Integrated Micro Photoionization Detector with Ultra-Thin Ultraviolet Transmission Window

Abstract

An integrated microfluidic photoionization detector (PID) is provided, the integrated microfluidic photoionization detector comprising: a microfluidic ionization chamber, a microfluidic ultraviolet radiation chamber configured to generate ultraviolet photons. An ultra-thin transmission window is disposed between the microfluidic ionization chamber and the microfluidic ultraviolet radiation chamber and permits the ultraviolet photons to enter the microfluidic ionization chamber from the microfluidic ultraviolet radiation chamber. A detection system for one or more VOC analytes is also provided, the detection system comprising: a gas chromatography (GC) unit, the gas chromatography unit including at least one gas chromatography column; and an integrated microfluidic photoionization detector (PID) disposed downstream of the gas chromatography (GC) unit.

Description

[0001] Government Support

[0002] This invention was made with government support under EPA RD-83564401-0 awarded by the United States Environmental Protection Agency. The government has certain rights in the invention.

[0003] Cross-Reference to Related Applications

[0004] This application claims the benefit of U.S. Provisional Application No. 62 / 740,583, filed Oct. 3, 2018. The entire disclosure of the above application is incorporated herein by reference. Technical Field

[0005] This disclosure relates to an integrated microphotoionization detector having an ultrathin ultraviolet transmission window. Background Art

[0006] This section provides background information related to the present disclosure that is not necessarily prior art.

[0007] Gas chromatography (GC) is widely used for analyzing volatile organic compounds (VOCs) and other analyte compounds. When used for analysis, a GC system typically also includes an analyte detector. A flame ionization detector (FID) is a commonly used vapor detector for benchtop GC instruments. However, FIDs and μFIDs (micro FIDs) are destructive and thus have limited use. For example, FIDs and μFIDs cannot be placed in the middle of the vapor flow path to monitor multidimensional GC separations. Instead, FIDs and μFIDs can only be used at the end of a GC instrument. In addition, FIDs require the use of hydrogen, which has hindered their widespread acceptance in μGC instruments. Thermal conductivity detectors (TCDs) and μTCDs (micro TCDs) have been used with GC as vapor detectors. Thermal conductivity detectors and μTCDs are non-destructive and have a flow-through design. However, TCDs have low sensitivity (nanograms) and require helium. Electron capture detectors (ECDs) are another type of non-destructive vapor detector. Although electron capture detectors are very sensitive, they have a limited dynamic range and require the use of radioactive materials for analyte ionization.

[0008] A photoionization detector (PID) is another type of vapor detector. The PID is sensitive (picogram), non-destructive, and applicable to a wide range of vapors. In a PID, vapor molecules are ionized in an ionization chamber by UV radiation generated by a UV lamp. The ions generated in the ionization chamber are then driven to electrodes to generate a current. The UV lamp is typically filled with low-pressure argon, krypton, or other gases to generate UV light under external electrical excitation. In the UV lamp, a sealed window (i.e., a UV-transmissive window) is made of a special material such as calcium fluoride, magnesium fluoride, or lithium fluoride. These materials have a relatively high transmission coefficient in the wavelength range of the UV radiation of interest. However, these materials are relatively expensive and are susceptible to water erosion, crystal solarization, and yellowing effects due to UV damage, all of which reduce the performance of the UV lamp (and thus the PID) and shorten its service life. Additionally, this conventional UV lamp is not compatible with microfabrication processes. Thus, it is difficult to microfabricate a PID using this conventional UV lamp. SUMMARY OF THE INVENTION

[0009] This section provides a general overview of the disclosure and not an exhaustive disclosure of its full scope or all of its features.

[0010] In some aspects, the present disclosure provides an integrated microfluidic photoionization detector (PID) that includes a microfluidic ionization chamber having an inlet for receiving a fluid sample and an outlet through which the fluid sample exits the microfluidic ionization chamber. The integrated microfluidic PID further includes a first electrode and a different second electrode that are in electrical communication with the microfluidic ionization chamber. A microfluidic ultraviolet radiation chamber is configured to generate ultraviolet photons. The integrated microfluidic PID further includes an ultrathin transmissive window disposed between the microfluidic ionization chamber and the microfluidic ultraviolet radiation chamber and that permits ultraviolet photons to enter the microfluidic ionization chamber from the microfluidic ultraviolet radiation chamber. In some variants, the ultrathin transmissive window permits greater than or equal to about 5% of the ultraviolet photons to pass through.

[0011] In one aspect, the transmissive ultrathin window comprises a material selected from the group consisting of silica, fused silica, quartz, sapphire, magnesium fluoride, calcium fluoride, lithium fluoride, and combinations thereof.

[0012] In one aspect, the transmissive ultrathin window is defined as one or more selected regions on a plate.

[0013] In one aspect, the transmissive ultrathin window is disposed on a support plate, and the transmissive ultrathin window is defined within a selected region of the support plate.

[0014] In one aspect, the integrated microfluidic photoionization detector (PID) further includes a stack of layers, the stack of layers including a first layer and a second layer. A transmissive ultra-thin window is defined within the first layer, and one or more regions corresponding to the transmissive ultra-thin window of the second layer are absent.

[0015] In one aspect, the transmissive ultra-thin window has a thickness less than or equal to about 20 μm and is configured to transmit greater than or equal to about 5% of ultraviolet photons.

[0016] In one aspect, the transmissive ultra-thin window has a thickness greater than or equal to about 250 nm to less than or equal to about 500 nm.

[0017] In one aspect, the microfluidic ultraviolet radiation chamber has an inlet for receiving an ultraviolet-generating fluid.

[0018] In one aspect, the microfluidic ultraviolet radiation chamber includes an ultraviolet-generating fluid selected from the group consisting of krypton, argon, helium, and combinations thereof.

[0019] In one aspect, the microfluidic ionization chamber is one or more microfluidic channels.

[0020] In one aspect, the one or more microfluidic channels have a total volume less than about 10 μL.

[0021] In one aspect, a first electrode and a different second electrode are formed in a layer of conductive material, and the one or more microfluidic channels are disposed in the layer to electrically insulate the first electrode from the different second electrode.

[0022] In some other aspects, the present disclosure provides a detection system for one or more VOC analytes, the detection system including: a gas chromatography (GC) unit including at least one gas chromatography column; and an integrated microfluidic photoionization detector (PID) disposed downstream of the gas chromatography (GC) unit. The integrated microfluidic photoionization detector (PID) includes a microfluidic ionization chamber having an inlet for receiving a fluid sample and an outlet through which the fluid sample exits the microfluidic ionization chamber. The integrated microfluidic photoionization detector (PID) further includes a first electrode and a different second electrode in electrical communication with the microfluidic ionization chamber. The integrated microfluidic photoionization detector (PID) further includes a microfluidic ultraviolet radiation chamber configured to generate ultraviolet photons. The integrated microfluidic photoionization detector (PID) further includes a transmissive ultra-thin window disposed between the microfluidic ionization chamber and the microfluidic ultraviolet radiation chamber, the transmissive ultra-thin window allowing greater than or equal to 5% of ultraviolet photons to enter the microfluidic ionization chamber from the microfluidic ultraviolet radiation chamber. The microfluidic photoionization detector (PID) analyzes the sample processed in the gas chromatography (GC) unit.

[0023] In one aspect, the transmissive ultra-thin window comprises a material selected from the group consisting of silica, fused silica, quartz, sapphire, magnesium fluoride, calcium fluoride, lithium fluoride, and combinations thereof.

[0024] In one aspect, the transmissive ultra-thin window is defined as one or more selected regions on a plate.

[0025] In one aspect, the transmissive ultra-thin window is disposed on a support plate, and the transmissive ultra-thin window is defined within a selected region of the support plate.

[0026] In one aspect, the integrated microfluidic photoionization detector (PID) further comprises a stack of layers having a first layer and a second layer. The transmissive ultra-thin window is defined within the first layer, and one or more regions of the second layer corresponding to the transmissive ultra-thin window are absent.

[0027] In one aspect, the transmissive ultra-thin window has a thickness less than or equal to about 20 μm and is configured to transmit ultraviolet photons greater than or equal to about 5%.

[0028] In one aspect, the microfluidic ionization chamber is one or more microfluidic channels.

[0029] In one aspect, a first electrode and a different second electrode are formed in a layer of conductive material, and one or more microfluidic channels are disposed in the layer to electrically insulate the first electrode and the different second electrode.

[0030] Based on the description provided herein, other fields of application will become apparent. The description and specific examples in this summary of the invention are intended for illustrative purposes only and are not intended to limit the scope of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The drawings described herein are for illustrative purposes only for the selected embodiments and not for all possible implementations and are not intended to limit the scope of the present disclosure.

[0032] Figure 1 A schematic diagram of an exemplary detection system is shown, the detection system comprising: a gas chromatography (GC) unit including at least one gas chromatography column; and an integrated microfluidic photoionization detector (PID) disposed downstream of the gas chromatography (GC) unit.

[0033] Figure 2 An illustration of an integrated microfluidic photoionization detector (PID) having an ultra-thin transmissive window in accordance with certain aspects of the present disclosure is shown.

[0034] Figure 3 is Figure 2 a cross-sectional view of the integrated microfluidic photoionization detector (PID) taken along line 3-3.

[0035] Figure 4 It is a photograph of an ultrathin ultraviolet (UV) transmission window patterned on a thermally oxidized silicon wafer.

[0036] Figure 5 It is a graph comparing the UV transmission ability of an ultrathin ultraviolet (UV) transmission window prepared according to certain aspects of the present disclosure with that of an air window.

[0037] In several views throughout the drawings, corresponding reference numerals represent corresponding components. Detailed Description

[0038] Providing exemplary embodiments will make the present disclosure thorough and will fully convey the scope to those skilled in the art. Many specific details, such as specific compositions, components, devices, and methods, are set forth by way of example to provide a thorough understanding of the embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details are not required, and the exemplary embodiments may be embodied in many different forms and should not be construed as limiting the scope of the present disclosure. In some exemplary embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.

[0039] The terms used herein are for the purpose of describing particular exemplary embodiments only and are not intended to be limiting. As used herein, unless the context clearly dictates otherwise, the singular forms "a", "an" and "the" may also be intended to include the plural forms. The terms "comprising", "including" and "having" are inclusive and thus specify the presence of the stated features, elements, compositions, steps, wholes, operations and / or components, but do not preclude the presence or addition of one or more other features, wholes, steps, operations, elements, components and / or groups thereof. Although the open-ended term "comprising" should be understood as a non-limiting term used to describe and claim the various embodiments set forth herein, in some instances, the term may alternatively be understood as a more restrictive and limiting term such as "consisting of" or "consisting essentially of". Thus, for any given embodiment reciting a composition, material, component, element, feature, whole, operation and / or process step, the present disclosure also specifically includes embodiments consisting of or consisting essentially of the stated composition, material, component, element, feature, whole, operation and / or process step. In the case of "consisting of", alternative embodiments do not include any additional compositions, materials, components, elements, features, wholes, operations and / or process steps, while in the case of "consisting essentially of", any other compositions, materials, components, elements, features, wholes, operations and / or process steps that materially affect the basic and novel characteristics are excluded from such embodiments, but any compositions, materials, components, elements, features, wholes, operations and / or process steps that do not materially affect the basic and novel characteristics may be included in such embodiments.

[0040] Unless explicitly identified as an order of execution, any method steps, processes and operations described herein should not be construed as necessarily requiring them to be performed in the particular order discussed or illustrated. It should also be understood that additional steps or alternative steps may be employed unless otherwise indicated.

[0041] When a component, element or layer is referred to as being "on", "engaged to", "connected to" or "coupled to" another element or another layer, the component, element or layer may be directly on, engaged to, connected to or coupled to another component, another element or another layer, and there may be intervening elements or intervening layers. In contrast, when an element is referred to as being "directly on", "directly engaged to", "directly connected to" or "directly coupled to" another element or another layer, there may be no intervening elements or intervening layers. Other words used to describe the relationship between elements should be interpreted in a similar manner (e.g., "between" versus "directly between", "adjacent" versus "directly adjacent", etc.). As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0042] Unless otherwise stated, although the terms first, second, third, etc. may be used herein to describe various steps, elements, components, regions, layers, and / or sections, these steps, elements, components, regions, layers, and / or sections should not be limited by these terms. These terms may be used only to distinguish one step, element, component, region, layer, or section from another step, element, component, region, layer, or section. Terms such as "first", "second", and other numerical terms used herein do not imply an order or sequence unless the context clearly indicates otherwise. Thus, the first step, element, component, region, layer, or section discussed below may be referred to as the second step, element, component, region, layer, or section without departing from the teachings of the exemplary embodiments.

[0043] For ease of description, spatial or temporal relative terms such as "before", "after", "inside", "outside", "below", "under", "lower", "above", "upper", etc. may be used herein to describe the relationship of one element or feature to another element or elements or features as shown in the figures. Spatial or temporal relative terms are intended to cover different orientations of the device or system in use or operation in addition to the orientation depicted in the figures.

[0044] Throughout this disclosure, numerical values represent approximate measures or limitations of ranges to cover minor deviations from a given value, as well as embodiments that approximately have the recited value and embodiments that exactly have the recited value. Except for the working examples provided at the end of the detailed description, all numerical values of the parameters (e.g., quantities or conditions) in this specification, including the appended claims, should be understood to be modified in all instances by the term "about", whether or not "about" actually appears before the numerical value. "About" means that the recited numerical value allows some slight imprecision (being accurate to that value in some way; approaching or reasonably close to that value; nearly). If the imprecision provided by "about" is not understood in the ordinary sense in the art, then "about" as used herein means at least the variation that can be caused by the ordinary methods of measuring and using such a parameter. For example, "about" may include variations of less than or equal to 5%, optionally less than or equal to 4%, optionally less than or equal to 3%, optionally less than or equal to 2%, optionally less than or equal to 1%, optionally less than or equal to 0.5%, and in some aspects, optionally less than or equal to 0.1%.

[0045] In addition, the disclosure of a range includes the disclosure of all values and further divided ranges within the entire range, including the endpoints and sub-ranges given for the range.

[0046] Exemplary embodiments will now be described more fully with reference to the accompanying drawings.

[0047] A detection system using gas chromatography can be used to detect the presence of specific analyte compounds (such as VOCs or other compounds). In such a system, like the vapor detector prepared according to certain aspects of the present teachings, a vapor detector in the form of a microfluidic PID is used in combination with a gas chromatography (GC) column. First, the vapor sample to be analyzed is introduced into the GC column. In certain aspects, the GC column can be miniaturized to a micro GC (μGC). Subsequently, the sample is transported through the column by an inert gas carrier, and the sample is separated in the GC column according to the physical properties of each compound (analyte) in the sample. Each eluted compound exits the GC column and enters the vapor detector unit, which can be a photoionization detector (PID) or a microfluidic PID (μPID) as described below. In other designs, the PID or μPID vapor detector unit can be used at an intermediate position within the GC column to detect the analyte. As discussed herein, unless otherwise stated, PID and μPID are used interchangeably.

[0048] PID typically uses high-energy photons (e.g., within the ultraviolet (UV) wavelength range) to break down eluted analyte molecules into positively charged ions. PID typically uses a discharge gas, such as an inert gas (e.g., krypton and argon), which is ionized within a discharge lamp chamber. External energy (e.g., RF energy) can be absorbed by the discharge gas, causing the atoms in the discharge gas to transition to an excited state. In the discharge chamber of the UV lamp, each ion can combine with another atom to emit one or more photons. The eluted compound enters the PID device in the ionization chamber. The ionization chamber and the discharge lamp chamber are typically separated from each other by an optically transparent window that allows high-energy photons to enter the ionization chamber. A typical transmission window is formed of a material such as magnesium fluoride, calcium fluoride, or lithium fluoride and has a thickness of about 0.1 mm (about 100 μm) to about 10 mm. Subsequently, the ionization chamber containing the eluted compound is bombarded by photons generated by the ionized discharge gas from the discharge lamp chamber.

[0049] Subsequently, the photons / energy are absorbed by the analyte molecules, which transition to an excited state and are ionized in a separate ionization chamber, ultimately forming positively charged ions. Thus, based on the relative retention times in the GC column, different analyte molecules in the sample are separated, elute at different times, and then enter the chamber where these molecules are ionized by the photons emitted from the ionized discharge gas.

[0050] The gas is thus charged, and the ions generate a current that becomes an output related to the concentration of the ionized analyte molecules. A current is generated when each ionized compound passes over one or more collection electrodes adjacent to the ionization chamber. In this manner, the analyte compounds can be identified based on the retention time of the analyte compounds and quantified by the PID signal (or current PID generation).

[0051] In various aspects, the present disclosure contemplates an integrated photoionization detector (PID), which in certain variations as further discussed below can be a microfluidic PID (μPID). The μPID includes a microfluidic ionization chamber for receiving and processing a fluid sample. A first electrode and a different second electrode are in electrical communication with the microfluidic ionization chamber. The μPID also includes an integrated microfluidic ultraviolet radiation chamber configured to generate ultraviolet photons. A transmissive ultra-thin window is disposed between the microfluidic ionization chamber and the microfluidic ultraviolet radiation chamber, and the transmissive ultra-thin window permits ultraviolet photons to enter the microfluidic ionization chamber from the microfluidic ultraviolet radiation chamber.

[0052] The present disclosure in certain aspects provides a detection system 20 for one or more volatile organic compounds (VOCs) or other target analytes, such as Figure 1 the detection system shown. In the detection system 10, a gas chromatography (GC) unit 20 includes at least one gas chromatography column 22. The use of the term "column" is intended to broadly include various flow paths through which a fluid can flow, such as a patterned flow field from microfeatures defined in one or more substrates or other fluid flow paths recognized by those skilled in the art. An integrated microfluidic photoionization detector (μPID) 30 is disposed downstream of the gas chromatography (GC) unit 20.

[0053] In some aspects, the GC unit 20 is a microfluidic GC (μGC), and the PID 30 is a microfluidic PID (μPID). In various aspects, the present disclosure provides forming methods and devices that have features or channels at the microscale and are thus microfluidic. In some aspects, features such as channels or chambers are optionally smaller than the microscale, such as nanoscale structures. As used herein, "micron-scale" refers to a structure having at least one dimension that is less than about 500 μm, optionally less than about 400 μm, optionally less than about 300 μm, optionally less than about 200 μm, optionally less than about 150 μm, and in certain variations, optionally less than about 100 μm. A "nanoscale" structure has at least one dimension that is less than or equal to about 50 μm, optionally less than or equal to about 10 μm (10,000 nm), optionally less than or equal to about 1 μm (1,000 nm), optionally less than or equal to about 0.1 μm (100 nm), optionally less than about 50 nm, and optionally less than about 10 nm. As used herein, references to the microscale, microchannels, microfluidic channels, or microstructures include smaller structures, such as equivalent nanoscale structures.

[0054] A microfluidic channel is a microchannel formed in or on a substrate, the cross-sectional area and volume of which are sufficient to allow the microfluidic channel to receive, transfer, and / or store materials including fluids. Fluids include gases, vapors, and liquids, etc. Thus, microfluidic channels typically have dimensions such that the length of the structure forms the largest dimension, for example, a groove (open shape) or a channel (structurally closed geometry). In certain variations, as further described herein, the microfluidic channel can be a completely enclosed structure that defines a void region that allows fluid communication through the structure. Microfluidic channels can have a variety of cross-sectional shapes including annular, circular, or elliptical (forming a tubular or cylindrical shape), rectangular, etc.

[0055] A gas chromatography-based detection system 20 typically has at least five components: (1) a carrier gas supply source 24; (2) a sample fluid injection system 26; (3) one or more gas chromatography columns 22; (4) a detector, such as a microfluidic PID 30; and (5) a data processing system (not shown). The carrier gas (also known as the mobile phase) is a high-purity and relatively inert gas, such as helium, hydrogen, nitrogen, argon, or air. The carrier gas can flow through the GC column 22 simultaneously with the sample fluid to be tested (throughout the separation process). The sample fluid injection system 26 introduces a predetermined volume of a sample mixture containing one or more target analytes to be tested (e.g., in gaseous form) into the column by mixing the sample mixture with the flowing carrier gas from the carrier gas supply source. Generally, separation is achieved within the chromatography column 22 because the inner surface of the column is coated (or the interior of the column is filled) with a material that serves as the stationary phase. The stationary phase adsorbs different target analytes in the sample mixture to varying degrees. The differences in adsorption result in different delays, which in turn cause different mobilities of different chemicals as they travel down the column, thus affecting the physical separation of the target analytes in the sample mixture. It should be noted that although only shown as a single GC column 22, the gas chromatography (GC) unit 20 can include multiple columns through which the sample fluid can pass. Additionally, such a detection system can include various other components, such as a modulator, etc.

[0056] A detector such as the microfluidic PID 30 is located downstream of the outlet 32 of one or more GC columns 22. The μPID 30 is integrated with the gas chromatography (GC) unit 20 and is used to detect various chemicals or target analytes in the sample that elute or flow out of the column 22 at different times. The μPID 30 includes a microfluidic ionization chamber 40 having an inlet 42 for receiving the fluid sample and an outlet 44 through which the fluid sample exits the microfluidic ionization chamber 40. A first electrode and a different second electrode (not shown) are also in electrical communication with the microfluidic ionization chamber 40. The microfluidic ultraviolet radiation chamber 50 is configured to generate ultraviolet photons. A transmission window 60 is disposed between the microfluidic ionization chamber 40 and the microfluidic ultraviolet radiation chamber 50 and allows the ultraviolet photons to enter the microfluidic ionization chamber 50 from the microfluidic ultraviolet radiation chamber 40. Thus, the μPID 30 analyzes the sample processed in the gas chromatography (GC) unit 20. Although not shown, the data processing system typically also communicates with the μPID 30 to be able to store, process, and record the separated test results.

[0057] Figure 2 and Figure 3 A schematic and cross-sectional view of a representative integrated microfluidic photoionization detector (μPID) 100 prepared in accordance with certain aspects of the present disclosure is shown. The μPID 100 includes a substrate 110. One or more microfluidic channels 118 can be formed in or on the substrate 110. InFigure 2 and Figure 3 In Figure 3 , a wall structure 112 is formed on a substrate 110 to define one or more microfluidic channels 118. The substrate 110 can be formed of an inorganic material or a polymer. In some aspects, the substrate 110 can be glass (e.g., silica or borosilicate). In some variations, the substrate 110 includes multiple layers.

[0058] In some variations, the wall structure 112 can be a layer or a selected region of a conductive material (e.g., a conductive silicon material) formed on the substrate 110. In this way, in some variations, when the wall structure 112 is conductive, the wall structure 112 can be used as a positive electrode and / or a negative electrode. Thus, at least one layer of the wall structure 112 can include a conductive material. The conductive material can be formed of a conductive material or a semiconductor material (such as a doped semiconductor material). In some aspects, the conductive material includes a material selected from the group consisting of: silicon (Si) (e.g., doped silicon), aluminum (Al), indium tin oxide (ITO), gold (Au), silver (Ag), platinum (Pt), iridium (Ir), palladium (Pd), tungsten (W), stainless steel (SS), zinc (Zn), titanium (Ti), their alloys and oxides, and combinations thereof. At least two of the multiple layers can have different compositions. For example, the first layer on the substrate 110 can include a doped semiconductor material, such as doped silicon, and the second layer covering the first layer can include a conductive metal. Alternatively, the conductive material can be embedded as an electrode in the wall structure 112 in contact with one or more microfluidic channels 118.

[0059] The wall structure 112 can be selectively formed in certain regions or alternatively removed in selected regions in a pattern that forms one or more microfluidic channels 118. In some aspects, one or more microfluidic channels 118 can define a spiral pattern. One or more microfluidic channels 118 can thus define a serpentine pattern on the substrate. "Serpentine" refers to a flow-through design in which the fluid channel is meandering and has at least two 180° direction changes in the course of the fluid path. Thus, the fluid path defined by one or more microfluidic channels 118 is curved and can avoid direction changes that result in dead zones or reduced fluid flow. Such a serpentine path can define a spiral structure or an interdigitated structure. In an embodiment, one or more microfluidic channels 118 can define an Archimedean spiral. One or more microfluidic channels 118 can be etched or formed in a conductive silicon wafer or layer, e.g., as an Archimedean spiral channel formed in such a material. In other variations, one or more microfluidic channels can have other flow path configurations including linear straight flow paths.

[0060] In some variations, one or more microfluidic channels 118 (or ionization chambers) have a total volume that is less than or equal to about 10 μL, and in some preferred aspects, less than or equal to about 9 μL, optionally less than or equal to about 8 μL, optionally less than or equal to about 7 μL, optionally less than or equal to about 6 μL, optionally less than or equal to about 5 μL, optionally less than or equal to about 4 μL, optionally less than or equal to about 3 μL, optionally less than or equal to about 2 μL, and in some variations, optionally less than or equal to about 1.5 μL. For example, in one variation, one or more microfluidic channels 118 define an ionization chamber volume of only about 1.3 μL.

[0061] In addition, in some aspects, the μPID 100 device has a negligible total dead volume within the microfluidic pathway. The total dead volume of one or more microfluidic channels 118 can be less than or equal to about 1% of the total volume of the microfluidic channel. For example, when the total volume of one or more microfluidic channels is 5 μL, a dead volume that is less than or equal to 1% would be a dead volume that is less than or equal to about 0.05 μL or 50 nL. In some other variations, the total dead volume of one or more microfluidic channels 118 can be less than or equal to about 0.9% of the total volume of one or more microfluidic channels, optionally less than or equal to about 0.7% of the total volume of one or more microfluidic channels, optionally less than or equal to about 0.6% of the total volume of one or more microfluidic channels, and in some variations, less than or equal to about 0.5% of the total volume of one or more microfluidic channels. In some other variations, the dead volume of one or more microfluidic channels can be less than or equal to about 30 nL, optionally less than or equal to about 25 nL, optionally less than or equal to about 15 nL, optionally less than or equal to about 10 nL, optionally less than or equal to about 5 nL, optionally less than or equal to about 4 nL, optionally less than or equal to about 3 nL, and in some variations, optionally less than or equal to about 2 nL.

[0062] The microfluidic channel can have a width greater than or equal to about 50 μm to less than or equal to about 200 μm, optionally a width greater than or equal to about 100 μm to less than or equal to about 200 μm, and in some aspects, optionally a width greater than or equal to about 125 μm to less than or equal to about 175 μm. In some other variants, the height or depth of the microfluidic channel is greater than or equal to about 100 μm to less than or equal to about 600 μm, optionally greater than or equal to about 200 μm to less than or equal to about 500 μm, optionally greater than or equal to about 300 μm to less than or equal to about 400 μm, and in some aspects, optionally greater than or equal to about 350 μm to less than or equal to about 400 μm. The total length of the microfluidic channel can be greater than or equal to about 0.5 cm to less than or equal to about 10 cm, optionally greater than or equal to about 1 cm to less than or equal to about 5 cm, and in some aspects, optionally greater than or equal to about 2 cm to less than or equal to about 3 cm. The thickness of the wall (e.g., between individual channels adjacent to each other in the microfluidic channel) can be greater than or equal to about 10 μm to less than or equal to about 100 μm, optionally greater than or equal to about 25 μm to less than or equal to about 75 μm, and in some aspects, optionally greater than or equal to about 40 μm to less than or equal to about 60 μm. In one embodiment, the microfluidic channel has a cross-section of 150 μm (width) × 380 μm (depth), a wall thickness of 50 μm, and a length of 2.3 cm.

[0063] As noted above, the microfluidic photoionization detector (PID) 100 also includes a first electrode and a second electrode of opposite polarities. For example, the wall structure 112 can define a first electrode region 114 and a distinct second electrode region 116. The first electrode region 114 and the second electrode region 116 can be formed in selected regions of one or more layers of the wall structure 112 on the substrate 110. The wall structure 112 can have one or more non-conductive regions 108. In some variants, the substrate 110 includes selected regions of a first layer of doped silicon and a second layer of conductive metal, where the selected regions are distinct regions corresponding to the respective first electrode region 114 and second electrode region 116. The first electrode region 114 can be separated and electrically isolated from the second electrode region 116 by one or more microfluidic channels 118. The microfluidic channels 118 can be formed in the wall structure 112 and thus separate and define the first electrode region 114 and the second electrode region 116. In some aspects, the bottom surface of the microfluidic channels 118 can be the substrate 110, or alternatively, although not shown, one or more microfluidic channels can be formed entirely within the wall structure 112 such that sidewalls and a bottom surface / bottom are defined therein.

[0064] The first electrode region 114 and the second electrode region 116 can be connected to the positive and negative leads of an external power supply (not shown). Although not shown, a power drive circuit connected to the power supply can be connected to the first electrode region 114 and the second electrode region 116 with opposite polarities. The first electrode region 114 and the second electrode region 116 can be connected to an amplifier (not shown) to form a closed circuit. In some aspects, the power supply can be a low-voltage power supply having a maximum voltage of direct current (VDC) less than or equal to about 20 volts. Thus, the electrodes defined by the first electrode region 114 and the second electrode region 116 provide the ability to measure electrical signals generated by ionized analytes within one or more microfluidic channels 118 when the one or more microfluidic channels 118 are bombarded and excited by photons.

[0065] The μPID 100 device further includes an inlet 122 to one or more microfluidic channels 118, which serve as a microfluidic ionization chamber. There is also an outlet 124 to the one or more microfluidic channels 118. Thus, a carrier gas containing one or more target analytes can leave the GC column and enter the inlet 122, where the carrier gas travels through the one or more microfluidic channels 118. As discussed below, analytes in the fluid flowing through the microfluidic ionization chamber (one or more microfluidic channels 118) can be ionized and the charge measured. The first electrode region 114 and the second electrode region 116 can detect the current generated by the target analytes during UV ionization. Subsequently, the fluid can leave the microfluidic ionization chamber (one or more microfluidic channels 118) through the outlet 124.

[0066] The μPID 100 device further includes an electromagnetic radiation source or light source, which can be a microfluidic ultraviolet radiation chamber or a microfluidic discharge chamber 130 configured to generate ultraviolet photons. The microfluidic discharge chamber 130 can have an inlet 132 and can be filled with an ultraviolet-generating fluid, such as krypton, argon, helium, and other pure or mixed gases known in the art, to generate ultraviolet light. In one aspect, the ultraviolet-generating fluid can be selected from the group consisting of krypton, argon, helium, and combinations thereof. The inlet 132 can be sealed after being filled with the ultraviolet-generating fluid. Alternatively, the microfluidic discharge chamber 130 can have an outlet 134 such that the ultraviolet-generating fluid can flow into the inlet 132 and leave the microfluidic discharge chamber 130 via the outlet 134. The microfluidic discharge chamber 130 can also have a non-conductive cap 136 disposed at the end edge of the open chamber.

[0067] The integrated microfluidic discharge chamber 130 generates light or electromagnetic radiation within the integrated lamp and subsequently generates photons that are directed towards the contents of one or more microfluidic channels 118. As shown, the first excitation electrode 140 and the second excitation electrode 142 of opposite polarities are arranged in a pattern in the overlay 144, the pattern generally corresponding to the pattern of one or more microfluidic channels 118 below. Thus, when a current or potential is applied to the first excitation electrode 140 and the second excitation electrode 142, the ultraviolet-generating fluid is excited in the regions corresponding to the one or more microfluidic channels 118 and generates photons.

[0068] Particularly suitable light falls within the ultraviolet electromagnetic radiation spectrum. In certain variants, the light can be ultraviolet radiation (UV) having a wavelength greater than or equal to about 10 nm to less than or equal to about 400 nm (including ultraviolet A, ultraviolet B, ultraviolet C, near ultraviolet, middle ultraviolet, far ultraviolet, extreme ultraviolet, and vacuum ultraviolet). In other variants, the light can be ultraviolet radiation in the range greater than or equal to about 100 nm to less than or equal to about 400 nm (including ultraviolet A, ultraviolet B, ultraviolet C). In particular, the light can be filtered, focused, polarized light, or can be hyperspectral or a mixture of different wavelengths.

[0069] In certain variants, as further described below, the UV transmission window can be ultrathin and thus can have a thickness in the sub-micron range in certain embodiments. Thus, an ultrathin transmission window 150 is provided between the microfluidic ionization chamber in the form of one or more microfluidic channels 118 and the microfluidic ultraviolet radiation chamber in the form of the microfluidic discharge chamber 130. The ultrathin transmission window 150 allows a sufficient amount of ultraviolet photons to enter the one or more microfluidic channels 118 from the microfluidic discharge chamber 130, and the one or more microfluidic channels serve as a microfluidic ionization chamber to excite one or more target analytes to a detectable level. In certain aspects, transmission means that the ultrathin window is transparent to the target range of wavelengths of electromagnetic energy, for example, transparent in the above ultraviolet wavelength range. Thus, in certain aspects, the transmission window transmits greater than or equal to about 5% of the electromagnetic energy in a predetermined wavelength range, optionally greater than or equal to about 10%, optionally greater than or equal to about 20%, optionally greater than or equal to about 30%, optionally greater than or equal to about 40%, optionally greater than or equal to about 50%, optionally greater than or equal to about 60%, optionally greater than or equal to about 70%, optionally greater than or equal to about 80%, optionally greater than or equal to about 90%, and in certain aspects optionally greater than or equal to about 95% of the electromagnetic energy in a predetermined wavelength range (e.g., in the ultraviolet range of the spectrum). In certain variants, the transmissive ultrathin window has a thickness less than or equal to about 20 μm and is configured to transmit greater than or equal to about 5% of the ultraviolet photons or any of the above transmission levels of ultraviolet photons.

[0070] The thickness of the ultra-thin transmission window 150 can be less than or equal to about 20 micrometers (μm), optionally less than or equal to about 10 μm, optionally less than or equal to about 5 μm, optionally less than or equal to about 4 μm, optionally less than or equal to about 3 μm, optionally less than or equal to about 2 μm, and in certain variations, optionally less than or equal to about 1 μm. In certain selected variations, the thickness of the ultra-thin transmission window 150 can be less than or equal to about 500 nm, optionally less than or equal to about 450 nm, optionally less than or equal to about 400 nm, optionally less than or equal to about 350 nm, optionally less than or equal to about 300 nm, optionally less than or equal to about 250 nm, optionally less than or equal to about 200 nm, optionally less than or equal to about 150 nm, optionally less than or equal to about 100 nm, and in certain variations, optionally less than or equal to about 50 nm. In certain variations, the thickness of the ultra-thin transmission window 150 can be greater than or equal to about 50 nm to less than or equal to about 20 μm, optionally greater than or equal to about 50 nm to less than or equal to about 10 μm, optionally greater than or equal to about 50 nm to less than or equal to about 5 μm, optionally greater than or equal to about 50 nm to less than or equal to about 4 μm, optionally greater than or equal to about 50 nm to less than or equal to about 3 μm, optionally greater than or equal to about 50 nm to less than or equal to about 2 μm, optionally greater than or equal to about 50 nm to less than or equal to about 1 μm, optionally greater than or equal to about 50 nm to less than or equal to about 500 nm, optionally greater than or equal to about 50 nm to less than or equal to about 250 nm, optionally greater than or equal to about 50 nm less than or equal to about 200 nm, optionally greater than or equal to about 50 nm to less than or equal to about 150 nm, and in certain variations, optionally greater than or equal to about 50 nm to less than or equal to about 100 nm.

[0071] As discussed below, the ultra-thin transmission window 150 can be an ultra-thin plate or can be one or more selected ultra-thin regions or layers defined in a thicker plate or material layer. In some aspects, the ultra-thin transmission window 150 can be disposed above one or more microfluidic channels 118. In some variations, the ultra-thin transmission window 150 is placed above at least a portion of one or more microfluidic channels 118 so as to form an upper wall or top wall (e.g., the fourth side of a three-sided channel) to enclose the microfluidic channels 118. However, the ultra-thin transmission window 150 does not need to contact one or more microfluidic channels 118 but can be positioned near the microfluidic channels with a small gap, e.g., positioned less than a few millimeters to less than about 10 μm from one or more microfluidic channels 118. Thus, a UV light source in the form of a microfluidic discharge chamber 130 is positioned and configured to direct photons to a sample fluid that may be present within one or more microfluidic channels. One or more microfluidic channels 118 thus serve as an ionization chamber for analyte compounds present in and flowing through the one or more microfluidic channels.

[0072] Notably, in some variations, the ultra-thin transmission window can be microfabricated from a material such as silica which is generally considered unsuitable for such applications because the material is considered opaque to UV radiation. Silica is known to have an extremely low transmission coefficient (or extremely high extinction coefficient) in the UV spectrum of interest (e.g., wavelengths from about 140 nm to about 70 nm, corresponding to UV photon energies from 9 eV to 17.5 eV). Thus, silica is generally not considered a material that can be used as a UV transmission window for a PID device. However, when certain microfabrication techniques described herein are used, silica can be formed as part of a transmission window that has an ultra-thin thickness and is thus made transparent in the target UV spectrum. More specifically, the proportion of the transmitted UV photon flux is determined by the formula: (1 - A×t), where A is the extinction coefficient of silica and t is the thickness of the transmission window. Although A is large, when t is extremely small (e.g., when t is a sub-micron thickness), A×t becomes relatively small (meaning it becomes transparent to UV). However, the present disclosure contemplates forming ultra-thin transmission windows not only from materials such as silica (e.g., silicon dioxide), fused silica, silicon but also from various other materials such as magnesium fluoride (MgF 2 )、calcium fluoride (CaF 2 ) and lithium fluoride (LiF). In some variations, the ultra-thin transmission window comprises a material selected from the group consisting of silica, fused silica, silicon, quartz, sapphire, magnesium fluoride, calcium fluoride, lithium fluoride and combinations thereof.

[0073] In some variants, for mechanical integrity, the transmissive window is designed as a plate with one or more selected regions having an ultra-thin thickness to allow transmission of UV electromagnetic radiation / photons, while the remaining regions of the plate outside the one or more selected regions can remain relatively thick as they are opaque. The thicker opaque regions have a thickness greater than the thickness of the ultra-thin transmissive regions. In some variants, the ratio of the average first thickness of the ultra-thin transmissive regions to the average second thickness of the remaining thicker opaque regions can be less than or equal to about 1:2, optionally less than or equal to about 1:3, optionally less than or equal to about 1:4, optionally less than or equal to about 1:5, and in some variants, optionally less than or equal to about 1:5.

[0074] In some variants, the thicker regions have a thickness greater than about 500 nm, optionally greater than or equal to about 600 nm, optionally greater than or equal to about 700 nm, optionally greater than or equal to about 750 nm, optionally greater than or equal to about 800 nm, optionally greater than or equal to about 900 nm, optionally greater than or equal to about 1 μm, optionally greater than or equal to about 2 μm, optionally greater than or equal to about 3 μm, optionally greater than or equal to about 4 μm, optionally greater than or equal to about 5 μm, optionally greater than or equal to about 10 μm, and in some variants, optionally greater than or equal to about 20 μm.

[0075] In other aspects, the transmissive window can be a component including multiple layers. For example, one layer can be an ultra-thin layer while another layer can be one or more thicker layers. Thus, the transmissive window can be formed as a stack of layers including a first layer and a second layer. The transmissive ultra-thin window is defined in the first layer and one or more regions corresponding to the transmissive ultra-thin window in the second layer are absent. Thus, the selected regions of one or more thicker layers can be removed to allow transmission of UV radiation through the ultra-thin layer in the selected regions. The removal can be achieved by nano-patterning, etching, photolithography or lithography techniques. In some aspects, the material forming the transmissive window can be a material capable of being processed via such photolithography, lithography or nano-fabrication techniques, such as silica, silicon, quartz, fused quartz, etc.

[0076] Figure 4 A photograph of an ultra-thin transmissive window made according to certain aspects of the present disclosure is shown. A sub-micron (e.g., 500 nm) thick silica UV transmissive window is formed on a silicon wafer having a thermal oxide coating. Thermal oxidation occurs after exposing the silicon wafer to an oxidant combination (and optionally heating) to produce a layer containing silicon dioxide (SiO 2) or a thermal oxide layer of silica. To maintain mechanical integrity, a transmissive window containing silica can be designed such that certain portions are very thin (e.g., a thickness less than or equal to about 500 nm), while the remaining portions can remain relatively thick. To ensure mechanical strength, a periodic pattern of small opening regions is created and etched from the silicon side (i.e., the side opposite the side with the thermal oxide coating). The thermal oxide layer (i.e., the silica layer) serves as an etch stop layer. The thickness of the silica transmissive window is controlled by the thermal oxide layer on the silicon wafer. Thus, the etched pattern creates a pattern of selected regions of silica, and the pattern of the selected regions of silica defines a transmissive window for UV radiation / photons. By way of non-limiting example, such a transmissive window can be formed in a clean room by standard lithography methods.

[0077] Example

[0078] Test the UV transmission ability of the ultrathin silica window. In this study, a UV lamp for conventional PID was purchased from Baseline-Mocon and used as the UV source (UV photon energy 10.6 eV, wavelength approximately 120 nm). The lamp was placed directly on top of the ultrathin silica UV transmissive window formed according to certain aspects of the present disclosure. Toluene was used as the analyte and flowed through a microfluidic ionization chamber below the silica window. The signal obtained ([ Figure 5 The bar on the right). For comparison, in Figure 5 the bar on the left, the silica window was replaced with a silicon wafer with a gas window having the same UV transmissive area as the silica window (meaning a portion of the silicon wafer was completely etched away). A signal of 0.05 V was obtained under the same test conditions as the silica window. The above comparison shows that the ultrathin silica window can effectively transmit UV light.

[0079] For purposes of illustration and description, the foregoing description of the embodiments has been provided. It is not intended to be exhaustive or to limit the disclosure. The individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but are interchangeable where applicable and can be used in selected embodiments even if not specifically shown or described. Likewise, they can be varied in many ways. Such variations should not be regarded as departing from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

Claims

1. An integrated microfluidic photoionization detector (PID), the integrated microfluidic photoionization detector comprises: a microfluidic ionization chamber having an inlet for receiving a fluid sample and an outlet through which the fluid sample exits the microfluidic ionization chamber; a first electrode and a different second electrode electrically connected to the microfluidic ionization chamber; an integrated microfluidic ultraviolet radiation chamber configured to generate ultraviolet photons; and a transmission window disposed between the microfluidic ionization chamber and the microfluidic ultraviolet radiation chamber, the transmission window defining a plate having one or more ultra-thin transmission regions with a thickness less than or equal to about 500 nm, the one or more ultra-thin transmission regions being disposed in selected regions of the plate, wherein the plate has a thickness greater than or equal to about 20 microns in regions outside the ultra-thin transmission regions, wherein the one or more ultra-thin transmission regions comprise materials selected from the group consisting of silica, fused silica, silicon, quartz, and combinations thereof, and the one or more ultra-thin transmission regions permit the ultraviolet photons to enter the microfluidic ionization chamber from the microfluidic ultraviolet radiation chamber.

2. The integrated microfluidic photoionization detector (PID) according to claim 1, wherein the transmission window further comprises materials selected from the group consisting of sapphire, magnesium fluoride, calcium fluoride, lithium fluoride, and combinations thereof.

3. The integrated microfluidic photoionization detector (PID) according to any one of claims 1 to 2, wherein the transmission window further comprises a stack of layers including a first layer and a second layer, wherein the one or more ultra-thin transmission regions are defined where the second layer is absent within the first layer.

4. The integrated microfluidic photoionization detector (PID) according to any one of claims 1 to 2, wherein the plate of the transmission window is configured to transmit greater than or equal to about 5% of the ultraviolet photons outside the one or more ultra-thin transmission regions.

5. The integrated microfluidic photoionization detector (PID) according to any one of claims 1 to 2, wherein the one or more ultra-thin transmission regions comprise materials selected from the group consisting of silica and fused silica and have a thickness greater than or equal to about 250 nm to less than or equal to about 500 nm.

6. The integrated microfluidic photoionization detector (PID) according to any one of claims 1 to 2, wherein the microfluidic ultraviolet radiation chamber has an inlet for receiving an ultraviolet generating fluid.

7. The integrated microfluidic photoionization detector (PID) according to any one of claims 1 to 2, wherein the microfluidic ultraviolet radiation chamber comprises ultraviolet generating fluids selected from the group consisting of krypton, argon, helium, and combinations thereof.

8. The integrated microfluidic photoionization detector (PID) according to any one of claims 1 to 2, wherein The microfluidic ionization chamber is one or more microfluidic channels, and the integrated microfluidic photoionization detector (PID) further includes a third electrode and a different fourth electrode formed in a layer disposed above and in electrical communication with the microfluidic ultraviolet radiation chamber, wherein the third electrode and the fourth electrode define a second pattern corresponding to a first pattern of one or more microfluidic channels of the microfluidic ionization chamber disposed below the microfluidic ultraviolet radiation chamber.

9. The integrated microfluidic photoionization detector (PID) according to any one of claims 1 to 2, wherein, the total volume of the microfluidic ionization chamber is less than or equal to about 10 μL, and the total volume of the microfluidic ultraviolet radiation chamber is less than about 10 μL.

10. The integrated microfluidic photoionization detector (PID) according to any one of claims 1 to 2, wherein, the first electrode and the different second electrode are formed in a layer of conductive material, and one or more microfluidic channels are disposed in the layer to electrically insulate the first electrode from the different second electrode.

11. A detection system for one or more VOC analytes, the detection system comprising: (i) a gas chromatography (GC) unit, the gas chromatography (GC) unit including at least one gas chromatography column; and (ii) an integrated microfluidic photoionization detector (PID) disposed downstream of the gas chromatography (GC) unit, the integrated microfluidic photoionization detector including: a microfluidic ionization chamber having an inlet for receiving a fluid sample and an outlet through which the fluid sample exits the microfluidic ionization chamber; a first electrode and a different second electrode in electrical communication with the microfluidic ionization chamber; an integrated microfluidic ultraviolet radiation chamber configured to generate ultraviolet photons; and a transmission window disposed between the microfluidic ionization chamber and the microfluidic ultraviolet radiation chamber, the transmission window defining a plate having one or more ultra-thin transmission regions with a thickness less than or equal to about 500 nm, the one or more ultra-thin transmission regions being disposed in selected regions of the plate, wherein the plate has a thickness greater than or equal to about 20 microns in regions outside the ultra-thin transmission regions, and the one or more ultra-thin transmission regions include materials selected from the group consisting of silica, fused silica, silicon, quartz, and combinations thereof, the one or more ultra-thin transmission regions allowing the ultraviolet photons to enter the microfluidic ionization chamber from the microfluidic ultraviolet radiation chamber, wherein the microfluidic photoionization detector (PID) analyzes the sample processed in the gas chromatography (GC) unit.

12. The detection system according to claim 11, wherein, the transmission window further includes materials selected from the group consisting of sapphire, magnesium fluoride, calcium fluoride, lithium fluoride, and combinations thereof.

13. The detection system according to any one of claims 11 to 12, wherein, The transmissive window of the integrated microfluidic photoionization detector (PID) further includes a stack of layers, the stack of layers including a first layer and a second layer, wherein one or more ultra-thin transmissive regions are defined where the second layer is absent within the first layer.

14. The detection system according to any one of claims 11 to 12, wherein, the plate of the transmissive window is configured to transmit greater than or equal to about 5% of the ultraviolet photons outside the one or more ultra-thin transmissive regions.

15. The detection system according to any one of claims 11 to 12, wherein, the microfluidic ionization chamber is one or more microfluidic channels, and the integrated microfluidic photoionization detector (PID) further includes a third electrode and a different fourth electrode formed in a layer disposed above and in electrical communication with the microfluidic ultraviolet radiation chamber, wherein the third electrode and the fourth electrode define a second pattern corresponding to a first pattern of one or more microfluidic channels of the microfluidic ionization chamber disposed below the microfluidic ultraviolet radiation chamber.

16. The detection system according to claim 15, wherein, the first electrode and the different second electrode are formed in a layer of conductive material, and the one or more microfluidic channels are arranged in the layer to electrically insulate the first electrode from the different second electrode.

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