Fused Silica Fibers in the Extraction of Organics from Water Matrix Samples and their Rapid Transfer to Capillary Columns.

Transcription

1 Robert P. Belardi and Janusz B. Pawliszynl The Application of Chemically Modified Fused Silica Fibers in the Extraction of Organics from Water Matrix Samples and their Rapid Transfer to Capillary Columns Abstract A technique for water sample analysis is presented which uses chemically modified fused silica optical fibers as micro solid phase extractors. The small size of the fibers (100 pm 0.d.) allows for direct introduction into the on column injector port of a high resolution gas chromatograph (GC), where the analytes are thermally desorbed. This eliminates the need for solvents and syringes which are used in liquid-liquid or solid phase extraction techniques. Thus, this method lowers cost and analysis time per sample, as well as eliminates possible sources of error. Results show that the efficiency and selectivity of this technique are dependent upon the thickness and polarity of the stationary phase. Initial results indicate that the limit of detection for 2-naphthol and FID detection is approximately 5 ng/g with a linear response range of 5 orders of magnitude. Introduction The ability to analyze organics in water matrix samples in the shortest period of time and in the most cost efficient manner is a key concern to any environmental analytical chemist. Currently, the method of liquid-liquid extraction is the most popular form of analysis of such samples. However, this procedure is time consuming and very expensive requiring high purity solvents. In addition, this method is labour intensive and very difficult to automate. * Department of Chemistry and Waterloo Centre for Groundwater Research, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1

2 180 R. P. BELARDI AND J. B. PAWLISZYN Recently, the techniques of solid phase extraction (SPE) have been developed (Rohm and Haas 1975,1978; Saner et al. 1979; Chalk and Merritt 1973; Ryan and Fritz 1978; Junk and Richard 1988) which improve the analysis time per sample and allows for more selective analysis. The method employs disposable plastic cartridges packed with materials which are commonly used in high performance liquid chromatography (HPLC) columns. Various types of chemical moieties constituting the stationary phase are covalently bonded to high surface area silica particles. The type of stationary phase gives SPE its selective properties. The procedure of SPE involves passing volumes of the sample water through the cartridge. The selectivity of the absorbent facilitates the retention of the specific analytes while allowing the water to flow through. Although this approach is a significant improvement over liquid-liquid extraction, by shortening time and cost, the isolated analytes must still be recovered via solvent elution and introduced into a capillary column by a syringe. Below, a new microscaled approach to solid phase extraction is described. This method employs chemically modified fused silica fibers (MFSF) as solid phase extractors. The unique feature of these fibers, due to their small size (ranging from 0.05 or 1.0 mm), is the ability to place them directly into a water sample (Figure 1) and then directly into the capillary column. This method of sample introduction eliminates the need for solvents and syringes which are possible sources of errors. In addition, the elimination of the sample preparation steps will reduce the sampling time, thus increasing time and cost efficiency. Furthermore, there is also a greater potential for automation using this technique. The fused silica fibers are modified by either chemically bonding or polymerizing an organic layer to the tip of the fiber. The analytes to be extracted from the polar aqueous solution are absorbed into the less polar stationary phase. However, the technique is not a complete extraction technique, instead it is based on the equilibrium that is formed between the concentration of the compound in the organic layer and the concentration of the compound in the aqueous medium based on the analytes partitioning coefficient. Therefore, the analysis is independent of the sample size, which eliminates another possible source of error (Pawliszyn ACS Conference, Toronto, Canada 1988). Experimental Preparation of the Fibers The step index fused silica fibers, 100 pm 0.d. (Newport Corpo-

3 Figure 1. Chemically modified fused silica fibers are placed in the water sample for the extraction of trace organics. ration), were all treated by etching 1 cm of the tip of the fiber with concentrated HF (except the PMVCS fiber) for 15 minutes followed by successive rinsing with methylene chloride, acetone and methanol. Once the fibers were rinsed and air dried, they were ready to be coated. Application of Stationary Phase 1) Carbowax 1540: (Wilkens Instrument and Research Inc.) The etched tip of the fiber was immersed in 10 ml of a 10% solution of Carbowax in chloroform for 30 minutes. The fiber was then placed in a curing vessel at 400 C under a continuous flow of N2. After baking the fiber for 4 hours, the fiber was again washed with methylene chloride, acetone, and methanol. 2) Octadecyltrichlorosilane (ODs): (Pierce Chemical Company) The same procedure was followed as in the Carbowax fiber. 3) Polymethylvinylchlorosilane (PMVCS): (Petrarch Systems Inc.) In order to remove acidic traces present on the fused silica surface,

4 182 R. P. BELARDI AND J. B. PAWLISZYN the untreated fiber was purged for 30 minutes under He. The fiber tip was then placed in 10 ml of 5% PMVCS solution in hexane and 1% (of polymer) dicumyl peroxide. A stream of N2 gas was applied to the solution in order to statically coat the fiber. After 5 minutes of purging the fiber with N2, it was placed in the curing vessel for 2 hours at 240 C. 4) Liquid Crystalline Polyacrylates (LCPA): Same procedure was followed as the Carbowax fiber. All the fibers were stored under N2 or He when not in use in order to prevent absorption of volatile organics presence in the air. Injection and Operating Procedure 2-Naphthol (Fisher Scientific Company) was chosen as the compound to be studied due to its solubility in water. All the water used was distilled, deionized water. A Varian 3500 GC equipped with a modified "on column" capillary injector, FID detector and Nz carrier gas, was used for the analysis. Unlike typical on column injection techniques, where the syringe is removed after the sample is dispensed, the fiber must remain in the injector during the course of the run in order to desorb the organics. Therefore, the valve which is usually closed after the injection is made, must remain open, so as not to sever the fiber. Hence, an additional septum was inserted (Figure 2). To pass the fiber through the septum without breaking it, a metal sheath, which surrounded the fiber, was used to penetrate the septum. When the metal sheath was pushed through the septum (Figure 2), the fiber tip was lowered until it reached the injector oven. The metal sheath was then removed, allowing the septum to close around the fiber, resulting in a closed system. The chromatographic runs were temperature programmed from 50 C to 176OC at 20 C/min and held for 15 minutes. The injector was also programmed from 50 C to 176OC at 20 C/min and held for 5 minutes. The chromatograms were recorded on a Spectra Physics 4290 integrator. Results and Discussion Four different organic layers were used to explore the properties and limitations of this new and unique technique. They

5 Hotel Shrsth Closr Figure 2. A cross-section of the modified Varian "on column" capillary injector, showing the presence of the added septum and the silica fiber. were: 1) Carbowax; 2) Octadecyletrichlorosilane (ODs); 3) Polymethylvinylchlorosilane (PMVCS); and, 4) Liquid Crystalline Polyacrylate (LCPA) (Figure 3). Carbowax is a relatively polar stationary phase, due to its repeating unit of ethylene glycol. It is a relatively common stationary phase which is readily available. The ODs stationary phase, which is bonded to the fiber via the silicon site, has been shown to be relatively stable with little or no mobility when chemically bonded to borosilicate columns (Folstead et al. 1987). The long hydrophobic 18 carbon chain in the ODs structure should result in a more efficient extraction ability than the Carbowax fiber. Although PMVCS does not have as long a carbon chain as ODs, it is still a relatively non-polar phase. The appealing feature of PMVCS is that not only can it be chemically bonded to the fused silica glass via the chlorine sites, but its vinyl group will lead to polymerization. Therefore, it should be a very immobile stationary phase. The unique feature of the LCPA fiber is that it polymerizes into definitive linear planes which are capable of more effectively

6 184 R. P. BELARDI AND J. B. PAWLISZYN Liquid Crystalline Polyacrylate (LCPA) Figure 3. Various stationary phases applied to the fused silica fibers. absorbing linear molecules, while more bulkier compounds are less apt to be absorbed. This provides a unique selective property to the LCPA phase (Jadhav et al. 1987). In addition, the presence of phenyl groups in the repeating unit of the LCPA should prove to be more effective in attracting aromatic compounds which are regularly analyzed in groundwater. All of the stationary phases investigated in this report have been previously evaluated as effective stationary phases in coating GC capillary columns with little or no mobility (Folstead et al. 1987; Jadhav et al. 1987; Jorgensen 1983). This, however, could not be taken for granted in this technique due to the fact that the fibers were exposed to a harsher environment. This proved to be the case for the fiber with the ODS layer. Even at temperatures as low as 70 C, bleeding from the fiber was apparent, possibly due to the stationary phase's poor ability to bond to the fused silica surface. The remaining three types of coatings performed well with little bleeding up to temperatures of 200 C. A study of the extraction efficiency versus sampling time of various types of fibers was conducted. Figure 4 compares the extraction efficiency of the LCPA coating with the Carbowax coating for a 100 pg/g 2-naphthol solution. As illustrated in Figure 4, the amount of

7 - LCPA Figure 4. Sampling time vs. peak area study of 100 pg/g 2-naphthol solution. The Carbowax fiber achieved extraction equilibrium in 5 minutes while equilibrium occurred in 15 seconds for LCPA fiber. the analyte absorbed by both stationary phases increases with the increase in extraction time. This trend continues until the point of saturation is achieved which is observed by the level region of the curve in Figure 4. This saturation point signifies a state of equilibrium between the concentration of the analyte in the stationary phase and in the aqueous medium, which is dependent upon their relative distribution coefficients. The time for equilibrium to occur differed significantly between the two types of stationary phases. The LCPA fiber reached equilibrium in 15 seconds while 5 minutes were required for the Carbowax layer to stabilize. The difference in times for equilibrium to occur between the two stationary phases are due to the thickness of each layer. The LCPA fiber stabilized very quickly, this is the result of rapid penetration of the analyte into the stationary as a result of the thinness of the LCPA layer. On the other hand, with the thicker layer of the Carbowax, the rate of penetration of the analyte is significantly reduced, which translates in an increase

8 in time for the state of equilibrium to occur. Assuming the diffusion constants (D) for 2-naphthol in Carbowax and LCPA are equal, a ratio, Equation (1) (Carslaw and Jaeger 1986) between the times of equilibrium can be utilized to approximately determine the relative thickness of each corresponding polymer layer. Where t is the time of equilibration; d is the thickness of the stationary phase; C is the Carbowax and L is LCPA. This equation estimates the Carbowax layer is 4.5 times as thick as the LCPA layer. These observations demonstrate that the thinner coatings of the stationary phase, the time for optimum extraction efficiency to occur will be less. Therefore, minimizing the analysis time per sample. In addition, Figure 4 reveals the selectivity that can be achieved with various stationary phases. This is illustrated by assuming the partitioning coefficients (A') between both stationary phases and the aqueous phase are equal for 2-naphthol. If this is true, the amount of 2-naphthol extracted by the Carbowax fiber would be 4.5 times greater than that of LCPA due to the differences in thickness. In fact, however, the LCPA fiber absorbed slightly more 2-naphthol than the Carbowax fiber. This clearly indicates that the partitioning coefficients of the two stationary phases differ, with LCPA having a greater affinity (K value) than Carbowax for 2-naphthol. Figure 5 illustrates again the optimum extraction time's dependence on the thickness of the stationary phase. It compares the amount 2-naphthol extracted after a 2 minute sample time. The extraction ratio between the Carbowax layer (A) and the LCPA layer (B) is 1 to 5 respectively. A series of identical water samples containing 100 pg/g of 2- naphthol were analysed using the MFSF technique (in this case the LCPA fiber was used) in order to determine its reproducibility. The following results were obtained. Number of Samples 6 Mean Peak Area 9836 units2 Maximum Peak Area units2 Minimum Peak Area 6932 units2 Standard Deviation (S) 1951 (18.1%) Confidence Interval (95%) f 2047 (20.8%) During the analysis of this set of water samples it was observed that the peak area decreased slightly after each run. This suggests that some of the stationary phase was being desorbed, causing a

9 Figure 5.The chromatographic peaks corresponding to Znaphthol (marked by the arrow) after 2 minutes sampling, using two different fibers: A) Carbowax fiber; B) LCPA fiber. The ratio of their extraction efficiency after a sampling time of 2 minutes is 1 to 5 respectively. decrease in the stationary phase layer, and thus less absortion. However, with more stable stationary phases and improvements in the coating procedures this variance should decrease significantly. The concentration curve in Figure 6 illustrates the linear dynamic range of 5 orders of magnitude, from 20 ng/g to 100 pg/g of 2-naphthol. These preliminary findings indicate that concentration curves for the presence of organics in water samples can be established. More importantly, these findings reveal that the limit of detection for this technique is approximately 5 ng/g for 2-naphthol. However, with less polar samples, such as naphthalene, it is believed that this detection limit may be significantly lower.

10 188 R. P. BELARDI AND J. B. PAWLISZYN Figure 6. The calibration curve for the LCPA fiber using a 2 minute sampling time. An effluent sample taken from a waste water treatment plant was extracted using a modified fused silica fiber and analyzed by GC/FID. Its chromatogram, Figure 7a, was compared with that of a liquid-liquid extraction and on column injection procedure, Figure 7b. As illustrated in Figure 7, the chromatogram corresponding to liquid-liquid isolation technique produced a very large solvent peak which can mask compounds with similar retention times. However, with the silica fiber the use of the solvent is eliminated, therefore removing the presence of this possible interference. In addition, the total time of analysis was substantially decreased with the MFSF method due to the elimination of various additional steps involved in the liquid-liquid extraction procedure, such as mixing of the two liquid phases and the solvent evaporation step. Recommendations and Conclusions More work is needed to make the MFSF technique a ready-touse and reliable method for the analysis of water samples. Methods of improving the coating procedure for the various fibers, such that

11 Figure 7. A comparison between the chromatograms using the PMVCS fused silica fiber as the extractor (a) and using a common solid phase extractor (b) of a waste water treatment plant water sample.

12 190 R. P. BELARDI AND J. B. PAWLISZYN bleeding of the organic layer is minimized, are currently underway. These involve stricter controls in the flowrate of the inert gas during the coating procedure, which would also greatly improve the extraction consistency from fiber to fiber. The conditioning of the fibers should be carefully controlled to ensure that the organic layers are not oxidized if the temperature is too high. In addition, new designs for a fiber holding device which will encase the entire fiber as it is guided all the way to the injection oven are currently under development. This will minimize any chance of the absorbed organics inadvertently sticking to the side of the walls of the injector as the fiber travels down to the injector oven and minimize breakage of the fiber during storage, sample extraction and introduction. The MFSF extraction/introduction technique not only decreases the analysis time, but it can also decrease cost of analysis per Sample. Furthermore, the MFSF extraction/introduction method can be easily automated. In addition, this unique procedure can be used for the analysis of volatile components in air samples, as well as to study the kinetics of liquid-liquid and liquid-solid equilibria. Acknowledgement Dr. K.P. Naikwadi and Dr. F. Onuska provided helpful advice during the course of the research. Shi Lui performed analysis of the waste water treatment plant sample. This work was partially supported by a grant from Imperial Oil. Key words: Fused Silica Fibers, Solid Phase Extraction, Water Matrix, Rapid Sample Isolation. References Carslaw, H.S., J.C. Jaeger (1986). Conduction of Heat in Solids 2nd Ed., Clarendon Press, Oxford, 510 p. Folestead, S. B. Josefsson, M. Larsson (1987), "Performance and Preparation of Immobilized Polysiloxane Stationary Phases in 5-55 pm I.D. Open-Tubular Fused Silica Columns for Liquid Chromatography". Journal of Chromatography, 391, Jadhav, A.L., K.P. Naikwadi, S. Rokushika, H. Hatano, M. Ohshina (1987), "A Study of Chromatographic Properties of Crystalline Polyacrylates Using WCOT Columns in Gas Chromatography". Liquid High Resolution Chromatography and Capillary Chromatography, 10,

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