Some Practical Aspects of the Acceleration—Deceleration Method for Ion Kinetic Energy Focusing in Matrix-Assisted Laser Desorption/Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

A new acceleration–deceleration (AD) method for reducing and focusing ion kinetic energies in matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry (MALDI-FTICR) mass spectrometry has been developed. The aim of the method is to equalize and then reduce the initial kinetic energy of the MALDI-produced ions, distributed over a broad energy range, by means of delayed acceleration and deceleration of the ions. The focusing of the energy results in a more efficient trapping of high mass ions and, at the same time, improves the precision of mass measurements in the ICR trap. The experimental parameters for the acceleration–deceleration ion kinetic energy focusing were predicted by means of the SIMION 3D software, incorporating both the electric and magnetic fields. The deceleration of ions both outside and inside the ICR trap was studied. The theoretical treatment shows that by applying the acceleration–deceleration method it is possible to focus the initially high kinetic energies of 100,000 Da ions into a 1 eV range and register their mass spectrum at low trapping potentials. In order to put the new method into practice, the existing FTICR mass spectrometer was equipped with a MALDI source inside the magnet. To compare the acceleration– deceleration method with other methods, the optimal mass spectrum measurement conditions for several proteomic biopolymers were studied. By means of the acceleration–deceleration method, the MALDI-FTICR mass spectrum of substance P (protonated mass 1347.736 Da) was registered at 3 × 10⁻⁸ mbar with mass resolution close to 70,000. A resolution of 45,000 was achieved by the gated trapping method. Bovine insulin B-chain (protonated mass 3494.65 Da) was used to compare the gated trapping method with the deceleration and acceleration–deceleration methods (which also include the gated trapping method as an essential part) at different matrix-to-analyte ratios. The acceleration–deceleration method does not reject high energy ions and has therefore inherently high sensitivity particularly at low analyte concentration. Bovine insulin (protonated mass 5732.6 Da) is so far the highest mass ion registered in our spectrometer. It was possible to measure this ion only by the acceleration–deceleration method.