Patent Application
20070267575
Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures.
A gas sample may contain many, e.g., several hundred, different gases. According to one embodiment of the present invention, the terahertz absorption spectra of the various gases of a gas sample are separated in a manner that simplifies analysis thereof. That is, instead of all of the spectra being present during a single spectroscopic measurement, only a portion of the spectra are present during each measurement. This is accomplished by performing spectroscopic measurements upon the species or components of the gas sample one component or group of components at a time.
The spectroscopic measurements of the individual components or groups of components may be performed sequentially. In this manner, a single, comparatively complex spectrum is divided into a plurality of substantially less complex spectra. The less complex spectra may provide enhanced resolution and inherently easier recognition of individual gas species.
An embodiment of the present invention uses a cryogenically cooled gas sample cell. By controlling the temperature of the gas sample within the sample cell, the gases of the sample can be made to condense and then to selectively evaporate.
Thus, a spectroscopic measurement may be performed upon a selection of the gases according to a predetermined algorithm. That is, selected gases may be caused to boil off or evaporate by raising the temperature of a condensed sample above the freezing point of the selected gases. In this manner, the composite complex gas sample may be divided into its components or groups of components for enhanced spectroscopic measurements.
According to an embodiment of the present invention, a sample cell may be configured to be cryogenically cooled. Such cryogenic cooling may be effected by cooling the entire sample cell or by cooling a portion of the sample cell, e.g., an interior surface thereof. For example, cryogenic cooling coils may be wrapped around the cell, in intimate contact therewith, to effect such cooling. As a further example, a cold finger or cryogenic tube may extend into the sample cell and may either be in contact with a surface upon which sample gases condense or may define that surface itself. In either instance, the cold finger may define a condensation surface within the sample cell.
Initially, substantially all of the components of the gas sample may be condensed upon one or more surfaces of the sample chamber. The temperature may then be increased according to an accurately defined schedule. The schedule may depend upon the suspected components of the gas sample. Alternatively, the schedule may be generic, so as to contemplate the potential presence of a large number of components.
As the temperature of the condensed gas sample is increased, more of the components evaporate from the inside surface(s) into the volume of the test chamber (for example, into the inside volume of a Terahertz Fabry-Perot resonant cavity). An absorption spectrum is recorded. The absorption spectrum may be recorded digitally.
Each time that the gas sample's temperature is increased by a predetermined amount or step, Δt, some new gas components may evaporate within sample chamber and another absorption spectrum may be measured. The new gases that were added to any previously evaporated gases are known to belong to a group of gases having boiling points within the present Δt temperature range. The temperature at which evaporation occurs may provide some additional information regarding the identification of the new gas components this information may be used in the analysis thereof.
The previously recorded absorption spectrum (which may comprise digital data) may be subtracted from this new absorption spectrum (which may also comprise digital data) and may be analyzed to determine new types of gas components that are now present. During each such temperature step, the new spectrum (the one determined by subtracting one spectrum from another) may be compared to the already known absorption spectra of those gases with boiling points within the current Δt temperature interval. Thus, the gas components may be more readily identified since their boiling points are known. This procedure may be repeated until all of the gases are boiled off of the cryogenically cooled surface(s).
As the chamber temperature increases to the boiling points of N2 and O2, the noise floor of the absorption spectrum may increase substantially. However, much of this noise floor may be subtracted from the composite absorption spectrum. The absorption spectra of oxygen and nitrogen gases are well known.
Water vapor (from moisture in the air) will generally remain frozen until the top of the Δt ramp is reached. Water vapor, if present, may interfere substantially with the absorption spectra of many gases especially since many gases may only be present in very small quantities, i.e., parts per trillion.
Referring now to
Since the spectra are separated, a higher sweep rate may be used. The absorption bands are expected to be farther apart when the spectra are the result of the presence of one or just a few different gases. Thus, the need to enhance resolution by reducing the sweep rate may be substantially mitigated.
Any desired terahertz frequency range may be used. For example, to perform absorption spectra measurements on complex molecules, a frequency range of approximately 0.1 terahertz to approximately 10 terahertz may be used.
Terahertz source 11 may comprise a single tunable source, multiple tunable sources, or multiple discrete (non-tunable) sources. Terahertz source 11 may comprise any desired combination of tunable and discrete (non-tunable) sources.
Sample cell 12 may be cooled, such as via a cooling jacket, cooling tubes, or other means for cryogenically cooling thereof. For example a tube may be wrapped around sample cell 12 such that the tube is in intimate contact therewith. A cryogenic gas may then be caused to flow through the tube so as to cool sample cell 12 and cause the sample gas to condense upon a surface or surfaces thereof. As a further example, a cold finger may be made of diamond plates (or another material with superior thermal conductivity), with heater filaments (thin film, etc) deposited on at least one surface to facilitate accurate electronic control of its surface temperature. Materials with high thermal conductivity (such as diamond) can provide highly uniform temperature distribution over their entire surfaces.
Sample cell 12, i.e., more specifically, the cold finger surfaces, may also be warmed, such as in discrete Δt steps, so as to raise the temperature of the gas sample according to a predetermined schedule, as discussed above. Thus, a cryogenic cooling/heating system 13 may be used to precisely control and vary the temperature of sample cell 12.
Terahertz radiation from terahertz source 11 passes through sample cell 12, encountering the sample gas therein, and can be detected by a terahertz detector 14. As the frequency of terahertz source 11 is swept, terahertz detector 14 measures the terahertz radiation transmitted through sample cell 12 so as to facilitate the measurement of an absorption spectrum. This process may be repeated, at increasing temperatures of sample cell 12, so as to provide a plurality of absorption spectra having enhanced resolution.
Since the spectra are separated, and thus each spectrum is less complex, the resolution requirements of the terahertz source and the detector may be mitigated. Less resolution is required because there is less need to be able to separate closely spaced absorption bands in less complex spectra.
However, although the resolution requirements of the terahertz source and the detector may be mitigated, at least in some instances it may be desirable to maintain superior resolution capability. Thus, in such instances terahertz sources and detectors having higher resolution may be used. This may be beneficial, for example, in those instances where hundreds different gases are present in the sample chamber simultaneously.
Referring now to
As those skilled in the art will appreciate, an increased path length effectively provides more of the sample gas for the terahertz radiation to travel through and be absorbed by, so as to enhance sensitivity of absorption spectra measurement. After traveling through chamber 25, the terahertz radiation exits sample cell 12 via exit window 22 and is incident upon a terahertz detector (such as terahertz detector 14 of
Sample gas is provided to sample cell 12 via gas input port 28. Inside of chamber 25, the sample gas may be cooled by cold finger 26 such that it condenses thereon. Cryogenic fluid inlet 31 provides a cryogenic gas or cryogenic fluid to cold finger 26 and cryogenic gas or fluid outlet 32 facilitates gas or fluid flow from cold finger 26. Gas outlet port 29 facilitates the flow of sample gas out of chamber 25.
Alternatively, the entire sample cell 12 or any portion thereof may be cooled such that the sample gas condenses upon a surface thereof. Sample cell 12 may be cooled by any desired acceptable means.
Condensed sample gas 27 (such as that condensed upon cold finger 26) may subsequently be caused to evaporate one component or group of components at a time by slowing warming condensed sample gas 27. Condensed sample gas 27 may be warmed by varying the temperature of the fluid that flows through cold finger 26, by an electric heating element 37 within chamber 25, by an electric heating element outside of chamber 25, or by a heating element (such as a thin film heater deposited on a cold finger surface) or by any other desired means.
For example, condensed sample gas 27 may be warmed simply by discontinuing the flow of cryogenic fluid through cold finger 26. The flow of cryogenic fluid through cold finger 26 may be varied, such as by momentarily discontinuing such flow, in a manner that regulates the temperature of condensed sample gas 27. One consideration is the need to keep temperatures on all surfaces inside the test chamber substantially uniform.
A cryogenic temperature sensor 36, such as a thermistor or a thermocouple, may be used to monitor the temperature within chamber 25. The temperature sensor can be inside of the cold finger. For example, the temperature sensor can be integrated with the cold finger.
Referring now to
Procedures may be implemented to prevent liquefied and/or frozen gas of the gas sample from undesirably, e.g., prematurely, evaporating due to reduced pressure in the sample cell. For example, the sample cell may be sufficiently cooled so as to mitigate such undesirable evaporation and/or the pressure within the sample cell may be maintained at a level that mitigates such undesirable evaporation. One way of maintaining pressure within the cell is by adding an inert gas thereto.
Referring now to
Alternatively, the temperature may be raised by any other desired amount. For example, an operator may choose to raise the temperature by a different amount based upon experience. As a further example, automated control equipment may raise the temperature a different amount based upon results of prior analyses, e.g., prior absorption spectra, or based upon any other criteria.
A first or earlier absorption spectra of the gas sample is measured, as indicated in block 52. Next, the temperature is again raised, either by a fixed amount Δt, or by any other desired amount, as indicated in block 53. Then a second or later absorption spectra of the gas sample is measured, as indicated in block 54.
When two consecutive (or non-consecutive) absorption measurements have been made, then the earlier, e.g., first absorption spectrum may be subtracted from the later, e.g., second, absorption spectrum to define a new absorption spectrum, as indicated in block 55. The new absorption spectrum contains information relating to the sample gas components that boiled off at the new temperature. The process of raising the temperature of the gas sample, measuring the absorption spectrum, and determining a new absorption spectrum using two previously performed absorption spectra is repeated across the desired temperature range.
In this manner, the terahertz absorption spectrum of a complex gas sample may be divided into a plurality of separate absorption spectra. Each of the divided absorption spectra may be substantially simpler (have fewer absorption bands) than a composite absorption spectrum taken without such varying of the temperature of the sample gas.
One or more embodiments of the cryogenic terahertz spectroscopy method of the present invention may be used in a variety of different applications. For example, the present invention may be used to identify hazardous, toxic, or dangerous gases in the air. Applications include the battlefield detection of poisonous gases, use in mines to detect deadly gases, and use in laboratory analysis.
By performing a series of separate absorption spectroscopic measurements upon a corresponding series of separate components or groups of components of a sample gas, the resulting spectra are likewise separated. Because the spectra are separated, the resolution requirements of the spectroscopy system are reduced. Each individual absorption spectrum is substantially less complicated.
This inherently reduces the resolution necessary to distinguish absorption bands. Thus, a Time Domain Terahertz Spectroscopy (TDTS) system may use terahertz radiation sources and detectors having reduced resolution capabilities. Since the resolution requirements are mitigated, less expensive and more readily available terahertz sources and detectors may be used.
Although embodiments of the present invention are described as being used in terahertz frequency domain, the present invention may be used in other bands of the electromagnetic spectrum (millimeter waves, sub millimeter waves, infrared, even in the visible wavelength range), as well as in other types of chemical analysis. As such, description of the present invention as being used in terahertz spectroscopy is by way of example only, and not by way of limitation.
The terms evaporate, vaporize, and boil may be used interchangeably herein and may refer to sublimation. The frozen or condensed sample gas may be either a solid, a liquid, or some combination thereof. In any instance, the terms evaporate, vaporize, and boil may refer to the condensed sample gas becoming gaseous.
Embodiments described above illustrate but do not limit the invention. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present invention. Accordingly, the scope of the invention is defined only by the following claims.
