1. Field of the Invention
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.
2. Description of the Related Art
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.