Reagent gas control for an ion trap mass spectrometer used in the chemical ionization mode

A reagent gas flow control system for use with an ion trap mass spectrometer is shown. The gas reagent gas flows from a source through a first gas flow restrictor connected to the inputs of second and third gas flow restrictors. The output of the second restrictor is connected to the ion trap where reagent gas is used, and the output of the third restrictor is connected to a vacuum pump, which may be the roughing pump used by the ion trap. At least one of the three restrictors is a variable restrictor. The configuration of the present invention allows the use of simple and inexpensive parts to provide exacting flow control.

FIELD OF THE INVENTION 
The present invention relates to gas flow control systems and is 
particularly useful in connection with flow control of reagent gases used 
in ion trap mass spectrometers operating in the chemical ionization mode. 
BACKGROUND OF THE INVENTION 
The quadrupole ion trap, sometimes referred to as an ion store or an ion 
trap detector, is a well-known device for performing mass spectroscopy. A 
ion trap comprises a ring electrode and two coaxial end cap electrodes 
defining an inner volume. Each of the electrodes preferably has a 
hyperbolic surface, so that when appropriate AC and DC voltages are placed 
on the electrodes, a quadrupole trapping field is created. Typically, an 
ion trap is operated by introducing sample molecules into the ion trap 
where they are ionized. Depending on the operative trapping parameters, 
ions may be stably contained within the trap for relatively long periods 
of time. Under certain trapping conditions, a large range of masses may be 
simultaneously held within the trap. Various means are known for detecting 
ions that have been so trapped. One convenient method is to scan one or 
more of the trapping parameters so that ions become sequentially unstable 
and leave the trap where they may be detected using an electron 
multiplier. Another method is to use a resonance ejection technique 
whereby ions of consecutive masses can be scanned out of the trap and 
detected. 
Several methods are known for ionizing sample molecules within the ion 
trap. Perhaps the most common method is to expose the sample to an 
electron beam. The impact of electrons with the sample molecules cause 
them to become ionized. This method is commonly referred to as electron 
impact ionization or "EI". 
Another commonly used method of ionizing sample with an ion trap is 
chemical ionization or "CI". Chemical ionization involves the us of a 
reagent gas which is ionized, usually by EI within the trap, and allowed 
to react with sample molecules to form sample ions. Commonly used reagent 
gases include methane, isobutane, and ammonia. Chemical ionization is 
considered to be a "softer" ionization technique. With many samples CI 
produces fewer ion fragments than the EI technique, thereby simplifying 
mass analysis. Chemical ionization is a well known technique that is 
routinely used not only with quadrupole ion traps, but also with most 
other conventional types of mass spectrometers such as quadrupole mass 
filters, etc. 
Most mass spectrometer systems used today include a gas chromatograph 
("GC") as a sample separation and introduction device. When using a GC for 
this purpose, sample which elutes from the GC continuously flows into the 
mass spectrometer, which is set up to perform periodic mass analyses. Such 
analyses may, typically, be performed once a second. When performing CI 
experiments in such a system, a continuous flow of reagent gas is 
maintained. 
Mass spectrometers operate at pressures that are greatly reduced below 
atmospheric pressure. A typical quadrupole ion trap operates at a pressure 
of 2.times.10.sup.-2 Torr helium, and thus requires a continuous vacuum 
pumping system to maintain the desired vacuum level. When operating in the 
chemical ionization mode, reagent gas is introduced into the ion trap at 
0.1 to 100 microTorr. This pressure range is far lower than the reagent 
gas pressure associated with other conventional mass spectrometers which 
typically operate using a reagent gas pressure of 0.5 to 50 Torr. One 
reason the reagent gas pressure is so much lower in an ion trap mass 
spectrometer is that the much longer residence time of the reagent ions in 
the trap allows for a much longer reaction period. In other conventional 
types of mass spectrometers, the reagent ions are present a much shorter 
time and, thus, much higher concentrations of reagent gas must be used to 
insure that sufficient numbers of sample ions are created by CI. 
In a typical commercially available ion trap configuration, the vacuum 
enclosure is continuously pumped at a rate of 40 to 60 liters per second. 
To attain the desired partial pressure of reagent gas, a volumetric 
reagent gas flow rate of 0.0003 to 0.3 atmosphere ml/min is required. The 
reagent gas is typically supplied from a pressurized bottle, with the 
source pressure being substantially above atmospheric pressure. A high 
source pressure is required for the pressure regulator to properly 
function and provide a stable pressure from a restricting mechanism. The 
large pressure differential which is placed across the gas flow control 
mechanism, coupled with the requirement of extremely small reagent gas 
flow rates has lead to the use, in prior art systems, of expensive and 
complicated mechanical variable restrictor valves. These valves, often 
called "micro leak valves", must be fabricated from high precision 
mechanical components. The components that are used in commercially 
available valves have high temperature coefficients, so that such valves 
are highly temperature sensitive. 
In prior art reagent gas flow control systems for ion trap mass 
spectrometers, a solenoid valve is used in series with and downstream from 
the micro leak valve to turn the gas flow on and off. For example, the 
reagent gas might be turned off while the user of the ion trap conducts EI 
mass spectrometry. Later the valve may be turned on to allow the user to 
conduct a CI experiment. When the solenoid valve is in the off position, 
i.e., the reagent gas flow is turned off, pressure builds up behind the 
valve. Thereafter, when the solenoid valve is switched "on" to allow the 
reagent gas to flow, there will be a significant pressure "surge" into the 
ion trap due to the build up of pressure behind the solenoid valve. This 
large pressure surge requires that the electronics of the ion trap be 
turned off before, and for some time after the valve is turned on. 
Otherwise, the rf electronics, the electron multiplier, and the electron 
emission filament are subject to potential damage. This disruption causes 
drift and instability for a period of time. 
Accordingly, it is the object of the present invention to provide a reagent 
gas flow control system which is less expensive and more reliable than 
those of the prior art. 
Another object of the present invention is to provide a gas flow control 
system for delivering reagent gas to an ion trap mass spectrometer which 
can be turned off and on without causing pressure surges within the ion 
trap. 
Yet another object of the present invention is to provide a gas flow 
control system which delivers a controlled low volume of gas at low 
pressure using simple, readily available component parts. 
Still another object of the present invention is to provide a gas flow 
control system which is less temperature dependent, yet just as accurate 
as those of the prior art. 
SUMMARY OF THE INVENTION 
The foregoing objects of the present invention, and others that will be 
apparent to those skilled in the art, are realized in the new gas flow 
control system described herein. The system consists of a source of a 
desired gas which is to be introduced into a vacuum chamber where it is to 
be used. The gas source is linked to the vacuum chamber by a conduit 
containing a switching means for turning the gas flow on and off. At least 
two gas restrictors are also positioned along the conduit, and a pressure 
re means is connected to the conduit between the two gas flow restrictors. 
In a preferred embodiment the pressure reducing means comprises a second 
conduit connecting the first conduit, at a point between the two 
restrictors, to the vacuum pump for the overall system. A third gas flow 
restrictor may be positioned in the second conduit, and at least one of 
the restrictors may be a variable restrictor.

DETAILED DESCRIPTION 
FIG. 1 schematically shows a prior art gas flow control system for 
introducing a reagent gas into an ion trap mass spectrometer for 
performing mass analysis in the chemical ionization mode. A source of a 
suitable reagent gas 5, typically contained in a pressurized bottle, is 
connected to the ion trap 10, by way of gas conduit 15. Ion Trap 10 is 
held at a greatly reduced pressure by a vacuum pumping system, which may 
consist of a roughing pump 20 and a turbomolecular pump 25, as is well 
known in the art. Typically, the operating pressure within the ion trap is 
about 2.times.1O.sup.-3 Torr helium. Sample is introduced into the ion 
trap from a source of sample gas, which may consist of a standard gas 
chromatograph 30. A gas chromatograph is useful for this purpose because 
it separates complex samples into their components, thereby making the 
mass analysis much easier to perform. 
Along the flow path of conduit 15, positioned between the reagent gas 
source 5 and the ion trap 10, are a variable restrictor 40, also referred 
to as a metering or micro leak valve, and a solenoid actuated gate valve 
50. As explained above, in order to provide reagent gas at a suitably low 
flow rate to achieve the proper reagent gas pressure level in the ion 
trap, restrictor 40 comprises a high precision micro leak valve. The gas 
control metering valve must be of the type which functions over a very 
large pressure drop. Not only are such valves relatively expensive but, 
due to the components from which they are fabricated, micro leak valves 
are quite temperature sensitive. Accordingly, it is necessary to operate 
system so that restrictor 40 is held at a relatively constant temperature. 
Gate valve 50 is required so that the flow of reagent gas can be shut off. 
For example, it is necessary to shut off the flow of reagent gas when the 
ion trap is being used to conduct experiments in the electron impact 
ionization mode. It will be apparent to those skilled in the art that when 
gate valve 50 is closed, pressure will build up behind the valve until 
both sides of restrictor 40 reach equilibrium. Thereafter, when gate valve 
50 is opened, there will be a pressure surge of reagent gas into the ion 
trap. As explained above, this requires that some of the system 
electronics be turned off to avoid damage until the pressure within the 
ion trap is again reduced to its operating level. Gate valve 50 must be 
located downstream of restrictor 40 in order to achieve adequate on/off 
control of the reagent gas flow. If gate valve 50 were to be positioned 
upstream of restrictor 40, then the flow of gas into the ion trap would 
continue for an appreciable time after shutting of the gate valve until 
pressure equilibrium across the micro leak valve were reached. 
Turning now to FIG. 2, a preferred embodiment of the present invention is 
shown in schematic form. Those components of the system which are the same 
as the components shown in FIG. I are given the same numbers. The system 
of FIG. 2 includes a source of reagent gas 5 which is connected to the ion 
trap 10 by conduit 15 having two portions 15a and 15b. Also connected to 
the ion trap are a gas chromatograph 30 and a vacuum pumping system 
comprising roughing pump 20 and turbomolecular pump 25. In one embodiment, 
roughing pump 20 is an Alcatel pump with a two cubic feet per minute 
pumping capacity. The turbomolecular pump is a Varian model V-60, with a 
pumping speed of 60 liters/sec. Those skilled in the art will recognize 
that other types of vacuum pumps may be substituted. 
Between conduit portions 15a and 15b is a tee connector 60 which connects a 
second conduit 65 between the tee connector and the roughing pump 20. In 
the embodiment of FIG. 2, a solenoid actuated gate valve 50 is used to 
turn the flow of reagent gas into the ion trap on and off. Within first 
conduit portion 15a, i.e., between the reagent gas source 5 and the tee 
connector 60, is a first fixed gas flow restrictor 80. Likewise, within 
second conduit portion 15b, i.e., between the gate valve 50 and the tee 
connector 60, is a second fixed gas flow restrictor 70. First and second 
fixed restrictors 70 and 80 may be simply and inexpensively constructed; 
for example, from lengths of stainless steel capillary tubing. Stainless 
steel tubing having an inner diameter of 5 mils, an outer diameter of 60 
mils and a length in the range of 10 to 25 inches has been found to be 
suitable for this purpose. 
A variable restrictor 90 is positioned along conduit 65, between tee 
connector 60 and roughing pump 20. Variable restrictor 90 may be a simple 
needle valve; for example, a needle valve of the type commercially 
available from Porter Precision Valve, model number HR 3. In operation, by 
selecting a first fixed restrictor of a proper value and adjusting the 
variable restrictor valve, it is possible to establish a predetermined 
vacuum level at tee connector 60. For example, variable restrictor 90 may 
be adjusted to provide a vacuum level of 100-0.1 Torr at tee connector 60. 
Accordingly, the pressure differential across fixed restrictor 80 is 
greatly reduced. Likewise, since a major portion of the pressure in the 
system in dropped across first fixed restrictor 70, the pressure drop 
across variable restrictor 90 is greatly reduced and, thus, restrictor 90 
need not have the precision of those used in the prior art. 
When gate valve 50 is off, the pressure at the upstream end of second fixed 
restrictor 80, i.e., at tee connector 60, remains relatively low, so that 
when the gate valve is opened the pressure surge is negligible. This is 
due to the fact that shutting gate valve 50 does not stop the flow of gas 
through restrictors 70 and 90 to vacuum pump 20. This allows actuation of 
gate valve 50 without concern for whether the system electronics are on or 
off. 
A second embodiment of the flow control system of the present invention is 
shown schematically in FIG. 3, wherein similar parts are, again, given the 
same numbers. The design of this embodiment is similar to that of the FIG. 
2 embodiment, except that a fixed restrictor 95 is used between tee 
connector 60 and vacuum pump 20 and a variable restrictor 110 is used 
between the tee connector and ion trap 10. Again, by selecting properly 
sized fixed restrictors 70 and 95, it is possible to obtain a generally 
predetermined vacuum level at tee connector 60. In the design of FIG. 3, 
rather than use a separate variable restrictor and gate valve, an 
integrated linear valve 100 may be utilized. Such a valve, which may be 
purchased from General Valve, model no. 9-556, may be electronically 
controlled to be fully open, fully closed, or at some intermediate 
position which provides a desired level of flow restriction. Valve 100 is 
equivalent to the variable flow restrictor and a gate valve shown within 
the dashed lines of FIG. 3 and numbered 110 and 120 respectively. Again, 
since the pressure level at tee connector 60 is well below atmospheric 
pressure, opening and closing linear valve 100 does not cause any 
appreciable pressure surge into the ion trap. 
Another embodiment of the present invention is shown in FIG. 4, wherein 
similar parts are, again, given the same numbers. In this embodiment, a 
first fixed restrictor 95 is positioned between tee connector 60 and 
vacuum pump 20 and a second fixed restrictor 80 is positioned between tee 
connector 60 and ion trap 10. A linear valve 130, similar to that 
described above in reference to FIG. 3, is positioned between reagent gas 
source 5 and tee connector 60. Again, linear valve 130 may be depicted as 
a combination of a variable restrictor 140 and a gate valve 150. It should 
be noted that in the designs of FIGS. 2 and 3, the flow of reagent gas is 
continuous through the system to the vacuum pump 20, even when gate valves 
50 or 120 is closed. This assures that the pressure level at tee connector 
60 remains constant. However, these configurations result in wasted gas. 
Moreover, in all of the embodiments that have been described, a 
significant portion of the reagent gas flows to the vacuum pump without 
ever entering the ion trap. Nonetheless, the reagent gases used in CI are 
normally relatively inexpensive so that the advantages of the present 
system outweigh any added expense associated with wasted reagent gas. The 
FIG. 4 embodiment mitigates the loss of reagent gas by placing a gate 
valve 150 between the gas source 5 and vacuum pump 20. This embodiment is 
less preferred, however, because the pump out time for gas in the portion 
of the conduit 15a downstream of the solenoid and in conduit 15b is 
limited by the conductance through restrictors 80 and 95. Thus, in this 
arrangement there is a delay between the time that the gate valve or 
solenoid is shut and the stoppage of reagent gas flow into the ion trap. 
In each of the embodiments of the present invention that have been 
described, the variable restrictor operates at flow rates that are orders 
of magnitude higher that those used in the prior art, and thus are simpler 
and lower in cost than the prior art devices. Moreover, much of the flow 
restriction is provided by fixed restrictors that inherently have lower 
temperature coefficients than variable restrictors. It can be shown, for 
example, that in the embodiment of FIG. 2, the flow into the ion trap 
depends on the ratio of fixed restrictor 80 (R.sub.3) to variable 
restrictor 90 (R.sub.2). Since fixed restrictors have inherently lower 
temperature coefficients than variable restrictors, the effect of the 
temperature coefficient of variable restrictor 90 is reduced in proportion 
to the ratio of restrictor 80 to restrictor 90 (R.sub.3 /R.sub.2). 
In comparisons between the prior art design and the design of the present 
invention, the absolute responses of the prior art device were found to 
vary between 15 and 20 percent, while those of the present invention were 
found to vary between 1 and 5 percent. 
FIGS. 5A and B are chromatograms, for masses 69 and 414, respectively, 
which show the effect of turning the flow of reagent gas into the ion trap 
on and off. It can be seen that no pressure surge is observable when the 
gate valve is turned on, so that the electronics can be left on during the 
process without any detrimental effects. FIGS. 5C and D ar enlargements of 
a portion of the chromatograms of FIGS. 5A and B, showing specifically the 
response between scans 60 and 61. The scan rate was set at one per second, 
and thus the opening of the valve and the stabilization occurs in less 
than one second. Scan 60 was performed with the reagent gas off and scan 
61 was performed with the reagent gas on. Mass 414 was monitored in the 
sample of perfluortributyl amine, since it has a small response in the 
absence of reagent gas and a large response when subject to chemical 
ionization. 
While the present invention has been described in connection with several 
preferred embodiments thereof, those skilled in the art will recognize 
that other changes and embodiments may be made without departing from the 
spirit thereof. Accordingly, the scope of the present invention should be 
construed only with reference to following claims.