|
Ion Cyclotron Resonance Mass Spectrometry
The Beckman Institute Mass Spectrometry Resource Center, run by the Beauchamp group, has an external ion source FT-ICR mass spectrometer. The instrument has three differentiallly pumped regions. The first region houses the ion source, the second an octopole ion guide and the third the ICR detection cell. These regions are pumped by 3 APD-6 cryopumps, rated at a speed of 1000L/s. In this configuration, typical operating pressures above the third pump are 5x10-10 torr, as measured by an internal Granville-Phillips ionization gauge just above the cryopump adjacent to the magnet.
An Oxford Mark II 7-Tesla (70,000 Gauss) superconducting magnet is used to supply uniform magnetic field. A uniform 7 T at the ICR cell, the magnetic field extends beyond the outer case of the magnet, requiring specialized methods to protect experiments and apparatus from the magnetic field. For example, shields for the cryopump expanders have been fabricated using 6" cold rolled steel tubing with 1/4" wall so that the expander piston travel is truly vertical and is not perturbed, which would cause premature wear. The octopole ion guide, designed and constrcted in our laboratory, efficiently transports ions from the source region to the ICR detection cell which is located approximately 47" away in the high magnetic field. The ICR detection cell is constructed from stainless steel plates, has dimensions of 2"x 2"x 3", and is located in the center of the homogeneous region of the magnet.
Collision gases are delivered directly into the ICR detection cell either through an inlet maniford equipped with two Varian leak valves (Model 951-5106) or through a pulsed valve (VAC-1250PSIA, General Valve Co., Fairfield, NJ). This allows for reactions inside the ICR cell or CID (collision induced dissociation) studies. An OMEGA data system manufactured by IonSpec Corporation is used to provide control pulses for the cesium ion gun, rf octopole ion guide, split injection electrodes and the ICR detection cell in addition to digitizing and processing time domain spectra. The instrument is designed to carry out fundamental studies of the energetics and reactivity of biological molecules in the gas phase.
|
