A SAMPLE TRAPPING ION MOBILITY SPECTROMETER FOR PORTABLE MOLECULAR DETECTION

Abstract: A "cold" ion mobility spectrometer (IMS) device (100) deliberately operates at a temperature low enough that sample vapors condense, i.e., are trapped, in the IMS (100) after their introduction with a condensing liner (117) in a reaction region (102) of the IMS (100). The deliberate trapping of the vapors in the IMS (100) effectively removes the sample from the ionization process because after condensation, the vapor pressure of the compound at the operating temperature of the IMS (100) is negligible. Since the compound is no longer present in vapor form, ion production no longer takes place at a sufficient rate to be detectable.

Description: A SAMPLE TRAPPING ION MOBILITY SPECTROMETER FOR PORTABLE MOLECULAR DETECTION CROSS-REFERENCE TO RELATED APPLICATIONS The following patent application is based on and claims the benefit of U.S. Provisional Patent Application Serial No. 60/062,682 filed October 22, 1997. FIELD OF THE INVENTION The present invention relates generally to ion mobility spectrometers and, particularly, to a novel sample trapping ion mobility spectrometer device for portable molecular detection. BACKGROUND OF THE INVENTION There are several types of ion mobility spectrometers (IMS) in existence which differ from each other in the way that a sample substance is ionized, separated, and detected. Sample transport into a conventional IMS device 10, such as the example IMS device 10 shown in Figure 1, is normally performed by converting the sample into a vapor and injecting the vapor sample in a carrier gas into the sample inlet 12. A part of the sample is ionized in an ionizer device 16 and the rest of the sample is carried out of the IMS via a sample outlet port 14. A description of existing ion mobility spectrometer devices of this design may be found in issued U.S. Patent No. 3,621,240 to Cohen, U.S. Patent No. 4,311,669 to Spangler, U.S. Patent No. 3,845,301 to Thekkadath, and U.S. Patent No. 5,083,019 to Spangler. For example, in U.S. Patent No. 5,089,019 to Spangler, the sample is introduced via the sample inlet port 12 in a condensed form into the reaction region, such as reaction region 15 of the IMS device 10 of Figure 1, and then directly vaporized for subsequent ionization and detection. It should be noted that in all these patent disclosures, care is taken to remove residual samples after ionization by providing a continuous flow of the carrier and drift gas 20 and by keeping the IMS at a high enough temperature with respect to the sample condensation temperature. Thus, the sample is efficiently carried out of the IMS device 10 via the drift and sample output port 14, and the history of the sample injection is removed. For samples of drugs and explosives which are commonly detected by using the IMS device 10 of Figure 1, this means that the IMS and the inlet and outlet ports have to be kept at temperatures as high as 200 to 250 degrees Celsius to avoid the condensation of the sample in the inlet port or in the IMS. The condensation can lead to contamination of the system and loss in efficiency of detection. Thus, in the above mentioned disclosures relating to conventional IMS devices, the vapor pressures of the compounds in the IMS are high enough that ionization of the compound in the vapor form produces enough number of ions for detection as a signal above noise. The temperature of the IMS and the inlet and outlet ports are kept high enough so as to remove the residual vapors from the IMS. It would be highly desirable to provide an IMS device which deliberately operates at a temperature low enough such that the sample vapors introduced into the IMS actually condense in the IMS after their introduction. Such a low temperature IMS device is hereinafter characterized as a low power consumption IMS device. SUMMARY OF THE INVENTION It is an object of the present invention to provide a novel IMS device that operates at lower temperatures and is consequently characterized by its lower power consumption. It is another object of the present invention to provide a novel IMS device which deliberately operates at a temperature low enough such that the sample vapors introduced into the IMS device immediately condenses in the IMS after their introduction, thus avoiding ionization reactions in the vapor phase. In accordance with the preferred embodiments of the invention, there is provided a "cold" IMS device which deliberately operates at a temperature low enough such that the sample vapors introduced into the IMS actually condense in the IMS in a fraction of a second after their introduction. The deliberate trapping of the vapors in the IMS effectively removes the sample from the ionization process because after condensation, the vapor pressure of the compound at the operating temperature of the IMS is so low as to be negligible. Since the compound is no longer present in the vapor form, ion production no longer takes place at a sufficient rate as to be detectable. The process of sample introduction in such a cold IMS is different from sample introduction in conventional IMS devices. The sample is first transported in the vapor form to the entrance of the reaction region at a temperature several tens of degrees higher than the temperature of the reaction region and the temperature of the carrier gas flowing in the reaction region. As the sample vapor enters the reaction region, it encounters the colder gas in the region and starts to cool down. Before the cooling takes place, however, the reactant ions present in the reaction region rapidly convert a portion of the vapor sample into product ions which are subsequently swept away by an electric field into a drift region. The rest of the sample condenses on the walls of the reaction region and is no longer available for ionization reactions in the vapor phase. Advantageously, the IMS of the present invention operates at essentially ambient temperatures, thus, making it ideal for hand-held and portable types of drug and explosive detection systems powered, for example, by batteries. BRIEF DESCRIPTION OF THE DRAWINGS Further features and advantages of the invention will become more readily apparent from a consideration of the following detailed description set forth with reference to the accompanying drawings, which specify and show preferred embodiments of the invention, wherein like elements are designated by identical references throughout the drawings, and in which: Figure 1 illustrates an Ion Mobility Spectrometer (IMS) device of conventional design. Figure 2 is an illustrative cross -sectional view of the low power IMS device according to the invention. Figure 3 is a schematic diagram of an example hand-held drug detection system implementing the low power, sample trapping IMS device of the invention. Figures 4 (a) and 4 (b) depict a process flow diagram for the sample trapping IMS of the invention implemented in a battery powered portable molecular detection system. DETAILED DESCRIPTION OF THE INVENTION Figure 2 depicts the process of sample introduction and ionization in the cold IMS device 100 of the invention. As shown in Figure 2, the reaction region 102 is essentially cylindrical having an ionizing source 111 at one end of the cylinder and an electrode assembly 112 at the other end of the cylinder for creating an electric field that will transport the ions to the drift region 103. Additionally shown are two gas inlet ports 106, 107 and a gas outlet port 105. Preferably, there are three gas flows into the reaction region. A first gas flow 121 input from gas inlet port 106 is the drift gas flow comprising a buffer gas such as air or nitrogen which starts at the detector end 104 of the IMS 100 and flows into the reaction region 102 and out through the gas outlet port 105. In the preferred embodiment, a drift gas flow rate is of a value required to keep the drift region free of any unwanted vapors and to provide a constant background buffer gas for the ions to drift in. For example, this drift gas flow rate may be about lOcc/min. A second gas flow 122 input from gas inlet port 107 provided near the top end of the reaction region 102 has a dual purpose: 1) for carrying the reactant gas which is required to provide an efficient reaction pathway for the sample species; and, 2) for functioning as an "air curtain" to prevent the condensing sample species from condensing on the ionization source end. The exit of this gas flow is additionally via the outlet port 105. A third flow 123 is the sample flow containing vapors of the sample substance. The sample inlet 116 for receiving and directing the sample gas flow 123 is normally at the same temperature as the reaction region 102 and the gas flowing into the inlet has the same temperature as that of the other two gas flows 121,122. However, according to the invention, when the sample injection takes place, the inlet tube 116 is heated to bring its temperature up. Figure 2 illustrates a pulsed direct current source 125 for heating the sample inlet 116. The temperature to which the sample inlet port 116 is heated preferably is a function of time and the nature of the sample. For example, when analyzing the drugs cocaine and heroin, the temperature may be ramped from about 50° C to about 230° C in six seconds. The ramp is typically proportional to the square of the time elapsed but in general is a function programmed into the computer including a steady temperature (usually 180° C) . The inlet tube is designed in such a way that there are no cold spots on it, especially at the location 118 where the inlet tube 116 joins the reaction region. When the temperature of the sample inlet tube 116 is high enough, the vapors of the sample, e.g., drugs, get efficiently transported into the reaction region 102 without condensing on the walls of the inlet tube. The inlet tube is then cooled rapidly, i.e., the heat source is removed within seconds, to prevent any further injection of the sample into the reaction region. The sample vapor in the reaction region 102 is then subjected to reactions with the charged species present in the region 102. The nature of the reactions and their ionization rates depend upon the ionizing species from the ionizing source 111, and the compound being ionized. In general, the reactions occur on a time scale in the order of microseconds, with the sample vapors still in their vapor state. Condensation of the vapors on the walls of the tube starts to take place only after several tens of milliseconds after their introduction into the reaction region 102 and may be varied by adjusting the flow rates of the various gas streams in the reaction region. In the preferred embodiment, after a sufficient number of product ions have accumulated in the reaction region, an electric field of the correct polarity and magnitude is established between the reaction 102 and drift regions 103 to pulse the ions into the drift region. This pulse VI is typically applied to the electrode 111 with respect to the voltage V2 on electrode 112 and has a relative amplitude with respect to V2 of several hundred volts and a duration of 200 to 500 microseconds. This creates an ion packet to be injected into the drift region. The constituents of the ion packet are separated by their mobility in the drift region as in any IMS device, typically using an electric field created by ring electrodes at different potentials indicated as V3 , V4,...,V7 in Figure 2. The detection of the separated ion packets can also be done conventionally as in a typical IMS or can be injected into other apparatus using electric fields for further processing. It should be noted that ion injection into the drift region 103 may also be carried out using the Nielson-Bradbury shutter 125 as shown in the conventional IMS device (Figure 1) . Several detailed variations on the foregoing description are now provided. In a first variation, if the sample to be analyzed has a substantial vapor pressure at room temperature, the sample may be removed from the reaction region 102 by cooling the reaction region 102 and keeping its temperature lower than the temperature of the sample. Means for cooling the reaction region 102 may include thermo-electric cooling or, using maintaining a drift gas 121 at a cooler temperature. This temperature reduction reduces the vapor pressure of the sample in the reaction region to a low enough value so as to be negligible for producing measurable quantities of ions, as required by the invention. Another way of achieving this is to provide adsorbing media 130 for the sample vapor on the inside walls of the reaction region such as shown in Figure 2. Once the sample reaches the adsorber, it is trapped and the net effect is the same as a lowering of the sample temperature, and thus its vapor pressure. The sample inlet drive 116 shown in Figure 2 is normally used in a pulsed mode in order to reduce the loading of the reaction region with too much sample. Thus, in another embodiment, the inlet tube 116 comprises a gas chromatographic column which normally sits at a low temperature so that the sample is trapped at the inlet end of the column. The column may then be heated at a certain rate using a pulsed direct electric current through the column if it is metallic or by an indirect means, e.g., infra-red or hot air envelope, if it is non-metallic. This causes the sample to travel down the column into the drift region of the IMS for analyzation as described above. Since the constituents in the sample are separated by the column, the IMS analyzes each constituent at a different time and thus the IMS mobility spectra will vary in time. Once the sample is analyzed, the column is rapidly cooled and prepared for trapping the next sample in the column. Since the reaction region 102 acts as a condensing location for the sample, it eventually becomes loaded with the condensed sample and becomes unusable. The reaction region electrode 111 is made in such a way that it has an inner condensing liner 117 which, when loaded with sufficient sample residue, can easily be replaced with a new one. Under normal circumstances of sampling, replacement of the inner condensing liner 117 may occur only after several thousand hours of operation since each sample is only a few ten to a few hundred nanograms in weight. Figure 3 is a schematic diagram of an example hand-held (portable) drug detection system implementing the low power, sample trapping IMS device of the invention. As shown in Figure 3, power to the sample trapping IMS system 100 may be provided by a battery 150, for example, a 12V battery. A column heating and sampling gas input system 180 is controlled by a microprocessor-based control system depicted in Figure 3 as computer system 175 comprising Digital I/O, analog I/O, a keyboard, CPU, and display. The sample inlet itself 116 is depicted in Figure 3 as a GC column with a sample intake system 180 comprising a sealed rotatable preconcentrator device having sampling media 180 including, for example, target sample adsorbent material, and having a first sample input end 181 and a second heater end 182. Preferably, the preconcentrator device is a sealed container in which the sampling media rotating between a first sample input end 181 in communication with a computer controlled sampling pump system 170 for periodically retrieving samples to be analyzed, and the second end 182 in communication with the GC column inlet 116 where a gas flow containing desorbed sample is injected into the inlet port or GC column 116. Figures 4 (a) and 4 (b) depict a process flow diagram 200 for the sample trapping IMS of the invention implemented in a battery powered portable molecular detection system. As shown in Figure 4(a), a first step 202 is to check the battery state, and, at step 204, to determine whether the battery voltage is normal. If the battery voltage is not normal, the operator is so warned at step 206 and the process terminates at step 208. If the battery voltage is sufficient, the portable detection device displays a device ready indication at step 210 and waits for the sampling signal 215 from the CPU system 175 (Figure 3) at step 217. When the start sampling signal is received, the computer controlled sampling pump system 170 is calibrated and a gas flow for the IMS and column is started at step 220. The sampling cycle is then executed at step 225 as will be described in further detail with reference to Figure 4 (b) . Finally, at step 230, the results at the output of the IMS detector are analyzed and displayed, and the process returns to step 202 for the next sample cycle. The sample execution cycle depicted at step 225 in Figure 4 (a) , is now described in further detail with reference to Figure 4(b) . As shown at step 250, the first step is to seal the preconcentrator housing, and, at step 253, to start the sampling pump system 170 (Figure 3) . At step 254, the preconcentrator is terminated, and at step 259, the housing seal is broken and the preconcentrator wheel device rotated to place the sample media containing the adsorbed sample to the GC column input end 182. Then, at step 260, a heated gas flow is input to the preconcentrator at the IMS/GC column sample inlet end 182 to enable desorption and injection of the sample to the column. It should be understood that the heating and desorption time is dependent upon a variety of factors including: the type of target sample compound, e.g., explosives, narcotics, etc., and the sample adsorbing material employed, etc. Next, at step 263, the desorption and injection port heaters are turned off for a predetermined amount of time. At step 265, the column or IMS inlet port is heated, in the manner as described herein, for example, by pulse d.c. current applied directly to the column or inlet port. For example, when used for detecting certain types of narcotic compounds, the GC column may be subjected to computer- controlled pulsed d.c. current ranging, for example, from 0 V to about 24 V at 100 kHz, at a duty cycle ranging anywhere from 0% to 80% depending upon how much heat is required to control elution of the desorbed sample compounds within the column. Preferably, in the portable sample trapping IMS detection system, the temperature of the GC column is monitored using a thermocouple attached to it (not shown) and the heating of the column is regulated by the CPU 175 by varying the pulse width of the current flowing through the metal part of the column. Simultaneously therewith, as indicated at step 275, the sample trapping action of the trapping IMS system 100 of the invention gathers its data for a controlled time period that depends upon the retention time of the compound within the GC column, i.e., the time it takes the target compound to travel to the trapping IMS column as the GC column is heated. Finally, the pulsed current for supplying heat to the GC column or inlet 116 (Figure 3) , is terminated, and the process returns to step 230, Figure 4(a). Further details regarding the operation of the programmed sampling and the "heat -on-demand" sampling technique, may be found in commonly owned, co- pending U.S. Provisional Application 60/074,195 entitled "A VALVELESS GAS CHROMATOGRAPHIC SYSTEM WITH PULSED INJECTION AND TEMPERATURE PROGRAMMED ELUTION," the contents and disclosure of which is incorporated by reference as if fully set forth herein. The advantage of the cold IMS and the heat- on- demand sampling technique is in the savings in power as opposed to conventionally heating the IMS and the sampling device continuously so as to keep the device at operating temperatures of typically 200° Celsius, e.g., for drugs. Typical power savings are in the order of 10 to 20 watts, which is very important for a battery operated IMS devices. Another advantage is the increased resolution of the IMS since the diffusion broadening of the IMS signal peaks is reduced at the lower temperatures (the peak width being proportional to the square root of the absolute temperature of the drift gas) . Thus, for a temperature drop of 200° Celsius from 220° Celsius (i.e., 20°Celsius) , the resolution of the IMS is increased by thirty percent (30%) . The practical advantages of using the cold IMS are also evident as weight and size savings and in a simpler design due to the lack of a temperature controlled heater for the IMS. The reliability of the IMS is improved since as there are no heat stressed parts in the IMS. The foregoing merely illustrates the principles of the present invention. Those skilled in the art will be able to devise various modifications, which although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope.

Claims: WHAT IS CLAIMED IS: 1. A sample ion mobility spectrometer comprising: an inlet port for inputting a sample vapor having molecules to be detected; a first reaction region structure for receiving said input sample vapor of gas, said inlet port at a temperature greater than a temperature of said reaction region; an ionization source for ionizing selected molecules of said vapor sample to be detected; means for a transporting said ionized selected molecules across a drift region for subsequent detection, whereby remaining sample vapors introduced into the reaction region condense in the reaction region after their introduction therein to effectively eliminate sample vapors and further ionization reactions thereof in the IMS. 2. The sample ion mobility spectrometer as claimed in Claim 1, wherein said inlet port includes a metallic tube that is heated by application of pulsed direct current. 3. The sample ion mobility spectrometer as claimed in Claim 1, further comprising a hot air jacket surrounding said inlet port. 4. The sample ion mobility spectrometer as claimed in Claim 1, further comprising means for cooling said reaction region to a temperature below that of said inlet port carrying said vapor sample, said temperature sufficient to cause condensation of said vapor sample within said reaction region. 5. The sample ion mobility spectrometer as claimed in Claim 1, wherein said sample vapor condenses a predetermined amount of time after introduction into said reaction region. 6. The sample ion mobility spectrometer as claimed in Claim 1, wherein said drift region includes an ion detection device, said spectrometer further comprising means for generating an electric potential between said reaction region and said detector for transporting said ionized selected molecules to said detection device. 7. The sample ion mobility spectrometer as claimed in Claim 1, further comprising adsorbing media located on inside walls of said reaction region for trapping remaining sample vapors after ionization in said reaction region and reducing sample vapor pressure within said reaction region. 8. The sample ion mobility spectrometer as claimed in Claim 1, further comprising a replaceable inner condensing liner means for receiving sample residue. 9. The sample ion mobility spectrometer as claimed in Claim 1, wherein said inlet port comprises a gas chromatographic column. 10. A method for detecting molecules of interest in a sample ion mobility spectrometer device comprising the steps of inputting a sample vapor having molecules to be detected into a reaction region of said spectrometer device, said sample vapor being input to said device via an input port; heating said input port to a temperature greater than a temperature of said reaction region; ionizing selected molecules of said vapor sample to be detected; and transporting said ionized selected molecules across a drift region for subsequent detection; and - condensing remaining sample vapors in the reaction region after their introduction therein to effectively eliminate sample vapors and further ionization reactions thereof in the IMS.

Inventors: HALEY LAWRENCE, THEKKADATH GOVINDANUNNY, HALEY, LAWRENCE, THEKKADATH, GOVINDANUNNY
Applicants: HALEY LAWRENCE, IDS INTELLIGENT DETECTION SYST, THEKKADATH GOVINDANUNNY, HALEY, LAWRENCE, IDS INTELLIGENT DETECTION SYSTEMS, INC., THEKKADATH, GOVINDANUNNY
Application year: 1998
Main classification: H01J49
IPC codes: B01D59, G01N27, H01J49