U.S. patent number 3,576,992 [Application Number 04/759,645] was granted by the patent office on 1971-05-04 for time-of-flight mass spectrometer having both linear and curved drift regions whose energy dispersions with time are mutually compensatory.
This patent grant is currently assigned to The Bendix Corporation. Invention is credited to Charles J. Moorman, John Q. Parmater.
United States Patent |
3,576,992 |
Moorman , et al. |
May 4, 1971 |
TIME-OF-FLIGHT MASS SPECTROMETER HAVING BOTH LINEAR AND CURVED
DRIFT REGIONS WHOSE ENERGY DISPERSIONS WITH TIME ARE MUTUALLY
COMPENSATORY
Abstract
A mass spectrometer is disclosed which is based on the linear
time-of-flight principle. Initial ion energies are compensated for
by combining a curved drift region with a linear drift region so
that ions of the same mass but differing energies reach the
collector at the same time.
Inventors: |
Moorman; Charles J.
(Cincinnati, OH), Parmater; John Q. (Cincinnati, OH) |
Assignee: |
The Bendix Corporation
(N/A)
|
Family
ID: |
25056426 |
Appl.
No.: |
04/759,645 |
Filed: |
September 13, 1968 |
Current U.S.
Class: |
250/287 |
Current CPC
Class: |
H01J
49/22 (20130101); H01J 49/40 (20130101) |
Current International
Class: |
H01J
49/40 (20060101); H01J 49/00 (20060101); H01J
49/34 (20060101); H01j 039/36 () |
Field of
Search: |
;250/41.9 (1)/ ;250/41.9
(3)/ |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Borchelt; Archie R.
Assistant Examiner: Church; C. E.
Claims
We claim:
1. In combination with a time flight mass spectrometer of the type
having an ion source, an ion collector, an energy focusing ion path
between the ion source and collector, and means for periodically
accelerating ions from the ion source to the energy focusing ion
path, the improvement wherein said energy focusing ion path
comprises:
a linear drift region providing a first ion time dispersion
according to the mass-to-charge ratio; and
a curved drift region providing a second ion time dispersion
depending on the variation in ion angular velocity with ion energy,
said second time dispersion being opposite said first time
dispersion wherein one drift region compensates for the adverse
effect of energy spread in the other drift region whereby all ions
of the same mass and differing energy that leave the ion source at
the same time reach the collector at the same time.
2. A time-of-flight mass spectrometer comprising,
an ion source,
means for accelerating the ions from said source in a specified
direction,
an ion collector,
an ion path between said source and said collector, said ion path
comprising,
a curved ion drift region including means providing a uniform
electrostatic field deflection therein, wherein the time of flight
of an ion through said curved drift region is proportional to the
square root of the kinetic energy of the ion,
a linear ion drift region wherein the time of flight of an ion
through said linear drift region is inversely proportional to the
square root of the kinetic energy of the ion whereby changes in
time of flight caused by variations in ion kinetic energy cancel
one another and thereby providing equal times of flight for ions of
equal mass and differing energy between said source and
collector.
3. A mass spectrometer as set forth in claim 2 further
comprising,
electrostatic plates providing said uniform electrostatic field,
and
means at the exit end of said curved drift region for limiting the
acceptable ion energy spread of transmitted ions.
4. A mass spectrometer as set forth in claim 2 in which said curved
ion drift region precedes said linear ion drift region.
5. A mass spectrometer as set forth in claim 2 in which said linear
ion drift region precedes said curved ion drift region.
6. A mass spectrometer as set forth in claim 2 in which said linear
ion drift region is divided whereby one part precedes and the other
part follows said curved ion drift region.
7. A mass spectrometer as set forth in claim 2 in which said curved
ion drift region is divided whereby one part precedes and the other
part follows 8In an energy focusing time of flight mass
spectrometer of the type having an ion source, an ion accelerating
means, an ion collector, and ion path between said ion source and
said ion collector, the improvement in energy focusing means
wherein said ion path comprises:
a curved ion drift region;
ion-deflecting means providing a uniform ion deflection in said
curved ion drift region wherein ions of equal mass and different
energies are made to follow different paths therein whereby
variations in ion angular velocity depend on ion energy to provide
an ion time dispersion; and
a linear ion region providing an ion time dispersion opposite the
time dispersion created by said curved ion drift region whereby
ions of equal mass and differing energies leaving said ion source
at the same time will travel different paths and have equal times
of travel and thereby reach
said ion collector at the same time. 9. A mass spectrometer as set
forth in claim 8 in which said ion deflecting means comprise,
electrostatic plates providing a uniform electrostatic deflection
field in said curved drift region wherein said ions assume
different radii whereby ions of high energy have an angular
velocity about the center of curvature of said curved drift region
less than that of ions of lower energy and thereby ions of lower
energies leave said curved drift region before ions of higher
energy that enter said curved drift region with said ions of low
energy, and
means at the exit end of said curved ion drift region for limiting
the acceptable ion energy spread of transmitted ions.
Description
BACKGROUND OF THE INVENTION
This invention pertains to mass spectrometers and more particularly
to an energy focusing time-of-flight mass spectrometer.
There are many different types of mass spectrometers. Magnetic mass
spectrometers separate ions according to their mass-to-charge by
noting the angular deflection while passing the beam through a
magnetic field. Radio frequency mass spectrometers utilize a
combination of electrostatic and radio frequency field in such a
way that ions of only one mass-to-charge ratio may pass through.
The present invention is particularly concerned with linear
time-of-flight mass spectrometers.
In its simplest embodiment, the known type of time-of-flight mass
spectrometer comprises an ion source and a collector disposed at
opposite ends of an evacuated field-free drift tube. Upon the
introduction of gas molecules into the ionization region of the
spectrometer source, ions are formed, usually be electron
bombardment, which are periodically pulsed out of the source by
source grids toward the collector by either one or several electric
fields established between appropriately spaced grids. Since the
velocity of the ions in the field-free drift tube region is a
function of the mass-to-charge ratio, ion separation occurs. The
amount of separation depends strongly on the length of the tube.
Therefore, when the ions reach the collector they have separated
into ion bunches with the lightest group reaching the collector
first. As such, proper electrical circuitry connected to the
collector will show a complete mass spectrum (in time) of the gas
molecules ionized in the source.
In previous linear time-of-flight mass spectrometers, all ions are
accelerated through a field to the same energy or to the same
momentum. They are then allowed to drift in a field-free region.
Since they all have the same energy but have different masses, they
will separate into bunches according to their mass. These bunches
are then collected at a fixed point in the field-free region and
their variations in arrival time are measured as a measure of
atomic mass or more properly mass-to-charge ratio.
One serious problem has always existed in this type of instrument.
This problem is that all ions are not at rest before the initial
accelerating pulse is applied and therefore all ions of the same
mass-to-charge ratio may have different energies. Thus, ions of the
same mass will not reach the collector at the same time and the ion
bunches may overlap one another. The ions have initial ion energies
which arise from several sources. The most easily understood source
of energy spread is that associated with thermal energy. Certain
classes of molecules exhibit energies greater than thermal. This
energy is imparted to them during the ionizing process. While this
mechanism is not completely understood, it has been experienced by
many investigators. Variations in starting positions within the
initial region also contributes to variations in energy.
This invention provides a time-of-flight mass spectrometer which
compensates for slight variations in initial energy so that ions of
equal mass-to-charge ratio but with slightly different initial
energies will arrive at the collector or detector simultaneously.
This invention also provides a means for limiting the amount of
energy spread of ions which are transmitted through the
spectrometer ion flight path.
SUMMARY OF THE INVENTION
A time-of-flight mass spectrometer having energy focusing means is
provided which compensates for ions of equal mass having differing
energies so that all ions of the same mass reach a collector
simultaneously. The ion path comprises a curved region and a linear
drift region. The present invention relies on the variation in
angular velocity with ion energy to give a time dispersion in a
curved region which is opposite the time dispersion in the linear
drift region. In this way, one region compensates for the adverse
effect of energy spread in the other.
DESCRIPTION OF THE DRAWINGS
An illustrative embodiment of the present invention is shown in the
following drawings, in which:
FIG. 1 is a schematic illustration of a standard prior art linear
time-of-flight mass spectrometer,
FIG. 2 is a schematic illustration of a time-of-flight mass
spectrometer according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The equipment schematically illustrated in FIG. 1 is a typical
prior art time-of-flight mass spectrometer 10. The mass
spectrometer 10 consists of four basic elements which are: an ion
source 12 to produce ions of the molecular species entering the
ionization region 13, a grid system 14--15 that draws the ions from
the ion source and accelerates the ions down the drift tube, a
field free linear drift tube or region 16 for confining the ion
"packet" and supplying the distance required to separate ions
according to mass-to-charge ratio, and an ion collector 18 to
collect the ions and measure their intensity.
Region 13 designates the region of ion formation typically
accomplished by passing an electron beam 20 from a cathode 22
through a gas to be analyzed which is introduced into the region
13. A power supply 24 is shown supplying a heater current to the
cathode 22. Cathode 22 has a negative potential bias supplied by
source 24 to provide ionizing electrons with energy sufficient to
ionize the gas in region 13.
Upon application of a voltage pulse to grid 14 from the pulse
source 26, the heterogeneous group of ion masses formed in region
13 are accelerated, each ion acquiring substantially the same
kinetic energy but differing in exit velocity through grid 15 in
proportion to the square root of its mass-to-charge ratio, the
heavier ions having lower velocities and therefore becoming
separated from the lighter ions. As a result, discrete ion bunches
of different mass-to-charge ratios pass through grid 15 and either
drift or accelerate, depending on the specific potentials through
drift region 16 to collector 18. If only ions of the same energies
are present, the lightest group reaches the collector 18 first,
followed by groups of successively heavier mass. However, as
previously described, all ions of equal mass will not have equal
energies and thus ions of equal mass and differing energies will
not reach the collector simultaneously and the groups or bunches of
ions may overlap one another.
An illustrative embodiment of the novel construction of the present
invention is shown in FIG. 2. In this embodiment, an electrostatic
sector 28 is combined with a field free linear drift tube or region
30 to form the ion path between an ion source region 32 and an ion
collector 34. As previously described, ions are created by electron
bombardment in the source and are accelerated towards the
electrostatic sector 28 by means of applying a pulse to one of the
accelerating grids within the source region 32.
The electrostatic sector 28 has a curved construction and has
ion-deflecting means, such as electrostatic plates or electrodes 36
and 38, therein which produce a uniform electrostatic field within
the curved sector or region. The electrodes are typically of
cylindrical shape and have an angle .theta.; however, other shapes
such as toroidal, spherical, etc., may be utilized.
It is the property of an electrostatic sector of this type that
ions of different energy or velocity passing therethrough take
different paths. The faster ions take longer and therefore ions
which enter sector 28 with high velocities leave later than ions
which enter at the same time with lower velocities.
The electrostatic field existing within the sector makes the ions
follow a curved path. Hence, when two ions of the same mass but
with slightly different energies or velocities are made to follow a
circular path by a uniform electrostatic field, they will assume
different radii. The faster ion will take a radius such that its
angular velocity about the center of curvature of the field will be
less than that of the slower ion. The path of the high energy ion
and the low energy ion are represented by 40 and 42 respectively.
Therefore, if a high energy and a low energy ion of equal mass
enter the electrostatic field at the same time, the slower ion will
exit first.
At the end of the sector 28 is placed an energy slit 44 which
limits the lowest and highest energy ions and passes only the
chosen energy span. At this point ions of equal mass but slightly
different velocities are dispersed along the ion path with the high
velocity ions trailing behind the lower velocity ions. From here
the ions continue through the field free linear drift region 30
with the faster ions again overtaking the slower ions, just as both
ions reach the detector.
The motion of ions traveling in the radial electrostatic field and
the field free linear drift region may be mathematically explained
as follows, in which the following symbols will be used:
m = mass of ion
F = centripetal force of ion
v = rectilinear velocity of ion
r = radius of ion path
K = kinetic energy of ion
w = angular velocity of ion
.theta. = angle of electrostatic sector, radians
t.sub.1 = time of flight through electrostatic sector
t.sub.2 = time of flight through linear region
T= t.sub.1 +t.sub.2
E.sub.(r) = field intensity in the electrostatic sector
Vf = voltage applied to electrostatic sector
R.sub.1 = radius of inner sector plate
R.sub.2 = radius of outer sector plate
d = length of linear region
A body undergoing uniform circular motion follows a path of radius
r, given by
The kinetic energy of the body is given by
2K= mv.sup.2 (2)
the rectilinear velocity by
the angular velocity by
when equations (1), (2), and (3) are combined.
The time t.sub.1 is expressed by
indicating t.sub.1 is proportional to the square root of the
kinetic energy.
The time t.sub.2 is expressed by
indicating t.sub.2 is inversely proportional to the square root of
the kinetic energy.
When the total time
T= t.sub.1 +t.sub.2 (7)
is expressed as
it becomes apparent that changes in time of flight caused by
variations in
kinetic energy tend to cancel one another.
The centripetal force F is generated by an electrostatic field E
(r), described by the equation
It is also seen from the above calculations that other structural
arrangements are possible. For instance, the linear drift region 30
can be placed in front of the electrostatic sector 28 or the linear
drift region 30 can be divided and one part thereof placed on each
end of the electrostatic sector 28. It is also possible to split
the electrostatic sector 28 and place one part thereof on each end
of the linear drift region 30. Thus, the total effective length of
the ion path resulting from the combination of a field free linear
drift region and a curved electrostatic region provides equal
times-of-flight between the source and collect or for ions of equal
mass and differing energy.
The present invention relies on the variations in angular velocity
with ion energy to give a time dispersion which is opposite the
time dispersion in the linear drift region. In this way, one
section of the instrument compensates for the adverse effect of
energy spread in the other, so that the two together do what
neither can do alone.
Therefore, a linear time-of-flight mass spectrometer is now
provided which limits the energy spread of transmitted ions and
compensates for the energy spread of the ions which are
transmitted.
While the form of apparatus herein described constitutes a
preferred embodiment of the invention, it is to be understood that
the invention is not limited to this precise form of apparatus.
* * * * *