Gas Detection by Ion Mobility Segregation

The field of the present invention is gas detection and particularly gas detection determined by ion mobility range segregation.

Historically, gas detection utilizing ion molecule reactions and subsequent detection of the resulting analyte ions by their differing ion mobilities have been accomplished by two different detection methods. In each method, the reactant ions, formed by a variety of techniques, undergo ion molecule reactions with the sample molecules whereby stable positive or negative product ions are produced. These detection methods for the product ions are:

(1) Plasma chromatography. Here, ion identification is achieved by selective ion gating followed by statistical analysis of the time required for the ions to reach a collector. Since different ions have different mobilities in the presence of an electric field, the collection time can be used for ion identification.

(2) Detection by diffusion. Here, ions are contained in a constant flow stream, and travel through a complex geometric labyrinth in which they are separated by radial diffusion and subsequent neutralization before reaching the collector. The larger product ions which are able to reach the collector before neutralization are detected.

These prior methods are limited in practicability in that their functional use requires complex geometries for each type of ion requiring detection or complex time base data collection techniques and analysis for mobility identification.

The present invention is directed to an advancement in commercial gas detection techniques wherein ion segregation based on discrete ion mobility ranges reduces system complexity to a constant flow stream through controlled electric fields.

Theoretically, this technique is based on the phenomenon that electric fields can be used to influence the motion of mobile ions which have been formed by any standard ionization process. In the case of this invention, the electric fields are formed by applying a voltage potential to electrodes spaced along a flow stream. The forces acting on the ions in said flow stream are then of two origins: flow forces and field forces. Proper operation of the device is dependent upon controlling flows and fields so that the interaction of flow and field forces yields the desired ion motion. The flow forces are primarily directed axially along the tube and are constant in the direction of the flow. The field forces are complex and their effect on the ions vary in magnitude and direction as the ions travel through the field. The positive pole of the field will attract a negatively-charged ion and repel a positively-charged ion and the opposite is true for the negative pole of the field. The interaction of these forces on the ions is evidenced by a measurement of the current generated by ion/electrode charge transfer as the ions collide with the electrodes. Positive ion collision generates a positive current, while negative ion collision generates a negative current. Thus, the entrained ion charges are collected by the appropriately charged pole of the electric field, wherein the induced current is measured.

The present invention provides segregation of the ion stream into discrete ranges of ionic mobility wherein said discrete mobility range charge transfers can be measured at a specific field electrode. Normally, measurement of only one particular polarity and mobility range is made at one time. However, several collection stages can be used in a series arrangement to collect ions in either polarity in mobility ranges from high to low. The device would then behave as a series of bandpass filters with each stage collecting charge transfers for a particular range of mobilities.

FIG. l is a schematic view of a device of the invention, configured to detect a gas forming primarily positively-charged ions of a specific mobility species.

FIG. 2 is a schematic view of a device similar to FIG. 1 configured to detect a gas forming primarily negatively-charged ions of a specific mobility species.

FIGS. 3a and 3b illustrate the intensity versus mobility of clean air and of clean air with a gas sample (G).

Apparatus is shown in FIG. 1, which may be used to determine the presence of a contaminant gas by segregating ions into mobility ranges where the contaminant gas forms positively-charged ions by chemical reaction with ionized atmospheric air produced reactant ions. The apparatus shown in FIG. 2 performs the same function for a contaminant gas forming negatively-charged ions.

The apparatus or device for gas detection by ion mobility segregation shown in FIG. 1 includes a flow tube 10. The flow tube 10 has a first open end or inlet 11 and a second end or outlet 12 connected to a blower 13. The blower 13 pulls an atmospheric air sample through the flow tube 10 at a fixed average flow stream velocity. A pump could just as well be used to push an air sample through the flow tube 10.

Adjacent the inlet 11 a radiation source 14 ionizes gases within the atmospheric air stream. The gases in the air stream are ionized (electrically charged) to produce reactant ions, primarily (H.sub.2 O).sub.n H.sup.+ and (H.sub.2 O).sub.m O.sub.2.sup.-. The values of n and m depend on the amount or concentration of water vapor in the atmosphere air stream. Accordingly, the humidity can be controlled to establish different mobility ranges. Atmospheric air is a mixture of known gases and may also contain other contaminating gases which can react with the reactant ions to form stable product ions. These stable product ions of the contaminant gas are the ions of interest for identification and detection.

Referring to FIG. 1, a field E.sub.1 or first electric field 15 and a field E.sub.2 or second electric field 16 are formed along the flow tube 10. These fields 15 and 16 are formed generally axially but may be of any other complex geometry or direction. The first electric field 15 is formed between a first position 17 spaced from a second position 18 to segregate and collect ions from the gas stream within the flow tube 10 which have a first mobility range.

The second electric field 16 is positioned downstream of the first electric field 15 and is formed between a third position 19 spaced from a fourth position 20 to segregate and collect ions from the gas mixture which have a second mobility range. In FIG. 1, the second position 18 of the first electric field 15 and the third position 19 of the second electric field 16 are spaced from each other. The second position 18 may also be about the same position as the third position 19 or may be the same position.

In the preferred embodiment, the first electric field 15 includes a first electrode 21 positioned along or annularly about the flow tube 10 at the first position 17 and a second electrode 22 also positioned along or annularly about the flow tube 10 at the second position 18 and spaced from the first electrode 21. The second electric field 16 includes a third electrode 23 positioned along or annularly about the flow tube 10 at the third position 19 and a fourth electrode 24 positioned along or annularly about the flow tube 10 at the fourth position 20. In the preferred embodiment, the second electrode 22 and the third electrode 23 are separated by some distance along the flow tube 10 as shown in FIG. 1. However, electrode 22 and electrode 23 may be combined into a single electrode. Additionally, in the case where the space is large between electrodes 22 and 23 as compared to the space between the electrode sets 21 and 22, and 23 and 24, the direction of field E.sub.2 for the electric field 16 is unimportant.

A first power supply 25 provides a voltage (V.sub.A) to the first electrode 21 and a second power supply 26 provides a voltage (V.sub.B) to the second electrode 22. This provides a field E.sub.1 or first electric field 15 along the flow tube 10 having a differential voltage .DELTA.V.sub.AB between the first electrode 21 and the second electrode 22. A third power supply 27 also provides a voltage (V.sub.B ') to the third electrode 23. The fourth electrode 24 is maintained at or about ground potential. The field E.sub.2 or second electric field 16, has a voltage .DELTA.V.sub.B'C between the third electrode 23 and fourth electrode 24. In the preferred embodiment of FIG. 1 the voltage (V.sub.B) is greater than the voltage (V.sub.A), the voltage (V.sub.B') is greater than the voltage (V.sub.C), and the voltage difference .DELTA.V.sub.B'C is greater than the voltage difference .DELTA.V.sub.AB. The system may also be configured such that V.sub.B =V.sub.B' or further that power supplies 26 and 27 are the same power supply.

To detect the presence of a contaminant gas G of interest which forms primarily positive product ions from a gas phase ion molecule reaction with the reactant ions present in ionized atmospheric air, the device of FIG. 1 is used. The first electric field 15 and the second electric field 16 are generated by applying specific voltage potentials to the four electrodes 21 through 24. It is the polarity and strength of these electric fields which perform the segregation or separation and collection of the reactant and product ions contained in the flow tube. The first electric field 15 separates and collects those ions having higher mobility than the product ions of gas of interest G. The positive ions (charges) are collected on electrode 21 and the negative ions (charges) are collected on electrode 22 for these ions having a mobility greater than the mobility range of the gas G of interest. The ions of the gas G of interest continue to travel down the tube and are separated and collected in the second electric field 16. The product ions which have a mobility range less than the mobility range of the reactant ions collected in the first stage of the apparatus controlled by the electric field 15, pass through the first stage and flow down the flow tube 11 entering the second stage electric field 16. In the second stage electric field 16 the negative product ions (charges) are collected on electrode 23 and the positive product ions (charges) are collected on electrode 24. The collection of positive product ion charges on electrode 24 induces a current which is measured by an ampmeter 33 which is connected to electrode 24. An increase in current i.sub.C indicates the presence of a gas G of interest. In the same manner in which the strength of electric field 15 is set to separate and collect those ions within the reactant ion mobility range the strength of electric field 16 can be set to collect only those ions of a specific product ion mobility range. It then follows that many sets of electric fields can be used in series with each successive electric field collecting a family of ions of lower mobility range than those separated and collected in the preceding electric field (stage). Any electrode in this arrangement can be attached to an ampmeter to detect a presence of ions of the specific mobility range being collected at any electrode.

FIGS. 3a and 3b show the change in mobility when a contaminant gas G (in this case, dimethyl methyl phosphonate or DMMP) is added to clean air. FIG. 3a shows the intensity vs. mobility for the reactant ions produced from clean air. FIG. 3b shows the intensity vs. mobility of ions produced when DMMP is added. Note the formation of product ions DMMP (H.sub.2 O).sub.r H.sup.+ and (DMMP).sub.2 (H.sub.2 O).sub.s H+ where none was indicated with clean air. Also note the reduction in intensity of the reactant ions. The first electric field 15 would be used to collect all the reactant ions. This would allow the product ions to be collected by the electrodes forming the second electric field 16 where an increase in current (i) would indicate the presence of the contaminant gas DMMP.

In the device shown in FIG. 1, the diameter of the flow tube 10 is about one-quarter inch, the spacing between the first and second electrode rings 21 and 22 is about one-quarter inch and the spacing between the third and fourth electrode rings 23 and 24 is about one-quarter inch. The voltage V.sub.A is 200 volts, the voltage V.sub.B equals the voltage V.sub.B ' and is 500 volts and the voltage V.sub.C at electrode 24 is about ground. This provides a .DELTA.V.sub.AB of 300 volts and a .DELTA.V.sub.B '.sub.C of 500 volts. The flow stream velocity is about 1.2 meters per second with a flow volume of 1.3 liters per minute.

The apparatus shown in FIG. 2 includes identical or substantially equivalent components as that of FIG. 1. Identical components are identified by the same reference numbers and equivalent components are identified by reference numbers having the subscript (a). The field E.sub.4 is functionally equivalent to the field E.sub.1 except reversed to collect negatively-charged reactant ions and the field E.sub.4 is functionally equivalent to the field E.sub.2 except reversed to collect negatively charged product ions.

Thus, a device and method is described for segregating or separating and collecting ions of either polarity within specific ranges of ion mobility to determine the presence of a gas G of interest. While embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art that many more modifications are possible without departing from the inventive concepts herein. The invention therefore, is not to be restricted except in the spirit of the appended claims.