WO2009143623A1

WO2009143623A1 - Method and system for vacuum driven differential mobility spectrometer/mass spectrometer interface with adjustable resolution and selectivity

Info

Publication number: WO2009143623A1
Application number: PCT/CA2009/000741
Authority: WO
Inventors: Bradley Schneider; Thomas Covey; Erkinjon Nazarov
Original assignee: Mds Analytical Technologies; Life Technologies Corporation
Priority date: 2008-05-30
Filing date: 2009-05-28
Publication date: 2009-12-03
Also published as: CA2720244A1; EP2281297A1; JP5767581B2; US8084736B2; US20090294650A1; EP2281297B1; CA2720244C; EP2281297A4; JP2011522363A

Abstract

A mass spectrometer system including a differential mobility spectrometer and a mass spectrometer at least partially sealed to, and in fluid communication, with, the differential mobility spectrometer, together with a related method, are provided. The mass spectrometer system can be operable to, and method can comprise, a) maintaining the differential mobility spectrometer at an internal operating pressure; b) providing ions to the differential mobility spectrometer; c) maintaining the mass spectrometer at a vacuum pressure lower than the internal operating pressure to draw a gas flow including the ions through the differential mobility spectrometer and into the vacuum chamber; and, d) modifying the gas flow between the differential mobility spectrometer and the mass spectrometer to change a gas flow rate through the differential mobility spectrometer.

Description

TITLE: METHOD AND SYSTEM FOR VACUUM DRIVEN DIFFERENTIAL

MOBILITY SPECTROMETER/MASS SPECTROMETER INTERFACE WITH

ADJUSTABLE RESOLUTION AND SELECTIVITY

INTRODUCTION

[0001] The present invention relates generally to methods and systems involving both a mass spectrometer and a differential mobility spectrometer.

[0002] In differential mobility spectrometer/mass spectrometer systems, a drift gas is typically supplied from a compressed gas source upstream of the differential mobility spectrometer. This drift gas acts as a carrier gas flow through the differential mobility spectrometer. The delivery of the drift gas to the differential mobility spectrometer can be controlled by flow restriction valves. Sensitivity is related to the transmission efficiency of the system - what percentage of the ions end up being actually detected. Selectivity or resolution refers to the detector's ability to distinguish between similar ions.

[0003] Differential mobility spectrometry, also referred to as high field asymmetric waveform ion mobility spectrometry (FAIMS) or Field Ion Spectrometry (FIS), is a variant of ion mobility spectrometry (IMS). IMS separates ions by the difference in the time it takes for them to drift through a gas, typically at atmospheric pressure, in a constant electrostatic field of low field strength applied along the axial length of a flight tube. Ions are pulsed into the flight tube and their flight times are recorded. The time of flight is inversely related to the mobility of an ion. Ions have a single motion of direction (axial) and are separated according to their mobility through the gas under these low field conditions (E<1000V/cm). The drift time and thus mobility is a function of the size and shape of an ion and its interactions with the background gas.

[0004] Differential mobility spectrometry differs from IMS in the geometry of the instrumentation and adds an additional dimension to the separation theory. RF voltages, often referred to as separation voltages (SV), are applied across the ion transport chamber, perpendicular to the direction of the transport gas flow. Ions will migrate toward the walls and leave the flight path unless their trajectory is corrected by a counterbalancing voltage, a DC potential often referred to as a compensation voltage (CV). Instead of recording the flight time of an ion through the chamber, the voltage required to correct the trajectory of a particular ion is recorded. Ions are not separated in time as with an IMS: instead, the mobility measurement is a function of the compensation voltage used to correct the tilt in ion trajectory caused by the difference between high field and low field ion mobilities. As such, ions are not pulsed into the analyzer but instead introduced in a continuous fashion and the compensation voltage is scanned to serially pass ions of different differential mobility or set to a fixed value to pass only ion species with a particular differential mobility.

SUMMARY OF THE INVENTION

[0005] Typically, there is a tradeoff between selectivity and sensitivity, both of which are linked to the residence time of the ions in the differential mobility spectrometer. Specifically, increasing the residence time of the ions in the differential mobility spectrometer may increase selectivity, but at the price of reducing sensitivity.

[0006] As described above, in the description that follows, sensitivity is related to the transmission efficiency of the system - what percentage of the ions end up being actually detected. Selectivity or resolution refers to the detector's ability to distinguish between similar ions. [0007] In accordance with an aspect of an embodiment of the invention, there is provided a mass spectrometer system comprising: a) a differential mobility spectrometer for receiving ions from an ion source, the differential mobility spectrometer having an internal operating pressure, electrodes, and at least one voltage source for providing DC and RF voltages to the electrodes; b) a mass spectrometer at least partially sealed to, and in fluid communication with, the differential mobility spectrometer for receiving the ions from the differential mobility spectrometer; c) a vacuum chamber surrounding the mass spectrometer for maintaining the mass spectrometer at a vacuum pressure lower than the internal operating pressure, such that the vacuum chamber is operable to draw a gas flow including the ions through the differential mobility spectrometer and into the vacuum chamber; and, d) a gas port for modifying a gas flow rate through the differential mobility spectrometer, the gas port being located between the differential mobility spectrometer and the mass spectrometer.

[0008] In accordance with an aspect of a further embodiment of the invention, there is provided a method of operating a mass spectrometer system including a differential mobility spectrometer and a mass spectrometer contained in a vacuum chamber at least partially sealed to, and in fluid communication, with, the differential mobility spectrometer. The method comprises: a) maintaining the differential mobility spectrometer at an internal operating pressure; b) providing ions to the differential mobility spectrometer, and providing a selected separation RF voltage and a selected compensation DC voltage to the differential mobility spectrometer; c) maintaining the mass spectrometer at a vacuum pressure lower than the internal operating pressure to draw a gas flow including the ions through the differential mobility spectrometer and into the vacuum chamber; and, d) modifying the gas flow between the differential mobility spectrometer and the mass spectrometer to change a gas flow rate through the differential mobility spectrometer. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The skilled person in the art will understand that the drawings, described below, are for illustration purposes only. The drawings are not intended to limit the scope of the applicant's teachings in any way.

[0010] Fig. 1, in a schematic diagram, illustrates a differential mobility spectrometer/mass spectrometer system including a juncture chamber to which a throttle gas is added between the differential mobility spectrometer and the mass spectrometer in accordance with an aspect of a first embodiment of the present invention.

[0011] Fig. 2, in a schematic view, illustrates a differential mobility spectrometer/mass spectrometer system including a juncture chamber to which a throttle gas is added between the differential mobility spectrometer and the mass spectrometer in accordance with an aspect of a second embodiment of the present invention.

[0012] Fig. 2A, in a graph, plots the signals for various ions against the

DMS offset potential relative to a potential at the vacuum chamber inlet or mass spectrometer inlet.

[0013] Fig. 3, in a schematic view, illustrates a differential mobility spectrometer/mass spectrometer system including a juncture chamber to which a throttle gas is added between the differential mobility spectrometer and the mass spectrometer, and in which gas flow into the differential mobility spectrometer is restricted, in accordance with an aspect of a third embodiment of the present invention. [0014] Fig. 4, in a schematic view, illustrates a differential mobility spectrometer/mass spectrometer system in which a controlled leak is provided at a juncture of the differential mobility spectrometer and the mass spectrometer to adjust the gas flow rate through the differential mobility spectrometer in accordance with an aspect of a fourth embodiment of the present invention. [0015] Fig. 5, in a schematic view, illustrates a differential mobility spectrometer/mass spectrometer system including a juncture chamber to which the throttle gas is added between the differential mobility spectrometer and the mass spectrometer and in which the differential mobility spectrometer includes a heated tube in accordance with an aspect of a fifth embodiment of the present invention.

[0016] Fig. 6, in a schematic view, illustrates a differential mobility spectrometer/mass spectrometer system similar to that described in Figure 2, and in which bubblers are provided for adding liquid modifiers to the curtain gas provided to the curtain chamber, in accordance with an aspect of a sixth embodiment of the present invention.

[0017] Fig. 7, in a schematic view, illustrates a differential mobility spectrometer/mass spectrometer system similar to that of Figure 6, but also including an additional conduit branch for providing curtain gas directly to the curtain chamber without any liquid modifiers, in accordance with an aspect of a seventh embodiment of the present invention.

[0018] Fig. 8, in a schematic view, illustrates a differential mobility spectrometer/mass spectrometer system including a juncture chamber to which a throttle gas is added between the differential mobility spectrometer and the mass spectrometer, and in which bubblers are provided to add various modifiers to the throttle gas, in accordance with an aspect of a eighth embodiment of the present invention.

[0019] Fig. 9, in a schematic view, illustrates a differential mobility spectrometer/mass spectrometer system including a juncture chamber from which a bleed gas is drawn between the differential mobility spectrometer and the mass spectrometer in accordance with an aspect of a ninth embodiment of the present invention.

[0020] Fig. 10, in a series of graphs, illustrates different resolutions of a

CV scan for separation of a sample containing ephedrine and pseudoephedrine using a fixed differential mobility spectrometer geometry with a variable amount of throttle gas added at the juncture of the differential mobility spectrometer and the mass spectrometer.

DESCRIPTION OF VARIOUS EMBODIMENTS

[0021] Referring to Figure 1 , there is illustrated in a schematic view, a differential mobility spectrometer/mass spectrometer system 200 in accordance with an aspect of a first embodiment of the present invention. The differential mobility spectrometer/mass spectrometer system 200 comprises a differential mobility spectrometer 202 and a first vacuum lens element 204 of a mass spectrometer (hereinafter generally designated mass spectrometer 204). Mass spectrometer 204 also comprises mass analyzer elements 204a downstream from vacuum chamber 227. Ions can be transported through vacuum chamber 227 and may be transported through one or more additional differentially pumped vacuum stages prior to the mass analyzer indicated schematically as mass analyzer elements 204a. For instance in one embodiment, a triple quadrupole mass spectrometer may comprise three differentially pumped vacuum stages, including a first stage maintained at a pressure of approximately 2.3 Torr, a second stage maintained at a pressure of approximately 6 mTorr, and a third stage maintained at a pressure of approximately 10^~5 Torr. The third vacuum stage may contain a detector, as well as two quadrupole mass analyzers with a collision cell located between them. It will be apparent to those of skill in the art that there may be a number of other ion optical elements in the system that have not been described. This example is not meant to be limiting as it will also be apparent to those of skill in the art that the differential mobility spectrometer/mass spectrometer coupling described can be applicable to many mass spectrometer systems that sample ions from elevated pressure sources. These may include time of flight (TOF), ion trap, quadrupole, or other mass analyzers as known in the art. [0022] The differential mobility spectrometer 202 comprises plates 206 and an electrical insulator 207 along the outside of plates 206. The plates 206 surround a drift gas 208 that drifts from an inlet 210 of the differential mobility spectrometer to an outlet 212 of the differential mobility spectrometer 202. The insulator 207 supports the electrodes and isolates them from other conductive elements. For example, the insulator may be fabricated from ceramic or Teflon™. The outlet 212 of the differential mobility spectrometer 202 releases the drift gas into a juncture or baffle chamber 214 defined by baffles 216, which juncture chamber 214 defines a path of travel for ions between the differential mobility spectrometer 202 and the mass spectrometer 204. In some embodiments, the outlet 212 of the differential mobility spectrometer 202 is aligned with the inlet of the mass spectrometer 204 to define the ion path of travel therebetween, while the baffles 216 are spaced from this path of travel to limit interference of the baffles 216 with the ions 222 traveling along the path of travel.

[0023] The differential mobility spectrometer 202 and juncture chamber

214 are both contained within a curtain chamber 218, defined by curtain plate (boundary member) 219 and supplied with a curtain gas from a curtain gas source 220. The curtain gas source 220 provides the curtain gas to the interior of the curtain chamber 218. Ions 222 are provided from an ion source (not shown) and are emitted into the curtain chamber 218 via curtain chamber inlet 224. The pressure of the curtain gas within the curtain chamber 218 provides both a curtain gas outflow 226 out of curtain gas chamber inlet 224, as well as a curtain gas inflow 228 into the differential mobility spectrometer 202, which inflow 228 becomes the drift gas 208 that carries the ions 222 through the differential mobility spectrometer 202 and into the juncture chamber 214. The curtain plate 219 may be connected to a power supply to provide an adjustable DC potential to it.

[0024] As illustrated in Figure 1 , the first vacuum lens element 204 of the mass spectrometer 204 is contained within a vacuum chamber 227, which can be maintained at a much lower pressure than the curtain chamber 218. In accordance with an aspect of an embodiment of the present invention, the vacuum chamber 227 can be maintained at a pressure of 2.3 Torr by a vacuum pump 230 while the curtain chamber 218 and an internal operating pressure of the differential mobility spectrometer 202 can be maintained at a pressure of 760 Torr. As a result of the significant pressure differential between the curtain chamber 218 and the vacuum chamber 227, the drift gas 208 is drawn through the differential mobility spectrometer 202, the juncture chamber 214 and, via vacuum chamber inlet 229, into the vacuum chamber 227 and first vacuum lens element 204. As shown, the mass spectrometer 204 can be sealed to (or at least partially sealed), and in fluid communication with the differential mobility spectrometer, via the juncture chamber, to receive the ions 222 from the differential mobility spectrometer 202.

[0025] As shown, the baffles 216 of the curtain chamber comprise a controlled leak or gas port 232 for admitting the curtain gas into the juncture chamber 214. Within the juncture chamber 214, the curtain gas becomes a throttle gas that throttles back the flow of the drift gas 208 through the differential mobility spectrometer 202. Specifically, the throttle gas within the juncture chamber 214 modifies a gas flow rate within the differential mobility spectrometer 202 and into the juncture chamber 214, thereby controlling the residence time of the ions 222 within the differential mobility spectrometer 202. By controlling the residence time of the ions 222 within the differential mobility spectrometer 202, resolution and sensitivity can be adjusted. That is, increasing the residence times of the ions 222 within the differential mobility spectrometer 202 can increase the resolution, but can also result in additional losses of the ions, reducing sensitivity. In some embodiments it can therefore be desirable to be able to precisely control the amount of throttle gas that is added to the juncture chamber 214 to provide a degree of control to the gas flow rate through the differential mobility spectrometer 202, thereby controlling the tradeoff between sensitivity and selectivity. In the embodiment of Figure 1 , the inflow of throttle gas from the curtain chamber 218 can be controlled by controlling the size of the leak provided by the gas port 232. [0026] The baffles can be configured to provide a randomizer surface member, and the gas port 232 can be oriented to direct the throttle gas at least somewhat against the baffles 216 and randomizer surface to disburse the throttle gas throughout the juncture chamber 214. In one embodiment, the gas port 232 introduces the throttle gas without disrupting the gas streamlines between the differential mobility spectrometer 202 and the mass spectrometer inlet 229.

[0027] As described above and as known in the art, RF voltages, often referred to as separation voltages (SV), can be applied across an ion transport chamber of a differential mobility spectrometer perpendicular to the direction of drift gas 208 (shown in Figure 1). The RF voltages may be applied to one or both of the DMS electrodes comprising the differential mobility spectrometer. The tendency of ions to migrate toward the walls and leave the path of the DMS can be corrected by a DC potential often referred to as a compensation voltage (CV). The compensation voltage may be generated by applying DC potentials to one or both of the DMS electrodes comprising the differential mobility spectrometer. As is known in the art, a DMS voltage source (not shown) can be provided to provide both the RF SV and the DC CV. Alternatively, multiple voltage sources may be provided. [0028] Similarly, a DC declustering or inlet potential can be provided to the vacuum chamber inlet 229 (again as shown in Figure 1) by an inlet potential voltage source (not shown), again as known in the art. This vacuum chamber inlet may be a orifice, or, alternatively, may be a capillary, heated, capillary or an ion pipe. [0029] In embodiments of the present invention in which the vacuum chamber inlet 229 is smaller then an outlet of the differential mobility spectrometer 202, it can be advantageous to provide a braking potential to the vacuum chamber inlet 229 relative to the differential mobility spectrometer 202. This braking potential can be provided by providing a DMS DC offset voltage to the plates or electrodes of the DMS relative to the declustering or inlet potential provided to the vacuum chamber inlet 229. By slowing down the ions prior to them entering the vacuum chamber 229, the braking potential can increase the extent to which these ions are entrained within the gas flows, thereby increasing the likelihood that the ions will actually pass through the vacuum chamber inlet, instead of impacting on the sides of the vacuum chamber inlet 229.

[0030] Alternatively, in some embodiments of the present invention, such as, for example without limitation, embodiments in which the vacuum chamber inlet 229 is larger relative to the slit or outlet from the differential mobility spectrometer 202, it can be desirable to adjust the DMS DC offset voltage. In particular this DC offset voltage may actually be positive to speed up ions as they pass through the vacuum chamber inlet 229, if it is not desirable to slow them down to improve transmission from the differential mobility spectrometer 202 into the vacuum chamber 227.

[0031] This DMS DC offset can also be adjusted based on a mass of the ions being selected in a differential mobility spectrometer 202. This could be part of a two-stage process. Specifically, the declustering voltage provided to the vacuum chamber inlet 229 can first be adjusted based on the mass of the ions being selected in the differential mobility spectrometer 202. Then, relative to this declustering potential provided to the vacuum chamber inlet 229, the DMS DC offset voltage could be adjusted to enhance transmission from the differential mobility spectrometer 202 through the vacuum chamber inlet 229. Alternatively, the DMS offset DC potential may be selected for a given ion. In some embodiments, a voltage source controller can be set to automatically adjust the DMS electrode DC offsets to maintain the same potential difference relative to the orifice potential. Then the declustering potential or inlet potential may be adjusted, That is, in these embodiments the DMS offset voltage is merely the difference between the DC potential applied to the electrodes as an offset and the inlet voltage. Say, for example, that a preferred DMS offset voltage is -3 V. Then, when the inlet voltage is tuned, the control system can, in these embodiments, maintain that -3 V offset regardless of the current inlet voltage. For instance, if the inlet potential is initially 50 V, the DC potential on the DMS electrodes can be automatically maintained at 47 V (CV = 0 situation). If the inlet potential is tuned up to 100 V, the DC applied to the DMS electrodes can be automatically changed to 97 V.

[0032] Referring to Figure 2, there is illustrated in a schematic view, a differential mobility spectrometer/mass spectrometer system 300 in accordance with an aspect of a second embodiment of the present invention. For clarity, the same reference numerals used in Figure 1 , with 100 added, are used in Figure 2 to designate elements analogous to the elements of Figure 1. For brevity, the description of Figure 1 is not repeated with respect to Figure 2.

[0033] It is important to note that due to the compensation voltage provided to the plates or electrodes of the differential mobility spectrometer, the actual DC potential of one or both of the electrodes of the differential mobility spectrometer may not differ by the DMS DC offset amount from the declustering potential applied to vacuum chamber inlet element. For example, say that a declustering potential is applied to vacuum chamber inlet element 329. This declustering potential (DP) is determined based on the m/z of the ion being selected by the differential mobility spectrometer, and this determination of the DP is known in the art. Then, a DC offset voltage is applied to the plate or electrodes 306 of the differential mobility spectrometer 302. In addition, the CV will be applied to the electrodes 306. Application of a CV may proceed in different ways. For example, say that there is a CV of 10 volts, then 5V can be applied to one electrode, while -5V are applied to the other electrode. Alternatively, 10V can be applied to one electrode and no volts to the other electrode.

[0034] Consider an example where all of the CV is applied to one electrode. Then, say that a DP of 100V is first determined for the vacuum chamber inlet. The offset between the vacuum chamber inlet and the differential mobility spectrometer is determined to be -5V. The CV for the differential mobility spectrometer is 10V. Then, one electrode of the differential mobility spectrometer would have a potential of 100V - 5V + 10V or 105V, while the other electrode would have a potential of 100V - 5V = 95V.

[0035] As noted above, the DC offset voltage need not be negative. Specifically, where the orifice or inlet dimension more closely matches the slit dimension for the differential mobility spectrometer, there may be no need to slow the ions down to properly entrain them in the gas flow so that they can flow through the orifice. Instead, it could even be desirable to speed the ions up. [0036] Referring to Figure 2A, the effectiveness of braking potentials applied to electrodes of dimension 1x10x30 mm is plotted in a graph for various mass to charge ratios. For all the ions tested, the optimal DMS offset voltage appears to be negative - that is, the optimal DMS potential should be slightly lower than the vacuum chamber inlet potential to establish a braking potential. The magnitude of the optimal offset voltage and the widths of the optimal voltage range both increase with the mass to charge ratio of the ion of interest, likely reflecting the known decrease in the ion mobility constant for higher m/z ions. The data plotted in Figure 2A demonstrates that the transfer of ions from a slotted DMS analyzer to a circular mass spectrometer or vacuum chamber inlet may be improved by slowing down the ions to give them a longer time period in which to be influenced by the bending gas streams converging on the inlet, thereby reducing losses in the interface region or juncture of the differential mobility spectrometer and mass spectrometer. [0037] As with the system 200 of Figure 1 , in the system 300 of Figure

2 drift gas 308 can be drawn through the differential mobility spectrometer 302 and into the vacuum chamber 327 and the first vacuum lens element 304 by the much lower pressure maintained in the vacuum chamber 327. As with the system 200 of Figure 1 , the vacuum chamber 327 of the system 300 can be maintained, say, at a pressure of 2.3 Torr, for example, while the pressure in the curtain chamber 318 can be maintained at a pressure of 760 Torr. [0038] As with the system 200 of Figure 1 , the resolution or selectivity of system 300 can be adjusted by adding a throttle gas to a juncture chamber 314 between the differential mobility spectrometer 302 and the vacuum chamber inlet 329. In the system 300 of Figure 2, a common source is provided for both the curtain gas and the throttle gas; however, separate sources may also be provided. For example, this gas could be nitrogen. The throttle gas flows through a conduit branch 320a into the juncture chamber 314. Again, this gas is called a throttle gas because it throttles back the flow through the differential mobility spectrometer. In some embodiments, the gas can be added in a coaxial manner to reduce the likelihood of a cross beam of the throttle gas interfering with the ion beam trajectory or ion path of travel between the differential mobility spectrometer 302 and the mass spectrometer, as interference with this ion beam trajectory could potentially diminish transmission efficiency. For example, as shown in Figure 2, gas ports 332 are oriented or inclined such that the throttle gas flows into the juncture chamber 314 at an orientation that is toward the vacuum chamber 327 and away from the differential mobility spectrometer 302. Optionally, the juncture chamber 314 can be enlarged or may include extra structures to reduce the linear velocity of the throttle gas to reduce its interference with the ion beam at the point of entry into the mass spectrometer. Further, the gas ports 332 may optionally be oriented such that the throttle gas flows along the sidewalls of the juncture chamber, somewhat parallel to the ion path of travel. In another embodiment, the juncture chamber may be designed with a much larger diameter than the diameter of the insulator, and the gas port may be oriented such that the gas stream through the inlet is directed along the wall.

[0039] Please note that schematic figures 1-9 are not to scale. That is, from a functional perspective the juncture chamber can be made substantially larger than what is shown in the figures, to reduce the risk of the throttle gas inflow disrupting ion flow through the juncture chamber. Further, the throttle gas can be introduced so as to be directed along the wall, thereby reducing disruption, and increasing sensitivity, as compared to the case in which the throttle gas is provided in a cross-flow to the ion motion in the juncture chamber.

[0040] Conduit branch 320a comprises a controllable valve 320b that can be used to control the rate of flow of the throttle gas into the juncture chamber 314. For example, to increase resolution or selectivity, at the price of an acceptable loss in sensitivity, the controllable valve 320b could be opened to admit more throttle gas into the juncture chamber 314 via conduit branch 320a to reduce the gas flow rate within the differential mobility spectrometer 302. This, in turn, can increase the residence time of the ions 322 within the differential mobility spectrometer. The increased residence time manifests itself as narrower mobility peak widths, and therefore, improved selectivity. At the same time, the increased residence time lowers sensitivity somewhat due to increased diffusion losses. At the same time, because of the increased residence time within the differential mobility spectrometer, more of the ions can be lost.

[0041] As shown, Figure 2 also can comprise a valve 320c for controlling the rate of flow of the curtain gas into the curtain chamber 318. It is important to control the curtain gas flow rate to ensure proper declustering of ions upstream of to the DMS. Clusters can have different mobilities than dry ions, and can therefore have different compensation voltage (CV) values. These clusters can be filtered and lost while transmitting an ion of interest, leading to reduced sensitivity. As shown in Figure 2, the system may comprise a common gas supply to provide both the curtain gas and the throttle gas flows. The curtain gas outflow 326 from the curtain plate 319 aperture can be defined by the sum of the volumetric flow rates for the curtain gas and the throttle gas minus the volumetric flow rate through the gas conductance limiting aperture 329. The curtain gas outflow 326 is typically optimized for a given compound and set of conditions. Therefore, with the configuration illustrated in Figure 2, the curtain gas outflow 326 may be maintained constant regardless of what portion of the total flow is provided through passage 320 (curtain gas) or 320a (throttle gas), provided that the total volumetric flow rate is constant.

[0042] As the differential mobility spectrometer 302 is sealed, or at least partially sealed (no seal is perfect) to the mass spectrometer, or at least to the first vacuum chamber 327, The mass spectrometer can comprise a circular orifice to receive the ions 322 from the differential mobility spectrometer 302. This is enabled by the streamlines resulting from sealing the differential mobility spectrometer 302 to the mass spectrometer. The gas streamlines exiting the differential mobility spectrometer 302 converge on the orifice inlet 329, and these bending streamlines can transport ions through the inlet 329. It can be desirable to maintain a circular orifice to ensure high transmission efficiency through subsequent vacuum stages and lenses.

[0043] Referring to Figure 3, there is illustrated in a schematic view, a differential mobility spectrometer/mass spectrometer system 400 in accordance with an aspect of a third embodiment of the present invention. For clarity, the same reference numerals used in Figure 2, with 100 added, are used in Figure 3 to designate elements analogous to the elements of Figure 2. For brevity, the descriptions of Figures 1 and 2 are not repeated with respect to Figure 3. [0044] As with the system 300 of Figure 2, the resolution or selectivity of system 400 of Figure 3 can be adjusted by adding throttle gas to a juncture chamber 414 between the differential mobility spectrometer 402 and the vacuum chamber inlet 429. As with system 300 of Figure 2, a common source can be provided for both the curtain gas and the throttle gas. [0045] In addition, gas restriction plates 434 are provided at an inlet

410 of the differential mobility spectrometer 402. These gas restriction plates 434 can facilitate tuning the pressure of the differential mobility spectrometer 402 for further optimization of selectivity, in an analogous fashion to Nazarov et al. (Nazarov EG, Coy SL, Krylov EV, Miller AR, Eiceman G., Pressure Effects in Differential Mobility Spectrometry, Anal. Chem., 2006, 78, 7697- 7706). Specifically, when the gas restriction plates 434 are provided to restrict the flow of drift gas into the differential mobility spectrometer 402, pumping at the back of the differential mobility spectrometer 402, by providing the lower pressure in the vacuum chamber 427, can lower the pressure within the differential mobility spectrometer to provide an extra degree of selectivity or an extra parameter to adjust for tricky separations. The diameter of the aperture in the gas restriction plate 434 can be adjustable to allow an operator to tune the pressure within the differential mobility spectrometer 402 for the vacuum draw established with a fixed mass spectrometer inlet diameter.

[0046] Referring to Figure 4, there is illustrated in a schematic view a differential mobility spectrometer/mass spectrometer system 500 in accordance with an aspect of a fourth embodiment of the present invention.

For clarity, the same reference numerals used in Figure 1 , with 300 added, are used in Figure 4 to designate elements analogous to the elements of

Figure 1. For brevity, the descriptions of Figure 1 to 3 are not repeated with respect to Figure 4.

[0047] The system 500 of Figure 4 is a hybrid of the systems 200 and

300 of Figures 1 and 2 respectively. That is, similar to the system 200 in Figure 1 , the curtain gas supply 520 provides a curtain gas to the curtain chamber 518, and a controlled leak 532 is provided from the curtain chamber 518 into the juncture chamber 514. However, similar to the system 300 of Figure 2, the flow of throttle gas into the juncture chamber 514 of the system 500 of Figure 4 can be controlled independently of the pressure of the curtain gas within the curtain chamber 518 by adjusting gas flow restrictors 533. That is, gas flow restrictors 533 can be adjusted to adjust the size of controlled leak 532, thereby adjusting the amount of throttle gas sucked into the juncture chamber 514, without mechanically breaking the seal with the mass spectrometer 504. In contrast, the controlled leak 232 of the system 200 of Figure 1 is provided by mechanically altering the leak provided by the seal with the mass spectrometer as shown in Figure 1. Specifically, baffles 216 of the system 200 of Figure 1 are adjustable to control the size of the leak shown in Figure 1. [0048] Referring to Figure 5, there is illustrated in a schematic view, a differential mobility spectrometer/mass spectrometer system 600 in accordance with an aspect of a fifth embodiment of the invention. For clarity the same reference numerals used in Figure 2, with 300 added are used in Figure 5 to designate elements analogous to the elements of Figure 2. For brevity, the descriptions of Figures 1 and 2 are not repeated with respect to Figure 5.

[0049] In a system 600 of Figure 5, a heated tube 602a is installed at the inlet 610 of the differential mobility spectrometer 602. The heated tube 602a can be sealed to the inlet 610 of the differential mobility spectrometer 602. The heated tube 602a can facilitate additional declustering of the ions 622 prior to the ions 622 entering the differential mobility spectrometer. During use with high flow rate high performance liquid chromatography (HPLC), for example clusterring can be a problem that decreases sensitivity. This additional declustering can also help with other ion sources, such as atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI), atmospheric pressure chemical ionization (APCI), Desorption electrospray ionization (DESI), and Direct Analysis in Real Time (DART), for example. While these sources have been provided as examples, it will be apparent to those of skill in the art that this approach can improve performance for any ionization source that generates ions as well as clusters. The heated tube 602a can also facilitate laminar flow conditions, and increase the uniformity of the electric field at the inlet 610 of the differential mobility spectrometer 602, and by doing so can facilitate ion transmission into the differential mobility spectrometer 602.

[0050] In some embodiments of the present invention, such as the system 600 illustrated in Figure 5, increases in the volumetric flow of throttle gas into the juncture chamber 614 can be balanced by corresponding reductions in the volumetric flow rate of curtain gas into the curtain chamber 618 . For example, the total rate of flow of, say, nitrogen, into both the curtain chamber and the juncture chamber (the curtain gas flow rate and the throttle gas flow rate respectively) can be kept substantially constant by balancing changes in one of the curtain gas flow rate and the throttle gas flow rate with opposite changes in the other of these flow rates. This can be desirable.

[0051] Specifically, if the flow of throttle gas into the juncture chamber is increased while the flow of curtain gas into the curtain chamber is kept constant, then the outflow of curtain gas away from the inlet of the differential mass spectrometer can be expected to increase. This can be undesirable. That is, as shown in Figure 5 for example, a particular outflow 626 of curtain gas, in the opposite direction to the flow of gas through the differential mobility spectrometer and into the mass spectrometer, may have been selected for a particular group of ions sharing a common m/z to decluster these ions. Thus, the curtain gas outflow rate desired may depend on the group of ions of interest and what counterflow is desired to help to decluster them. If the flow of throttle gas into the juncture chamber 614 is increased, then, other things equal, the outflow 626 from the boundary member 619 can also be expected to increase beyond what was selected, which can be undesirable. Accordingly, in some embodiments it can be desirable to reduce the inflow of the curtain gas into the curtain chamber proportionally to balance an increase in the inflow of throttle gas into the juncture chamber. [0052] Figure 5 illustrates one way in which this can be achieved.

Specifically, for system 600 a common source is shown for both the curtain gas and the throttle gas. Thus, for example, if a greater flow from this common source is used to increase the rate at which throttle gas flows into the juncture chamber, then there may be a corresponding reduction in the flow rate of the curtain gas from this common source into the curtain chamber. This reduction of the flow rate of curtain gas into the curtain chamber can help to balance the increase in the rate of flow of throttle gas into the juncture chamber, such that the outflow 626 of curtain gas away from the inlet of differential mass spectrometer 602 is substantially unchanged. [0053] This balancing of increases in throttle gas flow with proportional decreases in curtain gas flow can also be achieved using other means in connection with other embodiments of the present invention. For example, in the case of the system 500 of Figure 4, increasing the curtain gas flow rate into the curtain chamber 518 could, on its own, also increase the rate at which throttle gas flows into the juncture chamber 514, resulting in a significant increase in the outflow 526 from the boundary member 519 relative to the outflow initially selected. However, this effect can be overcome, and the rate at which throttle gas flows into the juncture chamber even reduced, by adjusting gas flow restrictors 533 to reduce the flow of throttle gas into the juncture chamber 514 via controlled leak 532. Of course, where the throttle gas flow rate decreases, the curtain gas flow into the curtain chamber can be proportionally increased, to maintain the outflow of curtain gas away from the inlet of the differential mass spectrometer substantially constant.

[0054] Referring to Figure 6, there is illustrated in a schematic view a differential mobility spectrometer/mass spectrometer system 700 in accordance with an aspect of a sixth embodiment of the present invention. For clarity, the same reference numerals used in Figure 2, with 400 added, are used in Figure 6 to designate elements analogous to the elements of Figure 2. For brevity the descriptions of preceding Figures, including Figures 1 and 2, are not repeated with respect to Figure 6. [0055] As shown in Figure 6, the system 700 is quite similar to the system 300 of Figure 2. However, the system 700 of Figure 6 comprises additional elements. Specifically, as with the system 300 of Figure 2, a curtain gas supply 720 comprises a controllable valve 720b that can be used to control the rate of flow of the throttle gas into the juncture chamber 714 via conduit branch 720a. Conduit or curtain gas supply 720 also comprises a valve 720c for controlling the rate of flow of the curtain gas that will ultimately end up in the curtain chamber 718. The flow of the curtain gas downstream of valve 720c is divided into two branches 72Od and 72Oe. The flow of the curtain gas within branch 72Od is controlled by valve 72Of. Similarly, the flow of the curtain gas within branch 72Oe is controlled by valve 72Og. [0056] The flow of the curtain gas through branch 72Od passes into a bubbler 72Oh, which can be used to add a modifier liquid to the curtain gas/drift gas, which passes through branch 72Od and will ultimately be pumped into the differential mobility spectrometer 702 by the vacuum maintained in the vacuum chamber 727. Similarly, a separate modifier can be added to the curtain gas flowing through branch 72Oe in bubbler 72Oi. The curtain gas outflows from the bubblers 72Oh and 72Oi can be controlled by outlet valves 72Oj and 720k respectively, after which the two branches 72Od and 72Oe merge and then release the curtain gas with the modifiers into the curtain chamber 718. As noted above, the curtain gas and drift gas are one and the same; thus, adding the modifiers to the curtain gas adds simplicity to the system 700. Modifiers can be vapors that provide selectivity by clustering with ions to different degrees, thereby shifting the differential mobility. Examples of modifiers can include alcohols such as isopropyl alcohol, water, as well as hydrogen and deuterium exchange agents, such as deuterated water or methanol, which can be used, amongst other things, to count the number of exchangeable protons on a molecule. In general, a modifier may be defined as any additive to the drift gas that changes the observed compensation voltage for a peak at a given AC amplitude. The compensation voltage is related to the ratio of high to low field mobility. Modifiers can act in other ways as well as clustering phenomena. For instance, changing the polarizability of the drift gas can also change the observed compensation voltage. Clustering and polarizability changes are two examples of mechanisms that modifiers may use to change compensation voltage optima; however, there may also be many other mechanisms.

[0057] Referring to Figure 7, there is illustrated in a schematic view a differential mobility spectrometer/mass spectrometer system 800 in accordance with an aspect of a seventh embodiment of the present invention. For clarity, the same reference numerals used in Figure 6 with 100 added are used in Figure 7 to designate elements analogous to the elements of Figure 6. For brevity the descriptions of preceding figures, including Figures 1 , 2 and 6, are not repeated with respect to Figure 7. [0058] As shown in Figure 7, the system 800 is very similar to the system 700 of Figure 6. However the system 800 of Figure 7 comprises an additional conduit branch 821. Conduit branch 821 can provide curtain gas, nitrogen in the present case, directly to the curtain chamber 818 without passing through bubblers 82Oh and 82Oi for modifier liquids to be added. Alternatively conduit branch 821 can provide curtain gas directly to chamber 818, while an additional gas fraction can also be added containing one or more modifiers.

[0059] Referring to Figure 8, there is illustrated in a schematic view a differential mobility spectrometer/mass spectrometer system 900 in accordance with an aspect of an eighth embodiment of the present invention.

For clarity, the same reference numerals used in Figure 2, with 600 added, are used in Figure 8 to designate elements analogous to the elements of

Figure 2. For brevity, the descriptions of preceding figures, including Figures 1 and 2 are not repeated with respect to Figure 8.

[0060] As shown in Figure 8, the system 900 is quite similar to the system 300 in Figure 2. However, the system 900 in Figure 8 comprises additional elements. Specifically, as with the system 300 of Figure 2, a curtain gas supply 920 comprises a controllable valve 920c for controlling the rate of flow of the curtain gas into curtain chamber 918. However, unlike the embodiments of Figures 2 and 7, separate sources are provided for the curtain gas and the throttle gas. Specifically, the system 900 further comprises a throttle gas source 940 that divides into two branches 942 and 944. The flow of the throttle gas within conduit branch 942 is controlled by controllable valve 946. Similarly, the flow of the throttle gas within branch 944 is controlled by valve 948. Optionally, as described below, auxiliary supplies for supplying auxiliary substances to the juncture chamber via the gas port 932 can be provided.

[0061] The flow of throttle gas through branch 942 passes into a bubbler 950, which can be used to add a modifier liquid to the throttle gas passing through branch 942. Similarly, a separate liquid modifier can be added to the throttle gas flowing through branch 944 by bubbler 952. The throttle gas/liquid modifier outflows from the bubblers 950 and 952 can be controlled by outlet valves 954 and 956 respectively, after which the two branches 942 and 944 merge into common branch 958. The flow of the throttle gas and modifier liquids added by bubblers 950 and 952 through conduit 958 and eventually into juncture chamber 914 can be controlled by controllable valve 960.

[0062] The various controllable valves 946, 948, 954 and 956 enable liquid modifiers to be added to the throttle gas by bubblers 950 and 952 in a controlled manner to facilitate selectivity by clustering and reacting ions to different degrees thereby shifting their masses observed in the mass spectrometer 904. As described above, the modifiers added may also include hydrogen and deuterium exchange agents, such as deuterated water or methanol, used, amongst other things, to count the number of exchangeable protons on the ions prefiltered with the differential mobility spectrometer.

[0063] In the differential mobility spectrometer/mass spectrometer systems of Figures 1 to 8, the differential mobility spectrometers can be dimensioned to provide, initially, a relatively short residence time for the ions within the differential mobility spectrometer, and a relatively high gas flow rate of the drift gas within the differential mobility spectrometer. This initial bias of the differential mobility spectrometer in favor of sensitivity at the expense of selectivity can be subsequently offset by providing a throttle gas to the juncture chambers as described above to decrease the flow rate of the drift gas. In the aspects of the embodiments illustrated in Figure 9, the opposite approach is taken.

[0064] Referring to Figure 9, there is illustrated in a schematic view a differential mobility spectrometer/mass spectrometer system 1000 in accordance with an aspect of ninth embodiment of the present invention. For clarity, the same reference numerals used in Figure 1 , with 800 added, are used in Figure 9 to designate elements analogous to the elements of Figure 1. For brevity, the description of preceding figures, including Figure 1 , are not repeated with respect to Figure 9.

[0065] As shown in Figure 9, the system 1000 is quite similar to the systems 200 and 300 of Figures 1 and 2 respectively. However, instead of adding a throttle gas to the juncture chamber 1014, the system 1000 of Figure 9 comprises a gas outlet 1032 including a vacuum pump 1040 for drawing a bleed gas out of the juncture chamber 1014. As the quantity of bleed gas drawn out of the juncture chamber 1014 increases, the gas flow rate of the drift gas 1008 within the differential mobility spectrometer 1002 will increase, which can diminish selectivity and resolution, while, at the same time, increasing sensitivity. For this reason, in the system 1000 of Figure 9, the differential mobility spectrometer 1002 can be dimensioned to provide, initially, a relatively long residence time for the ions 1022 within the differential mobility spectrometer 1002, and a relatively low gas flow rate of the drift gas 1008 within the differential mobility spectrometer 1002. This initial bias of the differential mobility spectrometer 1002 in favor of selectivity at the expense of sensitivity can be subsequently offset by increasing a vacuum draw provided by vacuum pump 1040 to increase the rate at which bleed gas is drawn from the juncture chamber 1014 to increase the flow rate of the drift gas. [0066] The bleed gas may also be useful for DMS/MS systems where the mass spectrometer inlet is sized to provide either a discontinuous gas flow into vacuum, or a very low gas flow rate. As known in the art, a very small diameter orifice can provide a very low gas flow rate into the vacuum system, and an inlet diaphragm or adjustable orifice dimension may provide a discontinuous or variable gas flow into the mass spectrometer vacuum system. Under these conditions, as described below in more detail, the bleed gas draw can provide a continuous flow of carrier gas through the DMS cell regardless of the flow rate into the vacuum system of the mass spectrometer system. [0067] For example, drawing a bleed gas from the juncture of a differential mobility spectrometer and a mass spectrometer can be used to match the higher flow capacity of the differential mobility spectrometer with the lower flow capacity of a low-flow, low-cost, portable mass spectrometer. Because pumping capacity can be the primary limitation in reducing the size and weight of a mass spectrometer, this pumping capacity can be sacrificed to provide a smaller mass spectrometer. To compensate for this lower pumping capacity, a shutter can be provided at the orifice or inlet to the vacuum chamber. This shutter might have a duty cycle of, say, 1%, so that it is open for 10 milliseconds, and then closed for one second (1000 milliseconds), to reduce the load on the vacuum pump. [0068] However, the flows through the differential mobility spectrometer can be, and preferably are, continuous. Thus, to avoid turbulence or other problems, as shown in Figure 9 a bleed gas can be drawn from the juncture of the differential mobility spectrometer with the mass spectrometer. By extracting gas from this juncture region, a high fraction of the differential mobility spectrometer-filtered ions can enter the vacuum chamber inlet, while excess flow through the differential mobility spectrometer can be exhausted as bleed gas. This bleed gas flow can prevent or reduce turbulence in the differential mobility spectrometer and maintain a constant differential mobility spectrometer resolution. [0069] Referring to Figure 10, there is illustrated in a series of graphs, the effect of throttle gas on the resolution and peak width for a sample containing ephedrine and pseudoephedrine. The differential mobility spectrometer electrode dimension was 1x10x300mm. RF was set to approximately 4000 V peak to peak amplitude. In each graph, CV voltage is plotted on the x axis, while original signal intensity is plotted on the y axis normalized to the signal intensity achieved with no throttle gas provided.

[0070] As shown the compensation voltage peak width decreases

(improved selectivity) as more throttle gas is added to the juncture chamber, while sensitivity correspondingly diminishes. That is, the top trace in Figure 10, trace 100, shows that data generated when the DMS electrode set is optimized for sensitivity, and no throttle gas is provided. In this case, the mobility peaks for pseudoephedrine and ephedrine are merged to give a single peak with very broad half width. As the volumetric flow of throttle gas increases, from the trace 102 representing a throttle gas flow rate of 0.4L/min, to trace 104, representing a throttle gas flow rate of 0.8 L/min two distinct peaks begin to become apparent as a result of the selectivity improvement achieved by narrowing each of the mobility peaks. When the throttle gas is set to approximately 1.4 L/min, the peak centers are sufficiently resolved to achieve separation of these two components, although the sensitivity has decreased by approximately a factor of 2. Further increases in the throttle gas flow provide higher resolution, although the sensitivity loss also becomes greater, as shown in trace 108, representing a throttle gas flow rate of 1.8L/min.

[0071] Accordingly, according to some aspects of these embodiments of the present invention, a throttle gas can be added to the juncture chamber until an acceptable compromise between sensitivity and selectivity is reached, such that sensitivity remains at a level to enable the peaks to be discerned, while selectivity has been improved to enable the peaks to be readily distinguished.

[0072] According to some aspects of some other embodiments of the present invention, in which no throttle gas is provided, but instead a bleed gas is drawn from the juncture chamber, the initial mass spectrum obtained may show peaks that are distinguishable, but which represent very faint signals, given the loss of sensitivity due to the very high residence times within the differential mobility spectrometer. According to these aspects of the present invention, increasing amounts of bleed gas can be drawn from the juncture chamber to increase the gas flow rate through the differential mobility spectrometer, thereby reducing the residence time of ions within the differential mobility spectrometer (the electrode geometry having been selected to provide this long residence time). As this occurs, the peak height will increase, representing the greater sensitivity, but may also become broader and overlap. By observing this process, an operator can stop increasing the bleed gas flow rate at a point where the peaks are still readily distinguishable and sensitivity is still acceptable.

[0073] According to some aspects of various embodiments of the present invention, a method of operating mass spectrometer systems as defined above is provided in which the differential mobility spectrometer is maintained at an internal operating pressure (the curtain chamber operating pressure), while the mass spectrometer is maintained at a vacuum pressure that is substantially lower than the internal operating pressure. The differential mobility spectrometer is also in fluid communication with the mass spectrometer to draw a gas flow, including ions provided to the differential mobility spectrometer, through the differential mobility spectrometer and into a vacuum chamber containing the mass spectrometer. A gas flow between the differential mobility spectrometer and the mass spectrometer can be modified to change the gas flow rate within the differential mobility spectrometer without changing the total volumetric flow rate into the mass spectrometer. As described above, this gas flow rate can be modified, for example, by adding a throttle gas at a throttle gas flow rate to the gas flow between the differential mobility spectrometer and the mass spectrometer to decrease the gas flow rate through the differential mobility spectrometer. Optionally, the throttle gas flow rate can be varied to vary the decreases in the gas flow rate.

[0074] Optionally the method further comprises detecting the ions drawn into the mass spectrometer to provide a mass spectrum. Initially, the electrode geometry of the differential mobility spectrometer may be selected to provide good sensitivity but poor selectivity. Then, an operator can select a selected resolution for the mass spectrum and determine and then adjust the gas flow rate to provide the selected resolution. The operator can then vary the throttle gas flow rate to decrease the gas flow rate to provide the adjusted gas flow rate to provide the selected resolution for the mass spectrum, by increasing a residence time of the ions within the differential mobility spectrometer. This can also have the result of decreasing sensitivity somewhat, however. [0075] Optionally, an outlet of the differential mobility spectrometer can be connected to an inlet of the mass spectrometer to define an ion path of travel for ions therebetween using a juncture chamber. In such embodiments, the throttle gas can be directed into the juncture chamber and away from the ion path of travel to reduce disruption of the ion path of travel by the throttle gas. Alternatively, the throttle gas can simply be dispersed throughout the juncture chamber.

[0076] Optionally, the selected resolution for the mass spectrum and the adjusted gas flow rate for providing this selected resolution can be determined substantially contemporaneously. For example, these steps can be performed substantially contemporaneously with the step of varying the throttle gas flow rate, whereby an operator can simply observe how the resolution of the mass spectrum changes (along with the sensitivity) as the throttle gas flow rate is increased. Then, after an operator reaches an acceptable resolution (while retaining acceptable sensitivity), the throttle gas flow rate can be maintained at a constant level, thereby determining the adjusted gas flow rate to provide the selected resolution of the mass spectrum.

[0077] According to aspects of other embodiments of the present invention, instead of supplying a throttle gas to a juncture chamber between the differential mobility spectrometer and the mass spectrometer, a bleed gas can be drawn from the gas flow between the differential mobility spectrometer and the mass spectrometer at a bleed gas flow rate to increase a gas flow rate through the differential mobility spectrometer. The bleed gas flow rate can be varied to vary the increase in the gas flow rate. That is, in embodiments in which a bleed gas is drawn from the gas flow between the differential mobility spectrometer and the mass spectrometer, an electrode geometry of the differential mobility spectrometer can initially be selected to provide good selectivity at the price of poor or very poor sensitivity. Then, sensitivity can be improved, while selectivity is diminished, by increasing the bleed gas flow rate of the bleed gas drawn from the gas flow between the differential mobility spectrometer and the mass spectrometer.

[0078] According to some aspects of some embodiments of the present invention, an operator can determine a selected transmission sensitivity, determine an adjusted gas flow rate to provide the selected transmission sensitivity, and vary the bleed gas flow rate to provide the increase in the gas flow rate to provide the adjusted gas flow rate to provide the selected transmission sensitivity. Optionally, the steps can be performed altogether. That is, an operator can gradually increase a vacuum pump speed connected to the juncture chamber to increase the bleed gas flow rate, observing at the same time from the mass spectrum how the selected transmission sensitivity improves. Then, once an acceptable transmission sensitivity has been reached (and while selectivity is still acceptable) the bleed gas flow rate can be maintained to provide the adjusted gas flow rate to provide the selected transmission sensitivity. For example, for a given separation, an operator may try to optimize the sensitivity by seeing how much selectivity is required to eliminate an interference, and then maximizing the sensitivity while still removing the interference.

[0079] While the Applicant's teachings are described in conjunction with various embodiments, it is not intended that the Applicant's teachings be limited to such embodiments. On the contrary, the Applicant's teachings encompass various alternatives, modifications and equivalents as will be appreciated by those of skill in the art. For example, instead of a curtain wall other boundary members could be provided, such as a tube-like structure that directs the flow of the curtain gas to the inlet of the mass spectrometer. In addition to heating devices mounted in front of the DMS, heating devices may also be mounted elsewhere such as in the ion source, in the curtain chamber or around the mass spectrometer inlet, and the curtain gas and/or throttle gas may also be heated to facilitate declustering. Also, with respect to embodiments of the present invention such as that shown in Figure 6, where bubblers are either auxiliary supply elements or provided to add auxiliary substances to the throttle gas provided to the juncture chamber, these auxiliary substances need not be limited to liquid modifiers. For example, mass calibrants can also be added, as can exchange reagents. The mass calibrants added can be used to calibrate mass analysis, while the exchange reagents added can react with the ions based on the structure of the ions to provide additional information in the mass spectrum. All such alternatives, modifications and equivalents are believed to be within the sphere and scope of the invention as defined by the claims appended hereto.

Claims

Claims:

1. A mass spectrometer system comprising: a differential mobility spectrometer for receiving ions from an ion source, the differential mobility spectrometer having an internal operating pressure, electrodes, and at least one voltage source for providing DC and RF voltages to the electrodes; a mass spectrometer at least partially sealed to, and in fluid communication with, the differential mobility spectrometer for receiving the ions from the differential mobility spectrometer; a vacuum chamber surrounding the mass spectrometer for maintaining the mass spectrometer at a vacuum pressure lower than the internal operating pressure, such that the vacuum chamber is operable to draw a gas flow including the ions through the differential mobility spectrometer and into the vacuum chamber; and, a gas port for modifying a gas flow rate through the differential mobility spectrometer, the gas port being located between the differential mobility spectrometer and the mass spectrometer.

2. The mass spectrometer system as defined in claim 1 further comprising a boundary member; a curtain gas supply for providing a curtain gas directed by the boundary member to an inlet of the differential mobility spectrometer to dry and decluster the ions; and, at least one heater for heating at least one of the curtain gas, the throttle gas and a heated zone upstream of the inlet of the differential mobility spectrometer to decluster the ions.

3. The mass spectrometer system as defined in claim 1 wherein the gas port comprises a gas outlet for drawing a gas outflow from the gas flow located between the differential mobility spectrometer and the mass spectrometer to increase the gas flow rate.

4. The mass spectrometer system as defined in claim 3 wherein the gas outlet is adjustable to vary the gas outflow from the gas flow and vary the increase in the gas flow rate.

5. The mass spectrometer system as defined in claim 1 wherein the gas port comprises a gas inlet for adding a throttle gas to the gas flow between the differential mobility spectrometer and the mass spectrometer to reduce the gas flow rate.

6. The mass spectrometer system as defined in claim 5 wherein the gas inlet is adjustable to vary an inflow of the throttle gas to vary the decrease in the gas flow rate.

7. The mass spectrometer system as defined in claim 6 further comprising a juncture chamber connecting an outlet of the differential mobility spectrometer to an inlet of the mass spectrometer to define an ion path of travel therebetween, the gas port being located in the juncture chamber.

8. The mass spectrometer system as defined in claim 7 wherein the outlet of the differential mobility spectrometer is aligned with the inlet of the mass spectrometer to transmit the ions substantially along the path of travel to the inlet of the mass spectrometer; the juncture chamber comprises a side wall spaced from the path of travel; and, the gas port is oriented to direct the throttle gas along the side wall and away from the ion path of travel.

9. The mass spectrometer system as defined in claim 7 wherein the juncture chamber comprises a randomizer member; and, the gas port is oriented to direct the throttle gas at the randomizer member to disperse the throttle gas throughout the juncture chamber.

10. The mass spectrometer system as defined in claim 7 further comprising an auxiliary supply for supplying an auxiliary substance to the juncture chamber, the auxiliary supply being connected to the gas port.

11. A method of operating a mass spectrometer system including a differential mobility spectrometer and a mass spectrometer contained in a vacuum chamber at least partially sealed to, and in fluid communication, with, the differential mobility spectrometer, the method comprising: a) maintaining the differential mobility spectrometer at an internal operating pressure; b) providing ions to the differential mobility spectrometer, and providing a selected separation RF voltage and a selected compensation DC voltage to the differential mobility spectrometer; c) maintaining the mass spectrometer at a vacuum pressure lower than the internal operating pressure to draw a gas flow including the ions through the differential mobility spectrometer and into the vacuum chamber; and, d) modifying the gas flow between the differential mobility spectrometer and the mass spectrometer to change a gas flow rate through the differential mobility spectrometer.

12. The method as defined in claim 11 wherein modifying the gas flow between the differential mobility spectrometer and the mass spectrometer to change the gas flow rate through the differential mobility spectrometer comprises adding a throttle gas at a throttle gas flow rate to the gas flow between the differential mobility spectrometer and the mass spectrometer to decrease a gas flow rate through the differential mobility spectrometer.

13. The method as defined in claim 12 wherein d) further comprises varying the throttle gas flow rate to vary the decrease in the gas flow rate.

14. The method as defined in claim 13 further comprising detecting the ions drawn into the mass spectrometer to provide a mass spectrum; selecting a selected resolution for the mass spectrum; determining an adjusted gas flow rate to provide the selected resolution; and, varying the throttle gas flow rate to provide the decrease in the gas flow rate to provide the adjusted gas flow rate to provide the selected resolution.

15. The method as defined in claim 13 further comprising connecting an outlet of the differential mobility spectrometer to an inlet of the mass spectrometer to define an ion path of travel therebetween using a juncture chamber, wherein d) comprises directing the throttle gas into the juncture chamber and away from the ion path of travel.

16. The method as defined in claim 13 further comprising connecting an outlet of the differential mobility spectrometer to an inlet of the mass spectrometer to define an ion path of travel therebetween using a juncture chamber, wherein d) comprises dispersing the throttle gas throughout the juncture chamber.

17. The method as defined in claim 14 wherein selecting the selected resolution for the mass spectrum and determining the adjusted gas flow rate to provide the selected resolution are substantially contemporaneous.

18. The method as defined in claim 15 further comprising providing an auxiliary substance to the juncture chamber.

19. The method as defined in claim 18 wherein the auxiliary substance comprises at least one of the modifier liquid, a mass calibrant for calibrating mass analysis, an exchange reagent for reacting with at least some of the ions,

20. The method as defined in claim 11 wherein modifying the gas flow between the differential mobility spectrometer and the mass spectrometer to change the gas flow rate through the differential mobility spectrometer comprises drawing a bleed gas at a bleed gas flow rate from the gas flow between the differential mobility spectrometer and the mass spectrometer to increase a gas flow rate through the differential mobility spectrometer.

21. The method as defined in claim 20 wherein d) further comprises varying the bleed gas flow rate to vary the increase in the gas flow rate.

22. The method as defined in claim 21 further comprising determining a selected transmission sensitivity; determining an adjusted gas flow rate to provide the selected transmission sensitivity; and, varying the bleed gas flow rate to provide the increase in the gas flow rate to provide the adjusted gas flow rate to provide the selected transmission sensitivity.

23. The method as defined in claim 22 wherein selecting the transmission sensitivity and determining the adjusted gas flow rate to provide the selected transmission sensitivity are substantially contemporaneous.

24. The method as defined in claim 11 wherein the vacuum chamber comprises a vacuum chamber inlet for receiving the ions from the differential mobility spectrometer, the method further comprising: providing an inlet DC potential to the vacuum chamber inlet and the sum of the inlet DC potential and a DMS DC offset voltage to the differential mobility spectrometer, in addition to the selected separation RF voltage and the selected compensation DC voltage.

25. The method as defined in claim 24 wherein the inlet DC potential provided to the vacuum chamber inlet is determined, and then the DMS DC offset voltage is adjusted relative to the inlet DC potential to enhance ion transmission from the differential mobility spectrometer into the vacuum chamber by changing an ion flow rate between the differential mobility spectrometer and the vacuum chamber inlet.

26. The method as defined in claim 24 further comprising changing the inlet DC potential provided to both the vacuum chamber inlet and the differential mobility spectrometer such that the potential of the differential mobility spectrometer tracks the potential of the vacuum chamber inlet, offset by the DMS DC offset voltage.

27. The method as defined in claim 12 further comprising: providing a curtain gas at a selected volumetric flow rate to an inlet of the differential mobility spectrometer to provide a gas flow through the differential mobility spectrometer and into the mass spectrometer, and a curtain gas outflow away from the inlet of the differential mobility spectrometer and outside the differential mobility spectrometer to decluster the ions; and adjusting the selected volumetric flow rate of the curtain gas inversely and proportionately with changes in the throttle gas flow rate to maintain a substantially constant rate of the curtain gas outflow.

28. The method as defined in claim 11 further comprising modifying the gas flow between the differential mobility spectrometer and the mass spectrometer, by drawing a bleed gas at a bleed gas rate from the gas flow between the differential mobility spectrometer and the mass spectrometer, to maintain a substantially constant flow through the differential mobility spectrometer.