Laser desorption studies of high mass biomolecules in Fourier-transform ion cyclotron resonance mass spectrometry.

Abstract

Matrix-assisted laser desorption ionization is used to obtain Fourier-transform ion cyclotron resonance mass spectra of model peptides (e.g., gramicidin S, angiotensin I, renin substrate, melittin, and bovine insulin). Matrix-assisted laser desorption ionization yields ions having appreciable kinetic energies. Two methods for trapping the high kinetic energy ions are described: (') the ion signal for [M + H]+ ions is shown to increase with increasing trapping voltages, and (ii) collisional relaxation is used for the detection of [M + H]+ ions of bovine insulin.Ionization methods such as californium plasma desorption (1), matrix-assisted laser desorption (MALD) (2), and electrospray ionization (ESI) (3) have greatly expanded the role of mass spectrometry of high mass biomolecules. Biomolecules having molecular masses >100,000 Da can be ionized to yield a single-charged (e.g., [M + H]+) or multiple-charged (e.g., [M + nH]n+) ion. Although most of the MALD and ESI work has utilized time-of-flight, quadrupole mass spectrometers and/or quadrupole ion traps, MALD (4) and ESI are adaptable to Fourier-transform ion cyclotron resonance (FT-ICR). The advantage of FTICR (and the quadrupole ion trap) is that tandem mass spectrometry experiments [e.g., collision-induced dissociation and photodissociation (5)] and ion molecule reactions can be implemented for structural characterization. The best high mass performance [-60,000 mass resolution at m/z 6000 (6)] of FTICR has been achieved with laser desorption ionization of nonpolar polymers. McLafferty and coworkers (7) have made considerable progress with the development of ESI for FTICR, and a highsensitivity ESI/FTICR system was recently described.* Several groups (9, 10) have suggested that the performance of FTICR with biological molecules (samples that have relatively low ionization probabilities) and molecular masses greater than -2500 Da may be adversely affected by inefficient trapping of the ions or inherent limitations of the ion detection. For example, the mass resolution for samples with molecular masses greater than -2500 Da is far less than the theoretically predicted values (11). On the basis of our experience with desorption ionization and high mass FTICR, we suggested that the major problem with high-resolution FTICR at high mass is due to the high kinetic energies of desorbed ions (9). We have recently evaluated two experimental methods for improving the trapping efficiency of high mass ions: (i) the effects of increasing the electric trapping field and (ii) the effects of collisional relaxation of the laser-desorbed species prior to trapping the ions in the ion cyclotron resonance (ICR) cell. In this article we present the results from both experimental methods. The results show that increasing the trapping potentials improves the ion trapping efficiency, but detection of [M + H]+ ions of bovine insulin (m/z = 5734) requires collisional relaxation of ions prior to their introduction into the ion cell. MATERIALS AND METHODSFTICR Mass Spectrometer. The...

Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry for quantitation and molecular stability assessment of insulin entrapped within PLGA nanoparticles

“…The sample, containing a mixture of insulin and dimer, led thus to a spectrum with an additional peak at 11554 m / z , ascribed to CID. This peak ([2M + H] + ) is similar to that one already observed by MALDI in a previous work 24. CIDs were found only at trace levels, i.e., below the quantitation limit of the MALDI‐TOF MS (<1% of entrapped insulin; results not shown).…”

Evaluation of different combinations of gated trapping, RF-only mode and trap compensation for in-field MALDI Fourier transform mass spectrometry

“…The first is to modify the MALDI desorption to attenuate the translational velocities and internal energies of the desorbed ions. Examples of these efforts include the use of a comatrix [2] or a "waiting room," [3] which in the limit becomes high-pressure MALDI [4]. An alternative is the use of an external source [5], which permits the use of quadrupole and higher order traps or ion guides to focus and thermalize the MALDI-produced ions before introducing them into the FTMS trap.…”

Accurate mass measurements by Fourier transform mass spectrometry

“…The problem would be minimized if the translational energy of the peptides could be moderated. Solouki & Russell (1992) developed an external housing or ''waiting room'' device so that the MALDI-generated ions could be translationally relaxed by collisions before transferring them to the analyzer cell. Castoro & Wilkins (1993) further developed this method, and they achieved for small peptides a mass resolving power in excess of 100,000.…”