Approaches to separation and characterization of ions based on their mobilities in gases date back to the 1960s. Conventional ion mobility spectrometry (IMS) measures the absolute mobility, and field asymmetric waveform IMS (FAIMS) exploits the difference between mobilities at high and low electric fields. However, in all previous IMS and FAIMS experiments ions experienced an essentially free rotation; thus the separation was based on the orientationally averaged cross-sections Omega(avg) between ions and buffer gas molecules. Virtually all large ions are permanent electric dipoles that will be oriented by a sufficiently strong electric field. Under typical FAIMS conditions this will occur for dipole moments >400 D, found for many macroions including most proteins above approximately 30 kDa. Mobilities of aligned dipoles depend on directional cross-sections Omega(dir) (rather than Omega(avg)), which should have a major effect on FAIMS separation parameters. Here we report the FAIMS behavior of electrospray-ionization-generated ions for 10 proteins up to approximately 70 kDa. Those above 29 kDa exhibit a strong increase of mobility at high field, which is consistent with predicted ion dipole alignment. This effect expands the useful FAIMS separation power by an order of magnitude, allowing separation of up to approximately 10(2) distinct protein conformers and potentially revealing information about Omega(dir) and ion dipole moment that is of utility for structural characterization. Possible approaches to extending dipole alignment to smaller ions are discussed.

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A systematic study of the effects of Ti doping on the electrical and optical properties of GaAs and lnP has been carried out employing both melt and solution-grown crystals. Utilizing deep level transient spectroscopy, Hall effect measurements, photoconductivity, and optical absorption measurements, it was found that Ti introduces two deep levels in GaAs at Ee -0.23 eV and Ee -1.00 eV which were identified as the Ti3+ ITi2+ acceptor level and the Ti 4 + lye + donor level, respectively. In lnP the Ti 4 + ITe +-donor level was found near midgap at Ee -0.63 eV, while the Tj3l-/Ti 2 + acceptor level was found to He within the conduction band. As a consequence of the midgap position of this donor level, we developed a formulation for producing semi-insulating InP based on doping with Ti to compensate shallow acceptors.Resistivities in excess of 10 7 n em can easily be obtained using this technique, This is the first semi-insulating IU-V compound having a compensation mechanism based on a deep donor impurity. In view of the fact that Ti is expected to have 11 very low diffusivity in InP, Ti-doped semi-insulating InP should exhibit far greater thermal stability than Fe-doped InP and thus it should prove technologically significant.

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