Ion trap performances were investigated based on digital ion trap technology with different collision gases and their pressures. Collision gases of helium (M = 4 amu), nitrogen (M = 28 amu) and argon (M = 40 amu) with various pressures were adopted in ion excitation and dissociation stages for comparing the ion trap performances, including mass resolution, signal intensity, analysis ability of tandem mass spectrometric and low-mass cut off (LMCO) effect. The results indicated that when argon was used, the kinetic energy could be efficiently transferred and the LMCO effect was improved with higher ion capture and dissociation efficiencies except lower mass resolution. Higher mass resolution was realized with helium as collision gas. Furthermore, at the same gas pressure, heavier gas was beneficial to abundant fragment ions and structural information of precursor ion.The effects of the species and pressures of collision gases on the performance of ion trap were investigated based on digital ion trap technique.Download high-res image (72KB)Download full-size image

•Faster mass separation is obtained with the use of stability islands of special shape (Xband) formed by two AC excitations. Only few cycles of the base AC excitation frequency ν is sufficient to eject unstable ions and it possible to obtain about 10000 mass resolution with ion residence time of only 100 RF cycles.•Mass separation of ions happens in only one direction which increases acceptance of the mass filter.•The instability bands which are used for mass separation appear only in the vicinity of the upper tip of the first stability region, so it possible to use the delayed DC ramp technique to improve signal sensitivity.•The operation with the use of additional AC excitations is not sensitive to many nonlinear field distortions. So the new method might be applied to quadrupole analyzers with lower mechanical accuracy for improving their mass resolving power.A new possibility of improving the resolving power of quadrupole mass filters has been studied theoretically in this work. The results show that with the use of two AC excitations, in addition to the main RF supply, it is possible to modify the first stability diagram for mass filtering by creating a narrow and long band of stability along the X boundary near the tip of first stability region. These newly developed stability regions (the X-band) are similar to higher stability regions, and offer high mass resolution and fast mass separation features. This approach overcomes the many limitations of the normal operation of quadrupole analyzers, while retaining the advantages of using the first stability region. The new operation mode could achieve up to 10,000 mass resolving power with the ion residence time of only 100 RF cycles. In addition, the ion transmission efficiency with the use of the X-band is not only compromised, but is greater than in the normal operation mode. Furthermore, the new mode features one-dimensional mass filtering (in the X direction only) that is not sensitive to nonlinear field distortions, which are particularly problematic for quadrupole mass filters which built with circular rods. Faster mass separation has been confirmed in simulations and theoretical computations of the exponential increment of the trajectory instability. Due to the location of the X-band near the tip of the first stability region, the new operation mode can still have the benefits of traditional techniques (delayed DC ramp) for overcoming the negative effects of fringe fields and improving the ion transmission efficiency. The theoretical simulations show that the method of improving the performance of quadrupole mass filters does not require any modifications of mechanical structures, and only needs different and a little more sophisticated method of electric applications.

AbstractOwing to the ion storage capacity far superior to that of three-dimensional ion trap of the same volume, toriodal ion trap is deemed to be another useful candidate of miniaturization of ion trap mass analyzer in recent years. To further optimize the performance of mass spectrometry of toriodal ion trap, especially the ion detection efficiency and the mass resolving power, a new toriodal ion trap built with triangular electrodes was proposed and investigated theoretically. The new toroidal ion trap was composed of two identical ring electrodes with triangular cross section and two cylindrical electrodes of different sizes. Ions were extracted from the ion trap through resonance excitation for mass analysis. The asymmetrical triangular electrode structure was obtained through theoretical simulation and optimization of ion detection efficiency and mass resolving power. The mass resolution (M/ΔM) of ∼1500 at m/z 609 was obtained for a toriodal ion trap with an optimal electrode structure.The new toroidal ion trap was composed of two identical ring electrodes with asymmetrical triangular cross section and two cylindrical electrodes. The mass resolution (M/ΔM) of ∼1500 at m/z 609 was obtained with an optimal electrode structure.

The theoretical simulation and optimization of a new linear ion trap mass analyzer-ladder electrode ion trap mass analyzer (LeLIT) was described. The LeLIT was consisted of two pairs of ladder electrodes and one pair of end-electrodes. Compared with conventional rectilinear ion trap (ReLIT) which is built with plate electrodes, LeLIT can adjust and optimize the electric field distribution to obtain optimal performance. Meanwhile, the structure of LeLIT is more similar to the geometrical structure of hyperbolic electrode than that of ReLIT, but it is much easier for processing compared with hyperbolic electrode. According to the simulation results, the optimization of electric field distribution inside the ion trap could be realized through adjusting its geometric parameters such as height, width and field radius ratio of ladder electrodes, which was expected to further optimize the mass spectrometry performance of ion trap. As indicated by theoretical simulation results, mass resolution of 10150 at the scanning speed of 225 Da s−1 was obtained with a LeLIT (dimension of X0 × Y0 = 9 mm × 5 mm). LeLIT can significantly improve the mass resolution while maintaining its simple structure. As indicated by the preliminary experiment results, LeLIT has a good analyzing performance as tandem mass spectrometry.The research mainly foucuses on investigation of ladder electrode ion trap mass analyzer (LeLIT). As compared with conventional rectilinear ion trap, the electric field distribution inside LeLIT can be adjusted and optimized for optimum performance. LeLIT can significantly improve the mass resolution while maintaining its simple structure.

Collision induced dissociation (CID) is one of the most established techniques for tandem mass spectrometry analysis. The CID of mass selected ion could be realized by ion resonance excitation with a digital rectangular waveform. The method is simple, and highly efficient CID result could be obtained by optimizing the experimental parameters, such as digital waveform voltage, frequency, and q value. In this work, the relationship between ion trapping waveform voltage and frequency at preselected q value, the relationship between waveform frequency and the q value at certain ion trapping voltage for optimum CID efficiency were investigated. Experiment results showed that the max CID efficiency of precursor reserpine ions can be obtained at different trapping waveform voltage and frequency when q and β are different. Based on systematic experimental analysis, the optimum experimental conditions for high CID efficiency can be calculated at any selected β or q. By using digital ion trap technology, the CID process and efficient fragmentation of parent ions can be realized by simply changing the trapping waveform amplitude, frequency, and the β values in the digital ion trap mass spectrometry. The technology and method are simple. It has potential use in ion trap mass spectrometry.

The ion enhanced activation and collision-induced dissociation (CID) by simultaneous dipolar excitation of ions in the two radial directions of linear ion trap (LIT) have been recently developed and tested by experiment. In this work, its detailed properties were further studied by theoretical simulation. The effects of some experimental parameters such as the buffer gas pressure, the dipolar excitation signal phases, power amplitudes, and frequencies on the ion trajectory and energy were carefully investigated. The results show that the ion activation energy can be significantly increased by dual-direction excitation using two identical dipolar excitation signals because of the addition of an excitation dimension and the fact that the ion motion radius related to ion kinetic energy can be greater than the field radius. The effects of higher-order field components, such as dodecapole field on the performance of this method are also revealed. They mainly cause ion motion frequency shift as ion motion amplitude increases. Because of the frequency shift, there are different optimized excitation frequencies in different LITs. At the optimized frequency, ion average energy is improved significantly with relatively few ions lost. The results show that this method can be used in different kinds of LITs such as LIT with 4-fold symmetric stretch, linear quadrupole ion trap, and standard hyperbolic LIT, which can significantly increase the ion activation energy and CID efficiency, compared with the conventional method.

Collision-induced dissociation (CID) in linear ion traps is usually performed by applying a dipolar alternating current (AC) signal to one pair of electrodes, which results in ion excitation mainly in one direction. In this paper, we report simulation and experimental studies of the ion excitation in two coordinate directions by applying identical dipolar AC signals to two pairs of electrodes simultaneously. Theoretical analysis and simulation results demonstrate that the ion kinetic energy is higher than that using the conventional CID method. Experimental results show that more activation energy (as determined by the intensity ratio of the a4/b4 fragments from the CID of protonated leucine enkephalin) can be deposited into parent ions in this method. The dissociation rate constant in this method was about 3.8 times higher than that in the conventional method under the same experimental condition, at the Mathieu parameter qu (where u = x, y) value of 0.25. The ion fragmentation efficiency is also significantly improved. Compared with the conventional method, the smaller qu value can be used in this method to obtain the same internal energy deposited into ions. Consequently, the “low mass cut-off” is redeemed and more fragment ions can be detected. This excitation method can be implemented easily without changing any experimental parameters.

In this work, protonated and sodiated leucine-enkephalin (LE) were investigated by gas-phase hydrogen–deuterium exchange (HDX) performed on a linear ion trap time-of-flight mass spectrometer. It is found that more hydrogen atoms are exchanged in protonated LE than in sodiated LE, indicating the different conformations of the two peptide ions. To clarify further the experimental results, the conformations were calculated by using density functional theory, which shows that the terminal amino group is the most thermodynamically stable protonation site, while the sodium ion coordinated to four carbonyl oxygen atoms forms the most favourable sodium adduct. Limited HDX reactions of sodiated LE are explained by the rigid conformation and fewer exchangeable acidic hydrogen atoms from sodium coordination.

A fast mass analysis method using digital ion trap technology was reported. The mass analysis was realized by scanning the frequency of digital RF voltage and dipole ion resonance voltage during ion injection period without the conventional ion cooling, ion resonance ejection or ion cleaning process. It can decrease about three fourth of traditional analysis time using ion trap mass spectrometry. The ion signal was optimized through the modification of frequency scanning rate, ion gate voltage and other experimental parameters. For example, an optimal mass spectroscopy of reserpine (m/z 609) and arginine (m/z 174) were obtained when the ion gate voltage was 9 V and the digital RF frequency was scanned from 1 MHz to 400 kHz at rate of 2385 Th s−1. The results indicated that the obtained mass spectra were in very good agreement with conventional analytical method using ion trap mass analyzer.Fast mass analysis was realized by scanning the frequency of digital RF voltage and dipole ion resonance voltage during ion injection period without the conventional ion cooling, ion resonance ejection or ion cleaning process. It can decrease about three fourth of traditional analysis time using ion trap mass spectrometry.

A novel linear ion trap mass analyzer built with four triangular electrodes, the triangular-electrode linear ion trap (TeLIT), has been built and its performance has been characterized. The TeLIT has all the properties of a conventional LIT mass analyzer, performing ion trapping, mass analysis, and tandem mass spectrometry functions. The TeLIT was constructed with four identical triangular cross-section shaped electrodes and two planar electrodes. Unlike commercial LITs, which have very well-defined hyperbolic shaped electrodes, the TeLIT electrodes have a much simpler geometric structure and larger mechanical tolerances. The electric field distribution inside the IT region was simulated and there are more quadrupole field components and less higher order fields compared with those in other simplified ITs, such as cylindrical ion trap and rectilinear ion trap; hence, the instrument would potentially offer a relatively high mass resolution. In routine measurement, mass analysis with a resolving power of over 1500 at m/z = 609 Th was obtained. The TeLIT was shown to perform basic mass spectrometer functions such as mass-selected isolation, mass-selected ejection and collision-induced dissociation (CID) of ions comparable to other available LITs. Moreover, given the small size of the TeLIT and its simple structure and good analytical performance, further miniaturization and use as a portable mass spectrometer are envisaged.

Collision-induced dissociation (CID) of ions by resonance activation in a quadrupole ion trap is usually accomplished by resonance exciting the ions to higher kinetic energy, whereby the high kinetic energy ions collide with a bath gas, such as helium or argon, inside the trap and dissociate to fragments. A new ion activation method using a well-defined rectangular wave dipolar potential formed by dividing down the trapping rectangular waveform is developed and examined herein. The mass-selected parent ions are resonance excited to high kinetic energies by simply changing the frequency of the rectangular wave dipolar potential and dissociation proceeds. A relationship between the ion mass and the activation waveform frequency is also identified and described. This highly efficient (CID) procedure can be realized by simply changing the waveform frequency of the dipolar potential, which could certainly simplify tandem mass spectrometry analysis methods.

A mesh-electrode linear ion trap (ME-LIT) mass analyzer was developed and its performance was primarily characterized. In conventional linear ion trap mass analyzers, the trapped ions are mass-selected and then ejected in a radial direction by a slot on a trap electrode. The presence of slots can strongly affect the electric field distribution in the ion trapping region and distort the mass analysis performance. To compensate for detrimental electric field effects, the slot is usually designed and fabricated to be as small as possible, and also has very high mechanical accuracy and symmetry. A ME-LIT with several mesh electrodes was built to compensate for the effects caused by slots. Each mesh electrode was fabricated from a plate electrode with a relatively large slot and the slot was covered with a conductive mesh. Our preliminary experimental results show that the ME-LIT could considerably diminish the detrimental electric field effects caused by slots, and increase the mass resolving power and ion detection efficiency. Even with 4-mm-wide slots, a mass resolution in excess of 600 was obtained using the ME-LIT. Mass resolution could be remarkably improved using mesh electrodes in ion traps with asymmetric electrodes. The stability diagram of the ME-LIT was mapped, and highly efficient tandem mass spectrometry was demonstrated. The ME-LIT was qualified as a LIT mass analyzer. The ME-LIT can improve the mass resolution and decrease the requirements of mechanical accuracy and symmetry of slots, so it shows potential for a wide range of practical uses.

A digital ion trap (DIT) and rectilinear ion trap (RIT) have been proven to be very useful technology in the past years. In this work, the digital ion trap technology was combined with the ceramic-based rectilinear ion trap (cRIT) system. The rectangular waveform was used for ion trapping. A dipolar excitation waveform which was formed by dividing down the trapping rectangular waveform was used for the ion ejection. We found that the high efficient collision-induced dissociation (CID) procedure could be obtained by simply manipulating the duty cycle of the dipole excitation waveform, and it could significantly simplify the tandem mass spectrometry analysis method and procedure with an ion trap, since the dipolar direct current (dc) voltage could be easily produced and applied to one of the pair of electrodes, which was fully controlled by the computer software and does not need any hardware modification.

An ion trap (IT) mass analyzer can be simply built with low cost material—the printed circuit board (PCB). A printed circuit board ion trap (PCBIT) can perform ion trapping, mass analysis, and tandem mass spectrometry as a conventional ion trap mass analyzer. In a PCBIT, each PCB electrode was fabricated to specially designed patterns with several separate electric strips. The strips’ electrodes were insulated from each other and applied with different voltages during the experiment. Therefore, the electric field distribution inside the ion trap region may be adjusted and optimized by simply adjusting the voltage on each strip. The performance of the PCBIT can also be optimized since the property of an ion trap is strongly dependent on the field distribution. The fabrication, operation, and performance of the PCBIT are described and characterized in this paper. A prototype PCBIT was built with two pairs of 64 mm × 12 mm PCB rectangular plates and one pair of 10 mm × 10 mm stainless steel square plates. A mass analysis with a resolving power of over 1500 and a mass range of around 3000 Th was observed. The mass-selected isolation and collision-induced dissociation (CID) of ions were also tested using the homemade PCBIT system. The adjustable electric field distribution, simple structure, and low cost of PCBIT make it certainly suitable for the further miniaturization of the portable mass spectrometer.

The analytical performance of rectilinear ion trap array made up of printed circuit board material has been investigated. The experimental results indicated the intensity of ions determined were dramatically affected by ion kinetic energy, as in 18 eV, the arginine (Arg) ion (m/z 175.2) dominated in the mass spectrum. The ion trap array can perform multiple sample ion storage, mass-selected ion isolation, ion ejection, collision induce dissociation. The ions of m/z 175.2, 117.3 and 71.8 were isolated by SWIFT notch 50-60 kHz, 75-85 kHz and 130-145 kHz, respectively. The ions of m/z 175.2, 117.3 and 71.8 were selectively ejected using 55, 80 and 135 kHz sine wave. In helium gas, the fragment ions of m/z 157.2, 130.3 and 117.3 were obtained by the fragmentation reaction of Arg precursor ion (m/z 175.2) using 102 kHz sine wave. The ion detection efficiency was investigated by electric current integration method, which indicated that the ion detection efficiency was estimated to be 46.3% in 256 kHz AC resonant ejection and was about 9.7% in boundary ejection.The analytical performance of rectilinear ion trap array made up of printed circuit board material has been investigated. The experimental results indicated that the intensity of ions determined were dramatically affected by ion kinetic energy. The ion trap array can perform multiple sample ion storage, mass-selected ion isolation, ion ejection, collision induce dissociation.

A novel ion trap array (ITA) mass analyzer with six ion trapping and analyzing channels was investigated. It is capable of analyzing multiple samples simultaneously. The ITA was built with several planar electrodes made of stainless steel and 12 identical parallel zirconia ceramic substrates plated with conductive metal layers. Each two of the opposing ceramic electrode plates formed a boundary of an ion trap channel and six identical ion trapping and analyzing channels were placed in parallel without physical electrode between any two adjacent channels. The electric field distribution inside each channel was studied with simulation. The new design took the advantage of high precision machining attributable to the rigidity of ceramic, and the convenience of surface patterning technique. The ITA system was tested by using a two-channel electrospray ionization source, a multichannel simultaneous quadruple ion guide, and two detectors. The simultaneous analysis of two different samples with two adjacent ITA channels was achieved and independent mass spectra were obtained. For each channel, the mass resolution was tested. Additional ion trap functions such as mass-selected ion isolation and collision-induced dissociation (CID) were also tested. The results show that one ITA is well suited for multiple simultaneous mass analyses.

We report the computer simulation results of a tandem RF-only quadrupole mass filter. This mass filter consists of two short quadrupole rod sets, Q1 and Q2. The first quadrupole, Q1, is operated as a higher mass reject filter to remove lower q ions. The main filter, Q2, is a RF-only quadrupole, and it is used to give suitable narrow pass band on q axis. Quadrupole excitation by additional RF voltage leads to instability bands along q axis between q = 0 and q = 0.2, and this instability band can be used to remove low q ions with m = M0 − ∞ mass range. Quadrupole excitation creates a narrow pass band near q0 = 0.9080 in Q2. R0.1 = 1150 could be achieved at short separation time n = 40 in Q2.A powerful instability band located near β = ν = 1/10 is created with auxiliary low frequency quadrupole excitations to filter low q ions.

In this work, protonated and sodiated leucine-enkephalin (LE) were investigated by gas-phase hydrogen–deuterium exchange (HDX) performed on a linear ion trap time-of-flight mass spectrometer. It is found that more hydrogen atoms are exchanged in protonated LE than in sodiated LE, indicating the different conformations of the two peptide ions. To clarify further the experimental results, the conformations were calculated by using density functional theory, which shows that the terminal amino group is the most thermodynamically stable protonation site, while the sodium ion coordinated to four carbonyl oxygen atoms forms the most favourable sodium adduct. Limited HDX reactions of sodiated LE are explained by the rigid conformation and fewer exchangeable acidic hydrogen atoms from sodium coordination.