Compact spectroscopic absorption cell

Abstract

A spectroscopic absorption cell suitable for use with a divergent light source, the cell including: an elongate chamber having reflective sidewalls; a first mirror positioned at a first end of the chamber, the first mirror having a first aperture for receiving light into the chamber; a second mirror positioned at an opposite second end of the chamber, the second mirror having a second aperture for allowing light to exit the chamber. The first and second mirrors have reflective surfaces opposing each other and the reflective sidewalls cooperate with the first and second mirrors to contain light within the chamber, such that a majority of light rays entering the chamber make multiple passes of the cell between the mirrors.

Description

[0001] COMPACT SPECTROSCOPIC ABSORPTION CELL

[0002] Field of the Invention

[0003] The present invention relates to a compact spectroscopic absorption cell. It has been developed primarily for use in a portable gas detector.

[0004] Background

[0005] Portable gas detectors are ubiquitous in both civilian and military applications for detecting potentially harmful gases, such as carbon monoxide, ammonia, sulfur dioxide, hydrogen sulfide, phosphine, VOCs etc. A range of different technologies are known for gas detection, such as electrochemical, colorimetric, infrared absorption etc. Ideally, a portable gas detector should be capable of detecting a range of gases and have high sensitivity for detection of gases at low concentrations. Colorimetric detectors are popular due to their compact form factor, but suffer from the disadvantages of: detection of a narrow range of gases, a relatively short shelf-life and poor sensitivity (at least for some gases). Infrared spectroscopy enables the detection of a wide range of gases, but current technologies suffer from the disadvantages of size, cost and sensitivity.

[0006] Spectroscopic absorption cells are commonly used to contain and detect gases by passing light (e.g. IR light) through the cell. Absorption of wavelengths corresponding to characteristic spectral lines in the gases can be used to determine the concentrations of different chemical species according to the well-known Beer Lambert law: log Io / I = ale, where Io is the intensity of the light entering the cell, / is the intensity of the light exiting the cell, a is the wavelength-specific absorption coefficient, I is the distance travelled by the light within the cell and c is the concentration of the gas within the cell.

[0007] In order to increase sensitivity to low concentration gases, the path length I may be increased or the concentration c may be increased by pressurizing the gas cell with the sample gas.

[0008] Since longer cells are usually impractical, I may be increased by using mirrors within the cell to create a compact multi-pass cell. Several multi-pass cell designs are known in the art including, for example, the Pfund, Herriot, White, and Circular cells (https: / / en.wikipedia.org / wiki / Multipass_spectroscopic_absorption_cells). Alternatively, a hollow core fiber may be coiled to create a long path length within a compact form factor, with the hollow core fibre containing both the sample gas and the interacting light, the flexible fiber acting as a light pipe. With the notable exception of the White cell, the majority of multi-pass cell designs known in the art require a laser with a well collimated beam to efficiently couple the source light into the cell and, in the case of the mirrored designs, maintain that collimation over multiple passes of the cell. However, lasers are problematic for mid-infrared spectroscopy, especially in portable, broad spectrum gas detectors, because mid-infrared lasers are typically expensive and / or bulky with a narrow spectral emission.

[0009] Relatively cheap, broad spectral emission in a small form factor can be obtained with a divergent black body source, such as a glow bar (“Globar”) commonly used in laboratory FTIR machines. FTIR gas cell designs are typically based on the White cell, where two concave mirrors at the far end of the cell are used to repeatedly refocus the divergent beam onto a concave mirror at the near end of the cell. While this arrangement works well in a laboratory instrument, the mirrors at the far end of the cell need to be large to span the divergent beam, and the resulting cells are necessarily relatively large making them poorly suited for use in portable gas detectors.

[0010] It would be desirable to provide an alternative spectroscopic absorption cell suitable for detecting gases using, for example, IR absorption. It would be particularly desirable to provide a portable gas detector having a small form factor, low cost and high sensitivity to a wide range of gases.

[0011] Summary of the Invention

[0012] In a first aspect, there is provided a spectroscopic absorption cell suitable for use with a divergent light source, the cell including: an elongate chamber having reflective sidewalls; a first mirror positioned at a first end of the chamber, the first mirror having a first aperture for receiving light into the chamber; a second mirror positioned at an opposite second end of the chamber, the second mirror having a second aperture for allowing light to exit the chamber, wherein: the first and second mirrors have reflective surfaces opposing each other; and the reflective sidewalls cooperate with the first and second mirrors to contain light within the chamber, such that a majority of light rays entering the chamber make multiple passes of the cell between the mirrors.

[0013] In a second aspect, there is provided a portable gas detector comprising the cell as described as herein. The term “cell” or “spectroscopic absorption cell” is taken to mean an assembly suitable for increasing spectroscopic absorption sensitivity. As used herein, the “cell” or “spectroscopic absorption cell” may additionally include a light source and / or a detector, said assemblies often being referred to in the art as spectrometers.

[0014] As used herein, the term “portable gas detector” means a gas detector having a size and weight such that it can be readily carried by a person without assistance or machinery. For example, the “portable gas detector” may have the approximate dimensions of a regular briefcase.

[0015] Brief Description of the Drawings

[0016] One or more embodiments of the present invention will now be described with reference to the drawings, in which:

[0017] Figure l is a front perspective of a spectroscopic absorption cell according to the first aspect;

[0018] Figure 2 is a front perspective of the spectroscopic absorption cell with the chamber removed;

[0019] Figure 3 is a rear perspective of the spectroscopic absorption cell with the chamber removed;

[0020] Figure 4 is perspective of a first mounting plate and holding plates with the first mirror removed;

[0021] Figure 5 is a side sectional view of the holding plates and first mirror;

[0022] Figure 6 is a perspective of a second mounting plate with the second mirror and a detection unit;

[0023] Figure 7 shows a schematic ray trace though the spectroscopic absorption cell; and Figure 8 shows various performance characteristics of the spectroscopic absorption cell.

[0024] Detailed Description

[0025] Referring to Figures 1 to 6, there is shown a spectroscopic absorption cell 1 according to one embodiment of the present invention. The cell 1 comprises an elongate chamber 3 in the form of a cylindrical hollow pipe supported between first and second mounting plates 5 and 7 at opposite longitudinal ends thereof. Brace rods 9 interconnect the first mounting plate 5 and the second mounting plate 7 to provide additional rigidity to the cell 1. A first end of the chamber 3 is received in an opening of the first mounting plate 5 and capped by a planar first mirror 11 supported within the opening so as to form a seal between the first mirror and the first end of the chamber. Likewise, a second end of the chamber 3 is received in an opening of the second mounting plate 7 and capped by a planar second mirror 13 supported within the opening so as to form a seal between the second mirror and the second end of the chamber. The circular first and second mirrors 11 and 13 have the same diameters as the chamber 3 and gas-tight seals are provided therebetween via O-ring seals.

[0026] The first and second mirrors 11 and 13 are typically IR-reflective mirrors having respective reflective surfaces opposing each other at opposite ends of the chamber 3. For example, the mirrors may comprise a gold, silver or aluminum coating for optimum IR- reflectivity, as is well-known the art. Likewise, the chamber 3 has inner surfaces adapted for optimizing IR-reflectivity. For example, the chamber 3 may be formed of an IR-reflective material such as copper or aluminium, which has been electropolished to achieve excellent IR reflectivity in a cost-effective manner.

[0027] Alternatively or additionally, the inner surfaces of the chamber 3 may be coated with a suitable IR-reflective material, such as electroplated gold or an electroless gold coating. For optimal reflectivity, the inner surfaces of the chamber have a maximum surface roughness of 2 / 6 or less, wherein 2 is the lowest wavelength of light detectable using the cell.

[0028] The first mirror 11 has a corresponding first aperture 15, which allows light from a suitable IR light source 17 to enter the chamber 3. As shown in Figure 5, the light source 17 comprises a Globar 19 equipped with a parabolic reflector 21 so as to transmit divergent light towards the first aperture 15. The light source 17 is supported within a window 22 defined between first and second holding plates 23 and 25 fastened to a rear side of the first mounting plate 5. The window 18 may be sealed with an IR-transparent pane (not shown) so as to hermetically seal the light source 17 from gas entering the chamber 3 via the first aperture 15. Suitable IR-transparent materials, such as zinc selenide, silicon, quartz etc, will be well known to the person skilled in the art.

[0029] Still referring to Figure 5, sample gas (e.g. ambient gas) enters the chamber 3 via a gas inlet / outlet port 27 defined in the first and second holding plates 23 and 25. A frontside recessed portion 29 of the first holding plate 23 provides a gap between the window 22 and the first mirror 11, thereby allowing the gas to flow from the port 27, through the recessed portion 29 and out through the first aperture 15 (as indicated by the dotted arrow marked as “gas” in Figure 5). In this way, the first aperture 15 allows sample gas to enter the chamber 3 from the port 27 as well as light from the light source 17. The port 27 may be connected to a suitable pump arrangement (not shown) for pressurizing the chamber 3 with sample gas, thereby providing a higher concentration of gas for detection. The pump arrangement (not shown) may also be used for evacuating the chamber 3, thereby enabling the cell to be calibrated against a known absorption reference under vacuum for each wavelength of interest.

[0030] Referring to Figures 3 and 6, the second mirror 13 is mounted in an opening of the second mounting plate 7 and positioned opposite the first mirror 11. The second mirror 13 has a corresponding second aperture 30 coaxially aligned with the first aperture 15 along a central longitudinal axis of the chamber 3. The second aperture 30 typically has a smaller diameter than the first aperture 15.

[0031] Light exiting the chamber 3 via the second aperture 30 is detected by means of a detection unit 32 having a suitable detector 34 aligned with the second aperture. Suitable detection units will be well known to the person skilled in the art. For example, a detection unit incorporating a plasmonic filter and detector array, suitable for the detection of different analytes, is described in WO2022 / 047549.

[0032] A pair of sealing plates 36 are positioned between the detector 34 and the second mounting plate 7 to provide a gas-tight seal at the second end of the chamber 3. Thus, sample gas can only enter and exit the chamber 3 via the inlet / outlet gas port 27 enabling pressurization and evacuation of the chamber, as required. The sealing plates 36 together define a window, which supports an IR-transparent pane of a suitable material (e.g. zinc selenide, silicon, quartz etc) so to allow exiting light rays to reach the detector 34.

[0033] In use, the cell 1 combines the benefits of known multi-pass cells with hollow core fibers to provide a compact, inexpensive, broad spectrum spectrometer. In particular, the cell may be used in portable gas detectors for detection of different gas at low concentrations.

[0034] The compact form factor of the cell 1 is possible through the use of the first and second apertured mirrors 11 and 13 in combination with the reflective chamber sidewalls. Accordingly, light from the divergent (e.g. black body) light source 17 is contained within the cell by reflecting off-axis rays from the inside sidewalls of the chamber 3. Reflective loss is minimized through judicious choice of materials and / or coatings for the chamber sidewalls, as well as relatively smooth surfaces (that is, smooth relative to the wavelengths of interest) to provide specular reflection.

[0035] With small aperture diameters relative to mirror diameters (e.g. less than 25% relative to mirror diameters), there is a high probability of light rays being reflected by the mirrors at each end of the chamber 3 and, hence, relatively long path lengths can be achieved within a small form factor. Typically, the cylindrical chamber (and mirrors) have a diameter of about 2 inches, thereby providing a good compromise between maximizing reflections at each end of the chamber 3, minimizing the number of lossy off-axis reflections from the sidewalls of the chamber, and minimizing the overall volume, size and weight of the cell 1. By way of comparison, a typical White cell diameter is much larger by a factor of at least three or four, making such prior art cells far less suitable for use in portable gas detectors having the desirable size of a briefcase or similar.

[0036] Equally, the cell 1 described herein has several advantages over hollow core fiber light pipes known in the art, namely: (i) the cell 1 is rigid, allowing evacuation and pressurization of the cell for increased sensitivity to low gas concentrations; (ii) a larger diameter compared to a hollow core fiber light pipe, which minimizes a number of lossy reflections from the sidewalls of the cell; and (iii) a divergent light source, which can be more readily coupled to a cell of this diameter.

[0037] As foreshadowed above, the cell 1 may be sealed for evacuation and pressurization of the cell and, therefore, the light detected at each wavelength of interest can be compared with a reference measurement at increased pressure using a suitable look-up table or via calculation. For example, a piston pump may be connected to the gas inlet / outlet port 27 providing high flow rates with rapid evacuation and pressurization.

[0038] Significantly, and in common with other multi-pass spectroscopic absorption cells, the cell 1 provides much longer path lengths relative to the length of the chamber 3 for increased sensitivity at low gas concentrations. Figure 7 is a schematic ray tracing showing light entering through the first aperture 15 in the first mirror, reflecting off the first and second mirrors and the inner sidewalls of the chamber 3 multiple times, before exiting the chamber through the second aperture 30 in the second mirror.

[0039] In contrast with prior art multi-pass cells (e.g. the White cell), rather than controlling a known number of light reflections through the cell to provide a singular, predetermined path length, the cell 1 increases the average and median path lengths using a range of different path lengths through the cell - a small number of light rays may pass through the cell only once (i.e. from the first aperture 15 directly to the second aperture 30), some light rays pass through the cell with one reflection thereby increasing the path length by a certain amount, some light rays pass through the cell with two reflections thereby by increasing the path length by a greater amount . . . some light rays pass through the cell with 80 reflections etc. Provided that the cell 1 is calibrated for use in the field and the average (or median) path length is known for a given wavelength, then the cell can still be used for quantitative measurement of gas concentrations using well-known spectroscopic techniques. Figure 8 shows: (a) a histogram of ray reflection count; (b) a histogram of ray (path) length; (c) cumulative power contribution vs. ray length; and (d) a histogram of ray incident angles. From Figure 8, it can be seen, for example, that the median path length is about 4 m for the chamber 3 having a length of 30 cm. It will be appreciated that this increased median path length, together with the facility to pressurize the cell, enables detection of different gases at low concentrations via IR spectroscopy in a compact form suitable for use in portable gas detector.

[0040] The foregoing describes only some embodiments of the present invention, and modifications of detail may be made thereto without departing from the scope of the invention, the embodiments being illustrative and not restrictive.

Claims

CLAIMS1. A spectroscopic absorption cell suitable for use with a divergent light source, the cell comprising: an elongate chamber having reflective sidewalls; a first mirror positioned at a first end of the chamber, the first mirror having a first aperture for receiving light into the chamber; a second mirror positioned at an opposite second end of the chamber, the second mirror having a second aperture for allowing light to exit the chamber, wherein: the first and second mirrors have reflective surfaces opposing each other; and the reflective sidewalls cooperate with the first and second mirrors to contain light within the chamber, such that a majority of light rays entering the chamber make multiple passes of the cell between the mirrors.

2. The cell of claim 1, wherein the elongate chamber is a cylindrical pipe.

3. The cell of claim 2, wherein the first and second mirrors are plane mirrors capping the first and second ends of the chamber.

4. The cell of claim 3, wherein the first and second apertures are coaxial with a central longitudinal axis of the chamber.

5. The cell of claim 2, where the reflective sidewalls of the pipe have a reflectivity of at least 98% at angles of incidence less than 45 degrees.

6. The cell of claim 1, wherein the chamber sidewalls have a maximum surface roughness of 2 / 6 or less, wherein z is a lowest wavelength of light detectable using the cell.

7. The cell of claim 6, wherein the chamber is a cylindrical pipe comprised of a material selected from the group consisting of: copper, aluminum, brass, stainless steel and polymers.

8. The cell of claim 6, wherein an inner surface of the pipe is electropolished.

9. The cell of claim 6, wherein an inner surface of the pipe is coated with an electroless gold coating or electroplated gold.

10. The cell of claim 1 further comprising the divergent light source, the light source being positioned for emitting light rays through the first aperture.

11. The cell of claim 10, wherein the light source includes a parabolic reflector or a lens for reducing a divergence of emitted light rays.

12. The cell of claim 11, wherein the light source is positioned adjacent to and aligned with the first aperture, and wherein a gap between the light source and the first aperture allows gas to enter the chamber via the first aperture.

13. The cell of claim 11, wherein the first aperture has a diameter less than 25% of a diameter of the first mirror and between 80% and 120% of a diameter of the reflector or lens.

14. The cell of claim 1, wherein the light source is hermetically sealed from the gas.

15. The cell of claim 1, wherein the gas enters the chamber only via the first aperture.

16. The cell of claim 1, wherein the cell is sealable for pressurizing the chamber with the gas and / or evacuating the chamber.

17. The cell of claim 16, further comprising a single gas inlet / outlet port positioned at a first end of the cell having the first mirror.

18. The cell of claim 11, further comprising a detector positioned at the second end of the chamber.

19. The cell of claim 18, wherein the detector is isolated from gas in the chamber.

20. The cell of claim 19, wherein a transparent window is positioned between the chamber and the detector.

21. The cell of claim 18, wherein the second aperture has a diameter within 5 mm of a maximum dimension of the detector’s active element.

22. The cell of claim 1, wherein the second aperture has a smaller diameter than the first aperture.

23. The cell of claim 1 having one or more of the following dimensions: the chamber length is from 150 to 500 mm the chamber diameter is from 20 to 100 mm; and the first and second mirror diameters are from 20 to 100 mm,24. A gas detector comprising the cell according to any one of the preceding claims.

Images