In-source H/D exchange and ion-molecule reactions using matrix assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry with pulsed collision and reaction gases

Abstract

Controlled in-source ion-molecule reactions are performed for the first time in an external matrix assisted laser desorption ionization (MALDI) source of a Fourier transform ion cyclotron resonance mass spectrometer. The MALDI source with a hexapole ion guide that was originally designed to incorporate pulsed gas to collisionally cool ions (Baykut, G.;Jertz, R.; Witt, M. Rapid Commun. Mass Spectrom. 2000, 14, 1238 -1247 has been modified to allow the study of in-source ion-molecule reactions. Upon laser desorption, a reaction gas was introduced through a second inlet and allowed to interact with the MALDI-generated ions trapped in the hexapole ion guide. Performing ion-molecule reactions in the high pressure range of the ion source prior to analysis in the ion cyclotron resonance (ICR) cell allows to maintain the ultra high vacuum in the cell which is crucial for high mass resolution measurements. In addition, due to the reaction gas pressure in the hexapole product ion formation is much faster than would be otherwise possible in the ICR cell. H/D exchange reactions with different peptides are investigated, as are proton-bound complex formations. A typical experimental sequence would be ion accumulation in the hexapole ion guide from multiple laser shots, addition of cooling gas during ion formation, addition of reaction gas, varied time delays for the ion-molecule reactions, and transmission of the product ions into the ICR cell for mass analysis. In this MALDI source H/D exchange reactions for different protonated peptides are investigated, as well as proton-bound complex formations with the reaction gas triethylamine. Amino acid sequence, structural flexibility and folding state of the peptides can be seen to play a part in the reactivity of such ions. [1-5] and matrix assisted laser desorption ionization (MALDI) [6 -9] has provided mass spectrometry (MS) with two very important tools for investigating biomolecules in the gas phase. ESI and MALDI are fairly mild ionization processes, generally involving cation transfer (usually a proton), that generates positive or negative gas-phase ions while retaining the structural integrity of the analyte. More and more structural information of peptides is emerging from gas-phase studies utilizing mass spectrometry.MALDI has become one of the fastest growing techniques in mass spectrometry due to its application to biological and biochemical research [9 -14]. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) [15][16][17][18][19][20] is known for its unparalleled mass accuracy and mass resolution and has become an important tool in many areas of analytical life sciences [21,22]. For this reason, the FT-ICR MS technique has been combined with the soft ionization technique MALDI using internal [23][24][25][26][27][28] as well as external [29 -37] ion sources. One of the striking advantages of the MALDI FT-ICR technique is high sensitivity as well as high throughput capability.Many ion chemistry studies show similarities between the behavior of gas-...

H/D exchange of gas phase bradykinin ions in a linear quadrupole ion trap

“…Recently, Purves et al found four conformers of BK ϩ 2H ϩ ions with a combination of H/D exchange and high-field asymmetric waveform ion mobility spectrometry (FAIMS) [33]. The hydrogen exchange level of BK ϩ 2H ϩ ions formed by MALDI [21] was found to be same as that for BK ϩ 2H ϩ ions formed by ESI, observed in an ICR cell [11].…”

“…The hexapole exit potential was set to prevent ions from being transmitted to an FTICR and ions exchanged for periods from zero up to 60 s. Recently, Witt et al described the H/D exchange of several peptide ions in a modified hexapole trap coupled to an ICR cell. The hexapole was used as an ion guide to collisionally cool ions generated by MALDI [21]. Ions were trapped for up to 60 s when D 2 O or CD 3 OD vapor was infused into the trap.…”

Linear ion traps in mass spectrometry

“…H/D exchange in a linear hexapole trap of 10 different peptide ions generated by MALDI, combined with FTMS, was described by Witt, Fuchser, & Baykut (2002). After ions were cooled with Ar, the deuterating agent CD 3 OD or D 2 O was added and ions exchanged for up to 60 sec.…”

A combined ion source for fast switching between electrospray and matrix‐assisted laser desorption/ionization in Fourier transform ion cyclotron resonance mass spectrometry

“…The operating principle and conditions of the MALDI part of this combined ion source are very similar to the MALDI source described previously 37,. 39 A significant improvement in the MALDI hardware is the use of the 384‐spot microtiter plate format sample plates, which can be positioned by a computer‐controlled high precision x‐y stage. The insertion of the sample plates into the source occurs through the computer‐controlled vacuum lock.…”

“…Pulsing gases into the source is not limited to inert collision gases for cooling the ions during their formation. It is shown that by pulsing reactant gases into this region, ion‐molecule reactions can be observed and studied 39. Another FTICR‐MALDI source desorbing ions into a multipole ion guide using even higher pressures in the MALDI region was developed by O'Connor and Costello 40.…”