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Dr.  Bert Wang

   Fourier transform mass spectrometry (FTMS) has received considerable attention for its ability to make mass measurements with a combination of resolution and accuracy that is higher than any other mass spectrometer, most recently for biomolecules ionized by electrospray ionization (ESI) and matrix-assisted laser desorption/ onization (MALDI). It is a versatile instrument that can be adapted to a variety of analytical and physical chemistry measurements and to examine ion chemistry and photochemistry. Its versatility follows from the fact that it is an ion trapping instrument, as is the rf quadrupole ass spectrometer. The FTMS instrument mass analyzes and detects ions using methods which are unique among mass spectrometers.

    Fourier transform mass spectrometry derives from ion cyclotron resonance (ICR) spectrometry. The theory of cyclotron resonance was developed by Lawrence in the 1930s. Lawrence built the first cyclotron accelerator and used to study the fundamental properties into a mass spectrometer called the omegatron, by Sommer et al. Other designs followed over the next 15 years, producing instruments that were used principally to study ion-molevule reactions. In 1978, Comisarow and Marshall adapted Fourier transform methods to ICR spectrometry and built the first FTMS instrument. Since that time, interest in FTMS has increased exponentially, as has the number of FTMS instruments.

    Several types of FTMS instruments can be found in laboratories throughout the world. Many have been built by investigators to perform experiments that demand high performance under a variety of conditions, such as ultra-precision mass measurements of low molecular mass ions or high molecular mass ions, while interfacing sources with supersonic jet expansion, atmospheric ionization or laser microprobe capabilities. All FTMS instruments have in common four main components. First is a magnet, which can be either a permanent magnet, an electromagnet or a superconducting magnet. Permanent magnets have low field strengths that limit the performance of an FTMS instruments and only a few systems have been built with these. Electromagnets are limited to field strengths below 2 T, although 1 T is most common. At these fields, FTMS instruments are capable of achieving high performance for ions of relatively low mass-to- charge ratio. The performance of the FTMS instrument improves as the magnetic field strength increases and so the trend is to design instrument with stronger fields using superconducting magnets. These are solenoidal magnets with relatively wide bores compared with those used for NMR spectroscopy. The NbTi superconducting magnets used for FTMS have field strengths ranging from 3 to 9.4 T. The practical and economical high end FTMS magnetic field at present time is 7 T to 9.4 T, however.

    The second component common to FTMS instruments is the analyzer cell. The cell is the heart of the FTMS instrument, where ions are stored, mass analyzed and detected. Several analyzer cell design have been developed. Two common types  of analyzer cells are cubic cell and open end cylindrical cell.

    The cubic cell was the first type of analyzer used for FTMS and is still widely used today. It is composed of six plates arranged in the shape of a cube. This cell is oriented in the magnetic field so that one opposing pair of plates is orthogonal to the direction of the magnetic field lines and two pair of plates lie parallel to the field. The plates that are perpendicular to the field are called the trapping plates. It is common for trapping electrodes of cubic cells to have openings that permit electrons or ions to enter the cell along the magnetic field lines. The four remaining plates are used for ion excitation and ion detection.

    The open-ended cylindrical cell differs in appearance from the cubic cell but has six electrodes that perform the same function as those of the cubic cell. This cell is oriented so that the principal axis of the cylinder aligns with the magnetic field. The trapping electrodes in this design are the two cylinders at the ends of the cell. The center cylinder is divided into four electrodes that function as excitation and detection plates. The shape and dimensions of this type of cell make it more suitable to fit into the bore of a superconducting magnet, while the cubic cell is better matched with the narrow gap between the pole caps of an electromagnet.

    The third feature required of FTMS instruments is an ultra-vacuum system. While all mass spectrometers require vacuum for the analysis and detection of ions, the performance of the FTMS instrument is more sensitive to pressure than other instruments. High vacuum is required to achieve high resolution. A vacuum of 10-910-10 Torr (1 Torr = 133.3 Pa) is required. To achieve these low pressures, cryogenic pumps or turbomolecular pumps are used more frequently then diffusion pumps. Low pressure is required only when ions are detected. Ion formation and detection occur at different times in an FTMS experiment, so that ion formation of these instruments can also operate at elevated pressure provided that high vacuum is restored during ion detection. Most FTMS vacuum systems are equipped with pulsed valves so that the pressure can be increased for a brief duration, for example to introduce a sample for ion molecule studies or to collisionally activate trapped ions.

    The fourth feature that is shared y all FTMS instruments is a sophisticated data system. Many of the components of the data system are similar to those used for FT-NMR. They include a frequency synthesizer, delay pulse generator, broadband r.f. amplifier and preamplifier, a fast transient digitizer and a computer to coordinate all of the electronic devices during the acquisition of data, as well as to process and analyze the data. The tremendous growth and development of the semiconductor industry have benefited FTMS. Electronics hardware costs for FTMS have remained level over the last decade, while performance has increased dramatically.

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Last updated:  08/15/02