Anal. Chem. 1990, 62, 2145-2148
2145
Solid Phase Microextraction with Thermal Desorption Using Fused Silica Optical Fibers Sir: Solid phase extraction (SPE) is an effective alternative to liquid-liquid extraction (I,2). SPE involves absorbing the analyte from the sample onto a modified solid support. The analyte is then desorbed either by thermal means or by using a solvent. The primary advantage of SPE is the reduced consumption of high-purity solvents, thereby reducing laboratory costs and diminishing the need for solvent disposal. Thermal desorption of a solid phase adsorbent can eliminate solvents entirely but current methods require extensive modification of the gas chromatographic injector or the addition of a desorption module ( 3 , 4 ) . The time required in S P E methods to isolate the analyte of interest is greatly reduced when compared to classical liquid-liquid extraction methods. However, solid phase extractions often suffer from high blank values ( 5 , 6 ) there , is considerable variation between the products offered by different manufacturers, and lot to lot variation can be a problem. SPE cartridges are normally constructed from plastic, which can adsorb the analyte and increase interferences in the analysis. Solid phase microextraction (SPME) onto chemically modified fused silica fibers with thermal desorption eliminates the problems associated with S P E while retaining the advantages; solvents are completely eliminated, blanks are greatly reduced, and extraction time can be reduced to a few minutes. In this procedure a small diameter fiber coated with a stationary phase is placed in an aqueous sample. The analytes partition into the stationary phase and are then thermally desorbed, on-column, in the injector of a gas chromatograph. The technique is an extension of laser desorption from fused silica fibers (7), which provided extremely fast chromatography but did not address the time required for sample preparation. The technique is experimentally simpler than thermal desorption from solid phase packings primarily because the cylindrical surface geometry of the fibers is very well defined and less complex than the surface of S P E packings. This allows easy access of the analytes to and from the surface, which facilitates efficient extraction and desorption. The small dimensions on the fiber (100-300 pm) are convenient for on-column injections. Secondly, the method does not require complete removal of the analyte from the liquid matrix and can therefore be applied to a broader range of applications than SPE, which requires an exhaustive extraction. A wide variety of solid phases can be designed or the fibers can be used uncoated, maximizing the potential applications of this device. Possible solid phases that have been investigated in this lab include polyimide, fused silica, liquid crystal polyacrylate (8), Carbowax, cross-linked Langmuir-Blodgett layers, and graphite. In this communication the construction and performance of a SPME device that can be used to adsorb analytes from solution will be described. Applications will be discussed and possible explanations for the observed behavior will be proposed. EXPERIMENTAL SECTION
SPME Construction. The SPME consists of a fused silica fiber contained inside a Hamilton 7000 series syringe (Figure 1). The metal plunger wire assembly is removed and replaced with a fused silica optical fiber. The syringe is used to introduce the fiber into the injector of a gas chromatograph and protects the fiber from damage during storage. Prior to placing the fiber inside the plunger, a drop of 5 min epoxy is placed on the end of the fiber and allowed to harden. This holds the fiber in position in 0003-2700/90/0362-2145$02.50/0
the syringe. The length of the fiber was determined experimentally and depends on the injector of the gas chromatograph as “hot spots” may vary from instrument to instrument. For the modified, on-column injector used in this research, the optimum fiber length was 21.5 cm. Fiber Preparation. Fused silica fibers were obtained from PolyMicro Technologies, Inc., Phoenix, AZ. These were coated with polyimide and had an outer diameter of 171 f 5 wm. To use the polyimide film as a stationary phase, it had to be thermally activated by heating at 350° for 4 h. A 2-cm section of polyimide was burned off, leaving a 1-2 mm section of polyimide at the end of the fiber and the remaining 19.4 cm intact. The char was removed with gentle scraping. In all cases the polyimide was burned off after the fiber had been inserted in the syringe and trimmed to the correct length. After burning, the fiber became fragile and had to be handled carefully. The normal lifetime for a prepared fiber was 5-6 weeks with regular use. The SPME is quite inexpensive as the fiber cost is about $2 per meter. The polyimide coating was used for the analysis of light, chlorinated hydrocarbons. Uncoated fused silica was obtained by burning off the polyimide coating and gently scraping off the charred portion. A 1-cm section of fused silica was used to absorb polychlorinated biphenyls and substituted benzenes. Apparatus and Reagents. Analyses were done on a Varian 3500 gas chromatograph equipped with a modified on-column injector. The modification involved the removal of the on-column tower and left the valve in the open position. The tower assembly was replaced with an ordinary injector nut and required two septa to provide a gas-tight seal. The on-column insert was also inverted. This allowed the injector to operate either as a modified septum programmable injector (SPI) or as an on-column injector. For fiber use, an on-column injection technique was used, where the fiber descends into the column in the bottom 2 cm of the injector. For a syringe injection the syringe needle is above the restriction in the insert. All analyses were done with a 30 m X 0.25 mm fused silica column with 0.25-pm DB-5 coating and an electron capture detector. conditions for the individual components are shown on the chromatograms. In all cases, the fiber was desorbed in an isothermal injector. RESULTS AND DISCUSSION The SPME does not require a sophisticated coating system to be a useful technique. Either the uncoated, fused silica or the polyimide fiim that the optical fibers are shipped with can be a suitable stationary phase. The method was initially tested by using these two coatings. A typical analysis would involve exposing the fiber to a stirred solution for 1-2 min. The fiber would be removed and immediately transferred to the injector of the gas chromatograph for thermal desorption. The fiber was left inside the injector of the gas chromatograph for the duration of the analysis. Volatile Chlorinated Organic Chemicals in Water. The first application used a fiber coated with a 1 mm wide strip of polyimide that was exposed to a solution of l,l,l-trichloroethane (TCEA), trichloroethene (TCEY), and perchloroethylene (PCE). These chemicals are common groundwater contaminants (9). The polyimide had to be thermally treated at 350 “C to obtain a good response. A chromatogram of a typical fiber desorption is shown in Figure 2. Some tailing does occur when compared to a syringe injection. The tailing is thought to be caused by desorption from within the bulk of the film, which would be slower than the initial, fast desorption of layers adsorbed onto the surface of the film. When no film is used, tailing decreases (Figure 5). 0 1990 American Chemical Society
2146
ANALYTICAL CHEMISTRY. VOL. 62, NO. 19. OCTOBER 1. 1990
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spectively. Other conditions were as follows: He carrier. 29 cmls: N, make-up, 24 mllmin: injector. 275 'C: oven, 60 OC isothermal; detector. 325 "C. 3oooooo~
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