Abstract:
A method of analyzing a sample gas for the presence of at least one gas impurity by combining a stream of sample gas with a stream of carrier gas to provide a combined stream of gas, directing the combined stream of gas through a column which preferentially removes the sample gas from the combined stream to produce a retentate stream of gas, and analyzing the retentate stream of gas for the presence of the at least one gas impurity.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for analyzing a gas stream, for example, a hydrogen or oxygen gas stream, under continuous flow conditions to detect and quantify the concentration of one or more gaseous contaminants. 
     2. Related Background Art 
     Ultra high purity supplies of process gases are essential in the manufacture of large scale integrated circuits. Measurement and control of impurities at the parts per billion (ppb) level are critical with the process gases utilized by semiconductor manufacturers in the production of integrated circuit devices. Semiconductor manufacturers utilize commercial purifiers to remove impurities from the process gases. Some of the more important impurities removed by these purifiers include oxygen, water, carbon monoxide, carbon dioxide, hydrogen, methane and nitrogen. Continuous monitoring of the process gas stream under continuous flow conditions is necessary to ensure that the gas stream maintains stringent purity requirements. 
     The gases of interest according to the present invention include, but are not limited to hydrogen, oxygen, nitrogen and air. Although not used in as great a volume as argon or nitrogen, hydrogen and oxygen are used in several key processing steps. Consequently, analysis of impurities in these gases is also important. 
     However, several sensitive analytical techniques for providing ppb limits of detection for various impurities cannot be applied to the impurity analysis of hydrogen and oxygen in gas streams under continuous flow conditions. These analytical techniques include emission spectroscopy and gas chromatography (GC) using a discharge ionization detector (DID). In addition, atmospheric pressure ionization mass spectrometers (APIMS) cannot be used to analyze impurities in oxygen gas. Additionally, these analytical techniques cannot analyze large volumes (generally flow rates greater than 10 cc/min) of sample gas streams under continuous flow conditions. 
     The DID detectors and the APIMS cannot be used for oxygen analysis because these techniques require that the sample gas have a higher ionization potential than that of the gaseous impurity to be determined. Common oxygen impurities have higher ionization potentials than oxygen. 
     Emission spectroscopy, on the other hand, cannot be used to analyze impurity levels in diatomic gases such as hydrogen, nitrogen and oxygen. Monoatomic gases such as argon, helium and the like readily transfer energy to lower ionization potential impurities which then can be detected. Diatomic gases have additional vibrational and rotational pathways to dissipate the energy from a plasma and hence do not transfer the energy to the impurities of interest. Consequently, the emission lines of the impurities cannot be detected in diatomic gases. Instead, only the spectrum of the sample is observed in most cases. 
     Previous attempts to solve this problem focused primarily on the use of GC-DID analyzers for hydrogen and oxygen sample gases and, more recently, on emission spectroscopy for detecting nitrogen in either hydrogen or oxygen gases. 
     With GC techniques, the typical carrier gas is purified helium. A small injection (e.g., 1-2 cc) of the sample gas (e.g., hydrogen) is made into the carrier gas stream. The 1-2 cc “slug ” of sample gas is then moved to a device to handle the slug of sample gas. In the case of hydrogen sample gas, the device is typically a hot palladium membrane which selectively allows only the hydrogen gas to pass through it. The impurities are, therefore, retained in the helium carrier gas. A GC column is used to separate the impurities, and because they are contained in the helium carrier gas, a DID detector can be used for this analysis. GC techniques are, however, limited to batch analysis of the sample gas and do not allow analysis of a sample gas under continuous flow conditions. 
     Problems may arise when, for example, the oxygen gas sample must be consumed in a trap. The traps have a finite capacity for oxygen gas and are themselves consumed over time. Most commercial instruments currently available may accommodate only about 80-100 injections before they must be replaced, which may be equivalent to as little as one day of operation. To overcome this problem, dual traps may be employed with an automated regeneration sequence. While this approach minimizes the trap regeneration problem, it may add considerable expense and complexity to the process. 
     Newer trap materials with higher capacity for oxygen, may extend the number of injections possible between trap regenerations. Trap materials which exhibit reversible oxygen adsorption may eliminate the need for dual traps and separate high temperature regeneration steps involving hydrogen or carbon monoxide addition. Such a trap would receive an injection of oxygen sample containing the impurities of interest. The trap material would hold up the oxygen while allowing the impurities to pass through. Before the oxygen breaks through the trap material, and affects the detector&#39;s response, carrier gas is flowed in the reverse direction to sweep the oxygen off the trap to vent. This process continues while the impurities separate on the analytical column and are quantified by the DID detector. If all of the oxygen can be purged off the trap material in the time required to analyze the sample the process can be repeated indefinitely and only a single trap is required. While this modification represents an improvement to the GC-DID analysis of UHP oxygen samples it still is a batch or discrete analysis. 
     It would be highly desirable to provide a continuous, simple and reliable method for analyzing one or more impurities in a gas stream under continuous flow conditions while minimizing the difficulties associated with the systems previously described. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for analyzing a sample gas for the presence of at least one gas impurity. The method comprises the steps of: (a) combining a stream of a sample gas with a stream of a carrier gas to generate a combined stream of gas; (b) directing the combined stream of gas through a column which preferentially removes the sample gas from the combined stream of gas to produce a retentate stream of gas; and (c) analyzing the retentate stream of gas by emission spectroscopy for the presence of at least one gas impurity. 
     In another aspect, the present invention provides a method for analyzing a sample gas for the presence of at least one gas impurity in the sample gas. The method comprises the steps of: (a) directing a stream of carrier gas through a column; (b) directing a stream of sample gas to the column which allows selective permeation of the at least one gas impurity from the stream of sample gas into the stream of carrier gas to produce a permeate stream of gas; and (c) analyzing the permeate stream of gas by emission spectroscopy for the presence of the at least one gas impurity. 
     This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiments thereof in connection with the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an impurity detection system of the present invention for analyzing gaseous impurities contained within a hydrogen gas stream; 
     FIG. 2 is a schematic diagram of an impurity detection system of the present invention for analyzing gaseous impurities contained within an oxygen gas stream; and 
     FIG. 3 is a schematic diagram of an impurity detection system of the present invention configured for analysis of a pre-selected gas impurity. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The impurity detection system of FIG. 1 provides a system for detecting and quantifying gaseous impurities in a gas stream under continuous flow conditions. The sample gases of interest according to the present invention include, but are not limited to hydrogen, oxygen, nitrogen and air. The gas impurities of interest according to the present invention include, but are not limited to methane, water, carbon monoxide, carbon dioxide, nitrogen and oxygen. The impurity detection system comprises a carrier gas pathway  10 , a carrier gas purge pathway  20 , a sample gas pathway  30 , an exchange column  40 , a retentate pathway  50  and an analyzer  60 . 
     The carrier gas pathway  10  comprises a source of carrier gas  65  in communication with a carrier gas getter  70 , a carrier gas branch point  80 , a carrier gas pressure gauge  90 , a carrier gas flow control unit  100 , a junction  110  and the carrier gas purge pathway  20 . The carrier gas purge pathway  20  communicates between the carrier gas branch point  80  and the exchange column  40 . In this embodiment, a mass flow controller is used as the carrier gas flow control unit  100 . However, any means for regulating flow control may be used, such as a pressure regulator and/or a fixed orifice restriction. 
     The term “getter” refers to a device that is capable of selectively removing chemical impurities from a gas stream. 
     The sample gas pathway  30  comprises a valve manifold  120  in communication with a sample gas vent  130 , a sample gas flow control unit  140  and the junction  110 . The valve manifold  120  further comprises a span gas source  150 , a sample gas source  160  and a zero gas source  170 . In this embodiment, a mass flow controller is used as the sample gas flow control unit  140 . However, any means for regulating flow control may be used, such as a pressure regulator and/or a fixed orifice restriction. 
     The retentate pathway  50  communicates between a retentate stream outlet port  180  of the exchange column  40  and the analyzer  60 . 
     The exchange column  40  comprises a hollow tube  200  with a gas inlet end  210  and a gas outlet end  220 , a feed stream inlet port  230 , a purified carrier gas inlet port  280 , the retentate stream outlet port  180 , a permeate stream vent  240  and a membrane system  250 , which generally comprises a semi-permeable membrane preferentially permeable to the sample gas  160 . The materials of which the semi-permeable membrane  250  of the present invention is constructed include, but are not limited to polysulfone, ceramic and palladium. 
     The configuration of the membrane system  250  differs depending upon the source of the sample gas  160  under analysis. When the source of the sample gas  160  is hydrogen, the membrane system  250  comprises a series of hollow fibers  260  that are potted at both the gas inlet end  210  and the gas outlet end  220 . An annular space  270  surrounds the series of hollow fibers  260  and is bounded by the inside of hollow tube  200 , the gas inlet end  210  and the gas outlet end  220  of the hollow tube  200 . 
     In one embodiment, the source of the carrier gas  65  is argon, and the source of the sample gas containing impurities  160  is hydrogen. A continuous stream of argon gas enters the carrier gas pathway  10  at a rate ranging from about 20 cc/min to about 200 cc/min and a pressure ranging from about 10 psig to about 150 psig, which is regulated by the carrier gas flow control unit  100 . The continuous stream of argon gas passes through the getter  70  and reaches the branch point  80  where the argon gas not filling the carrier gas pathway  10  fills the carrier gas purge pathway  20 , which supplies a continuous argon gas stream to the annular space  270  of the exchange column  40  via purified carrier gas inlet port  280 . A continuous hydrogen gas sample containing impurities enters the sample gas pathway  30  at a rate ranging from about 20 cc/min to about 250 cc/min, which is regulated by the sample gas flow control unit  140 . The carrier gas pathway  10  and the sample gas pathway  30  intersect at the junction  110  thereby causing the argon and hydrogen gas streams to combine and form a combined gas feed stream  290 . The ratio of hydrogen sample gas to the argon carrier gas in the combined gas feed stream  290  ranges from about 4:1 to about 1:4 and preferably from about 2:1 to about 1:2 and more preferably is about 1:1. 
     The combined gas feed stream  290  enters the exchange column  40  at the feed stream inlet port  230  at a pressure ranging from about 50 psig to about 150 psig. A pressure difference between the series of hollow fibers  260  and the annular space  270  serves as the driving force moving hydrogen gas from the series of hollow fibers  260  into the annular space  270 . The pressure difference ranges from about 70 psig to about 140 psig and preferably from about 80 psig to about 120 psig. Hydrogen gas diffuses from the series of hollow fibers  260  into the annular space  270  as the combined gas feed stream  290  passes through the exchange column  40 . The diffused hydrogen gas and argon gas combine within the annular space  270  to form a permeate stream  300 . If hydrogen gas is allowed to build up in the annular space  270 , the diffusion rate of hydrogen gas rapidly will decrease. The carrier gas purge pathway  20  supplies purified argon gas in the form of a purge stream  310  to the annular space  270  within the exchange column  40  at a rate of about one 1/min. The purge stream  310  is introduced at atmospheric pressure into the annular space  270  at the purified carrier gas inlet port  280  of the exchange column  40 . The purge stream  310  sweeps the diffused hydrogen gas located within the annular space  270  out of the exchange column  40  through the permeate stream vent  240 . In this fashion, the hydrogen gas concentration within the annular space  270  is maintained near zero, thereby maximizing the hydrogen gas diffusion rate from the series of hollow fibers  260  into the annular space  270 . 
     Gas flow exiting the exchange column  40  at the retentate stream outlet port  180  is referred to as a retentate stream  320 . The retentate stream  320  exits the exchange column  40  and enters the retentate pathway  50  at a flow rate ranging from about 50 cc/min to about 500 cc/min. A back-pressure regulator  190  is located along the retentate pathway  50  between the exchange column  40  and the analyzer  60 . 
     In this embodiment, the analyzer  60  is an emission spectrometer. In another embodiment, the analyzer is an emission spectrometer such as that described in U.S. Pat. No. 3,032,654. In still another embodiment, the analyzer is an emission spectrometer as described in U.S. Pat. Nos. 5,412,467 and 5,831,728 the disclosures of which are incorporated herein by reference. In yet another embodiment, the analyzer  60  is an atmospheric pressure ionization mass spectrometer. 
     The retentate pathway  50  communicates with the exchange column  40  and the analyzer  60 . The back-pressure regulator  190  is located within the retentate pathway  50  between the retentate stream outlet port  180  and the analyzer  60 . The back-pressure regulator  190  functions to maintain proper pressure of the combined gas feed stream  290  as it enters the exchange column  40  and flows through the series of hollow fibers  260 . The back-pressure regulator  190  also serves to maintain a constant pressure within the retentate pathway  50  which, in turn, allows constant inlet pressure to the analyzer  60 , thereby stabilizing the response of the analyzer  60 . 
     The retentate stream  320  contains the impurities of interest originally in the hydrogen sample gas now in the argon gas carrier which is suitable for introduction and analysis into the emission spectroscopic analyzer  60 . Preferably, less than about 2% residual hydrogen gas remains within the retentate stream  320  as it flows to the analyzer  60 . 
     If the impurity detection system were an ideal system, all of the hydrogen gas within the combined gas feed stream  290  would diffuse into the permeate stream  300  and none would remain in the retentate stream  320 . Similarly, in the ideal system, none of the impurities within the combined gas feed stream  290  would diffuse into the permeate stream  300  and all the impurities would remain in the retentate stream  320 . In this ideal case, the analyzer  60  could be calibrated using purified argon gas as the zero gas source  170  and spanned using a span gas source  150  containing known concentrations of the impurities of interest in an argon gas carrier. 
     Because the impurity detection system operates under non-ideal conditions, however, some hydrogen gas remains in the retentate stream  320  and each impurity diffuses to some extent into the permeate stream  300  and is lost to the analyzer  60 . To compensate for this leaching, calibration of the analyzer  60  is performed under identical conditions as the analysis of the sample gas  160 . Also, the zero gas source  170  must be hydrogen gas with all the impurities of interest removed, and the span gas source  150  must contain hydrogen gas as the balance gas. 
     For example, if the analyzer  60  is affected by residual hydrogen gas in the retentate stream  320 , the effect would be demonstrated when the hydrogen zero gas  170  enters the carrier gas pathway  10 . Consequently, an electronic adjustment could be made within the analyzer  60  to compensate for this during initial calibration of the analyzer  60 . Similarly, if 20% of an impurity diffused into the permeate stream  300 , 20% of the impurity would also diffuse when measuring the span gas source  150  and the gain in the analyzer  60  could be increased as compensation during span calibration of the analyzer  60 . The analyzer  60  would continue to give an accurate impurity concentration as long as the percentage of hydrogen gas remaining in the retentate stream  320  remains constant, and the percentage of each impurity leaching to the permeate stream  300  also remains constant. 
     An alternate embodiment of the invention for analyzing a gas stream under continuous flow conditions to detect and quantify the concentration of one or more pre-selected gaseous contaminants is illustrated in FIG.  2 . In this alternate embodiment, the configuration of the impurity detection system is basically similar to that of FIG. 1 with like reference numerals used to identify corresponding components. The differences in these systems will be discussed in more detail below. 
     The impurity detection system of FIG. 2 provides a system for detecting and quantifying gaseous impurities in, for example, an oxygen gas stream under continuous flow conditions. 
     Because the molecular weight of oxygen is close to the molecular weight of argon, the membrane system  250 ′ located within the exchange column  40  is different than the membrane system  250  utilized in the embodiment of FIG. 1 in which the sample gas source  160  is hydrogen. For analysis of impurities when the sample gas source  160  is oxygen gas, the series of hollow fibers  260  is preferably replaced with a high temperature ceramic membrane  265  that is selectively permeable for oxygen and is potted at both the gas inlet end  210  and the gas outlet end  220 . One example of such a high temperature ceramic device useful in this invention is a solid electrolyte ionic or mixed conductor, also known as a “SELIC” device such as described in U.S. Pat. Nos. 5,557,951, 5,837,125 and 5,935,298, the disclosures of which are incorporated herein by reference. An annular space  270  surrounds the high temperature ceramic membrane  265  and is bounded by the inside of hollow tube  200 , the gas inlet end  210  and the gas outlet end  220  of hollow tube  200 . Utilizing the high temperature ceramic membrane  265  allows selective diffusion of oxygen gas from the permeate stream  300 . Alternatively, the membrane system  250 ′ may be replaced with a high capacity oxygen adsorbent to selectively consume the oxygen gas which is then replaced with argon gas. 
     A disadvantage arising when using the high temperature ceramic membrane  265  is that the system operates at high temperatures, typically from about 800□C to about 1000□C. At such high temperatures, carbon-containing compounds such as methane, higher aliphatic hydrocarbons, and carbon monoxide will likely react with the excess oxygen sample gas and be combusted to generate carbon dioxide. Consequently, it becomes difficult to quantify the concentrations of each impurity individually; instead, a carbon dioxide concentration related to the total amount of combustible carbon compounds can be reported. Reporting the total carbon content in a sample as carbon dioxide may be acceptable to most semiconductor customers. 
     In this embodiment, the source of the carrier gas  65  is argon, and the source of the sample gas containing impurities  160  is oxygen. A continuous stream of argon gas enters the carrier gas pathway  10  at a rate ranging from about 20 cc/min to about 200 cc/min and a pressure ranging from about 10 psig to about 150 psig, which is regulated by the carrier gas flow control unit  100 . The continuous stream of argon gas passes through the getter  70  and reaches the branch point  80  where the argon gas not filling the carrier gas pathway  10  fills the carrier gas purge pathway  20 , which supplies a continuous argon stream to the annular space  270  of the exchange column  40  via purified carrier gas inlet port  280 . A continuous stream of oxygen sample gas containing impurities enters the sample gas pathway  30 . The carrier gas pathway  10  and sample gas pathway  30  intersect at junction  110  thereby causing the argon and oxygen gas streams to combine and form a combined gas feed stream  290 . The ratio of oxygen sample gas to the argon carrier gas in the combined gas feed stream  290  ranges from about 4:1 to about 1:4 and preferably from about 2:1 to about 1:2 and more preferably is about 1:1. 
     The combined gas feed stream  290  enters the exchange column  40  at the feed stream inlet port  230  at a pressure ranging from about 50 psig to about 120 psig. Oxygen gas diffuses from the high temperature ceramic membrane  265  into the annular space  270  as the combined gas feed stream  290  passes through the exchange column  40 . The diffused oxygen gas and argon gas combine within the annular space  270  to form a permeate stream  300 . If oxygen gas is allowed to build up in the annular space  270 , the diffusion rate of the oxygen gas rapidly will decrease. The carrier gas purge pathway  20  supplies purified argon gas in the form of a purge stream  310  to the annular space  270  within the exchange column  40  at a rate of about one 1/min. The purge stream  310  is introduced at atmospheric pressure into the annular space  270  at the purified carrier gas inlet port  280  of exchange column  40 . The purge stream  310  sweeps the diffused oxygen gas located within the annular space  270  out of exchange column  40  through permeate stream vent  240 . In this fashion, the oxygen gas concentration within the annular space  270  is maintained near zero, thereby maximizing the oxygen gas diffusion rate from the high temperature ceramic membrane  265  into the annular space  270 . 
     Gas flow exiting the exchange column  40  at the retentate stream outlet port  180  is referred to as a retentate stream  320 . The retentate stream  320  exits the exchange column  40  and enters the retentate pathway  50  at a flow rate ranging from about 50 cc/min to about 500 cc/min. The retentate pathway  50  communicates with the exchange column  40  and the analyzer  60 . 
     A back-pressure regulator  190  is located within the retentate pathway  50  between retentate stream outlet port  180  and the analyzer  60 . The back-pressure regulator  190  functions to maintain proper pressure of the combined gas feed stream  290  as it enters the exchange column  40  and flows through the high temperature ceramic membrane  265 . The back-pressure regulator  190  also serves to maintain a constant pressure within the retentate pathway  50  which, in turn, allows constant inlet pressure to the analyzer  60 , thereby stabilizing the response of the analyzer  60 . The retentate stream  320  contains the impurities of interest originally in the sample oxygen gas now in the argon carrier gas which is suitable for introduction and analysis into the emission spectroscopic analyzer  60 . 
     In yet another embodiment, the impurity detection system shown in FIG. 3 is configured such that a single impurity of interest diffuses from a sample gas  160  into a carrier gas  65  in which analysis can be performed. The diffusion of the single impurity of interest is accomplished through use of a membrane system  250 ″ that is selectively permeable for the impurity of interest. The configuration is basically similar to that of FIG. 1 with like reference numerals used to identify corresponding components. 
     As shown in FIG. 3, the impurity detection system for measuring a single impurity of interest comprises a carrier gas purge pathway  20 , a sample gas pathway  30 , an exchange column  40 , a retentate pathway  50 , a permeate pathway  330  and an analyzer  60 . 
     The carrier gas purge pathway  20  comprises a source of carrier gas  65  in communication with a carrier gas getter  70 , a carrier gas pressure gauge  90  and the exchange column  40 . 
     The sample gas pathway  30  comprises a valve manifold  120 , in communication with a sample gas vent  130 , a sample gas flow control unit  140  and the exchange column  40 . The valve manifold  120  further comprises a span gas source  150 , a sample gas source  160  and a zero gas source  170 . 
     The retentate pathway  50  communicates between a retentate stream outlet port  180  of the exchange column  40  and a retentate pathway vent  340 . A back-pressure regulator  190  is located along the retentate pathway  50  between exchange column  40  and the retentate pathway vent  340 . 
     The permeate pathway  330  communicates between a permeate stream outlet port  350  of the exchange column  40  and the analyzer  60 . In this embodiment, the analyzer  60  can be an emission spectrometer of the type discussed above with respect to the embodiment shown in FIG.  1 . 
     The exchange column  40  comprises a hollow tube  200  with a gas inlet end  210  and a gas outlet end  220 , a sample gas inlet port  360 , a purified carrier gas inlet port  280 , the retentate stream outlet port  180 , a permeate stream outlet port  350  and a membrane system  250 ″. 
     The membrane system  250 ″ of the exchange column  40  differs depending upon the identity of the impurity of interest, which may include, but is not limited to water, methane, carbon dioxide and oxygen. Generally, the membrane system  250 ″ will be selectively permeable for the impurity of interest. When the impurity of interest is water, for example, the membrane system  250 ″ comprises a selectively permeable membrane  370  which is selectively permeable for water and is potted at both the gas inlet end  210  and the gas outlet end  220  of the exchange column  40 . An annular space  270  surrounds the selectively permeable membrane  370  and is bounded by the inside of hollow tube  200 , the gas inlet end  210  and the gas outlet end  220  of hollow tube  200 . 
     In one embodiment, the source of the carrier gas  65  is argon, the source of sample gas  160  is nitrogen and the impurity of interest is water. A continuous stream of argon gas enters the carrier gas pathway  10  at a rate ranging from about 50 cc/min to about 500 cc/min and a pressure ranging from about 10 psig to about 150 psig. The continuous stream of argon gas passes through the getter  70  and enters the annular space  270  of the exchange column  40  via the purified carrier gas inlet port  280 . A continuous stream of nitrogen gas containing water enters the sample gas pathway  30  at a rate ranging from about 50 cc/min to about 200 cc/min and a pressure ranging from about 10 psig to about 150 psig, which is regulated by the sample gas flow control unit  140 . The continuous stream of nitrogen enters the exchange column  40  via the sample gas inlet port  360 . 
     Once inside the exchange column  40 , water passes through the membrane system  250  into the annular space  270  where the water mixes with the carrier gas argon to form a permeate stream  300 . The permeate stream  300  exits the exchange column  40  via the permeate stream outlet port  350  and enters the permeate pathway  330  which flows into the analyzer  60  for analysis at a rate ranging from about 50 cc/min to about 500 cc/min. 
     The nitrogen sample gas, with the water removed, exits the exchange column  40  through the retentate stream outlet port  180  and enters the retentate pathway  50  where it is vented to the outside environment via the retentate pathway vent  340  at a pressure ranging from about 10 psig to about 150 psig. 
     Calibration of the analyzer  60  is accomplished by supplying the zero gas source  120 , which is the sample gas with all impurities removed, and the span gas source  150  to the exchange column  40 . Adjustments may be made in the analyzer  60  for any sample gas which diffuses into the permeate stream  300  and affects the baseline of the analyzer  60 ; similarly, adjustments may be made to compensate for less than 100% transfer of water into the permeate stream  300 . 
     While the present invention is described above with respect to what is currently considered to be its preferred embodiments, it is to be understood that the invention is not limited to that described above. To the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.