Patent Application
20080142698
1. Field of the Invention
This invention generally relates to an ion source for a mass spectrometer, and more specifically to an ion transfer tube for transporting ions between regions of different pressure in a mass spectrometer.
2. Description of Related Art
Ion transfer tubes, also referred to as capillaries, are well-known in the mass spectrometry art for transporting ions from a spray chamber, which typically operates at or near atmospheric pressure, to a region of reduced pressure. Generally described, an ion transfer tube typically consists of a narrow elongated conduit having an inlet end open to the spray chamber, and an outlet end open to the reduced-pressure region. Ions formed in the spray chamber (e.g., via an electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) process), together with partially desolvated droplets and background gas, enter the inlet end of the ion transfer tube, traverse its length under the influence of the pressure gradient, and exit the outlet end as a supersonic expansion. The ions subsequently pass through an aperture in a skimmer cone through regions of successively lower pressures and are thereafter delivered to a mass analyzer for acquisition of a mass spectrum. The ion transfer tube may be heated to evaporate residual solvent (thereby improving ion production) and to dissociate solvent-analyte adducts.
The number of ions delivered to the mass analyzer (as measured by peak intensities or total ion count) is partially governed by the flow rate through the ion transfer tube. It is generally desirable to provide relatively high flow rates through the ion transfer tube so as to deliver greater numbers of ions to the mass analyzer and achieve high instrument sensitivity. The flow rate through the ion transfer tube may be increased by enlarging the tube bore (inner diameter). However, increasing the cross-sectional area through which the ions and gas are transported has a detrimental effect on the efficiency of heat transfer to the ion/gas flow. Enlargement of the ion transfer tube beyond a certain point achieves no further gains in sensitivity, because the benefit produced by increased flow rate is offset by significantly reduced desolvation/adduct dissociation rates. Of course, the heat transfer to the ion/gas flow may be increased by raising the tube wall temperature, but the maximum temperature at which the tube may be operated will be limited by material considerations, as well as the tendency of certain analyte molecules to undergo thermal dissociation.
U.S. Pat. Nos. 6,583,408 and 6,803,565 by Smith et al. disclose a mass spectrometer having a parallel arrangement of multiple heated capillaries for transporting ions from an ESI spray chamber to an ion funnel. The multiple capillary configuration enables both high flow rates and good heat transfer efficiencies. However, the ion/gas flows emerge from the exit ends of the capillaries as a geometrically complex set of multiple expansions, which (although suitable for use with the ion funnel) could not be easily interfaced to a conventional skimmer structure having a single aperture.
U.S. Pat. Application No. 2006/0186329 by Gebhardt et al. discloses an ion inlet of an ion source for a mass spectrometer having a multichannel plate that functions similar to the multiple heated capillaries of the Smith patents described above. That is, the multiple channels in the multichannel plate receive and guide ions and background gas and provide a large area entrance from the source into an ion funnel. Also in this case, the multichannel plate could not be easily interfaced with a conventional skimmer structure having a single aperture.
Another consideration is that with increased wall surface area in a multiple capillary or multichannel arrangement, more ions will be lost due to discharge when they come into contact with the wall surface area.
In view of the foregoing discussion, there is a need for an ion transfer tube that enables high flow rates while maintaining good heat transfer efficiency, and is capable of being interfaced to a conventional skimmer or similar structure.
In a simple form, a first embodiment of the invention includes a spray probe for introducing a spray of droplets of a sample solution into a first chamber and an ion transfer tube extending between the first chamber and a second chamber maintained at a reduced pressure relative to the first chamber. The ion transfer tube includes a first segment, and a second segment connected to the first segment. The first segment has an inlet end opening to the first chamber and the second segment has an outlet end opening to the second chamber. The first segment has a plurality of channels such that ions generated from the droplets are divided among the plurality of channels as the ions flow through the first segment. It is to be understood that the plurality of channels may be substantially parallel or may have other orientations relative to each other. The second segment has a common channel in fluid communication with each of the plurality of channels. The common channel may thus receive and carry a combined ion flow from the plurality of channels in the first segment. The ion transfer tube may have a heater structure associated therewith for heating at least a portion of the first segment in order to evaporate residual solvent flowing together with any associated gases through the ion transfer tube.
By dividing the ion flow among a plurality of channels in the first segment of the ion transfer tube, high ion/gas flow rates may be obtained without having a substantial adverse effect on heat transfer efficiency and consequent desolvation, thereby allowing relatively large numbers of ions to be delivered to a downstream mass analyzer. Further, by combining the ion/gas flow in a common channel in the second segment of the ion transfer tube, a single gas stream expansion is generated, which may be interfaced with a single aperture in a plate, or a skimmer structure.
The ion transfer tube 30 may be supported in one or more of the first chamber 15 and the second chamber 18. The ion transfer tube 30 is positioned so that the inlet end 27 is open to the interior of the first chamber 15. The ion transfer tube 30 also has an outlet end 33 that is open to the second chamber 18. Thus, ions, together with partially desolvated droplets and atoms or molecules of background gas (gas introduced into the first chamber 15 for nebulizing or focusing the droplets, solvent vapor, and ambient gases) are introduced into the inlet end 27 of the ion transfer tube 30, and traverse the length of the ion transfer tube 30. Thus, ions and background gas pass from the first chamber 15 at a relatively higher pressure through the ion transfer tube 30 and out the second end 33 into the second chamber 18 at a lower pressure.
The ion transfer tube 30 may be heated by a heater block 36. The heater block 36 may also be supported in the second chamber 18. The heater block may have one or more heating elements 39 thermally connected thereto for heating the heater block 39 and the ion transfer tube 30. Heating the ion transfer tube 30 in this manner during operation helps to evaporate residual solvent in partially desolvated droplets carried from the spray 24 into the ion transfer tube 30. The heater block 36 may be adapted with a bore to receive and hold the ion transfer tube 30. The heater block 36 may also be in sealed contact with an interior of the second chamber 18. The heater block 36 may include a sealing mechanism for sealing the second chamber when the ion transfer tube 30 is removed. The sealing mechanism may include a ball 42 movably supported in a recess 45 within the heater block 36 such that when the ion transfer tube 30 is removed from the second chamber 18, the ball 42 drops into sealing engagement with a seat in the recess, for example. Thus, the ion transfer tube 30 may be inserted, removed for cleaning or other purposes, and replaced without breaking the vacuum seal in the mass spectrometer. This sealing mechanism may be similar to that shown and described in U.S. Pat. No. 6,667,474 to Abramson et al., the entire specification of which is incorporated herein by reference.
Once the ions pass out of the second end 33 and into the second chamber 18, they may be focused by a tube lens 48 during a single gas stream expansion. The gas stream expansion may be interfaced with a single aperture in a plate, which may take the form of a conventional skimmer 51 as the stream proceeds toward a mass analyzer, for example.
The ion transfer tube 30 may be coupled to a wall 54 of one or both of the first and second chambers 15, 18 by threads 57 on the ion transfer tube 30 that engage in complimentary threads in the wall 54 or in a nut fixed to the wall 54. A flange 60 may be drawn into contact with the wall 54 by the threaded coupling of the threads 57 with the wall or nut. This engaging contact of the flange 60 may thus provide structural support for a coupling that has greater strength and stability. The ion transfer tube 30 may be coupled to the vacuum chambers in any conventional manner. With the ion transfer tube thus configured with both segments integrated as a mountable unit or assembly, repeatable alignment and positioning of the segments relative to each other and the overall systems is facilitated.
Conductance in all embodiments is dependent on length, cross sectional flow area (which depends on diameter for round capillaries/tubes), and temperatures in the plurality of channels of the first segment and the common channel of the second segment. Flow and throughput are dependent on conductance and a pressure differential between the inlet and outlet for the transfer tube. In one embodiment the conductance in the second segment is to be greater than or equal to the sum of the conductances in the first segment. It is to be understood that one having ordinary skill in the art would be capable of generally calculating the needed lengths, cross sectional flow areas, and temperatures for each of the first and second segments in order to yield a desired flow across a selected pressure differential.
It is to be understood that in another embodiment, the constriction at the tip may be purposely selected to dominate the overall conductance. Alternatively or additionally, the constriction may be provided for reasons other than controlling flow or throughput for all of the embodiments of the invention. For example, the constriction may be incorporated to provide the advantage of improving the unifying effect of the second segment 69 to form the combined stream from the plurality of streams coming from the first segment 66. Also, it is to be understood that the tip 98 may be a separate piece, or may be provided as one piece together with the outer sleeve 74 or the second segment tube 95 without departing from the spirit and scope of the invention.
One of the advantages provided by the plurality of capillary tubes 77 within the outer sleeve 74 is that the walls of the capillary tubes increase the surface area that is in contact with a sample fluid 83 (which comprises a combination of gas and ion flow) within the channels 80 as the sample fluid 83 passes through the capillary tubes 77. Thus, the convective heat transfer from the walls of the capillary tubes 77 into the sample fluid 83 is increased. As shown in
Another advantage of the outer sleeve 74 receiving and supporting the capillary tubes 77 is that the combination of the inner capillary tubes 77 and the outer sleeve 74 forms a strong and rigid ion transfer tube that has the needed structural integrity to maintain alignment during assembly and installation of the ion transfer tube 30 in the ion source and mass spectrometer. Each of the added materials 104, 107, 113 further serves to structurally strengthen ion transfer tube 30. Among other things, the ion transfer tube 30 is thus made strong enough to engage the ball 42 and move it away from a seated, sealed position without bending or other adverse effects on the ion transfer tube 30 when the ion transfer tube is inserted initially or after cleaning. Thus, very thin walled capillary tubes may be incorporated for the further advantage set forth below. It is to be understood that insertion and removal of the ion transfer tube 30 in this manner may be accomplished without breaking the vacuum seal.
In an alternative or additional expression of the advantageous structure of the present invention, the walls of the capillary tubes extend radially inwardly relative to the inner surface 110 of the outer sleeve 74 that would otherwise form a single channel in the first segment 66. That is, the walls extend to a central location within a perimeter of the path of the gaseous sample fluid 83. In ion transfer tubes without the benefits of the capillary tubes of the present invention, portions of the sample fluid 83 in a boundary layer near the inner walls of the outer sleeve 74 would form an insulative layer gas through which heat would have to be convectively transferred in order to reach centrally located portions of the fluid 83. Thus, the boundary layer would actually insulate the centrally located portions of the gaseous sample fluid 83 against heat transfer. This is an increasing concern as the ion transfer tube diameter is increased in an effort to increase throughput, as will be described below. On the other hand, with the capillary tubes 77, heat may be conductively transferred along and through walls of the capillary tubes 77 to a central region within the outer sleeve 74 of the ion transfer tube 30. Thus, more of the sample fluid 83 can be heated more effectively by providing capillary tubes within the outer sleeve 74.
One of the considerations in configuring the ion transfer tube 30 is that areas outside of the capillary tubes may not contribute to the flow or throughput. These areas may be considered dead spaces. If a large dead space is located at a center of the outer sleeve, then a large loss of flow or throughput may be the result. Hence, a configuration with minimal dead space may be utilized. To further lessen the loss of flow and throughput, the capillary tubes 77 may be provided with thin walls.
From
It is possible to provide the plurality of channels along an entire length of the ion transfer tube. However, it was discovered that doing so resulted in adverse interaction between the ion streams once they left the output end of the plurality of channels and entered the second chamber. That is, during the expansion of the relatively high pressure gas and analyte ions that occurs as they enter the second chamber, plural streams of gas and ions can interact with each other to form a complex flow geometry, resulting in a reduction in the number of ions being passed through the skimmer 71 or similar structure. On the other hand, extending the outer sleeve 74 beyond the outlet ends of the plurality of channels 80, or adding a common channel tube beyond the outlet ends as shown in
Analogous to the embodiments of
The second segment 149 may be considered to include a portion of the head 128 that receives the common capillary tube 137, and thus the second segment 149 may be directly connected to the first segment 146. The common capillary tube 137 may be abutted with or otherwise connected to the plural channel insert 131 for a direct connection between the first and second segments analogous to the embodiment of
In the embodiments of
The same advantages of increased throughput or TIC without the detrimental effects of reduced desolvation can be achieved with the ion transfer tubes 125 of
The embodiments and examples set forth herein were presented in order to best explain the present invention and its practical application and to thereby enable those of ordinary skill in the art to make and use the invention. However, those of ordinary skill in the art will recognize that the foregoing description and examples have been presented for the purposes of illustration and example only. The description as set forth is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the teachings above without departing from the spirit and scope of the forthcoming claims. For example, walls may be of any shape and may be extended radially inwardly from an inner wall of an ion transfer tube in order to form any plurality of channels or to provide conductive heat transfer to portions of the sample fluid that would otherwise be more remote from a heater block or some other heat source used to enhance desolvation.
Any number of capillary tubes may be provided in the first segment. For example without limitation, the number of capillary tubes may be five, six, seven, or eight. The number and relative orientations of capillaries may be selected depending on ion spray characteristics and geometries. For example, an elongate plume from a particular ion spray probe would interface well with a linear array of capillary tubes in the ion transfer tube. The ion spray characteristics of a sample may also call for other changes such as lower temperatures and less heat transfer from the heater block, for example. Lengths of the individual capillary tubes and the length of the overall ion transfer tube may be selected based on different characteristics/specifics of the sample or different pressure differentials between the first and second chambers. The length of the ion transfer tube over which the sample is heated may be selected based on flow characteristics and other sample characteristics such as ionization state.
It is to be understood that the flow characteristics may be different for different charge states of the same sample, whether the charge states are single or any of a variety of multiple charges per ion. Furthermore, some analyte compounds may interact more with the walls of the channels and result in greater loss of ions per length of the channels due to discharge. As such, there is a benefit in incorporating the common channel of the second segment as is done in the embodiments of the present invention. The benefit is that the surface area per unit length in the second segment is less than the surface area per unit length in the first segment such that there will be less discharge of the ions per unit length in the second segment than in the first segment.
The length of the first segment or the plural bore portion of the ion transfer tube may be small or large in comparison to the overall length of the ion transfer tube. This relationship may be expressed in terms of a ratio of the length of the first segment (or plurality of bores) to the overall length. For example, in an ion transfer tube having a length of one hundred millimeters, a short first segment could have a length of three fourths of a millimeter in a direction of flow. Thus, the ratio could be expressed as 0.0075. In one broad range, the ratio may be from 0.002 to 0.95. It is to be understood that the ratio of the first segment or plurality of bores to the overall length of the ion transfer tube may have any intermediate ratio including, but not limited to, one eighth, one fourth, one third, one half, two thirds, and three fourths. The embodiments of
Many of the exemplary embodiments of the figures show round capillaries, round ion transfer tube segments, and generally circumferential distributions of capillary tubes. However, it is to be understood that the shapes of the capillary tubes and/or ion transfer tubes need not be round. These shapes may include elliptical, square, triangular, or any other shape. The distribution of the capillary tubes need not be circumferential. Furthermore, the sizes and/or shapes of the capillary tubes in any given ion transfer tube may vary. Still further, the capillary tubes may be positioned in any symmetrical or nonsymmetrical way about a central axis of the ion transfer tube. Still further, the capillary tubes may form a linear or curved array, or may be distributed in a rectangular or hexagonal distribution with horizontal or diagonal rows at any desired angle. A seven capillary tube arrangement having six capillary tubes surrounding a central capillary tube is also contemplated.
The capillary tubes, outer sleeves, heads, and inserts may also include any of various materials. For example, one or more of these elements may be formed of Titanium, stainless steel, brass, or other metal, ceramic or composite material. Titanium and brass have the advantage of being good heat conductors. In one embodiment, the capillary tubes may be formed as grooves or drilled holes in a block of silicon nitride or other ceramic material. Thus, heaters may be embedded directly into the block. In one embodiment, grooves may be provided in a surface of a first block, and a second block may be added on top of the first block to close the grooves and form the capillary channels. A variety of surface characteristics on inner walls of the capillary tubes may be incorporated. For example, a less smooth surface that causes turbulence in boundary layers of the sample may actually result in less interaction between streams of the sample that are exiting a first segment of an ion transfer tube. For the silicon nitride ceramic or other examples having an array or other configuration of capillary tubes, the plume from the ion probe could be configured to have a corresponding flat or other configuration, such as by shaping with gas streams.
