the Remeasurement Experiment in Fourier Transform Ion Cyclotron Resonance Mass Spectrometry
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A single population of multiply charged protein ions formed by electrospray ionization is held in a trapped ion cell and remeasured continuously by Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry while undergoing multiple reactions with diethylamine. In a first example, electrosprayed horse myoglobin with an av. of 16 attached protons is reacted with base at a pressure of 1.5 x 10^-8 Torr for a period of 60 s. A total of 31 spectra acquired with a duty cycle of 2 s exhibit the charge state-dependent formation of up to three diethylamine adducts and removal of up to nine protons. A real-time measuring expt. is then conducted over a 1 h period to observe charge stripping of electrosprayed myoglobin ions, leaving as few as seven charges. Real-time monitoring is used to evaluate the effect of reagent gas pressure on adduct formation and is used in conjunction with high mass resolution. FTICR detection to resolve the isotopic peaks within individual charge states of adduct spectra. The attractive features of real-time reaction monitoring include a dramatic reduction. in experiment time and sample consumed while concurrently observing long-lived intermediates and reaction products as they form.

Campbell, Victoria L.; Guan, Ziqiang; Vartanian, Victor H.; Laude, David A.
Cell Geometry Considerations for the Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Remeasurement Experiment
Source:---------------
Three cell geometries, closed cubic, closed elongated, and open elongated, are evaluated for optimum remeasurement performance in the Fourier transform ion cyclotron resonance mass spectrometry (FTICR) experiment. The advantages and disadvantages of each cell during normal FTICR operation are discussed. Unit remeasurement efficiency is obtained for the closed elongated cell at .apprx.71% of the cell radius, whereas the open elongated cell achieves unit remeasurement at only .apprx.18% of the cell radius. The closed cubic cell does not achieve unit remeasurement efficiency at a detectable signal level. A correlation between remeasurement efficiency and the characteristic radial elec. field of each trapped ion cell is established. Radial dispersion of the ion cloud is considered the dominant signal loss mechanism in the remeasurement experiment, and cell geometries are examined for optimized sensitivity and unit remeasurement efficiency.

Guan, Ziqiang; Hofstadler, Steven A.; Laude, David A., Jr.
Remeasurement of electrosprayed proteins in the trapped ion cell of a Fourier transform ion cyclotron resonance mass spectrometer
Source
A single population of multiply charged protein ions formed by electrospray ionization (ESI) is subjected to multiple Fourier transform ion cyclotron resonance (FTCIR) excitation and detection events without promoting ion loss from the trapped ion cell. This nondestructive approach to mass spectrometric detection will allow detection limits to be significantly reduced by averaging the repetitive time-domain response that follows each excitation event. Under appropriate conditions, unit remeasurement efficiency is obsd. for large proteins; for example, 250 consecutive remeasurements of a single population of bovine albumin dimer ions (MW 132,532) yield the expected 16-fold improvement in signal-to-noise ratio compared to a single measurement. Examples presented include a 14-mol sample of horse myoglobin (MW 16,951), which yields a 15:1 S/N for 50 remeasurements compared to a single scan S/N of 2:1 and a 30-fmol sample of bovine albumin dimer which exhibits a S/N of 25:1 for 50 remeasurements compared to a single scan S/N of 3:1. Of both practical and theor. interest is the observation of a rapid collision-mediated homogeneous relaxation process that can return these large ions to the center of the cell within a few hundred milliseconds after excitation.

Hofstadler, S. A.; Beu, S. C.; Laude, D. A., Jr.
Band-pass kinetic energy filter for postionization separation of proteins by electrospray ionization/Fourier transform ion cyclotron resonance mass spectrometry
Anal. Chem., 65 (3), 1993, pp. 312-16
A postionization technique is developed for sepg. electrosprayed protein mixts. that have been introduced to a Fourier transform ion cyclotron resonance mass spectrometer (FTICR). Factors detg. the relative order of trapping at increasing accumulation potentials in the range 1-10 V at the cell are the potential applied to the skimmer cone and ion velocity in the expansion. A model for optimum trapping potential is developed and found to depend primarily on mass to charge ratio within the electrospray charge envelope but also on protein-specific features including velocity slip in the expansion and effective potential experienced upon exiting the skimmer cone. For a three component mixt. of cytochrome c, bovine albumin, and chicken egg white lysozyme, optimum accumulation potentials of 3.5, 5.0, and 6.5 V, resp., are employed to generate ESI/FTICR spectra of the individual proteins.

Zhou, Feimeng; Yau, Shueh Lin; Jehoulet, Christophe; Laude, David A., Jr.; Guan, Ziqiang; Bard, Allen J.
Electrochemistry of fullerene (C60) films: quartz crystal microbalance and mass spectrometric studies
J. Phys. Chem., 96 (11), 1992, pp. 4160-2
The incorporation of tetra-n-butylammonium (TBA+) upon electroredn. of films of C60 on an electrode in MeCN solns. was studied by electrochem. quartz crystal microbalance (EQCM) and laser desorption mass spectrometry (LDMS) methods. EQCM allowed estn. of the extent of dissoln. of the films upon redn. to the C60- and C602- forms and the increase in mass caused by incorporation of TBA+. LDMS anal. also indicated the presence of TBA= after redn. Both methods suggested that TBA+ remains in the lattice after reoxidn. of the reduced forms to C60, indicating a new type of ionic charge trapping in these films.

Hogan, Jeremiah D.; Laude, David A., Jr.
Mass discrimination in laser desorption/Fourier transform ion cyclotron resonance mass spectrometry cation-attachment spectra of polymers
Anal. Chem., 64 (7), 1992, pp. 763-9
A mass discrimination effect is demonstrated for the title mass spectral anal. (FTICR) for low-no.-av.-mol.-wt. polyethylene glycol (PEG). This effect is attributed to factors influencing the relative overlap of gas-phase neutral and ion populations reacting to form the cation-attached products detected by FTICR. The factors include laser power d. and reactant masses which det. velocity distributions, trap potential which serves as an energy filter for detected product ions, and distance between the trapped ion cell and desorption site. PEG samples of mol. wts. 600, 1000, and 1500 vary by 7, 10, and 12%, resp., as the desorption site is displaced over a 10-cm distance from the cell. The polydispersity value for a std. PEG-1000 sample, as detd. by vapor-phase osmometry, is achieved at probe distances of 5.4, 7.2, and 8.2 cm, resp., when KF, KBr, and KI are mixed with the polymer. All data are consistent with an ionization mechanism in which a distribution of desorbed low-energy polymer neutrals reacts in the cell with the low-energy tail of a salt adduct population. The well-defined polymer distribution provides effective time resolution in defining the gas-phase reactions occurring in the FTICR experiment, but are not otherwise discernible on the time scale of the detection process.

Hofstadler, Steven A.; Laude, David A., Jr.
Electrospray ionization in the strong magnetic field of a Fourier transform ion cyclotron resonance mass spectrometer
Anal. Chem., 64 (5), 1992, pp. 569-72
A new electrospray source for Fourier transform ion cyclotron resonance mass spectrometry (FTICR) is positioned in the bore of a superconducting magnet adjacent to the trapped ion cell. With this interface an electrospray ion current of 375 pA from gramicidin-S is detected at the cell. This two order of magnitude increase in current compared to external source ESI/FTICR designs is attributed to the strong radial confining nature of the magnetic field. Five stages of differential pumping within a series of concentric tubes of increasing radius in a 25 cm length region of the vacuum chamber are used to reduce atm. pressure at the source to the mid-10-7 torr range at the analyzer trapped ion cell. Spectra are presented for three proteins including bovine albumin dimer with a mol. wt. of 132,532. For the dimer, the spectrum exhibits a signal-to-noise of 71 for the base peak in a charge envelope ranging from +54 to +66 in the m/z range from 2645 to 2007. At m/z 1100, pressure-limited mass resoln. values are .apprx.110 and do not vary for proteins in the mol. wt. range from 1140 to 132,532 Da. This resoln. provides sufficient charge state sepn. to obtain mol. wt. ests. for the three proteins, with av. results that are within 0.07% of actual values.

Beu, Steven C.; Laude, David A., Jr.
Elimination of axial ejection during excitation with a capacitively coupled open trapped-ion cell for Fourier transform ion cyclotron resonance mass spectrometry
Anal. Chem., 64 (2), 1992, pp. 177-80
A Fourier transform ion cyclotron resonance (FTI-CR) mass spectrometer trapped-ion cell is constructed that eliminates the axial ejection of ions during excitation of cyclotron motion. The cell features an open geometry in which the trapping electrodes are extended in the planes of excitation and detection electrodes. A unique aspect of the cell is the capacitive coupling of excitation electrodes to the trap plates to distribute the excitation fields beyond the boundaries of the trapping-potential well. Not terminating excitation elec. fields at trap plates that are positioned perpendicular to the magnetic field and thus bounding the potential well eliminates the axial component of the excitation field that is responsible for ejection. Profiles of broad-band mass spectra acquired at increasing excitation energies demonstrate that, even at 0.4 V trap potentials, only mass-independent radial ejection occurs when the excitation is distributed throughout the open cell. In contrast, cells of similar dimension, including both a conventional closed cell at 1.0 V trap potentials and an uncoupled open cell at 0.4 V trap potentials, exhibit severe low-mass ejection during high-power excitation. The open cell is used to acquire laser desorption/ionization mass spectra as evidence of a design simplicity that facilitates immediate integration into more intricate FTICR experiments.

Hofstadler, Steven A.; Laude, David A., Jr.
Isolated dual trapped ion cell assembly for Fourier transform ion cyclotron resonance mass spectrometry
Anal. Chem., 63 (18), 1991, pp. 2001-7
An elec. isolated dual trapped ion cell positioned in the strong magnetic field of a Fourier-transform ion cyclotron resonance (FTICR) mass spectrometer is demonstrated to overcome an inefficient electron ionization duty cycle that is common to previous dual cell configurations. A repeller grid positioned between differentially pumped trapped ion cells allows continuous ion formation to occur in the source while excluding electrons from the analyzer cell during concurrent data acquisition. In a systematic study of dual cell performance including transfer efficiency, mass selectivity, and signal enhancement, the high-field isolated dual cell design was compared to conventional dual cell and fringing field external cell arrangements. Several ion trapping and manipulation schemes were investigated. For the high-field isolate cell, gated transfer and equil. pulse sequence transfer efficiencies routinely exceeded 80% and 30%, resp. These values were similar to the performance achieved with the conventional dual cell design. In contrast, transfer efficiencies for the fringing field source were a factor of 5 poorer. However, when time-of-flight-based mass selection was desired, superior performance was derived from the fringing field source because of increased path length. Finally, large ionization duty cycle increases were achieved for continuous beam experiments in the isolated dual cell with consequent signal/noise improvements as high as a factor of 30 compared to the conventional dual cell in otherwise identical high-resolution measurements.

Hogan, Jeremiah D.; Laude, David A., Jr.
Precursor ions for gas-phase cation-attachment reactions in laser desorption/Fourier transform ion cyclotron resonance mass spectrometry
Anal. Chem., 63 (19), 1991, pp. 2105-9
The mechanism by which cation-attached org. compds. are formed and trapped for detection in the IR laser desorption ionization (LDI)/Fourier transform ion cyclotron resonance (FTICR) mass spectrometry expt. is evalauated. A combination of time-of-flight (TOF), variable trap potential, and double-resonance expts. offers evidence that these ions result from gas-phase reactions in the trapped-ion cell. LDI spectra of KBr-doped org. samples show that as the desorption site is displaced from the trapped-ion cell, the (M + K)+ signal decreases and the K+ signal increases. Optimum LDI/FTICR trapping potentials for (M + K)+ and K+ are less than 3 V and greater than 17 V, resp., which indicates that substantial differences exist in kinetic energy distributions for these ions. In contrast, salt adduct ions formed by LDI, K2Br+ for example, exhibit trapping profiles that are similar to (M + K)+. Double-resonance expts. to eject suspected precursor ions indicate that it is these adduct ions rather than the bare cation which react with the neutral to form the cation-attached org. species. For example, in LDI/FTICR expts. on a mixt. of KCl and dilaurylthiodipropionate (DLTDP), ejection of K+ yields an abundant (M + K)+ ion while continuous ejection of K2Cl+ precludes formation of any product ion species.

Beu, S. C.; Laude, D. A., Jr.
Modular data system for selective wave-form excitation and trapping experiments in Fourier transform mass spectrometry
Anal. Chem., 63 (19), 1991, pp. 2200-3
A modular personal-computer-based data system for Fourier-transform mass spectrometry is described. Primary components are an arbitrary waveform generator which controls transmitter plate and trap plate waveform outputs along sep. channels, and a digital storage oscilloscope for data acquisition from receiver plates. Advantages of the system are a redn. in expense compared to integrated com. systems, simplified assembly and operation that obviates the need for circuit board design or software development, and increased versatility in execution of excitation and trapping waveforms. This versatility is exhibited by implementing broadband stored waveform inverse Fourier transform excitation with multiple suppression windows and a linear adiabatic ramp of trapping potentials to compress the z-amplitude of an ion cloud and thereby alleviate axial ejection during excitation.

Hogan, Jeremiah D.; Beu, Steven C.; Laude, David A., Jr.; Majidi, Vahid
Probe-mounted fiber optic assembly for laser desorption/ionization Fourier transform mass spectrometry
Anal. Chem., 63 (14), 1991, pp. 1452-7
A probe-mounted fiber optic interface for laser desorption/ionization (LDI) Fourier transformation mass spectrometry (FTMS) is demonstrated as an alternative to conventional optical assemblies for mass spectral anal. of nonvolatile and large mol. wt. compds. Among advantages of the fiber optic interface are low cost and ease of alignment and repair. Power densities to 2 .times. 108 W/cm2 are obtained at the probe tip for 1064-nm light from a Nd:YAG laser. Results are similar to those obtained with pulsed CO2 lasers in that cationized mol. species dominate the spectra of peptides, porphyrins, polymers, and polymer additives in the 500-2000-Da range. The effects of sample prove displacement from the trapped-ion cell on LDI/FTMS performance are also evaluated toward an improved understanding of the mechanism by which LDI-generated ions are trapped in the cell. For metal ions such as Au+, signal intensity is invariant at distances up to 35 cm from the cell. In contrast, potassium-attached org. mol. ions such as gramicidin S, polyethylene glycol 1000, and dilauryl thiodipropionate exhibit a redn. in spectral signal/noise with increasing probe displacement. This is in keeping with a gas-phase mechanism for ion formation in the trapped-ion cell. From a practical perspective, spectral reproducibility and quality are enhanced for both metal and org. samples by positioning the sample probe several centimeters from the cell to minimize spectral distortion due to space charge effects.

Riegner, D. E.; Hofstadler, S. A.; Laude, David A., Jr.
Mass discrimination due to z axis ion cloud coherence in the Fourier transform mass spectrometry trapped-ion cell
Anal. Chem., 63 (3), 1991, pp. 261-8
A mass discrimination effect common to Fourier transform mass spectroscopy (FTMS) expts. involving the introduction of externally generated ions to the analyzer trapped-ion cell is described. The combination in inhomogeneous excitation fields with the coherent-z axis motion of focussed-ion packets is shown to introduce time-dependent variations in apparent relative mass abundances that are as large as 60%. This effect is evaluated for the dual-cell ion equil. expt. by modeling the z-axis motion of isomass packets between adjacent trapped-ion cells during the pulse sequence. Predicted oscillations in the FTMS signal profile with period equal to half the expected cubic cell trapping period are then verified exptl. Mechanisms for abrupt termination of the mass-dependent oscillations at a time 2-25 ms after initiation are also considered; for example, ion-neutral and ion-ion interactions are discounted, and correlation with the onset of large amplitude magnetron motion is established. The merits of the dual-cell ion equil. pulse sequence as a direct probe of z axis evolution of the ion cloud during the FTMS expt. are demonstrated.

Hogan, Jeremiah D.; Laude, David A., Jr.
Suspended trapping procedure for alleviation of space charge effects in gas chromatography/Fourier-transform mass spectrometry
Anal. Chem., 62 (5), 1990, pp. 530-5
A suspended trapping pulse sequence is implemented in Fourier-transform mass spectrometry detection of capillary gas chromatog. effluent as a means to alleviate space charge effects in the trapped ion cell. The combination of intense ionization conditions and a suspended trapping delay extends the working range of gas chromatog./Fourier transform mass spectrometry (GC/FTMS) for which high-performance spectra are generated to 5 orders of magnitude, from detection limits of 10-100 pg to the limit of gas chromatog. column capacity. This corresponds to a factor of 103 improvement compared to conventional trapping methods. Shifts in cyclotron frequency over the eluting GC peak profile are also reduced from as much as 210 Hz to less than 3 Hz over the same range of neutral analyte concns., which indicates accurate mass calibration can be achieved independent of initial ion population in the trapped ion cell. This capability is demonstrated as frequency assignments with low part-per-million error are obtained by GC/FTMS for mixt. components of varying concn. from a single suspended trapping calibration table.

Laude, David A., Jr.; Beu, Steven C.
A suspended trapping pulse sequence for simplified mass calibration in Fourier-transform mass spectrometry
Anal. Chem., 61 (21), 1989, pp. 2422-7
A new pulse sequence for Fourier transform mass spectrometry is demonstrate to simplify wide-band mass calibration through a self-regulating procedure that reduces any initial ion population in the trapped ion cell to a level below the space charge limit. The novel feature of the pulse sequence is the addition. of an event following ionization during which the trap plates of the FTMS cell are grounded to permit the longitudinal efflux of excess ions from the cell. The actual ion population detected then depends not on the initial neutral population or ionization conditions, but rather on the suspending trapping delay time. At suspended trapping delays of 1-2 ms for both single- and dual-section trapped ion cells, the no. of ions detected is reduced to a level at which the ion d. elec. field contribution to the effective cyclotron frequency is negligible. Low-part-per-million error mass calibration tables are generated when suspended trapping is applied to initial ion populations which extend orders of magnitude beyond the space charge limit of the cell. This is contrasted with marked deterioration in mass calibration performance for identical initial ion populations if trapping voltages are sustained.