Atmospheric pressure chemical ionization ion source
An ion source for chemical ionization of analytes at atmospheric pressure with a non-radioactive electron source in a vacuum chamber, includes, a reaction chamber at atmospheric pressure, and a window with an electron-permeable and essentially gas-impermeable membrane in between. The window may be a structured window membrane, i.e. a window membrane with a structured form comprising a multitude of structural elements, between the reaction chamber and the vacuum chamber.
1. An atmospheric pressure chemical ionization (APCI) ion source, comprising:
a vacuum chamber having an electron source;
a reaction chamber under atmospheric pressure coupled to the vacuum chamber; and
a window having a window membrane that is permeable to electrons and essentially impermeable to gas and that is three-dimensionally structured at a thickness less than 100 nanometers, the window separating the vacuum chamber and the reaction chamber.
2. The ion source of claim 1 , wherein the structured window membrane further comprises a plurality of structural elements in a lateral plane of the window.
3. The ion source of claim 2 , wherein the structural elements has a size between 0.1 and 100 micrometers.
4. The ion source of claim 2 , wherein the ratio of the lateral size of the structural element to its depth is between 5:1 and 1:10.
5. The ion source of claim 2 , wherein the spacing between neighboring structural elements is between 0.2 and 100 micrometers.
6. The ion source of claim 2 , wherein at least one of the thickness of the structured window membrane, the material of the structured window membrane, and the shape and the dimensions of the structured elements of the structured window membrane change in at least one lateral dimension.
7. The ion source of claim 1 , wherein the structured window membrane comprises a material having a thermal conductivity greater than 10 Watts per meter-Kelvin (W/(m·K)).
8. The ion source of claim 1 , wherein the structured window membrane has an electrical conductivity that prevents the development of electrostatic charge on the window membrane.
9. The ion source of claim 1 , wherein the mean atomic number of the materials used in the structured window membrane is at least one of equal to and less than 33, the materials comprising at least one of silicon, silicon nitride, silicon carbide, boron nitride, carbon in at least one of an amorphous and crystalline phase, and titanium nitride.
10. The ion source of claim 1 , wherein the structured window membrane comprises a plurality of layers bonded together.
11. The ion source of claim 1 , wherein the structured window membrane further comprises a plurality of folds.
12. The ion source of claim 1 , wherein the structured window membrane further comprises a plurality of bulges, each bulge having one of a dome and truncated cone shape.
13. The ion source of claim 1 , wherein the structured window membrane has a radial symmetry.
14. The ion source of claim 1 , wherein the vacuum chamber further comprises an electrical acceleration region with an accelerating voltage of between 2 and 200 kV.
15. The ion source of claim 14 , wherein the electron source is connected to a negative pole and the window is connected to a positive pole of the electrical accelerating region.
16. The ion source of claim 1 , wherein the electron source comprises at least one of a thermionic cathode, a field emitter cathode and a photocathode.
17. A method for atmospheric pressure chemical ionization (APCI), comprising:
generating electrons from an electron source in a vacuum chamber;
permeating the electrons through a window having a window membrane that is essentially impermeable to gas and that is three-dimensionally structured at a thickness of less than 100 nanometers, the window separating the vacuum chamber and a reaction chamber under atmospheric pressure;
filling the reaction chamber with a carrier gas having analyte molecules; and
ionizing the analyte molecules with the electrons.
18. The method of claim 17 , further comprising accelerating the electrons in the vacuum chamber.
19. The method of claim 17 , wherein the structured window membrane comprises a plurality of structural elements in a lateral plane of the window.
20. The method of claim 17 , further comprising measuring an electrical current generated by the ionized analyte molecules impinging on an electrode.
21. The method of claim 17 , further comprising generating an electric field to move the ionized analyte molecules through the reaction chamber.
22. The method of claim 21 , further comprising permitting the ionized analyte molecules to pass from the reaction chamber, through a switchable grid and into a drift chamber.