Radiant energy – Ionic separation or analysis – Ion beam pulsing means with detector synchronizing means
Reexamination Certificate
2002-01-29
2004-09-14
Lee, John R. (Department: 2881)
Radiant energy
Ionic separation or analysis
Ion beam pulsing means with detector synchronizing means
C250S289000, C250S294000
Reexamination Certificate
active
06791079
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the fields of measurement instruments, in particular to mass spectrometers used for analyses of substances based on results of determination of masses of their ions or spectra of masses.
BACKGROUND OF THE INVENTION
In order to understand the present invention, terminology used in the description, and novelty of the present invention over the prior art, it would be advisable first to explain the general principles of construction and operation of mass spectrometers and their classification.
A mass spectrometer is an instrument for separation of ionized particles (such as atoms, molecules, cluster formations) by their masses, more specifically, by a ratio of ion mass m to its charge. The separation is carried out under the effect of magnetic and electric fields. Furthermore, mass spectrometer is used for determining masses of ions and relative contents of specific ions in a substance, i.e., spectrum of masses.
A typical mass spectrometer consists of the following parts: a system for preparation and introduction of an a substance to be analyzed into the instrument; an ions source where the aforementioned substance is ionized at least partially and where an ion beam is formed; a mass analyzer where the ions are separated in accordance with an m/e ratio, focused, and are emitted from the ion source in various directions within a small space angle; a ion receiver or collector where ion current is measured or converted into an electrical signal; and a device for amplification and registration of the output signal. In addition to amount of ions (ion current), the registration unit also receives information about ion mass. Other units included into a mass spectrometer are power supplies, measurement instruments, and a vacuum system. The latter is required for maintaining the interior of the mass spectrometer under high vacuum, e.g. of about 10
−3
to 10
−7
Pa. Operation is normally controlled by a computer, which also stores the acquired data.
A mass spectrometer is characterized by its resolution capacity, sensitivity, response, and a range of measured masses. The aforementioned response is a minimal time required for registration of mass spectrum without the loss of information within the limits of so-called decade of atomic mass units (1-10, 10-100, etc.). Normally such time is 0.1 to 0.5 sec. for static mass spectrometers and 10
−3
for dynamic (time-of-flight) mass spectrometers.
A substance to be analyzed is introduced into the mass spectrometer with the use of so-called molecular or viscous flow regulators, load ports, etc.
By methods of ionization, ion sources of mass spectrometers can be divided into various categories, which are the following: 1) ionization caused by collisions with electrons; 2) photo-ionization; 3) chemical ionization due to ionic-molecular reactions; 4) field ion emission ionization in a strong electric field; 5) ionization due to collisions with ions; 6)atomic-ionization emission due to collisions with fast atoms; 7) surface ionization; 8) spark discharge in vacuum; 9) desorption of ions under effect of laser radiation, electron beam, or products of decomposition of heavy nuclei; and 10) extraction from plasma.
In addition to ionization, in mass spectrometer an ion source is used also for forming and focusing an ion beam.
More detail general information about types and constructions of ion sources suitable for use in mass spectrometers can be found in “Industrial Plasma Engineering” by Reece Roth, Vol. 1, Institute of Physics Publishing, Bristol and Philadelphia, 1992, pp. 206-218.
By types of analyzers, mass spectrometers can be divided into static and dynamic. Static mass spectrometers are based on the use of electric and magnetic fields which remain, during the flight of ions through the chamber, practically unchanged. Depending on the value of the m/e ratio, the ions move along different trajectories.
The most popular static mass spectrometer is a conventional mass spectrograph in which ion beams with different e/m ratios are focused in different areas of a photo-sensitive element, e.g., a photo-sensitive plate located in a focal plane of the instrument. Since the outlet opening of the ion source is made in the form of a slit, after development, the points hit by ions are seen on such a plate in the form of strips. In a static mass spectrometer the ion beam is focused onto the slit of the ion receiver. If the electric or magnetic field varies smoothly, the ion beams with different e/m ratios will sequentially pass through the aforementioned slit. Continuous registration of the ion current will produce a graph with ion peaks on the mass spectrum. If necessary, the obtained mass spectrum can be used for quantitative evaluation by methods of photometry.
In a static mass spectrometer with a homogeneous magnetic field, ions are emitted from the ion-source slit in the form of a diverging beam. When the diverging beam enters the magnetic field, it is divided into beams with different elm ratios. Such beam can be registered on a photo-sensitive plate or in any other registering device. Static mass spectrometers, in turn, can be divided into various types such as static mass spectrometers with a non-homogeneous magnetic field, with ion prisms that separate the beam into sub-beams with different elm ratios, and with double-focusing of the ion beam. Various combinations of the aforementioned mass spectrometers are also possible.
It should be noted that static mass spectrometers are static installations which are heavy in weight, complicated in construction, an operation with them require the use of skilled personnel.
In dynamic mass spectrometers, the ions are separated on the basis of different times of flights through the given distance. Furthermore, the ions can be separated under the effect of pulse or RF electromagnetic fields with periods equal to or shorter than the time of flight of ions through the analyzer. Among the dynamic mass spectrometers, most popular ones are time-of-flight types, RF types, quadrupole types, magnetic-resonance type, and ion-cyclotron resonance types of mass spectrometers.
In time-of-flight mass spectrometers, ions formed in the ion source are injected into the analyzer via a grid in the form of short pulses of ion current. The analyzer comprises an equipotential space. On its way to the collector, the pulse is decomposed into several sub-pulses of the ion current. Each such sub-pulse consists of ions with the same elm ratios. The aforementioned decomposition occurs because in the initial pulse all ions have equal energies, while the speed of flight V and, hence, the time of flight t through the analyzer with the length equal to I are inversely proportional to m
1/2
:
T=I
(
m/
2
eV
)
1/2
.
A series of pulses with different e/m ratios forms a mass spectrum that can be registered, e.g., with the use of an oscilloscope. Resolution capacity of such an instrument is proportional to length l.
An alternative version of the time-of-flight mass spectrometer is a so-called mass-reflectron, which allows an increase in resolution capacity due to the use of an electrostatic mirror.
Energies of ions collected in each packet are spread over the temperature of the initial gas. This leads to broadening of peaks on the collector. Such broadening is compensated by the electrostatic mirror that prolongs the time of flight for slow ions and shortens the time of flight for fast ions. With the drift path being the same, the resolution capacity of a mass reflectron is several times the resolution capacity of a conventional time-of-flight mass spectrometer.
In the ion source of an RF mass spectrometer, ions acquire energy eV and pass through a system of several stages arranged in series. Each stage consists of three spaced parallel grids. An RF voltage is applied to the intermediate grid. With the frequency of the applied RF field and energies eV being constant, only those ions can pass through the space between the first and intermediate grids that have a predetermined m
Lee John R.
Quash Anthony
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