Search

A time-of-flight mass spectrometer (TOF-MS) that incorporates an ion storage trap as a pulsed extraction source and sample preconcentrator has been developed for air monitoring. This new instrument configuration is designed for real-time monitoring of changes in trace volatile and semivolatile compound concentrations in air. Typically, detection limits are in the low parts per billion (v/v) range. This ion store/TOF-MS configuration permits a relatively large volume air flux to be drawn directly into the ion trap cavity, creating a local pressure that exceeds 10-4 Torr. Operating the ion trap at this pressure increases the extent of chemical ionization with the abundant N2+ and O2+ that are formed by electron impact. The air background also serves as a suitable collisional damping matrix, helping to compress the ion cloud to the center of the trap, which is ideal for TOF-MS analysis. Ions are typically accumulated in the trap for 400 ms, at which point they are extracted through the exit end cap and analy...

A version of the atmospheric sampling glow discharge ionization (ASGDI) source was developed to study the role of gas dynamics on anion formation. This source, which is used in conjunction with mass spectrometry for direct air monitoring, was designed so several key instrumental dimensions as well as operating parameters could be readily changed. Such flexibility permitted the study of ionization processes in ASGDI and the parameters that can be controlled to favor a particular ion product. One aspect of ASGDI that was found to influence ionization yield was the hydrodynamic properties of the sample inlet free-jet expansion. From these investigations, it was found that mean molecular flow of species expanding toward the skimmer could be manipulated to favor kinetically fast reactions over more thermodynamically preferred reactions. In the case of 2,4-dinitrotoluene, observation of the M[sup [minus]] ion could be enhanced relative to anions derived from the air-matrix background. The primary ionization mechanism for this compound is believed to be associative electron capture. 26 refs., 9 figs., 2 tabs.

Matrix-assisted laser desorption (MALD), which has been proven to be an effective ionization technique for biological molecules, has been implemented on a quadrupole ion trap mass spectrometer (ITMS). In the instrumental configuration used for this work both the sample probe and the laser beam are brought through holes in the ring electrode, thereby enabling MALD-generated ions to expand directly into the ion trap cavity. This approach for directly introducing MALD-generated ions compliments the capabilities of the ITMS to obtain low detection limits and to perform tandem mass spectrometric analysis. For example, detection limits in the single-unit femtomole regime have been achieved for small polypeptides such as leucine enkephalin, bradykinin, and neuromedin U-8. Furthermore, structural information has been acquired via multiple stages of mass spectrometry. One limitation that currently exists is an unanticipated drop in sensitivity and resolution as the mass/charge ratio for ions exceeds 3000. 42 refs., 11 figs., 1 tab.