Characterization of FT-ICR Mass Spectrometer using ESI M.V. Gorshkov et al., Eur. J. Mass Spectrom. 8, 169–176 (2002) High performance electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry at low magnetic field Michael V. Gorshkov Institute of Energy Problems of Chemical Physics, Russian Academy of Sciences, Moscow, Russia 117829 Harold R. Udseth, Gordon A. Anderson and Richard D. Smith * Pacific Northwest National Laboratory, PO Box 999, Richland, WA 99352, USA A comprehensive characterization of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer utilizing a 1 tesla magnet with an electrospray ionization (ESI) source is presented. A resolution of up to 10,000,000 and a ppb-level mass accuracy has been achieved for atomic ions and light molecules produced with electron ionization. It has also been demonstrated that a resolution of up to 1,000,000 can be achieved for signals from biomolecule ions obtained with an ESI source, with the highest molecular weight for which the 13 C isotopic pattern was completely resolved being that of ubiquitin (8565 Da). Several features of FT-ICR mass spec- trometry at low magnetic field are discussed and approaches to overcome the low-field constraints are presented. Keywords: ion cyclotron resonance, electrospray, mass spectrometry Introduction Fourier transform ion cyclotron resonance mass spec- trometry (FT-ICR MS), in combination with an electrospray ionization (ESI) source, provides the highest achievable res- olution and accuracy for the characterization of biologically important molecules and/or non-covalent complexes. 1-3 Since the first FT-ICR mass spectrum was obtained with a resistive-coil magnet providing a field of 0.32 tesla, 4 it was recognized that the FT-ICR performance improves signifi- cantly with increasing magnetic field. 5,6 The highest-field FT-ICR mass spectrometer, utilizing an 11.4 tesla supercon- ducting magnet, is presently operated at the Pacific North- west National Laboratory in Richland, WA, USA. 7 The highest full-width at half maximum (FWHM) resolution of 400,000 for bovine serum albumin ions (MW = 66.4 kDa) and a record sensitivity of 30 zmol (30 × 10 –23 mol), based on sample consumption, have been obtained on this instru- ment. 7,8 Note also that preliminary FT-ICR results have been reported using a resistive 20 tesla magnet. 9 Due to its high mass accuracy, FT-ICR mass spectrometry is also becoming a method of choice in proteomics applications. 10 However, the use of high-field superconducting magnets brings added cost and complexity to the instruments, thus limiting the rou- tine utility of FT-ICR mass spectrometry in the wider range of biochemistry and/or organic chemistry applications. In this paper, we present the results from the evaluation of a 1 tesla FT-ICR mass spectrometer with an ESI source, which provides the foundation for future low-field FT-ICR instru- mentation as a lower cost alternative in many modern mass spectrometry applications. Experimental The home-built FT-ICR mass spectrometer incorpo- rated a resistively heated stainless steel inlet capillary ESI source (0.6 mm i.d., 140 mm long), 11 a vacuum system con- sisting of five stages of differential pumping, a 273 mm bore superconducting magnet (Oxford Instruments) providing 1 tesla field strength, an electrostatic ion guide and a 100 mm gold-plated cubic ICR trap. The ESI source was located inside the homogeneous region of the magnetic field (inter- nal ESI source 12 ). The pumping stages were assembled from © IM Publications 2002, ISSN 1356-1049 M.V. Gorshkov et al., Eur. J. Mass Spectrom. 8, 169–176 (2002) 169 * Corresponding author for reprints