Anal. Chem. 2001, 73, 4162-4170
A Multicapillary Inlet Jet Disruption Electrodynamic Ion Funnel Interface for Improved Sensitivity Using Atmospheric Pressure Ion Sources Taeman Kim, Keqi Tang, Harold R. Udseth, and Richard D. Smith*
Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352
A new multicapillary inlet and ion funnel interface for electrospray ionization-mass spectrometry has been developed and demonstrated to achieve higher ion transmission efficiency compared to a single-capillary inlet and ion funnel interface. Even though the distance between the end of the ESI inlet capillary and the exit of the ion funnel (10 cm) is significantly longer than that of the conventional interface (typically a few millimeters), a significant part of the directed inlet gas flow persists into the first stage of pumping and results in an increased gas load to the second chamber. A jet disrupter made of a circular metal disk placed on axis in the ion funnel enhanced the dispersion of the directed gas flow from a multicapillary inlet and was also found to improve the ion transmission. The ion funnel with the jet disrupter demonstrated a 15% improvement in ion transmission (compared to that without the jet disrupter) and simultaneously reduced the pumping speed required for the first or second stage by a factor of 2-3. Compared to the sensitivity with the standard mass spectrometer interface (an API 3000, Sciex, Concord, ON, Canada) in MS/MS operation using an interface equipped with the jet disrupter and ion funnel, a 5.3-10.7-fold enhancement in signal was observed for samples with concentrations of 100-500 pg/µL and 10.2 to 14.1-fold enhancement for concentrations of 10 to 50 pg/µL. The decreased enhancement at higher concentrations is attributed to space charge effects and detector saturation. Electrospray ion sources (including conventional electrosprays, microelectrosprays, and nebulizing gas-assisted electrosprays) are widely used with mass spectrometry for biological research because of their gentleness, sensitivity, and broad applicability.1,2 The analyte ions are created at near-atmospheric pressure and must be transferred to the high-vacuum region of a mass spectrometer where their mass-to-charge ratios are measured. Several stages of differential pumping are generally required to * Corresponding author: (tel) 509-376-0723; (fax) 509-376-2303; (e-mail)
[email protected]. (1) Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. Mass Spectrom. Rev. 1990, 9, 37-70. (2) Smith, R. D.; Loo, J. A.; Loo R. R. O.; Busman, M.; Udseth, H. R. Mass Spectrom. Rev. 1991, 10, 359-451.
4162 Analytical Chemistry, Vol. 73, No. 17, September 1, 2001
achieve the vacuum conditions required for the mass analyzer. The major design issues are broadly related to optimizing overall ion transmission efficiency through the sequentially lower pressure stages. Different ion optic arrangements have been described for improving ion transmission efficiencies through different vacuum stages, including the electrodynamic (rf) ion funnel at higher interface pressures (∼1-10 Torr)3-6 and rf-only multipole ion guides at lower interface pressures (∼ 1-50 mTorr).7 There is no efficient ion focusing mechanism at atmospheric pressure for the ion currents relevant to electrospray ionization (ESI) where space charge effects dominate, and overall charge transmission between an ion source and the first vacuum stage is primarily dependent upon the proximity of the emitter and gas conductance of the interface inlet. However, it should be noted that lower ion currents (e.g., as observed with high-field asymmetric waveform ion mobility spectrometry (FAIMS)) can be effectively focused even at atmospheric pressure, if not disrupted due to gas dynamic effects.8-10 One approach to improve the transmission efficiency is to reduce the distance between the electrospray emitter tip and inlet, which is facilitated by low flow rate electrosprays (nanoelectrospray) where desolvation time is shortened.11-13 However, this approach requires greater optimization, is not amenable to automation, and is not applicable to higher flow rate HPLC-MS. An alternative approach for improving the ion transmission between a conventional electrospray and the first vacuum chamber is to use a larger conductance inlet. In this approach, the challenge is to focus and transfer ions effectively (3) Shaffer, S. A.; Tang, K.; Anderson, G. A.; Prior, D. C.; Udseth, H. R.; Smith, R. D. Rapid Commun. Mass Spectrom. 1997, 11, 1813-1817. (4) Shaffer, S. A.; Prior, D. C.; Anderson, G. A.; Udseth, H. R.; Smith, R. D. Anal. Chem. 1998, 70, 4111-4119. (5) Shaffer, S. A.; Tolmachev, A.; Prior, D. C.; Anderson, G. A.; Udseth, H. R.; Smith, R. D. Anal. Chem. 1999, 71, 2957-2964. (6) Kim, T.; Tolmachev, A. V.; Harkewicz, R. H.; Prior, D. C.; Anderson, G.; Udseth, H. R.; Smith, R. D.; Bailey, T. H.; Rakov, S.; Futrell, J. H. Anal. Chem. 2000, 72, 2247-2255. (7) Douglas, D. J.; French, J. B. J. Am. Soc. Mass Spectrom. 1992, 3, 398-408. (8) Buryakov, I. A.; Krylov, E. V.; Nazarov, E. G.; Rasulev, U. Kh. Int. J. Mass Spectrom. Ion Processes 1993, 128, 143-148. (9) Carnahan, B.; Tarassov, A. U.S. Patent Number 5,420,424, 1995. (10) Guevremont, R.; Purves, R. W. Rev. Sci. Instrum. 1999, 70, 1370-1383. (11) Gale, D. C.; Smith, R. D. Rapid Commun. Mass Spectrom. 1993, 7, 10171021. (12) Wilm, M.; Mann, M. Anal. Chem. 1996, 68, 1-8. (13) Geromanos, S.; Freckleton, G.; Tempest, P. Anal. Chem. 2000, 72, 777790. 10.1021/ac010174e CCC: $20.00
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from a larger conductance inlet and through the next conductance limit to a lower pressure region (at
