Laser desorption in transmission geometry inside a Fourier-transform ion cyclotron resonance mass spectrometer.

We report here the first application of laser desorption (LD) in transmission geometry (backside irradiation of the sample through a transparent support) inside a Fourier-transform ion cyclotron resonance mass spectrometer (FT-ICR). A probe-mounted fiber optic assembly was used to simplify the implementation of this LD technique. This setup requires little or no instrument modifications, has minimum maintenance requirements, and is relatively inexpensive to build. The performance of the probe was tested by determining the molecular weight of a commercial polystyrene standard from its matrix-assisted laser desorption/ionization (MALDI) spectrum. The measured average molecular weight is comparable to that obtained for the same sample by MALDI in the conventional top-illumination arrangement (reflection geometry) and by the manufacturer of the sample by gel permeation chromatography. The average velocities measured for ions evaporated by transmission mode LD of several neat samples are about half the velocity of those obtained by using the reflection geometry. Therefore, transmission mode irradiation of the sample holds promise to desorb ions that are easier to trap in an ICR cell. An oscillating capillary nebulizer was adapted for the deposition of analytes to improve sampling reproducibility.

A pulsed-leak valve for use with ion trapping mass spectrometers.

A pulsed-leak valve that allows the introduction of a prolonged, flat, and controllable pulse of gas is described. Test results from the valve that utilized a Fourier transform ion cyclotron resonance mass spectrometer with Ar and C2H6 as the sample gases indicate that the valve functions as expected and yields basically rectangular pressure profiles in the cell region. The rise and fall times are similar to those of just the stand-alone pulsed valve and are believed to be determined mainly by the design of the vacuum system, rather than the design of the pulsed-leak valve. Kinetic data for the reaction of Nb(+) with C2H6, acquired with the use of the pulsed-leak valve to introduce the C2H6 gas, demonstrates the practical application of this valve for kinetic and other analogous studies. Use of the pulsed-leak valve greatly reduces the loss of the reactant ion signal during the cooling period.