Patent Number: 
Section: description

Before describing the invention in detail, it must be noted that, as used in this specification and the appended claims, the singular forms xe2x80x9ca,xe2x80x9d xe2x80x9can,xe2x80x9d and xe2x80x9cthexe2x80x9d include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to xe2x80x9ca materialxe2x80x9d includes combinations of materials, reference to xe2x80x9ca compoundxe2x80x9d includes admixtures of compounds, reference to xe2x80x9ca disulfidexe2x80x9d includes more than one disulfide, reference to xe2x80x9ca nitridexe2x80x9d includes a plurality of nitrides, and the like. In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below. The term xe2x80x9cionization chamberxe2x80x9d is used herein to refer to solid structure that substantially encloses a volume in which the sample, typically a gas, is ionized. The solid structure may also constitute part of a mass analyzer; for example, an ion trap wherein electron impact or chemical ionization occurs inside the trap. The term xe2x80x9cinner surfacexe2x80x9d as used herein refers to any surface within the chamber that can be subject to undesirable interaction with the analyte. The term encompasses surfaces of a component that may not be a part of the chamber but that is disposed within the chamber, such as means for sample introduction. The term xe2x80x9cmicrostructurexe2x80x9d is used herein to refer to a microscopic structure of a material and encompasses concepts such as lattice structure, degrees of crystallinity, dislocations, grain boundaries and the like. The term xe2x80x9cnitride compoundxe2x80x9d is used in its conventional sense and refers to a compound containing nitrogen and at least one more electropositive element. Typically, nitrides exhibit a high degree of hardness and may have a wurtzite-like microstructure. The term xe2x80x9cresistivityxe2x80x9d is used in its conventional sense and refers to a material""s opposition to the flow of electric current. Unless otherwise specified, resistivity is measured in ohm-cm and is the inverse of xe2x80x9cconductivityxe2x80x9d which is measured in siemens/cm. A material""s resistivity may vary according to temperature, and unless otherwise specified, resistivity is measured at room temperature. Semiconductors are considered to be relatively nonconductive at room temperature and at normal temperatures of operation of ion sources ( less than 300xc2x0 C.). The term xe2x80x9cdisulfide compoundxe2x80x9d is used in its conventional sense and refers to a compound containing two sulfur atoms for each at least one more electropositive element. Typically, disulfides exhibit lubricating properties and may have a layered microstructure. The term xe2x80x9cmetallicxe2x80x9d as used herein refers to a material that has a low resistivity (less than 10xe2x88x921 or 0.1 ohm-cm), that exhibits hardness and resistance to abrasion in thin film form, and that is inert toward the compounds described below. In particular, metallics are distinguished from insulators and ordinary semiconductors, which have resistivities much greater than 101 or 10.0 ohm-cm. Metallics are further distinguished from pure metals, such as chromium, tungsten, iron, gold, molybdenum and their oxides, and compounds containing metalloids such as silicon nitride and nonmetals such as boron nitride. The invention is described herein with reference to the figures. The figures are not to scale, and in particular, certain dimensions may be exaggerated for clarity of presentation. FIG. 1A schematically illustrates a quadrupole mass spectrometer. Although the present example or diagram illustrates an EI source, the invention should not be construed narrowly to only this particular source and can be applied to other sources known in the art. An EI source 10 typically comprises an ionization housing or substrate 11, a repeller electrode 12 and inner surfaces 13 that define a chamber 22 (See FIG. 1A). Housing or substrate 11 may comprise any of the nitride and disulfide materials discussed below. In a second embodiment of the invention, inner surfaces 13xe2x80x2 may be applied as a coating to substrate or housing 11 (See FIG. 1B). Coating 13xe2x80x2 may comprise any of the nitride and disulfide materials discussed below. In this embodiment of the invention, substrate or housing 11 may comprise an electrically-conducting material. In the case of EI, the analyte gas 17 typically is introduced as a sample stream from a GC apparatus (not shown) into the chamber through an inlet orifice (not shown). An electron beam 15 that passes through orifices 19 into the chamber 22, from a filament 14 to an electron collector 16, interacts with the analyte molecules 17 of the analyte gas stream. The interaction results in formation of analyte ions 18 that are repelled by the repeller electrode 12 that is charged to a repelling voltage with respect to the ions. The repelling voltage has the same polarity as that of the analyte ions. The repelling force drives the ions through a lens system 20 and a mass analyzer 30 that selects the ions by mass-to-charge ratio. When the ions 18 reach the detector system 40, their abundance is measured to produce a mass spectrum for the sample. The quadrupole mass filter is preferred for the invention but various types of analyzers are also known in the art, e.g., ion traps, time-of-flight instruments and magnetic sector spectrometers. It has now been discovered that inorganic, conductive nitride compounds unexpectedly render surfaces within an ionization chamber more inert with respect to certain known reactive analytes than typical chamber surface materials such as stainless steel, gold, nickel, chromium and chromium oxides, fused silica, aluminum oxide and molybdenum. Those reactive analytes include, but are not limited to, acetophenone, 2-acetylaminofluorene, 1-acetyl-2-thiourea, aldrin, 4-aminobiphenyl, aramite, barban, benzidine, benzoic acid, benzo(a)pyrene, 1,4-dichlorobenzene, 2,4-dinitrophenol, hexachlorocyclopentadiene, 4-nitrophenol, N-nitroso-di-n-propylamine, and other compounds that occur in various solid waste matrices, soils, and water samples. The conductive nitride compound may be a titanium nitride, or a mixed metal nitride such as an aluminum-titanium nitride. Titanium nitride exhibits exceptionally inert properties with respect to many such analytes. Other nitrides include, but are not limited to, titanium carbon nitride, titanium aluminum nitride, aluminum titanium nitride, chromium nitride, zirconium nitride and tungsten nitride. In addition, nitrides in general exhibit other properties that are particularly beneficial for mass spectrometry applications. For example, nitrides when coated on surfaces of ionization chambers are extremely hard and allow parts coated therewith to be cleaned using relatively hard abrasives. Nitrides of the present invention exhibit hardness greater than about 2000 kg/mm Knoop or Vicker Microhardness, typically about 2500 to about 3500. This translates to about 85 Rc. In addition, some nitrides exhibit microstructural polymorphism that may or may not depend on the stoichiometry of the compound. Polymorphism may be the result of how the compound is formed. Alternatively, a preferred inner surface for an ionization chamber is a conductive disulfide compound. The disulfide compound may exhibit a layered microstructure. Examples of conductive disulfide layered compounds include, but are not limited to, tungsten disulfide, molybdenum disulfide, iron disulfide, copper disulfide, and titanium disulfide. These layered compounds are generally chemically inert at elevated temperatures. In particular, tungsten disulfide has unexpectedly been found to exhibit excellent inert properties in mass spectrometry applications. When surfaces of an ionization chamber are coated with a layered material such as tungsten disulfide, the layered compound provides lubrication that in turn facilitates assembly of components that formn the ionization chamber or that are disposed within the ionization chamber. Surprisingly, these materials have also been found to be inert with respect to certain known reactive analytes and to be hard and mechanically robust. If the ionization chamber is coated with a dielectric, static charge will accumulate on the dielectric during the ionization process. Such charging will cause arcing resulting in a false signal, or such charge distribution may distort the field, thereby altering the ability of the ionization chamber to produce ions. Thus, if an inert coating is employed on any inner surface of the ionization chamber, it is preferred that the coating is sufficiently electrically conductive to allow dissipation of charge, as disclosed below. Materials having a lower resistivity may be deposited in a thicker coating on an inner surface of the ionization chamber. Irrespective of the resistivity of the coating, the coating should be uniformly deposited to insure that there are no uncoated areas or pinholes as well as to provide sufficient coverage to mask active sites on the surface. As is evident, any surface of the ionization chamber, including the surfaces of the electrodes, is subject to reaction with the uncharged reagent gas or the analyte. In addition to unexpected inertness toward certain important reactive analyte substances, the compounds disclosed herein for use on ionization chamber inner surfaces exhibit certain other advantages. These compounds, having electrical resistivities no greater than about 10xe2x88x921 ohm-cm, preferably no greater than about 10xe2x88x923 ohm-cm, provide a conductive surface that resists charging by ion bombardment more than materials with higher resistivity. In particular, it is known that when typical insulating or semiconducting materials are used to provide a coating for ionization chamber surfaces, such coating usually cannot exceed about a thousand angstroms before an undesirable degree of electrical charging occurs due to accumulation of ions on the surface of the coating. The optimum thickness for avoiding charging is less than about two hundred angstroms. However, it is generally difficult to provide uniform coverage of a thin film coating over a surface; typically, thin coatings can contain pinholes or areas that are too thin to mask the reactive properties of the surface beneath the coating. Moreover, even if uniform coverage of a thin film is possible, thin films are less scratch resistant than thick films. Conducting films can be applied in any thickness without danger of charging, thus, conducting films are preferred over thin non-conducting films. In addition, since nitride compounds are harder than most metals, coatings of the present invention resist scratching better than metals and alloys that also exhibit low electrical resistivity. As an aside, for some ionic films deposited on a substrate surface, e.g., titanium nitride on a metal substrate, it has been observed that the hardness of the film depends on the hardness of the substrate. Many ionic compounds do not exhibit electrical resistivity lower than about 10 ohm-cm. Typical ionic compounds, e.g., aluminum oxide, silicon nitrides and boron nitride, exhibit an electrical resistivity greater than about 1013 ohm-cm. Examples of metal nitrides with low resistivity include, but are not limited to, titanium nitride, zirconium nitride, chromium nitride and mixed-metal nitrides such as an aluminum-doped titanium nitride. In some conductive ionic materials, stoichiometry and microstructure can greatly affect the resistivity. However, one of ordinary skill in the art, through routine experimentation, can determine the optimum stoichiometry for any of the conductive compounds of the present invention, which can be produced using any of a number of techniques as disclosed herein. Preferably, the coating consists essentially of a nitride or disulfide compound with low resistivity as disclosed above. There are many methods that can be employed to coat the compounds of the present invention onto the inner surface of an ionization chamber. One method involves a two-step process: depositing a thin layer of a metal or alloy on the surface of interest and exposing the surface to an appropriate element under reaction conditions effective to form the desired compound. There are many ways in which a thin layer of metal can be deposited, e.g., by evaporation, sputtering, electroplating, chemical vapor deposition (CVD), physical vapor deposition (PVD), etc, as is known in the art. It is notable, though, that not all methods of metallic layer deposition can be employed with ease for any particular metal. For example, a metal with a low melting point or boiling point temperature is particularly suitable for deposition through evaporation. Conversely, metals with a high melting point such as tungsten are not easily deposited through evaporation. Once a layer of metal is deposited, the layer can be exposed to a source of an appropriate electronegative element under suitable conditions to form the desired compound. For example, metal layer surfaces may be exposed to glow discharge plasma. With nitrides, a substrate having a metal layer surface is placed in a vacuum chamber. Then, ionized nitrogen gas is combined with other gases and a high voltage is applied to strike a glow to react with the substrate. It is evident that proper film formation conditions may involve high temperature processing; therefore, the material on which the surface is to be converted must be able to withstand all processing conditions. In addition, conversion of a metal layer into a compound of the present invention depends on the diffusion rate of the negatively charged species into the metal layer, and such conversion may be inefficient for some compounds of the present invention. Alternatively, the compounds of the present invention may be deposited on the surface in vacuum processes that do not involve two discrete steps as described above. Such vacuum processes include, but are not limited to, cathodic arc PVD, electron-beam evaporation, enhanced arc PVD, CVD, magnetronic sputtering, molecular beam epitaxy, combinations of such techniques and a variety of other techniques known to one of ordinary skill in the art. One of ordinary skill in the art will recognize that CVD usually involves heating a substrate surface to a sufficiently high temperature to decompose gaseous organic species to form the desired film. Such heating usually precludes the use of plastic as a surface on which the film is deposited. PVD, on the other hand, does not necessarily exclude plastics as a substrate and allows for masked film deposition. However, the method coats only surfaces that are within the xe2x80x9cline of sightxe2x80x9d of the source of the coating material, and xe2x80x9cblindxe2x80x9d spots are not coated. In addition, some substrate heating may be employed in physical vapor deposition to promote film adhesion. In the case of titanium nitride, hollow cathode discharge ion plating has been widely used. This method involves depositing titanium in the presence of nitrogen gas as a reactive gas. In hollow cathode discharge ion plating, dense films can be formed as titanium molecules are evaporated while nitrogen gas is introduced. Care must be taken, however, to ensure optimal deposition. If energy in the process is too low, the evaporated titanium does not react with the nitrogen and the resultant film does not adhere well to the surface. On the other hand, excessive energy results in re-evaporation from the substrate or damages to the surface. The highly conductive surface of the invention can be provided using the above methods. As discussed above, the coating of the highly conductive material is thicker than ordinary semiconductor or insulator coatings. Generally, the coating of the invention can be deposited having a thickness from about 1000 angstroms to about 10 microns. Thicknesses achieved with PVD are normally about 0.5 to about 2 microns, and CVD processes normally result in thicknesses of about 2 to about 5 microns. It is notable that adhesion between the compound of the present invention and the surface tends to be of marginal quality at very high thicknesses. In addition, differences in thermal expansion coefficient between the coating layer and the surface on which the coating is deposited can also contribute to adhesion problems if the surfaces are subject to drastic changes in temperature. The particular coating technique used generally affects the microstructure, morphology, and other physical characteristics of the deposited material. In addition, when the aforementioned deposition techniques are employed, variations in processing parameters can substantially change the morphology of the deposited film. In general, it is desirable to produce a smooth film of generally uniform thickness. Smooth films tend to provide a lower surface area, thereby rendering the film kinetically unfavorable for reaction with analytes. Smoothness of the film will, however, be highly dependent on, and in general determined by, the smoothness of the underlying surface. As another alternative, the surface coating material can be applied as a powder. One method of powder application involves providing the conductive compound in powdered form and employing high pressure to spray the powder entrained in a fluid at high velocity such that the powder mechanically adheres to the surface. Another method involves suspending the powder in a solvent to form a paint, applying the paint onto the surface, and evaporating the solvent. The solvent can be a relatively inert carrier or one that facilitates chemical bonding between the powder particles or between the powder and the surface. In addition, heat can be applied to evaporate the solvent or to promote chemical bonding. Typically, no organic binder is used because organic materials generally outgas at sufficiently high vapor pressure to produce a gas phase that is ionized along with the sample, producing a high background in the mass spectrum. However, the film of the present invention does not necessarily preclude inclusion of a small amount of an organic binder if overall outgassing is sufficiently low. Typically, powder application is well suited for disulfides such as molybdenum disulfide, tungsten disulfide, chromium disulfide, etc. However, one drawback to this method is that the resulting coating does not withstand abrasive cleaning as well and may have to be reapplied over time. Variation of the foregoing will be apparent to those of ordinary skill in the art. For example, while these coatings may be applied to surfaces composed of stainless steel, such coatings can also be applied to other surfaces such as aluminum or other structural materials that are typically used to form an ionization chamber or other components of a mass spectrometer. In addition, some compounds will be especially inert with respect to some analytes, and a particular coating may be applied to a surface that is designed for exposure to a specific analyte. For example, dinitrophenols are particularly reactive to components of conventional mass spectrometers. In contrast to the insulating and even conductive compounds used in the prior art, the conductive compounds of the invention, e.g., titanium nitride and various disulfides such as tungsten disulfide, have been found to exhibit unexpected inertness with,respect to dinitrophenols. Titanium nitride also exhibits unexpected inertness with respect to less reactive compounds than dinitrophenols. It is to be understood that while the invention has been described in conjunction with the preferred specific embodiments thereof, that the foregoing description as well as the examples which follow is intended to illustrate and not limit the scope of the invention. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains. All patents, patent applications, and publications mentioned-herein are hereby incorporated by reference in their entireties. A freshly cleaned inner surface of a 316 stainless steel ionization chamber was provided in an ion source of a mass spectrometer made by Agilent Technologies. The inner surface was cleaned by abrasion. Acenaphthene-d10, a calibration standard, in a standard concentration, Cis, of 40 ng/xcexcL, was analyzed using the mass spectrometer. The response of the mass spectrometer at mass 164 was used for the detection of the acenaphthene-d10. The analysis produced a peak area, Ais, for the internal standard. Then a series of analyte solutions were prepared that contained 2,4-dinitrophenol in concentrations, Cs, of 160, 120, 80, 50, 20 and 10 ng/xcexcL. The response of the mass spectrometer at mass 184 was used for the detection of 2,4-dinitrophenol. Each solution was analyzed by the mass spectrometer, resulting in a series of peak areas, As. For each solution, a relative response factor (RRF) was determined according to the following equation: RRF=(Asxc3x97Cis)/(Aisxc3x97Cs).xe2x80x83xe2x80x83(I) The RRF for each solution is reported in FIG. 2. These RRFs provide a standard against which the inertness of coatings is evaluated. An inner surface of the ionization chamber of Example 1 was coated with titanium nitride. The coating was applied by a commercial vendor. The series of analyte solutions containing 2,4-dinitrophenol was analyzed in the mass spectrometer. For each solution, RRF was determined according to equation (I). The RRF for each solution is reported in FIG. 2. It is evident that for all concentrations of 2,4-dinitrophenol, RRF was greater when a titanium nitride coating was employed. This indicates that the titanium nitride surface is less reactive with respect to 2,4-dinitrophenol than a freshly cleaned 316 stainless steel surface with no coating. An inner surface of the ionization chamber of Example 1 coated with a layer of tungsten disulfide was provided in the mass spectrometer of Example 1. The coating was applied by subjecting the ion source to ajet of tungsten disulfide particles. The coating was sufficiently thick to obscure the shine of the stainless steel. The series of analyte solutions of Example 1 was analyzed in the mass spectrometer. For each solution, an RRF was determined according to equation (I). The RRF for each solution is reported in FIG. 2. It is evident that for all concentrations of 2,4-dinitrophenol, RRF was greater when a tungsten disulfide coated 316 stainless steel surface was employed. This indicates that the tungsten disulfide surface is less reactive with respect to 2,4-dinitrophenol than a freshly cleaned 316 stainless steel ion source with no coating.