| Summary: | A collisional damping interface for orthogonal injection of either electrospray ionization (ESI) or matrix-assisted laser desorption/ionization (MALDI) ions into a reflecting TOF mass spectrometer (MANITOBA TOF III) has been designed. Ions produced by either ESI or MALDI are injected into an RF-quadrupole ion guide operating at pressure $\sim$70mTorr. As they pass through the quadrupole their motion is constrained by the RF field. Meanwhile, they lose energy by collisions with the buffer gas molecules. Computer simulations and experiments performed in the instrument with the new interface indicate that collisional cooling of ions improves the quality of the primary ion beams orthogonally injected into the mass spectrometer. Important characteristics of the mass spectrometer, such as resolution and sensitivity were greatly improved. In addition, mass discrimination was reduced, providing an observable mass range greater than 1 MDa with an electrospray ion source and up to $\sim$12,300 Da with a MALDI ion source. These improvements appeared to be very useful for the study of large biological compounds, noncovalent complexes in particular. The results of investigation of several noncovalent complexes are presented to illustrate the capabilities of the instrument with the new interface. The system studied consist of Echerichia coli citrate synthase dimers and hexamers interacting with an allosteric inhibitor, the reduced form of nicotinamide adenine nucleotide.
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