Not applicable.
Not applicable.
The present invention relates to ion mobility spectrometers. More particularly, the present invention is concerned with an improved, simple and low cost method for using an ion mobility spectrometer that separates impurity ions by their mobilities.
In the past, ion mobility has been used to detect trace impurities in bulk inert gases. However, due to the low resolution of the ion mobility spectrometer, the device suffers from interference from mobility peaks generated by the bulk inert gas itself. In addition, due to the limitations of charge transfer mechanism, an ion mobility spectrometer has not been successfully used to analyze trace impurities in bulk oxygen.
Ion mobility spectrometry, as the name suggests, uses the separation of ions based on their mobilities. The separation occurs, in time, by allowing the ions to drift in a bath of gas, the drift gas, under the influence of a uniform electric field. The ions interact with the molecules of the drift gas, and this interaction is dependent on the mobility of the ions, the electric field, and the nature of the drift gas. The separation of the ions is somewhat analogous to the separation of molecules in gas chromatography, and, therefore, ion mobility spectrometry is sometimes referred to as plasma chromatography.
The prior art teaches that an ion mobility spectrometer can be used to detect trace level impurities in air. For example, U.S. Pat. No. 4,238,678 refers to the use of ion mobility spectrometer to detect the presence of very small concentrations of vapors and gases in air or other gaseous backgrounds. In an ion mobility spectrometer, the interaction time for the ions to interact with the trace impurity atoms/molecules is relatively large. This leads to the formation of cluster ions. The mobilities of the different ions produced in the interaction region, are not necessarily very different. Moreover, the resolution of a typical ion mobility spectrometer is not very large. This leads to the situation where two or more trace impurities will produce ions with similar mobility. This can prevent the unambiguous detection of the trace level impurity of interest.
It has long been recognized that water, which is omnipresent and which has a great propensity to form cluster ions, is one of the main interfering species. U.S. Pat. No. 5,457,316 refers to the use of a hermetically sealed ion mobility spectrometer for the detection of trace gases. This patent recognizes the interference problems caused by the presence of water and teaches us the need for purifying the drift gas. The ""316 patent also teaches purifying the sample gas and using the purified sample gas as the drift gas. The ""316 patent also teaches the use of a hermetically sealed ion mobility spectrometer so that the presence of water in the ion mobility spectrometer can be drastically reduced. The use of this invention dramatically reduces the interference problems due to water clusters. However, interfering ions generated by other co-existing trace level impurities are still present. One example of such an interfering ion is the nitrogen cluster ions N3+, N5+. In an ion mass spectrometer used to determine impurities in nitrogen, the determination of trace levels of O2 will be hindered by the presence of these nitrogen cluster ions, since O2+ has the same mobility as these nitrogen cluster ions.
U.S. Pat. No. 4,551,624 refers to the use of a reagent gas to improve the specificity of an ion mass spectrometer, with the reagent gas chosen so that the electron affinity or proton affinity or acidity of the reagent gas is higher than that of the interfering species and lower than of the trace impurity of interest. This method implicitly assumes that the proton affinity, electron affinity or acidity of the interfering species is less than that of the trace impurity of interest.
U.S. Pat. No. 5,095,206 refers to the use of sulfur dioxide dopant to overcome interference problem with the detection of acid gases in air.
U.S. Pat. No. 5,283,199 provides a method for using an ion mobility spectrometer where a controlled concentration of an amine such as methylamine is added to the air carrier gas stream. The amine suppresses the chlorine peak, thereby improving the specificity of the ion mobility spectrometer to chlorine dioxide.
Finally, U.S. Pat. No. 3,621,239 generally provides methods of ion detection and separation by use of different species of reactants on a sample gas for producing predictable reactions.
It is principally desired to provide a method for improving the performance of an ion mobility spectrometer used to detect trace atmospheric impurities in gases.
It is further desired to provide a method for improving the performance of an ion mobility spectrometer used to detect trace atmospheric impurities in gases that is relatively simple and low cost.
It is further desired to provide a method for improving the performance of an ion mobility spectrometer used to detect trace atmospheric impurities in bulk inert gases where interference from mobility peaks generated by the bulk inert gas itself is minimized.
It is further desired to provide a method for improving the performance of an ion mobility spectrometer used to detect trace atmospheric impurities in bulk inert gases to analyze trace impurities in bulk oxygen.
It is still further desired to provide a method for improving the performance of an ion mobility spectrometer used to detect trace atmospheric impurities in bulk inert gases where the method provides for the separation of the mobility peaks due to interfering ions from the mobility peak due to the trace impurity of interest, so that the trace impurity of interest can be determined unambiguously.
Finally, it is desired to provide a method for improving the performance of an ion mobility spectrometer used to detect trace atmospheric impurities in bulk inert gases where a reagent gas alters the nature of the ions formed by the bulk inert gas to shift the location of a bulk inert gas mobility peak such that the bulk inert gas mobility peak does not overlap with an impurity mobility peak of the ion of a trace impurity of interest, whereby bulk inert gas ions are quenched and a cluster of the reagent gas and the bulk gas is formed.
The present invention is directed to a method for operating an ion mobility spectrometer that uses a bulk gas and a drift gas where the spectrometer includes an analyzer cell having an ionization region, an interaction region, and a drift region, where the ionization region has an ionization source, and where the spectrometer also includes a shutter grid separating the interaction region and the drift region. The drift region has an ion current detector for detecting ions transiting the drift region and means for measuring the transit times through the drift region of ions generated in the ionization region and released into the drift region through the shutter grid. The method includes the steps of applying a drift gas stream to an inlet in the drift region, mixing a selected reagent gas with a bulk gas to create a doped bulk gas stream, applying the doped bulk gas stream to an inlet in the ionization region to carry a test sample of the doped bulk gas stream into the interaction region to form ions of the doped bulk inert gas. The process further includes measuring an ion current at the ion current detector at a time corresponding to the transit time through the drift region, of ions generated by the test sample in the interaction region. The reagent gas is selected to cause ions generated by the doped bulk gas stream in the interaction region to have transit times through the drift region different from the transit times through the drift region of ions generated by the test sample. The reagent gas is also selected for its capacity to alter the nature of the ions formed by the bulk inert gas to shift the location of a bulk inert gas mobility peak such that the bulk inert gas mobility peak does not overlap with an impurity mobility peak of the ion of a trace impurity of interest, whereby bulk inert gas ions are quenched and a cluster of the reagent gas and the bulk gas is formed.
One embodiment of the method for operating an ion mobility spectrometer includes providing the bulk inert gas as N2, where the trace impurity of interest is O2, and the reagent gas is Ar. Another embodiment includes providing the bulk inert gas as O2, where the trace impurity of interest as H2O, and the reagent gas is Ar and H2.
Alternatively, rather than applying the reagent gas to the bulk gas stream, the reagent gas may be supplied as the drift gas in the ion mobility spectrometer.