Research Equipment
Fourier Transform Mass Spectrometer (FTMS)
The principle and fucntion of the Fourier Transform Mass Spectrometer (FTMS) are explained as Q and A.
FTMS apparatus installed on the 1st floor of the Electron Microscopy Building
Q. What is mass spectrometry?
A. Mass spectrometry is the measurement of inherent mass in all materials. Strictly speaking, the mass of ion constituting of atoms and molecules can be measured by the spctrometry. Because, mass of an electron is negligibly small; no more than 1/1800 different than the mass of a hydrogen atom.
In general, a mass spectrometer divides into three parts of compounents; an "ion source," where the ions are generated by ionization of the neutral molecules. The generated ions are then introduced to an "analyzer" through electrical force, and the analyzer separates the ions according to the spatial difference in movement under electrical or magnetic field. Finally, all ions separated through the analyzer are detected and recorded by a "detector" spatially or time-dependently. The ion cyclotron resonance method differs greatly from the usual mass analysis methods (sector type or quadrupole type) in view of its analyzer and detector.
In the analyzer, the ions are trapped circularly with a high vacuum (at a pressure approximately one trillionth of the atmospheric pressure) and strong magnetic field (approximately 150,000 times the earth's magnetism)viz. the ions are forced into a circular motion (ion cyclotron motion) in the direction of the magnetic field according to balance of centripetal and centrifugal forces. The generated frequency of the circular motion provides superposition of alternating electric currents. As a result, the frequencies are detected by the detector. Therefore, the increases in ionic radius gyration imply the higher masses. To obtain the signals, Fourier transformation is carried out by from time-domain to frequency-domain as in the case of FT-NMR.
Q. What is the "analyzer cell"?
A. It is simple to explain the analyzer cell by use of a box-type compartment. Refer to Figure 1 for an idea of what it looks like. The Trap Plate I can trap the ions in the analyzer cell under an electrode perpendicular to the magnetic field, whereas the Transmitter Plate II organizes the circular rotation of the ions on the plane perpendicular to the magnetic field. Finally, the Receiver Plate III detects ions with an electrode parallel to the magnetic field but perpendicular to the Trap Plate I. The following processes occur within the analyzer cell. First of all, voltage is adjusted in Trap Plate I, and all produced ions are trapped there. Then the ions moving in circular motions are excited in Transmitter Plate II to give them in coherence. And finally, their signals are detected in Receiver Plate III.
Q. What is "ion cyclotron motion"?
A. Recall what you would learn in high school physics. Fleming's left hand rule indicates the thumb(force), index finger(magnetic), and middle finger(electrical) of the left hand all pointing as shown in Figure 2 below. If positively-charged particles are put into a magnetized atmosphere, those particles will move circularly in plane perpendicular to the magnetic field. This is called "ion cyclotron motion." That is, the direction of an electrical current (of a positively charged particle) will be perpendicular to the magnetic field. As the centripetal force pulls them toward the center of the magnetic field, they will be traveling perpendicular to the electrical current. However, the particle undertakes a centrifugal force to push it toward the outside. As a result, both forces are balanced. Phenomenally, a positively charged particle with mass travels around the magnetic field in a circular motion (a negatively charged particle will also travel in a circular motion, but in the opposite direction). Particles with a higher mass and traveling at a lower frequency will travel further to the outside of the circle.
Q. How does this movement allow you to measure the mass?
A. First let's think about a certain type of particle (particles with the same mass at m/z). Inside the analyzer cell, the particles scatter and move in a circular motion as shown in Figure 3. Once AC voltage is applied to the particles with a frequency that coincides with the circular motion by the Transmitter Plate, these ions is in resonance with the AC voltage and the particles are coherent, and they travel in a circular motion in phase as shown in Figure 4. (Excitation) Once these particles congregate and orbit in a circular motion, they repeatedly move in close and then far away from the Receiver Plate. This phenomenon ultimately produces a sine motion in the inductive current. Fourier transformation is carried out from this induced current change to a resonant frequency providing a mass spectrum. If many types of ions are present, measuring induced currents that overlaps the sine wave give you superposition of mass spectra. This aspect resembles theFourier-transform NMR method of measurement.
Q. What is characteristic of FT-MS?
A. Let's talk about the performance of FT-MS. The high precision of FT-MS is allegorically expressed. It is well-known that the population of Ishikawa Prefecture is about 1 million people. If there was even a single person from outside of Ishikawa Prefecture, the device could distinguish that individual from the rest. That is, the resolution of the machine is more than one million. Furthermore, the FT-MS has high sensitivity. In principle, it can even measure a single molecule. Usually it is possible to measure the spectra of the sample with amount of one pico gram (about femto mol). The characteristics of the FT-MS, ultrahigh sensitivity and, precision, enable researchers to obtain extremely accurate measurements of the molecular mass.
However, it is necessary to obtain the structural information on functional groups of the molecule. In order to acquire the information, molecular ions are decomposed by collision of inert gasses (such as nitrogen gas or helium gas). This technique will provide you with their spectrum (MS/MS). The selected ion, i.e. the molecular ion among the mixed ions, is dissociated to provide fragment ions relating to the original ion in the mass spectrum. Furthermore, the obtained ion can be repeatedly dissociated after MS2 (MS/MS). The reputation is generally supposed as MSn (really speaking n is less than ten). For an example, the sequence of amino acids in peptide can be obtained with this method. In FT-MS the MS/MS technique can be operated as SORI (sustained off-resonance irradiation) or IRMPD (infrared multiphoton dissociation).
Here, let's overview the ionization in the FT-MS of JIAST.
ESI (electrospray ionization): A sample that is soluble in water or water/methanol, etc. is sprayed under high voltage to give the multi-charged particles which are decomposed in Coulomb explosion. The resulted cluster ions without solvation are analyzed in the spectrometer. The ionization offers the following merits:
- Highly polar or nonvolatile molecules.
- Highly charged ions.
SIMS (secondary ion mass spectrometry): Mix the sample with a glycerol matrix onto the sample holder, and then spatter that sample layer with a high-speed ion beam (such as cesium ion Cs+). The spattered molecules are ionized and introduced into the analyzer. The ionization provides the ions; (M+H)+, (M-H)-, (M+Na)+, (M+K)+ etc.
MALDI (matrix-assisted laser desorption ionization): First dry a mixed solution of sample and matrix to crystallize it, and then irradiate it with a pulse laser beam. This method even produces protonated molecule of high-mass chemical compounds such as proteins.
EI (electron ionization): This is the most common method of ionization. The electrons are substracted from vaporized sample molecules by electron beam to furnish the molecular ions. Excess of the electron-energy produces the fragment ions which can be observed at many peaks in the spectra.
