Patent Application
20120161029
This invention relates to an ion transfer arrangement, for transporting ions within a mass spectrometer, and more particularly to an ion transfer arrangement for transporting ions from an atmospheric pressure ionisation source to the high vacuum of a mass spectrometer vacuum chamber.
Ion transfer tubes, also known as capillaries, are well known in the mass spectrometry art for the transport of ions between an ionization chamber maintained at or near atmospheric pressure and a second chamber maintained at reduced pressure. Generally described, an ion transfer channel typically takes the form of an elongated narrow tube (capillary) having an inlet end open to the ionization chamber and an outlet end open to the second chamber. Ions, together with charged and uncharged particles (e.g., partially desolvated droplets from an electrospray or APCI probe, or Ions and neutrals and Substrate/Matrix from a Laser Desorption or MALDI source) and background gas, enter the inlet end of the ion transfer capillary and traverse its length under the influence of the pressure gradient. The ion/gas flow then exits the ion transfer tube as a free jet expansion. The ions may subsequently pass through the aperture of a skimmer cone through regions of successively lower pressures and are thereafter delivered to a mass analyzer for acquisition of a mass spectrum.
There is a significant loss in existing ion transfer arrangements, so that the majority of those ions generated by the ion source do not succeed in reaching and passing through the ion transfer arrangement into the subsequent stages of mass spectrometry.
A number of approaches have been taken to address this problem. For example, the ion transfer tube may be heated to evaporate residual solvent (thereby improving ion production) and to dissociate solvent-analyte adducts. A counterflow of heated gas has been proposed to increase desolvation prior to entry of the spray into the transfer channel. Various techniques for alignment and positioning of the sample spray, the capillary tube and the skimmer have been implemented to seek to maximize the number of ions from the source that are actually received into the ion optics of the mass spectrometers downstream of the ion transfer channel.
It has been observed (see, e.g., Sunner et. al, J. Amer. Soc. Mass Spectrometry, V. 5, No. 10, pp. 873-885 (October 1994)) that a substantial portion of the ions entering the ion transfer tube are lost via collisions with the tube wall. This diminishes the number of ions delivered to the mass analyzer and adversely affects instrument sensitivity. Furthermore, for tubes constructed of a dielectric material, collision of ions with the tube wall may result in charge accumulation and inhibit ion entry to and flow through the tube. The prior art contains a number of ion transfer tube designs that purportedly reduce ion loss by decreasing interactions of the ions with the tube wall, or by reducing the charging effect. For example, U.S. Pat. No. 5,736,740 to Franzen proposes decelerating ions relative to the gas stream by application of an axial DC field. According to this reference, the parabolic velocity profile of the gas stream (relative to the ions) produces a gas dynamic force that focuses ions to the tube centerline.
Other prior art references (e.g., U.S. Pat. No. 6,486,469 to Fischer) are directed to techniques for minimizing charging of a dielectric tube, for example by coating the entrance region with a layer of conductive material connected to a charge sink.
Another approach is to “funnel” ions entering from atmosphere towards a central axis. The concept of an ion funnel for operation under vacuum conditions after an ion transfer capillary was first set out in U.S. Pat. No. 6,107,628 and then described in detail by Belov et al in J Am Soc Mass Spectrom 200, Vol 11, pages 19-23. More recent ion funneling techniques are described in U.S. Pat. No. 6,107,628, in Tang et al, “Independent Control of Ion transmission in a jet disrupter Dual-Channel ion funnel electrospray ionization MS interface”, Anal. Chem. 2002, Vol 74, p5431-5437, which shows a dual funnel arrangement, in Page et al, “An electrodynamic ion funnel interface for greater sensitivity and higher throughput with linear ion trap mass spectrometers”, Int. J. Mass Spectrometry 265 (2007) p244-250, which describes an ion funnel adapted for use in a linear trap quadrupole (LTQ) arrangement. Unfortunately, effective operation of ion funnel extends only up to gas pressures of approximately 40 mbar, i.e 4% of atmospheric pressure.
A funnel shaped device with an opening to atmospheric pressure is disclosed in Kremer et al, “A novel method for the collimation of ions at atmospheric pressure” in J. Phys D: Appl Phys. Vol 39(2006) p5008-5015, which employs a floating element passive ion lens to focus ions (collimate them) electrostatically. However, it does not address the issue of focusing ions in the pressure region between atmospheric and forevacuum.
Still another alternative arrangement is set out in U.S. Pat. No. 6,943,347 to Willoughby et al., which provides a stratified tube structure having axially alternating layers of conducting electrodes. Accelerating potentials are applied to the conducting electrodes to minimize field penetration into the entrance region and delay field dispersion until viscous forces are more capable of overcoming the dispersive effects arising from decreasing electric fields. Though this is likely to help reducing ion losses, actual focusing of ions towards the central axis would require ever increasing axial field which is becomes technically impossible at low pressures because of breakdown.
Yet other prior art references (e.g., U.S. Pat. No. 6,486,469 to Fischer) are directed to techniques for minimizing charging of a dielectric tube, for example by coating the entrance region with a layer of conductive material connected to a charge sink.
While some of the foregoing approaches may be partially successful for reducing ion loss and/or alleviating adverse effects arising from ion collisions with the tube wall, the focusing force is far from sufficient for keeping ions away from the walls, especially given significant space charge within the ion beam and significant length of the tube. The latter requirement appears from the need to desolvate clusters formed by electrospray or APCI ion source. In an alternative arrangement, the tube could be replaced by a simple aperture and then desolvation region must be provided in front of this aperture. However, gas velocity is significantly lower in this region than inside the tube and therefore space charge effects produce higher losses. Therefore, there remains a need in the art for ion transfer tube designs that achieve further reductions in ion loss and are operable over a greater range of experimental conditions and sample types.
Against this background, and in accordance with a first aspect of the present invention, there is provided
an ion transfer arrangement for transporting ions between a relatively high pressure region and a relatively low pressure region, comprising:
an ion transfer conduit having an inlet opening towards a relatively high pressure chamber, an outlet opening towards a relatively low pressure chamber, and at least one sidewall surrounding an ion transfer channel, the sidewall extending along a central axis between the inlet end and the outlet end; and
a plurality of apertures formed in the longitudinal direction of the sidewall so as to permit a flow of gas from within the ion transfer channel to a lower pressure region outside of the sidewall of the conduit.
According to a second aspect of the present invention, there is provided method of transporting ions between a first, relatively high pressure region and a second, relatively low pressure region, comprising the steps of:
admitting, from the relatively high pressure region, a mixture of ions and gas into an inlet opening of an ion transfer conduit having or defining an ion transfer channel;
removing a portion of the gas in the ion transfer channel, through a plurality of passageways in a conduit wall located intermediate the inlet opening and an outlet opening of the ion transfer conduit; and
causing the ions and the remaining gas to exit the ion transfer conduit through the exit opening towards the relatively low pressure region.
In a simple form, an interface for a mass spectrometer in accordance with embodiments of the present invention includes an ion transfer tube having an inlet end opening to a high pressure chamber and an outlet end opening to a low pressure chamber. The high and low pressure chambers may be provided by any regions that have respective higher and lower pressures relative to each other. For example, the high pressure chamber may be an ion source chamber and the low pressure chamber may be a first vacuum chamber. The ion transfer tube has at least one sidewall surrounding an interior region and extending along a central axis between the inlet end and the outlet end. The ion transfer tube has a plurality of passageways formed in the sidewall. The passageways permit the flow of gas from the interior region to a reduced-pressure region exterior to the sidewall.
In another simple form, embodiments of the present invention include an ion transfer tube for receiving and transporting ions from a source in a high pressure region to ion optics in a reduced pressure region of a mass spectrometer. The ion transfer tube includes an inlet end, an outlet end, and at least one sidewall surrounding an interior region and extending along a central axis between the inlet end and the outlet end. The ion transfer tube may also include an integral vacuum chamber tube at least partially surrounding and connected to the ion transfer tube. The integral vacuum chamber tube isolates a volume immediately surrounding at least a portion of the ion transfer tube at a reduced pressure relative to the interior region. The sidewall has a structure that provides at least one passageway formed in the sidewall. The at least one passageway permits a flow of gas from the interior region to the volume exterior to the sidewall. The structure and passageway are inside the integral vacuum chamber tube. The structure of the sidewall may include a plurality of passageways.
In still another simple form, embodiments of the present invention include a method of transporting ions from an ion source region to a first vacuum chamber. The method includes admitting from the ion source region, a mixture of ions and gas to an inlet end of an ion transfer tube. The method also includes removing a portion of the gas through a plurality of passageways located intermediate the inlet end and an outlet end of the ion transfer tube. The method further includes causing the ions and the remaining gas to exit the ion transfer tube through the outlet end into the first vacuum chamber. The method may also include sensing a reduction in latent heat in the ion transfer tube due to at least one of removal of the portion of the background gas and an associated evaporation, and increasing an amount of heat applied to the ion transfer tube through a heater under software or firmware control.
The embodiments of the present invention have the advantage of reduced flow of gas through an exit end of the ion transfer tube. Several associated advantages have also been postulated. For example, the reduced flow through the exit end of the ion transfer tube decreases the energy with which the ion bearing gas expands as it leaves the ion transfer tube. Thus, the ions have a greater chance of travelling on a straight line though an aperture of a skimmer immediately downstream. Also, reduction of the flow in at least a portion of the ion transfer tube may have the effect of increasing the amount of laminar flow in that portion of the ion transfer tube. Laminar flow is more stable so that the ions can remain focused and travel in a straight line for passage through the relatively small aperture of a skimmer. With gas being pumped out through a sidewall of the ion transfer tube, the pressure inside the ion transfer tube is reduced. Reduced pressure can cause increased desolvation. Furthermore, latent heat is removed with the gas is pumped out through the sidewall. Hence, more heat may be transferred through the ion transfer tube and into the sample remaining in the interior region resulting in increased desolvation and increased numbers of ions actually reaching the ion optics.
Further features and advantages of the present invention will be apparent from the appended claims and the following description.
a, 4b and 4c together show examples of envelopes of shaped embodiments for the ion entry region of
a, 9b, 9c and 9d show alternative embodiments of an ion transfer arrangement in accordance with the present invention.
A more detailed explanation of the configuration of components in the ion transfer arrangement 20 of
Ion transport is characteristically different in the different pressure regions in and surrounding the ion transport arrangement 20 of
Region 1. This is the region where entrance ion optics of MS1 is situated, with pressures below approx. 1-10 mbar. This region is not addressed by the present invention.
Region 5. This is the atmospheric pressure region and is mostly dominated by dynamic flow and the electrospray or other atmospheric pressure ionization source itself. As with Region 1, it is not directly addressed by the present invention.
This leaves Regions 2, 3 and 4.
Region 4: This is in the vicinity of the entrance orifice 30 to the ion transport arrangement 20.
Region 2: This is the region in which the conduit 60 is situated, which abuts the exit aperture 70 of the ion transport arrangement 20 into MS1. Finally,
Region 3: This is the region between the entrance orifice 30 (Region 4) of the ion transport arrangement 20, and Region 2 as described above.
Measurements of the ion current entering the ion transport arrangement (at the entrance orifice 30) of a typical commercially available capillary indicate that it is in the range of I0≈2.5 nA. Hence, knowing the incoming gas flow value Q=8 atm·cm3/S, and the inner diameter of the conduit of 0.5 mm, the range of the initial charge density □0 may be estimated as 0.3−1*10−9 C/cm3=(0.3 . . . 1)*10−3 C/m3. Knowing the dwell time of the ions inside the conduit, t=0.113 m/50 m/s·2*10−3 s, and the average ion mobility value at atmospheric pressure K=10−4 m2/s, the limit of the transmission efficiency because of the space charge repulsion can be determined from:
Thus to improve ion current (which is an aim of aspects of the present invention), the ion mobility and ion dwell time in the conduit are preferably optimized.
An essential part of the ion loss in an atmospheric pressure ionization (API) source takes place in the ionisation chamber in front of the entrance orifice 30 of the interface. This proportion of the ion loss is determined by the ion/droplet drift time from the Taylor cone of the API source to the entrance orifice 30. The gas flow velocity distribution in vicinity of the entrance orifice 30 is
where d is the diameter of the conduit, and R is the distance from the point to the entrance orifice 30, C is a constant and ΔP is pressure drop. The ion velocity is Vion=Vgas+KE, where K is the ion mobility, and E is the electrical field strength. Assuming that K˜10−4 m2/s, and E˜5·105 V/m, the velocity caused by the electrical field is ˜50 m/s. The gas flow velocity inside the 0.5 mm ID conduit is about the same value, but at a distance 5 mm from the entrance orifice 30, ions travelling with the gas are about 10 times slower than their drift in the electrical field. Hence, the ion dwell time in this region is in the range of 10−4 s, which results in an ion loss of about 50% because of space charge repulsion according to equation (2) above.
In other words, analytical consideration of the ion transfer arrangement suggests that space charge repulsion is the main ion loss mechanism. The main parameters determining the ion transmission efficiency are ion dwell time t in the conduit, and ion mobility K. Thus one way to improve ion transport efficiency would be to decrease t. However, there is a series of limitations on the indefinite increase of t:
1. The time needed to evaporate droplets;
2. The critical velocity at which laminar gas flow transforms into turbulent gas flow; and
3. The appearance of shock waves when the gas flow accelerates to the speed of sound. This is especially the case when a big pressure drop is experienced from regions 5 to 1 (1000 to 1 mbar approximately).
Returning now to
The first regions to consider are regions 4 and 3 which define, respectively, the vicinity of the entrance aperture 30 and the expansion chamber 40.
In order to address ion losses in front of the entrance orifice 30, it is desirable to increase the incoming gas flow into the entrance orifice 30. This is in accordance with the analysis above—for a given ion current, a higher gas flow rate at the entrance to the ion transport arrangement allows to capture larger volume of gas and, given that gas is filled with ions up to saturation, more ions. Decreasing the dwell time in regions 3 and 4 conditions the ion stream to a high but not supersonic velocity.
Thus improvements are possible in Regions 4 and 3, by optimising or including components between the API source 10 and the entrance to the conduit 60. Regions 4 and 3, which interface between Region 5 at atmosphere and Region 2, desirably provide a gas dynamic focusing of ions which are typically more than 4-10 times heavier than nitrogen molecules for most analytes of interest.
A first aim is to avoid a supersonic flow mode between regions 5 and 2, as this can cause an unexpected ion loss. This aim can be achieved by the use of an entrance funnel 48, located in the expansion chamber 40. Such a funnel 48 is illustrated in
The expansion chamber 40 is preferably pumped to around 300-600 mbar by a diaphragm, extraction or scroll pump (not shown) connected to a pumping port 45 of the expansion chamber. By appropriate shaping of the ion funnel 48, expansion of ions as they enter the expansion chamber 40 can be arranged so as to control or avoid altogether shock wave formation.
As shown in the above referenced paper by Sunner et. al, even at low spray currents, atmospheric pressure sources (e.g. electrospray or APCI) are space-charge limited. It has been determined experimentally by the present inventors that, even with application of the highest electric fields, API sources are not capable of carrying more than 0.1-0.5*10−9 Coulomb/(atm.cm3). To capture most of this current even for a nanospray source this requires that the entrance aperture 30 has a diameter of at least 0.6-0.7 mm and is followed by strong accelerating and focusing electric field (though it is necessary to keep the total voltage drop below the onset for electric breakdown).
As a development to the simple arrangement of
Various different shapes can be described by the array of plate electrodes 100: in the simplest case the funnel towards the conduit is just flared (linear taper). This is shown schematically in
Thus the effect of the arrangements of
A very simple example of jet separation, which is just one example for an aerodynamic lens is discussed below in connection with some of the embodiments in
As still further additions or alternatives to the arrangement of regions 4 and 3 of the preferred embodiment, the ion funnel 48 may include auxiliary pumping of a boundary layer at one or more points inside the channel, the pressure drop along the channel may be limited, and so forth. To sustain a strong electric field along such a funnel 48, these pumping slots could be used as gaps between thin plates at different potentials.
Referring again to
The conduit 60 located in the vacuum chamber 50 and defining region 2 of the ion transfer arrangement is formed from three separate components: a heater 110, a set of DC electrodes 120 and a differential pumping arrangement shown generally at 130 and described in further detail below. It is to be understood that these components each have their own separate function and advantage but that they additionally have a mutually synergistic benefit when employed together. In other words, whilst the use of any one or two of these three components results in an improvement to the net ion flow into MS1, the combination of all three together tends to provide the greatest improvement therein.
The heater 110 is formed in known manner as a resistive winding around a channel defined by the set of DC electrodes which extend along the longitudinal axis of the conduit 60. The windings may be in direct thermal contact with the channel 115, or may instead be separate therefrom so that when current flows through the heater 110 windings, it results in radiative or convective heating of the gas stream in the channel. Indeed in another alternative arrangement, the heater windings may be formed within or upon the differential pumping arrangement 130 so as to radiate heat inwards towards the gas flow in the channel 115. In still another alternative, the heater may even be constituted by the DC electrodes 120 (provided that the resistance can be matched)—regarding which see further below. Other alternative arrangements will be apparent to the skilled reader.
Heating the ion transfer channel 115 raises the temperature of the gas stream flowing through it, thereby promoting evaporation of residual solvent and dissociation of solvent ion clusters and increasing the number of analyte ions delivered to MS1 80.
Embodiments of the set of DC electrodes 120 will now be described. These may be seen in schematic form and in longitudinal cross section in
Referring to
It will be appreciated that, while
The electrodes are arranged with a period H (the spacing between successive LFE's or HFE's). The width (longitudinal extent) of HFE's 205 is substantially smaller than the width of the corresponding LFE's 210, with the HFE's typically constituting approximately 20-25% of the period H. The HFE width may be expressed as H/p, where p may be typically in the range of 3-4. The period H is selected such that ions traveling through ion transfer channel 115 experience alternating high and low field-strengths at a frequency that approximates that of a radio-frequency confinement field in conventional high-field asymmetric ion mobility spectrometry (FAIMS) devices. For example, assuming an average gas stream velocity of 500 meters/second, a period H of 500 micrometers yields a frequency of 1 megahertz. The period H may be maintained constant along the entire length of the tube, or may alternatively be adjusted (either in a continuous or step-wise fashion) along the channel length to reflect the variation in velocity due to the pressure gradient. The inner diameter (ID) of ion transfer channel 115 (defined by the inner surfaces of the LFE's 205 and HFE's 210) will preferably have a value greater than the period H.
One or more DC voltage sources (not depicted) are connected to the electrodes to apply a first voltage V1 to HFE's 205 and a second voltage V2 to LFE's 210. V2 has a polarity opposite to and a magnitude significantly lower than V1. Preferably, the ratio V1/V2 is equal to −p, where p (as indicated above) is the inverse of the fraction of the period H occupied by the LFE width and is typically in the range of 3-4, such that the space/time integral of the electric fields experienced by an ion over a full period is equal to zero. The magnitudes of V1 and V2 should be sufficiently great to achieve the desired focusing effect detailed below, but not so great as to cause discharge between adjacent electrodes or between electrodes and nearby surfaces. It is believed that a magnitude of 50 to 500 V will satisfy the foregoing criteria.
Application of the prescribed DC voltages to HFE's 205 and LFE's 210 generates a spatially alternating pattern of high and low field strength regions within the ion transfer channel 115 interior, each region being roughly longitudinally co-extensive with the corresponding electrode. Within each region, the field strength is at or close to zero at the flow centerline and increases with radial distance from the center, so that ions experience an attractive or repulsive radial force that increases in magnitude as the ion approaches the inner surface of the ion transfer tube. The alternating high/low field strength pattern produces ion behavior that is conceptually similar to that occurring in conventional high-field asymmetric ion mobility spectrometry (FAIMS) devices, in which an asymmetric waveform is applied to one electrode of an opposed electrode pair defining a analyzer region (see, e.g., U.S. Pat. No. 7,084,394 to Guevremont et al.)
As has been described in detail in the FAIMS art, the net movement of an ion in a viscous flow region subjected to alternating high/low fields will be a function of the variation of the ion's mobility with field strength. For A-type ions, for which the ion mobility increases with increasing field strength, the radial distance traveled in the high field-strength portion of the cycle will exceed the radial distance traveled during the low field-strength portion. For the example depicted in
The above-described technique of providing alternating DC fields may be inadequate to focus ions in regions where gas dynamic forces deflect the ions' trajectory from a purely longitudinal path or the mean free path becomes long enough (i.e., where collisions with gas atoms or molecules no longer dominate ion motion). For example, gas expansion and acceleration within ion transfer channel 115 due to the pressure differential between the API source 10 at atmospheric pressure and MS1 80 at high vacuum (<1 mbar) may cause one or more shock waves to be generated within the ion transfer channel interior near its outlet end, thereby sharply deflecting the ions' paths. For electrodes disposed at the distal portions of ion transfer channel 115, it may be necessary to apply an RF voltage (either with or in place of the DC voltage) to provide sufficient focusing to avoid ion-channel wall interactions. In this case, RF voltages of opposite phases will be applied to adjacent electrodes.
An alternative approach to suppress shock waves is to differentially pump the conduit 60 (
Generally we consider a flow as viscous as opposed to molecular flow when the mean free path of the ions is small compared to the dimensions of the device. In that case collisions between molecules or between molecules and ions play an important role in transport phenomena.
For devices according to the invention with a typical diameter of a few millimeters or up to a centimeter and an overall length of a few centimetres or decimeters, and a pressure gradient from approximately atmospheric pressure to pressures of about one hpa, we have viscous flow conditions throughout the inventive device.
Actually the viscous flow condition of the Knudsen number K=lambda/D being less than 1 we have viscous flow down to pressures of approx. 1 to 10 pa, depending on the analytes and dimensions (1 pa for small molecules like metabolites in a 1 mm diameter capillary).
Focusing/guide structure 300 is composed of a first plurality of ring electrodes (hereinafter “first electrodes”) 305 interposed in alternating arrangement with a second plurality of ring electrodes (hereinafter “second electrodes”) 310. Adjacent electrodes are electrically isolated from each other by means of a gap or insulating material or layer. In contradistinction to the embodiment of
In this arrangement as well as in the other inventive arrangements, the run length H is preferentially small, with dimensions around 0.1 to 20 mm, typically about 1 mm, such that the mean free path of ions is usually shorter than the relevant dimensions of the conduit.
As opposed to the arrangement of
A similar effect can be achieved by adjustment of the
In an alternative mode of operation the apparatus of
The arrangement of first and second electrodes of the focusing/guide structure may be modified to achieve certain objectives. For example,
Referring back to
As has been discussed, conventional inlet sections having atmospheric pressure ionization sources suffer from a loss of a majority of the ions produced in the sources prior to the ions entering ion optics for transport into filtering and analyzing sections of mass spectrometers. It is believed that high gas flow at an exit end of the ion transfer arrangement is a contributing factor to this loss of high numbers of ions. The neutral gas undergoes an energetic expansion as it leaves the ion transfer tube. The flow in this expansion region and for a distance upstream in the ion transfer tube is typically turbulent in conventional inlet sections. Thus, the ions borne by the gas are focused only to a limited degree in the ion inlet sections of the past. Rather, many of the ions are energetically moved throughout a volume of the flowing gas. It is postulated that because of this energetic and turbulent flow and the resultant mixing effect on the ions, the ions are not focused to a desirable degree and it is difficult to separate the ions from the neutral gas under these flow conditions. Thus, it is difficult to separate out a majority of the ions and move them downstream while the neutral gas is pumped away. Rather, many of the ions are carried away with the neutral gas and are lost. On the other hand, the hypothesis associated with embodiments of the present invention is that to the extent that the flow can be caused to be laminar along a greater portion of an ion transfer tube, the ions can be kept focused to a greater degree. One way to provide the desired laminar flow is to remove the neutral gas through a sidewall of the ion transfer tube so that the flow in an axial direction and flow out the exit end of the ion transfer tube is reduced. Also, by pumping the neutral gas out of the sidewalls to a moderate degree, the boundary layer of the gas flowing axially inside the ion transfer tube becomes thin, the velocity distribution becomes fuller, and the flow becomes more stable.
One way to increase the throughput of ions or transport efficiency in atmospheric pressure ionization interfaces is to increase the conductance by one or more of increasing an inner diameter of the ion transfer tube and decreasing a length of the ion transfer tube. As is known generally, with wider and shorter ion transfer tubes, it will be possible to transport more ions into the ion optics downstream. However, the capacity of available pumping systems limits how large the diameter and how great the overall conductance can be. Hence, in accordance with embodiments of the present invention, the inner diameter of the ion transfer channel 115 (
Even if it is found in some or all cases, that turbulent flow results in increased ion transport efficiency, it is to be understood that decreased pressure in a downstream end of the ion transfer channel and increased desolvation due to the decreased pressure may be advantages accompanying the embodiments of the present invention under both laminar and turbulent flow conditions. Furthermore, even with turbulent flow conditions, the removal of at least some of the neutral gas through the sidewall of the ion transfer tube may function to effectively separate the ions from the neutral gas. Even in turbulent flow, the droplets and ions with their larger masses will most likely be distributed more centrally during axial flow through the conduit 60. Thus, it is expected that removal of the neutral gas through the sidewalls will effectively separate the neutral gas from the ions with relatively few ion losses under both laminar and turbulent flow conditions. Still further, the removal of latent heat by pumping the neutral gas through the sidewalls enables additional heating for increased desolvation under both laminar and turbulent flow conditions.
Region 2 containing the conduit 60 is preferably pumped from pumping port 55. As may be seen in
A sensor may be connected to the ion transfer conduit 60 and to a controller 58 for sending a signal indicating a temperature of the sidewall or some other part of the ion transfer conduit 60 back to the controller 58. It is to be understood that a plurality of sensors may be placed at different positions to obtain a temperature profile. Thus, the sensor(s) may be connected to the ion transfer conduit 60 for detecting a reduction in heat as gas is pumped through the plurality of passageways 140 in the sidewall of the ion transfer conduit 60.
In an alternative arrangement, shown in
In an application of both external force and coulomb explosion disruption, both removal and addition of gas may be applied in one ion transfer tube. For example, as shown in
The wall of the differential pumping arrangement 130 in the embodiments of
As a further detail
The multiple pumping arrangement shown in
It will be noted from the introductory discussion above that the various parts of the ion transfer arrangement seek to keep the gas flow velocity upon exit from the conduit 60 to below supersonic levels so as to avoid shock waves. One consequence of this is that a skimmer is not necessary on the entrance into MS1 80—that is, the exit aperture 70 from Region 2 can be a simple aperture. It has been observed that the presence of a skimmer on the exit aperture can result in a reduction in ion current so the subsonic velocity of the gas leaving the conduit 60 in fact has a further desirable consequence (a skimmer is not needed).
Though most of the embodiments described above preferably employ ion transfer conduits of circular cross-section (i.e. a tube), the present invention is not limited to tubes. Other cross-sections, e.g. elliptical or rectangular or even planar (i.e. rectangular or elliptical with a very high aspect ratio) might become more preferable, especially when high ion currents or multiple nozzles (nozzle arrays) are employed. The accompanying significant increase in gas flow is compensated by the increase in the number of stages of differential pumping. This may for example be implemented by using intermediate stages of those pumps that are already employed.
Ion transfer channels described in this application lend themselves to be multiplexed into arrays, with adjustment of pumping as described above. Such an arrangement could become optimum for multi-capillary or multi-sprayer ion sources.
The present application is a continuation under 35 U.S.C. §120 and claims the priority benefit of co-pending U.S. patent application Ser. No. 12/513,944, filed May 7, 2009, which is a National Stage application under 35 U.S.C. §371 of PCT Application No. PCT/EP07/09642 filed on Nov. 7, 2007, which claims the priority benefit of U.S. Provisional Patent Application Ser. No. 60/857,737, filed Nov. 7, 2006. The disclosures of each of the foregoing applications are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12513944 | May 2009 | US |
| Child | 13413568 | US |
