MASS SPECTROMETRY RESOURCE

Theory and History:

     Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS or just FT-MS) is often thought of as being the most complex method of mass analysis and detection. When considered at first principles though, it is fairly simple to describe - although this does ignore many of the 3 dimensional ion perturbation effects which make the technique far more complex. It is not necessary to discuss those in any detail here. The technique of ICR-MS was first published in the mid. 1950's [1] where it was demonstrated for measurement of very small mass differences at very high precision. The technique remained a largely academic tool until the application of FT methods [2] by Alan Marshall and Melvin Comisarow in the early 1970's [3]. It is now one of the most sensitive methods of ion detection in existence and has almost unlimited resolution - >10⁷ is possible, with most experiments taking place in the 10⁵ to 10⁶ range.

    In the basic FT-MS instrument, the ions are generated in the source (as usual) and then pass through a series of pumping stages at increasingly high vacuum. When the ions enter the cell (ion trap) pressures are in the range of 10^-10 to 10^-11 mBar with temperatures close to absolute zero. The cell is located inside a spatial uniform static superconducting high field magnet (typically 4.7 to 13 Tesla) cooled by liquid helium and liquid nitrogen. When the ions pass into the magnetic field they are bent into a circular motion in a plane perpendicular to the field (see Fig. 1) by the Lorentz Force (see equation 1 below). They are prevented from precessing out of the cell by the trapping plates at each end (see Fig. 1).

F is the Lorentz Force observed by the ion when entering the magnetic field B is the magnetic field strength (constant) v is the incident velocity of the ion wc is the induced cyclotron frequency m is the mass of the ion z is the charge on the ion

  
    The frequency of rotation of the ions is dependent on their m/z ratio (equation 2). At this stage, no signal is observed because the radius of the motion is very small. Excitation of each individual m/z is achieved by a swept RF pulse across the excitation plates of the cell. Each individual excitation frequency will couple with the ions natural motion and excite them to a higher orbit where they induce an alternating current between the detector plates. The frequency of this current is the same as the cyclotron frequency of the ions and the intensity is proportional to the number of ions. When the RF goes off resonance for that particular m/z value, the ions drop back down to their natural orbit (relax) and the next m/z packet is excited. Although the RF sweep is made up of a series of stepped frequencies, it can be considered as all frequencies simultaneously. This results in the measurement of all the ions in one go producing a complex frequency vs. time spectrum containing all the signals - the FID. Deconvolution of this signal by FT methods results in the deconvoluted frequency vs. intensity spectrum which is then converted to the mass vs. intensity spectrum (the mass spectrum) by equation 3. It is also usual to correct for mass errors at this stage by applying a calibration.