Pulsed discharge ionization source for miniature ion mobility spectrometers

ABSTRACT

A method and apparatus is disclosed for flowing a sample gas and a reactant gas () past a corona discharge electrode () situated at a first location in an ion drift chamber (), applying a pulsed voltage waveform comprising a varying pulse component and a dc bias component to the corona discharge electrode () to cause a corona which in turn produces ions from the sample gas and the reactant gas, applying a dc bias to the ion drift chamber () to cause the ions to drift to a second location () in the ion drift chamber (), detecting the ions at the second location () in the drift chamber (), and timing the period for the ions to drift from the corona discharge electrode to the selected location in the drift chamber.

INFORMATION

DETAILED DESCRIPTION OF THE INVENTION

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made under Contract DE-AC05-000R22725 between the U.S. Department of Energy and the assignee of the present invention. The Government has certain rights in this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the present invention is practiced in a miniature ion mobility spectrometer (IMS) employing a pulsed corona discharge ion source as shown in FIG. .

FIG. 2 shows a second miniaturized embodiment of the apparatus featuring a microelectronic CPU .

In FIGS. 1 and 2, the device has a cylindrical body comprised of ten () stacked, annular metal electrodes -, and which are separated by annular spacers (5-mm thick and 8 mm ID) of a dielectric material such as Teflon. This forms a drift channel which can be in the range from 1.7 mm-2.5 mm in diameter and 35-50 mm in effective length. In FIG. 1, the drift channel is specifically 2.5 mm in diameter and 47 mm in length, respectively.

Nine miniature resistors (not shown), each with 2 MΩ resistance, 1% tolerance, are connected between the electrodes -, and to form a voltage divider. The first electrode is biased with a power supply to provide an ion drift voltage, with the voltage being distributed to the intermediate electrodes -, and through these resistors. The last electrode is connected to an electrical ground . The next to the last electrode is connected to a 470-pf capacitor to suppress transients. An ion detector electrode is located in the drift chamber between the last electrode and the next to last electrode . Positive or negative potentials can be applied to the detection electrode for detecting positive and negative ions, respectively.

A nickel-tipped electrode of non-radioactive (non-doped) material with an end radius of curvature of approximately 25 μm is mounted at the entrance of the drift chamber . The second drift channel electrode is used as the counter electrode for corona discharge with the distance to the tip being larger than the threshold distance for discharge zone as illustrated in FIG. . The corona-producing tip , together with the second electrode of the IMS channel, formed a tip-ring corona discharge element.

A sample gas is supplied from reservoir in FIG. 1 through a flow meter to an inlet into the corona discharge end of the drift chamber . A carrier gas, in this case, nitrogen, is supplied from a source through a filter and a second flow meter to an inlet into the detection end of the drift chamber . These gases exit the drift chamber through valve and outlet . In FIG. 2, where parts similar to FIG. 1 have the same number, a sample gas is received from a source , while dry air enters from a supply into an entrance at the opposite end of the drift chamber . The dry air includes both drift gas and reactant gas. All of these gases exit from exit .

A corona is produced at the electrode by applying an electrical pulse having a width of from 40 ns to 100 μs, a pulse height varying from 0.2-3.3 kV and a repetition rate (frequency) of 20 Hz. The pulse is generated as a base dc voltage component originating at a high voltage source and a varying pulse component generated by a pulse generator comprising high voltage source , amplifier and pulse generator , which generates pulses on the order of 5 volts before they are amplified. These pulses are summed with a base dc voltage through capacitor C. The resulting amplified high-voltage pulse is applied to the corona tip electrode , which is seen in FIG. . During the high voltage pulse, ions are generated in the vicinity of the tip . After the pulse, the ions move along the drift channel through the carrier gases under the influence of the drift field bias provided by voltage supply .

The corona discharge pulse also provides a start signal for timing the ion mobility movements. For each pulse, ions are separated according to their travel time to reach the ion detector located at the end of the channel . There, an ion current is produced and is transmitted to a current amplifier connected to electrode . The time difference between the start signal and arrival of ions is detected by a time-to-digital converter (TDC) and is transmitted to a computer for analysis. If a digital oscilloscope is used instead of time-to-digital converter , the start pulse triggers the oscilloscope. The ion arrival signal is recorded by the scope and sent to the computer .

The detector is connected to an amplifier in FIG. 1 which amplifies the signals. The oscilloscope is connected to an Apple Macintosh computer running a Labview application program in FIG. . This is a lab prototype embodiment for demonstrating the operation of the invention. In FIG. 2, the components in FIG. 1 are designed for reduced size in a commercial embodiment.

Ion mobility spectra of both positive and negative ions were measured as a function of pulse width. For positive ions, the ion current increased with pulse width and saturated. For negative ions, the ion current peaked rapidly and then decayed with increased pulse width.

Ion mobility spectra of negative ions produced by pulsed corona discharge and by ionization of air were measured as a function of drift bias voltage from −600 VDC to −1700 VDC as seen in FIG. . The pulses had 1.08 μs width and +2600V amplitude. The sample air was at atmospheric pressure and room temperature. The drift gas was N2, which was fed from a source through a filter and flow meter at the detector end of the IMS channel with a flow rate of 20 sccm (standard cubic centimeter per minute).

A typical mobility spectrum of positive ions generated by pulsed corona discharge ionization of air is shown in FIG. . For producing positive ions, the pulse potential applied to the tip was also positive, the same polarity as used for generating negative ions, with a height of 3100 VDC and a width of 14.5 μs.

The corona discharge properties depend on the distance between the tip and the counter electrode . The counter electrode can be either a ring or a tip. This is illustrated in FIG. . For distances less than 1.96 mm, no ionization occurred until a threshold of potential, about 1900 VDC was reached. At and above the threshold, spark breakdown occurred, which preceded the establishment of a stable corona. The voltage threshold was found to increase as a function of distance, as shown in FIG. 5, up to 2400 volts at 1.96 mm. Stable corona discharge conditions could not be found in this distance range. When the distance was larger than 1.96 mm, corona discharge occurred at a threshold that was a function of the drift bias.

Corona discharge was also generated by a combination of a base dc potential in combination with a pulsed voltage potential. As seen in FIG. 1, a dc voltage supply is connected to a dc pulse generator , an amplifier and a second dc supply through capacitor C. As seen in FIG. 2, dc voltage supply is connected to a pulse amplifier and a pulse height control circuit through a capacitor . In FIG. 2, the pulse is commanded by the microelectronic CPU through a digital-to-analog converter . The base dc potential, which varied from 0 to 3000 volts, was superimposed on the pulsed potential. The combined potentials permit independent variation of the dc potential, pulse height, and pulse width to the corona tip. For a given pulse height, the ion mobility spectrum current can be measured as a function of dc bias voltage. For a higher pulse voltage, the current exhibited a threshold for the dc bias and increased to a saturation level. The dc threshold was found to linearly decrease from 3000 VDC to 200 VDC as the pulse height was increased from 200 VDC to 3000 VDC, as shown in FIG. . Therefore, ions could be generated with lower voltage pulses if the dc base voltage were raised. The detector in FIG. 2 is connected in close proximity to an amplifier which amplified the small signal. This signal is then digitized by digitizer to filter noise, and is then read by the microelectronic CPU . For a specific substance, thresholds are set, and if a threshold is exceeded, a visual indication is provided to a user through an alarm display , such as by illuminating an icon or changing the color of an object on a display screen. The electronic circuits and - in FIG. 2 can be made quite compact and can be mounted on circuit boards. These can be packaged with the drift chamber body in a package the size of a lightweight notebook computer of the type having a titanium case.

The pulsed corona ionization source of the present invention eliminates the need for the ion gate of the prior art near the ion source. It also provides for a smaller drift chamber and a smaller body for housing the drift chamber. The invention also provides a method for timing the movement of the ions between the source and the detector. The use of a dc voltage comprising a pulse element and a base voltage element reduces the pulse component, which reduces noise and power consumption.

This has been a description of detailed examples of the invention. It will apparent to those of ordinary skill in the art that certain modifications might be made without departing from the scope of the invention, which is defined by the following claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a first embodiment of an apparatus for practicing the method of the present invention;

FIG. 2 is a schematic view of a second embodiment of an apparatus for practicing the method of the present invention;

FIG. 3 is a graph of ion detection current vs. time vs. dc bias voltage;

FIG. 4 is a graph of ion detection current vs. drift time for moist air and for nitrogen supplied to the drift chamber;

FIG. 5 is a graph of arcing threshold voltage vs. distance between two electrodes for generating an ion-producing corona; and

FIG. 6 is a graph of arcing threshold voltage vs. pulse height for generating an ion-producing corona.

CLAIMS

1. A method of pulsed discharge for an analytical instrument, comprising: flowing a sample gas and a reactant gas past a corona discharge electrode of non-radioactive material situated at a first location in an ion drift chamber; applying a pulsed voltage to the corona discharge electrode to cause a corona which in turn produces ions from the sample gas and the reactant gas; applying a dc bias to the ion drift chamber to cause the ions to drift to a second location in the ion drift chamber through a medium provided by a drift gas without assistance of an ion gating structure; detecting the ions at the second location in the drift chamber; timing a period for the ions to drift from the corona discharge electrode to the second location in the drift chamber; and using the timed period to determine an identity of the sample gas.

2. The method of claim 1, wherein applying the pulsed voltage to the corona discharge electrode further includes generating a pulsed voltage comprising a controllable base dc component and a controllable varying pulse component.

3. The method of claim 1, wherein said pulsed voltage has a selected pulse width within a range from 40 ns to 100 μs.

4. The method of claim 1, wherein said pulsed voltage has a selected pulse height in a range from 0.2-3.3 kV.

5. The method of claim 1, wherein said pulse voltage has a frequency of approximately 20 Hz.

6. The method of claim 1, further comprising flowing the drift gas into the drift chamber proximate to the second location in the drift chamber.

7. The method of claim 1, further comprising recording the time at which ions from the sample gas arrive at the detector, comparing ions from the sample gas detected at the detector with a threshold, and when the threshold is exceeded, providing a visual display to a user indicating detection of a substance associated with the threshold.

8. An analytical instrument comprising: a body forming an elongated chamber for reaction of gases and for movement of gases, said chamber having a first entrance for receiving a sample gas and having a second entrance for receiving a reactant gas and a drift gas; a corona discharge electrode of non-radioactive material and a counter electrode positioned in the body at a first location in the chamber in a path of flow for the sample gas; an ion detector at a second location the chamber spaced from the corona discharge electrode; wherein a pair of electrodes are provided for applying a dc bias voltage along a length of the chamber; and an electronic control for controlling application of a voltage to the corona discharge electrode and for timing an interval beginning with the application of the corona discharge voltage and ending with detection of the ions at the ion detector; and wherein the instrument does not have an ion control gate and does not confine the ions produced by corona discharge element at an end of the drift chamber where ions are produced.

9. The instrument of claim 8, wherein the body forming the chamber has at least four electrode rings spaced apart and separated by dielectric spacer rings, said electrode rings and spacer rings having central openings which together form at least a portion of the chamber.

10. The instrument of claim 8, wherein the corona discharge electrode is disposed either axially or transversely to the elongated chamber through an opening in an end electrode and has a tip that is spaced from a next to the last electrode ring, which forms the counter electrode for the corona discharge element.

11. The instrument of claim 8, wherein the electronic control is operable for applying a voltage to the corona discharge electrode which further comprises a controllable base dc component and a controllable varying pull component.

12. The instrument of claim 8, wherein the electronic control further comprises means for applying a voltage to the corona discharge electrode which further comprises a base dc component and a varying pulse component.

13. The instrument of claim 8, further comprising means for flowing a second gas into the drift chamber proximate to the second location in the drift chamber and means for flowing a reactant gas into the chamber.

14. The instrument of claim 8, wherein the electronic control further comprises a microelectronic CPU for generating a start pulse, said CPU being connected to the detector through an amplifier and to a digitizer to receive detected ion signals.

15. The instrument of claim 14, wherein the electronic control is connected to a visual display, and wherein the electronic control identifies sample gas by timing an ion drift time and compares ions detected at the detector with a threshold, and when the threshold is exceeded, provides a signal to a user through the visual display to indicate detection of a substance associated with the threshold.

16. An analytical instrument comprising: a body forming an elongated chamber for reaction of gases and for movement of gases, said chamber having a first entrance for receiving a sample gas and having a second entrance for receiving a reactant gas and a drift gas; a corona discharge electrode of non-radioactive material and a counter electrode positioned in the body at a first location in the chamber in a path of flow for the sample gas; an ion detector at a second location the chamber spaced from the corona discharge electrode; wherein a pair of electrodes are provided for applying a dc bias voltage along a length of the chamber; and an electronic control further comprising a microelectronic CPU for controlling application of a controllable base dc component and a controllable varying pulse component to the corona discharge electrode; wherein the microelectronic CPU times an interval beginning with the application of the corona discharge voltage and ending with detection of the ions at the ion detector; wherein the microelectronic CPU controls application of a dc bias voltage to the pair of electrodes for applying the dc bias voltage along a length of the chamber; and wherein the instrument does not have an ion control gate at an end of the drift chamber where ions are introduced.

COPYRIGHT

User acknowledges that Fairview Research and its third party providers retain all right, title and interest in and to this xml under applicable copyright laws. User acquires no ownership rights to this xml including but not limited to its format. User hereby accepts the terms and conditions of the License Agreement.