Abstract:
A high speed mass spectrometer system capable of detecting in real-time multiple compounds in complex environments. This system includes a continuous ionization source coupled to a quadrupole ion trap to store ions, to filter ions for detection, to resonantly excite the ion trajectories to cause them to dissociate for more detailed analysis. This system includes a dual ionization configuration to cover broad and disparate classes of compounds. A glow discharge source is used to attach electrons to molecules with high electrons affinity. A photoionization source is used to detach electrons from molecules with low ionization potentials.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a mass spectrometer which has a glow discharge ionizer and a photoionizer that are coupled to a mass detector(s) by quadrupole ion traps. 
     2. Background Information 
     Terrorists have been known to use explosives to hijack commercial aircraft. For this reason, there has been a desire to provide an explosive detection system that can be operated “on-site” at an airport terminal. An on-site detection system must be capable of detecting extremely low concentrations of an explosive(s) material in a relatively fast time frame to minimize the time delays in air travel for the passengers. 
     U.S. Pat. No. 5,854,431 issued to Linker et al. and assigned to Sandia Corporation (“Sandia”) discloses a preconcentrator system that generates a flow of air to dislodge explosive material from a passenger. The dislodged explosive material is captured by a screen of the system. The air flow across the passenger is temporarily terminated to allow the captured explosive material to be removed from the screen by a secondary flow of air. The explosive material removed from the screen is directed into a particle detector. The preconcentrator disclosed in the Sandia patent increases the concentration of explosive material provided to the detector. 
     U.S. Pat. No. 4,849,628 issued to McLuckey et al. (“McLuckey”) discloses a mass detection system that can detect relatively low concentrations of a trace molecule(s). McLuckey utilizes a glow discharge ionizer which ionizes an “atmospheric” sample. Providing an air sample at atmospheric pressures increases the density of the sample and the number of ionized molecules. Increasing the number of ions improves the sensitivity of the detector. 
     The glow discharge ionizer includes a pair of electrodes separated by a chamber. A voltage potential is created between the electrodes to induce a glow discharge which ionizes a gas sample within the chamber. The glow discharge ionizer of McLuckey is coupled to a quadrupole mass spectrometer that can detect a trace molecule such as an explosive material. 
     The quadrupole mass spectrometer includes a scanning circuit which provides a continuously varying voltage field across the poles of the spectrometer. The continuously varying voltage field sequentially ejects ionized molecules from the quadrupole to a detector. The excitation circuit and detector can be coupled to a computer which correlates detected molecules with the excitation voltage. Explosive materials will provide detection at a predetermined voltage(s). The computer can correlate detection with an explosive material and inform an operator that an explosive has been detected. 
     Quadrupole mass spectrometers are relatively slow because of the time required to vary the excitation voltage to sequentially eject the ionized trace molecules. The prior art does include time of flight mass spectrometers, which simultaneously accelerate all of the ionized molecules toward a detector and then detect the different times when the molecules arrive. The mass of the molecules varies with the different arrival times. Time of flight mass spectrometers are not effective when used with a continuous ionization source such as a glow discharge ionizer. It would be desirable to provide a monitor that can quickly detect trace molecules in relatively low concentrations. 
     Glow discharge ionizers are efficient in ionizing molecules with high electron affinity but are not generally effective for molecules with low ionization potentials, which generally have low electron affinity. It would also be desirable to provide a monitor that can quickly detect a variety of different trace molecules in relatively low concentrations. For example, it would be desirable to provide an on-site airport terminal detector that can detect explosives as well as other threats and contraband such as chemical weapons and drugs. 
     SUMMARY OF THE INVENTION 
     The present invention includes an embodiment of a monitor for detecting a trace molecule from a gas sample. The monitor may include a glow discharge ionizer and a threshold photoionizer, which can ionize a trace molecule from the gas sample. The ionized trace molecule is trapped within a quadrupole ion trap. The quadrupole ion trap is coupled to a mass detector which can detect the ionized trace molecule. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of an embodiment of a monitor of the present invention; 
     FIG. 2 is a schematic representation of an alternate embodiment of the monitor; 
     FIG. 3 is a schematic showing the complementary function of photoionization and discharge ionization 
     FIG. 4 is a schematic representation of an alternate embodiment of the monitor; 
     FIG. 5 is a schematic representation showing fluid flow through the monitor; 
     FIG. 6 is a graph which shows signal levels as a function of residence time to show the potential to achieve high dynamic range; 
     FIG. 7 is a graph which shows reduced ion-molecule association in a quadrupole ion trap using air sampling as measured by ion trap residence time; 
     FIG. 8 is a diagram showing the ion collection and ion scan periods for operation by a QIT/TOFMS and an ITMS. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings more particularly by reference numbers, FIG. 1 shows an embodiment of a monitor  100  of the present invention. The monitor  100  is typically used to measure trace molecular constituents from a direct air sample, a sample collector, a preconcentrator, a process line, or from other sources. For purposes of discussion, trace molecular constituents could include small quantities of explosives or chemical agents or other threat compounds, or any other molecules that are to be monitored. The monitor  100  is contained in a vacuum housing that has a pumping device and other standard vacuum components. 
     The monitor  100  may include a glow discharge ionizer  102  that can receive a gas sample. The glow discharge ionizer  102  may include a first electrode  104  and a second electrode  106  that are separated by a chamber  108 . The gas sample may enter the chamber  108  through an aperture  110  in the first electrode  104  and exit the chamber  108  through an aperture  112  in the second electrode  106 . The electrodes  104  and  106  may be connected to an electrical circuit(s) (not shown). The electrical circuit may generate a voltage potential between the electrodes  104  and  106  which creates a glow discharge that ionizes trace molecules within the chamber  108 . 
     By way of example, the space between the electrodes  104  and  106  may be approximately about 2 cm, but can be other dimensions. Typically the voltages of electrodes  104  and  106  are about −350 V and 0 V, respectively. The glow discharge ionizer  102  may be similar to the ionizer disclosed in U.S. Pat. No. 4,849,628 issued to McLuckey et al., which is hereby incorporated by reference. 
     The ionizer  102  may include a port  114  that is in fluid communication with a pump  116 . The pump  116  may be used to pump out the ionization chamber in order to keep the residence time of the sample to a minimum, which improves detection time response, and to handle relatively large sample volumes. The ionizer  102  can also operate with the port  114  sealed, in which case, the residence time in the ionization volume is dictated by the flow through aperture  112 . Ions may exit the aperture  112  along with the neutral gas. The ions can be drawn through the aperture  112  by the potential field across the electrodes  104  and  106 . This invention also allows for focusing elements in the ionizer  102  to increase the yield of ions that exit aperture  112 . 
     The ions that exit the aperture  112  may be steered into a quadrupole ion trap  118  by electrostatic focusing elements  120 . The quadrupole ion trap  118  may trap ions created within the ionizer  102 . The quadrupole ion trap  118  may include a ring electrode  122  that is separated from a pair of endcap electrodes  124  and  126  by dielectric material  127 . The elements  120  and endcap  124  may function as an einsel lens. Typically, the first and last element of the einsel lens (the first element of  120  and electrode  124 ) can be biased at the same voltage, such as ground potential, and the middle element (the second element of  120 ) is biased at a different potential. Alternatively, the lenses can have progressively decreasing potential to accelerate the ions. Other focusing elements have been tested and may include multipole ion guides, such as quadrupole, hexapole, or octapole ion guides. The device can also operate without focusing elements, by relying on the ion velocities through the aperture  112  to carry them to the ion trap  118  or by applying a potential difference from electrodes  106  and  124  to accelerate the ions toward the ion trap  118 . The elements  120  and electrodes  122 ,  124  and  126  may be connected to an electrical circuit(s)  128 . 
     The ions enter the quadrupole ion trap  118  through an aperture  129  in the entrance electrode  122  and are stabilized and stored within the trap  118  by the application of an alternating current to the ring electrode  126  in a manner known in the art. The endcaps  122  and  124  are usually held at a constant voltage, such as ground potential, however, auxiliary oscillating current may be applied. The range of ion masses that are stored efficiently depends on the frequency and amplitude of the current applied to the ring electrode, it is typically of radio frequency (such as 1 MHz) and a few hundred to a few thousand volts peak to peak, but can have other values. The ions may continuously accumulate within the trap  118 . Waveforms can be applied to one or both endcap electrodes  122  and  124  and/or to the ring electrode  126  to excite specific ion masses in the trap  118  in order to eject them from stable orbits, to prevent them from accumulating, or to excite them to more energetic orbits to cause them to dissociate with background gas in order to produce fragment ions of the selected ions. Each ion mass has a distinct resonance condition. Many different ion masses may be excited simultaneously by applying a superposition of many frequencies. The frequency spectrum may be generated by a variety of prior art methods. In this embodiment, the arbitrary waveform is formed by superimposing the sum of individual periodic waveforms corresponding to the frequency and amplitude most suited for exciting each ion mass to the desired effect. In one embodiment this waveform may be applied to the exit endcap  126 , although it is to be understood that effective excitation may be achieved by application of the waveform to other electrodes in the ion trap as noted above. 
     Following accumulation of ions and the optional manipulation of ions consisting of selective ion ejection and selective collision-induced ion dissociation, the remaining ions in the quadrupole ion trap  118  are mass analyzed by ejecting all the ions into a mass detector  130 . The mass detector  130  may be a time of flight mass spectrometer. Ion ejection to the mass detector  130  may be accomplished by applying a high voltage pulse to the ion trap exit endcap  126 . Alternatively, a high voltage pulse may be applied to the entrance endcap  124  to “push” the ions into the detector  130 , or two oppositely-phased pulses may be applied to both endcaps  124  and  126  in a “push-pull” manner to extract ions into the detector  130 . 
     The extracted ions can be accelerated to a higher energy by an acceleration grid  132 . The accelerated ion pulse may be focused and collimated by a electrostatic lens assembly  134 . This is shown as a three-element einsel lens, however other configurations may be used, such as a two-element assembly. The third element in the einsel lens configuration can make use of the back plate of a detector  136 . 
     The accelerated and collimated ion packet passes through a hole  138  in the coaxial detector  136 . A cylinder  139  may be provided in the detector hole  138  to keep a uniform voltage potential for the traversing ions and is electrically isolated from the detector plates themselves (described below). The ions travel through a drift tube  140  under field-free conditions where ions of different mass travel at different speeds and spread out in space. The ions may then reach a reflectron section  142  of the mass detector where they are reversed in direction. This operation acts to focus ions of different initial energies in the usual manner. The ions then travel back toward the front of the detector  136  where they impact and are recorded as a signal in the normal manner. The resulting signal from the detector is measured with electronics that can distinguish the different arrival times of different ion masses as is known in the art. Although a reflectron  142  is shown with a coaxial detector in monitor  100 , it is to be understood that the reflectron  142  may also be of an off-axis design, or the mass detector  130  may be of a linear design with the detector plate  136  at the end of the drift tube  140 . 
     FIG. 2 shows an alternate embodiment of a monitor  200  that incorporates a glow discharge ionizier  202  and a photoionizer  204 . The glow discharge ionizer  202  may ionize molecules which have a high electron affinity. The photoionizer  204  may be used to detach electrons from molecules which have low ionization potentials. 
     As shown in FIG. 3 drugs and chemical weapons tend to have a low ionization potential while explosive materials tend to have a high electron affinity. The inclusion of both the photoionizer  204  and the glow discharge ionizer  202  provides a single monitor which can effectively ionize a number of different trace molecules to detect a plurality of substances. Such a monitor would be particularly useful when used to detect both explosives, chemical weapons, and drugs at an airport terminal. Photoionization and glow discharge electron attachment are complementary ionization methods that significantly increase the range of compounds that can be detected in one device. 
     Referring again to FIG. 2, each ionizer  202  and  204  is connected to a corresponding quadrupole ion trap  206  and  208 , respectively, and mass detectors  210 . Each detector  210  may include a coaxial detector and focusing lens assembly  214 , a reflectron section  216 , and other components. The operation of the combined glow discharge ionizer  202 , ion trap  206  and mass detector  210  may be similar to the system shown in FIG.  1 . 
     The photoionizer  204 , ion trap  208  and detector  210  may function in a manner similar to the glow discharge section of the monitor  200 . The photoionizer  204  typically operates in positive ion mode compared to the glow discharge ionizer  202 , which typically operates in a negative ion mode. The monitor  200  may include partitions  218  that separates the ion source vacuum region from the mass detector vacuum region and allows each region to be separately pumped and to have different operating pressures. 
     Many designs are possible for the photoionizer  204 . In the embodiment shown, atmospheric air or other gaseous mixture may be allowed to enter the ionizer through a valve, or aperture, and/or a thin tube. The pressure in the photoionizer  204  may be sub-atmosphere, typically being about 1 torr, but can operate from 10 −3  torr to greater than 10 torr, even up to atmosphere. Ions that are formed in the photoionizer  204  are extracted through an aperture that leads to the ion trap  208 . The ions are steered and accumulated and ejected from the trap  208  and into the detector  212  in a manner similar to the description given for monitor  100  shown in FIG.  1 . 
     The photoionizer may include a light source which emits a light beam which has a wavelength so that photo-energy between 8.0 and 12.0 electron volts (eV) is delivered to the gas sample. Photo-energy between 8.0 and 12.0 is high enough to ionize most trace molecules of interest without creating much molecular fragmentation within the sample. By way of example the light source may be a Nd:YAG laser which emits light at a wavelength of 355 nanometers (nm). The 355 nm light may travel through a frequency tripling cell that generates light at 118 nms. 118 nm light has an energy of 10.5 eV. Such a light source is described in U.S. Pat. No. 5,808,299 issued to Syage, which is hereby incorporated by reference. Alternatively, the light source may include continuous or pulsed discharge lamps which are disclosed in U.S. Pat. No. 3,933,432 issued to Driscoll; U.S. Pat. No. 5,393,979 issued to Hsi; U.S. Pat. No. 5,338,931 issued to Spangler et al. and U.S. Pat. No. 5,206,594 issued to Zipf, which are hereby incorporated by reference. 
     FIG. 4 shows an embodiment of a monitor  300  which has a glow discharge ionizer  302  and a photoionizer  304  coupled to the same mass detector  306 . Each ionizer  302  and  304  can be coupled to the mass detector  306  by a quadrupole ion trap  308  and  310 , respectively. The ion traps  308  and  310  can be connected to the ionizers  302  and  304  so that the ions exiting the ionization source directly enter the traps without requiring ion focusing elements. In this and other embodiments, the spacers that seal the ion traps  308  and  310  from the surrounding vacuum chamber,  312  in FIG. 4 and 127 in FIG. 1 may be removed so that the traps  308  and  310  can be pumped out. This may be used very effectively for the directly coupled ionizer/trap configuration to allow increased sample throughput into the traps. The quadrupole ion traps  308  and  310  may have ports  314  or open area that are coupled to a pump (not shown). The ports can be used to pump out the ion traps, or introduce a gas other than the sample gas, such as helium, which has been shown in previous work to effectively cool ions in the traps. 
     The monitor  300  in FIG. 4 shows an embodiment whereby the ions that exit each trap  308  and  310  enter the same mass detector  306 . The mass detector  306  may be a time of flight mass spectrometer which includes a drift tube  316 , reflectron  318  and detector plate  320 . In order to separate the recorded mass spectrum from each ion source, the ions from each ion trap  308  and  310  can be pulsed into the mass detector  306  at different times. The monitor  300  may have electrostatic steering optics such as a simple deflector  322  for this purpose to steer the ions from the traps  308  and  310  in a direction that will insure detection by plate  320 . 
     If ions of the same charge are detected from each quadrupole ion trap  308  and  310 , then the detection follows the prescription described earlier and the operation of the mass detector  306  may operate in a conventional manner. If ions of different charge exit each trap  308  and  310 , such as is the case for the glow discharge ionizer  302  in electron attachment, negative ion mode, and the photoionizer  304  in positive ion mode, then the voltages on the drift tube  316 , the reflectron segment  318  and the detector  320  must be switched according to the conditions that are appropriate for the charge being detected. Standard electronic methods may be used to achieve switching in the time period after the recording of one mass spectrum and before the extraction of the ions from the other trap  308  or  310 . 
     The monitor  300  may have separate sample inlet ports  324  and  326  for the glow discharge ionizer  302  and photoionizer  304 , respectively. In one embodiment, these sample inlets  324  and  326  are connected so that the same sample is split and enters both ionizers  302  and  304 . It is also possible to use each ionizer and mass analyzer for separate samples. 
     The embodiments in FIG. 4 represent a variety of options that may be applied separately or in combination to achieve a variety of configurations tailored for specific applications. 
     FIG. 5 shows an embodiment of sample gas flow partitioning systems for a photoionizer  402  and a glow discharge ionizer  404  that achieve high sample throughput while minimizing the gas load on the vacuum systems. A sample consisting of trace compounds in air or other gases can be delivered to the inlet system of the glow discharge ionizer  404  or the photoionizer  402 . A sample may be introduced through tubes  406  which have inlets  408  that are coupled to a pump (not shown) which draws in a sample. Alternatively, the sample may be delivered by exposure to ambient air without a sampling tube, a preconcentrator device such as a momentum impactor device for particles, a mesh, an electrostatic precipitator for particles and vapor, or by other means. 
     A portion of the air sample flow, constituting the first stage of partitioning enters the ionizers  402  and  404  through an aperture  410  for glow discharge ionizer  402  and through either an aperture or a jet separator  412  for the photoionizer  404 . If a jet separator is used then the usual skimmed flow is pumped away along an airstream  414 . For the glow discharge ionizier  402 , the air entering the ionizer chamber is pumped away along airstream  416 . This bypass pumping and the resultant advantages were described earlier when referring to number  114  in FIG.  1 . The glow discharge partitioning  414  and the optional photoionizer jet separator partitioning  416  constitute the second stage of flow partitioning. 
     The third stage of flow partitioning occurs in the regions between the ionizer exit apertures and the quadrupole ion trap entrance apertures of the ion traps  418  and  420 , along flow streams  422  and  424 , respectively. The traps  418  and  420  may be coupled to mass detectors  426  and  428 , respectively. The mass detectors  426  and  428  can be evacuated by flow streams  430  and  432 , respectively. Only a small fraction of the neutral background air or gas enters the traps  418  and  420  and hence the final gas load on the mass detectors  426  and  428  is minimized. The requirements for vacuum partitioning denoted by  422  and  430  for the photoionizer section, and  424  and  432  for the glow discharge section can be met by available split-flow or multi-ported turbo-molecular pumps, although other pumps and separate pumping may also be used. An example of operating pressures in the source and mass detector regions is about 10 −3  torr and about 10 −5  torr, respectively, although these regions can operate at higher or lower pressures. For the embodiment  400  described by FIG. 5, the source and mass detector vacuum sections can be connected, such that a single multi-ported pump  433  and corresponding manifold  434  can be used for the glow discharge source and the dual photoionizer configuration. 
     The intent for each stage of flow partitioning is to achieve enrichment of the target compounds and ions in the background air or gas. In the sample delivery stage, this may be effected by using a preconcentrator or other device as noted above. In the second stage, a jet separator achieves mass focusing whereby higher molecular weight compounds are enriched along the centerline, which is the portion that enters into the photoionizer  404 . A similar effect occurs for the glow discharge ionizer  402  in which higher molecular weight ions may be enriched along the centerline, which is aligned with the exit aperture. The third stage achieves very effective enrichment because the ions exiting the ionizers  402  and  404  can be focused into the traps  418  and  420 , whereas the exiting neutral gas disperses and is mostly pumped along  422  and  424 . 
     FIGS. 6 and 7 show results which demonstrate the benefits of the combined quadrupole ion trap/time of flight mass spectrometer (“QIT/TOFMS”) vs. an ion trap mass spectrometer (“ITMS”)for use with continuous ionization sources such as glow discharge and photoionization. To achieve the highest levels of sensitivity and dynamic range, it is advantageous to use a method of mass analysis that has a high duty cycle for ion collection. The use of a quadrupole ion trap as an interface between a continuous ionization source such as glow discharge and a pulsed mass analyzer such as time of flight mass spectrometer has significant advantages over the use of an ITMS, or an orthogonal extraction time of flight mass spectrometer. 
     The advantage of ion trap over orthogonal extraction TOFMS is (1) higher ion collection efficiency, and (2) capability to perform specific ion rejection and specific collision-induced dissociation (CID). The QIT/TOFMS mass analyzer and ITMS operate similarly with regard to ion collection, ion rejection and CID. However as noted above, the principal difference is in the method of mass analysis. The ITMS uses mass-selective instability to sequentially scan out ions of increasing mass from the trap, whereas QIT/TOFMS uses a high voltage pulse to inject all the ions into a TOFMS for mass analysis. There are three main advantages of QIT/TOFMS compared to ITMS: 
     (1) The ion ejection time is significantly less for QIT/TOFMS vs ITMS (about 5-10 microsecond vs 1-100 millisecond, respectively). Because ion collection must be turned off during the mass analysis period, the longer mass analysis period for ITMS limits how the high repetition rate may be set before the duty cycle for ion collection becomes small. Referring to FIG. 8 the detection duty for an ITMS is 1−(t/T) where t is the mass scan out time and T is the time between collection periods. By way of example a 50% duty cycle corresponds to a repetition rate of 10 Hz for a 50 ms mass analysis time and to 50 Hz for a 10 ms analysis time. QIT/TOFMS achieves nearly a 100% ion collection duty cycle up to repetition rates as high as and greater than 1 kHz. Excellent signal linearity is observed in FIG. 6 for collection periods (inverse repetition rate) ranging from 3 ms to 60 ms. If MS/MS is employed, then the duty cycle for ion collection will decrease due to the finite time required to effect CID in the trap. By using air as a carrier gas and operating the trap at relatively high pressures (a few m-torr), we anticipate time periods for sufficient CID to be about 10 ms, based on some preliminary observations. An alternative method to avoid the ion collection “down time” is to use notch filtering whereby a selected set of parent and daughter ion masses is stabilized and the remaining masses destabilized using the appropriate RF waveform. In this case, the parent ions would be excited to induce CID, without fully ejecting them from the trap. Other multiplexed MS/MS routines may be incorporated and benefit from the high repetition rates achievable by QIT/TOFMS. 
     (2) Mass resolution and mass analysis ejection efficiency is less sensitive to space charge repulsion for QIT/TOFMS than for ITMS allowing a higher ion storage capacity. The ITMS method requires that the Mathieu parameter q for each ion mass be constant for high mass resolution in the RF scan out. Space charge repulsion can broaden the apparent q value and consequently the mass resolution. For QIT/TOFMS a HV pulse out is less sensitive to space charge repulsion for the following reasons: (i) The broadening effects of the energy spread of the ions due to space charge repulsion can be minimized by increasing the drift length and TOF voltage. (ii) Broadening due to the ion energy spread can be refocused using a reflectron TOFMS. Significantly greater QIT ion capacities have been measured by us for QIT/TOFMS than reported by commercial instrument manufacturers for ITMS. A possible limitation on trap capacity may occur when using the selective ion rejection mode for ion collection because severe space charge may broaden the resonance spectrum of each ion mass. However, to the extent that unit mass resolution is not needed for resonance ejection, a higher ion capacity may be used compared to the limit value for ITMS instruments. 
     (3) Ion mass signals appear within significantly narrower time window for QIT/TOFMS vs ITMS (typically 50-100 nanoseconds vs about 100 microseconds, respectively), leading to potentially better signal-to-noise ratios for the former technique. A given number of ions will produce a larger peak current (or voltage) if they are detected in a shorter period of time. Narrower time bins per unit mass also lead to proportionately better noise immunity since less instrument noise is collected. However, note that chemical noise, defined as signal from ions of the same mass as the analyte, is not reduced by using narrower time bins. 
     The potential for operating at higher repetition rates by QIT/TOFMS vs. ITMS also offers the potential for higher detection dynamic range by the former method. The repetition rate enables control over the maximum number of ions that accumulate in the trap. Too many ions in the QIT can lead to spectral broadening and other known undesirable effects. By increasing the maximum repetition rate, the range of detectable ions per unit time is increased proportionately. This feature in addition to the greater number of ions that may be stored in the QIT using TOFMS analysis vs. mass-selective instability scanning as described earlier, leads to a potential improvement in dynamic range of about 1-2 orders of magnitude for QIT/TOFMS vs ITMS. 
     FIG. 7 shows QIT/TOFMS mass spectral intensities recorded for repetition rates ranging from 17 Hz to 304 Hz (residence time of 59 and 3 ms, respectively) for a sample of 5 ppm DIMP (180 molecular weight) and 9 ppm DMMP (124 molecular weight) in room air. Excellent linearity is observed over this range for all signals (97 and 139 amu are fragments of DIMP parent ion at 180 amu; DMMP is observed as a protonated ion at 125 amu, and ion clustering is observed at 249 and 263 amu). Ion molecule reactions, such as clustering and dissociation can occur in the QIT, which may be undesirable for certain applications. The extent of reaction can be tested by varying the residence or reaction time in the QIT. FIG. 7 shows the ratio of the DIMP fragment ions at 97 and 139 amu relative to the parent ion at 180 amu. These ratios show a slight dependence, however for the typical condition of 40 Hz (25 ms residence time), the extent of reaction is not significant. The effect of ion-molecule association (clustering) is also not significant in the QIT as measured by the ratio of DMMP dimer ion to monomer ion (protonated signal 249 amu/125 amu) and to the DIMP 139 amu fragment and DMMP parent cluster (263 amu/125 amu). The ion clusters have been shown to occur in the PI source and can be minimized by reducing the PI source pressure. 
     While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art.