Note: Descriptions are shown in the official language in which they were submitted.
L6~
The quadrupole mass filter of W. Paul et al described in United
States Patent No. 2,939,952, issued June 7, 1960, consists of four sub-
stantially parallel hyperbolic sheet electrodes (or cylindrical rods~,
symmetrically disposed about an axis. Opposite rods are electrically
connected. On one pair of electrically connec~ed oppositely disposed
electrodes a dc voltage, U~ and an ac voltage of amplitude, V, are placed.
On the other pair of electrically connected oppositely disposed electrodes
identical voltages, except having an electrical polarity opposite to the
first pair, are placed. With proper settings of the dc voltage and the
amplitude of the ac voltages, ions of a given charge-to-mass ratio have
stable trajectories and oscillate about the axis whereby they do not collide
with the eleckrodes; ions of other than the given charge-to-mass ratio are
on unstable trajectories whereby they strike the electrodes. If ions are
injected along the axis of the electrode structure, those with the given
charge-to-mass ratio do not strike the electrodes and emerge from the
opposite end of the electrode structure; however~ ions with other than the
given charge-to-mass ratio are accelerated in the transverse directions so
that they collide with the electrodes and therefore do not emerge from the
opposite end of the electrode structure. In this manner~ the electrode
structure functions as an ion "mass filter."
As no~ed in United States Patent No. 3,129,327 to W.M. Brubaker
of April 14, 1964, an ion entering the electrode structure must pass through
fringe fields near and beyond the end o-f the electrode struckure. The ions
must also pass through a similar fringe field in emerging from the opposite
end of khe electrode structure. As pointed out in the aforesaid patent of
W.M. Brubaker, the ratio of the dc field strengths to ac field strength in
the fringe fields is the same as in the electrode structure itself. Also,
.
as disclosed in the aforesaid patent, an ion of the given charge-to-mass
ratio, which is stable within the electrode structure proper, is on an unstable
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trajectory when it is in the fringe fields. Thus, although an ion would be
stable within the electrode structure proper, it may not be received in the
electrode structure proper due to its unstable trajectory in the fringe
fields. This greatly reduces the transmission of ions of a given charge-to-
mass ratio due to their rejection while within the fringe fields.
Patent No. 3,129,327 further teaches that the ion trajectories
can be stabilized on passage through the fringe fields provided that the
ratio of the dc voltage (U) to the ac voltage amplitude (V) is reduced to a
lower value than appropriate for use within the electrode structure proper.
The aforesaid patent indicates several ways in which this can be accomplished
in the case of quadrupole mass filters which have conventional metal
electrodes. But the patent does not address itself to the broader problem of
spatial separation of ac and dc fields emanating from the same metallic
electrodes.
~q~ ,c, t~
The copending Canadian patent application Serial No. 234,491~filed -
August 29, 1975 teaches that separation of the high frequency ac fringe fields
from the low frequency (including dc) fringe fields, is possible by placement
of a tube or other appropriate geometrical configuration near an end or ends
of a quadrupole mass filter wherein the tube is composed of a material which
appears as a dielectric to the high frequency ac fringe fields and as a
conductor to the low frequency (including dc) fringe fields. A required
characteristic of such material is that the parameter 4~/f~ be much less
than unity in value, where a is the dc electrical conductivity of the
material and is the material's dielectric constant. The angular frequency,
G~ is equal to 2~f where f is the frequency of the high-frequency ac fields.
-.:
Materials having the necessary physical characteristics exist and
are readily available. Among such materials is a Nickel-zinc type ferrite
manufactured by StackpoleCarbon Company of St. Mary's, Pennsylvania, known
as " ~erramag C/12," which has a volume resis-tivity at 25 C. of about 3 x 10
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ohm-cm and a dielectric constant at l.OMHz of about 10. When ions or
electrons or both pass inside and along the length of a tube of such mate-
rial, some of said ions or electrons or both strike the tube's walls
producing effectively dc electrical currents within the material which may
affect the dc potential of the tube. I'he dc potential of the tube can thus
be changed through the IR drop of such currents, so that trajectories of
ions which do not strike the tube's walls but pass on into the quadrupole
mass filter are affected adversely. Another nickel-zinc ferrite manufactured
by Stackpole Carbon Company which may be utilized in the invention, Cerramag
C/ll, has a resistivity of 2 x 10 ohm-cm. A still further substance which
~- has been tested and found operable is slate, the test sample having a
resistivity of 1.0 x 106 ohm-cm along two axes and 2.2 x 106 ohm-cm along
the third axis. The dielectric constant of slate is 6.0 - 7.5. The basic
- formula for the ferrites, Cerramag C/12 and Cerramag C/ll, may be found
in the patent of Zerbes No. 3,036,009 wherein other characteristics of the
ferrite not important to the instant invention are discussed.
It is essential that the shield in the instant invention must
appear to high-frequency ac fields as a dielectric and to low frequency,
including direct current, fields as a conductor. Thus, for frequencies on
the order of one megahertz, the resistivity must be substantially greater
than 105 ohm-cm. However, the materials utilized for the shielding effect
are to be contrasted with good dielectrics which have resistivities of 10 2
ohm-cm and higher. It is important that the resistivity of the material be
not so high as to be unable effectively to conduct away current caused by
ions or electrons which may strike the material during their transmission
or receipt thereinq Also, there are practical upper limits on the
resistivity which basically relate to the sweep rate 'which may be utilized
in the quadrupole mass filter. In this connection, it is to 'be understood
~ that the substantially dc fields may be fields in a mass filter up to about
; -3-
loss llz because this is about the limit of the sweep rate of thc quadrupole
mass filter. Thus, it is desirable that the material act like a conductor
at a frequency up to 1000 Hz but as a dielectric at one million Hz. As a
practical matter, materials having resistivities up to about 108 ohm-cm are
operable even at the maximum sweeping rate. By reducing the sweep rate and
in many applications a sweep rate of about 10 Hz is all that is desirable,
materials with resistivities up to about 101 ohm-cm are operable. Still
further, there are bona fide applications wherein resistivities up to about
1011 ohm-cm. However, a resistivity up to roughly 108 ohm-cm offers improve-
ment in every application of a quadrupole mass filter. From the foregoingit will be understood by those skilled in the art that the upper limit of
resistivity of the shielding material varies depending upon the sweep rate
desired in the mass filter, the configuration of the shield a~l the total ion
current necessary for the shield to carry off.
The present invention relates to an improved spatial separation
of fields emanating from electrodes wherein such fields are produced from
superpositions of the dc or low-frequency ac and high frequency voltages
placed on the electrodes, such fringe fields being produced in the vicinity
of the ends of the electrode structure of a quadrupole mass filter. More
particularly, the invention involves the biasing of tha material affecting
the spatial separation in a dc fashion away from ground potential whereby
the effects of dc currents produced within the material are overcome to
; permit a free passage of ions into the quadrupole mass filter.
In accordance with this invention there is provided a me~thod of
mass analysis which utilizes a quadrupole mass filter and comprises the
steps of producing positive ions with an initial kinetic energy Ei rom an
ion source having an electrical potential Us, introducing said ions into the
space between the poles of the quadrupole mass filter wherein the electric
potential along the axis of said poles is U , causing the transmission of
only those ions of a selected mass-to-charge ratio through the space between
said poles, providing a selective shielding at least one end of said poles
which functions substantially as a conductor to the substantially dc fields
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and substantially as a dielectric to ac fields, said shielding having an
~ electric connection at a potential of UE, and maintaining said potential UE
less than said potential U , and said potential U less than said potential
of Us plus Ei/e where e is the charge of the ion.
In accordance with another aspect of the invention there is provided
a device for improving the efficiency of injection and/or transmission of
ions passing through a quadrupole mass filter, said device comprising an ion
source having an electric potential Us, means for producing ions from said
ion source having an initial kinetic energy of Ei, a selective shielding at
at least one end of the poles of the quadrupole mass filter adapted to receive
said ions therethrough, said shielding functioni.ng substantially as a con-
ductor to the substantially dc fields and substantially as a dielectric to
the ac fields produced by said poles, said shielding having electric connec-
tion to a potential of UE, means for causing the transmission of said ions
through said quadrupole mass filter, means for maintaining an electric
potential along the axis of said poles of Up, and biasing means in said
quadrupole mass filter for maintaining said potential UE less than said
potential Up and said potential Up less than said potential Us ~ Ei/e
wherein e is the charge on the ion.
Other adaptabilities and capabilities of the invention will be
~ appreciated by those skilled in the art with reference to the following
- description and the drawing wherein a schematic representation of the
invention is shown.
: With reference to the Figure, a selective electric field shielding
means composed of a material which is a conductor to dc and low-frequency
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64
electrical ac fields and a dielectric to high-frequency electric ac fields,
is shown as tube 17, one end of which protrudes a short distance into the
space between the four rods, or hyperbolic sheets, of a quadrupole mass
filter, the forward two of which are designated 11+ and 12-, and the more
rearwardly two as seen in the Figure are designated 1~ and 1~-. The other
end of tube 17 is mounted in an end plate 18 which is electrically conducting
and in dc electrical contact with tube 17 at their juncture.
Ions produced from an ion source 21 are drawn from source 21, pass
through the opening 20 in end plate 18 and enter the interior of tube 17.
With tube 17 composed of material having the proper ratio o-f electrical
conductivity to dielectric constant, as described in the aforesaid copending
patent application Serial No. 234,491, some ions follow trajectories such as
l9a and enter the massfilter. mese ions are those having masses greater
than about 0.77 times the mass of the ions to be transmitted through the
mass filter at any given setting of its fields. Ions with masses less than
about 0.77 times the mass of the ions to be transmitted by the mass filter
(and any electrons that might accompany the ions) are caused by the ac fields
within the interior of tube 17 to strike its interior walls. Such
trajectories are designated by reference characters l9b and 19c.
Inasmuch as tube 17 is constructed of a material with high
resistivity within a range of above 6 x 10 to less than 109 ohm-cm~
typically of the order of 10 ohm-cm, tube 17 has an electrical resistance.
Thus the ions or electrons or both striking the interior walls of tube 17
produce electric currents which are of necessity, conducted to the circuit
common 30 or ground through the tube material itself. Typical dimensions
of tube 17 are about one or two centimeters in length, an interior diameter
; of from two to six millimeters and wall thicknesses of two or three
millimeters; the total resistance of the tube is several tens of megaohms
:
and reaches values of the order of 10 ohms. Thus, with currents reaching
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the interior walls being as high as a few times 10 amperes, voltages along
tube 17 of in the range of several volts result.
Referring to the electrical portions of the Figure, when positive
ions are produced inside an ion source 21 at a potential Us with respect
to the circuit common 30 potential, they have a kinetic energy of eUs ~ Ei,
e being the value of the charge of the ion and Ei the kinetic energy with
which the ion is initially produced, at any point where the existing
potential is that of the circuit common 30. In particular, if the value of
UE, provided by a dc voltage supply, is zero, the ions have a kinetic energy
of eUs + Ei in passing through opening 20 in end plate 18. Under these
circumstances, positive ions striking the interior walls of tube 17 cause
positive dc voltages to build up along the length of the tube extending into
the mass filter. These voltages, if small, cause the tube to act as an
unwanted lens which defocuses the ion beam in unwanted ways. Moreover, if
the ion currents become sufficiently large, potentials within the tube 17 may
increase to the value of Us, in which case ions that would normally follow
a trajectory such as l9a lose their kinetic energy within the tube and are
not transmitted. The possibility exists that this situation may develop
when end plate 18 is grounded as taught by copending Patent Application Serial
No. 3~6,250.
A solution to the problem lies in increasing the value of Us to
where the ion's kinetic energy is necessarily greater than the product of
the maximum current from the ion source times the resistance of tube 17
divided by the charge on the ions. However, for ion sources which produce
ion currents in excess of 10 amperes, this solution has serious drawbacks
as will be discussed.
It is customary to operate the quadrupole mass filter by applying
.
thereto a combination of radio frequency ac voltages generated by source V
of the Figure and dc voltages represented by the battery symbols in the
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~48~6~
Figure and designated U therein. Blocking condensers, Cl and C2, and radio
frequency chokes RFCl and RFC2 prevent the ac and dc voltage sources from
shorting each other out and permit application of both the ac and dc
voltages to the four poles 11 +~12 -, 13 + and 14 -.
It is also usual to include two voltage sources of equal values,
sources U on either side of a point 31. By this means, the dc potential
at the axis of the mass filter is the same as the potential, U31 at point 31.
The kinetic energy of the ions during their passage through the
- mass filter is given by Ei plus the ion charge, e times (Us ~ U31), both
o potentials Us and U31 being measured with respect to the circuit common. If
US is increased to excessively high values, the ion kinetic energy during
passage through the mass filter is increased to the point where the ion
executes too few oscillations at the ac frequency within the mass filter to
permit the mass filter to provide good mass resolution. Raising the potential
US to the values required to overcome the voltages on tube 17 in the case of
large ion currents leads to having excessive ion energy within ~he massfilter
under these normal operating conditions.
With an understanding of the foregoing two improved solutions are
presented:
The first is to place end plate 18 not at the circuit common
potential but at a lower dc potential UE as represented by the battery
symbol in Figure 1. Because it is only the potential difference Us ~ UE
which determines whether ions pass through the tube 17, the difference can
- be made large by increasing UE while holding Us at a low enough value to give
the required low energy of the ions in passing through the mass filter.
Referring to the Figure, this solution is appropriate to having the pole
potential U be ~ero, so that the potential of the mass filter axis, which
is equal to U31, is at the potential of the circuit common.
A second solution is to let the potential of the end plate 18 be
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the same as the circuit common, by letting the value of UE be zero. This
requires that the value of Us be relatively high in order to have ions be
able to be transmitted through the tube 17. However~ by placing potential
U at a value slightly less than the value of Us, the ions are slowed down
to a low energy, e(Us - U ), for their passage through the mass fi]ter,
which may be sufficiently low to obtain good resolution in the mass filter.
Either solution or both in combination have been used with success.
Where negative ions rather than positive ions are involved, the
same solutions to the problem are applicable by merely reversing the signs
of the potentials Us, UE, and U .
Tests have been conducted using an Extranuclear Laboratories
quadrupole mass filter, Model 324-9, which has poles 3/4~' diameter. A tube
17 of Ceramag C/12, of length 1.5 cm, outside diameter of 1.2 cm and inside
diameter 0.6 cm, and with an end-to-end resistance of 6 x 10 ohms, was
mounted on a stainless steel end plate 18 by means of pressing tube 17 into
a recess that was cut into the end plate. An electron impac~ ioni~ation
source, Extranuclear Laboratories, Model 041-1, was used to make both
positive and negative ions. The positive ion currents produced by this
ionizer are in excess of 10 milliamperes per torr of gas being ionized, so
that when operating in a vacuum of 10 torr, a current of 10 amperes is
caused to pass through tube 17. ~fost of this current is from ions of low
mass ranging from twelve through eighteen amu (carbon ions through water ions)
and from twenty-eight through ~orty-four amu (nitrogen carbon dioxide).
When the mass filter is set to examine ions at higher masses, such as
mercury ions at 200 amu, the ac fields within tube 17 cause the lighter ions,
which constitute almost all of the 10 amps of ions coming from the ionizer
to strike the interior walls of tube 17. This current, being carried to the
.
end plate through the tube material, theoretically caused the potential
within the tube to increase to about 6 x 10 ohms times 10 amps, or 6 volts.
.
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In the first test, both the potentials of the end plate 18 and
of the mass filter axis were placed at ground potential by grounding the
circuit common and setting UE = Up = 0. It was found that to cause any ions
to pass through the mass filter, it was necessary to set the ion source
potential Us in excess of eight volts. By reducing the total ion current
coming from the source through using a reduced electron current, and thus
; reducing the total current of ions reaching the interior of tube 17, it was
found that Us could be reduced to as low as one volt, and still cause ions
to pass through the mass filter. Under the first circumstances it is
calculated that of the minimum eight volts required, some two ~olts were
~ required to compensate for electron space charge within the ionizer and the
;~ remaining SiY volts are attributed to voltages built up in the tube 17 by
ion currents reaching its interior wallsO
It was also found that at the eight volts value of Us the shape
of the mass peaks and their resolution showed the typical effects of having
an excessive ion energy for transmission through the mass filter.
~ wo further tests were then performed. In the first the
potential of the axis of the mass filter was increased by application of
positive voltage U . It was found that U could be increased to
approximately six volts without appreciable loss of ion current and that
such increases improved the shapes of the mass peaks and the resolution.
In the second test the mass filter axis was again placed at ground
potential, by making U = 0, and the ion source potential placed at Us was
two volts. ~ negative potential UE was then applied to the end plate 19.
It was found that to cause ion current to pass through the mass filter, it
was necessary that the value of U~ be at least six volts negative. Under
these conditions the mass peaks showed both good shape and good resolution
.
as expected for the low energy of the ions during transmission through the
mass filter. Making the magnitude of UE less than six volts away from ground
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seriously reduced the transmitted ion current. Making the magnitude of UE
greater than six rolts away from ground has only minor effects on the ion
current and virtually no effect on the peak shape and resolution of the
mass filter. These observations are interpreted as confirming the theory
described abovc.
In a second set of tests, negative ions were produced in the
source. The negative ion examined was 0 produced by dissociative attach-
ment of electrons of 2 molecules. This process has its maximum cross-
section at an electron energy of about 6.5 eV, and this was the energy of the
electrons used in the tests. At this low energy, the accelerating fields
of the ion source used to extract the negative ions also cause some of the
electrons to be extracted so that the ion beam presented to the mass filter
is a mixture of negative ions and electrons with the vast majority of the
particles being electrons.
It was found that with the axis of the mass filter at ground
~ potential and with the end plate 18 grounded, under certain ion optics
- tuning conditions it was necessary to raise the potential Us of the ion
source to a value of negative twenty volts in order to observe ions
transmitted by the mass filter. At this potential, the peak shapes and mass
resolution of the emerging ions were seriously distorted in a manner
characteristic of excess ion energy. It was found that placing a bias
voltage at Up of a negative fifteen volts considerably improved the peak
shapes and resolution without engendering a loss of 0 ion current in
accordance with the theory.
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