Apparatus and method for detecting negative ions

Abstract

.[.A negative ion detector converts negative ions to positive ions by means of a conversion anode which is maintained at a relatively high positive voltage. The resultant positive ions are detected by a standard continuous dynode electron multiplier which has its detection signal output at ground potential..]. .Iadd.Apparatus for detecting the abundance of negative ions from a source of negative ions, having a first conversion dynode and an electron multiplier, the electron multiplier having a second conversion dynode. The first conversion dynode is operated at a high positive potential to attract the negative ions whereby the negative ions impact the first conversion dynode with a substantial portion of the negative ions being converted to secondary positive ions. The second conversion dynode is operated at a potential which is less positive than the positive potential on the first conversion dynode to attract the secondary positive ions and to cause the secondary positive ions to be converted to electrons which are multiplied in the electron multiplier to provide an output signal indicative of the abundance of negative ions emanating from the source. .Iaddend.

What is claimed is: 
     
       1. Apparatus for detecting the abundance of negative ions from a source of such ions comprising: conversion means maintained at a high positive voltage for attracting and accelerating only said negative ions whereby the negative ions impact the conversion means with sufficient kinetic energy to produce a proportional amount of secondary positive ions, and electron multiplier means having an input operated at a negative potential to attract said secondary positive ions and for providing an output signal indicative of the abundance of said negative ions. 
     
     
       2. Apparatus as in claim 1 wherein said .Iadd.high .Iaddend.positive voltage is in the order of magnitude of 2000 volts. 
     
     
       3. Apparatus as in claim 1 where said output signal is referenced to ground potential. 
     
     
       4. In a mass spectrometer of the type in which both positive and negative ions are produced, mass analyzed and applied to detection means to provide output detection signals indicative of the composition of a chemical compound, the improvement comprising a .[.signal.]. .Iadd.single .Iaddend.electron multiplier means including a first stage operated at .Iadd.a .Iaddend.negative potential to attract positive ions and provide an output signal indicative of the abundance of said positive ions and .Iadd.conversion dynode .Iaddend.means for receiving the negative ions and converting them to secondary positive ions and whereby said secondary positive ions are attracted to said electron multiplier means to provide an output signal indicative of the abundance of said negative ions whereby a single .[.detection.]. .Iadd.electron multiplier .Iaddend.means serves to provide output signals indicative of the abundance of negative or positive ions. 
     
     
       5. A mass spectrometer as in claim 4 wherein said electron multiplier means provides output signals referenced to ground potential. 
     
     
       6. A mass spectrometer system comprising: an ionizing region where a chemical composition to be analyzed is ionized to form positive and negative ions, a mass analyzer disposed to receive and analyze at least said negative ions; .Iadd.conversion dynode .Iaddend.means for converting said negative ions from said analyzer to secondary positive ions, and electron multiplier means having an input operated at a negative potential to attract said secondary positive ions from said conversion means and provide an output signal with respect to ground potential indicative of the abundance of said negative ions. 
     
     
       7. Apparatus as in claim 6 where said negative ions are polyatomic and said amount of secondary positive ions for polyatomic ions is substantially 100% of said negative ions. .Iadd.8. Apparatus for detecting the abundance of negative ions from a source of such ions comprising: conversion dynode means maintained at a high positive voltage for attracting and accelerating only said negative ions whereby the negative ions impact the conversion dynode means with sufficient kinetic energy to produce a substantially proportional amount of secondary positive ions, and electron multiplier means having an input operated at a potential to attract said secondary positive ions and for providing an output signal indicative of the abundance of said negative ions. .Iaddend. .Iadd.9. Apparatus as in claim 8 wherein said high positive voltage is in the order of magnitude of 3000 volts. .Iaddend. .Iadd.10. Apparatus as in claim 8 where said output signal is referenced to ground potential. .Iaddend. .Iadd.11. In a mass spectrometer of the type in which both positive and negative ions are produced, mass analyzed and applied to detection means to provide output detection signals indicative of the composition of a chemical compound, the improvement comprising a single electron multiplier means including a first stage operated at a potential to attract positive ions and provide an output signal indicative of the abundance of said positive ions and conversion dynode means operated at a positive potential substantially more positive than the potential on the first stage of the electron multiplier means for receiving the negative ions and converting them to secondary positive ions and whereby said secondary positive ions are attracted to said electron multiplier means to provide an output signal indicative of the abundance of said negative ions whereby said single electron multiplier means serves to provide output signals indicative of the abundance of negative and positive ions. .Iaddend. .Iadd.12. A mass spectrometer as in claim 11 wherein said electron multiplier means provides output signals referenced to ground potential. .Iaddend. .Iadd.13. A mass spectrometer system comprising: an ionizing region where a chemical composition to be analyzed is ionized to form positive and negative ions, a mass analyzer disposed to receive and analyze at least said negative ions; conversion dynode means operated at a high positive potential for converting said negative ions from said analyzer to secondary positive ions, and electron multiplier means having an input operated at a potential which is less positive than the potential on said conversion dynode means to attract said secondary positive ions from said conversion dynode means and provide an output signal with respect to ground potential indicative of the abundance of said negative ions. .Iaddend. .Iadd.14. Apparatus as in claim 13 where said negative ions are polyatomic and said amount of secondary positive ions for polyatomic ions is a high proportion of said negative ions. .Iaddend. .Iadd.15. In an apparatus for detecting the abundance of negative ions from a source of negative ions, a first conversion dynode operated at a high positive potential for attracting the negative ions from said source of negative ions and causing a substantial portion of the negative ions impacting said first conversion dynode to be converted into secondary positive ions, electron multiplier means including a second conversion dynode operated at a potential which is less positive than the potential on the first conversion dynode for attracting the secondary positive ions created at said first conversion dynode and converting the secondary positive ions to electrons, said electron multiplier means multiplying the electrons emitted from the second conversion dynode to provide an output signal indicative of the abundance of negative ions emanating from the source. .Iaddend. .Iadd.16. Apparatus as in claim 15 together with means for causing said output signal to be referenced to ground potential. .Iaddend. .Iadd.17. Apparatus as in claim 15 wherein said second conversion dynode is operated at a potential which is negative with respect to ground potential. .Iaddend. .Iadd.18. Apparatus as in claim 15 wherein said electron multiplier means is a continuous dynode electron multiplier. .Iaddend. .Iadd.19. Apparatus as in claim 15 wherein said electron multiplier means is comprised of a multiplicity of discrete dynode stages. .Iaddend. .Iadd.20. In a method for detecting the abundance of negative ions from a source of negative ions, attracting the negative ions with a high positive potential converting a substantial portion of the negative ions to secondary positive ions, attracting the secondary positive ions and converting the secondary positive ions to electrons, and multiplying the electrons to provide an output signal indicative of the abundance of negative ions emanating from said source. .Iaddend. .Iadd.21. A method as in claim 20 wherein in the output signal is referenced to ground 
     
     
        potential. .Iaddend. .Iadd.22.  A method as in claim 20 wherein the second conversion dynode is operated at a potential which is negative with respect to ground potential. .Iaddend.