Patent Grant
8039795
Mass spectrometry is an important tool in the analysis of components (or “analytes”) in a sample. In a mass spectrometric analysis, a sample has to be ionized to generate ions of the analytes; the ions are then separated based on their mass-to-charge ratios by a mass analyzer, and detected by a detector. There are many different techniques for ionizing samples, such as electrospray ionization (ESI), chemical ionization (CI), photoionization (PI), inductively coupled plasma (ICP) ionization, and matrix assisted laser desorption ionization (MALDI). Although all the techniques listed above share a common aspect, that a solid or liquid sample must be converted to a plume of molecules, atoms or ions, their mechanisms of ionization differ. As a result, the compounds that can be ionized by each of these techniques are not identical.
In the earliest implementation of electrospray, a sample plume was sprayed into a high electrical field without pneumatic or ultrasonic nebulization. This is referred to as “pure electrospray.” Pure electrospray had the problem of low flow capabilities (0.1 to 10 μl per minute). Therefore, it was difficult to use pure electrospray with liquid chromatography (LC), which has a much higher flow rate (typically up to 2 ml per minute). When the electrospray flow rate is above 100 μl per minute, it is usually impossible to maintain a sample plume, due to unstable spray formation. The ionization efficiency of pure electrospray thus decreases at higher flow rates, and sensitivity is completely lost at typical chromatographic flow rates. Therefore, the interface between LC and pure electrospray routinely splits the sample flow by a factor of 10 or more, sacrificing sensitivity, resolution and reproducibility.
The development of pneumatically assisted electrospray (or “ion spray”; see, e.g., U.S. Pat. No. 4,861,988) alleviated the flow limitation to some extent. This technique employs a concentric nebulizing gas around the central liquid delivery capillary, and enables a flow rate up to several hundred micro liters per minute, with a moderate loss of sensitivity. As discussed below, various improvements have been made to this technique.
A few years after U.S. Pat. No. 4,861,988, a heater was mounted directly on the pneumatic sprayer to assist ionization with heat and heated gas. This thermally assisted electrospray interface improved sensitivity by three times, and a flow rate of up to 500 μl per minute was demonstrated (U.S. Pat. No. 4,935,624). However, the heated nebulizer was prone to sample degradation and clogging, due to difficulty of regulating the temperature at the tip of the nebulizer.
Another implementation (Vestal, 1992) used moderately heated concentric air to assist ion formation within the electrospray plume, but, because the sprayer was deeply buried inside the concentric heated chamber, adjustment or service of the sprayer region was difficult.
At about the same time, U.S. Pat. No. 5,352,892 disclosed another way of heating the spray plume, wherein a heated disk with a central opening was placed in between a pneumatically assisted electrospray nebulizer and the ion sampling inlet to a mass analyzer. In this arrangement, a fraction of the nebulizing gas would be preheated at the opening of the heated disk body. This heated gas was then remixed with the central portion of the spray plume prior to the ion sampling inlet. In this device, heat transfer was sufficient to achieve ion formation at flow rates as high as 2 ml per minute, but the drawback was contamination of the heated disk, which required frequent cleaning.
In a design described in U.S. Pat. No. 5,412,208, the nebulization and ion sampling process was assisted by preheated gas that intersected the flow of the nebulized sample. This turbulent mixing helped to evaporate droplets of the sample, as well as push the electrospray plume in the direction of the ion sampling inlet. The main disadvantage of this design is non-uniform and limited heat exchange between the heated gas flow and the ESI plume. A newer design, described in U.S. Pat. No. 6,759,650, used two heated gas flows that intersected with the sample flow to promote turbulent mixing, but the design was complicated and less cost effective.
U.S. Pat. No. 5,495,108 discloses an ion source in which a heated drying gas is directed to a spray plume that is orthogonal to the ion sampling inlet. For example, the ion sampling inlet 236 may be positioned at 90 degrees with respect to the direction of nebulization (
Another design, described in U.S. Pat. No. 7,199,364, includes a second, laminar gas flow that is heated, wherein the nozzle for the second gas flow is behind the nebulization nozzle in a semi-circular pattern. This design achieved limited heat transfer and only a moderate improvement in sensitivity.
In summary, there is a constant need for further improvements in ion source design and higher ionization efficiency.
This invention provides, inter alia, ion sources that generate significantly higher ion density. Furthermore, the resulting ion distribution maintains sharp and non-tailing chromatographic peaks, indicating uniform ion formation and better resolution among different analytes. In some embodiments, the ion source comprises a capillary for sample intake from one end and spraying the sample into droplets from the other end. The droplets, along with a first gas that is supplied to a location near the droplets, form a plume, which is confined by the flow of a second, heated gas. The heated gas can be delivered in close proximity to the spray end of the capillary, resulting in flash vaporization of the sprayed droplets in a confining flow of heated gas. In some of the embodiments, the nozzle that releases the heated gas is electrically connected to a power supply, and is capable of providing an electrical field at the spray end of the capillary. When solvents are removed from the droplets, the analytes in the droplets become ions. The nozzle can comprise multiple electrodes, and different parts of the nozzle may operate at different electrical potentials, but the combined effects, along with other electrical forces in the ion source, can result in an electrical field to charge at least some of the droplets. In some embodiments, the capillary and/or the tube for supplying the first gas are at ground potential, and are thus safer for the user to handle.
In some embodiments, the ion source comprises a heat shield between the second, heated gas and the first gas. In some of the embodiments, the heat shield is heat-conductive and configured to transmit heat away from the ion source, thus the heated gas can be heated to a higher temperature without damaging other parts of the ion source. For the same reason, the heated gas can be located closer to the sample intake capillary without thermally degrading the sample in the capillary.
In some embodiments, the first and second gas flows are both parallel to, or even concentric with, the capillary. In some embodiments, the first or second gas is directed at a point some distance beyond the end of the capillary. Thus, the first gas flow or the second, heated gas flow meets the flow of the sample at an angle. In some other embodiments, the first and second gas flows are parallel to the flow of the sample.
Prior to describing the invention in further detail, the terms used in this application are defined as follows unless otherwise indicated.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Definition
It should be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a mass analyzer” includes combinations of mass analyzers, and reference to “a tube” includes combinations of tubes, and the like.
An “electrospray ion source” is a device that can ionize a sample by electrospray. In an electrospray process, a liquid sample containing analytes is sprayed into droplets. The droplets are subjected to an electrical field, and at least some of the droplets are electrically charged. Upon removal of solvent from the droplets (“desolvation”), some of the analytes in the charged droplets become ionized.
As used herein, when a part (part A) “surrounds” another part (part B), part A appears in all or almost all directions of part B, although holes or gaps may exist (partial surrounding, see below). Surrounding may be direct or indirect, and complete or partial. For example, if a layer surrounds a tube, the layer may be in contact with the tube (surrounding directly), or it may be separated from the tube by at least one object or space (surrounding indirectly). Furthermore, the layer may completely surround the perimeter or length of the tube, or it may surround the tube only partially lengthwise and/or circumferentially. When part A does not completely surround part B circumferentially, at least 55, 60, 65, 70, 75, 80, 85, 90, 91, 92, 93, 94, 95, 96, 97, 98, or 99% of the perimeter of part B should be surrounded.
A “nebulizing gas” is a gas used to help a liquid to form an aerosol. The gas is preferably an inert gas, usually nitrogen.
As used herein in the context of mass spectrometry, “atmospheric pressure (AP)” is a pressure above the vacuum level, usually between about 100 Torr and about twice the local atmospheric pressure, or higher.
Exemplary Ion Sources and Methods of Use
Thus, one aspect of the present invention provides a device comprising:
It is contemplated that the description above encompasses the embodiments in which the tube is a group of tubes which collectively surround the capillary and transmit the first gas. Similarly, the conduit may be a group of conduits which collectively surround the tube and transmit the heated gas. Furthermore, as illustrated in
It should be noted that the flows of the sample (in capillary 26), the first gas (in tube 28), and the sheath gas (between nozzles elements 46 and 48) can be concentric. In some other embodiments, the flows may have parallel axes but not concentric. In some embodiments, the sprayer tip 51 is positioned approximately flush with the opening of the nozzle elements 46 and 48. It is possible to position the sprayer tip 51 slightly extended beyond the opening of the nozzle elements 46 and 48, which may affect the strength of the charging field. It is also possible to position the sprayer tip 51 slightly recessed from the nozzle opening; however, this may result in sample deposition on to the internal nozzle surfaces, which may increase the required cleaning frequency.
In some embodiments, the exit region between the inner nozzle element 48 and outer nozzle element 46 is angled. The angle, as defined by the smallest angle between a hypothetical line extended from the end part of nozzle element 46 and a hypothetical line extended from capillary 26, is typically 50 degrees or less, such as 50, 45, 40, 35, 30, 25, 20, 15, 10, 5 degrees or less. An angle of 0 degrees would deliver a parallel flow. It should be noted that a divergent flow (negative angle) can be used in the devices of the present invention as well. Such a flow is still confining, but does not focus the plume very much. In some cases, a positive angle will direct the gas flow to a region below the spray tip 51 (as illustrated in
In some other embodiments, the nozzle elements 46 and 48 are both parallel to the capillary 26 in the exit region (as illustrated in
The sizes of the parts can be decided according to knowledge in the art, economic concerns, and goal of the user. In many embodiments, the inside diameter (ID) of the inner or outer nozzle element (46, 48) is 2-25 mm, particularly 2-5 or 5-10 mm. For example, the ID of the inner nozzle element 48 can be 7 mm. The outside diameter (OD) of the inner nozzle element 48 can be 8 mm and the ID of the outer nozzle element 46 can be 9 mm, providing a 0.5 mm circular opening for the sheath gas. These dimensions were chosen to be relatively small to minimize sheath gas flow and maximize the effect of the charging field generated by the nozzle electrodes. In general, when the ID of the nozzle is decreased, there is a higher chance of bringing the heated sheath gas into proximity of the spray tip 51, resulting in undesired sample boiling and signal drop outs. However, as described herein, this invention provides multiple features to insulate the sample from the nozzle and the sheath gas thermally, electrically, or both. Therefore, the nozzles can be brought close to the sample capillary. In some embodiments, the distance between the spray tip 51 and the nearest part of the nozzle releasing the sheath gas is less than about 10, 9, 8, 7, 6, 5, 4, 3, or 2 mm, a feature that could not be achieved by prior devices without thermally degrading the sample or causing arching. Since these embodiments allow high-temperature sheath gas and close proximity between the sheath gas and the sample, flash vaporization of the sample and a confined plume can be achieved.
In some embodiments, the sheath gas flows quickly as a jet stream. Thus, the velocity of the sheath gas, in some embodiments, can be about 35-55, 25-60, 25-80, or 15-70 meters per second. For example, the velocity can be 35, 40, 45, 50, 55, or 60 meters per second. The velocity can also be lower or higher as decided by the user.
The ion source may further comprise an inlet to a mass spectrometer or an ion mobility separating device. The inlet may be any structure known or apparent in the art. Exemplary inlets include, without being limited to, an orifice, a short tube, and a capillary. The MS inlet in
With the presence of the heat shielding layer, it is possible to increase the temperature of the sheath gas above 250° C., such as up to about 400° C. (measured where the sheath gas is released from the nozzle to the chamber), without boiling the sample in the tip of the nebulizer. In fact, the sheath gas temperature may be even higher if the sample solvent is less volatile (such as aqueous) and provides more protection to the sample from boiling. Note that the sheath gas cools down in the conduit before it reaches the nozzle, so the gas can be heated to a temperature significantly higher than 400° C. (for example, 500° C. or above) by heater 14 or as a pre-heated gas in order to be released to the chamber at about 400° C. The actual temperature decrease in the conduit should be determined by the user, as it depends on many factors, including the length of the conduit, the material of the parts, and the speed of the sheath gas flow.
In some embodiments, the heat shielding layer (such as the thermally conductive tube 74) comprises a copper layer that is coated with an inert material or a material with low surface emissivity. For example, gold has low surface emissivity and tends to reflect heat rather than absorbing it, and this property helps to prevent heat transfer from the heated gas to the sample capillary. In addition, gold is chemically inert and capable of protecting copper from oxidation, erosion, or other damages. Other low-surface emissivity, inert materials include, without being limited to, platinum, rhodium, and titanium nitride.
In addition to or in lieu of the heat shielding layer described above, the ion source may comprise a space between the nebulizing gas tube and the sheath gas conduit. In some embodiments, the space may be optionally connected to a cooling gas supply to run a cooling gas through the space, which helps to remove the heat from the nebulizer. In the embodiments wherein there are both a heat shielding layer and a space, any combination of these parts can be employed, for example, nebulizer—heat shielding layer—space—sheath gas conduit, nebulizer—space—heat shielding layer—sheath gas conduit, nebulizer—space—heat shielding layer—space—sheath gas conduit, and the like.
Another cooling tool that can be included in the heat shielding layer or the space is a heat pipe, which comprises a liquid that undergoes phase change at a relatively low temperature, e.g., 60° C. The liquid can be sealed in the space or the center of the heat shielding layer. When the liquid is heated near the phase change temperature, many bubbles are formed and flow upwards, while the remaining liquid flows down, resulting in vigorous mixing and heat exchange. The upper part of this reservoir can be connected to a heat sink, cooled by a fan, or the like, to increase the heat exchange.
All voltages can be optimized for maximum amounts of ions delivered to the mass spectrometer. For example,
At present, the reasons for these observations are not well understood, but without limiting the invention, it appears there may be different dynamics for ion formation from the droplets when the spray plume is confined by a sheath gas at elevated temperatures. The electrospray plume under operating conditions appears much more confined, focused and compressed in the radial dimension. Without limiting the scope of invention, this potentially can be attributed to the thermal gradient focusing that can be described as the balance of heat transfer to the border between the condensed phase plume and the encompassing heated sheath gas. Heat flow (Q) to the plume is proportional to the temperature difference (ΔT) between the sheath gas and the boiling temperature of the liquid in the condensed phase within the plume. Heat flow (Q) is proportional as well to the total area (S) of the condensed phase plume.
Q˜ΔTS (1).
At the same time, Q is constant and is equal to the total heat needed to evaporate the sprayed condensed phase, thus resulting in an inversely proportional relationship of the total condensed phase plume area (S) vs. ΔT. Depending on the particular plume geometry, which can range from spherical to cylindrical, the surface area (S) is either proportional to R2 or to the first degree of R, where R is the characteristic radial dimension of the sprayed condensed phase plume. Thus Equation (1) can be rewritten as:
R˜1/ΔTα (2),
where α is between 0.5 and 1 depending on the particular spray plume geometry. Equation (2) describes the observed focusing of the sprayed condensed phase plume in the radial dimension with increased sheath gas temperature. A tighter, more focused spray can result in higher droplet concentrations and therefore higher ion concentrations at the border of the spray, thus resulting in the enhanced sensitivity observed in the device of the present invention.
The absolute intensity of peak 82 (
a and 9b illustrate an additional advantage of the source of the present invention, which is the ability to maintain sharp, non-tailing chromatographic peaks. Peak 94 (
Additional embodiments of the present invention could be extended to low flow ESI ion sources that operate in a “pure electrospray” mode (no pneumatic or ultrasonic nebulization) such as the Nanospray Source or the HPLC-Chip MS Interface from Agilent Technologies (www.agilent.com).
It is also recognized that in some embodiments, running the nozzle elements 46 and 48 at different potentials can further optimize droplet charge density and ion transport, as illustrated in
The ions sources of the present invention may be part of a larger system or device, such as a mass spectrometer system or an ion mobility spectrometer.
A mass spectrometer typically comprises an ion source, a mass analyzer, an ion detector and a data system. The ion source contains an ion generator which generates ions from a sample, the mass analyzer analyzes the mass/charge properties of the ions, the ion detector measures the abundances of the ions, and the data system processes and presents the data. Pumps for creating vacuum in certain parts of the system, and ion optics for directing the movement of ions, may also be included. The mass analyzer may be any mass analyzer (including mass filters), for example, a quadrupole, time-of-flight, ion trap, orbital trap, fourier transform-ion cyclotron resonance (FT-ICR), or combinations thereof. The mass spectrometer system may also be a tandem MS system, comprising more than one mass analyzer configured in tandem. For instance, the tandem MS system may be a “QQQ” system comprising, sequentially, a quadrupole mass filter, a quadrupole ion guide, and a quadrupole mass analyzer. The tandem MS system may also be a “Q-TOF” system that comprises a quadrupole and a time-of-flight mass analyzer. A particular class of MS systems is a combination of a mass spectrometer and an ion mobility spectrometer, comprising an ion mobility separating device and a mass analyzer in series. The mass spectrometer system may further comprise a sample separation device, such as a liquid chromatography column or a capillary electrophoresis device.
An ion mobility spectrometer typically comprises an ion source and an ion mobility separating device, such as a field asymmetric ion mobility spectrometer (FAIMS).
Surprisingly, it was discovered that the ion sources and methods of the present invention can be used to ionize many analyte compounds that have been considered not amenable to ionization by electrospray. In general, polar compounds are ionized more efficiently by electrospray, and less polar compounds are traditionally ionized by chemical ionization, because they do not respond well to electrospray. In the past, in order to ionize analyte compounds of a broader range, multimode ion sources were invented to ionize samples with two or more different mechanisms, such as an ion source having an electrospray portion and a chemical ionization portion that has a corona discharge needle (see, e.g., U.S. Pat. No. 6,646,257). However, our data shows that the ion source of the present invention can successfully ionize less polar compounds that are traditionally ionized by chemical ionization (Example 1).
Therefore, the present invention provides a method of generating ions from an analyte that is less polar and traditionally not amenable to electrospray ionization by using the ion sources described in this disclosure. In particular, ionization of these analytes can be achieved without adding a chemical ionization corona discharge needle or a UV light source.
The reason for this broader compound range is uncertain. Without wishing to be limited by a theory, we believe having a high charge density and a high temperature sheath gas contributes to efficient charge transfer at the border between the confined plume and the sheath gas.
The following abbreviations have the following meanings in this disclosure. Abbreviations not defined have their generally accepted meanings.
To compare the effect of different ion sources, various analyte compounds were analyzed by LCMS using an ion source as described in
The experimental conditions were as follows:
LC Conditions (except for Ergocalciferol Positive MeOH/Water, in which a gradient was used):
Ergocalciferol Positive MeOH/Water Gradient
MS Condition:
A. P. Bruins, Mass spectrometry with ion sources operating at atmospheric pressures, Mass Spec Review, 1991, 10, 53-77.
W. M. A. Niessen, Advances in instrumentation in liquid chromatography—mass spectrometry and related liquid-introduction techniques. J. Chromatography A, 794 (1998) 407-435.
U.S. Pat. No. 4,861,988.
U.S. Pat. No. 4,935,624.
M. L. Vestal, JASMS, 1992, 3, 18-26.
U.S. Pat. No. 5,352,892.
U.S. Pat. No. 5,412,208.
U.S. Pat. No. 5,495,108.
U.S. Pat. No. 6,759,650.
U.S. Pat. No. 7,199,364.
U.S. Pat. No. 6,998,605.
All of the publications, patents and patent applications cited in this application are herein incorporated by reference in their entirety to the same extent as if the disclosure of each individual publication, patent application or patent was specifically and individually indicated to be incorporated by reference in its entirety.
In addition to the embodiments described elsewhere in this disclosure, exemplary embodiments of the present invention include, without being limited to, the following:
A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention.
This application claims the benefit of U.S. Provisional Patent Application No. 61/042,703, filed Apr. 4, 2008. The entire disclosure of this prior application is hereby incorporated by reference.
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