Low-vacuum mass spectrometer

Details Low-vacuum mass spectrometer Low-vacuum mass spectrometer

1998-07-29

1999-08-17

Williams, Hezron

Measuring and testing

Gas analysis

Gas chromatography

250287, 313400, G01N 3002, E04G 300, H01J 3100

Patent

active

059396130

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The present invention relates to a low-vacuum mass spectrometer, for use as a gas analyzer.


BACKGROUND OF THE INVENTION

There exist two types of gas analyzers. The first and most commonly used is a mass spectrometer (MS). In the MS, a sample of material is ionized, accelerated by an electric field in a vacuum of about 10.sup.-6 Torr, mass-selected and than detected by a charge detector (e.g., Faraday cap, electron multiplier or microchannel plate). The high vacuum is essential for the operation of MSs of any type. Recently, MSs were modified to include atmospheric pressure ionization capability. In these devices, a pressure transducer is used to reduce the pressure as the ions accelerate towards the detector.
A second type of gas analyzer is the Ion Mobility Spectrometer (IMS). In the IMS, a sample is ionized and is drifted inside a tube kept at a pressure of about 1 atm. The interaction of the ions with the gas in the tube determines its time of arrival at the charge detector. In this device, the charge detector is bound to be a Faraday cap, since no other charge detector can operate at atmospheric pressure.
Due to the high vacuum requirements, the MS is typically a large system operating with expensive and large vacuum pumps. The data obtained is based on the mass of the ions arriving at the detector. In order to achieve better analytical capability, the MS is sometimes coupled to gas chromatography (GC). In this case, the affinity between the analyzed gases and the material in the columns of the GC serves for achieving better analytical capability.
Unlike the MS, the IMS is compact in size and easy to operate. Its operation at atmospheric pressure makes it inexpensive. Two main problems, however, exist in attempting to use it as a widely used analytical tool. Since it works at atmospheric pressure, the results are difficult to reproduce from place to place and from time to time. This is due to the different pressure depending on height above sea level and weather conditions. A second severe drawback is the limited dynamic range. Since the ions are produced at low velocity, space charge limits the number of ions that can be created within the ionizing volume. This is the upper limit on signals that can be obtained. The low sensitivity of the Faraday cap used as the detector puts a lower limit on the number of ions that must be produced for a signal to become measurable. It has been established that, as a result from these two limits, the dynamic range typical for IMS is about 50, much too low for many analytical applications.
It is thus one of the objects of the present invention to overcome the limits and drawbacks of existing gas analyzers by combining the dynamic range and sensitivity typical of the MS, and the ease of operation and low cost characteristic of the IMS.


SUMMARY OF THE INVENTION

According to the invention, this is achieved by providing a low-vacuum mass spectrometer for use as gas analyzer, comprising a drift tube having a gas inlet and an ionization region at one end thereof, a power source for supplying the required high tension, a vacuum pump connectable to said drift tube to maintain a level of vacuum of up to 1 Torr, and a detector located at the other end of said drift tube, wherein said detector is a multisphere plate comprised of a multilayer arrangement of beads bonded together in a substantially close-packing order.
In the present device, the gas to be analyzed is introduced into the ionizing area. It is ionized in a small volume by an ionizer means, such as an electron beam, a radioactive source or a laser. A voltage is applied to a drift tube, through which the ions are moving towards the detector. The drift tube is filled with gas up to a pressure of about 0.1 Torr. While drifting through the tube, the ions can either collide with the gas, whereby their velocity will be reduced, or they can actually react with the gas to form new ions. Passing along the drift tube, the ions impinge on a microsphere plate (MSP) detector which is capable of operat

REFERENCES:
patent: 4482836 (1984-11-01), Washington et al.
patent: 5189301 (1993-02-01), Thekkadath
patent: 5591969 (1997-01-01), Park et al.
patent: 5770859 (1998-06-01), Bielawski

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