Radiant energy – Ion generation – Electron bombardment type
Reexamination Certificate
1998-10-16
2001-10-09
Anderson, Bruce (Department: 2881)
Radiant energy
Ion generation
Electron bombardment type
C313S362100
Reexamination Certificate
active
06300637
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to mass spectrometers and more particularly to the increase in the ionization efficiency in such a spectrometer.
DESCRIPTION OF THE PRIOR ART
Mass spectrometry is a technique for determining the mass of individual molecules by manipulating electrically charged (i.e., ionized) forms of these species in the gas phase. This information may be used to determine both the chemical composition and molecular structure of the ionized species. Because molecules are electrically neutral it is necessary to ionize the gas phase species of interest prior to mass analysis.
While there are many different techniques to accomplish ionization, the most widely used technique is to bombard the species of interest with energetic electrons. This process is referred to as electron ionization. In most cases the collision of an energetic electron with a molecule leaves the molecule with a positive charge as the exchange of energy results in the ejection of an electron from the molecule. There are some molecules that are capable of capturing an electron and therefore being left with a negative charge as a result of electron ionization; however, very low energy electrons must be used to obtain that result.
The energy delivered to a molecule during an electron ionization event may vary over a large range depending on the electron energy and the dynamics of a particular collision. Typically the energy delivered to the molecule is sufficient to not only accomplish ionization but to also fragment the initially created molecular ion by breaking chemical bonds. The original electrical charge must be distributed on the resulting fragments and thus the subsequent mass analysis will reveal the mass of the charged fragments. A molecule usually has many different fragmentation pathways available and the probability of following a particular pathway is determined by the internal distribution of the deposited energy and the relative strengths of different chemical bonds.
Because electron ionization typically results in the ionization of many molecules, the statistical distribution of charge and energy during fragmentations will result in a mixture of fragment and molecular ions that is characteristic of the original molecules. A plot of the mass versus relative number of this mixture of ions is referred to as a mass spectrum. Such a spectrum may be interpreted using known mechanisms and statistics of fragmentation to deduce the composition and structure of the original molecules. Alternatively, the spectrum may be compared to a library of known spectra in an attempt to find a matching spectrum and thereby determine the identity of the original molecules.
Electrons are usually created by an electron source such as a wire filament. Alternative means for producing electrons exist and include various other thermionic emitters, as well as field emitter arrays, etc.
When a wire filament is used as the electron source, the filament is heated to incandescence thereby causing electrons to be emitted from the filament. These electrons will have low kinetic energy and must be accelerated to a kinetic energy sufficient to ionize the sample molecules. The acceleration is provided by an electric field applied to the ionization region. The accelerated electrons are directed through a chamber which contains the gas sample to be ionized. The electrons that exit the chamber are typically collected by an electrode employed to measure the total electron flux or current. If the ionization chamber is at ground potential the electrons can be accelerated by placing a negative potential, typically in the order of −70V, on the filament.
Referring now to
FIG. 1
, there is shown a simplified diagram of a prior art mass spectrometer
10
. The spectrometer
10
includes an electron source such as filament
12
which is connected to a power supply
14
which is used to heat the filament to incandescence. The filament
12
is also connected to a source
16
of negative potential with respect to the ionization chamber
18
. The filament
12
is placed opposite an opening
18
a
in chamber
18
. The chamber has another opening
18
b
through which the gas sample to be ionized enters the chamber and an opening
18
c
which is opposite opening
18
a
and adjacent an external collector
20
. The collector
20
is connected through an ammeter
22
to ground potential. The accelerated electrons enter chamber
18
through opening
18
a
and exit the chamber through opening
18
c.
Spectrometer
10
also includes a magnet
24
which functions to constrain the electrons and increase the path length as the electrons travel a helical path between filament
12
and collector
20
.
Each electron emitted from filament
12
has a low probability of encountering a sample molecule during its traversal of chamber
18
. Therefore, the mass spectrometer of
FIG. 1
has a low ionization efficiency which makes it difficult to obtain a flux or quantity of ions sufficient for detection of low concentration constituents of the sample stream.
Several techniques have been employed in the prior art to increase the ionization efficiency. One such technique is to increase the sample pressure and thereby increase the number density of molecules in the ionization region. This has the disadvantage of decreasing filament lifetime and may also require isolation and increased pumping of the mass analyzer region to avoid degradation of analyzer performance. Another such technique is to increase the electron emission by heating the filament
12
to higher temperature; however, this technique also has the disadvantage of reducing the life of filament
12
.
Yet another technique is to increase the effective length of the electron trajectory through chamber
18
by immersing chamber
18
in a magnetic field to thereby effect a spiral electron trajectory through the chamber. The magnetic field is provided by magnet
24
and is a typical component of most electron ionization sources. As will be described in the description of the preferred embodiment(s), the ionization source of the present invention applies a reflecting voltage to the collector
20
to increase ionization efficiency. While the ionization source of the present invention does include magnet
24
, the magnetic field of the magnet is used primarily in the present invention for radial focusing to ensure confinement of the electrons to a path connecting filament
12
and collector
20
.
SUMMARY OF THE INVENTION
The present invention is a method for increasing ionization efficiency in a mass spectrometer (MS) that has an electron source for producing electrons. The MS also has an electron collector opposite said electron source to thereby effect an ionization region between the electron source and the electron collector. The method has the step of connecting to the electron source a first potential relative to the ionization region. The first potential has a predetermined amplitude and a predetermined polarity to cause electrons emitted by the electron source to traverse the ionization region.
The method also has the step of connecting to the electron collector a second potential relative to the ionization region having a predetermined amplitude and a predetermined polarity identical to the first potential predetermined polarity. This creates a potential well between the electron source and the electron collector.
The present invention is also an apparatus for increasing ionization efficiency in a mass spectrometer (MS). The MS has an electron source for producing electrons and an electron collector opposite the electron source to thereby effect an ionization region between the electron source and the electron collector. The apparatus has a first switch which when closed connects to the electron source a first potential relative to the ionization region. The first potential has a predetermined amplitude and a predetermined polarity to cause electrons emitted by the electron source to traverse the ionization region. The apparatus also has a second switch which when closed connec
Arnold Robert W.
Beu Steven C.
Anderson Bruce
Siemens Energy & Automation Inc.
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