Multi-pass palladium hydride optical cavity for detection of hydrogen

Abstract

An apparatus for measuring a hydrogen concentration in a gas sample includes a multi-pass optical cavity having an optical element including a mirror that supports a multi-pass optical path inside the multi-pass optical cavity. The laser is configured to generate a laser beam that propagates along a multi-pass optical path so as to be reflected multiple times from the mirror. An intensity of the laser beam exiting the multi-pass optical cavity is detected, and a signal processor determines a hydrogen concentration in the gas sample in the optical cavity from the measured intensity of the laser beam. The optical element may also include one or more transmissive windows.

Description

Technical Field

[0001] The present invention generally relates to methods and apparatus for detecting hydrogen gas. More particularly, the present invention relates to hydrogen gas sensing using optical methods. Background Art

[0002] Hydrogen is one of the most important fuels for a fossil-free future. One of the biggest challenges in developing a hydrogen-based infrastructure is that hydrogen is explosive and highly susceptible to leaks. To help address this challenge, considerable research is being conducted to detect hydrogen leaks.

[0003] An active method for hydrogen sensing is the use of palladium hydride (PdH x ) material. When exposed to hydrogen, palladium absorbs hydrogen and changes various physical properties, including its optical properties. The amount of change in the optical properties of palladium depends on the concentration of hydrogen present. This allows the hydrogen concentration to be determined, for example, by measuring the drop in the intensity of light reflected from a palladium-coated mirror or transmitted through a tapered optical fiber coated with palladium. However, the sensitivity of these existing optical methods of detecting hydrogen is limited to ppm-level detection. Despite attempts to develop improved palladium materials for these purposes, greater sensitivity is still needed in order to detect trace levels of hydrogen. Summary of the invention

[0004] The present invention provides a hydrogen sensor capable of detecting hydrogen at concentration levels of ppb or lower. x The sensitivity of the optical detection uses a multipass optical cavity. The laser beam passes through the free space in the cavity and undergoes a large number of reflections. Along the optical path of the laser beam in the cavity, the beam interacts multiple times with the palladium-containing coating on the reflective and / or transmissive optical elements in the cavity. The coated optical elements are exposed to the gas in the free space in the cavity.

[0005] Multi-pass optical cavities have been used in gas detectors based on laser absorption spectroscopy. In these devices, a laser beam interacts directly with the gas along a free space path, and then the laser beam is reflected between two mirrors. The mirrors are used to increase the free space path length and do not play any active role in sensing. In contrast to the multi-pass optical cavity in the laser absorption spectroscopy sensor, the multi-pass optical cavity of the present invention includes optical elements that play an active role in gas sensing. Specifically, the palladium material coated on the optical elements inside the cavity interacts optically with the laser beam and actively participates in the sensing of hydrogen in the cavity through the hydrogenation of palladium. Therefore, in this design, it is the number of times the laser beam interacts with these palladium-coated optical elements that is used to improve the sensitivity of the sensor, rather than the free space path length. Therefore, the mirrors can be positioned close to each other without reducing the sensitivity, allowing a compact design. The sensitivity of the sensor can be further improved by inserting an optically transmissive material coated with palladium in the cavity so as to intersect the multi-pass path of the laser.

[0006] Thus, in one aspect, the present invention provides a laser-based hydrogen sensor comprising: a) a laser adapted to generate a laser beam at an operating wavelength; b) an optical multi-reflection cavity comprising a reflective optical element having a coating comprising palladium, and optionally further comprising a transmissive optical element having a coating comprising palladium; c) optionally, a sealed gas chamber for enclosing a gas sample, wherein the sealed gas chamber has fittings to facilitate gas flow; d) a laser beam detector configured to detect the laser beam after passing through the cavity; and e) a signal processor adapted to quantify the hydrogen concentration in the sample in the cavity based on measurements from the laser beam detector. In some embodiments, the cell is provided with a porous wall to facilitate diffusion-based sampling and reduce particulate entrainment. In some embodiments, the gas chamber may be equipped with a particulate filter or an interfering gas reduction catalyst to prevent degradation of the optical surface and reduce cross-substance interference.

[0007] In another aspect, the present invention provides an apparatus for measuring the concentration of hydrogen in a gas sample, the apparatus comprising: a multi-pass optical cavity comprising an optical element, wherein the optical element comprises a reflector supporting a multi-pass optical path in an interior of the multi-pass optical cavity; a laser configured to generate a laser beam, the laser beam entering the multi-pass optical cavity and propagating along the multi-pass optical path in an interior of the multi-pass optical cavity such that the laser beam is reflected multiple times from the reflector of the multi-pass optical cavity; a detector configured to detect the intensity of the laser beam leaving the multi-pass optical cavity; and a signal processor configured to determine the concentration of hydrogen in the gas sample in the optical cavity from the measured intensity of the laser beam; wherein the multi-pass optical cavity comprises a surface layer comprising palladium on the optical element; whereby hydrogenation of the surface layer comprising palladium by hydrogen in the gas sample changes the optical properties of the surface layer comprising palladium.

[0008] The surface layer comprising palladium may include a palladium alloy with gold, cobalt, tantalum, titanium or hafnium. The surface layer comprising palladium may include nanoparticles or a polymer coating. The surface layer comprising palladium may have a thickness of 30nm or 20nm. The surface layer comprising palladium may have a thickness in the range of 5nm-200nm. The surface layer comprising palladium may have a 160nm alloy layer of 67% palladium and 33% cobalt or gold.

[0009] The surface layer comprising palladium may be on a reflector. The surface layer comprising palladium may be on a surface of a transmission window positioned between the reflectors to intersect the multi-pass optical path, wherein the transmission window is included in the optical element. For example, the transmission window may be a contrast enhancement glass slide. The transmission window may include a palladium layer or a palladium alloy layer with a thickness of 2 nm. The transmission window may include Pd x Co 100–x The transmission window may be a translucent glass slide coated with a nanoparticle layer comprising palladium or an alloy with gold, titanium, cobalt, tantalum, hafnium or tungsten. The transmission window may include nanoparticles or nanostructures with a diameter in the range of 50nm-1000nm or 200nm-500nm. The transmission window may include an anti-reflective coating. The transmission window may include a polymer coating of PMMA and / or PTFE. The transmission window may include a polymer coating less than 100nm thick.

[0010] The mirrors may be configured such that the laser beam reflects from the mirrors of the multi-pass optical cavity at least 5 times, or more preferably at least 10 times, or even more preferably at least 66 times from the mirrors of the multi-pass optical cavity.

[0011] The multi-pass optical cavity can be contained in a cell with porous walls configured to facilitate diffusion-based sampling and reduce particulate entrainment. The multi-pass optical cavity can be contained in a sealed gas chamber. The sealed gas chamber can contain a particle filter or an interfering gas reduction catalyst to prevent degradation of optical surfaces and reduce cross-matter interference.

[0012] The laser beam may have a wavelength in the range of 400-1000 nm. The laser beam may have a wavelength in the range of 1000-8000 nm. The laser beam may have a wavelength of 5.051 μm. The laser beam may have a wavelength of 1300 nm or 1550 nm. The laser may be a mid-infrared laser. The laser may be a diode laser, a quantum cascade laser, or an interband cascade laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1A is a schematic diagram showing a hydrogen detection system according to one embodiment of the present invention.

[0014] Figure 1B is a cross-sectional view of a multi-pass palladium hydride optical cavity for use in a hydrogen sensor according to one embodiment of the present invention.

[0015] Figure 2 is a graph of transmittance and reflectance versus wavelength for a palladium gold alloy with and without exposure to 4% hydrogen.

[0016] Figure 3 is a cross-sectional view of a multi-pass palladium hydride optical cavity including a plurality of transmissive windows with a coating including palladium according to an embodiment of the present invention.

[0017] Figure 4 2 are perspective views of a reflector according to an embodiment of the present invention and cross-sectional views of two variations of a coating comprising palladium on the surface of the reflector.

[0018] Figure 5 is a perspective view of a multi-pass palladium hydride optical cavity including two reflectors according to an embodiment of the present invention, showing the multi-pass propagation paths between the reflectors.

[0019] Figure 6 is a perspective view of a multi-pass palladium hydride optical cavity including two transmissive windows with a coating including palladium according to an embodiment of the present invention.

[0020] Fig. 7A is a graph of the reflectivity of palladium versus wavelength, showing that the reflectivity of palladium increases significantly in the infrared region.

[0021] Figure 7B is a graph of absorbance versus wavelength for different film-compatible polymers.

[0022] Figure 7C is a graph of absorbance versus wavelength showing simulated infrared spectra of H2O and CO2.

[0023] Figure 8 is a graph of absorbance versus wavelength for water, showing the selected operating wavelength for hydrogen detection to avoid water interference. DETAILED DESCRIPTION

[0024] Figure 1A A hydrogen detection system according to one embodiment of the present invention is shown. The hydrogen detection system comprises a laser 101 emitting a beam 102 which enters an optical cavity 103, bounces between a plurality of mirrors 104a, 104b containing a special palladium coating, and exits the optical cavity, wherein the intensity of the exit beam is measured by a photodetector 108. A sample gas enters the optical cavity 103, interacts with the palladium coatings on various reflective optical elements 104a, 104b and transmissive optical elements 105a, 105b, 105c through a gas inlet 106, and then exits through an outlet 107.

[0025] The system also includes a laser controller 109 that controls the operation of the laser 101. In addition to controlling the laser, the controller can also control the external temperature of the cavity through a resistive heater or a thermoelectric cooler. Since the absorption and desorption rates are significantly faster at elevated temperatures, active control of the cavity temperature can further improve the temporal response of the sensor.

[0026] The system also includes a signal processor 110 that determines the hydrogen concentration from the intensity measurements of the photodetector 108. A sampling system, such as a pump, may be provided at the outlet 107, or a compressor may be provided at the inlet 106. The sampling system may also be replaced by an open path system, where the sensor operates at ambient pressure.

[0027] Figure 1BA cross-sectional view of a multi-pass palladium hydride optical cavity (MHPOC) according to one embodiment of the present invention is shown. The cavity has two or more reflectors 151a, 151b, which are placed in a certain orientation so that the laser beam is reflected multiple times between reflective surfaces 152a, 152b. The reflective surface of each reflector is made of a thin film containing palladium or a thin film of an alloy of palladium and gold, cobalt, tantalum, titanium, hafnium or tungsten. The composition of the alloy can have a graded structure to enhance sensitivity without sacrificing its response time. The reflective surface can also contain nanoparticles and polymer coatings, as discussed below. The laser beam 153 enters through an entrance aperture 157 and reflects between surfaces 152a, 152b within a free space region 156 between the reflectors until the attenuated beam 154 leaves the cavity through an aperture 158. The assembly of these components forms a version of the MHPOC represented by 155. The cavity can be enclosed in an airtight sampling chamber to allow controlled flow of a gas sample. The cavity can also maintain an open path if environmental conditions are suitable to provide sufficient sensitivity.

[0028] The basic principle behind MHPOC is that the reflectivity and transmittance of a coating of material containing palladium changes when exposed to different concentrations of hydrogen. To illustrate this dependence, Figure 2 The reflectivity and transmittance of a palladium gold alloy without and with 4% H2 in air for a wavelength range of 400-1000 nm are shown. The reflectivity and transmittance will also depend on the thickness, structure and composition of the coating layer. The reflectivity and transmittance data shown here are for layer thicknesses of 30 nm and 20 nm, respectively. It can be seen that the reflectivity is reduced by the presence of hydrogen, while the transmittance is increased by the presence of hydrogen.

[0029] The hydrogen contrast ratio (HC) in this embodiment is obtained by the formula:

[0030]

[0031] where ΔI is the change in light intensity at the cavity outlet 154 due to the presence of H2, I is the intensity of light at the cavity outlet 154 in the absence of H2, R0 is the reflectivity of the mirror in the absence of any exposure to hydrogen (assuming both mirrors have uniform reflectivity), and R H is the modified reflectivity of the mirror in the presence of hydrogen, R H Depending on the hydrogen concentration, n is the total number of times the beam reflects between the two mirrors.

[0032] Figure 3Another embodiment of the MHPOC is shown. In this embodiment, a plurality of transmissive windows 302a, 302b, 302c, 302d having a palladium-containing coating are inserted into the optical path to increase the sensitivity of the sensor to trace amounts of hydrogen. These transmissive windows have a substrate (such as fused quartz or other optically transparent material) and a coating 303 containing palladium on one or both sides. The reflector coating 301 can be the same as the reflector coating in the previously discussed embodiments. This having n T The hydrogen contrast of the number of transmission window variants can be expressed as:

[0033]

[0034] where T0 is the transmittance of windows 302a, 302b, 302c, 302d in the absence of hydrogen (assuming all windows have the same coating), T H is the transmittance of windows 302a, 302b, 302c, 302d in the presence of hydrogen. It can be emphasized that the transmissive window can have two coated surfaces containing palladium, so the sensitivity of the sensor can be significantly improved with fewer passes. However, this is at the expense of transmission losses through the substrate, which are usually higher than reflection losses. This can be compensated by using higher power lasers or higher sensitivity photodetectors or applying anti-reflection (AR) coatings.

[0035] In a variation of this embodiment, the reflector coating 301 may be a high reflectivity reflector coating without palladium, in which case the transmission window provides the entire hydrogen signal contrast. In the specific case where the reflector is made of a high reflectivity hydrogen-invariant reflective material, the hydrogen contrast formula can be simplified to:

[0036]

[0037] pass Figure 4The coating structure is shown. The reflector substrate 401 can be made of fused quartz or any other suitable material. The transmissive substrate is optically transparent to the selected wavelength of the laser. The aperture 403 can be a physical hole in the reflector or an optically transparent area. The reflective and transmissive coating 402 can be a thin film and / or nanoparticle layer containing palladium or a palladium alloy. The structure of only thin films is relatively direct and inexpensive. Such reflective coatings are made of multiple layers of materials (404, 405, 407) deposited on a substrate 406 (e.g., fused quartz). Layer 405 is a thin bonding layer (e.g., titanium or nickel) for bonding other layers thereon. This layer is not mandatory for all material combinations. Layer 404 is a layer of palladium or any one of palladium and alloys of gold, cobalt, tantalum, hafnium, copper, etc. This layer serves as the main hydrogen sensitive element in the MHPOC. Layer 407 is a top layer made of a polymer such as polymethyl methacrylate (PMMA). This layer reduces the cross-material sensitivity and poisoning of layer 404 while also allowing H2 to diffuse. An alternative coating structure includes a nanoparticle layer 408. The coating can be more sensitive to both reflection and transmission and is preferably used for transmission windows. The nanoparticle layer can be made of a nano-patchy particle array. This layer greatly enhances the hydrogen sensitivity of the MHPOC. The layer is covered by a layer 409 of much smaller palladium alloy nanoparticles to form the main sensing element, and the layer 409 generally has a total thickness in the range of about 2-20nm. The layer can be further coated with a layer 412 to reduce cross-material interference and poisoning, and the layer 412 is generally a polymer (e.g., PMMA). When the palladium alloy layer is covered by a 30nm TAF (Teflon AF 2400) layer, the response time of the sensor is significantly enhanced.

[0038] Figure 5 The path of a light beam in an MHPOC according to an embodiment of the present invention is shown. In this embodiment, the MHPOC has two reflectors 501 and 502 facing each other. The laser beam enters the MHPOC through a physical aperture 503. The beam then continues to bounce 66 times in the cavity, thereby interacting with a reflective coating comprising palladium at spots including three spots 504a, 504b, and 504c. Thereafter, the beam leaves the MHPOC along path 505. The hydrogen contrast of this embodiment is given by formula (1), where n=66, i.e.:

[0039]

[0040] Figure 6 Another embodiment is shown, in which two transmission windows 601a, 601b are inserted Figure 5 The two surfaces 602 and 603 of 601a are coated with nanoparticles to enhance the sensitivity of MHPOC. The hydrogen contrast is given by formula (2), where n = 66 and n T =2, that is:

[0041]

[0042] For a calibration system, the optical unit can be partially evacuated periodically to measure the baseline intensity I in the absence of hydrogen. The intensity of this partial evacuation is compared with the intensity I H of the atmospheric sample to allow determination of ΔI = I - I H . The ratio ΔI / I provides the hydrogen partial pressure. This method is slower than contrast modulation created by other means, but can be very effective. This pressure effect can also be replaced by a thermal effect because the absorption of hydrogen depends on temperature. If the temperature changes, the amount of hydrogen trapped inside the palladium lattice changes significantly. Therefore, if the temperature of the modulation medium is varied, (if the thermal effect does not cause significant optical distortion) intensity changes will occur only when hydrogen is present. The temperature of the glass slide can be changed more quickly than the pressure in the MHPOC. However, the temperature effect may require periodic calibration.

[0043] For embodiments that include a transmissive slide inside the optical cavity, an important consideration is that the total power of the laser beam decreases with each pass. For example, an uncoated BK-7 glass slide transmits only 92% of the incident light at 633 nm. For 66 reflections and one transmissive slide, this results in only 0.4% light output compared to a cavity without a slide. The remaining light is reflected as stray reflections inside the cavity, which can also cause edge noise or an etaloning effect. To achieve an ultra-low detection limit (LOD), the limit ΔT << T0 should be satisfied. If T0 is close to one and ΔT is small, such conditions can be achieved. Although the absorption of the PS monolayer and polymer coating is negligible, the absorption of the glass substrate is typically large due to the difference in refractive index between the glass substrate and air. To mitigate such losses, a preferred embodiment includes a thin anti-reflection layer (e.g., TiO2 / SiO2-TiO2) on the transmissive slide. Thus, the transmittance through the glass substrate at a certain wavelength can reach 99%. By using a Pd x Co 100-x coating with a thickness of a few nanometers, a smaller ΔT can be achieved. When using a 2-nm thin palladium layer, the transmittance at a wavelength of 1000 nm is reduced by 7%. The thin Pd x Co 100-x coating also ensures that T0 is close to one and ensures that the sensor has a fast response. This can also be achieved by applying a nano-textured surface by dry / wet etching or molding processes.

[0044] The coating incorporating nanospheres will now be discussed in more detail. In one embodiment, nanosphere lithography (NL) and glancing angle co-deposition (GLACD) techniques can be used together to fabricate Pd on an array of polystyrene (PS) nanospheresx Co 100-x Nanopatch (NPD) arrays, polystyrene (PS) nanospheres with diameters D = 500 nm and 200 nm. In GLACD, the evaporation rates of palladium and cobalt can be increased by 10 -7 The two electron beam sources of the support are independently controlled. The incident angle θ and azimuth angle The combination of the two produced nanostructures with different shapes and sizes. The formation and composition of the alloy were experimentally confirmed by energy dispersive X-ray spectroscopy (EDS) elemental mapping. The vapor incidence angle θ = 50° was chosen to ensure that the film would not be deposited on the glass substrate. The lack of hysteresis response ensured that the accuracy of the sensor was better than 3%. In general, the same conditions as Pd 80 Ag 20 and Pd 80 Au 20 In comparison, Pd 80 Co 20 Has a faster response time (less than 1 second across the range), but lower sensitivity. When more cobalt is used, Pd 67 Co 33 NP 200 Shows better resistance to humidity. In a preferred embodiment, using Pd x M 100-x Sensors made of nanopatch arrays with alloys (M = Ag or Co) as coating materials have ppb-level LOD and fast response time. There are several advantages to using PS nanosphere arrays as substrates: (1) Such structures are very useful for light transmission readout because the thickness, shape, and morphology of the structure can be determined by the PS bead size, incident angle θ (e.g. Figure 4 a) and deposition thickness; (2) Due to the rapid hydrogen diffusion in PS, hydrogen can interact with the sensing composite through substrate sites. Therefore, the surface-to-volume ratio (SVR) in this structure is almost doubled compared to deposition on glass or silicon substrates, accelerating the H2 adsorption / desorption process. The unique design of the nanopatch sensor allows very high surface coverage (>90%), even in Very low values ​​can also result in considerable optical contrast. The curvature of the patch and the loose connection of the patch to the PS beads mitigate substrate effects and allow for fast response times and better mechanical stability during lattice expansion and contraction.

[0045] In order to obtain a fast H2 sensor with higher sensitivity and selectivity, it is preferred to use a polymer coating on the nanostructure, for example, PMMA and / or PTFE. The PMMA layer is known to filter gases that are toxic to hydrides and is used as an outer layer, while the PTFE layer modifies the surface energy of the metal alloy for faster H2 absorption kinetics. In principle, coatings of PMMA and PTFE less than 0.1 μm do not have a significant effect on the response time. The PMMA layer can be deposited by spin coating techniques, while the PTFE can be deposited at 10 -7 Deposition in an evaporation system at a base pressure of Torr.

[0046] We now discuss the choice of operating wavelength. The reflectivity of palladium increases steadily with longer wavelengths. However, the absorption of the polymer layer used to improve hydrogen selectivity also increases with longer wavelengths. The preferred operating wavelength is chosen by balancing these trade-offs.

[0047] The total light output at the cavity exit is exponentially related to the reflectivity of the mirror. As predicted by the Lorentz-Drude model, the reflectivity of palladium increases dramatically in the infrared ( Fig. 7A ), thereby improving the total light intensity available at the cavity exit. The wavelength dependence of reflectivity from mid-infrared to far-infrared using different coatings. For example, Figure 5 66 reflections of the Zephyr cavity at 93.26% reflectivity will result in only 1% laser intensity. This threshold is given by Fig. 7A The signal is indicated by the dashed line in the range marked >1% and occurs around 3.42 μm. A polymer coating can be applied on the mirror to significantly enhance the selectivity of the mirror for hydrogen absorption. Figure 7B The calculated absorbance for different thin film compatible polymers for the coating thicknesses achieved previously is shown. It can be seen that the extinction in the polymer layer has approximately the same trend as the reflectance of palladium with respect to wavelength. This means that there is a trade-off between increased reflectivity that must be managed and extinction from the polymer coating for enhanced selectivity. Not only that, but the selectivity of the sensor also depends on the selectivity for light absorption in the laser path due to direct absorption by atmospheric gases. Figure 7C The infrared spectra of H2O and CO2 in the wavelength range of 0.25-12 μm simulated using the HITRAN database are shown. This competition from other atmospheric gases narrows the wavelength selection range very significantly. Several other molecules (such as various hydrocarbons, carbon monoxide, nitrogen oxides, etc.) are also potential interferences. Figure 8 As shown, all of these factors lead to the selection of a preferred wavelength of 5.051 μm for embodiments using these materials. The calculated reflectivity of palladium at this wavelength is 0.973, while the absorbance from the 20 nm PMMA top layer is 1.8 × 10-4 For a 66-reflection cavity, a healthy 16% of the light output at the cavity exit is expected. Preferably, mid-IR lasers (quantum cascade and interband cascade lasers) close to this wavelength region are used. PdH can be further determined using Fourier Transform Infrared Spectroscopy (FTIR) x In summary, for pure palladium mirrors, the reflective element can benefit from selecting an operating wavelength above 3.4um. However, for alloys of palladium with gold, for example, this can change.

[0048] The transmission of light through ultra-thin layers of palladium is much more lenient in terms of wavelength selection. The choice therefore depends mainly on the wavelength sensitivity of the various layer thicknesses of the palladium alloy layer. Large-scale production of visible or telecommunication near-infrared diode lasers (600-1800 nm) can be used for transmission applications.

Claims

1. An apparatus for measuring the hydrogen concentration in a gas sample, the apparatus comprising: a multi-pass optical cavity comprising an optical element, wherein the optical element comprises a mirror supporting a multi-pass optical pathway within an interior of the multi-pass optical cavity; a laser configured to generate a laser beam that enters the multi-pass optical cavity and propagates along the multi-pass optical path in the interior of the multi-pass optical cavity such that the laser beam is reflected multiple times from the mirrors of the multi-pass optical cavity; a detector configured to detect an intensity of the laser beam exiting the multi-pass optical cavity; as well as a signal processor configured to determine the hydrogen concentration in the gas sample in the optical cavity from the measured intensity of the laser beam; Wherein the multi-pass optical cavity comprises a surface layer comprising palladium on the optical element; whereby hydrogenation of the surface layer comprising palladium by hydrogen in the gas sample changes the optical properties of the surface layer comprising palladium to allow optical detection of hydrogen.

2. The method of claim 1, wherein the surface layer comprising palladium is on the reflector.

3. The method of claim 1, wherein the laser beam reflects from the mirror of the multi-pass optical cavity at least 10 times.

4. The method of claim 1, wherein the optical element comprises a transmissive window positioned between the mirrors to intersect the multi-pass optical pathway.

5. The method of claim 1, wherein the optical element comprises a transmissive window, and wherein the surface layer comprising palladium is on at least one surface of the transmissive window.

6. The method of claim 1, wherein the multi-pass optical cavity is contained in a cell having porous walls configured to facilitate diffusion-based sampling and reduce particulate entrainment.

7. The method of claim 1, wherein the multi-pass optical cavity is contained in a sealed gas chamber.

8. The method of claim 7, wherein the sealed gas chamber contains a particulate filter or an interfering gas reduction catalyst to prevent degradation of optical surfaces and reduce cross-matter interference.

9. The method of claim 1, wherein the surface layer comprising palladium comprises palladium alloyed with gold, cobalt, tantalum, titanium, or hafnium.

10. The method of claim 1, wherein the surface layer comprising palladium comprises nanoparticles or a polymer coating.

11. The method of claim 1, wherein the laser beam has a wavelength in the range of 400-1000 nm.

12. The method of claim 1, wherein the surface layer comprising palladium has a thickness of 30 nm or 20 nm.

13. The method of claim 1, wherein the laser beam has a wavelength in the range of 1000-8000 nm.

14. The method of claim 1, wherein the surface layer comprising palladium has a thickness in the range of 5 nm to 200 nm.

15. The method of claim 1, wherein the surface layer comprising palladium comprises palladium alloyed with cobalt or gold.

16. The method of claim 1, wherein the transmissive window is a contrast enhancing slide.

17. The method of claim 1, wherein the laser beam reflects at least 5 times from the mirror of the multi-pass optical cavity.

18. The method of claim 1, wherein the laser beam reflects 66 times from the mirror of the multi-pass optical cavity.

19. The method of claim 1, wherein the laser beam has a wavelength of 5.051 μm.

20. The method of claim 1, wherein the laser is a mid-infrared laser 21. The method of claim 1, wherein the laser is a diode laser, a quantum cascade laser, or an interband cascade laser.

22. The method of claim 1, wherein the surface layer comprising palladium comprises a 160 nm layer of palladium alloyed with cobalt or gold.

23. The method of claim 1, wherein the transmissive window is a translucent glass slide coated with a nanoparticle layer comprising palladium or an alloy with gold, titanium, cobalt, tantalum, hafnium or tungsten.

24. The method of claim 1, wherein the transmissive window comprises an anti-reflective coating.

25. The method of claim 1, wherein the transmissive window comprises Pd x Co 100–x coating, where x is in the range of 50-100.

26. The method of claim 1, wherein the transmissive window comprises a layer of palladium or its alloys with a thickness of 2 nm.

27. The method of claim 1, wherein the transmissive window comprises nanoparticles or nanostructures having a diameter in the range of 50 nm to 1000 nm.

28. The method of claim 1, wherein the transmissive window comprises a polymer coating of PMMA and / or PTFE.

29. The method of claim 1, wherein the transmissive window comprises a polymer coating less than 100 nm thick.

30. The method of claim 1, wherein the laser beam has a wavelength of 1300 nm or 1550 nm.

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