MULTIPLE REMEASUREMENT OF IONS IN FOURIER-TRANSFORM MASS-SPECTROMETRY

For large ions, the sensitivity of Fourier-transform mass spectrometry (FTMS) can be greatly improved by making multiple measurements of the same ions. After excitation to a large orbit for measurement, high mass ions have the advantage that their collisions with small background molecules cause negligible scattering, so that the resulting loss of kinetic energy returns the ions to the center of their original orbits where the measurement process can be repeated. With continuous ion production using 252Cf plasma desorption, asignal enhancement of ~100× is obtained for 1000 measurements in which any remaining ions are not (versus are) removed from the cell before each measurement. With ions desorbed by a single laser pulse, a signal/noise enhancement of ~4× is obtained with 25 additional measurements. Remeasurement efficiencies of 98% have been achieved for m/z 2000 ions. The time required for this collisional deactivation process is-120 s for m/z 1164 peptide ions at 5 × 10-9 Torr and decreases with an increase in ion mass or cellpressure, or with a decrease in excitation power. Further, this relaxation time is dependent on ion structure; collisional cross sections are ~8× larger for the(M + Na)+ ion of gramicidin D (m/z 1904) than for Csl clusters of comparable mass, consistent with an extended linear structure for this pentadecapeptide ion. Extension of this remeasurement technique should make possible single ion detection with FTMS.© 1990, American Chemical Society. All rights reserved.