Laser desorption in a quadrupole ion trap. Mixture analysis using positive and negative ions (original) (raw)
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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.
International Journal of Mass Spectrometry and Ion Processes, 1994
We have developed an improved method, dynamic r.f. trapping, for capturing laser desorbed ions in a quadrupole ion trap mass spectrometer (ITMS). Trapping efficiency is enhanced by over an order of magnitude over previous methods. A 308 nm excimer laser pulse desorbs the sample-trimethylphenylammonium iodide (TPA-I) is used in most of the work reported-from a probe inserted through the ring electrode. The laser is fired as the r.f. trapping potential (risetime about 175 ps) is applied to the ring electrode. Laser desorbed ions penetrate the trap while the trapping potential is low, but cannot escape because the r.f. potential rises substantially during their transit across the trap. The trapping efficiency is found to depend critically on the kinetic energy of the laser desorbed ions, and on the r.f. amplitude, phase, and rate of change of the r.f. amplitude when the laser fires. Cation and anion signals are recorded as functions of coarse and fine steps in the laser-to-r.f. timing. Coarse and fine timing steps test the effects of laser-to-r.f. delay and phase respectively. We also report effects on trapping efficiency of buffer gas pressure and composition (He neat versus He : Xe mixtures) and the amplitude of the ring electrode steady state r.f. potential. The delay and phase dependence of the experimental data is analyzed with reference to an effective potential barrier model. Differences in the phase and delay dependences for anions and cations are attributed to differences in Debye shielding early in the expansion of the laser desorbed plume. Cation and anion mass spectra are presented for laser desorption/ionization of TPA-I and pyrene. For TPA-I desorption, reactions between laser desorbed cations and neutral TPA fragments in the early, high density portion of the laser plume lead to production of high mass cations.
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.
Analytical Chemistry, 1995
A modified ion trap detector has been utilized to obtain high-performance collision-induced dissociation (CID) mass spectra of peptide ions formed by matrix-assisted laser desorption/ionization (MALDI). MALDI ions are trapped while increasing the fundamental radio fiequency field, obviating the need for elevated helium gas pressures, Molecular ion isotopic clusters are then isolated by a reverse-forward-reverse scan sequence. A single species within the isotopic cluster (generally the monoisotopic mass) is then selected for activation. €ha&, modulation of the amplitude of the resonant excitation voltage on the end-cap electrodes, used previously to improve mass calibration in normal mass spectra, is now utilized to provide high mass accuracy for the product ions. The CID mass spectra of several protonated and sodium-cationized peptides are presented and are often characterized by a series of rearrangement ions that can be utilized in the determination of amino acid sequences.
Recent developments in analytical ion trap mass spectrometry
TrAC Trends in Analytical Chemistry, 1998
The current status of quadrupole ion trap mass spectrometry is reviewed, with particular emphasis on liquid chromatographic coupling, membrane inlet introduction, laser desorption / ionisation and selective chemical ionisation. The £exibility, high sensitivity and multi-stage tandem mass spectrometric capability of the quadrupole ion trap are all illustrated. z1998 Elsevier Science B.V. All rights reserved.
Factors Affecting Quantitative Analysis in Laser Desorption/Laser Ionization Mass Spectrometry
Analytical Chemistry, 2004
Microprobe laser desorption/laser ionization mass spectrometry (µL 2 MS) is a sensitive and selective technique that has proven useful in the qualitative and semiquantitative detection of trace organic compounds, particularly polycyclic aromatic hydrocarbons (PAHs). Recent efforts have focused on developing µL 2 MS as a quantitative method, often by measuring the ratio of signal strength of an analyte to an internal standard. Here, we present evidence of factors that affect these ratios and thus create uncertainty and irreproducibility in quantification. The power and wavelength of the desorption laser, the delay time between the desorption and ionization steps, the power of the ionization laser, and the ionization laser alignment are all shown to change PAH ratios, in some cases by up to a factor of 24. Although changes in the desorption laser parameters and the delay time cause the largest effects, the ionization laser power and alignment are the most difficult parameters to control and thus provide the most practical limitations for quantitative µL 2 MS. Variation in ratios is seen in both synthetic poly-(vinyl chloride) membranes and in "real-life" samples of Murchison meteorite powder. Ratios between similar PAHs vary less than those between PAHs that differ greatly in mass and structure. This finding indicates that multiple internal standards may be needed for quantification of samples containing diverse PAHs.
Ion/molecule reactions, mass spectrometry and optical spectroscopy in a linear ion trap
International Journal of Mass Spectrometry and Ion Processes
A linear-geometry, radio-frequency, quadrupole ion trap has been developed to generate, purify, accumulate and study atomic and molecular ions in the gas phase. By employing a trap-based system, both reactant and product ions can be stored for significant time periods, which can both enhance the efficiency of gas-phase reaction processes and create an environment to observe collision products after vibrational and rotational excitations have had time to relax. Relaxation occurs via viscous cooling with a dilute buffer gas or via laser cooling. Furthermore, the setup is particularly useful for performing optical spectroscopy on the trapped ions. Atomic and molecular ovens are used to generate thermal beams of neutral species, which are then ionized by electron bombardment. The ions can be trapped, or they can be collided with neutral molecules (e.g. C60) under well defined experimental conditions. The collision energies are variable over a range from nearly 0 to 200 eV. This feature makes possible studies of complex formation, charge transfer and collision-induced fragmentation as a function of kinetic energy. A wide range of masses of up to 4000 u can be stored and manipulated with this apparatus. Two mass spectrometric techniques for the analysis of trapped ionic species are presented. In one method, parametric excitation of the secular motion is used to generate mass spectra with resolutions as high as 1 part in 800 with a simple experimental setup. The second method is capable of quickly generating mass spectra over the entire range of trapped masses, but has only moderate resolution. These spectra are generated by linearly sweeping the rf-trapping voltage to zero and detecting ions as their trap depth falls below a threshold value. In the trapping volume, which is 10 cm in length and 200 #m in diameter, 106 ions can be loaded and stored, corresponding to an ion density above 108 cm-3. Such densities facilitate spectroscopy of the stored ions. Both laser-induced fluorescence and photodissociation measurements have been carried out with a cw laser system providing near-infrared, visible, and ultraviolet beams. Absolute, total cross-sections and branching ratios of the photodissociation of MgC~0 have been measured.
Rapid Communications in Mass Spectrometry, 2005
Using n-butylbenzene as a test molecule, evidence is provided that fast, efficient or highly energetic collision-induced dissociation (CID) can be achieved during the mass acquisition ramp of a commercially available quadrupole ion trap (QIT) mass spectrometer. The method of excitation is very similar to axial modulation for mass range extension except that lower amplitude waveforms are used to excite the precursor ions within the trap instead of ejecting them from the trap. ITSIM simulations verify that fast kinetic excitation followed by kinetic-to-internal energy transfer occurs on the rapid time-scale required for the recapture and mass analysis of product ions during the mass acquisition ramp. CID efficiencies larger than 50% can be obtained using this new approach and ratios of Th 91/92 of n-butylbenzene fragment ions as large as 9 are possible, albeit at significantly reduced efficiencies. These very large ratios indicate an internal energy above 7 eV for the precursor ions indicating that fragmentation of larger ions could also be possible using this new approach. The main benefits of the new method are that no extra time is required for fragmentation or cooling and that on-resonance conditions are guaranteed because the ions' secular frequencies are swept through the fixed frequency of excitation. Also presented are the effects of experimental variables such as excitation frequency, excitation amplitude and scan rate on the CID efficiencies and energetics.