High-sensitivity instrument to measure NVR in fluids

• US 5,918,258 A
• Filed: 07/11/1996
• Issued: 06/29/1999
• Est. Priority Date: 07/11/1996
• Status: Expired due to Fees

• Current Assignee: Femtometrics, Inc. (BAE Systems Plc)
• Original Assignee: Femtometrics, Inc. (BAE Systems Plc)
• Inventors: Bowers, William D.
• Primary Examiner(s): Oda, Christine K.
• Assistant Examiner(s): MILLER, ROSE MARY
• Application Number:
  US08/678,792
• Time in Patent Office: 1,083 Days
• Field of Search: 73/24.01, 73/24.06, 73/31.06, 73/32 A, 73/54.41, 73/599, 73/61.79, 310/313 D, 310/313 B, 310/313 R, 310/312
• US Class Current: 73/24.06
• CPC Class Codes: G01N 2291/0224 Mixtures of three or more liquids
• CPC Class Codes: G01N 2291/0256 Adsorption, desorption, surface mass change, e.g. on biosensors
• CPC Class Codes: G01N 2291/0422 Shear waves, transverse waves, horizontally polarised waves
• CPC Class Codes: G01N 2291/0423 Surface waves, e.g. Rayleigh waves, Love waves
• CPC Class Codes: G01N 2291/0426 Bulk waves, e.g. quartz crystal microbalance, torsional waves
• CPC Class Codes: G01N 2291/101 one transducer
• CPC Class Codes: G01N 29/022 Fluid sensors based on microsensors, e.g. quartz crystal-microbalance [QCM], surface acoustic wave [SAW] devices, tuning forks, cantilevers, flexural plate wave [FPW] devices microdevices per se B81B
• CPC Class Codes: G01N 29/036 by measuring frequency or resonance of acoustic waves
• CPC Class Codes: G01N 29/222 Constructional or flow details for analysing fluids optoacoustic fluid cells G01N29/2425

Abstract

A real time nonvolatile residue (NVR) monitor operates to efficiently detect molecular contamination in a given environment. The present NVR monitor utilizes surface acoustic wave (SAW) resonators in a controlled environment which efficiently promotes deposition of NVR on the sensor surface. The SAW resonators preferably operate at a resonant frequency of approximately 200 MHz-2,000 MHz which enables the NVR monitor to detect molecular contamination on the order of 10^-11 g-cm^-2 to 10^-13 g-cm^-2. The NVR monitor utilizes active temperature control of (SAW) resonators to achieve a stable resonant frequency and to thermally separate NVR from a sample fluid contacting the SAW. The temperature control system of the NVR monitor is able to directly heat and cool the SAW resonators utilizing a thermoelectric element to maintain the resonators at a preset temperature in accordance with optimal environmental conditions for separating NVR from the sample fluid. The sample fluid, comprising a carrier fluid and NVR, is furthermore processed to provide conditions, such as pre-cooling and/or pressurization, which enhance condensation of NVR on the sensor surface. Increasing the collection efficiency allows the NVR monitor to operate faster at a desired level of accuracy and provides more quantitative information for trace contaminant analysis.

Claims

1. An apparatus for detecting contamination in a fluid comprising:

a SAW sensor having a sensor surface;

a fluid inlet which receives a mixture of contaminants and an otherwise pure fluid from an external source;

a fluid dispenser which receives the mixture received by said inlet and dispenses said mixture with said contaminants in said fluid onto the sensor surface; and

a vessel which contains the fluid under pressure, said vessel being connected to supply said fluid under pressure to said inlet.

2. The apparatus of claim 1, wherein said pressure is greater than 100 psi.

3. The apparatus of claim 1, additionally comprising a temperature controller which controls the temperature of the mixture as said mixture flows from said fluid inlet to said fluid dispenser.

4. The apparatus of claim 1, additionally comprising a temperature controller comprising a heat sink in thermal contact with the SAW sensor and a thermoelectric element for heating and cooling the heat sink, the thermoelectric element in thermal contact with the heat sink.

5. The apparatus of claim 1, wherein the fluid comprises a compound of at least two elements.

6. The apparatus of claim 5, wherein said fluid compound comprises a halocarbon.

7. The apparatus of claim 6, wherein the halocarbon comprises a compound selected from the group consisting of C₂ F₆, CHF₃, and CF₄.

8. The apparatus of claim 5, wherein the compound comprises CO₂.

9. The apparatus of claim 1, wherein the fluid comprises a gas, said fluid dispenser comprises a fluid conduit, said apparatus additionally comprising a pressure controller comprising an orifice disposed within the fluid conduit.

10. The apparatus of claim 9, wherein the fluid dispenser is arranged to direct the sample in a substantially linear path from said fluid dispenser to said sensor surface.

11. The apparatus of claim 1, wherein the fluid comprises a liquid.

12. The apparatus of claim 11, wherein the liquid comprises isopropanol.

13. The apparatus of claim 11, wherein the fluid dispenser comprises a liquid dispenser disposed adjacent to the sensor surface for supplying a preselected volume of liquid to the sensor surface.

14. The apparatus of claim 11, wherein the liquid dispenser comprises an array of jets disposed in proximity to the sensor surface.

15. The apparatus of claim 1, comprising a reference SAW sensor and a detection circuit, said detection circuit producing a signal corresponding to a frequency difference between outputs of the reference SAW sensor and the SAW sensor.

16. The apparatus of claim 1, wherein the contaminants comprise bulk containment contaminants.

17. The apparatus of claim 16, wherein the bulk containment contaminants comprise an oil.

18. The apparatus of claim 16, wherein the bulk containment contaminants comprise a lubricant.

19. The apparatus of claim 1, additionally comprising a vessel which contains the fluid under pressure, said vessel being connected to supply said fluid under pressure to said inlet.

20. A method of detecting contaminants in a fluid, comprising:

inputting a mixture of said fluid and contaminants into a fluid inlet of a detection apparatus;

transporting said mixture of said fluid and said contaminants from said fluid inlet along a flow path to a SAW sensor of said apparatus without separating said fluid and said contaminants;

directing said mixture of said fluid and said contaminants from said flow path onto a surface of the SAW sensor;

thermally separating the contaminants from the fluid such that said contaminants are distributed on the surface of the SAW sensor; and

detecting changes in response of the SAW sensor due to the contaminants on the surface of the SAW sensor;

wherein the step of inputting comprises the step of connecting the fluid inlet to a vessel containing said mixture of said fluid and contaminants under pressure.

21. The method of claim 20, additionally comprising controlling the temperature of the mixture as said mixture flows along the flow path.

22. The method of claim 20, wherein the fluid comprises a liquid, and wherein the step of directing comprises coating the surface of the SAW sensor with a uniformly thick layer of said liquid.

23. The method of claim 22, wherein the step of coating comprises spraying the liquid onto the surface of the SAW sensor.

24. The method of claim 20, wherein the step of directing the fluid onto the surface of the SAW sensor comprises flowing the fluid through a fluid dispenser disposed adjacent to the surface of the SAW sensor such that the fluid travels in a substantially linear path from the fluid dispenser to the surface of the SAW sensor.

25. The method of claim 20, wherein the fluid comprises a liquid, and wherein the step of thermally separating the contaminants from the liquid comprises maintaining the SAW sensor at a temperature sufficient to induce evaporation of the liquid from the surface of the SAW sensor while leaving contaminants thereon.

26. The method of claim 20, wherein the fluid comprises a gas, and wherein the step of thermally separating the contaminants from the gas comprises maintaining the SAW sensor at a temperature sufficient to induce condensation of the contaminants onto the surface of the SAW sensor as the gas contacts the surface of the SAW sensor.

27. The method of claim 20, further comprising precooling the sample fluid prior to directing the fluid onto the surface of the SAW sensor.

28. The method of claim 27, wherein the step of precooling the sample fluid comprises substantially adiabatically cooling the sample fluid.

29. The method of claim 27, wherein the step of precooling the sample fluid comprises substantially isenthalpically cooling the sample fluid.

30. The method of claim 20, wherein the detecting step comprises detecting changes in frequency, response of the SAW sensor by detecting a beat frequency between said SAW sensor and a reference sensor.

31. The method of claim 20, further comprising the step of identifying molecular species of the contaminants by determining a mass spectrum of the molecular species.