Chemicals and Instrumentation
All mass spectra shown herein were taken on a Thermo LTQ linear ion trap mass spectrometer (Thermo Scientific, San Jose, CA, USA) over a mass range of m/z 1002000. Prior to use, the instrument was calibrated according to procedures outlined in the LTQ user manual. Solutions for positive ion calibration were prepared in-house and consisted of a mixture of caffeine, a short chain peptide salt (Met-Arg-Phe-Ala acetate, MRFA), and Ultramark 1621. For negative spectra sodium dodecyl sulfate and sodium taurocholate was included in the positive ion mixture. These solutions were prepared according to instructions provided in the user manual. Detection of the spatial distribution of ions at atmospheric pressure was accomplished through the use of an IonCCD detector system (OI Analytical, Pelham, AL, USA) with an integration time of 100 ms. Experiments in which the transport efficiency of ions was tested utilized a 1 mM solution of rhodamine B in 4:1 methanol:water (v:v). A Cary 300 UV-Visible spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) was used for the collection of absorption spectra. Chemicals and solvents were purchased from Sigma-Aldrich (St. Louis, MO, USA).