Radiant energy – Ionic separation or analysis – With evacuation or sealing means
Patent
1993-07-22
1994-10-04
Berman, Jack I.
Radiant energy
Ionic separation or analysis
With evacuation or sealing means
250281, 250282, 250296, 250298, H01J 4910, H01J 4932
Patent
active
053528930
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a mass spectrometer for the accurate determination of isotopic ratios which is fitted with either an inductively coupled plasma (ICP) or microwave induced plasma (MIP) ion source. In particular it provides a magnetic sector mass o spectrometer having an ICP or an MIP ion source and several ion collectors to enable the simultaneous monitoring of two or more mass-to-charge ratios.
In prior multiple-collector mass spectrometers for high precision determination of isotopic composition, samples are normally ionized by thermal ionization: A solution of the sample is coated on a filament which after drying is transferred into the ion source of the mass spectrometer. After a period of evacuation and preheating to stabilize ion emission, which may last several hours, the filament is heated (by passage through it of an electrical current) to a temperature sufficient for thermal ionization of the sample to take place and to produce ions in sufficient quantity, for the isotopic analysis to be carried out. Ions so generated are typically characteristic of the isotopes present in the sample. These ions are accelerated through a fixed potential gradient and are separated according to their mass-to-charge ratios by a magnetic sector mass analyzer which has at least two collectors disposed to receive ions of different mass-to-charge ratios. In this way the ratio of the intensities of two or more ion beams of different mass-to-charge ratios may be instantaneously determined and the effect of any time-dependent fluctuation in the ionization intensity or mass spectrometer stability can be minimized. In some cases, integration of the ion current signals from a single filament may be continued for several hours in order to reduce fractionation effects and to smooth noisy signals from isotopes present in small quantities so that high precision ratios can be determined.
Sampling handling and pretreatment procedures for thermal ionization mass spectroscopy, although highly refined and capable of yielding isotopic ratios of very high precision, are elaborate and time-consuming, and typically only a few samples can be analyzed in a working day. A significant part of the analysis time is required for outgassing and preheating of the coated filament after it has been mounted in the spectrometer's ion source, and current thermal ionization spectrometers comprise magazines which allow several filaments to be loaded in the vacuum system and automatically analyzed in turn. Typically, outgassing and preheating of filament next in line for analysis is carried out while another filament is being analyzed. However, magazine systems of this kind are complicated and expensive mechanical devices and do not completely solve the problem of limited sample throughput because of the long sample preheating and measurement times that are necessary for thermal ionization. A need therefore exists for a sample ionization technique suitable for use with multiple-collector magnetic sector mass analyzers and which enables high precision isotopic-ratio measurements to be made with greater facility. It is the object of this invention to provide an isotopic-ratio mass spectrometer incorporating such a sample ionization technique. It is another object to provide a method of isotopic-ratio mass spectrometry which is capable of a greater sample throughput than prior high-precision isotopic-ratio mass spectrometers.
The invention provides an isotopic-ratio mass spectrometer comprising ion source means, an electrostatic ion-energy analyzer, a magnetic sector ion-momentum analyzer wherein ions are dispersed at a first potential according to their mass-to-charge ratios, and ion detecting means comprising two or more ion collectors for receiving ions of different mass-to-charge ratios, wherein: discharge in an inert gas by the action of an electromagnetic field created by a radio-frequency or microwave generator; isotopic composition is to be determined; plasma, said sampling member having an aperture communicating between said plasma and a fi
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Berman Jack I.
Fisons plc
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