Patent Application
20090095917
This patent application claims priority from German patent application 10 2007 049 350.0 filed Oct. 15, 2007, which is hereby incorporated by reference.
The present invention relates to an ion source for chemical ionization of analytes at atmospheric pressure with a non-radioactive electron source in a vacuum chamber, a reaction chamber at atmospheric pressure and a window with an electron-permeable and essentially gas-impermeable membrane in between.
Chemical ionization at atmospheric pressure (APCI=Atmospheric Pressure Chemical Ionization) is a known method for creating analyte ions in ion mobility spectrometers (IMS) and mass spectrometers. In this context, atmospheric pressure refers to a pressure of between 6×104 and 1.2×105 Pascal.
In chemical ionization, the molecules of a gas containing analyte molecules, referred to below as the carrier gas, are first ionized by interaction with nuclear radiation (α, β, or γ radiation), or with electrons, X-ray quanta, UV light or in combination. In a cascade of primary reactions, the so-called reactant ions are created. The analyte molecules are ionized by secondary reactions with the reactant ions. These secondary reactions include the transfer of electrons, protons and other charged species from the reactant ions to the analyte molecules. Negative or positive analyte ions are created, depending on the properties of analyte and reactant molecules.
APCI ion sources are employed in mass spectrometry, in particular in combination with chromatographic separation processes such as gas chromatography (GC/MS) and liquid chromatography (LC/MS).
IMS devices with drift gas at atmospheric pressure are primarily employed for the detection of trace organic vapors from drugs, pollutants, warfare agents and explosives in the air and on surfaces. Apart from the most commonly used time-of-drift type (see
In almost all atmospheric pressure IMS systems used commercially, the analyte ions are generated by radioactive APCI ion sources; beta emitters such as tritium (3H) and, in particular, 63Ni are used, and also the alpha emitter 241Am. The mean kinetic energy of the electrons from beta emitters is between about 5 and 16 keV. Due to the restrictions that surround the use of radioactive sources, non-radioactive ionization methods have also been investigated since work began on IMS. This work has concentrated on photoionization and on corona discharge. Experience has shown that both methods involve different ionization processes from those of a 63Ni source, leading to different types of analyte ions: see, Dzidic et al.: “Comparison of Positive Ions Formed in Nickel-63 and Corona Discharge Ion Sources Using Nitrogen, Argon, Isobutene, Ammonia and Nitric Oxide as Reagents in Atmospheric Pressure Ionization-MS”, in: Anal. Chem., 1976, Vol. 48, No. (12), pages 1763-1768. Some analyte molecules cannot be ionized at all by these methods, which therefore do not represent equally effective alternatives to the radioactive electron sources 63Ni, 3H and the alpha emitter 241Am.
The patent specifications DE 196 27 620 C1 and DE 196 27 621 C2 from Budovich et al. disclose non-radioactive APCI ion sources in which electrons are generated in a vacuum chamber using a non-radioactive electron source and reach an electron capture detector (ECD) or the reaction region of an IMS by passing through a window that is permeable to electrons but impermeable to gas. The primary and secondary reactions of the chemical ionization take place in the ECD chamber or in the reaction chamber of the IMS after the electrons have entered through the window. In one embodiment, the window has a plane disk of mica between three and five micrometers thick which, having a diameter of five millimeters, is able to withstand a pressure difference of one atmosphere. It has, however, been found that the ion currents in IMS are significantly smaller than when a commercial 63Ni source with an activity of 100 MBq, corresponding to the currently permitted limit, is used. The result is a worse signal-to-noise ratio and a markedly poorer detection limit for analytes. The reason for this is that the mica disk is not sufficiently permeable to electrons with an energy of around 15 keV.
Electron sources with windows that are impermeable to gas but permeable to electrons are known from other applications, such as Ulrich et al. (“Anregung dichter Gase mit niederenergetischen Elektronenstrahlen” (“Excitation of Dense Gases with Low-Energy Electron Beams”), in: Physikalische Blätter, 56 (2000), No. 6, pages 49-52), and show that windows with a plane membrane of silicon nitride, having a thickness of only 200 to 300 nanometers, can be produced. A window of this type can withstand a pressure difference of one atmosphere if the surface area of the thin silicon nitride membrane does not exceed one square millimeter. A further electron source is described by F. Haase et al. (“Electron permeable membranes for MEMS electron sources”, in: Sensors and Actuators A: Physical, Vol. 132 (2006), No. 1, pages 98-103). In this, a plane membrane of silicon nitride, only 100 nanometers thick is mounted on a supporting honeycomb of silicon. The supporting structure is between 5 and 10 micrometers thick. The diameter of each honeycomb cell is around 10 micrometers.
A proportion of the electron energy always remains in the window when an electron passes through. The main effect of the absorbed energy is to heat the window up, but secondary electrons and X-ray quanta are also generated. The electron-permeable region of the window will be referred to below as the “window membrane”. The supporting frame for the window membrane (“window frame”) is significantly thicker than the window membrane, and therefore exhibits greater thermal conductivity than the membrane. In thermal equilibrium there is a temperature gradient between the hotter center of the window membrane and its supporting frame. The inhomogeneous temperature distribution generates mechanical stresses in the window, which the window membrane must be able to withstand. Furthermore, the heating-up of the window has the effect that the window membrane becomes more permeable to gas, and consequently the pressure in the vacuum chamber can rise to a point where the function of the electron source is no longer assured.
According to an aspect of the invention, an APCI ion source uses a window with a structured form between a vacuum chamber with a non-radioactive electron source and a reaction chamber.
“Window membrane with a structured form” or “structured window membrane” means a window membrane that is not plane; instead it is shaped to structures in a third dimension, for instance, like the folds of corrugated iron or as dome-like bulges. The structures can, for instance, have a V-shaped, sinusoidal, trapezoidal or rectangular cross-section. The folds of a window membrane can be concentric with the center of the window, may run parallel to the edges of the window membrane, or, as in corrugated iron, may all be parallel to one another. In contrast to windows having a plane membrane of the prior art, a window having a structured membrane offers several advantages:
In order to ensure both adequate mechanical stability and permeability to low-energy electrons simultaneously, the thickness of the structured window membrane is preferably between 20 and 1000 nanometers, most preferably between 30 and 100 nanometers and particularly at around 50 nanometers. The thickness of the window membrane should be understood to refer to the material thickness measured normal to the surface.
A structural element in a structured window membrane is predominantly characterized by its lateral dimension (width) and its depth (see
The permeability to electrons decreases with the atomic number of the material of the window membrane. In addition, the conversion efficiency for the undesirable X-ray radiation rises with the atomic number. The mean atomic number of the materials used in the window membrane is preferably less than or equal to 33, and particularly smaller than or equal to 15. On the basis of the established methods from microelectronics, suitable materials for the manufacture of structured window membranes are, in particular, silicon, doped silicon, silicon nitride and silicon carbide.
The parameters characterizing the structures of a structured window membrane can vary at different locations across the surface: for example, the thickness of the window membrane, the material of the window membrane and the dimensions of the structures.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.
The range of electrons in solid material can be estimated using Weber's empirical equation (cited in: G. Hertz: “Lehrbuch der Kernphysik” (“Textbook of Nuclear Physics”), Volume 1, B. G. Teubner Verlagsgesellschaft, Leipzig, 1966, page 189ff):
R
max=0.5·E·(1−0.983/(1+4.29·E)), ii.
where Rmax is the maximum range in grams per square centimeter (g/cm2) and E is the electron energy in megaelectronvolts (MeV). The range of electrons in the material is expressed, in this equation, as the product of the traveling distance L and the material density ρ [g/cm3], referred to as the area density [g/cm2]. For a given material, the distance L [cm] can be calculated.
Weber's equation is applicable to electron energies between 3 kiloelectronvolts and 3 megaelectronvolts. In Weber's empirical equation, the range Rmax corresponds to about 7 half-value thicknesses for the energy loss, i.e., after traveling the range Rmax, electrons have a residual energy equal to ½7 of their initial energy (i.e., about 0.8%).
For the silicon window membrane 10b with 300 nanometers thickness, it follows that with an electron energy of 15 kiloelectronvolts, which corresponds to the mean electron energy of a 63Ni beta emitter (about 16 kiloelectronvolts), around 50% of the electron energy is absorbed in the window membrane 10b.
The internal layer 20d includes silicon nitride and is only about 50 nanometers thick. The layers 20c, 20e includes titanium nitride (TiN) and are only about 10 nanometers thick. With a total thickness d of 70 nanometers, it is the layer 20d of silicon nitride which determines the mechanical strength of the membrane window 20b. The two titanium nitride layers 20c, 20e have high electrical conductivity, preventing the window 20 from becoming electrostatically charged. Membrane windows with shaped structures having more than three layers bonded together are also possible. Through the choice of materials for the layers and their thicknesses, it is possible to optimize properties such as thermal conductivity, emissivity for thermal radiation, elasticity (mechanical strength), permeability to electrons, gas permeability (leakage rate), and electrical conductivity. Within certain limits these properties can be optimized independently of one another. Patent publication WO 2004/097882 A1 (Wieser et al.) discloses plane window membranes of several layers, where the emissivity for electromagnetic thermal radiation is maximized to minimize heating of the window membranes. In the patent specifications from Budovich et al. mentioned above, the mica disk used has a vapor-deposited aluminum coating.
Weber's equation indicates that only about 20% of the electron energy is absorbed in the membrane window 20b if the electrons have an initial energy of 15 kiloelectronvolts. When subjected to the same pressure load and electron radiation, the structured window 20 heats up less than the plane window 10. In addition, the structured window 20 exhibits lower gas permeability, even though window membrane 20b is thinner than window membrane 10b.
The measuring cell 1 of the IMS includes the APCI ion source 2 and a drift chamber 3, separated from one another by a switchable grid 4a. The switchable grid 4a is connected to a source of voltage pulses, not shown. The APCI ion source 2 includes a vacuum chamber 21 and a reaction chamber 22, separated from another by a partition 23 with embedded window 20, as shown in
The pressure in the vacuum chamber 21 should be less than about 1/100 Pascal. The reaction chamber 22 is at atmospheric pressure. IMS devices do not generally incorporate vacuum chambers, unless they incorporate integrated pumping systems with which a vacuum chamber can be evacuated. Even passive pumping systems, not using any energy, for example sorption pumps, are critical. For IMS devices that operate with air as the carrier gas, sorption pumps in particular are only of very limited value because, on the one hand, helium, as a trace gas, passes very easily through thin window membranes while, on the other hand, a sorption pump does not have either adequate pumping capacity or an ability to bind helium. In order to maintain the function of an electron source in a vacuum for a commercially relevant duration, the leakage rate of the window 20 should be less than about 10−10 Pascal liters per second. Thickness, surface area, material and, in particular, the temperature of the window membranes determine the leakage rate. As the temperature of a structured window membrane is lower than that of plane window membranes, APCI ion sources according to the invention are able to operate significantly longer without an integrated vacuum system.
The housing of the reaction chamber 2 and the housing of the drift chamber 3 each include metal rings 9 separated by rings 10 of an insulating material such as ceramic. The metal rings 9 are connected to a high-voltage DC source via a voltage divider in such a way that an electrical drift field acting in the direction of a collecting electrode 5 is created in both chambers 22 and 3. A screen grid 4b is located directly in front of the collecting electrode 5, decoupling the collecting electrode 5 electrostatically from the drift chamber 3. To avoid complicating the diagram, the voltage divider, the high-voltage DC source and the electrical circuit are not illustrated.
The reaction chamber 22 incorporates a gas supply line 6 and a gas exit line 7 serving respectively to introduce and exhaust the carrier gas containing the analytes. At the end of the drift chamber 3 there is a gas supply line 8 for filtered drift gas that does not contain any analyte molecules. A gas flow in the direction of the reaction chamber 22 is created within the drift chamber 3, thereby preventing the carrier gas with the analytes from entering the drift chamber 3. The drift gas also leaves the measuring cell 1 of the IMS through the gas exit line 7.
The vacuum chamber 21 contains a thermal emitter 24 in the form of a tungsten filament 24 connected to a filament voltage source 25. The electrons in the vacuum chamber 21 can, however, also be generated in other ways, for instance using a field emission emitter (cold emitter) or a photo-emitter. It is also possible for the vacuum chamber 21 to contain a multitude of electron emitters, in which case a separate membrane window with shaped structures can be incorporated for each individual electron emitter.
The thermal emitter 24 is connected to the negative pole of an accelerating voltage source 28a, while the conductive partition 23 and the conductive titanium nitride layers 20c, 20e of the window 20 are connected to the positive pole of the accelerating voltage source 28a. The accelerating voltage 28a is preferably between 2 kV and 200 kV, most preferably between 5 kV and 50 kV, and particularly around 15 kV. The control electrode 27 is at a small negative potential, such as minus 10 V, provided by the voltage source 28b. The position of the control electrode 27 between the thermal emitter 24 and the partition 23, its dimensions and its potential 28b are designed so that the primary electrons are guided to the window 20.
The measuring cell 1 of the IMS operates as follows. The tungsten filament of the thermal emitter 24 is heated by a current from a filament voltage source 25, and emits primary electrons 26. After passing through the accelerating voltage between the thermal emitter 24 and the partition 23, the primary electrons 26 have a kinetic energy of around 15 kiloelectronvolts. The control electrode 27 operates as electron lens and focuses the primary electrons 26 onto the window 20.
Having passed into the reaction chamber 22, the electrons interact with the molecules of the carrier gas, and also with the analyte molecules. The range of the most energetic electrons in air at standard pressure is approximately 4 mm, with the mean range being around 1 mm. The region in which the APCI ion source 2 can generate reactant ions is therefore very restricted. Ionization of the analyte molecules in the reaction chamber 22 takes place predominantly through reactions with the reactant ions. The analyte molecules are thus ionized in the reaction chamber 22 itself. On the other hand, it is also possible for reactant ions to be generated in a first reaction chamber and then to be transferred to a second, spatially separate, reaction chamber, where the analyte molecules are then ionized by secondary reactions, as described in, for example, the patent specification DE 196 37 205 C2 by H. Hertle et al: “Massenspektroskopie-Verfahren” (“Mass Spectroscopy Methods”).
The voltages applied to the metal rings 9 create an electrical field, which moves the ions generated in the reaction chamber 22 (which may be positive or negative, depending on the polarity of the high-voltage DC source) toward the switchable grid 4a. Short, periodic voltage pulses (0.1 to 5 ms) are supplied to the switchable grid 4a by a pulsed voltage source (not illustrated). These voltage pulses open the switchable grid 4a, permitting an ion packet to enter the drift chamber 3. In the electrical field of the drift chamber 3, the ions move toward the screen electrode 4a and the collecting electrode 5. During the drift period, the ions become temporally separated due to their different ion mobilities. When they impinge on the collecting electrode 5, the ions generate an electrical current that is amplified by an electrical circuit and measured. The measured function of the ion current against the drift time is referred to as the ion mobility spectrum, and is specific to each analyte.
The window membrane 30b has corrugated folds and a radial symmetry. The folds are about 1 micrometer wide and spaced about 2 micrometers apart. The geometric dimensions of the shaped structural elements can, however, differ at different parts of the window membrane. For example, the depth of the folds here is about 2 micrometers in the center and decreases to about 0.5 micrometer toward the edge. In comparison with silicon nitride, silicon carbide has a lower specific electrical resistance (SiC: 102 to 106 Ohm-centimeters, Si3N4: 109 to 1015 Ohm-centimeters), as a result of which an electrostatic charge on the window 30 can be avoided without the need for additional layers. In addition, silicon carbide has a thermal conductivity some five times greater than that of silicon nitride (SiC: 30 to 250 Watts per meter-Kelvin, Si3N4: 7 to 70 Watts per meter-Kelvin). Generally speaking, the material used for a window membrane with a structured form should have a high thermal conductivity, preferably more than 10 Watts per meter-Kelvin (W/(m·K)), and particularly greater than 100 Watts per meter-Kelvin.
In contrast to the windows 20 and 30, the structured window membrane 40 is not folded, but has a large number of round, dome-shaped bulges 40c. The bases of the bulges can, however, be plane or conical. The bulges 40c have a width of approximately 1 micrometer and a depth of about 2 micrometers. The distance between neighboring bulges 40c is 4 micrometers.
In general, the materials of the window and the type, size, shape and spacing of the structural elements used for the ion source are chosen, depending on the particular application, on the basis of the electron-permeability, gas-impermeability, mechanical strength, and reliability and cost of manufacture. For the manufacturing process, a variety of methods known in the technical area of chip production can be used, for example, but not restricted to, pure silicon, doped silicon, and silicon containing materials like silicon alloys and silicon compounds.
The quantitative data given in connection with the individual example embodiments, particularly regarding materials, the general structure and the preferred dimensions are not restricted to these embodiments, but can be applied analogously to other cases that the specialist will recognize. The invention is not limited to the embodiments described. Persons skilled in the art will be able to develop modifications and combinations.
Although the present invention has been illustrated and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2007 049 350.0 | Oct 2007 | DE | national |
