The basics of a FT-ICR mass spectrometer

A tour of EMSL Electrospray Ionization FT-ICR mass spectrometry laboratory

Collecting a FT-ICR spectrum: a step-by-step procedure.

The basics of a FT-ICR mass spectrometer.

The basic components of a FT-ICR spectrometer are illustrated in figure 1.

They consist of an ionization source, a vacuum pumping system that can attain ultra-high vacuum (< 10^-9 torr), and a trapped ion analyzer cell that is situated in the homogeneous region of a large magnet. Superconducting magnets up to 12 tesla are currently being used, with new magnets up to 20-tesla being developed.

The High Field Mass Spectrometry Facility focuses on biochemical applicatons of mass spectrometry, including combinatorial chemistry and sequencing applications. This facility contains the world's first 11.5-Tesla Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FT-ICR-MS) as well as a state-of-the-art 7-Tesla FTICR spectrometer. Although the facility specializes in FTICR mass spectrometry, it has a broad array of mass spectrometers available for use.

A 7-tesla spectrometer is also available.

This spectrometer uses electrospray ionization as the ion source. The ions are produced in a modified Analytica (Branford, CT) ESI source equipped with a heated metal capillary inlet. Ions are injected through the fringing magnetic field using two sets of rf-only quadrupoles equipped with high speed mechanical shutters to enhance differential pumping and prevent the molecular beam from impinging on the trapped ion cell. A pressure reduction of > 12 orders of magnitude from the atmospheric pressure ion source to the trapped ion cell (base pressure <= 10^-10 Torr) is accomplished with 6 stages of differential pumping. The final stage of pumping is based on a custom cryopanel assembly consisting of two sets of concentric cylinders; the outer cylinders are cooled to 77 K and serve as radiative shields, while the inner cylinders are cooled to 15 K which provides a pumping speed of > 10⁵ L/s.

Ions were accumulated in the trapped ion cell by collisional cooling in which the applied trapping potential approximately matches the kinetic energy per charge (i.e. eV/q) of the ion beam. Accumulation efficiency is greatly enhanced by the pulsed introduction of nitrogen gas concurrent with the ion beam traversing the trapped ion cell. The pressure in the cell during accumulation is typically in the 10⁴ Torr range, and drops to <10 Torr after about a second with the use of the custom cryopumping assembly described above. All aspects of pulse sequence timing, excitation, and detection are controlled by an Extrel-FTMS (Madison, WI) Odyssey data station.

Step 1 - Ions are produced by the electrospray ionization source at atmospheric pressure. The ESI source consists of a very fine needle and a series of skimmers. A sample solution is sprayed into the source chamber to form droplets. The droplets carry charge when they exit the capillary and as the solvent vaporizes the droplets disappear leaving highly charged analyte molecules. ESI is particularly useful for large biological molecules that are difficult to vaporize or ionize.

Step 2 - Ions are then injected into the trapped ion cell through a series of quadrupoles and electrochemical shutters that aid in selecting ions of specific m/q.

Step 3 - Different pumping stages are used to obtain an operating pressure in the cell below 10^-9 torr.

Step 4 - Ions are excited to larger cyclotron orbits by a short radio-frequency pulse that increases linearly in frequency over time. The amplitude of the excitation voltage is optimized for each sample studied.

Step 5 - After the frequency sweep is discontinued, the image current induced by the different packets of ions is recorded, amplified, digitized and stored in memory.

Step 6 - The time domain decay signal is converted to frequency domain by Fourier transform. Finally the frequency spectrum is translated into a mass spectrum.

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