Abstract:
A flame photometric detector comprises a burner assembly configured to combust a sample of an effluent, the combusted sample emitting at least one excited molecule, an interchangeable selective optical filter configured to pass a plurality of selected optical wavelengths corresponding to the excited molecule, and a photomultiplier tube configured to quantify the concentration of the excited molecule.

Description:
BACKGROUND  
       [0001]     Gas chromatography is used to analyze and detect the presence of many different substances in a gaseous sample. Gas chromatography uses various types of detectors, depending on the specific element or compound sought to be detected. Different detectors are used to achieve selective and/or highly sensitive detection of specific elements or compounds, in particular chromatographic analyses.  
         [0002]     Typically, a flame photometric detector is used to detect the presence of sulfur or phosphorous in a particular sample, or analyte. A flame photometric detector uses what is referred to as a chemiluminescent reaction where compounds containing sulfur or phosphorous encounter a hydrogen-rich flame. Chemiluminescence uses quantitative measurements of the optical emission from excited chemical species to determine analyte concentration. Chemiluminescence is typically emission from energized molecule species. When burned, or combusted, in such a flame, sulfur is transformed into an emitting species referred to as “S 2 ” and phosphorous is transformed into an emitting species referred to as “HPO.” The emission wavelength range for excited S 2  includes, among others, the region from 320-405 nanometers (nm) and the wavelength range for excited HPO includes, among others, the range from 510-530 nm. The molecular emissions impinge on a photomultiplier tube, which converts photons to an electrical signal to quantify the concentration of a particular excited species.  
         [0003]     To selectively detect either the excited S 2  emission or the excited HPO emission, a narrow band-pass optical (interference) filter has typically been used between the flame and the photomultiplier tube to isolate the appropriate emission band. Unfortunately, a narrow band-pass optical filter limits the signal-to-noise ratio, and therefore the signal strength of the signal delivered to the photomultiplier, resulting in an inability to detect minute quantities of an analyte. For example, a narrow band-pass optical filter used in a conventional flame photometric detector to detect sulfur transmits a photon emission band ranging in wavelength from 385-400 nm and has 65% transmissivity. A disadvantage of such an optical filter is that it only passes one of many characteristic emission bands for sulfur, thereby limiting the signal supplied to the photomultiplier tube. Further, to detect the presence of different analytes, different filters must be interchanged.  
         [0004]     Therefore, it would be desirable to detect the presence of multiple elements without having to change interference filters in a flame photometric detector.  
       SUMMARY OF THE INVENTION  
       [0005]     According to one embodiment, a flame photometric detector comprises a burner assembly configured to combust a sample of an effluent, the combusted sample emitting at least one excited molecule, an interchangeable selective optical filter configured to pass a plurality of selected optical wavelengths corresponding to the excited molecule and a photomultiplier tube configured to quantify the concentration of the excited molecule.  
         [0006]     Other aspects and advantages of the invention will be discussed with reference to the figures and to the detailed description of the preferred embodiments. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0007]     The invention will be described by way of example, in the description of exemplary embodiments, with particular reference to the accompanying figures in which:  
         [0008]      FIG. 1  is schematic diagram illustrating a flame photometric detector employing a selective optical filter.  
         [0009]      FIG. 2  is a graphical illustration showing the spectral wavelength and filter transmission band of a conventional optical filter.  
         [0010]      FIG. 3  is a graphical illustration showing the optical characteristics of a selective optical filter constructed in accordance with an embodiment of the invention.  
         [0011]      FIG. 4  is a graphical illustration showing the optical characteristics of another embodiment of the selective optical filter.  
         [0012]      FIG. 5  is a graphical illustration showing the optical characteristics of another embodiment of the selective optical filter.  
         [0013]      FIG. 6  is a graphical illustration showing the optical characteristics of another embodiment of the selective optical filter.  
         [0014]      FIG. 7  is a flow chart illustrating a method for selectively detecting optical signals. 
     
    
     DETAILED DESCRIPTION  
       [0015]     While described below for use in detecting sulfur and phosphorous, the selective optical filter for use in a flame photometric detector can be modified to detect the presence of other elements. For example, by carefully designing the transmissive and non-transmissive wavelengths of a selective optical filter in accordance with embodiments of the invention, the presence of other elements can be detected and analyzed. Further, while particular wavelengths are described herein, the selective optical filter described below can be configured to be transmissive and non-transmissive for other wavelengths.  
         [0016]      FIG. 1  is a schematic diagram illustrating a flame photometric detector  100  employing a selective optical filter. The flame photometric detector  100  generally includes a body portion  146  and a sample supply element  106 . In one embodiment, the output of a gas chromatograph, illustrated at  102 , including a chromatographic column  104 , is supplied to an input port  108  of the sample supply element  106 . The sample material supplied at the input port  108  is referred to as the “effluent” and represents the output of the chromatographic column  104 . The sample supply element  106  also includes a port  112  through which a supply of hydrogen is supplied, and a port  114  through which a supply of oxygen or air is provided. The sample supply element  106  also includes a burner element  116  to provide a flame  118 . In accordance with the operation of the flame photometric detector  100 , the sample introduced via the chromatographic column  104  is volatized in a hydrogen rich flame  118 . Heating the sample in the hydrogen rich flame  118  excites the molecules in the sample and if present, causes excited species of sulfur (S 2  in this example) and phosphorous (HPO in this example) to be formed from the effluent.  
         [0017]     The flame  118  burns within a chamber  126 , which includes an exhaust port  122 . The exhaust port  122  is configured to carry away all combustion by-products from the chamber  126 . The chamber  126  also includes a chemiluminescence region  128 . The chemiluminescence region  128  is the area within the body  146  of the flame photometric detector  100  in which the sample volatized by the flame  118  forms excited molecules that will be detected by the photomultiplier tube  138 . The term chemiluminescence refers to the process by which an excited species of molecule emits light energy in the form of photons at a series of particular wavelengths. As will be described below, the photons pass through the thermal filter  134  and a selective optical filter  150  and are detected and amplified by the photomultiplier tube  138 .  
         [0018]     A thermal filter  134  separates the chemiluminescence region  128  and the chamber  126  from the selective optical filter  150 . In this example, the selective optical filter  150  is removable and interchangeable so that the wavelength of light generated in the chemiluminescence region  128  and passed to the photomultiplier tube  138  can be selectively detected. When the sample is burned in the chamber  126  the sample emits excited species that emit photons at one or more characteristic wavelengths. The selective optical filter  150  filters the light energy impinging thereon, and passes selected wavelengths to the photomultiplier tube  138 . The photomultiplier tube  138  receives a high voltage energy source on connection  142  and provides an output on connection  144  to an amplifier and other processing elements (not shown). The photomultiplier tube  138  converts photons to an electrical signal to quantify the concentration of a particular excited species. The photons that impinge on the photomultiplier tube  138  have a characteristic wavelength depending upon the material that has undergone a chemiluminescent reaction in region  128 . In this manner, the flame photometric detector  100  can determine, via the characteristic wavelength of the light energy that impinges on the photomultiplier tube  138 , whether a particular element is present in the sample.  
         [0019]     In a typical application, the flame photometric detector  100  is used to detect the presence of sulfur and/or phosphorous in a sample. When excited due to the chemiluminescent reaction, sulfur becomes a molecular species referred to as “S 2 ,” and phosphorous becomes a molecular species referred to as “HPO.” 
         [0020]     In a conventional flame photometric detector, the optical filter has historically been designed as a narrow band-pass optical filter, which is designed to pass a small portion of the electromagnetic spectrum that is characteristic of the material sought to be detected. In accordance with an embodiment of the invention, the selective optical filter  150  is fabricated as a broad band-pass optical filter and in an alternative embodiment, as a plurality of selective band-pass optical filters having selective transmissive and non-transmissive regions. Until a broad band-pass selective optical filter was implemented by the inventors, it was thought that such an optical filter would be incapable of detecting the S 2  and the HPO species with sufficient selectivity. However, it has been discovered that historical concerns about the expected poor filter selectivity when using a broad band-pass optical filter are not valid. In accordance with an embodiment of the invention, a broad band-pass selective optical filter provides both high selectivity and high sensitivity for detecting both the S 2  and the HPO species.  
         [0021]      FIG. 2  is a graphical illustration  200  showing the spectral wavelength and filter transmission band of a conventional optical filter. The horizontal axis  202  represents spectral wavelength in nanometers (nm) and the vertical axis  204  represents signal intensity through an optical filter. The trace  206  illustrates an interference signal containing multiple peaks  222 ,  224 ,  226 ,  228  and  232  of hydrocarbon or OH that is present in most combusted samples. The trace  208  represents the optical response of sulfur after it is combusted by the flame photometric detector  100 , and the trace  212  represents the optical response of phosphorous after it has been combusted by the flame photometric detector  100 . The curve  208  includes a plurality of characteristic wavelengths (364 nm, 374 nm, 384 nm and 394 nm (illustrated in  FIG. 2 )) of the excited S 2  molecule and the curve  212  includes a characteristic wavelength (525 nm) of the excited HPO molecule.  
         [0022]     The region indicated at  250  illustrates a narrow optical transmission band that includes the 394 nanometer wavelength that is one of the characteristic wavelengths of the excited S 2  sulfur molecule. The peak illustrated at point  214  represents the 394 nm characteristic emission of the sulfur S 2  molecule while the peak  216  illustrates the characteristic 525 nanometer wavelength emission of the phosphorous HPO. In this example, a conventional narrowband optical filter would pass only the 394 nm S 2  characteristic wavelength.  
         [0023]      FIG. 3  is a graphical illustration  300  showing the optical characteristics of a selective optical filter constructed in accordance with an embodiment of the invention. The horizontal axis  302  represents spectral wavelength in nanometers, and the vertical axis  304  represents signal intensity through the selective optical filter. The trace  306  represents interfering signals including a plurality of hydrocarbon and OH peaks  322 ,  324 ,  326 ,  328  and  332 , the trace  308  represents the optical response of the S 2  sulfur molecule after it is combusted by the flame photometric detector  100 , and the trace  312  represents the optical response of the HPO phosphorous molecule after it is combusted by the flame photometric detector  100 . The curve  308  includes a plurality of characteristic wavelengths (364 nm, 374 nm, 384 nm and 394 nm) of the excited S 2  molecule and the curve  312  includes a characteristic wavelength (525 nm) of the excited HPO molecule.  
         [0024]     In accordance with an embodiment of the invention, the selective optical filter  150  has a broad transmissive band illustrated as transmissive region  350 . In this example, the transmissive region encompasses wavelengths from approximately 335 to 405 nanometers. This broad band-pass filter characteristic captures most of the characteristic wavelengths of the S 2  molecule (364 nm, 374 nm, 384 nm and 394 nm) illustrated generally at  336 . Other characteristic wavelengths of the S 2  molecule include, for example, 342 nm, 350 nm and 359 nm. The broad band-pass filter characteristic illustrated using transmissive region  350  provides superior signal-to-noise ratio while taking advantage of a broad range of characteristic wavelengths  336  characterizing the S 2  sulfur molecule. The selective optical filter  150  is designed to be non-transmissive at wavelengths outside of the transmissive region  350 .  
         [0025]      FIG. 4  is a graphical illustration  400  showing the optical characteristics of another embodiment of the selective optical filter. The horizontal axis  402  represents a spectral wavelength in nanometers and the vertical axis  404  represents signal intensity through the selective optical filter. The trace  406  represents an interfering signal including a plurality of hydrocarbon and OH peaks  422 ,  424 ,  426 ,  428  and  432 , the trace  408  represents the response of the S 2  sulfur molecule after it is combusted by the flame photometric detector  100 , and the trace  412  represents the response of the HPO phosphorous molecule after it is combusted by the flame photometric detector  100 . The curve  408  includes a plurality of characteristic wavelengths (364 nm, 374 nm, 384 nm and 394 nm) of the excited S 2  molecule and the curve  412  includes a characteristic wavelength (525 nm) of the excited HPO molecule.  
         [0026]     The optical filter characteristic of a selective optical filter indicated using the graphical illustration  400  shows a transmissive region  442  and a transmissive region  444  with a non-transmissive region  446  there between. The transmissive region  442  is transmissive in the approximate wavelength region of 335 through 380 nanometers and the transmissive region  444  is transmissive in the approximate wavelength region of 405 through 425 nanometers. The non-transmissive region  446  occupies an approximate spectral wavelength encompassing 385 through 400 nanometers. In this example, there is an approximate 5 nm region between the non-transmissive region  446  and the transmissive regions  442  and  444  that is considered neither transmissive nor non-transmissive due to optical filter performance characteristics.  
         [0027]     The non-transmissive region  446  is referred to as a “notch” in that the wavelengths therein are notched out of the filter response. As shown in  FIG. 4 , the non-transmissive region  446  encompasses the hydrocarbon response of the interfering trace  406  shown at point  424 . By notching out the hydrocarbon response at this wavelength, both high selectivity and high sensitivity to detecting the S 2  sulfur molecule at the characteristic wavelengths 342 nm, 350 nm, 359 nm, 364 nm and 374 nm are provided by the selective optical filter  150 .  
         [0028]      FIG. 5  is a graphical illustration  500  showing the optical characteristics of another embodiment of the selective optical filter. The horizontal axis  502  represents spectral wavelength in nanometers and the vertical axis  504  represents signal intensity through the selective optical filter. The trace  506  represents an interfering signal including a plurality of hydrocarbon and OH peaks  522 ,  524 ,  526 ,  528  and  532 , the trace  508  represents the response of the S 2  sulfur molecule after it is combusted by the flame photometric detector  100 , and the trace  512  represents the response of the HPO phosphorous molecule after it is combusted by the flame photometric detector  100 . The curve  508  includes a plurality of characteristic wavelengths (364 nm, 374 nm, 384 nm and 394 nm) of the excited S 2  molecule and the curve  512  includes a characteristic wavelength (525 nm) of the excited HPO molecule. Other characteristic wavelengths of the HPO molecule may also be detected.  
         [0029]     The filter response shown in  FIG. 5  includes a transmissive region  550  and a transmissive region  560 . The transmissive region  550  encompasses the approximate wavelengths of 335 through 405 nanometers while the transmissive region  560  encompasses the approximate wavelengths of 520 through 580 nanometers. In this manner, a single selective optical filter can provide optical sensitivity and selectivity for detecting both the S 2  sulfur molecule and the HPO phosphorous molecule. As shown in  FIG. 5 , the characteristic wavelength emission (364 nm, 374 nm, 384 nm and 394 nm) of the excited S 2  molecule and the characteristic 525 nanometer wavelength emission of the excited HPO phosphorous molecule are easily captured within the transmissive regions  550  and  560 , respectively, while interfering hydrocarbon and OH signals illustrated at  522 ,  526 ,  528  and  532  lie outside of the transmissive regions  550  and  560 .  
         [0030]      FIG. 6  is a graphical illustration  600  showing the optical characteristics of another embodiment of the selective optical filter. The horizontal axis  602  represents spectral wavelength in nanometers and the vertical axis  604  represents signal intensity through the selective optical filter. The trace  606  represents an interfering signal including a plurality of hydrocarbon and OH peaks  622 ,  624 ,  626 ,  628  and  632 , the trace  608  represents the response of the S 2  sulfur molecule after it is combusted by the flame photometric detector  100 , and the trace  612  represents the response of the HPO phosphorous molecule after it is combusted by the flame photometric detector  100 . The curve  608  includes a plurality of characteristic wavelengths (364 nm, 374 nm, 384 nm and 394 nm) of the excited S 2  molecule and the curve  612  includes a characteristic wavelength (525 nm) of the excited HPO molecule.  
         [0031]     The optical response of the selective optical filter shown in  FIG. 6  includes transmissive region  650 , which encompasses an approximate wavelength range of 355 through 375 nanometers and the transmissive region  660 , which encompasses an approximate wavelength range of 520 through 540 nanometers. The transmissive region  650  is selectively defined to omit the hydrocarbon peak located at point  624  and the transmissive region  660  is selectively defined to omit the hydrocarbon peak located at point  632 . However, the transmissive region  650  still includes sufficient wavelength range to capture the 364 nm and the 374 nm S 2  molecule peaks and the 525 nm HPO molecule peak.  
         [0032]      FIG. 7  is a flow chart  700  illustrating a method for selectively detecting optical signals. In block  702 , the flame photometric detector  100  generates photons from a sample at a plurality of different wavelengths. In block  704 , the plurality of wavelengths are passed through a single selective optical filter  150 . In block  706 , the photomultiplier tube  138  detects an element associated with each of the plurality of wavelengths.  
         [0033]     The foregoing detailed description has been given for understanding exemplary implementations of the invention in the gas phase only and no unnecessary limitations should be understood therefrom as modifications will be obvious to those skilled in the art without departing from the scope of the appended claims and their equivalents.