Abstract:
The present disclosure relates to burner assemblies of flame-based detectors. These burner assemblies are configured to deliver decompressed mobile phase of supercritical fluid chromatography systems to the flame of a flame-based detector while providing for improved optimization of analyte response as well as enhanced flame stability during operation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of and priority to U.S. Provisional Application No. 61/994,353 entitled “Flame Ionization Detection Burner Assemblies for Use in Compressible Fluid-Based Chromatography Systems” filed May 16, 2014, which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to flame ionization detection burner assemblies and their use in compressible fluid-based chromatography systems. 
       BACKGROUND 
       [0003]    Flame ionization detection (FID) was originally developed for use in conjunction with gas chromatography and is nominally designed to operate at relatively low mobile phase flow rates (i.e., up to around 40 mL/min). However, FID is now also commonly employed in conjunction with compressible fluid-based chromatography, hereinafter referred to as “CFC,” in which the mobile phase, typically compressed carbon dioxide, is used. An example of CFC is supercritical fluid chromatography, hereinafter referred to as “SFC,” in which carbon dioxide in its supercritical state or near supercritical state is typically used as the mobile phase. When decompressed, the CFC mobile phase achieves much higher flow rates than that of gas chromatography. When interfacing a CFC system to an FID detector, a transfer line connected to the CFC system transports some or all of the mobile phase flow to the detector. Due to the compressed nature of the mobile phase, this transfer line must also function as a flow restrictor (i.e., to maintain system pressure). The compressed mobile phase enters the restrictor as a dense fluid and exits as a decompressed gas. Since the fluid expands as it transitions to a gas, the volumetric flow rate at the outlet of the restrictor is considerable. As a result of this expansion, precise positioning of the end of the restrictor within the FID burner is required to ensure stable flame operation and optimal analyte response. 
         [0004]    Conventional restrictors in FID burner assemblies are configured such that the flow stream exits the restrictor and into the burner in a direction substantially parallel to the longitudinal axis of the burner. This configuration can be accomplished by simply cutting the end of the restrictor at an angle perpendicular to its longitudinal axis (i.e., a “square cut” restrictor) and is done for ease of manufacture and to ensure restrictor-to-restrictor reproducibility. However, this restrictor design requires considerable burner length to allow for the mobile phase to fully decompress and slow in linear velocity so as to both maintain a stable flame and achieve an optimal analyte response. Thus, the use of “square cut” restrictors in FID burners results in a narrow window of distance from the flame in which the position of the restrictor must be precisely optimized. In a worst case scenario, the mobile phase flow rate out of the restrictor may be so great that the FID burner may not be long enough for optimal positioning of the restrictor at all. A further complication encountered when using such restrictors is that its position within the FID burner must be re-optimized when any change in restrictor flow rate is experienced, such as when the system pressure is changed (i.e., density programmed separations) while operating a split-flow interface to the FID or when the mobile phase flow rate is changed while employing a full-flow interface (i.e., changing column diameter). 
         [0005]    Thus, there exists a need for improved FID burner assemblies that do not require precise positioning and re-positioning of the restrictor in order to optimize analyte response and which provide for enhanced flame stability during operation. 
       SUMMARY OF THE INVENTION 
       [0006]    The present disclosure relates to FID burner assemblies and their use in CFC systems. In general and according to certain embodiments, FID burner assemblies of the present disclosure are configured to deliver a decompressed mobile phase fluid (e.g., CO 2 ) from a CFC system to the flame within the detector while providing for improved optimization of analyte response as well as enhanced flame stability during operation. 
         [0007]    In one embodiment, the present disclosure relates to a burner assembly of a flame-based detector. The burner assembly comprises a burner and a restrictor. The burner comprises a burner body having a fluid inlet for receiving combustion gases and a fluid outlet for delivering combustion gases to a flame position. The burner body defines a flow path extending from the fluid inlet to the flame position and having a longitudinal axis. The restrictor comprises a hollow body comprising a first end for receiving at least a portion of a mobile phase flow stream from a chromatography system and a second end for delivering at least a portion of the mobile phase flow stream as a decompressed mobile phase flow stream to the burner. During flame-based detection of one or more constituents of the at least a portion of the mobile phase flow stream (1) at least the second end of the restrictor is inserted into the burner and (2) the second end of the restrictor is adapted to deliver the decompressed mobile phase flow stream to the burner body flow path at an angle substantially non-parallel to the longitudinal axis of the burner. 
         [0008]    In another embodiment, the present disclosure relates to a burner assembly of a flame-based detector. The burner assembly comprises a burner and a restrictor. The burner comprises a burner body having a fluid inlet for receiving combustion gases and a fluid outlet for delivering combustion gases to a flame position. The burner body defines a flow path extending from the fluid inlet to the flame position and having a burner longitudinal axis. The restrictor comprises a hollow body comprising a first end for receiving at least a portion of a mobile phase flow stream from a chromatography system and a second end for delivering the at least a portion of the mobile phase flow stream as a decompressed mobile phase flow stream to the burner. The restrictor has a restrictor longitudinal axis. During flame-based detection of one or more constituents of the at least a portion of the mobile phase flow stream (1) at least the second end of the restrictor is located within the burner and (2) the restrictor longitudinal axis is substantially non-parallel to the burner longitudinal axis. 
         [0009]    In another embodiment, the present disclosure is directed to a burner assembly of a flame-based detector. The burner assembly comprises a burner and a restrictor. The burner comprises a burner body having a fluid inlet for receiving combustion gases and a fluid outlet for delivering combustion gases to a flame position. The burner body defines a flow path extending from the fluid inlet to the flame position and having a longitudinal axis. The restrictor comprises a hollow body having a first end for receiving at least a portion of a mobile phase flow stream from a chromatography system and a second end for delivering the at least a portion of the mobile phase flow stream as a decompressed mobile phase flow stream to the burner. During flame-based detection of one or more constituents of the at least portion of the mobile phase flow stream (1) at least the second end of the restrictor is positioned within the burner; and (2) the burner is adapted so that the at least a portion of the decompressed mobile phase flow stream travels through the flow path in one or more directions substantially non-parallel to the longitudinal axis. 
         [0010]    In another embodiment, the present disclosure is directed to a burner assembly of a flame-based detector. The burner assembly comprises a burner and a restrictor. The burner comprises a burner body having a fluid inlet for receiving combustion gases and a fluid outlet for delivering combustion gases to a flame position. The burner body has a longitudinal axis and further comprises an interior wall surface defining an inner perimeter of the burner body and one or more members extending from the interior wall surface at an angle substantially non-parallel to the longitudinal axis. The restrictor comprises a hollow body having a first end for receiving at least a portion of a mobile phase flow stream from a chromatography system and a second end for delivering the at least a portion of the mobile phase flow stream as a decompressed mobile phase flow stream to the burner. During flame-based detection of one or more constituents of the at least portion of the mobile phase flow stream (1) at least the second end of the restrictor is contained within the burner and (2) the one or more members extending from the interior wall are dimensioned and configured to deflect the decompressed mobile phase flow stream in a direction substantially non-parallel to the longitudinal axis. 
         [0011]    In another embodiment, the present disclosure is directed to a method of maintaining a flame in a burner assembly of a flame-based detector. The method comprises at least three steps. The first step of the method comprises providing the burner assembly. The burner assembly comprises a burner and a restrictor. The burner comprises a burner body having a fluid inlet for receiving combustion gases and a fluid outlet for delivering combustion gases to a flame position having a flame. The burner body defines a flow path extending from the fluid inlet to the flame position and having a burner longitudinal axis. The restrictor comprises a restrictor comprising a hollow body comprising a first end for receiving at least a portion of a mobile phase flow stream from a chromatography system and a second end for delivering the at least a portion of the mobile phase flow stream as a decompressed mobile phase flow stream to the burner. The second end of the restrictor is sized and inserted into the inner burner. The second step of the method comprises passing at least a portion of the mobile phase flow stream through the restrictor at a decompressed flow rate of 40 mL/min or greater. The third step of the method comprises delivering at least the portion of the mobile phase flow stream into the burner and to the flame position as the decompressed mobile phase flow stream at a force/velocity insufficient to extinguish the flame. 
         [0012]    In another embodiment, the present disclosure is directed to a method of maintaining a flame in a burner assembly of a flame-based detector. The method comprises at least three steps. The first step of the method comprises providing the burner assembly. The burner assembly comprises a burner and a restrictor. The burner comprises a burner body having a fluid inlet for receiving combustion gases and a fluid outlet for delivering combustion gases to a flame position having a flame. The restrictor comprises a restrictor comprising a hollow body comprising a first end for receiving at least a portion of a mobile phase flow stream from a chromatography system and a second end for delivering the at least a portion of the mobile phase flow stream as a decompressed mobile phase flow stream to the burner. The second end of the restrictor sized and inserted into the burner. The second step of the method comprises passing at least the portion of the mobile phase flow stream through the restrictor. The third step of the method comprises delivering at least a portion of the mobile phase flow stream into the burner and to the flame position as the decompressed mobile phase flow stream such that the decompressed mobile phase flow stream flows to the flame along a non-parallel fluid flow path. 
         [0013]    In another embodiment, the present disclosure is directed to a method of maintaining a flame in a burner assembly of a flame-based detector. The method comprises at least three steps. The first step of the method comprises providing the burner assembly. The burner assembly comprises a burner and a restrictor. The burner comprises a burner body having a fluid inlet for receiving combustion gases and a fluid outlet for delivering combustion gases to a flame position having a flame. The burner body defines a flow path extending from the fluid inlet to the flame position and having a burner longitudinal axis. The restrictor comprises a restrictor comprising a hollow body comprising a first end for receiving at least a portion of a mobile phase flow stream from a chromatography system and a second end for delivering the at least a portion of the mobile phase flow stream as a decompressed mobile phase flow stream to the burner. The second end of the restrictor is sized and inserted into the inner burner. The second step of the method comprises passing at least a portion of the mobile phase flow stream through the restrictor. The third step of the method comprises delivering at least the portion of the mobile phase flow stream into the burner and to the flame position as the decompressed mobile phase flow stream such that a stable flame is maintained and optimal analyte response is achieved regardless of the distance of the restrictor second end from the flame position. 
         [0014]    The above embodiments can include one or more of the following features. In some embodiment, the mobile phase flow stream can comprise carbon dioxide. In some of those embodiments, the second end of the restrictor can be adapted to deliver the decompressed mobile phase flow stream at an angle of at least 25 degrees with respect to the longitudinal axis of the inner burner. The second end of the restrictor in some of the above embodiments can also be adapted to provide radial decompression of the mobile phase. Alternatively, it can also comprise a frit or a pintle. In some embodiments, the restrictor is positioned relative to the burner such that the longitudinal axis of the restrictor is at a 90 degree angle relative to that of the burner. In certain embodiments, the flow path of the burner is tortuous or is packed with glass wool. In other embodiments, the interior wall surface of the burner defines a tortuous path between the second end of the restrictor and the flame. Alternatively or in addition thereto, the members extending from the interior wall surface of these burners can be baffles, porous, and/or tapered. 
         [0015]    The embodiments of the present disclosure provide advantages over the prior art based on their unique configurations and performance properties. For example, the “square cut” restrictors of conventional FID burner assemblies require precise positioning (and re-positioning when system pressure is changed) of the restrictor in order to optimize analyte response and to maintain flame stability during operation. In contrast, the burner assemblies of the present disclosure do not require precise positioning and re-positioning of the restrictor in order to optimize analyte response and which provide for enhanced flame stability during operation. For example, in certain embodiments of the present disclosure, optimal analyte response is achieved and flame stability maintained regardless of the where the restrictor is positioned inside the burner. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The foregoing and other features and advantages provided by the present disclosure will be more fully understood from the following description of exemplary embodiments when read together with the accompanying drawings. 
           [0017]      FIG. 1  schematically depicts an exemplary SFC system interfaced with an FID detector, including an enlarged cross-section detail of the FID detector burner assembly. 
           [0018]      FIG. 2  depicts a cross-section of a burner assembly with a conventional “square cut” restrictor. 
           [0019]      FIG. 3  depicts a cross-section of a burner assembly according to the present disclosure where the restrictor has a tip cut at an angle of less than 90 °. 
           [0020]      FIG. 4  depicts a cross-section of a portion of a burner assembly according to the present disclosure where the restrictor has a drilled tip. 
           [0021]      FIG. 5A  depicts a cross-section of a portion of a burner assembly according to the present disclosure where the restrictor has a fritted tip. 
           [0022]      FIG. 5B  depicts a cross-section of a portion of a burner assembly according to the present disclosure where the restrictor has a fritted tip, the top of which has been sealed. 
           [0023]      FIG. 6  depicts a cross-section of a portion of a burner assembly according to the present disclosure where the restrictor has a tip comprising a pintle. 
           [0024]      FIG. 7  depicts a cross-section of a burner assembly according to the present disclosure where the restrictor is inserted into a side of the burner. 
           [0025]      FIG. 8  depicts a cross-section of a burner assembly according to the present disclosure where the flow path of the burner is partially occluded by glass wool or a frit. 
           [0026]      FIG. 9  depicts a cross-section of a burner assembly according to the present disclosure where the flow path of the burner is partially occluded by baffles. 
           [0027]      FIG. 10A  depicts a cross-section of an integral restrictor according to the present disclosure. 
           [0028]      FIG. 10B  depicts a cross-section of a tapered restrictor according to the present disclosure. 
           [0029]      FIG. 10C  depicts a cross-section of a converging-diverging restrictor according to the present disclosure. 
           [0030]      FIG. 10D  depicts a cross-section of a linear hybrid restrictor according to the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    In various aspects, configurations, and embodiments, the present disclosure provides novel burner assemblies of flame-based detectors, as well as methods of maintaining a flame in such burner assemblies. 
         [0032]    As used herein, the phrase “chromatography system” refers to an assembly or array of interconnected components that is used to separate a mixture of compositions. The mixture is dissolved in a fluid mobile phase that carries it through a structure holding a stationary phase. The various constituents of the mixture travel at different speeds through the stationary phase, thereby causing them to separate. An example of such a chromatography system includes, but is not limited to, a compressible fluid-based (CFC) system. An example of a CFC system, includes, but is not limited to, CO 2 -based a supercritical fluid chromatography (SFC) system, which employs a supercritical fluid or near supercritical fluid as the mobile phase. That is, the mobile phase includes CO 2  (and potentially other modifiers) at or near supercritical conditions of the mobile phase for at least some portion of chromatographic process. In some embodiments the CO 2  mobile phase does not actually reach the supercritical state, but is highly compressed. The highly compressed CO 2  mobile phase provides similar advantages to supercritical CO 2  for the purposes of chromatography and is therefore considered to be near supercritical with respect to the performance of SFC. 
         [0033]    As used herein, the phrase “flame-based detection” refers to the detection energetic particles (e.g., electrons, photons, etc.) formed during combustion of compounds in a flame. Both organic and inorganic compounds can be ionized and detected. Examples of combustion gases used to produce the flame include, but are not limited to, mixtures of hydrogen and air/oxygen. Nitrogen (N 2 ) is commonly employed as a makeup gas in such mixtures. Examples of such flame-based detection include, but are not limited to, flame ionization detection (FID), flame photometric detection, chemiluminescence nitrogen detection, thermionic detection (e.g., nitrogen-phosphorus detection), and chemiluminescence sulfur detection. 
         [0034]    As used herein, the phrase “longitudinal axis” refers to the axis of symmetry that runs lengthwise from the fluid inlet, through the flow path, to the fluid outlet of the burner body or the axis of symmetry that runs lengthwise from the first end, through the hollow body, to the second end of the restrictor. 
         [0035]    As used herein, the phrase “decompressed” refers to the phase transition of the supercritical fluid of the mobile phase from a liquid, a supercritical fluid, or a highly compressed gas to a gas. 
         [0036]    As used herein, the phrase “substantially non-parallel” refers to the angle at which the mobile phase flow stream decompresses (i.e., the “angle of decompression”), relative to the longitudinal axis of the burner, sufficient to prevent the decompressed mobile phase flow stream from extinguishing the flame at the flame position. Such an angle can be any angle greater than 0° to 180°, relative to the longitudinal axis of the burner. Examples of such angles include, but are not limited to, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 160, 165, 170, and 175, and 180°. In some embodiments, the angle is greater than 22°. In other embodiments, the angle is 45, 67, 90, or 135°. 
         [0037]    An example of an SFC system that employs supercritical CO 2  as the mobile phase is illustrated in  FIG. 1 . SFC system  100  includes CO 2  pump  110 , which pumps CO 2  from CO 2  source  120  into system  100  at a pressure that maintains the CO 2  in a dense state, column  130 , back pressure regulator (BPR)  140 , and flame ionization detection (FID) detector  150 . The CO 2  mobile phase flows from CO 2  pump  110  to column  130 . Injector  160  for injecting mixtures to be separated is located in-line between CO 2  pump  110  and column  130 . Column  130  is housed in column oven  170 , which maintains column  130  at a constant temperature. Column oven  170  also functions to preheat the supercritical CO 2  mobile phase and the mixture for separation to the constant temperature of column  130  prior to their entry. Flow splitter  180  is located in-line between column oven  170  and BPR  140 . Flow splitter  180  operates to divert at least a portion of the mobile phase flow, which now contains the separated constituent compounds of the mixture, (i.e., the column effluent), to FID detector  150 , where one or more compounds are ionized via combustion in burner assembly  190  of FID  150 . 
         [0038]    As discussed above, one component of an FID detector is the burner assembly. A cross-section of an exemplary burner assembly is illustrated in  FIG. 2 . Assembly  200  includes burner  210  and restrictor  220 , a portion of which located inside burner  210 . A portion of burner  210  is, in turn, located inside burner housing  230 . Inlets  240  and  250  supply assembly  200  with fuel gas and oxidant gas, respectively, (together, combustion gases), which mix at flame position  260  and are then combusted. The flame produced at position  260  is a “diffusion” flame, the exterior of which is an oxidant-rich region and the interior of which is a fuel gas-rich region. The second end  224  of restrictor  220  can be positioned inside burner  210  at any distance  296  relative to flame position  260 . Column effluent  226  is fed to the FID detector, entering restrictor  220  at its first end  222  and exiting at its second end  224  at an angle substantially parallel to the longitudinal axis  212  of burner  210 , where it decompresses and travels through flow path  218  of burner  210  to flame position  260 , where one or more separated constituent compositions of the mixture are ionized via combustion. The electrons released during ionization are attracted to collector electrode  290 , where they induce a current, which is, in turn, fed to electrometer  292 . 
         [0039]    As can be seen in  FIG. 2 , the second end  224  of restrictor  220  is “square cut.” This geometry results in the mobile phase flow having a mean direction  216  substantially parallel to the longitudinal axis  212  of burner  210  as it exits the second end  224  of restrictor  220  and decompresses into flow path  218 . One problem with this restrictor geometry is that re-positioning of the second end  224  of restrictor  220  relative to flame position  260  may be required any time the flow rate of the mobile phase is changed in order to optimize detector response. Another problem is that, depending on the flow rate of the mobile phase, it may not be possible with this restrictor geometry to maintain a stable flame at certain distances  296  (or even regardless of the distance) of the second end  224  from flame position  260 . 
         [0040]    These problems are solved by the various burner assembly configurations of the present disclosure. 
         [0041]    All of the burner assembly configurations of the present disclosure comprises a burner and a restrictor. The burners of these configurations comprise a burner body having a fluid inlet for receiving combustion gases and a fluid outlet for delivering at least a portion of the combustion gases to a flame position. The burner bodies define a flow path extending from the fluid inlet to the flame position and having a longitudinal axis. The restrictors of these configurations comprise a hollow body comprising a first end for receiving at least a portion of a mobile phase flow stream from a chromatography system and a second end for delivering the at least a portion of the mobile phase flow stream as a decompressed mobile phase flow stream to the burner. In all configurations, at least the second end of the restrictor is inserted into the burner during flame-based detection of one or more constituents of the at least a portion of the mobile phase flow stream. 
         [0042]    During flame-based detection, the flame-based detector is typically maintained at a temperature above ambient temperature. For example, the flame-based detector can be maintained at a temperature in the range of from about 50 to 500° C. or higher. In certain embodiments, the flame-based detector can be held at a temperature of about 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340, 350, 360, 370, 380, 390, 400, 410, 420, 430, 440, 450, 460, 470, 480, 490, or 500° C. during flame-based detection. 
         [0043]    Both the burner and the restrictor of the burner assemblies of the present invention can be fabricated from any material capable of withstanding the temperatures at which the flame-based detectors are maintained. These materials also should not “de-gas” or otherwise introduce extraneous carbon containing compounds into the FID flame. Such materials include, but are not limited to metals, ceramics, glass, or polymers. In one embodiment, the burner is fabricated from a metal, such as steel. In one embodiment, the restrictor is fabricated from glass. Furthermore, the restrictor and its second end can be fabricated as separate, optionally disposable/replaceable, pieces that can be fitted together. These two pieces can be fabricated from the same or different materials. 
         [0044]    The cross-sectional dimensions of the burner and the restrictor can be any width and shape, with the only conditions being that the width and shape of the restrictor should be such that it can be inserted into the burner and there is space between the inner surface of the burner and the outer surface of the restrictor sufficient to allow combustion gas(es) to move freely towards the flame position. In certain embodiments, the cross-sectional width of the burner can be about 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0.762, 0.75, 0.5, 0.457, 0.28, or 0.25 mm. In certain embodiments, the cross-sectional width of the restrictor can be about 2.0, 1.9, 1.8, 1.7, 1.6, 1.5, 1.4, 1.3, 1.2, 1.1, 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0.090, 0.080, 0.070, 0.060, 0.050, 0.049, 0.048, 0.047, 0.046, 0.045, 0.044, 0.043, 0.042, 0.041, 0.040, 0.039, 0.038, 0.037, 0.036, 0.035, 0.034, 0.033, 0.032, 0.031, 0.030, 0.029, 0.028, 0.027, 0.026, 0.025, 0.024, 0.023, 0.022, 0.021, 0.020, 0.010, 0.009, 0.008, 0.007, 0.006, 0.005, 0.004, 0.003, 0.002, or 0.001 mm. In certain embodiments, the cross-sectional shape of the burner and/or the restrictor can be a circle, an oval, a square, a rectangle, or a triangle. In a microfluidic embodiment, the cross-sectional shape of the burner can be trapezoidal or “gumdrop” shaped. 
         [0045]    The longitudinal dimensions of the burner and the restrictor can be any length and shape. In certain embodiments, the length of the burner can be about 45, 44, 43, 42, 41, 40, 39, 38, 37, 36, 35, 34, 33, 32, 31, 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1 mm. In certain embodiments, the length of the restrictor can be about 200, 199, 198, 197, 196, 195, 194, 193, 192, 191, 190, 189, 188, 187, 186, 185, 184, 183, 182, 181, 180, 179, 178, 177, 176, 175, 174, 173, 172, 171, 170, 169, 168, 167, 166, 165, 164, 163, 162, 161, 160, 159, 158, 157, 156, 155, 154, 153, 152, 151, 150, 149, 148, 147, 146, 145, 144, 143, 142, 141, 140, 139, 138, 137, 136, 135, 134, 133, 132, 131, 130, 129, 128, 127, 126, 125, 124, 123, 122, 121, 120, 119, 118, 117, 116, 115, 114, 113, 112, 111, 110, 109, 108, 107, 106, 105, 104, 103, 102, 101, 100, 99, 98, 97, 96, 95, 94, 93, 92, 91, 90, 89, 88, 87, 86, 85, 84, 83, 82, 81, 80, 79, 78, 77, 76, 75, 74, 73, 72, 71, 70, 69, 68, 67, 66, 65, 64, 63, 62, 61, 60, 59, 58, 57, 56, 55, 54, 53, 52, 51, 50, 49, 48, 47, 46, 45, 44, 43, 42, 41, 40, 39, 38, 37, 36, 35, 34, 33, 32, 31, 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, or 10 cm. In certain embodiments, the longitudinal shape of the burner and/or restrictor can be cylindrical or tubular. In other embodiments, the longitudinal shape of the burner and/or restrictor can be cylindrical or tubular, pinched in one more positions along the length of the burner and/or restrictor, or undulates along the length of the burner and/or restrictor, such that the flow of the mobile phase and/or the combustion gas(es) converge and diverge along the flow path. In one embodiment, the restrictor can be longer than the burner. In another embodiment, the restrictor can be 35 cm long and the burner can be 25 mm long. 
         [0046]    In certain embodiments, the combustion gases can be hydrogen, as the fuel gas, in mixture with an oxidant gas, such as oxygen or air. Use of carbon-based gases should be avoided. Nitrogen can be used as a makeup gas. In one embodiment, the combustion gases are hydrogen and air or oxygen, with nitrogen as the makeup gas. 
         [0047]    The mobile phase can comprise any supercritical fluid. In certain embodiments, the supercritical fluid can be CO 2 , N 2 , Ar, Xe, a chlorofluorocarbon, a fluorocarbon, N 2 O, H 2 O, or SF 6 . The mobile phase can also comprise one or more FID-compatible modifiers. Example of such modifiers include, but are not limited to, formic acid, trifluoroacetic acid, and other highly oxidized carbon-containing compounds. 
         [0048]    In one of the above burner assembly configurations, the second end of the restrictor is adapted to deliver the decompressed mobile phase flow stream to the burner body flow path at an angle substantially non-parallel to the longitudinal axis of the burner. This adaptation of the second end of the restrictor can be achieved in any number of ways. For example, the second end can be “angled,” i.e., the second end can be cut at an angle less than or greater than perpendicular to the longitudinal axis of the restrictor (90°). The “angle of cut” includes any angle in the range of from greater than 0° to less than 90° and the range of from greater than 90° to less than 180°. Such an angle of cut results in an angle of decompression of the mobile phase flow stream that is substantially non-parallel to the longitudinal axis of the burner. Example of such angles of cut include, but are not limited to, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, and 175°. In some embodiments, the angle of cut results in an angle of decompression that is greater than 22°. In other embodiments, the angle of cut results in an angle of decompression that is 45, 67, 90, or 135°. Other examples of such adaptations include, but are not limited to, (1) a restrictor where the second end has been roughly cleaved, resulting in a jagged end surface, (2) a restrictor where the opening at the second end is sealed and a second opening is formed by drilling one or more holes into the side of the restrictor (i.e., a “drilled” restrictor), (3) a restrictor fitted with a frit (i.e., a “fritted” restrictor), wherein the top of the frit is optionally sealed, (4) a restrictor fitted with a pintle (i.e., a “pintled” restrictor) (5) an integral restrictor, (6) a restrictor where the second end has been tapered, such that the restrictor flow path dimensions perpendicular to the longitudinal axis of the restrictor decrease as the flow path approaches the second end, i.e., the flow path narrows as it approaches the second end, (7) a converging/diverging restrictor, and (8) a linear hybrid restrictor. Each of restrictor examples (5), (6), (7), and (8) have short decompression zones and are easily plugged. Illustrations of exemplary restrictors (5), (6), (7), and (8) are provided in  FIGS. 10A ,  10 B,  10 C, and  10 D, respectively. That is,  FIG. 10A  shows a cross-sectional view of an embodiment of an integral restrictor  1025 ;  FIG. 10B  shows a cross-sectional view of an embodiment of a restrictor with a tapered second end  1026 ;  FIG. 10C  shows a cross-sectional view of an embodiment of converging/diverging restrictor  1027 ; and  FIG. 10D  shows a cross-sectional view of an embodiment of a linear hybrid restrictor  1028 . 
         [0049]    An example of a burner assembly configuration where the second end of the restrictor is angled is illustrated in  FIG. 3 . Burner assembly  300  comprises a burner  310  and restrictor  320 . Burner  310  comprises a burner body having fluid inlets  340  and  350  for receiving combustion gases and a fluid outlet  314  for delivering at least a portion of the combustion gases to flame position  360 . Restrictor  320  comprises a first end  322  and a second end  324 . Second end  324  of restrictor  320  is located within burner  310 . A portion of burner  310  is, in turn, located inside burner housing  330 . Second end  324  of restrictor  320  can be positioned inside burner  310  at any distance  396  relative to flame position  360 . Column effluent  326  is fed to the FID detector, entering restrictor  320  at its first end  322  and exiting at its second end  324  at an angle  316  substantially non-parallel to the longitudinal axis  312  of burner  310 . The mobile phase of the column effluent decompresses and travels through flow path  318  of burner  310  to flame position  360 , where one or more separated constituent compositions of the mixture are ionized via combustion. These ions are attracted to collector electrode  390 , where they induce a current, which is, in turn, fed to electrometer  392 . 
         [0050]    An example of a burner assembly configuration having a “drilled” restrictor is illustrated in  FIG. 4 . In this example, the configuration of the burner assembly is identical to that of  FIG. 3  except for the adaptation to the second end of the restrictor. Column effluent exits at the second end  424  of restrictor  420  at an angle  416  substantially non-parallel to the longitudinal axis  412  of burner  410 . The mobile phase of the column effluent decompresses and travels through flow path  418  of burner  410  to the flame position. “Drilled” restrictors can be fabricated from a conventional “square cut” restrictor by sealing the end of the restrictor, followed by drilling one or more holes into its side until the interior channel of the restrictor is reached. The hole can be drilled using a laser or a mechanical drill. The holes can be drilled at any angle in the range of from greater than 0° to less than 180°, relative to the longitudinal axis of the restrictor. In one embodiment the one or more holes are drilled at an angle of 90°, relative to the longitudinal axis of the restrictor. 
         [0051]    Examples of a burner assembly configuration having a “fritted” restrictor is illustrated in  FIGS. 5A and 5B . In these examples, the configuration of the burner assembly is identical to that of  FIG. 3  except for the adaptation to the second end of the restrictor. Column effluent exits at the fritted second end  524  of restrictor  520  at an angle  516  substantially non-parallel to the longitudinal axis  512  of burner  510 . The tip of the fritted second end  524  can be sealed with a cap  517  ( FIG. 5B ) to force the mobile phase to decompress radially. The mobile phase of the column effluent decompresses and travels through flow path  518  of burner  510  to the flame position. 
         [0052]    An example of a burner assembly configuration having a “pintle” restrictor is illustrated in  FIG. 6 . In this example, the configuration of the burner assembly is identical to that of  FIG. 3  except for the adaptation to the second end of the restrictor. Column effluent exits at the second end  624  of restrictor  620  and is redirected by pintle  628  at an angle  616  substantially non-parallel to the longitudinal axis  612  of burner  610 . The mobile phase of the column effluent decompresses and travels through flow path  618  of burner  610  to the flame position. 
         [0053]    In another of the above burner assembly configurations, the second end of the restrictor, which has a longitudinal axis, is inserted into the burner such that the restrictor longitudinal axis is substantially non-parallel to the burner longitudinal axis. The restrictor used in such configurations can be a conventional “square cut” restrictor or one of the adapted restrictors described above. In one embodiment, the respective longitudinal axes of the restrictor and the burner are perpendicular. An example of such a burner assembly configuration is illustrated in  FIG. 7 . The burner assembly comprises a burner  710  and restrictor  720 . Longitudinal axis  729  of restrictor  720  is perpendicular to longitudinal axis  712  of burner  710 . Burner  710  comprises a burner body having fluid inlets  740  and  750  for receiving combustion gases and a fluid outlet  714  for delivering at least a portion of the combustion gases to flame position  760 . Restrictor  720  comprises a first end  722  and a second end  724 . Second end  724  of restrictor  720  is located within burner  710 . A portion of burner  710  is, in turn, located inside burner housing  730 . Column effluent  726  is fed to the FID detector, entering restrictor  720  at its first end  722  and exiting at its second end  724  at an angle  716  substantially non-parallel to the longitudinal axis  712  of burner  720 . The mobile phase of the column effluent decompresses and travels through flow path  718  of burner  710  to flame position  760 , where one or more separated constituent compositions of the mixture are ionized via combustion. These ions are attracted to collector electrode  790 , where they induce a current, which is, in turn, fed to electrometer  792 . 
         [0054]    In another of the above burner assembly configurations, the burner is adapted so that the at least a portion of the decompressed mobile phase flow stream travels through the flow path in one or more directions substantially non-parallel to the longitudinal axis. This can achieved by using a burner having a tortuous flow path. In one aspect, this tortuous flow path can be formed by packing the flow path of the burner with glass wool. In another aspect, the tortuous flow path can be formed in a burner having one or more members extending from its interior wall surface at an angle substantially non-parallel to its longitudinal axis and which are dimensioned and configured to deflect decompressed mobile phase flow stream in a direction substantially non-parallel to its longitudinal axis. Such members can take the form of a frit or one or more baffles extending from the interior wall of the burner. The members can be porous and/or tapered. The restrictor used in such configurations can be a conventional “square cut” restrictor or one of the adapted restrictors described above. 
         [0055]    An example of such a burner assembly configuration is illustrated in  FIG. 8 . The burner assembly  800  comprises a burner  810  and restrictor  820 . Burner  810  comprises a burner body having fluid inlets  840  and  850  for receiving combustion gases and a fluid outlet  819  for delivering at least a portion of the combustion gases to flame position  860 . Restrictor  820  comprises a first end  822  and a second end  824 . Second end  824  of restrictor  820  is located within burner  810 . A portion of burner  810  is, in turn, located inside burner housing  830 . Column effluent  826  is fed to the FID detector, entering restrictor  820  at its first end  822  and exiting at its second end  824  at an angle  816  parallel to the longitudinal axis  812  of burner  810 . The mobile phase of the column effluent decompresses and travels through flow path  818  of burner  810  through frit  814 . As a result of passing through frit  814 , the decompressed mobile phase travels on to flame position  860  at an angle substantially non-parallel to the longitudinal axis  812  of burner  810 , where one or more separated constituent compositions of the mixture are ionized via combustion. These ions are attracted to collector electrode  890 , where they induce a current, which is, in turn, fed to electrometer  892 . 
         [0056]    Another example of such a burner assembly configuration is illustrated in  FIG. 9 . The burner assembly comprises a burner  910  and restrictor  920 . Burner  910  comprises a burner body having fluid inlets  940  and  950  for receiving combustion gases and a fluid outlet  919  for delivering at least a portion of the combustion gases to flame position  960 . Restrictor  920  comprises a first end  922  and a second end  924 . Second end  924  of restrictor  920  is located within burner  910 . A portion of burner  910  is, in turn, located inside burner housing  930 . Column effluent  926  is fed to the FID detector, entering restrictor  920  at its first end  922  and exiting at its second end  924  at an angle  916  parallel to the longitudinal axis  912  of burner  910 . The mobile phase of the column effluent decompresses and travels through flow path  918  of burner  910  past baffles  914 . As a result of passing by baffles  914 , the decompressed mobile phase travels on to flame position  960  at an angle substantially non-parallel to the longitudinal axis  912  of burner  910 , where one or more separated constituent compositions of the mixture are ionized via combustion. These ions are attracted to collector electrode  990 , where they induce a current, which is, in turn, fed to electrometer  992 . 
         [0057]    The present disclosure is also directed to methods of maintaining a flame in a burner assembly of a flame-based detector. These methods comprises the following three steps:
       1) providing a burner assembly;   2) passing at least the portion of the mobile phase flow stream through the restrictor at a flow rate of 40 mL/min or greater; and either   3) delivering at least the portion of the mobile phase flow stream into the burner and to the flame position as the decompressed mobile phase flow stream at a force/velocity insufficient to extinguish the flame; or   3′) delivering at least the portion of the mobile phase flow stream into the burner and to the flame position as the decompressed mobile phase flow stream such that the decompressed mobile phase flow stream flows to the flame along a non-parallel fluid flow path.
 
The burner assembly comprises a burner and a restrictor. The burner comprises a burner body having a fluid inlet for receiving combustion gases and a fluid outlet for delivering at least a portion of the combustion gases to a flame position having a flame. The restrictor comprises a restrictor comprising a hollow body comprising a first end for receiving at least a portion of a mobile phase flow stream from a chromatographic system and a second end for delivering the at least a portion of the mobile phase flow stream as a decompressed mobile phase flow stream to the burner. The second end of the restrictor is sized and inserted into the inner burner. The effects of steps 3) and 3′) can achieved by employing any of the above-described burner assembly configurations as the burner assemblies of these methods.
       
 
         [0062]    The present invention is further defined in the following Examples. It should be understood that these Examples, while indicating preferred embodiments of the invention, are given by way of illustration only. From the above discussion and these Examples, one skilled in the art can ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various uses and conditions. 
       Examples 
       [0063]    The analytical chromatographic system used was a carbon-dioxide based system (UPC 2 ® System) commercially available from Waters Technologies Corporation, Milford, Mass., USA. The system included an autosampler, a column oven, a 3.0×100 mm ACQUITY UPC 2  1.8 μm HSS C18 SB chromatographic column commercially available from Waters Technologies Corporation, Milford, Mass., USA, and an automated back pressure regulator. The mobile phase was 100% carbon dioxide supplied to the system via a fluid delivery module and was maintained at a pressure of 138 bar. The column was heated to a temperature of 45° C. The flow rate was 1.5 mL/min. The sample injection volume was 0.5 μL. At the outlet of the column, and upstream of the backpressure regulator, a “T” fitting directed a portion of the mobile phase flow to the BPR and a portion of the mobile phase flow to a FID (SRI Model 110 FID commercially available from SRI Instruments, Torrance, Calif., USA). The output signal of the FID was analyzed using Empower® 3 Chromatography Data Software commercially available from Waters Technologies Corporation, Milford, Mass., USA. 
         [0064]    The measured, decompressed CO 2  flow rate directed to the FID was 100 mL/min. At this CO 2  flow rate, optimal response was achieved at hydrogen and air flow rates of 97 and 800 mL/min, respectively. The FID body was held at 350° C. The signal to noise ratio of an analyte peak was evaluated over a range of restrictor positions within the FID. A restrictor with a second end angle of decompression of 0 degrees (i.e., “square cut”) provided an optimal response when positioned about 15 mm from the tip of the burner. Positions further from or nearer to the burner tip resulted in a decrease in signal to noise. However, when a restrictor with a tip angle of 45 degrees was employed, optimal signal to noise was achieved over the entire range of restrictor position adjustment (i.e., response was independent of restrictor position). 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 FID conditions 
               
             
          
           
               
                 Outlet flow (mL/min) 
                 Hydrogen Flow (mL/min) 
                 Air Flow (mL/min) 
               
               
                   
               
             
          
           
               
                 40 
                 57 
                 560 
               
               
                 100 
                 97 
                 800 
               
               
                 200 
                 202 
                 1040 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Results Table 
               
             
          
           
               
                 Flow Rate 
                 Decompression angle (°) 
                   
               
             
          
           
               
                 (mL/min) 
                 ◯ 
                 22 
                 45 
                 67 
                 90 
                 135 
               
               
                   
               
             
          
           
               
                 40 
                 X 
                 X 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                 100 
                 X 
                 X 
                 ◯ 
                 ? 
                 N/A 
                 N/A 
               
               
                 200 
                 X 
                 X 
                 ◯ 
                 X 
                 N/A 
                 N/A 
               
               
                   
               
               
                 X—response dependent on position 
               
               
                 ?—response improved but not completely independent of position 
               
               
                 ◯—response independent of position 
               
               
                 N/A—no data