doi:10.1016/j.jasms.2007.08.006 
Copyright © 2007 American Society for Mass Spectrometry Published by Elsevier B.V.
Article
Probing the Mechanisms of an Air Amplifier Using a LTQ-FT-ICR-MS and Fluorescence Spectroscopy
R. Brent Dixona, David C. Muddimana, 
, 
, Adam M. Hawkridgea and A.G. Fedorovb
aW. M. Keck FT-ICR Mass Spectrometry Laboratory, Department of Chemistry, North Carolina State University, Raleigh, North Carolina, USA
bG. W. Woodruff School of Mechanical Engineering and Parker H. Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology, Atlanta, Georgia, USA
Received 27 June 2007;
revised 9 August 2007;
accepted 10 August 2007.
Available online 16 August 2007.
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We report the first quantitative assessment of electrosprayed droplet/ion focusing enabled by the use of a voltage-assisted air amplifier between an electrospray ionization emitter and a hybrid linear ion trap Fourier transform ion cyclotron resonance mass spectrometer (ESI-LTQ-FT-ICR-MS). A solution of fluorescent dye was electrosprayed with a stainless steel mesh screen placed in front of the MS inlet capillary acting as a gas-permeable imaging plate for fluorescence spectroscopy. Without use of the air amplifier, no detectable FT-ICR signal was observed, as well as no detectable fluorescence on the screen upon imaging using a fluorescence scanner. When the air amplifier was turned ON while electrospraying the fluorescent dye, FT-ICR mass spectra with high signal to noise ratio were obtained with an average ion injection time of 21 ms for an AGC target value of 5 × 105. Imaging of the screen using a fluorescence scanner produced a distinct spot of cross-sectional area 
33.5 mm2 in front of the MS inlet capillary. These experimental results provide direct evidence of aerodynamic focusing of electrosprayed droplets/ions enabled by an air amplifier, resulting in improved electrospray droplet/ion capture efficiency and reduced ion injection time. A second set of experiments was carried out to explore whether the air amplifier assists in desolvation. By electrospraying a mix of quaternary amines, ratios of increasingly hydrophobic molecules were obtained. Observation of the solvophobic effect associated with electrospray ionization resulted in a higher abundance of the hydrophobic molecule. This bias was eliminated when the air amplifier was turned ON and a response indicative of the respective component concentrations of the molecules in the bulk solution was observed.
Figure 1. Schematic of the experimental set-up and operating conditions. A Thermo LTQ-FT interface is shown at the left with standard capillary. The gas-permeable stainless steel mesh screen is inserted between the MS capillary inlet and the voltage assisted air amplifier. A pressure of 40 psi (at the regulator) was used to induce flow in the air amplifier, resulting in aerodynamic focusing of the electrosprayed ions. The electrospray emitter and syringe pump supplying the sample are also shown at the right.
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Figure 2. (a) Fluorescence image of the stainless steel mesh screen following electrosprayed rhodamine 6G containing droplets/ions, as detected by a BioRad imager (intensity key at right of imaged screen). No fluorescence is observed with the air amplifier OFF (left). A tightly spotted region of intense fluorescence is observed with the air amplifier ON (right), providing a measure of the area of focused electrosprayed ions. (b) With the air amplifier OFF (left, screen in place) the ion injection time reaches the maximum AGC time limit of 1000 ms, yet the ion abundance remains below the detectable limit. When the air amplifier is turned ON (right, screen in place), the ion injection time decreases to 21 ms and significant ion abundance is observed. (c) The FT-ICR mass spectrum observed with the air amplifier OFF (left, screen in place) demonstrates that no MS signal is observed without aid from the air amplifier, however, a clean spectrum of rhodamine 6G ions is observed with the assistance of an air amplifier (right, screen in place).
Figure 3. (a) Analysis of tetrabutyl (*) and tetraheptyl (**) ammonium halide. The response in standard ESI at room temperature results in a much lower response of tetrabutyl compared with tetraheptyl based on the much higher solvophobicity of tetraheptyl ammonium halide. With the air amplifier ON at room temperature, an equivalent response of the two molecules is observed, resulting in a true indication of the concentration of each molecule within the bulk solution. (b) The biased response observed in ESI is also minimized with the addition of heat as the body of the air amplifier and the inlet nitrogen is heated to 100 °C. With heat and the air amplifier ON, a nearly equivalent response is attained of the tetrabutyl and tetraheptyl ammonium halides.
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