An improved method for capturing laser desorbed ions in an ion trap mass spectrometer: dynamic r.f. trapping (original) (raw)
Related papers
A method of increasing the sensitivity for laser desorption in an ion trap mass spectrometer
Journal of The American Society for Mass Spectrometry, 1993
Laser desorption in an ion trap mass spectrometer shows significant promise for both qualitative and trace analysis. In this work, we explore various combinations of time-varying DC and radiofrequency (RF) fields in order to optimize laser-generated signals. By judicious choice of timing between the laser desorption pulse and the rise in the applied RF trapping potential, we observed over an order of magnitude enhancement in the trapped ion signal. This new method for laser desorption has enabled us to observe mass spectra of many compounds (e.g., pyrene, dichlorobenzene, and ferrocene) that are barely detectable using previous laser desorption methods. Effects of laser timing and the magnitude of the steady-state RF potential are discussed.
Laser desorption in a quadrupole ion trap. Mixture analysis using positive and negative ions
Analytical Chemistry, 1993
We demonstrate the generation of both negative and positive ions by laser desorption directly within a quadrupole ion trap and the application of this method to analyze complex samples containing compounds such as explosive residues, metal complexes, and polynuclear aromatic hydrocarbons. In some cases the ability to rapidly switch between positive and negative ion modes provides sufficient specificity to distinguish different compounds of a mixture with a single stage of mass spectrometry. In other experiments, we combine intensity variation studies\with tandem mass spectrometry experiments and positive and negative ion detection to further enhance specificity.
Axial introduction of laser-desorbed ions into a quadrupole ion trap mass spectrometer
Analytical Chemistry, 1992
Pulsed infrared laser desorption is accomplished with a quadrupole Ion trap by means of a fiber optic interface. The fiber optic probe lo located external to the ion trap device, and desorbed ions pats through the holm in one end-cap electrode and are trapped. This new method allows desorption and ionization by gabphase catlon attachment of biologically relevant molecules, including Gramicidin S which has a molecular weight of 1141. Trapping lo mort effective by uslng a hlgh helium buffer gas pressure (1 1 mlorr), a long storage Way (150 ms) prlor to detection, and a low rl trapping potential (350-650 V ,) during the desorption pulse. Ion-moiecuie reactions of neutrals with trapped laserdosorbed ions are also demonstrated. Aikaiknetai catlons or halide anlons may be formed by desorption of aikalknetai halide salts, and these ions may be selectively stored for subsequent attachment to volatile organics, such as polyethers. Additlonally, transitionmetal ions desorbed at higher laser power densities undergo selectlve attachment to aromatic substrates such as naphthalene.
Analytical and Bioanalytical Chemistry, 2008
A wide mass range trapping experiment using internal source matrix-assisted laser desorption-Fourier transform mass spectrometry (MALDI-FTMS) was evaluated. In this method, the front trap plate potential is ramped up and the rear trap plate potential is simultaneously decreased using a cubic cell to trap ions over a wide range of mass-to-charge ratios. To apply this to MS/MS experiments, a second ion ejection procedure would remove unwanted ions, with the selected remaining ions then fragmented by collision-induced dissociation. In measurements using a 7.2-T unshielded magnet presented here, an approximately equimolar mixture of a set of poly(ethylene glycol) (PEG) species for the ramped measurements had peak areas of 1.0:1.0:1.0:1.0, as did the previously described integral method which gave peak areas of 1.0:1.1:1.0:1.0, in good agreement with the known composition of the samples deposited on the MALDI probe tip. Comparative MALDI-TOF in reflectron mode results were of similar quality for the equimolar mixture, giving a ratio of 1.0:1.0:1.2:0.9. All methods failed to varying degrees when individual PEG compositions of the trial mixture were changed. However, the previously described integral method showed relatively better results for all but the PEG 8000 doubled mixture.
Rapid Communications in Mass Spectrometry, 2006
A specially constructed split sample probe was used to unequivocally demonstrate that gas-phase cationization occurs within the desorption plume during a matrix-assisted laser desorption/ionization experiment. Two separate samples were prepared for analysis: on side A, a mixture of poly(ethylene glycol) (PEG) 1500 analyte and 2,5-dihydroxybenzoic acid (DHB) matrix, and on side B a mixture of DHB matrix and lithium hydroxide (LiOH), the cationization reagent. Analysis of the data showed that when the ionization laser was focused on the split (so that both sides were illuminated), Li R -cationized PEG peaks were observed. Since the PEG analyte did not come into contact with Li R in either the solution or solid phase, the only possibility for the observed cationization was a reaction in the gas phase. Due to the difficulty in completely removing the adventitious cations (Na R and K R ) present in DHB and on sample surfaces, gas-phase cationization could not be demonstrated to be either the only or most important mechanism operating in the MALDI experiment.
International Journal of Mass Spectrometry and Ion Processes, 1992
The trapping of metal ions in laser desorption/ionization (LDI) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry is attributed to an electrostatic shielding mechanism promoted by the sustained quasi-neutral plasma behavior of the desorbed particle plume in strong magnetic fields. This shielding process allows low energy ions to penetrate the applied trapping potentials at the trapped-ion cell, thus resulting in the introduction of ions with energies less than the effective depth of the trapping potential well. Subsequent deshielding of these ions while in the cell exposes them to the trapping field and results in their retention. Data from time-of-flight (TOF) studies indicate that large spatially and temporally overlapped populations of high energy ions and low energy electrons are generated by LDI of a variety of metal targets when laser power density exceeds IO'-108W cm-'. The charge density in the desorbed plasma is shown to increase during flight along converging magnetic field lines but to dissipate rapidly on exiting the field. Retarding potential studies in the magnetic field performed with both TOF and FT-ICR detection indicate that the Debye shielding exhibited by these quasi-neutral populations is sufticient in some cases to allow ions with energies on the order of I eV to penetrate retarding potentials as high as 500 V. Further indication that such effects are present in the LDI FT-ICR experiment is given by TOF kinetic energy analysis of ions acquired in the trapped-ion cell from LDI and then dumped to an external detector. This analysis indicates that the average kinetic energy of such ions is typically only 60% of the applied trapping potential.
Review of Scientific Instruments, 2008
Efficient trapping and detection of intact peptide ions is demonstrated in a quadrupole ion trap ͑QIT͒ coupled to an external vacuum matrix-assisted laser desorption ionization ͑MALDI͒ source. Deactivation of metastable ions generated by MALDI is achieved in a pressure transient environment inside the QIT established by pulsing gas to access the higher pressures required for fast thermalization, without affecting vacuum conditions in the ion source and time-of-flight ͑TOF͒ mass analyzer. Pressure transients are experimentally determined and a threshold of ϳ10 mTorr is identified where internally excited ions, which commonly observed to dissociate upon injection in a QIT, are stabilized. Fragment-free spectra are presented for a set of peptides by using 2,5-dihydroxybenzoic acid ͑DHB͒ as a matrix, and significantly reduced fragmentation is observed by using a-cyano-4-hydroxycinnamic acid ͑CHCA͒. Intact peptide spectra of a protein tryptic digest are also recorded with CHCA. The process of translational cooling for externally injected ions in a dynamic pressure environment is visualized by using ion trajectory simulations that employ hard sphere collisions. Statistical theory of dynamic equilibrium of ions stored in rf fields is applied to our QIT to characterize a translationally thermalized ion cloud, to explain observed ejection efficiency into the TOF mass analyzer, and to further discuss collisional deactivation of metastable ions.
Journal of the American Society for Mass Spectrometry, 1997
A novel method was developed to measure the initial velocity of ions generated by matrix-assisted laser desorption ionization (MALDI). It is shown both experimentally and theoretically that with a delayed extraction (DE) technique, the flight time of an ion changes linearly with extraction delay. The initial velocity of the ion, a consequence of the desorption process, can be determined from the slope of this linear curve. Systematic study of the initial velocity was undertaken regarding its dependence on the matrix substance, molecular weight of the analyte, ion polarity, and wavelength of irradiation. It was found that the most important factor was the matrix material. Sinapinic acid and ~x-cyano-4-hydroxycinnamic acid matrices ejected slower peptide and protein ions than 2,5-dihydroxybenzoic acid or 3-hydroxypicolinic acid: ~ 300 versus ~ 550 m/s. Matrix ions themselves exhibited a similar order of initial velocities, but these were 15-40% higher than those of insulin ions. The molecular weight of protein samples (between 5 and 25 ku) was found to have little effect on the initial velocity, but for peptides below 5 ku a gradual transition was noted toward the velocity of the matrix ions. Also decreasing velocity with increasing molecular mass was observed for DNA samples in the 4-14-ku range. In the negative ion mode slightly lower velocities were observed than in the positive ion mode. No difference was found between 337-and 266-nm irradiation. Values of the initial velocities were used to correct systematic errors in the internal calibration observed in mass spectra with delayed extraction. These velocity corrections decrease mass errors substantially in the linear mode, in particular for multicomponent mixtures.
Laser desorption time-of-flight mass spectrometry using a 300-ps ultraviolet laser
Analytical Chemistry, 1988
An N, laser (A = 337 nm) was used wlth a h a r timeof-fllght mass spectrometer to study Ion formation from metal films, CsI, and sucrose. Improved technlques were Incorporated for fast Ion detection and data acqulsitlon. For Inorganic speclmens the mass resolution was-1500 (full width at halfmaximum) at m / z 400, a considerable Improvement over earlier reports for Instruments using a laser desorption Ion source and a linear Ion optlcal path. Catlonlred parent Ions and structurally slgnlficant fragments were obtained from 8uerose, In contrast to previous observatkns and predlctlons for subnanosecond UV laser desorptlon.
Rapid Communications in Mass Spectrometry, 2001
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 ms 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.