Abstract:
The present invention is directed to an enhanced sampling device, herein referred to as an ESD, for enhancing the collection efficiency of the SPME method by enhancing the flow of the analytes onto the sampling fiber. The ESD includes a tubular main body, used for a sampling shroud, which directs a flow of analytes to contact the fiber during collection. One end of the main body is open and faces the sample, allowing analytes to flow into the ESD and contact the fiber. A second piece of tubing branches from the other end of the main body and becomes an outlet port, possibly leading to a pump. The ESD permits more rapid transport and absorption of the analytes to the fiber for collection.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application relates to U.S. provisional application No. 60/854,126, filed on Oct. 25, 2006, which is incorporated herein by reference in its entirety. 
     
    
     STATEMENT OF GOVERNMENTAL INTEREST 
       [0002]    This invention was made with Government support under Department of the Navy contract N00024-03-D-6606. The Government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates to a device for enhancing the collection efficiency of the Solid Phase Microextraction (SPME) method of sample concentration, collection and injection for analysis by gas chromatography or liquid chromatography. More specifically, it relates to a device for enhancing the collection efficiency of the SPME method, which enhances the flow of the analytes onto the sampling fiber. 
         [0005]    2. Description of the Related Art 
         [0006]    Historically, the chief method of analyzing trace chemicals, also known as “analytes”, was to employ a syringe to collect a sample of the analyte and then inject it into a gas chromatograph (GC) or liquid chromatograph (LC). This resulted in the capture and injection of only small quantities of the analytes and thus yielded poor sensitivity. It was discovered that if large quantities of vapor or liquid analyte were drawn through a treated filter the components of interest would be concentrated on the filter paper. Solvents were used to selectively remove the chemicals of interest from the filter and a small portion of the solvent containing the traces was then injected into the GC or LC. Concentration of the sample via a filter was an improvement in sensitivity over straight analyte injection, but it added many processing steps to the analysis, which could increase errors. 
         [0007]    It was further discovered that gas samples could be drawn through tubing containing packing material, such as Tenax, to concentrate the sample on the packing. After sampling, the tubing was connected to the sampling inlet of the GC. If the packing was rapidly heated it would drive off the trace chemicals into the GC for analysis. This process was simpler than the filter process and yielded similar sample concentration enhancements as the filter paper method without the solvent extraction problems. However, this process was only compatible with GC techniques and not LC techniques. 
         [0008]    Certain materials selectively adsorb and absorb materials based upon their chemical properties. For instance, silica gel will absorb water and then release it when heat is applied. Other extraction materials will release to certain solvents. Solid phase extraction (SPE) is a process which may employ selective adsorbents and solvents to concentrate the components of interest within the sample and selectively remove components that may interfere with later analysis. SPE can be a known effective alternative to liquid-to-liquid extraction in the analysis of aqueous samples. SPE reduces the consumption of high purity solvents and also reduces the time required to isolate the analytes of interest. However, the fact that SPE continues to use solvents has a number of disadvantages, notable among them being the need to take extensive precautions to guard against interference of the solvent with the analytes in the analyses. 
         [0009]    Solid phase microextraction (SPME) is a similar process which allows the concentration of volatile or nonvolatile compounds from liquid samples or from headspace gas without the complicated apparatus and solvents of SPE. SPME may use a fiber mounted within a hollow needle sheath of a syringe. The fiber acts as a sponge which captures and concentrates the analytes. The fiber can then be inserted into the heated injector of a GC where the analytes may be thermally desorbed from the fiber and made available for analysis by the GC. SPME can achieve detection limits down to the parts-per-trillion range for a wide variety of compounds. 
         [0010]      FIG. 1  is a cross-sectional view of one SPME device  10  in a retracted position and adjacent to a sample vessel  20 . A thin fiber  30  may be coated with a substance that can enhance the selective absorption of the analytes, typically based upon the polarity of the molecules. Fiber  30  is attached to one end of a rod  40 , with the other end of rod  40  being attached to plunger  50 . Fiber  30  may be positioned inside of the needle sheath  60 . Needle sheath  60  may be attached to cylinder  70 . In the retracted position, cylinder  70  contains plunger  50  and partially contains rod  40 , allowing them to potentially move longitudinally therein as a force is applied to plunger  50  along its main axis. SPME holder  80  is generally cylindrical in shape and includes a septum  90  at one end and a rod support and seal  91  at the other end. Needle sheath  60  containing fiber  30  is positioned inside SPME holder  80  and adjacent to septum  90 . Sample vessel  20  may contain the material to be sampled, solid, liquid or gas, and may include a septum  100 . 
         [0011]    In operation, septum  90  of SPME holder  80  may be placed adjacent to septum  100  of sample vessel  20 . Cylinder  70  can be depressed, thus injecting needle sheath  60  into sample vessel  20 , as shown in  FIG. 2 . Next, plunger  50  is depressed which may extend fiber  30  through the free end of needle sheath  60 , and thus may expose fiber  30  to the liquid or headspace gas within the vessel, as shown in  FIG. 3 . In this manner, fiber  30  is likely not damaged by unwanted contact. The analytes diffuse onto the surface of fiber  30  where they can be absorbed. Once enough time has elapsed to allow sufficient sample to be captured by fiber  30 , fiber  30  is retracted into needle sheath  60  by retracting plunger  50  and needle sheath  60  is retracted via cylinder  70 . Having the fiber containing the analyte sealed behind septum  90  prevents contamination of the fiber and may prevent escape of the analyte. Using a similar process, fiber  30  can be then be inserted into the GC sample inlet (not shown). The analytes are then removed from fiber  30  by thermal desorption and injected into a GC or LC for analysis. Since fiber  30  can be protected by septum  90  while retracted inside of SPME holder  80 , this process protects fiber  30  while transporting field samples to a laboratory for the most detailed analyses. 
         [0012]    While the SPME process accomplishes its intended purposes of concentrating and collecting materials of interest, it suffers from a number of drawbacks. First, the SPME process is dependent upon diffusion to transport the analytes from solid or liquid on the bottom of the vessel to the fiber for collection. Since the time for diffusion and absorption of the analytes depends on many factors (e.g. the analytes themselves, as well as the type and thickness of any coating on the fiber), this process can take hours and even days to accomplish. Such long sampling times have been problematic because adsorption is a bi-directional process and, therefore, with a long sampling time, the analyte being adsorbed also begins to desorb from the fiber resulting in a non-linear response and making quantitization difficult. Therefore, techniques have been developed to reduce the absorption time. 
         [0013]    For example, it is known that stirring and/or forceful agitation or heat may be applied to the sample vessel to reduce the absorption time. Stirring can be accomplished through the placement of a magnetic bar within the analyte and the use of a standard magnetic stirrer. Another method of agitation is to apply ultrasonic vibrations to the vial. However, agitation techniques can lead to damage to sample vials, damage to the fiber and also to damage to mechanical and electrical parts. Moreover, stirring techniques, whether they are magnetic, ultrasound or other methods, often increase the risk of contamination. Also, heat applied, either directly or as a side-effect of forceful agitation, leads to a rise in the temperature of the sample. Since adsorption efficiency is temperature-dependent, this introduces an unwanted variable into the analysis. SPME can be very effective in allowing the identification of trace compounds due to its ability to selectively concentrate the analyte. However, there are so many variable affecting the capture efficiency of the SPME that absolute quantitative analysis can be very difficult. 
         [0014]    In the prior art, improvements on the SPME device have been proposed. For example, U.S. Pat. No. 7,131,341 to Wareham and Persaud discloses an instrument into which an SPME fiber and needle sheath is inserted. The holder/case contains a motor to present the fiber to the sample and retract the fiber into the needle sheath. This patent indicates that once the sample is on the fiber, the SPME can be either exchanged for a fresh one or immediately analyzed in the Wareham hosing. This patent is focused upon on-the-spot analysis of odor samples to determine if mold is within a wall or fungus samples to determine tree rot. This application is distinctly different than that of taking the SPME sample back to a lab for a more detailed analysis. First, if the SPME is removed from its housing for later analysis, as it is no longer sealed, it will likely leach sample and gather contamination. Though this patent discusses a tagging method, this tagging method appears incompatible with standard SPME holders, and thus leaves unaddressed a desirable next step of injecting the SPME into the laboratory analyzer for high resolution analysis. Further, this approach does not allow traceability of the analysis results to the quantity of air processed. 
         [0015]    In order to overcome these problems, what is needed is a device to enhance the efficiency of the SPME process, the device permitting more rapid transport and absorption of the analytes to the fiber for collection. Further, the device must allow injection of the SPME into a full-scale laboratory analyzer. These features thus address and solve problems associated with conventional SPME systems. 
       SUMMARY OF THE INVENTION 
       [0016]    The present invention is directed to an enhanced sampling device, herein referred to as an ESD, for enhancing the collection efficiency of the SPME method by enhancing the flow of the analytes onto the sampling fiber. The ESD includes a tubular main body, which may be used for a sampling shroud which directs a flow of analytes to contact the fiber during collection. One end of the main body can be open and may face the sample, allowing analytes to flow into the ESD and contact the fiber. A second piece of tubing may branch from the other end of the main body and can become an outlet port, possibly leading to a pump. This application describes several embodiments of the ESD. The ESD permits more rapid transport and absorption of the analytes to the fiber for collection by potentially passing the gas carrying the analyte coaxially over all surfaces of the SPME fiber in a closely confined area so that the majority of the analyte comes into intimate contact with the fiber where it may be captured for later analysis. 
         [0017]    It is an object of the invention disclosed herein to provide a new and improved device for the SPME technique, which provides novel utility and flexibility through the use of a unique design which permits more rapid transport and absorption of analytes from solid, liquid or gas to the fiber for collection. 
         [0018]    It is another object of the invention disclosed herein to provide a new and improved device for the SPME technique, which provides novel utility and flexibility through the use of a unique design which permits efficient collection of sample from a sample vessel or from ambient fluids, and in particular gases. 
         [0019]    It is an advantage of the invention disclosed herein to provide a new and improved device for the SPME technique, which does not agitate or stir the sample. 
         [0020]    It is a further advantage of the invention disclosed herein to provide a new and improved device for the SPME technique, which does not add additional heat to the sample. 
         [0021]    It is a further advantage of the invention that it constantly passes fresh gaseous sample containing the analyte in close proximity to the SPME fiber and that the flow rate and time of the flow can be easily measured which permits quantitation such that the results of the analysis can specify the quantity of analyte per volume of gas (i.e. ppm/liter). 
         [0022]    It is a further advantage of the invention that the required sampling time is reduced by more than 50%. 
         [0023]    It is a further advantage of the invention that due to the decrease in sampling time the linearity of the SPME technique is improved. 
         [0024]    These and other objects and advantages of the present invention will be fully apparent from the following description, when taken in connection with the annexed drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0025]    The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
           [0026]      FIG. 1  is a cross-sectional view of a conventional SMPE device in a retracted position; 
           [0027]      FIG. 2  is a cross-sectional view of a conventional SMPE in a partially extended position; 
           [0028]      FIG. 3  is a cross-sectional view of a conventional SMPE in a fully extended position; 
           [0029]      FIG. 4  is a cross sectional view of a first embodiment of an ESD for a SPME according to the principles of the present application; 
           [0030]      FIG. 5  is another cross sectional view of the first embodiment of an ESD for a SPME after insertion of a needle sheath according to the principles of the present application; 
           [0031]      FIG. 6 , consisting of  FIGS. 6A ,  6 B,  6 C, and  6 D, is a cross sectional view of a second embodiment of an ESD for a SPME according to the principles of the present application ( FIG. 6A ) and various embodiments of an alignment cone therefor ( FIGS. 6B-6D ); 
           [0032]      FIG. 7  is a cross sectional view of a third embodiment of an ESD for a SPME according to the principles of the present application; and 
           [0033]      FIG. 8  is a cross sectional view of a fourth embodiment of an enhanced sampling device for a SPME according to the principles of the present application. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0034]    Reference will now be made in detail to the exemplary embodiments of the present application, examples of which are illustrated in the accompanying drawings. It should be noted that both absorption and adsorption can occur on the sampling fiber. One process will predominate over the other depending upon the type of material on the fiber and the type of chemical being sampled. 
         [0035]      FIG. 4  shows a cross-sectional view of a first embodiment of an ESD according to the principles of the present application.  FIG. 4  shows the relative position of SPME  10  and ESD  200  prior to insertion of needle sheath  60  into ESD  200 , while SPME  10  is in a fully retracted position. In this embodiment, ESD  200  is a stand alone device, and is not attached to SPME  10 . ESD  200  includes a main body  210 , in a generally tubular shape and having two ends. The first end  220  is open and serves as a sample inlet, permitting sample to flow into main body  210 . Note that in  FIG. 4  sample inlet  220  is shown having a shape cut at an angle of approximately  45  degrees to the body axis of main body  210 . This particular shape is for illustrative purposes only and is not intended to limit the scope of this application as there are a wide variety of shapes which may serve the purpose of a sample inlet. The second end  230  of main body  210  may be open, or alternatively, may be sealed with a septum  240 , located at the second end  230  or recessed into the main body. Again, note that the second end  230  of main body  210  is shown with a funnel shape, though this particular shape is not intended to limit the scope of this application, and for this first embodiment a wide variety of shapes are possible. Main body  210  has an internal diameter larger than that of needle sheath  60 . An outlet port  250  is located toward the second end of main body  210  and can be connected to a pump (not shown). 
         [0036]      FIG. 5  shows the relative position of SPME  10  and ESD  200  after insertion of needle sheath  60  into ESD  200 , with SPME  10  in a fully extended position. Needle sheath  60  has been inserted through septum  90  of SPME  10  and also through (optional) septum  240  of ESD  200 , and fiber  30  has been extended through the opening of needle sheath  60 . In this way, sample may flow into inlet  220 , passing through the main body  210  of ESD  200 , in full coaxial contact with fiber  30 , and exit ESD  200  via outlet port  250  which can be connected to a pump. 
         [0037]      FIG. 6A  shows ESD  200  in a second embodiment. In this embodiment, ESD  200  has a funnel-shaped second end  230  and further includes a ribbed alignment cone  260  which attaches to the front of SPME  10  or is manufactured as an integral part of SPME holder  10 . Note that this embodiment does not include septum  240  which is optional in the first embodiment. Alignment cone  260 , shown in detail in a top view in  FIG. 6B , and in a side view in  FIG. 6C , has ribs  261  partway down the outermost edge so that the flow can reach and be vented via outlet port  250  which can be connected to a pump. Alignment cone  260  is solid at the wide end  262  to give a gas tight seal with the funnel shaped end  230  of main body  210 . The center hole  264  in the alignment cone  260  holds the SPME needle sheath  60  concentric with the ESD body  210 . Since the internal diameter of the main body  210  is very small, alignment of SPME needle sheath  60  concentric with the main body  210  can be difficult. The addition of alignment cone  260  makes the alignment easy to accomplish.  FIG. 6C  shows one embodiment of a side view of the alignment cone  260 , showing the ribs  261 , the surface  262  which seals with the ESD cone  230 , the through hole  264  through which the needle sheath passes, and the rear surface  263  which may be attached over the SPME septum  90 .  FIG. 6D  shows one embodiment of the flat rear surface  263  of the cone which attaches over the SPME septum. Attachment of the cone  260  to the SPME holder  10  can be done with any adhesive compatible with the materials. Another embodiment may use a cone  260  machined such that it is an integral part of the SPME holder  10 . 
         [0038]      FIG. 7  shows a third embodiment in which ESD  200  is permanently attached to the front of SPME device  10  as an integrated unit. An alignment cone  260  may optionally be a part of that attachment to insure the proper coaxial alignment of the needle sheath  60  and the ESD body  210  or the ESD  200  may be directly attached to the end of the SPME device  10 . The attachment may be accomplished through the use of an adhesive or through other means. In this embodiment ESD  200  may be injected in a sample vessel simultaneously with injection of SPME  10 . 
         [0039]      FIG. 8  shows a fourth embodiment in which ESD  200  is manufactured integrally to SPME  10 . This is accomplished by attaching one end of an outlet port  300  into the side of the body  80  of the SPME  10 . The junction of the SPME needle sheath  60  and its rod  70  may be modified to add a vent  310  which may allow gas entering the needle sheath  60  to vent into the sealed internal volume  92  of the SPME and out the vent port  300 . In this embodiment the SPME fiber  30  is not extended out of the end of the SPME needle sheath  60 . Gaseous or liquid sample is purged through the needle sheath  60  and out of the vent  300  for a metered volume. In this embodiment the SPME needle sheath  60  performs the function of the ESD body  210 . 
         [0040]    In order to use the present invention in its first embodiment as shown in  FIGS. 4 and 5 , an operator inserts ESD  200  into sample vessel  20  by piercing septum  100  with first open end  220 . The operator then inserts SPME  10  into ESD  200  by aligning needle sheath  60  with main body  210  and depressing cylinder  70 , thus injecting needle sheath  60  through optional ESD septum  240  and into main body  210 , as shown in  FIG. 5 . Next, the operator depresses plunger  50  which extends fiber  30  through the free end of needle sheath  60  into main body  210  in proximity to open end  220 , thus exposing fiber  30  to the liquid or headspace gas entering main body  210 . Alternatively, an automatic insertion device may be used to insert either or both ESD  200  into sample vessel  20  and/or SPME  10  into ESD  200 . Alternatively, in another embodiment the ESD  200  can be used with the SPME  10  to sample ambient air without the use of a container. 
         [0041]    In order to use the present invention in its second embodiment as shown in  FIG. 6 , the operator may align and insert funnel-shaped end  230  with alignment cone  260  mounted on front end of SPME  10 . At this point, the operator inserts ESD  200  into sample vessel  20 . Note that this may be accomplished in one embodiment manually or in another embodiment through the use of an automatic insertion device. The operator then depresses cylinder  70 , thus injecting needle sheath  60  through septum  90  and optional ESD septum  240  and into main body  210 . Next, in the current embodiment, the operator depresses plunger  50  which extends fiber  30  through the free end of needle sheath  60  into main body  210  in proximity to open end  220 , thus exposing fiber  30  to the liquid or headspace gas entering main body  210 . 
         [0042]    In order to use the present invention in its third embodiment as shown in  FIG. 7 , the operator may insert ESD  200  into sample vessel  20 . Note that this may be accomplished either manually or through the use of an automatic insertion device. The operator then depresses cylinder  70 , thus injecting needle sheath  60  through septum  90  and optional ESD septum  240  and into main body  210 . Next, in the current embodiment, the operator depresses plunger  50  which extends fiber  30  through the free end of needle sheath  60  into main body  210  in proximity to open end  200 , thus exposing fiber  30  to the liquid or headspace gas entering main body  210 . 
         [0043]    In order to use the present invention in its fourth embodiment as shown in  FIG. 8 , the operator may position the modified ESD  10  containing the integral vents  300  and  310  over the sample vessel  20 . Note that this may be accomplished in one embodiment manually but in another embodiment it may be accomplished through the use of an automatic insertion device. The operator may then depresses cylinder  70 , thus injecting needle sheath  60  through septum  90  and into the sample vessel and allowing gaseous sample containing the analyte to flow over fiber  30 . 
         [0044]    A further application of the ESD includes the sampling of air in proximity to a pipe. The Environmental Protection Agency (EPA) requires that chemical processing plants test all plumbing joints for possible trace leaks, called fugitive emissions. Depending upon the chemical in question, one might wipe the pipe fitting with a treated filter paper for later analysis. One might put a bag over a valve, which contains a dozen possible leak interfaces, and then take an air sample from the bag for a gross leak measurement. One might use a portable analyzer which contains a tube full of chemicals which change color with exposure and a pump to pull air samples through the tube. Since the EPA requires testing of the perimeter of each flange, each bolt perimeter, the valve stem, and any packing nuts around the stem, the ESD may be placed next to an interface (fitting) and retain a sample on the SPME. The ESD/SPME would be easier and more quantitative than the other methods of the prior art. 
         [0045]    Further applications include sampling process streams such as duct work for the central air supply of a building, or a line sampling the air in proximity to luggage to detect volatile chemicals. In all of these applications the rate of gaseous flow through the ESD  200  and the time of flow may be used to calculate the total volume of gas passing in close proximity to the SPME fiber. This then allows quantitative reporting of the results in parts of analyte per volume of gas. 
         [0046]    It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.