Patent Grant
7098452
This invention relates to mass spectrometry, and more particularly to the interface between an atmospheric pressure ion source and low pressure regions of a mass spectrometer.
Samples or analytes for analysis in mass spectrometers are often ionized in an atmospheric environment, and the ions are then introduced into a vacuum chamber that contains the mass spectrometer. An atmospheric pressure ion source provides advantages in handling of samples, but the introduction of ions from the ion source into the vacuum chamber often requires a proper interface disposed between the ion source and the vacuum chamber. For instance, one common family of ionization techniques includes electrospray and its derivatives, such as nanospray, which provides a low flow. In all such techniques, a liquid sample, containing the desired analyte in a solvent, is caused to form a spray of charged and neutral droplets at the tip of an electrospray capillary. Once the spray is produced, the solvent begins to evaporate and is removed from the droplet, which is a process commonly referred to as desolvation. Accordingly, an important step in generating ions is to ensure proper desolvation. The electrospray source is usually coupled with some means of desolvation in an atmospheric pressure chamber, where desolvation can be enhanced by heat transfer to the droplets (radiation, convection) or/and counter-current flow of dry gas. The spray generally consists of a distribution of droplet sizes, and subsequently, the degree of desolvation will be different for each droplet size. Consequently, after desolvation, there is a size distribution for desolvated particles where there are large and heavy charged particles that may contaminate the aperture or conductance limit, thereby preventing the long-term stable operation of the mass analysis region, and/or introducing additional noise to the ion detector. This additional source of noise reduces the signal to nose ratio and thus, the sensitivity of the mass spectrometer.
The ions and the accompanying solvent molecules (neutrals) and charged particles, are transferred from the atmospheric pressure region to the low-pressure chamber of the mass spectrometer. Generally, the mass spectrometer operates less than 10−4 Torr and requires stages of skimmers or apertures to provide step-wise pressure reduction. Various methods for allowing the ions to enter while preventing the neutrals from passing into the mass spectrometer are well known. In U.S. Pat. No. 4,023,398, assigned to the assignee of the present invention and the contents incorporated here, as represented in
U.S. Pat. Nos. 4,977,320, and 5,298,744, teach a method whereby a heated tube made from conductive or non-conductive material is used for delivering the ions/gas carrier/solvent flow into the low-pressure chamber. In such a configuration, the heated tube provides two distinct and separate functions; firstly, due to its significant resistance to gas flow, the tube configuration, namely its length and inner diameter, adjusts the gas load on the pumping system; secondly, the tube can be heated to effect desolvation and separation of ions from neutrals. With respect to the first function, this resistance can be provided, while keeping the tube length constant, to ensure laminar gas flow in the tube and the widest possible opening for inhaling the ion/gas carrier/solvent flow. Generally, a wider bore for the tube provides increased gas flow and hence more load on the pumping system; correspondingly, reducing the tube length provides less resistance to the gas flow, so as also to increase the gas flow and load on the pumping system. These two geometric parameters, bore and length, are obviously related and can be adjusted to provide the desired flow rate and flow resistance. The second function is provided by mounting a heater around the interface tube. The heat provided to the tube promotes desolvation of the ion flow, and also helps to reduce contamination of the surface of the tube, thereby reducing memory effects. An interface of this type is able to work best under strictly laminar flow conditions, limiting the variability of the tube length and tube bore. Additionally, the desolvation, which depends on temperature and residence time (inversely proportional to gas velocity through the tube) is related to the pumping requirements. As a rule, it is not possible to optimize all the desired parameters; in particular, it is desirable to minimize total mass flow to reduce pumping requirements, on the other hand to ensure best efficiency for transfer of ions into the mass spectrometer, a large diameter tube with high mass flow rates is desirable. In addition, the desolvation of ions is also affected by the diameter of the tube due to changes in residence time.
U.S. Pat. No. 5,304,798 attempts to satisfy both of these requirements by teaching a method whereby a chamber has a contoured passageway to provide both the desolvation function and the capillary restriction function. The opening of the passageway adjacent to atmospheric pressure has a wide and long bore while the opposite end of the passageway, ending within the vacuum chamber, has a smaller shorter bore. The electrospray source is place in front of the opening of the wide bore allowing the spray to pass directly into the passageway. The desolvation is performed within the wide bore region while the smaller bore provides the mass flow restriction. The entire spray is passed into the desolvation tube and any neutral or charged particulates or droplets not fully desolvated, will pass into the small bore. These particulates or droplets can accumulate in the small bore, which may cause blockage or they may pass through the small bore and enter the vacuum chamber leading to extensive contamination.
U.S. Pat. No. Re. 35,413 describes a desolvation tube and a skimmer arrangement where the exit of the desolvation tube is positioned off-axis to the skimmer. Offsetting the axis of the tube from the orifice of the skimmer is intended to allow the ions to flow through the orifice while the undesolvated droplets and particulates impinge upon the skimmer. This method does not take into consideration that the undesolvated droplets or charged particles, are not restricted to travel along the axis of the desolvation tube but follow a distribution across the bore. That is, this arrangement will only prevent undesolvated droplets and particulates traveling along the central axis from entering the orifice. An offset of the desolvation tube will not prevent droplets and charged particulates aligned with the offset location from entering the skimmer or to prevent an accumulation from building up around the orifice. In addition, it is expected that there would be a reduction of the ion current through the skimmer as a function of the offset.
In U.S. Pat. No. 5,756,994, a heated entrance chamber is provided, and is pumped separately. Ions entering this chamber through an entrance aperture are then sampled through an exit aperture that is located in the side of the chamber, off any line representing a linear trajectory from the entrance orifice. The intention of this off alignment is to prevent the neutral droplets or particles from entering the exit aperture. Pressure in this heated entrance chamber is maintained around 100 Torr. To the extent that this is understood, there is an independent pumping arrangement in the entrance chamber, and the shape of the chamber is not conducive to maintaining laminar flow, with the entrance aperture being much smaller than the cross-section of the main portion of the chamber itself. It is expected that significant loss of ion current to the walls of this chamber would occur in addition to obvious inefficiency of sampling from only one point of cylindrical flow through the exit aperture.
Another common type of atmospheric pressure ion sources uses the matrix-assisted laser desorption/ionization (MALDI) technique. In such a source, photon pulses from a laser strike a target and desorb ions that are to be measured in the mass spectrometer. The target material is composed of a low concentration of analyte molecules, which usually exhibit only moderate photon absorption per molecule, embedded in a solid or liquid matrix consisting of small, highly-absorbing species. The sudden influx of energy in the laser pulse is absorbed by the matrix molecules, causing them to vaporize and to produce a small supersonic jet of matrix molecules and ions in which the analyte molecules are entrained. During this ejection process, some of the energy absorbed by the matrix is transferred to the analyte molecules, thereby ionizing the analyte molecules. The plume of ions generated by each laser pulse contains not only the analyte ions but also charged particulates containing the matrix material, which may affect the performance of the mass spectrometer if not removed from the ion stream.
In view of the forgoing, the present invention provides a system for preparing ions to be studied by an ion mass spectrometer. The system has an atmospheric pressure ion source, such as an electrospray ion source or a MALDI source, a mass spectrometer contained in a vacuum chamber, and an interface for introducing ions from the ion source into the vacuum chamber. The interface includes an entrance cell and a particle discrimination cell.
In an embodiment where the atmospheric pressure ion source is an electrospray ion source, the entrance cell may function as a desolvation cell. The electrospray ion source operates in the atmosphere and provides a spray of charged droplets that contain ions to be studied. The spray is directed into a heated bore of the desolvation cell for drying the droplets in the spray to generate an ion stream, which contains undesirable particulates. A particle discrimination cell for discriminating against (i.e., removing) particulates is disposed downstream of the desolvation cell and before an aperture in a partition that separate the atmospheric pressure from the vacuum in the vacuum chamber. The particle discrimination cell has a bore for receiving the ion stream that is larger than the bore of the desolvation cell and has a central zone and a discrimination zone surrounding the central zone. Eddies are formed in the discrimination zone when the ion stream flows into the bore of the particle discrimination cell. The particle discrimination cell has a voltage applied thereto for generating a particle discrimination electric field in its bore. The electric field and the formation of eddies in the particle discrimination cell together provide the effect of removing particulates from the ion stream so that they do not enter the aperture of the partition.
The present invention also provides a method of interfacing an ion source that operates in the atmosphere with an ion mass spectrometer in a vacuum chamber. The ion source may be, for instance, an electrospray source or a MALDI source. An interface that contains an entrance cell and a charged particle discrimination cell is disposed between the atmospheric ion source and the vacuum chamber. When the ion source is an electrospray source, the entrance cell is used as a desolvation cell. A spray of charged ion droplets generated by the ion source is directed into a heated bore of a desolvation cell for drying the droplets in the spray to generate an ion stream, which contains undesirable particulates. The ion stream then is directed through a discrimination cell that is disposed downstream of the desolvation cell and upstream of an aperture in a partition that separates the atmosphere from the vacuum chamber containing the ion mass spectrometer. The discrimination cell has a bore that is greater than the bore of the desolvation cell and has a central zone and a discrimination zone surrounding the central zone. While flowing from the desolvation cell into the discrimination cell, the ion stream generates eddies in the discrimination zone of the discrimination cell. A voltage is applied to the discrimination cell to generate a discrimination electric field in the bore of the discrimination cell. The electric field and generation of eddies in the discrimination cell together provide the effect of removing undesirable charged particulates from the ion stream so that they do not enter the aperture of the partition.
While the appended claims set forth the features of the present invention with particularity, the invention, together with its objects and advantages, may be best understood from the following detailed description taken in conjunction with the accompanying drawings, of which:
Referring now to the drawings,
The ion source 1 can be a single or a multiple of the many known types of ion sources depending on the type of sample to be analyzed. For instance, the ion source may be an electrospray or ion spray device, a corona discharge needle, a plasma ion source, an electron impact or chemical ionization source, a photo ionization source, a MALDI source, or any multiple combinations of the above. Other desired types of ion sources may be used, and the ion source may operate at atmospheric pressure, above atmospheric pressure, near atmospheric pressure, or in vacuum. Generally, the pressure in the ion source is greater than the pressure downstream in the mass spectrometer 32. The ion source 1 produces a spray (in the case of an electrospray source) or a plume (in the case of a MALDI ion source), or plurality of sprays or plumes. The spray from an electrospray ions source initially comprises mostly charged droplets followed by the progressive formation of ions and particulates. When a MALDI ion source is used, the plume from a MALDI ion source typically comprises a mixture of ions and particulates where the particulates can be hydrated or simply charged or neutral particles (depending on the degree of thermal heating from the MALDI laser). Regardless of the ion source type, the presence of either undesolvated droplets or particulates may degrade the quality of the ion stream and interfere with the transmission of the ions through the aperture 4 of the mass spectrometer 32. As described below, the ion interface of the present invention enables the removal of the undesirable particulates from the ion stream before the ions enter the vacuum chamber containing the mass spectrometer.
For simplicity of description, the following description describes an embodiment in which the ion source is an electrospray source. It will be appreciated, however, that the ion interface of the invention is also effective in removing undesirable charged particulates from the plumes of ions generated by a MALDI source. Still referring to
The curtain plate 5 can take the form of a conical surface as in
The ions, the charged particles, the residual charged droplets, and a portion of the curtain gas 7 flow into an entrance cell 27, which is located within a heated chamber 26, having a bore 58. When an electrospray source is used, the entrance cell is heated to help desolvate the charged droplets from the electrospray source. For this reason, the entrance cell 27 is also referred to as the desolvation cell in the following description. Secondary desolvation occurs, a result of the heated chamber 26 convectively transferring heat to the residual charged droplets. Ions are released from the desolvated droplets but those charged droplets that form charged particulates are permitted to flow through the desolvation cell 27. Subsequently, the ions and the charged particulates emerging from the heated chamber exit 25 travel into a second particle discriminator cell 30, located between the heated chamber exit 25 and the partition 3 and confined by the spacer 29 in the radial direction. The inner diameter of the spacer 29 is greater than the internal bore 58 of the heated chamber 26, which is greater than the aperture 4 of the partition 3. Typically, the aperture 4 has diameter between 0.10 to 1.0 mm with wall thickness between 0.5 to 1.0 mm, the spacer 29 has diameter between 2 to 20 mm and the bore 58 of the heated chamber 26 has diameter between 0.75-3 mm. The curtain plate 5, the heated chamber 26, the spacer 29 and the partition 3 are electrically isolated from each other by appropriately known methods, having one pole (depending on the polarity of the ions desired) of voltage sources 40, 41, 42 and 43 connected to them respectively. As is conventional, the voltage sources 40, 41, 42 and 43, are configured for direct current, alternating current, RF voltage, grounding or any combination thereof. The spacer 29 can be fabricated from a non-conductive material such as ceramic, in which no potential is applied. As indicated previously, the pressure between the partition 3 and ion source 1 is substantially atmospheric and as such, the mating surface between the heated chamber 26 to the spacer 29 and the mating surfaces between the spacer 29 to the partition 3 do not require vacuum tight seals. However, because a net flow, comprising of the spray 2 and a portion of the curtain gas 7, in the direction from the ion source 1 to the aperture 4 is desired, a substantially leak free seal is preferable. The net flow at any point between the ion source 1 and aperture 4 may be supplemented by an additional source of gas, if the gas streamlines 18, described below, remain laminar.
In operation, the electric field and the gas flow dynamics that are present in the particle discriminator cell 30 create a charged particle discrimination effect that reduces the amount of undesirable charged particles entering the aperture 4. To better understand this process, a discussion of the gas flow dynamics and the electric field effects are independently presented by the following.
First, to illustrate the gas flow dynamics, reference is made to
In contrast, the CPD zone 37 serves to create a radial perturbation or longitudinal discontinuity between the heated chamber exit 25 and the aperture 4, and circulating streamlines 19 are formed. The circulating streamlines 19 are typically referred to as eddies having low flow velocities, about 2 m/s, while the streamlines 18 adjacent to the CPD zone 37 tend to converge 31 towards the aperture 4 at a greater gas flow velocity. Generally, the gas flowing through the heated chamber 26 and the center of the particle discriminator cell 30 is laminar, and all the gas flow is created by the vacuum draw from the mass spectrometer 32. Ions and charged particulates are distributed across the streamlines 18 with the large and heavy charged particulates traveling with the streamlines 18 in a region radially extending beyond line-of-sight of the aperture 4, breaking free of the streamlines 18 as the streamlines converge 31, and impact the partition 3 near the aperture 4. The charged particles nearest to the CPD zone 37 break free of the converging streamlines and tends to enter the circulating streamlines 19 of the CPD zone 37 while charged particles traversing along the central axis 20 in direct line-of-sight of the aperture, enter the aperture 4. As will be described later, these line-of-sight charged particles can be blocked from entering the aperture 4. On the other hand, small charged particles traversing in the region radially beyond line-of-sight of the aperture 4 are easily influenced by the gas flow and will converge 31 through the aperture 4 and pass into the mass spectrometer 32.
However, with the appropriate electric fields, a number of surprising effects are taking place, which includes; a) charged particulates are deflected away from the aperture 4; b) heavy charged particulates that would normally be impacting adjacent to the aperture 4 are drawn towards the circulating streamlines 19; and c) ions continue to traverse through to the aperture 4. The electric fields thus have the effect of reducing the amount of deposit collected near the aperture 4 while maintaining ion transmission to the mass spectrometer.
To illustrate the electric field effects, reference is now made to
The second region 50 of interest corresponds to a clear area surrounding the primary deposit. This area is generated because both the gas flow streamlines and the electric field are divergent relative to the partition 3, causing the charged particles to be directed away from this area. The final region 51 contains a light monodisperse layer of material deposited from the edge of the second region 50, out to the spacer surface 38. This light dusting occurs as a result of particles that become trapped within the swirling gas flow of the circulating streamlines 19 in the CPD zone 37. The gas flow properties cause particles within this region to swirl around until they strike the partition 3 and deposit there in a uniform fashion.
In accordance with an aspect of another embodiment,
Additionally, it can be appreciated that the location of blocking member 57 along the axis 20 is not limited to a position between the heated chamber exit 25 and the outlet 55 of the spacer 29. Similar results can be achieved by positioning the blocking member 57 within the bore 58 of the heated chamber 26.
From the above description, particle discrimination is achieved by a combination of electric field and gas flow contributions present within the spacer 29. The blocking member 57 removes charged particulates traversing on axis 20 in the direct line-of-sight with the aperture 4, while the electric field drives the charged particulates destined to impact the perimeter of the aperture 4 to flow into the CPD zone 37. This effect can become more pronounced by increasing the divergent nature of the electric field between the heated chamber exit 25 and the partition 3. It is also possible to vary the bore of the spacer 29 or by changing the shape of the spacer 29 to provide a larger region of circulating streamlines 19. For example, as shown in
In a preferred embodiment illustrated in
Additionally, an inverse mobility chamber can be created by applying the appropriate potentials to the heated chamber 26, spacer 29 and partition 3 so that the charged particle's mobility is directed towards the heated chamber exit surface 36. For example, the ion source 1 has a potential of +2000 volts, both the curtain plate 5 and heated chamber 26 have 0 volts, the spacer 29 is non conductive and the partition 3 is supplied with a potential of +30 volts. This combination of potentials generates an axially repellant electric field thereby preventing large charged particles from striking the aperture 3 while not affecting the count rate for ions. The selection of the potentials in the combination would depend on the diameters of the bore 58 and the bore 59, and to some extent the aperture 4. It is conceivable that with the appropriate combination of potentials, both ions and particulates can be diverted away from the aperture 4 to provide a convenient method of interrupting the stream of ions directed to the mass spectrometer. Similarly, reversing the polarity on the ion source 1 and partition 3 will repel negatively charged particles from the aperture 3. This is a significant advantage over the prior art because it substantially improves robustness, by decreasing contamination through the aperture thereby maintaining the gas conductance limit into the mass spectrometer.
While preferred embodiments of the invention have been described, it will be appreciated that changes may be made within the spirit of the invention and all such changes are intended to be included in the scope of the claims.
This application claims the priority of U.S. Provisional Application 60/447,655, filed Feb. 14, 2003.
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