Abstract:
Methods and related apparatuses and mixtures are described for chromatographic analysis. The described system includes a pressurized source of a mobile phase and a flow path in fluid communication with the pressurized source such that the mobile phase flows through the flow path. The system also includes an injector in fluid communication with the flow path and downstream of the pressurized source, the injector being configured to inject a sample into the flow path. A first column located downstream of the injector, contains a stationary phase, and forms part of the flow path. A first detector is positioned to detect properties of fluid in the flow path at a location downstream of the injector and upstream from the first column. A second detector is positioned to detect properties of fluid in the flow path at a location downstream of the first column.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This patent specification relates to Chromatography. More particularly, this patent specification relates to systems and methods for gas chromatography. 
     2. Background of the Invention 
     Chromatography is the field of separating chemicals based on differences in properties such as volatility, absorption, adsorption, size, etc. In this field, different rates of migration along a given flow path (gas, liquid, supercritical fluid, etc) result in the spatial separation of chemical analytes. This differential migration is achieved by differing rates of interaction with the separation column or by different values of analyte mobility.  FIG. 1  shows the configuration of a typical conventional gas chromatography system. As shown the configuration includes a pressure source  100 , injector  102 , a column  104 , a detector  106  and vent  108 . The injector  102  provides a sharp pulse of the sample (chemical mixture) into the system flow path. The column  104  provides the physical separation, and the detector detects analytes as they elute from the column. Some configurations employ other elements such as a focuser  110 , modulator  112 , and an additional column  114  to enhance performance or provide otherwise unattainable separations. 
     Modern chromatography has evolved substantially; many examples exist of advanced methods with various non-standard devices that perform a variety of tasks that provide enhanced chromatographic performance and/or analyte information, as well as hyphenated methods that bridge existing standards and protocols. Such devices include cryogenic focusers, adsorbent based preconcentrators, and band enhancement devices (similar to a focuser). Adding to this complexity are two-dimensional methods that use modulators to control injection into second columns in attempt to measure a second, independent retention time. Also of great significance is the integration of microfabricated devices and systems with traditional chromatographic systems. Microfluidics offer many advantages, but also have the potential for adding new sources of band broadening and other analytical errors. A growing problem common to these relatively recent methods and devices is how to diagnose problems within the system. To achieve the best separation performance, it is necessary to provide injections that are small with respect to the band broadening that will occur on the column. However, with the expected day to day changes in system elements, such as the devices mentioned above, the injection profile could easily change. 
     Quantitative analysis is typically based on comparison of the observed peak areas to the injected quantity of sample. This can result in a significant error if the sample volume varies (e.g. syringe error) or some injected components do not actually flow from the injector to the column (or between succeeding devices in the flow path). 
     U.S. Patent Application Publication No. US2005/0123452A1 discloses a chromatograph for analyzing natural gas having non-destructive detectors placed between columns in a multi-column combination. However, these detectors are used for detecting elutes from earlier columns that are discharged prior to a later column having a molecular sieve, so as not to contaminate the sieve. The detectors are not used for diagnosis of problems within the system. A detector is also disclosed in location before the first column. However, this detector is only used during a back-flushing operation where the detector can then detect eluents from the first column. 
     SUMMARY OF THE INVENTION 
     According to embodiments, a system for chromatographic analysis of a sample containing a plurality of components is provided. The system includes a pressurized source of a mobile phase and a flow path in fluid communication with the pressurized source such that the mobile phase flows through the flow path. The system also includes an injector in fluid communication with the flow path and downstream of the pressurized source, the injector being configured to inject a sample into the flow path. A first column located downstream of the injector, contains a stationary phase, and forms part of the flow path. A first detector is positioned to detect properties of fluid in the flow path at a location downstream of the injector and upstream from the first column. A second detector is positioned to detect properties of fluid in the flow path at a location downstream of the first column. A processor is configured to receive first measurement data from the first detector and second measurement data from the second detector and combine the first and second measurement data to calculate a property associated with at least one of the components of the sample. As used herein the terms analyte(s), compound(s) and component(s) refer to any separable components of a mixture. 
     According to embodiments, a method chromatographic analysis of a sample containing a plurality of components is provided. The method includes introducing a mobile phase into a fluid flow path, injecting a sample into the flow path at a location downstream from the location of mobile phase introduction, detecting a property of the fluid in the flow path with a first detector at a location on the flow path downstream of the location of injecting the sample thereby generating first measurement data. The mobile phase and sample flow through a first column located downstream of the first detector. A second detector is used to detect a property of fluid in the flow path at a location downstream of the first column thereby generating second measurement data. A property associated with at least one of the components of the sample is calculated by at least combining the first measurement data and the second measurement data. 
     Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein: 
         FIG. 1  shows the configuration of a typical conventional gas chromatography system; 
         FIG. 2  shows a multi-dimensional chromatographic system having multiple detectors according to embodiments; 
         FIG. 3  shows a simple single-dimension chromatography system with an additional detector, according to embodiments; 
         FIG. 4  shows a single-dimension chromatography system with two additional detectors, according to embodiments; 
         FIG. 5  shows a 2-dimensional separation system according to embodiments; and 
         FIG. 6  shows an additional detector on the vent line of a split-injection system according to embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice. Further, like reference numbers and designations in the various drawings indicated like elements. 
     The use of non-destructive detectors (i.e. detectors that do not destroy or modify the sample) in multiple locations throughout the flow path of a chromatograph (gas, liquid, etc) or other dynamic separation system is described herein. Traditionally, detectors are placed only at the end of such systems to detect eluting analytes. Generally, detectors are destructive (destroy or modify sample), expensive, have large internal dead volumes, any may require a makeup flow (inert mobile phase); also data acquisition bandwidth on control boards is usually limited. Therefore it has not been generally a high priority for the typical chromatographer to consider multiple detectors within a single chromatographic flow path. However, with modern advances in micro- and nano-technologies, a variety of chromatographic detectors are possible that can be placed along the sample flow path without modifying the sample or causing significant band broadening (e.g. thermal conductivity detectors, microsensors, microsensor arrays, etc.). Several useful applications are made possible by providing additional detectors as described herein. First of all, the diagnosis of many typical problems can be simplified by including these additional detectors. These diagnoses are of critical importance when considering micro-fabricated chromatographic systems with non-ideal flow paths and/or conditions. Second, new quantitative methods are possible by comparing peak areas at different stages of the system flow path. Third, by continuously comparing detector outputs (i.e. subtracting or dividing), new types of signals can be generated. 
       FIG. 2  shows a multi-dimensional chromatographic system having multiple detectors according to embodiments. The system is similar to that shown in  FIG. 1  in that it includes a pressure source  200 , injector  202 , a focuser  210 , a column  204 , a modulator  212 , second column  214 , detector  206  and vent  208 . According to embodiments, additional detectors  220 ,  222 ,  224 ,  226  and  228  are provided. Each of the detectors  220 ,  222 ,  224 ,  226  and  228  preferably has several qualifications. First of all, it should be non-destructive; it should not destroy a significant amount of the sample, and should modify the sample as little as possible. Second, it should be of low dead volume so that it does not cause significant band broadening of analytes passing through the device. Third, it should be either heated and/or fabricated from an inert material, such that no significant sorptive (retentive) interactions occur between the analyte and the detector. A detailed explanation of the advantages of having each of the detectors  220 ,  222 ,  224 ,  226  and  228  will now be discussed. 
     A detector in the location of detector  220  of  FIG. 2  is preferably not exposed to the sample. Therefore, assuming all detectors in the system are similar and calibrated in the same way, detector  220  will provide a representative background based on the current system flow conditions and mobile phase purity. In many cases, an analog circuit can be referenced the output of detector  220  to provide a built-in baseline subtraction amplification. Also, if detector  220  is a thermal conductivity detector (TCD), which is used for gas chromatography (GC), detector  220  can be used to determine the flow rate of the carrier gas. Although, flows may vary within the system due to pressure restrictions, knowing the flow at one point can be the basis for an estimate the flow at other points in the system with a fair degree of certainty. 
     A detector in the location of detector  222  is only be exposed to the sample immediately after injection. Therefore, detector  222  can serve as a diagnostic tool for monitoring the injection characteristics such as injection plug width. Detector  222  also can perform the same duties as a detector  220  once the injection plug has passed, which provides a second diagnostic capability. Mobile phase impurities are often introduced by residual contamination of the injector  202 . Therefore, a different baseline between detector  220  and detector  222  would be a clear identifier of this situation. 
     A detector in the location of detector  224  functions similarly to one in the location of detector  222 , although in this case detector  224  is capable of diagnosing the focuser module  210  rather than the injector  202 . Again, a difference in baseline between a detector  222  and detector  224  is an indication of contamination from the focuser  210 . Also, the purpose of focuser  210  is to sharpen the peaks from the injector, so by comparing the peak shape of detector  222  and detector  224 , monitoring the performance of focuser is provided. 
     A detector in the location of detector  226  has several unique advantages. First of all, it directly monitors the output of the first column  204 . For many systems, this may be the end of the flow path, so this would be the equivalent of the traditional detector. However, for multicolumn systems or 2D GCs, this location is the entry point to the modulator  212 . In many 2D-GC systems the modulator  212  requires a large amount of power and usually adds analysis time to the cycle. By monitoring what is going into the modulator  212 , the modulation cycle can be modified or even turned off to optimize the use of power and time. In some cases, the detector  226  may provide chemical selectivity, such that it can also identify or classify analytes. In this case, the detector  226  would also provide a means of mapping the flow path of certain compounds through the system. This could potentially eliminate the need for a modulator  212 , and open up even more complex chromatographic methods such as 3D GC or as many unique dimensions are available. 
     A detector in the location of detector  228  provides diagnostics for the modulator  212  in much the same way as detectors  222  and  224  would for the injector  202  and focuser  210 . One additional function of a detector  228  is to monitor the peak shape of the modulator output, especially for the case of a selective detector that provides the ability to identify vapors. For many compounds, the output profile from the modulator  212  may be unique to the chemistry between each analyte and the inner-surfaces of the modulator  212 , and therefore may contribute additional information that will help in identifying unknown compounds. 
     Detectors can also be located on system vents. Vents are often used as part of a split injection, modulation system, or flow adjustor system. A common problem with split injections is that the split ratio may vary slightly from day to day, and therefore an internal standard is often used to estimate the split ratio. By monitoring the sample fraction that exits through the split vent, the split ratio can be determined accurately and precisely without the need to “spike” the sample with an internal standard. Also, many diagnostic functions can be performed. For example, detectors on septum purge and inlet purge lines will indicate whether the exhausts are actually venting contamination. This will indicate contamination problems quickly and allow for advanced power, gas, and time saving features (i.e. turn off the septum purge if it is not necessary). 
     Different non-destructive detectors can also be placed in series or in parallel. Some detectors may have different strengths and weaknesses, therefore multiple detectors in series or parallel may provide more analyte information. In addition, composite signals (i.e. signals from multiple detectors that are subtracted, added, multiplied, or divided with one another) may provide more direct means of measuring sample properties based on differences in detector selectivity. 
       FIG. 3  shows a simple single-dimension chromatography system with an additional detector, according to embodiments. The system of  FIG. 3  includes pressure source  300 , flowline  342 , injector  302 , detector  322 , column  304 , detector  306  and vent  308 . Also shown is data storage  312  that records and stores measurement data from detectors  322  and  306 , and processor  310  which is programmed to process data from detectors  322  and  306 . Data storage  312  and processor  310  can be part of a general purpose computer, a network of computers, or a dedicated special purpose processor and storage, depending on the particular application. Data storage  312  and processor  310  can also be either co-located with the other system elements shown in  FIG. 3 , or can be locate remotely. The measurement data from detectors  322  and  306  can be transmitted to data storage  312  directly via an I/O interface (not shown), can be sent indirectly for example via an intermediate storage system (not shown). Although a processor and data storage are not shown in FIGS.  2  and  4 - 6 , it is understood that similar facilities are provided for storing and processing data from the detectors shown in those figures. 
     The system shown in  FIG. 3  is useful for many traditional separation applications, while providing improved long-term durability by virtue of detector  322 . First, this added detector provides a measurement of the injection pulse width from injector  302 , which is useful in diagnosing injection problems as well as monitoring changes in injection with various samples (injection width is often sample dependent). Secondly, additional quantitative strategies can be employed, for example, the peak area of the injection as measured with detector  322  can be compared to the total peak area of all eluted components as seen by detector  306 . The difference, assuming that the detectors are otherwise identical, is that due to compounds that have not eluted from the column. In some cases, those components may not be of importance, and therefore this is simply a measure of column contamination and can be used to recommend column cleaning (baking, washing, etc) or replacement. For example, in the arrangement of  FIG. 3 , processor  310  is used to compare peak area data from detector  322  with the sum of data from the corresponding peak areas from detector  306  to determine how much of the sample is still retained on column  304 . If the contaminating components are known and their effects characterized, this measure of column contamination could be used to correct for changes in retention times due to the contamination acting as additional stationary phase or competitive sorption between the contamination and other analytical components. 
     In compositional analysis methods, such as used in the oil industry for equation of state modeling, the relative mass fraction of each separated component is often of interest. However, in the case of un-eluted components, an error is created for compounds of unknown concentration, which is often called the “plus fraction.” By measuring the peak area of the injection plug, and using this for the denominator in mass-fraction calculations (rather than the sum of the eluted peak areas), some “plus fraction” related errors can be avoided. For example, in the arrangement of  FIG. 3 , processor  310  is used to divide the areas of each peak detected by detector  306  by the single peak area detected by detector  322 , thereby a yielding a more accurate mass fraction calculation. Subtracting the total of peak areas detected by detector  306  from the single peak area detected by detector  322  all divided by the single peak area detected by detector  322  yields a more accurate measure of the plus-fraction. 
       FIG. 4  shows a single-dimension chromatography system with two additional detectors, according to embodiments. The system of  FIG. 4  is similar system to that of  FIG. 3 , but with an added detector  430  at the end of the system. The rest of the system includes pressure source  400 , injector  402 , detector  422 , column  404 , detector  406  and vent  408 . A preferred use of the system shown in  FIG. 4  is to use non-destructive detectors as detector  422  and detector  406 , and a selective detector as detector  430 . By comparing the measured chromatograms of detectors  406  and  430 , the identification of eluted components is improved. For example, according to an embodiment, the system in  FIG. 4  is a gas chromatograph, detectors  422  and  406  are TCD type detectors, and detector  430  is a Nitrogen Phosphorus Detector. In this embodiment, detector  406  shows a chromatogram with all eluted components being detected, while detector  430  only shows nitrogen and phosphorus containing components. By comparing these two chromatograms, the non-nitrogen and non-phosphorus components are discerned. Other detectors that could be used in this embodiment for detector  430  include a flame ionization detector (FID), electron capture device (ECD), photoionization detector (PID), ion mobility spectrometer (IMS), differential mobility spectrometer (DMS), and a mass spectrometer (MS). 
       FIG. 5  shows a 2-dimensional separation system according to embodiments. The system of  FIG. 5  includes a pressure source  500 , injector  502 , detector  522 , a first column  504 , a detector  526 , a modulator  512 , detector  528 , a second column  514 , detector  506  and vent  508 . Detector  522  is working in the same fashion as detector  322  described above in conjunction with  FIG. 3 . Detectors  526  and  528  are measuring the input and output of the modulator  512 . Detector  526  can allow for “smart modulation,” that is to only cycle the modulator when it is “loaded” with eluted components from column  504 . Doing this saves time and energy during the separation, which is especially valuable in remote system applications. Detector  528  serves a similar function to detector  522  in that it monitors the output of the modulator  512 . Subtracting total peak areas measured with detector  506  from the input pulses on detector  528  will give you a measure of what has not eluted from column  514 , just as subtracting the total peak areas from detector  526  from the injection pulses measured from detector  522  will give you a measure of what has not eluted from column  504 . Comparing detectors  528  and  526  in the same way will also give you a measure of what has not been released from the modulator  512 . 
       FIG. 6  shows an additional detector on the vent line of a split-injection system according to embodiment. The system of  FIG. 6  is a 1-dimensional separation system which includes pressure source  600 , injector  602 , detector  622 , column  604 , detector  606  and vent  608 . Also included is vent line  640  leading to detector  632 , flow restrictor and/or metering valve  634 , and vent  636 . Split-injection systems are widely used, and are plagued by slight fluctuations in split ratio which lead to quantitative errors in sample analysis. Typically an internal standard is added to the sample to alleviate as much error as possible. However, “spiking” samples with internal standard can be incredibly difficult, especially in remote system applications. The peak area of detector  632  during a sample injection will be related to the mass of sample that was vented. Detectors  622  and  606  give measures of the sample that was injected. The ratios of the total peak areas allow direct calculation of the split ratio of the injection system. This is of considerable value as it can allow the use of methods that do not contain an internal sample. 
     As mentioned above, in selecting suitable detectors there are several important considerations. The detector should be non-destructive; it should not destroy, and should modify the sample as little as possible. The detector should be of low dead volume so that it does not cause significant band broadening of analytes passing through the device. Finally, the detector should be either heated and/or fabricated from an inert material, such that no significant sorptive (retentive) interactions occur between the analyte and the detector. Several methods and techniques have been propose that could be used for the purposes described herein. For example, see: D. Cruza, J. P. Chang, S. K. Showalter, F. Gelbard, R. P. Manginell and M. G. Blain,  Sensors and Actuators , B: Chemical, Volume 121, Issue 2, 20 Feb. 2007, Pages 414-422; Chen, K., Wu, Y.-E.,  Thermal analysis and simulation of the microchannel flow in miniature thermal conductivity detectors  (2000), Sensors and Actuators, A: Physical, 79 (3), pp. 211-218; Gajda, M. A., Ahmed, H.,  Applications of thermal silicon sensors on membranes , (1995) Sensors and Actuators, A: Physical, 49 (1-2), pp. 1-9; Kimura, Mitsuteru, Manaka, Junji, Satoh, Shigemasa, Takano, Shigeki, Igarashi, Norikazu, Nagai, Kazutoshi,  Application of the air - bridge microheater to gas detection  (1995) Sensors and Actuators, B: Chemical, B25 (1-3 pt 2), pp. 857-860; Laugere, F., Lubking, G. W., Berthold, A., Bastemeijer, J., Vellekoop, M. J.,  Downscaling aspects of a conductivity detector for application in on - chip capillary electrophoresis  (2001) Sensors and Actuators, A: Physical, 92 (1-3), pp. 109-114; Simon, I., Arndt, M.,  Thermal and gas - sensing properties of a micromachined thermal conductivity sensor for the detection of hydrogen in automotive applications  (2002) Sensors and Actuators, A: Physical, 97-98, pp. 104-108; Sorge, S., Pechstein, T.,  Fully integrated thermal conductivity sensor for gas chromatography without dead volume , (1997) Sensors and Actuators, A: Physical, 63 (3), pp. 191-195; Wu, Y. E., Chen, K., Chen, C. W., Hsu, K. H.,  Fabrication and characterization of thermal conductivity detectors  ( TCDs )  of different flow channel and heater designs  (2002) Sensors and Actuators, A: Physical, 100 (1), pp. 37-45; U.S. Pat. No. 5,756,878; and U.S. Pat. No. 4,909,078, all of which are incorporated herein by reference. 
     Additionally there is at least one TCD that can currently be obtained commercially from C2V which supplies Microsystems solutions from Concept to Volume, based in the Netherlands. It has been found that the micro TCD from C2V is suitable for many of the applications described herein. 
     Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that the particular embodiments shown and described by way of illustration are in no way intended to be considered limiting. For example, while some of the embodiments described herein refer to gas chromatography, the present invention is also applicable to other types of chromatographic analysis such as liquid chromatography and supercritical fluid chromatography. Further, the invention has been described with reference to particular preferred embodiments, but variations within the spirit and scope of the invention will occur to those skilled in the art. It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to exemplary embodiments, it is understood that the words, which have been used herein, are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.