US 20080290270 A1
An apparatus for producing analyte ions for detection by a mass spectrometer is described. The apparatus includes an ion source in which the surface of a target substrate for holding an analyte sample includes structured carbon nanotube material. The structured carbon nanotube material is structured in terms of being situated on a selected portion of the target support surface an/or in terms of being aligned in a selected orientation.
1. A mass spectrometer system comprising:
(a) an ion source for generating ions from a sample;
(b) a target support situated in the ion source having a surface for holding the sample, the surface including a structured carbon nanotube material;
(c) a laser for ionizing the sample on the structured carbon nanotube surface; and
(d) a detector situated downstream from the ion source for detecting the analyte ions.
2. The mass spectrometer system of
3. The mass spectrometer system of
4. The mass spectrometer system of
5. The mass spectrometer system of
6. The mass spectrometer system of
7. The mass spectrometer system of
catalyst material situated on a selected portion of the target support surface;
wherein the carbon nanotube material is situated on the selected portion containing the catalyst material.
8. The mass spectrometer system of
9. The mass spectrometer system of
10. The mass spectrometer system of
11. The mass spectrometer system of
12. A ion source for use in ionizing a sample, comprising:
(a) an irradiating source for ionizating the sample to form analyte ions; and
(b) a target support having a surface for holding the sample, the surface including a structured carbon nanotube material.
13. The ion source of
catalyst material situated on a selected portion of the target support surface, wherein the carbon nanotube material is coated over the catalyst material.
14. The ion source of
15. The ion source of
16. The ion source of
17. A method of producing a target support having a surface including structured carbon nanotubes for holding a sample in an ionization source:
coating catalyst material over the surface of the target support;
removing catalyst material except over a selected portion of surface of the target support;
growing carbon nanotubes selectively on the catalyst material using a carbon source; and
placing the sample on the carbon nanotubes.
18. The method of
19. The method of
prior to coating the target support with catalyst material, providing platforms on the selected portion of the target support surface.
20. The method of
growing the nanotubes in an alignment perpendicular to the target support surface.
21. The method of
22. The method of
growing the nanotubes in an alignment parallel to the target support surface.
23. The method of
(a) coating the carbon nanotubes on the target support with a material susceptible to alignment via an electric field;
b) subjecting the target support to an electric field; and
c) removing the alignable material.
24. The method of
25. The method of
26. The method of
In the preparation of samples for MALDI ionization, an analyte sample is typically intercalated in a matrix material on the surface of a support plate. A laser is then directed onto the targeted sample on the support plate in order to desorb energized matrix and analyte particles, which then causes ionization of the analytes through electron transfer reactions.
Carbon nanotubes have recently been used as a surface coating on MALDI sample target plates due to their exceptional physical and chemical properties, such as their hydrophobicity and their capacity to absorb ultraviolet radiation, which is the typical range of wavelengths used for the laser in the MALDI method. The carbon nanotubes are currently deposited over the target sample plate by chemical vapor deposition (CVD), which leaves a thin film of randomly ordered single or multi-walled carbon nanotubes that covers the entire exposed surface of the target plate. While the carbon nanotubes may be a useful material even in this spatially non-specific and relatively disordered state, it is believed that some of the beneficial properties of the carbon nanotubes are not necessarily being taken full advantage of in the MALDI process when used in this way.
The present invention provides a target support having a surface that includes structured nanotubes for holding a MALDI sample. The nanotubes may be structured spatially such that the nanotubes occupy a selected portion of the target support surface, and/or the nanotubes may be structured in terms of their alignment and orientation.
In a first aspect, the present invention provides a mass spectrometer system that comprises an ion source for generating ions from a sample, a target support situated in the ion source having a surface for holding the sample, the surface including a structured carbon nanotube material, a laser for ionizing the sample on the structured carbon nanotube surface, and a detector situated downstream for the ion source for detecting the analyte ions.
In a second aspect, the present invention provides an ion source for use in ionizing a sample that comprises an irradiating source for ionizating the sample to form analyte ions, and a target support having a surface for holding the sample, the surface including a structured carbon nanotube material.
In yet another aspect, the present invention provides a method of producing a target support having a surface including structured carbon nanotubes for holding a sample in an ionization source, the method comprising coating catalyst material over the surface of the target support, depositing carbon material over the catalyst material, removing catalyst material except over a selected portion of surface of the target support, growing carbon nanotubes from the carbon material over the selected portion including the catalyst material, and placing the sample on the carbon nanotubes.
In one implementation of the exemplary method of the present invention, the carbon nanotubes are grown such that they are aligned substantially either perpendicular or parallel to the surface of the target support.
In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below.
The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a portion of the surface of a target support” encompasses more than one portion of a surface of a target support.
The term “adjacent” means near, next to or adjoining. Something adjacent may also be in contact with another component, be spaced from the other component or contain a portion of the other component. For instance, a carbon nanotube that is adjacent to a surface or plate may be next to the surface, on the surface or plate, embedded in the surface or plate, fixed to the surface or plate, contact the surface or plate, surround the surface or plate, and/or comprise a portion of the surface or plate.
The term “ion source” or “source” refers to any source that produces analyte ions. Ion sources may comprise other sources besides AP-MALDI ion sources such as electron impact (EI), chemical ionization (CI) and other sources known in the art.
The term “structured” refers to a spatial positioning and/or alignment that is not random.
The ion source 3 may comprise a variety of different types of ionization mechanisms known in the art. For example, the ion source 3 may comprise a matrix-assisted laser desorption ionization source (MALDI) at either atmospheric (AP-MALDI) or non-atmospheric pressure. Other potential sources include electron ionization (EI), chemical ionization (CI), atmospheric pressure ionization (APPI), atmospheric pressure chemical ionization (APCI) and combinations of these devices. In general, the present invention may be applied to any ion sources which comprise a laser for the production of an ion plume, or perform a particular surface ionization or production of ion plume from a surface.
The transport system 5 is situated downstream and adjacent to the ion source for receiving analyte ions therefrom, and may comprise a variety of ion optics, conduits and other devices known in the art that are used to guide ions. These devices may or may not be under vacuum.
The ion detector 7 is situated downstream from the transport system and may comprise a mass filter for selecting ions according to their mass to charge ratio, and a device, such as a charge-coupled device (CCD) that detects the impact of analyte ions, and can therefore provide information as to their abundance.
The ionization region 15 is located between the ion source 3 and a capillary 6 that leads into the transport region 5. Analyte ions that enter the ionization region 15 may be guided towards the capillary by a gas dynamics provided by a gas source 7 through a conduit 9.
The target support 10 is designed to hold and maintain a target 13. The surface of the target support 10 is coated with carbon nanotubes structured according to the present invention. The surface of the target support 10 may be entirely covered with the target 13, or, more typically, the target 13 may be placed on a portion of surface of the target support.
Carbon nanotubes are graphene cylinders that have high chemical resistance and mechanical strength. In addition, carbon nanotubes in their natural state are extremely hydrophobic, i.e., they do not form hydrogen bonds with water molecules and thus combine with other hydrophobic materials in aqueous solution due to surface tension. Coating or modifying the surface of the target support 10 with nanotubes provides an extremely large surface area with good binding or attachment sites for analyte and/or matrix materials, particularly if they are hydrophobic or have hydrophobic side portions such as side chains. This property of nanotubes, along with its exceptional UV absorbtivity, promotes ionization and production of an ion plume in the MALDI process.
Both single-walled nanotubes (SWNTs) and multiple-walled nanotubes (MWNTs) can be synthesized and grown by various techniques known in the art. Typically, carbon nanotubes are grown on a “seed” layer of metal catalyst deposited on a substrate and then carbon is deposited over the catalyst using chemical vapor deposition (CVD) of a hydrocarbon gas. To create nanotube structures with specific locations on the target support, the positioning of the catalysts on the support is precisely controlled.
Platforms 22, 24 may be deposited, positioned or layered on the surface of the target support in any number of ways as commonly known in the art. The platforms 22, 24 may comprise a material that is generally resistant to certain chemical and heat treatments that are applied in subsequent steps as discussed below, such as silicon, silicon nitride, glass, quartz, mica and combinations thereof. The material of the platforms 22, 24 may also be functionalized with hydrophobic or hydrophilic compounds to promote binding.
A catalytic material layer 30 is then deposited over the surface of the target support 10, including the platforms 22, 24. The catalyst may include a variety of metals, such as Pt, Au, Al, Fe, Ni, Co, W, Ti, Ta, Cu and combinations thereof. The catalyst may be dispersed over the target support 10 in a number of ways. For example, the catalyst may be mixed with a polymeric binding material, such as polyvinylpyradine (PVP), and then spin-coated over the target support surface. This coating may then be annealed at a low temperature to adhere to the target support 10. In an alternative catalyst deposition process, an ion or electron beam incident upon a gaseous precursor, such as an organometallic compound, causes a catalyst film layer 30 to be deposited substantially uniformly over the surface of the target support 10. C7H7F6O2Au and C9H16Pt are examples of organometallic compounds which may be used in this context.
After the catalyst has been deposited over the target support surface, all of the catalyst directly contacting the target support surface is removed, while the catalyst adhered onto the platform surfaces remain. This may be accomplished by use of chemical etchants, high temperature sintering, or by patterning via lithography or ion beam. Since the platform surface may be functionalized, the binding between the catalyst layer and the platforms is stronger than the level of binding between the catalyst and the target support, so that the catalyst layer on the platforms is not degraded by the chemical and temperature treatments.
Nanotube growth (synthesis) conditions, in terms of temperature and chemical environment, can be adjusted so that mostly single-walled, or alternatively mostly multiple-walled nanotubes are produced. In particular, prevailing synthesis conditions affect the diameter of the nanotubes produced. For example, it has been found that a high hydrogen concentration during synthesis tends to result in smaller nanotube diameter sizes.
In one embodiment, the platforms are fabricated on the nanometer scale and have surfaces with dimensions on this scale. If such platforms are spotted on the surface of the surface of the target support in an array and a nickel catalyst layer is deposited onto the platform spots, it has been shown that single-walled carbon nanotubes can grow in an aligned manner from such spots. However, if the area of the catalyst upon which the nanotubes grow is large in comparison to the diameter of the nanotubes (as will be the case in many applications), the nanotubes will not all necessarily be initially aligned with respect to the surface or one another, either perpendicular to the platform (and target support) surface, or parallel to the platform surface. To create a structure array of nanotubes on a surface having a substantially uniform alignment in this case, the nanotubes can be aligned by growing the nanotubes in a template element, or by applying other alignment techniques.
In one embodiment, in which a perpendicular alignment is desired, a metallic template 50 having numerous cylindrical channels 55 dimensioned on the nanometer scale is positioned over the catalyst layer on the platforms of the target substrate. The channels are aligned upright, perpendicular to the surface of the platform and target support. An example of such a template 50 is shown in
Once the nanotubes have been synthesized within the channels of the template, etching procedures can be employed to dissolve or otherwise remove the template leaving aligned, free-standing nanotubes covering the surface of the platforms. Such a nanotube array, an example of which is shown in
Another advantage of using carbon nanotubes, particularly in the upright configuration, is that portions of the laser radiation incident on the sample which would otherwise be absorbed can be reflected back onto the sample for desorption and ionization because the tubes are hollow. As shown in
Having described the present invention with regard to specific embodiments, it is to be understood that the description is not meant to be limiting since further modifications and variations may be apparent or may suggest themselves to those skilled in the art. It is intended that the present invention cover all such modifications and variations as fall within the scope of the appended claims.