Radiant energy – Ionic separation or analysis – Ion beam pulsing means with detector synchronizing means
Reexamination Certificate
1999-12-02
2001-09-18
Berman, Jack (Department: 2881)
Radiant energy
Ionic separation or analysis
Ion beam pulsing means with detector synchronizing means
Reexamination Certificate
active
06291821
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to Ion Mobility Spectrometers (IMS), a class of chemical or mass analyzers used to identify trace constituents of a gas mixture by ionizing them, introducing a number of the resulting ions into a space to which an electric field is applied, and measuring the time taken for the ions to traverse the length of this drift space under the influence of the field. It particularly relates to the monitoring of the water vapour concentration in the air or gas supplied to an IMS, and the provision of advance warning to the operator of the need to service the air-drying sub-system of the instrument.
BACKGROUND OF THE INVENTION
Ion Mobility Spectrometers (IMS) identify trace constituents of a gas mixture by ionizing them, introducing a number of the resulting ions into a space to which an electric field is applied, and measuring the time taken for the ions to traverse the length of this drift space under the influence of the field. At the end of the drift space, the ions strike a collector electrode and produce current pulses, and the time history of the collector current, showing a series of pulses corresponding to the arrival of ions of different types, is known as a plasmagram. The length of time, t
d
, for an ion to traverse the drift space and reach the collector electrode is known as its drift time, and depends on the size and mass of the ion and the charge it carries, on the length of the drift space, and on the field strength, temperature and gas pressure therein. To a first approximation, this dependence can be described by the relation
t
d
=(1
/K
0
)*(
L/E
)*(273.5
/T
)*
P/
760) (1)
where T is the gas temperature (degrees Kelvin), P is the pressure (Torr), L is the length of the drift space, E is the electric field (assumed uniform over L), and K
0
, commonly called the reduced mobility, is a constant characteristic of the particular ion. Because of the statistical nature of the collision process, and the existence of a radial component to the electric field, there will actually be a narrow range of drift times for each species of ion. That is, even if all the ions of a given species enter the drift space simultaneously, they will reach the collector at different times, and produce a current pulse of finite width. In a practical instrument, the plasmagram peak is the convolution of this pulse with the shape of the voltage pulse used to gate ions from the ionization region into the drift region. The widths of the plasmagram peaks set a limit to the resolution of the IMS, that is, the ability to distinguish between ions with similar values of K
0
.
The operation of an IMS is strongly affected, in many ways, by the presence of water vapour in the gas in the ionization and drift regions. In particular, neutral water molecules will form clusters with many types of analyte ion, with the number of water molecules clustered with each ion depending on the nature of the ion, the gas temperature, and the concentration of water molecules. These clusters are larger than the original ion, and thus have lower values of K
0
. Clusters may grow or shrink during their drift time, and thus exhibit K
0
values intermediate between those for clusters with integer numbers of water molecules. The net effect is that, if the concentration of water increases, the drift time for a ion which forms clusters will increase and the peak will widen, which may cause the ion to be miss-classified, or to overlap and hide another peak. For this reason, most IMS instruments continuously purge the ionization and drift spaces with a gas flow which has been carefully dried to a water content of a few parts in 10
5
. This is an extremely low level of water vapour, and an operator needs to know if the drying system is not meeting the permissable maximum humidity level. Accordingly, there is a need for a device and method to monitor the performance of the drying system. This is especially true for instruments where the drift gas is atmospheric air dried by a consumable desiccant. Even with scheduled preventive maintenance, and the use of indicating desiccants, many factors, ranging from operator inattention to unusually high ambient humidity, may result in the IMS being operated under non-optimum conditions, with consequent loss of sensitivity and increase in false-alarm rate. Also, the humidity levels involved are below the range of conventional commercial moisture sensors.
The prior art proposes several solutions to the problem of instability in the IMS drift times of certain species due to variable formation of clusters with water. U.S. Pat. No. 5,405,781 teaches the use of a filter to remove water from the gas circulated through the IMS, and also the advisability, in IMS systems which use calibrants to compensate for temperature and like variables, of selecting, as calibrants, materials which have a minimal tendency to form clusters with water. U.S. Pat. No. 5,405,781 teaches the use of a supplementary thermal gas drying system to prolong the useful life of an absorption filter in an IMS. However, neither this patent nor other prior art reveal any means of monitoring the water vapour concentration in an IMS so as to be assured that it is within acceptable limits for the application.
SUMMARY OF THE INVENTION
The present invention is based on the realization that it is possible to make use of the change in drift time(s) of one or more known ion species to monitor the water concentration in an IMS and to generate a warning when this concentration approaches an undesirable level. The species used are ones which are deliberately injected, or are always present because they are normal atmospheric constituents.
It is a premise of the invention that certain ions will always be present in an IMS, in easily detectable amounts, either because they arise from normal constituents of atmospheric air, or because they are deliberately injected, and, further, that some of these ions will readily form clusters with water. As a result, their apparent drift times will increase, in a predictable, and easily detectable, manner, if the water vapour concentration in the IMS increases. The invention monitors the absolute drift time of one, or the difference in drift times of two of these species, and, when the measured time increases above a preset alarm value, issues an alert to the operator. The alarm value is chosen to correspond to a level of water vapour concentration below that which will cause unacceptable degradation in system performance.
REFERENCES:
patent: 5071771 (1991-12-01), Barbour et al.
patent: 5405781 (1995-04-01), Davies et al.
patent: 5457316 (1995-10-01), Cohen et al.
patent: 5510268 (1996-04-01), Doring et al.
patent: 5796099 (1998-08-01), Jackson
Danylewych-May Ludmila
Kuja Frank
Barringer Research Limited
Bereskin & Parr
Berman Jack
Smith Johnnie L
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