Simultaneous Mass Analysis of Positive and Negative Ions Using a Dual-Polarity Time-of-Flight Mass Spectrometer

Abstract

Positive and negative ions produced from matrix-assisted laser desorption/ionization (MALDI) were simultaneously measured using a newly developed dual-polarity time-of-flight mass spectrometer. This instrument is effective not only for express and comprehensive mass analysis but also for studying the ionization mechanisms of biomolecules. It comprises two identical time-of-flight mass analyzers located symmetrically about a MALDI ion source. The ion optics are arranged to be able to extract positive and negative ions synchronously with equal efficiency to each corresponding mass analyzer. Mass spectra of various proteins with molecular weights as large as that of myoglobin monomer and dimer were obtained. The spectral patterns obtained in this work are approximately mirror images with opposite polarities.

Incoherent production reactions of positive and negative ions in matrix-assisted laser desorption/ionization

“…Two home-made synchronized dual-polarity mass spectrometers were used in this work, namely (1) a dual-polarity time-of-flight mass spectrometer (DTOFMS) reported previously [11], and (2) a simplified DTOFMS. Both instruments were equipped with MALDI ion sources at the instrument centers.…”

“…Synchronized dual-polarity MALDI-TOF mass spectrometry was utilized in this work to record all ions produced under various ionization conditions. The use of synchronized dual-polarity MALDI-TOF mass spectrometry has been successfully demonstrated previously [11] to allow unambiguous examination of counter ions; it allows all ions from every MALDI event to be recorded simultaneously. This feature was essential in this work because uncertainties due to low reproducibility were eliminated.…”

“…The two reaction models seem not converge over time mainly due to the insufficient information provided by conventional mass spectrometric methods. It is thus desirable to develop our understanding of MALDI from novel perspectives, such as the correlation between primary positive and negative ions at the time of ionization.A recent report based on a synchronized dual-polarity MALDI-TOF method stated that the spectral pattern of MALDI-generated positive ions were approximately equal to those of the negative ions for some moderate-size proteins [11]. There was thus some speculation over whether the positive and negative ions are produced in a pair-wise fashion in MALDI [12,13], or whether they are the result of preformed ion-pairs in the crystal before laser irradiation [6].…”

Synchronized dual-polarity electrospray ionization mass spectrometry

“…Polarity switching [2] is an approach to monitor positive and negative ions alternatively by a mass spectrometer. It is utilized with liquid-chromatographic systems to identify protein modifications [3,4], metabolites [5], and environmental hazards [6], but the instrument performance declines due to duty cycle problems, and the few seconds required for signal recovery after every such switch.The best strategy to obtain positive and negative mass spectra is to employ contemporaneous dual-polarity (DP) ion detection [7,8]. The only DP ionization approach involving liquid samples is the field-induced droplet ionization (FIDI) [9], in which positive and negative liquid jets are produced from falling droplets within a strong d.c. electric field.…”

“…The best strategy to obtain positive and negative mass spectra is to employ contemporaneous dual-polarity (DP) ion detection [7,8]. The only DP ionization approach involving liquid samples is the field-induced droplet ionization (FIDI) [9], in which positive and negative liquid jets are produced from falling droplets within a strong d.c. electric field.…”