ELECTROSPRAY EMITTERS

FIELD OF THE INVENTION In general, this invention pertains to nano/electrospray technology. In particular, the present invention relates to nano/electrospray emitters and methods for manufacturing such emitters.

BACKGROUND OF THE INVENTION

Various analytical instruments can be used for analyzing proteins and other biomolecules. More recently, mass spectrometry has gained prominence because of its ability to handle a wide variety of biomolecules with high sensitivity and rapid throughput. A variety of ion sources have been developed for use in mass spectrometry. Many of these ion sources comprise some type of mechanism that produces charged species through spraying. One particular type of technique that is often used is Electrospray Ionization ("ESI"). One benefit of ESI is its ability to produce charged species from a wide variety of biomolecules such as proteins. Another benefit of ESI is that it can be readily used in conjunction with a wide variety of chemical separation techniques, such as High Performance Liquid Chromatography ("HPLC"). For example, ESI is often used in conjunction with HPLC for identifying proteins.

Electrospray ionization ("ESI") has revolutionized the use of mass spectrometry in bioanalytical chemistry because of its ability to transfer large macromolecules from solution into the gas-phase as intact multiply-charged molecular ions. A special advantage of ESI is the ease with which it may be coupled to liquid chromatography ("LC"). An attractive development in recent years has been the design of methods for decreased sample consumption in ESI by using much lower flow rates (nL/min, referred to as nanospray or nanoelectrospray) than with conventional ESI (μL/min).

Typically, nanospray emitters have been fabricated by pulling silica or glass substrates under heat to produce tapered emitters with small inner diameters. For nanospray ESI-MS emitters to be useful in coupling to LC the emitters must remain stable throughout the separation process. Failure of the emitter during the course of the separation is not acceptable. For quantification in particular, if calibration curves of multiple analyses at multiple concentration levels are to be constructed, single emitters with longer lifetimes or multiple emitters showing reproducible performance and ionization efficiency are needed.

EP 1 728 897 A1 discloses a method of producing an electrocast tube having a fine inner diameter. According to this document there is no special restriction on a material or a place where the metal is electrodeposited by the electroforming as long as the material has conductivity, but it is preferable to use a material having a satisfactory electric conductivity in order to easily electrodeposit the metal. It is possible to use, for example, iron, stainless steel, copper, gold, silver, brass, nickel, aluminum, carbon or the like. The document does not specifically disclose a rigid capillary, made of e.g. nickel, with a conductive and inert inner coating, wherein the end of the tube essentially consists of the inert coating material, and wherein the edge has been sharpened by e.g. electrolysis.

WO06090620A1 discloses a method for manufacturing an integrated ultra-fine nozzle using electrocast pipes having different inner and outer diameters. The tip end of the nozzle may be tapered or the inner and outer portions thereof may be plated with a metal such as gold, silver, and palladium.

WO06135057A1 also discloses a Ni electrocast ultra-thin tube (outer diameter from 5 μm to 1 mm), and that the tip end of the tube nozzle may be tapered or the inner and outer portions thereof may be plated with a metal such as gold, silver, and palladium. This present prior art reference does not disclose the use of the ultra-thin tube for electrospray ionisation purposes.

US07141807 discloses a capillary for a mass spectrometry system. The capillary comprises a channel and a tip, and at least one of the channel and the tip comprises a nanowire material. Examples of nanowires comprise those formed from metals, such as chromium, tungsten, iron, gold, nickel, titanium, molybdenum, and the like. For certain implementations, it is disclosed that the nanowire material can form a coating, which can be applied using any of a wide variety of techniques. For example, the nanowire material can be sprayed at high velocity onto a substrate, such that the nanowire material mechanically adheres to the substrate. As another example, the nanowire material can be dispersed in a suitable solvent to form a "paint," and this paint can be applied to the substrate. There is no disclosure of a method for manufacturing an electrospray needle in accordance with the present invention. Moreover, there is no explicit disclosure of a capillary coated with a layer on its inner surface.

US24265519A1 discloses a number of fluid vaporizing devices, which can be used to deliver fluids and/or generate aerosols including capillary tubes with finished or activated inner surfaces, can be manufactured. According to the document the device can be formed from a

metal or alloy having a low melting point, such as aluminum, zinc, silver, indium, tin, lead and mixtures thereof, including solders, brazes and the like. It further mentions that an activated inner surface layer material can be deposited using sputtering, chemical vapor deposition, plasma deposition, electroless plating, electroplating, dip-coating and electrophoresis to a total thickness of from about 1 to 100 nm and can comprise cobalt, nickel, copper, rhodium, palladium, silver, iridium, platinum, gold and alloys or combinations thereof. There is neither a specific disclosure of a gold plated nickel capillary nor a disclosure of how the edge of the capillary end could be treated to render it sharp and free from e.g. nickel. Also, the document does not contemplate that such capillaries would be suitable for electrospray ionisation.

WO05096720A2 discloses a nano-electrospray emitter including an emitter body which includes a fluid inlet, an outlet orifice, and a passage for fluid communication between the fluid inlet and outlet orifice. In one aspect of this embodiment, the passage facilitating communication between the inlet and outlet elements is comprised of a capillary column (i.e., a first capillary) that partially houses a second capillary. Although not a preferred embodiment the document discloses that the first and second capillaries may be comprised of ceramic glasses, borosilicate glasses, fused silica, aluminosilicate glasses, quartz, metals such as stainless steel, titanium, nickel, gold, platinum, other conductive materials and alike. There does not appear to be a specific disclosure of e.g. an emitter based on a first capillary of nickel and second capillary of gold, neither does the document envisage the use of a more rigid outer capillary coated on its inner surface with an inert metal.

Typically, ESI produces a spray of ions in a gaseous phase from a sample stream that is initially in a liquid phase. For a conventional ESI mass spectrometry system, a sample stream is pumped through a metal capillary, while a relatively high electric field is applied between a tip of the metal capillary and an electrode that is positioned adjacent to the tip of the metal capillary. As the sample stream exits the tip of the metal capillary, surface charges are produced in the sample stream, thus pulling the sample stream towards the electrode. As the sample stream enters the high electric field, a combined electro-hydrodynamic force on the sample stream is balanced by its surface tension, thus producing a "Taylor cone." Typically, the Taylor cone has a base positioned near the tip of the metal capillary and extends up to a certain distance away from the tip of the metal capillary, beyond which a spray of droplets is produced. As these droplets move towards the electrode, coulombic repulsive forces and desolvation lead to the formation of a spray of ions in a gaseous phase.

During operation of a conventional ESI mass spectrometry system, characteristics of a Taylor cone can affect characteristics of a spray of ions, which, in turn, can affect results of mass spectrometric analysis. Accordingly, it is desirable to produce Taylor cones with certain reproducible characteristics, such that results of mass spectrometric analysis have a desired level of accuracy and reproducibility.

While nanospray provides an avenue to achieve low-level detection limits with MS, even at high salt and/or buffer concentrations, most nanoliter-flow ESI emitters suffer from short operating lifetimes, poor durability, and/or low reproducibility. Additionally, if the internal diameter of the emitter is too large, or there is too much dead volume associated with coupling the emitter to the outlet of the column, then band broadening can often be a problem thereby compromising effective analysis. Presently, there is a need for an emitter that can overcome the deficiencies in the art as currently practiced.

SUMMARY OF THE INVENTION

The present invention relates to a novel nanospray emitter. The device includes a fluid inlet, an outlet orifice, and a passage for fluid: communication between the fluid inlet and outlet orifice. In one aspect of this embodiment, the passage facilitating fluid communication between the inlet and outlet elements is comprised of an outer capillary of a rigid material, such as nickel, that houses an inner capillary made from a highly conductive and inert material, such as gold; alternatively the inner material may constitute an inner coating of the outer capillary.

The invention provides a mass spectrometry system. The mass spectrometry system comprises an ion source comprising a capillary configured to pass a sample stream. The capillary comprises a portion that is exposed to the sample stream when the sample stream passes through the capillary, and the portion of the capillary comprises a inner capillary made from a highly conductive inert metal, such as gold.

The invention also provides an ion source for a mass spectrometry system. The ion source comprises a capillary configured to produce a spray of ions and comprising an inner capillary made from a highly conductive inert metal, such as gold. In another embodiment, the ion source comprises a capillary comprising a tip that comprises an inner capillary made from a highly conductive inert metal, such as gold. The ion source also comprises an electrode

positioned adjacent to the capillary. The ion source further comprises a power source in electrical connection with the capillary and the electrode, and the power source is configured to apply a voltage between the capillary and the electrode.

The invention further provides a capillary for a mass spectrometry system. The capillary comprises a channel and a tip, and at least one of the channel and the tip comprises an inner capillary made from a highly conductive inert metal, such as gold.

Accordingly, in a specific embodiment of the present invention there is provided an electrospray emitter comprising an outer capillary and a inner capillary, wherein said inner capillary is disposed within said outer capillary, said outer capillary being made from a robust conductive material, such as nickel, and said inner capillary being made from a highly conductive and inert material, such as gold, and a portion of said inner capillary protrudes from said outer capillary.

The outer capillary of the electrospray emitter preferably comprises a material selected from the group consisting of stainless steel, nickel, and titanium, whereas the inner capillary comprises a material selected from the group consisting of gold, silver, palladium and platinum.

The electrospray emitter of the present invention is made so that the inner capillary preferably protrudes from said outer capillary by approximately 0.1 to 5 mm, and the length of the emitter preferably ranges from 10 to 100mm.

The outer capillary of the emitter preferably has an inner diameter ranging from about 15 to 30 μm and since the inner capillary is perfectly fitted within the outer capillary the outer diameter of the inner capillary likewise ranges from about 15 to 30 μm. Preferably the thickness of the inner capillary ranges from about 0.5 to 5 μm.

In order to provide the electrospray emitter with sufficient stability and elasticity it is preferred that the outer capillary has Young's modulus of 93-191 GPa and a Vickers hardness of 300- 600.

The present invention also provides a process for manufacturing an electrospray emitter of the present invention or a microtube. The process comprises the steps of submerging a

microtube having an outer capillary housing and an inner capillary into an electrochemical etchant, whereafter an electrical field is established over the tip of the microtube and the wall of the well. The microtube is then electrochemically etched until the tip of the outer capillary has been removed.

In electrochemical etching, the etchant contains an electrolyte, which may not be capable of etching a material to be etched through a chemical reaction (i.e., the etchant does not etch merely through contact with the material). By applying an electric voltage to the etchant between the material and an electrode immersed in the etchant, an electrolytical process, however, is initiated, in which the material is one pole, (e.g., the anode), and the electrode the opposite pole. In the electrolytic process, electric current flows in the etchant, and ions in the etchant react in an etching manner with the material.

The process may further comprise the step of cleaning the emitter in a solution of e.g. water and methanol (50%/50% vol/vol), and optionally sonicating the solution.

Thus the present invention provides aprocess for the selective removal of the metal of the outer capillary, such as nickel, while leaving the inert metal, such as gold, of the inner capillary in undamaged form which comprises first immersing the microtube herein referred to into an electrolyte solution which will not attack the inert metal at a specified voltage condition. Such voltage condition is next imposed across the composite and potentiostatically controlled so as to etch the metal of the outer capillary without damaging the inert metal.

By this process it is possible to manufacture a tapered microtube comprising an outer capillary and a inner capillary, wherein said inner capillary is disposed within said outer capillary, said outer capillary being made from a robust conductive material, such as nickel, and said inner capillary being made from a highly conductive and inert material, such as gold, and a portion of said inner capillary protrudes from said outer capillary, wherein the microtube has been obtained by the process of the present invention.

Advantageously, embodiments of the invention allow Taylor cones to be produced with very reproducible characteristics, such that results of mass spectrometric analysis have a desired level of accuracy and reproducibility. For some embodiments of the invention, reproducibility of Taylor cones can be achieved by using certain materials that are highly hydrophobic, highly electrically conductive, highly robust, and highly inert with respect to typical analytes.

Moreover the ionisation efficiency also increases when using the electrospray emitter of the present invention.

Other aspects and embodiments of the invention are also contemplated. The foregoing summary and the following detailed description are not meant to restrict the invention to any particular embodiment but are merely meant to describe some embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the terms "electrically conductive" and "electrical conductivity" refer to an ability to transport an electric current. Electrically conductive materials typically correspond to those materials that exhibit little or no opposition to flow of an electric current. One measure of electrical conductivity of a material is its resistivity expressed in ohm.centimeter ("ω-cm").

Typically, the material is considered to be electrically conductive if its resistivity is less than

0.1 ω-cm. The resistivity of a material can sometimes vary with temperature. Thus, unless otherwise specified, the resistivity of a material is defined at room temperature.

As used herein, the terms "robust" and "robustness" refer to a mechanical hardness or strength. Robust materials typically correspond to those materials that exhibit little or no tendency to fragment under typical operating conditions, such as typical operating conditions of the electrospray capillaries described herein. One measure of robustness of a material is its Vicker microhardness expressed in kg/mm. Typically, the material is considered to be robust if its Vicker microhardness is greater than 1 ,000 kilogram/millimeter ("kg/mm").

As used herein, the terms "inert" and "inertness" refer to a lack of interaction. Inert materials typically correspond to those materials that exhibit little or no tendency to interact with a sample stream under typical operating conditions, such as typical operating conditions of the electrospray capillaries described herein. Typically, inert materials also exhibit little or no tendency to interact with a spray of droplets or a spray of ions produced from a sample stream in accordance with an ionization process. While a material is sometimes referred to herein as being inert, it is contemplated that the material can exhibit some detectable tendency to interact with a sample stream under certain conditions. One measure of inertness of a material is its chemical reactivity. Typically, the material is considered to be inert if it exhibits little or no chemical reactivity with respect to a sample stream.

The present invention relates to a nanospray emitter including an emitter body which includes a fluid inlet, an outlet orifice, and a passage for fluid communication between the fluid inlet and outlet orifice. In one aspect of this embodiment, the passage facilitating communication between the inlet and outlet elements is comprised of a first capillary made from a rigid material, such as nickel, that completely or partially houses a second capillary made from an inert metal, such as gold; alternatively the second capillary takes form of a thin inner coating of the first capillary.

Suitable materials for use in manufacturing the first capillary are metals such as stainless steel, titanium, nickel, or other conductive and rigid materials and alike.

A representative example of a microtube suitable for being processed in accordance with the process of the present invention may be purchased from LuzCom Inc., Japan. This company offers microtubes having an inner gold capillary with thickness of 2 μm and inner diameter 18μm perfectly fitted in an outer nickel capillary having an inner diameter of about 22μm and an outer diameter of 150μm. These microtubes may be purchased in various lengths; however for the present invention the microtubes should preferably be in the range 30mm to 1 10mm, preferably 30mm to 50mm, and most preferably 40mm.

DESCRIPTION OF THE FIGURES

Figures 1 and 2 depict the electrospray emitter of the present invention comprising an outer capillary and a inner capillary, wherein said inner capillary is disposed within said outer capillary, said outer capillary being made from a robust conductive material, and said inner capillary being made from a highly conductive and inert material, and a portion of said inner capillary protrudes from said outer capillary.

The outer capillary can be comprised of nickel. However, other suitable materials are also envisaged to be within the scope of this invention including, but not limited to, stainless steel aluminosilicate glasses, stainless steel, platinum, titanium, and other robust conductive materials and alike. In one aspect, the outer capillary has an outer diameter from about 20 to 200 μm with an inner diameter of about 10 to about 50 μm.

This inner capillary is comprised of an inert material. In this case, the inert metal is gold.

However, other suitable inert metals, such as silver and platinum, or other conductive and chemically inert materials are also envisaged to be within the scope of this invention. The

inner capillary is disposed thoughout the outer capillary, alternatively near or at the outlet orifice of the outer capillary. As stated elsewhere the inner capillary is disposed within the interior lumen of the outer capillary.

The inner capillary protrudes from the outer capillary in the area of the outlet. This protrusion can be from about 5 μm to about 5 mm, preferably from about 0.5 mm to about 4 mm, most preferably from 1 .5 mm to 3 mm. This protruding portion is contiguous with the rest of the inner capillary. As the outlet becomes obstructed, a practitioner can cleave a small section of the protruding portion containing the obstruction in such a manner so as to relieve the obstruction within the inner capillary and restore original flow.

Figure 2 illustrates a cross-sectional veiew of the nanospray emitter of the present invention. The emitter comprises an outer capillary and an inner capillary. In this embodiment, the inner capillary is fully housed within the outer capillary.

Also depicted in Figure 2 is a portion of the inner capillary protruding from the end of the outer capillary. The amount in which the inner capillary protrudes is relative. Initially, the protruding portion can extend from the end of the outer capillary from about 1 to about 10 mm. (It should be understood, however, that the inner capillary extends interiorly the full length of the emitter region.) However, as the apparatus is used over time, conceivably the inner capillary will become obstructed with material. Should the obstruction occur inside the "protruding" portion of the inner capillary near the tip, then a practitioner can remove the portion of the second capillary that is obstructed using any means well known to those skilled in the art. By removing the obstruction, the original flow is restored.

Figure 3 depicts an apparatus for performing the etching process of the present invention.