Radiant energy – Ionic separation or analysis – With sample supply means
Reexamination Certificate
2000-07-17
2002-08-06
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Ionic separation or analysis
With sample supply means
C250S286000, C250S42300F
Reexamination Certificate
active
06429426
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an ionization chamber with a non-radioactive ionization source, preferably of an ion mobility spectrometer, an electron capture detector, or a mass spectrometer with ionization at atmospheric pressure (APIMS), with a reaction compartment, a supply line to feed analyte into the reaction compartment, and a discharge line to remove the analyte, whereby the reaction compartment is separated from an evacuated compartment by a partition which is impervious to gas, whereby a non-radioactive electron source is installed in the evacuated compartment, and forms the negative pole of an acceleration section.
Such an ionization chamber is known from U.S. Pat. No. 5,969,349 for an ion mobility spectrometer (IMS) and from U.S. Pat. No. 6,023,169 for an electron capture detector (ECD).
Ion mobility spectrometers (IMS) were introduced in the early 1970s in order to analyze and detect organic vapors in air. An ion mobility spectrometer consists of the reaction chamber in order to generate ions of substances to be analyzed, and a drift chamber in order to separate the ions. In the reaction chamber radioactive materials are normally used to generate the ions to be analyzed, e.g. tritium,
63
Ni,
241
Am etc. The disadvantage of such an IMS is that the use of a radioactive ionization source can be hazardous for the environment and the health of the maintenance personnel.
In this connection a large number of attempts were made to design IMS setups with non-radioactive ionization sources in the reaction chamber, e.g. photo-emitters for the generation of electrons. However, in these experiments it was not possible to rule out contact between analyzed gas molecules and the surface of the source. This is one of the reasons for instability in detector displays because such contact can alter the operating characteristics of a non-radioactive source.
Known IMS setups consist of a reaction chamber, a drift chamber, a non-radioactive electron source which is integrated into the said reaction chamber, a supply line connected to the reaction chamber in order to feed an analyte, and a discharge line to remove the analyte, as well as a capture electrode integrated into the drift chamber (for example, see Begley P., Carbin R., Fougler B. F., Sammonds P. G., J. Chromatogr. 588 (1991) Page 239).
The disadvantage of this known IMS is that the analyte makes direct contact with the surface of the non-radioactive ionization source, which in turn alters the operating conditions of the said ionization source and can be one of the reasons for instabilities in detector display.
U.S. Pat. No. 5,021,654 describes how a radioactive ion source can be simply replaced by a non-radioactive one in the form of a thermionic emission source.
With the ionization chamber referred to at the beginning it is possible to create an IMS or ECD setup which avoids contacts between the analyte and the ionization source and permits operation with positive and negative ions.
Due to the fact that the electron source is accommodated in a separate, evacuated compartment, all contact between the gas and its surface is avoided and prevailing operating conditions are always uniform and controlled. On the other hand, the transparency of the partition for electrons makes it possible for them to pass into the second compartment of the reaction chamber, which forms part of the IMS gas circuit and where, after the electrons have entered through the partition, molecule ions are formed for positive or negative IMS operating modes, by means of reactions with the gas molecules. In a preferred embodiment the partition which divides the reaction chamber into two compartments is made from mica. This is a particularly suitable material both with a high level of electron transparency and sufficient imperviousness to gas. To avoid any bending in the partition due to differences in pressure, it should preferably be supported by a metal mesh, e.g. made from copper, with minimal scatter and absorption of electrons.
Although the known reaction compartment already solves a range of problems, there is still the serious problem that, for the partition to be adequately transparent for electrons it must be extremely thin. This involves the risk that despite the supporting measures mentioned the window may mechanically break or leak due to the difference in pressure, particularly in light of the additional load exerted by the intensive electron bombardment, which, among other things, leads to a local thermal load which can only be dissipated inadequately via the supporting mesh and an extremely thin metal film. Most of the electrons hitting the partition are still absorbed in the wall and as operation of the electron source progresses they cause irreversible changes to the partition, as a result of which its imperviousness is reduced. Only electrons in the sub-ppm range can penetrate the wall and ionize the air components in the reaction compartment, which is why only small measuring signals occur. Larger measuring signals could be achieved by increasing either the electron current which penetrates the partition or the voltage with which the electrons are accelerated in front of the partition. However, in both cases the energy input into the partition increases and, because the charges which have penetrated are only discharged inefficiently in the wall material (e.g. mica), it brings about a reduction of the life of the apparatus, which can be dramatic, depending on the composition of the wall material.
For this reason there is still the need for an ionization chamber with a non-radioactive source of the type referred to at the beginning and having a sufficient or even higher ionization rate for the required ion molecule reactions in the reaction compartment and, on the other hand, with a stable, vacuum-tight partition with a long service life in operation.
SUMMARY OF THE INVENTION
The problem is solved, on the one hand, by an ionization chamber of the type mentioned at the beginning in which the positive pole of the acceleration section is designed as an x-ray anode in the evacuated compartment, possibly as the surface of the partition, such that a) x-ray light generated in the x-ray anode by impinging electrons reaches the partition in the direction of the reaction compartment, b) the partition is essentially impervious to the electrons of the kinetic energy achieved by the acceleration voltage and largely permeable to the x-ray light generated in the x-ray anode, and c) in the reaction compartment, possibly as the surface of the partition, one or more electrodes are installed in order to generate photoelectrons from the x-ray light passing through the partition.
The problem is also solved by an ionization chamber of the type referred to at the beginning in which the positive pole is designed as an x-ray anode in the evacuated compartment, possibly as the surface of the partition, in such a way that a) x-ray light generated in the x-ray anode reaches the partition in the direction of the reaction compartment, b) the compartment is essentially impervious to electrons of the kinetic energy achieved by the acceleration voltage and is largely permeable to the x-ray light generated in the x-ray anode, c) the x-ray light largely comprises quantum energies below 2 keV, and preferably below 1 keV, when entering the reaction compartment, so that in the reaction compartment air constituents are effectively ionized by the x-ray quanta.
The invention also comprises a method for the ionization of air constituents in a reaction compartment at atmospheric pressure, particularly of an IMS, an ECD, or an APIMS, in which x-radiation is generated by electron bombardment in a vacuum outside the reaction compartment and this x-radiation passes into the reaction compartment through a stable, vacuum tight partition, which is largely transparent for the x-radiation generated, where it releases photoelectrons and/or lower-energy x-ray quanta, which ionize the air constituents, on one or more electrodes.
Finally the invention comprises a
Bruker Saxonia Analytik GmbH
Nguyen Kiet T.
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