3. Fourier Transform MS (FTMS)
In an FTMS instrument, detection of ions of interest is performed by applying a very fast frequency sweep voltage to the transmitter plates following ionization process. The frequency of this cyclotron motion is mass dependent. The coherent motion of the excited ions induces image currents in the receiver circuit. Positive ions approaching one receiver plate attract electrons. As they continue to move in their orbits they approach the opposite receiver plate and attract electrons on this surface. When the receiver plates are connected in a circuit the induced image current of ions can be detected in the form of a time domain signal resulting from the superposition of a number of individual frequencies produced by the different ion species coherently orbiting at the same time. A mass spectrum is obtained by amplification, digitization, and conversion of this time domain signal to a frequency domain spectrum using Fourier transformation.
4. Time of Flight
A scheme is displayed below.
A small number of ions is extracted from the source in a few /¿sec, accelerated with a few kV, and they are directed to a field free light tube. The process can be repeated 100,000 times per second. Kinetic energy is similar for every ion. Ions with higher velocities (light ions) will reach the end of the tube before heavy ions. Instruments have two tubes with a mirror in the middle and resolution may reach 5000.
In a tube of length L the time of flight t is connected to the velocity v t = L / v
Ions get a kinetic energy £k = 1 /2 mv2 = zV where V is the voltage. In a tube of length L the time of travel is t = L/ v thus t = L(m/2 Vz)1 ^2. The time of flight is proportional to the square root of the ion mass, which allows discrimination according to the m/z ratio.
As an example, if 3000 V are used to accelerate the ions in the flight tube of 1 m, an ion with m = 200 amu, and z = 1 the travel time is 18.597 msec, and with m = 201 amu, t = 18.643 msec. The difference is 48 nsec. It is thus necessary to use fast electronics. To cope with the small differences of velocities encountered by ions of the same m/z, a reflectron is used, which acts as a retarding electric field. It is a series of lenses with linearly increasing voltages. From ions of the same m/z value, those with the greater velocity will penetrate the reflectron further and take a longer time to turn around and leave the reflectron towards the detector. The ions of lower velocity will catch up with those of higher velocity and reach the detector at the same time. Orthogonal-acceleration reflectron TOF instruments combine the ability to perform accurate mass determination with an excellent fullscan sensitivity.
The hybrid quadrupole time of flight mass spectrometer (QTOF) was introduced as a mass spectrometer capable of tandem MS with particular emphasis on its applicability for protein and peptide analysis. It combines the simplicity of a quadrupole MS with the high efficiency of a detector tube
TOF analyzer. Key components of the instrument are the quadrupole, hexapole collision cell, and the reflectron-TOF analyzer. The sample is introduced through the interface and ions are focused using the hexapole ion bridge into the quadrupole. Here, the precursor ion is selected for later fragmentation and analysis with a mass window of approximately three mass units, which is a typical window to preserve the isotope envelopes into the product ion spectra. The ions are ejected into the hexapole collision cell, where argon is used for fragmentation. From this point, the ions are collected into the TOF region of the MS-MS. In an orthogonal TOF (oa-TOF) the flight path of the ions changes 90°. The ions are then accelerated by the pusher and travel about 1 m down the flight tube to the reflectron. Thus, the TOF side of the Q-TOF-MS achieves simultaneous detection of ions across the full mass range at all times. This continuous fullscan mass spectrum is in contrast to the tandem quadrupoles that must scan over one mass at a time. The Q-TOF-MS-MS is capable of 10,000 resolving power expressed at full width half maximum.
The electronics of the detector must record the complete mass spectrum within the flight time of the ions (1 to 100 /asec range) with peak widths in the ns range. This is possible since a high scan rate (up to 20,000 scans/sec) allows for the detection of narrow chromatographic peaks. There is virtually no limit on mass range and no ion loss. In TOF-MS there are instruments that can provide high resolution at a moderate scan speed, and instruments that can store 100 to 500 spectra/sec with unit mass resolution.
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