U.S. patent number 3,626,182 [Application Number 04/812,284] was granted by the patent office on 1971-12-07 for apparatus and method for improving the sensitivity of time of flight ion analysis by ion bunching.
This patent grant is currently assigned to Franklin GND Corporation. Invention is credited to Martin J. Cohen.
United States Patent |
3,626,182 |
Cohen |
December 7, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
APPARATUS AND METHOD FOR IMPROVING THE SENSITIVITY OF TIME OF
FLIGHT ION ANALYSIS BY ION BUNCHING
Abstract
Apparatus and methods for sorting and detecting ions in a drift
cell, the electric fields applied to different regions of the cell
being controlled at appropriate times to ensure the rapid
withdrawal of the ions from a reaction region to an analysis
region, the bunching of the ions in the analysis region, and
thereafter the separation of the bunched ions in accordance with
ion drift velocity, and detection of separated ion species.
Inventors: |
Cohen; Martin J. (West Palm
Beach, FL) |
Assignee: |
Franklin GND Corporation (West
Palm Beach, FL)
|
Family
ID: |
25209109 |
Appl.
No.: |
04/812,284 |
Filed: |
April 1, 1969 |
Current U.S.
Class: |
250/283; 250/287;
250/288 |
Current CPC
Class: |
G01N
27/622 (20130101) |
Current International
Class: |
G01N
27/64 (20060101); H01j 039/34 (); B01d
059/44 () |
Field of
Search: |
;250/41.9R,41.9SE,41.9SB |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lawrence; James W.
Assistant Examiner: Church; C. E.
Claims
The invention claimed is:
1. A method of ion analysis which comprises forming product ions by
reacting reactant ions with neutral trace particles, moving said
ions, concentrating said product ions into a bunch by decelerating,
and separating the bunched ions in accordance with their velocity
in a drift field, the recited steps being performed in a space
maintained at a pressure such that the length of the mean free path
of said ions is very much smaller than the dimensions of the
space.
2. A method in accordance with claim 1, wherein the ions are formed
during a first interval of time and are bunched during a second
interval of time shorter than the first.
3. A method in accordance with claim 2, wherein the method steps
are repeated cyclically, ions formed during one cycle being
separated while ions of the next cycle are formed.
4. A method in accordance with claim 1, wherein the ions are
subjected to a first drift field during formaction and to drift
fields stronger than the first field during concentration and
separating.
5. A method in accordance with claim 1, wherein the ions are formed
in a first region and are rapidly withdrawn to and held at a second
region during concentration.
6. A method of improving the sensitivity of ion analysis in a drift
cell having sequential ion formation and ion drift regions, which
comprises forming ions in the ion formation region during a first
interval of time, causing said ions to move into the drift region
during a second interval of time but to decelerate upon entering
the drift region and become bunched near the entrance to said drift
region, and thereafter moving the bunched ions through the drift
region and causing them to separate in accordance with their drift
velocity, the recited steps being performed in a space maintained
at a pressure such that the length of the mean free path of said
ions is very much smaller than the dimensions of the space.
7. A method in accordance with claim 6, further comprising passing
at least part of said separated ions to a signal detection
region.
8. A method in accordance with claim 6, wherein a first drift field
is applied across the ion formation region and a second drift field
across the ion drift region during said second interval of time,
the first field being stronger than the second field, and the
relative field strengths across said regions being reversed during
said first interval of time.
9. A method of operating a drift cell having an envelope with first
and second principal electrodes spaced therein, first and second
ion gates spaced apart between and spaced from the principal
electrodes, and an ionizing source associated with said first
principal electrode, which comprises applying, during a first
interval of time, a first drift field between the first ion gate
and the first principal electrode, opening the first ion gate at
the end of said first interval of time, applying, during a second
interval of time, a second drift field between said first principal
electrode and said first ion gate and a third field between said
first ion gate and said second ion gate, closing said first ion
gate at the end of said second interval of time, applying a fourth
drift field between said first ion gate and said second ion gate
during a third interval of time, opening said second ion gate
during said third interval, and applying a fifth drift field
between said second ion gate and said second principal electrode,
said first field being weaker than said second field, said third
field being weaker than said second field, said fourth field being
stronger than the third field, the recited steps being performed in
a space maintained at a pressure such that the length of the mean
free path of said ions is very much smaller than the dimensions of
the space.
10. Apparatus for ion measurements, comprising an envelope, a pair
of principal electrodes spaced apart in said envelope, a pair of
ion gates spaced between and from said electrodes, means for
introducing a gaseous sample into said envelope, means for forming
ions from said sample between one of said electrodes and one of
said gates, means for applying a first drift field between said one
electrode and said one gate during a first interval of time, means
for applying a second drift field between said one electrode and
said one gate and a third drift field between said one gate and
said other gate during a second interval of time, means for closing
said one gate during said first interval of time and for opening
said one gate during said second interval of time, means for
applying a fourth drift field between said gates during a third
interval of time, means for opening said other gate during said
third interval of time, and means for applying a fifth drift field
between said other gate and said other electrode, said first field
being weaker than said second field, said third field being weaker
than said second field, said fourth field being stronger than the
third field, and means for maintaining the pressure in said
envelope at a level such that the length of the mean free path of
said ions is very much smaller than the dimensions of the
envelope.
11. Apparatus in accordance with claim 10, wherein said means for
opening said one gate maintains said one gate open throughout said
second interval of time and wherein said means for opening said
other gate opens said other gate during a portion only of said
third interval of time.
12. Apparatus in accordance with claim 11, further comprising means
for varying the delay between the opening of said other gate and
said one gate.
13. Apparatus for analyzing ions, comprising an envelope, means for
forming ions at a first region of said envelope during a first
interval of time, means for rapidly withdrawing the ions formed
from said first region to a second region of the envelope during a
second interval of time, means for slowing the movement of the
withdrawn ions and forming them into a bunch near the entrance to
said second region during said second interval, means for more
rapidly moving the ions of said bunch through said second region
during a third interval of time and for causing them to separate in
accordance with their ion velocity, and means for maintaining the
pressure in said envelope at a level such that the length of the
mean free path of said ions is very much smaller than the
dimensions of the envelope.
14. Apparatus in accordance with claim 13, further comprising means
for passing a portion of the separated ions to a third region of
the envelope for detection.
15. A method in accordance with claim 6, wherein said ions are
formed by reacting other ions with neutral trace particles.
Description
BACKGROUND OF THE INVENTION
This invention relates to apparatus and methods of ion
classification and more particularly is concerned with enhancing
the sensitivity of ion measurements performed in a drift cell.
The copending application of Martin J. Cohen, David I. Carroll,
Roger F. Wernlund, and Wallace D. Kilpatrick Ser. No. 777,964,
filed Oct. 23, 1968 and entitled "Apparatus and Methods for
Separating, Concentrating, Detecting and Measuring Trace Gases,"
disclosed "Plasma Chromatography" systems involving the formation
reactant ions and reaction of these ions with molecules of trace
substances to form product ions, which may be concentrated,
separated, detected, and measured by virtue of the difference of
velocity or mobility of the ions in an electric field. The
production and analysis of ions take place in a chamber, the length
of the mean free path of the ions being very much less than the
dimensions of the chamber under operating pressure conditions, such
as atmospheric. The reactant ions may be produced by subjecting the
molecules of a suitable host gas, such as air, to ionizing
radiation, for example. The reactant ions are subjected to an
electric drift field, causing them to migrate in a predetermined
direction through a reaction space into which the sample or trace
gas is introduced. The resultant collisions between the reactant
ions and the trace gas molecules produce product ions of the trace
gas in much greater numbers than can be produced by mere electron
attachment, for example, to the trace gas molecules. The product
ions are also subjected to the electric drift field and may be
sorted in accordance with their velocity or mobility. A specific
system of the copending application employs a pair of successively
arranged ion shutter grids or gates for segregating the ion species
in accordance with their drift time. The opening of the first gate
is timed to pass a group of ions, which may comprise unreacted
reactant ions as well as product ions, and the opening of the
second gate is timed to pass a portion of the group to an ion
detection means. In accordance with the technique described in the
said copending application, the first shutter grid may sample the
ions in the reaction region by opening for 0.1 to 0.5 millisecond
every 50 milliseconds. Only those ions passed during the short
period when the first shutter grid is open form the mixed ion bunch
which is analyzed by ion drift time in the ion drift analyzer
space. Using a radioactive or other continuous ionizing source, 99
percent or more of the available ions are not utilized.
BRIEF DESCRIPTION OF THE INVENTION
The present invention is concerned with apparatus and methods for
ion measurements in which a much larger percentage of the available
ions is passed to the ion analysis region. It is accordingly a
principal object of the invention to provide apparatus and methods
of this type which are capable of signal improvements of the order
of 10 to 100 times when compared with the operation set forth in
the said copending application.
Briefly stated, the concept underlying the present invention
involves the rapid withdrawal of the ions from a reaction region to
an ion analysis region, the holding up of the ions in the analysis
region until substantially all of the ions are drawn out of the
reaction region, the concentration or bunching of the ions
withdrawn from the reaction region, and thereafter the analysis of
the ion bunch in accordance with the velocity of the various
species comprising the bunch, and the detection and measurement of
the ion species of interest.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be further described in conjunction with the
accompanying drawings, which illustrate a preferred and exemplary
embodiment of the invention, and wherein:
FIG. 1 is a diagrammatic longitudinal sectional view illustrating a
drift cell and potential supply employed in the invention;
FIG. 2 is a graphical diagram illustrating qualitatively the
electric field at different regions of the drift cell during
successive intervals of time;
FIG. 3 is a graphical diagram illustrating qualitatively the ion
density at different regions of the cell during successive
intervals of time;
FIG. 4 is a graphical diagram illustrating quantitatively the
voltages applied to different elements of the drift cell during the
successive intervals of time; and
FIG. 5 is a graphical diagram illustrating quantitatively the
electric field at different regions of the drift cell during the
successive intervals of time and the condition of the ion gates
during such intervals .
DETAILED DESCRIPTION OF THE INVENTION
The drift cell 10 which may be employed in the present invention is
of the type set forth in the said copending application and
comprises an envelope 12 enclosing a series of electrodes, which
may be of parallel plane geometry, for example. Principal
electrodes K and A may be arranged adjacent to opposite ends of the
envelope. When the apparatus is used to detect negative ions,
electrode K will be a cathode and electrode A an anode. When the
apparatus is used to detect positive ions, the polarities will be
reversed. Electrode K or the region of the envelope near this
electrode is provided with an ionizing source, such as a tritium
foil forming part of the electrode. Electrode A may be a collector
plate constituting an output electrode and may be connected to an
electrometer (not shown), such as a Cary Instruments Model 401
(vibrating reed) type with sensitivity of 10.sup.-.sup.15 amps. at
a time constant of 300 milliseconds. The drift cell employs a pair
of shutter grids or ion gates G1 and G2. Each gate comprises two
sets of interdigitated parallel wires, alternate wires of each grid
being connected together to form the two sets. Grids G1 and G2 are
arranged in spaced sequence between the electrodes K and A and
define therewith sequential regions within the envelope 12. In the
illustrative form of the invention the distance between grid G1 and
electrode K is 2 centimeters, between grids G1 and G2 is 8
centimeters, and between G2 and electrode A is 1 centimeter. Inlet
tube 14 permits the introduction of gas into the region between K
and G1, while outlet tube 16 permits gas to be exhausted from the
envelope.
As will be set forth more fully hereinafter, a static and dynamic
potential supply 18 provides static and dynamic potentials
appropriate to the various electrodes of the drift cell 10, which
may also include a series of guard rings 20 spaced along the
envelope for maintaining the uniformity of the drift field in the
different regions of the envelope. The guard rings may be connected
to taps of the static supply providing successively greater
potentials along the series of rings in each region.
Adjacent elements of each shutter grid are normally maintained at
equal and opposite potentials relative to a grid average potential
established by the static supply. Under these conditions the
shutter grid is closed to the passage of electrically charged
particles. At predetermined times all of the elements of the grid
are driven to the same potential, the grid average potential, by
the use of suitable grid drive circuits. The grid drive circuits
open the ion gates in sequence, the G1 drive circuit producing sync
pulses and the G2 drive circuit producing pulses delayed relative
to the G1 pulses. The dual grids per se and their drive circuits
are known in the prior art and do not constitute the present
invention.
In accordance with the techniques set forth in the said copending
application, molecules of a host gas, such as air introduced by
inlet 14 into the space K-G1 are ionized by the ionizing source,
such as the tritium foil on the electrode K, and these reactant
ions react with the molecules of the trace gas of the sample in the
space K-G1 to produce product ions, the pressure in the envelope 12
being such that the length of the mean free path of the ions is
very much less than the dimensions of the envelope. At a
predetermined instant the first shutter grid G1 is opened
momentarily for passing a group of ions from the mixed ion
population presented thereto from the space K-G1, and the ions of
this group migrate into the analysis region G1-G2 under the
influence of an electric drift field applied between principal
electrodes K and A. At a time delayed relative to the opening of
grid G1, grid G2 is opened momentarily for passing a selected ion
species to the detection space G2--A, the various ion species
having separated in the analysis region G1--G2 in accordance with
their velocity in the drift field. The ions which reach the
electrode A produce a current or signal in the output circuit which
may be measured, as by the electrometer referred to previously. By
scanning the opening of grid G2 relative to grid G1 a complete
spectrum of the ion population within the analysis region may be
produced for recording as a curve of output current versus time.
Peaks in this curve represent the different primary and secondary
ion species.
In accordance with the present invention, the techniques set forth
in the said copending application are modified to enhance the
sensitivity of the measurements, as will now be described in
conjunction with a specific example.
Electric fields which will be referred to hereinafter are defined
in terms of the O.sub.2.sup.- ion velocity at atmospheric pressure.
For example,
where
2.7 cm..sup.2 /volt sec. is taken as the mobility of
O.sub.2.sup.-.
The cell dimensions are as assumed previously.
At the beginning of a cycle, time t = 0.000, assume that the grid
G1 has just opened, and ions produced by the tritium source and
subsequent reactions have filled the space K-G1. At this instant,
an electric field which produces an O.sub.2.sup.- velocity of 800
cm./sec. is applied to the space K-G1. (Arbitrarily it may be
assumed that the heaviest ion of interest has a velocity of
one-half that of O.sub.2.sup.-.) For a 2 cm. space, the voltage
across this space is approximately 600 volts. At the same time in
the region G1-G2 the electric field may be made equivalent to 10
cm./sec. O.sub.2.sup.- velocity, which is 30 volts across the 8 cm.
space. Curve (a) of FIG. 2 shows the electric field conditions in
the reaction region K-G1 and the analysis region G1-G2 at time t =
0.000. The letter "O" under G1 indicates that this grid is open,
while "end scan" under G2 indicates that this grid has ended its
scan period, during which it is opened at the appropriate moment.
Curve (a) of FIG. 3 shows, by the shaded block, the formation of
ions throughout region K-G1.
Now suppose that these conditions remain for 5 milliseconds. During
this time, the heaviest ions produced in region K-G1 migrate
through G1 at a velocity of 1/2 .times. 800 cm./sec. As these ions
enter the G1-G2 region, they slow down to the lower velocity of 1/2
.times. 10 cm./sec. In a time of 0.005 second the O.sub.2.sup.-
ions move 10 cm./sec. .times. 0.005 sec. = 0.05 cm. Thus ideally a
bunch of ions approximately 0.05 cm. long is presented adjacent to
grid G1 in the region G1-G2 at the end of the 0.005 sec. interval,
as indicated by the shaded block, curve (b), FIG. 3. There is, of
course, a certain amount of sorting, with the O.sub.2.sup.- ions
ahead of the slower ions in the bunch.
At time t= 0.005 sec. the potentials are changed to favor the
analysis of the bunch of ions for 50 milliseconds in the region
G1-G2 and the reformation of the ion-molecule reaction cloud in the
region K-G1. The electric field conditions are shown by curve (b),
FIG. 2. In each region the field is selected for its objective. In
the region K-G1 the field is reduced at t= 0.005 sec. to produce a
velocity of 80 cm./sec. for O.sub.2.sup.-, so that the heavier ions
fill the 2 cm. space in 50 milliseconds. At time t= 0.005 sec.
shutter grid G1 is closed (indicated by "C" in FIG. 2, curve (b) ),
and this grid remains closed for the next 50 milliseconds. During
these 50 milliseconds the bunch of ions admitted to the analysis
region G1-G2 drifts rapidly through the analysis region under the
influence of the greatly increased field in this region, and the
ion species separate in accordance with their drift velocity in the
field between G1 and G2. Grid G2 is opened briefly at an
appropriate instant during the 50 millisecond scan period, and
passes selected species of ions to the detection region G2-A. By
opening grid G2 at different times delayed relative to the opening
of grid G1, different species of ions may be selected, as set forth
previously.
At time t= 0.055 sec. the cycle is complete, and during the next
0.055 sec. the cycle repeats the procedure commenced at time t=
0.00 sec., as shown by curves (c), (d), (e), FIGS. 2 and 3. There
is thus a 0.055 second period in which, in 5 milliseconds, those
ions which were formed during the previous 50 milliseconds are
bunched. By virtue of the invention the loss of ions is reduced to
less than 5 percent.
The basic period is unsymmetrical, 5 milliseconds devoted to ion
pulse forming and 50 milliseconds to ion analysis. The field is
high in the reaction region and low in the analysis region during
pulse forming. The reverse conditions prevail during ion analysis.
The values of the field are selected to collect the ions rapidly
from the space K-G1 and cause them to drift very little in the
space G1-G2 during the 5 millisecond bunching period. During the
analysis period the field in the reaction space K-G1 is reduced to
permit all the slow ions to form with a minimum number collected,
and the field in the analysis region is increased to provide the
relatively rapid ion separation required.
During the 5 millisecond bunching time, the ions in the analysis
region are in a very low field. Diffusion effect is relatively
small. For trace material at low concentrations the space charge
effect due to the trace ion is also small. If the reactant or
primary ion is not fully converted (by ion-molecule reactions), it
will contribute to the space charge. This condition may be
alleviated by utilizing the reaction region as a coarse
time-of-drift analyzer. For a fast reactant ion compared to a slow
product ion, the opening of grid G1 can be delayed past the time
used in the foregoing example in order to collect the unreacted ion
upon the grid G1.
FIG. 4 illustrates typical potentials of the various electrodes
during the intervals of time described above. For example, the
voltage V.sub.A of the electrode A may be 0 volts relative to
ground. The voltage V.sub.2 of grid G2 may be -100 volts. In the
intervals 0-5 milliseconds, 55-60 milliseconds, and 110-115
milliseconds, the voltage V.sub.1 of grid G1 may be -130 volts,
while the voltage V.sub.K of electrode K may be -730 volts. In the
intervals 5-55 milliseconds and 60-110 milliseconds the voltage
V.sub.1 may be -1300 volts and the voltage V.sub.K may be -1360
volts.
Curve (a) of FIG. 5 illustrates the electric field strength in the
reaction, analysis and signal regions during the successive
intervals. Curve (b) of FIG. 5 illustrates the opening and closing
of grid G1. Grid G2 may be closed during the opening of grid G1 and
opened at selected times during the G2 scan intervals
illustrated.
The static and dynamic potential supply 18 may be of conventional
design, a nominal potential for each element being established by a
bleeder resistor string from a negative power supply, for example.
Thus, V.sub.1 may have a static potential of -130 volts and V.sub.K
may have a static potential of -730 volts. V.sub.A may always be at
0 volts and V.sub.2 may always be at -100 volts. A 50 millisecond
pulse is generated with an amplitude of about -1170 volts for
V.sub.1 and -630 volts for V.sub.K. This pulse may be generated
with solid state circuitry using a transformer to carry an RF
frequency which is rectified at the electrode connection. The pulse
voltage can be clipped with Zener or corona tube regulation.
Additionally, of course, the grid opening and closing pulses for
the shutter grids G1 and G2 are superimposed upon the corresponding
grid elements. Thus, as pointed out above, when it is desired to
open a shutter grid, both sets of wires of that grid are driven to
the same potential, the grid average potential existing at that
time. When the grid is closed, the sets of grid wires are
maintained at equal and opposite potentials relative to the grid
average potential at the time.
Alternatively, the electrode potentials may be supplied by a
bleeder string having taps for explicitly providing all of the
voltages needed. A relay or motor-driven switch may then be
employed to connect the electrodes to the appropriate points of the
bleeder in sequence.
A special nonreactant or inert gas, such as nitrogen, may be
introduced to the analysis region as set forth in the copending
application of David I. Carroll, Martin J. Cohen, and Roger F.
Wernlund, Ser. No. 780,851, filed Dec. 3, 1968, and entitled
"Apparatus and Methods for Separating, Detecting, and Measuring
Trace Gases with Enhanced Resolution".
While a preferred embodiment of the invention has been shown and
described, it will be apparent to those skilled in the art that
changes can be made in this embodiment without departing from the
principles and spirit of the invention, the scope of which is
defined in the appended claims. In the claims reference is made to
first, second and third intervals of time, for ion formation, ion
bunching, and ion analysis, respectively, as described above. For
ease in correlating these intervals with the curves of FIGS. 2-5,
it should be noted that the first or ion formation interval occurs
between t= 0.005 and t= 0.055, the second or ion bunching interval
occurs between t= 0.055 and t= 0.060, and the third or ion analysis
interval occurs between t= 0.060 and t= 0.110. In repetitive cycles
ions for the next cycle are formed during the interval between t=
0.060 and t= 0.110, and at t= 0.00 such ions are ready for bunching
in the interval between t= 0.00 and t= 0.005. Of course with a
continuous ion source ions are actually formed continuously, but
the ions constituting a particular pulse or bunch are formed during
a finite interval.
* * * * *