1、 Characteristics and Performance of Supercritical Fluid Extraction (SFE) in the Analysis of Petroleum Hydrocarbons in Soils and Sludges Health Qualitative descriptions on SFE performance (primarily restric- tor plugging); Evaluate when presently-available SFE methods are viable alter- natives to con
2、ventional liquid solvent extraction; Determine development needs for SFE extraction and collection conditions, and develop extraction conditions for a wide range of petroleum-based hydrocarbons and PAHs. This effort included: Development of two SFE methods that can yield quantitative extraction and
3、recovery of gasoline- to diesel-range organics from soils, allowing BTEX and total petroleum hydrocarbons (TPH) to be determined using a single extraction method; Development of quantitative extraction conditions for PAHs and heavier hydrocarbons including heavy crudes and heavy resids; Determine ha
4、rdware development needs based on problems encountered with real-world samples, and identify commercially available instrumentation to meet those needs. Commercially available instrumentation and standard SFE approaches, such as the proposed Environmental Protection Agency (EPA) method for TPH, were
5、 used. Comparisons were made to standard liquid solvent 6 Purpose and typical total solvent use of less than 10 ml, compared to 150 ml for Soxhlet extraction. In nearly all of the samples studied, SFE yielded efficiencies similar to or higher than Soxhlet extraction; however, elevated temperature an
6、d/or organic modifiers were often needed to obtain high extraction effi- ciencies for organics beyond the gasoline- and diesel-range. It should be noted that SFE instruments continue to evolve, especially in the areas of improved restrictor and collection system designs, as well as systems offering
7、automated extraction of up to 20 samples without operator inter- vention. Such developments should further increase reliability and speed of SFE for petroleum hydrocarbon extractions from soils and sludges. GASOLINE-RANGE TPH, DIESEL-RANGE TPH, AND BTEX BY ON-LINE SFE/GC On-line SFE/GC methodology w
8、as developed to allow extraction and analysis of organics as volatile as n-butane from solids at part-per-billion (ppb) detection limits (Burford et al., 1994a). A solid-based calibration stan- dard, consisting of several n-alkanes and aromatic hydrocarbons spiked onto Tenax-TA, was successfully use
9、d to optimize the chromatographic parameters for coupled SFE/GC. A simple and reliable split SFE/GC sys- tem was developed utilizing a septumless injector installed on a split/split- 8 On-Line SFE/GCless injection port. The high gas flow rate generated inside the injection port during the SFE step w
10、as accommodated for by using the correct split ratio, so that high (1 ml/min liquid C02) SFE flow rates could be used. The use of thick-film (5 m film thickness) columns and cryogenic trapping tem- peratures in the GC oven as low as -50C allowed efficient trapping of species as volatile as n-butane,
11、 acetone, and methylene chloride. The chromatograms obtained using the optimized SFE/GC technique showed good peak shapes (comparable to those obtained using a conventional split injection) and typical peak area reproducibilities of 90%). Sorbent trapping yielded quantitative collections (:2: 88%) o
12、f n-alkanes as volatile as n-hexane, while the solvent trapping effectively collected n-alkanes as volatile as n-heptane (pressurized trapping system) and n-octane (normal trapping system). The quantification of BTEX, TPH, and individual species from contaminated soils obtained by both SFE systems a
13、greed well. However, because of the greater losses of BTEX and the volatile n-alkanes, Soxhlet extraction yield- ed significantly less BTEX, TPH, and compound-specific analytes than both SFE systems. This study demonstrated that commercially available SFE instrumentation can be used to determine BTE
14、X and TPH levels using a single extraction. HEAVY HYDROCARBON DETERMINATIONS BY SFE Heavy hydrocarbons are not extracted as readily as gasoline- and diesel- range organics using pure C02 at conventional temperatures (e.g., 50C) and pressures (e.g., 340 to 400 atm). Therefore, both elevated temperatu
15、re and the addition of organic modifiers to supercritical C02 have been eval- uated. SFE with C02 was used for the determination of TPH in real-world fuel-spill soil samples containing heavy fuel oil, diesel fuel, and light crude oil (TPH contents of 150, 15, 15 mg/g, respectively) (Eckert-Tilotta e
16、t al., 1993). Quantitative extraction by SFE was accomplished at 400 atm C02 and 150C extractor temperature, and TPH results were comparable (with- in standard deviations) with those obtained by Freon-113 Soxhlet extrac- tion (4 hr) for all samples. Comparable TPH results for the soil extracts were
17、obtained from analytes using gas chromatography with flame ioniza- PAH Determinations 11 tion detection and infrared spectrometry. Quantitative reproducibility for replicate SFE extracts was good (relative standard deviation of 2-10%), and the quantity of Freon-113 solvent was reduced from 150 ml fo
18、r Soxhlet to 80% for samples contaminated at 1 mg/kg or higher, while recoveries typ- ically ranged from 50 to 60% for samples contaminated at lower levels. A possible defect of the specified method was that it did not provide a static time to allow the modifier to contact the sample. Extensive work
19、 with other samples suggests that had the static time been provided, the recoveries would have been much higher for these samples. Three other studies have demonstrated that raising the temperature of the SFE step to 200C greatly enhances the extraction of PAHs and other organics, and high temperatu
20、re SFE typically yields quantitative recoveries of PAHs without the need for organic modifiers (Langenfeld et al., 1993; Practical Aspects and Theoretical Factors 13 Hawthorne et al., 1994b). For particularly difficult samples, combined high temperature and modifiers yield the highest recoveries (Ya
21、ng et al., 1994b). PRACTICAL ASPECTS AND THEORETICAL FACTORS CONTROLLING THE APPLICATION OF SFE Efficient methods development using SFE is greatly enhanced by an understanding of the chemical and instrumental parameters and their effect on recoveries. The development of quantitative SFE methods for
22、the recovery of organic pollutants from environmental samples requires three steps: quantitative partitioning of the analytes from the sample into the extraction fluid, quantitative removal from the extraction vessel, and quan- titative collection of the extracted analytes (Hawthorne et al., 1993b).
23、 While spike recovery studies are an excellent method to develop the final two steps, they are often not valid for determining extraction efficiencies from complex real-world samples such as soils and sediments, exhaust partic- ulates, and sludges. SFE conditions that yield quantitative recoveries o
24、f spiked analytes may recover 90%) the spiked deuterated-PAHs, but only extracted approximately 25-80% of the native PAHs. Similar differences were observed using conventional methylene chloride sonication, demonstrating that spike recovery studies are not valid for developing quantitative extractio
25、n methods for heteroge- nous environmental samples. While spikes should not be used to determine extraction efficiencies, they are very good to determine collection efficiencies. The collection of petro- Practical Aspects and Theoretical Factors 15 leum hydrocarbons as volatile as benzene and butane
26、 was discussed above and described in detail in Burford et al., 1994a; Burford et al., 1994b; and Yang et al., 1994a. Since some samples encountered in this study required that drying/dis- persing agents be added to avoid restrictor plugging during SFE, the use of 21 potential drying agents was inve
27、stigated (Burford et al., 1993b). Five (anhydrous and monohydrated magnesium sulfate, molecular sieves 3A and SA and Hydromatrix) were able to prevent restrictor plugging by water during off-line supercritical fluid extraction (e.g., 400 atm C02 at 60C) by retaining the majority of the water (but ge
28、nerally not the analytes of interest) in the extraction cell. Increasing the extraction temperature (e.g., to 150C) or adding a polar modifier (10% (v/v) methanol) to the C02 extraction fluid greatly reduced the amount of water the drying agents retained. However, when 10% (v/v) toluene was used for
29、 the extraction, the drying agents were able to retain the majority of the water (approximately 80% w/w). Polar and non-polar pollutants were quantitatively extracted from the wet drying agents, but nearly all of the drying agents selectively retained at least one of the polar analytes if used dry,
30、thus demonstrating the need for a spike recovery study to determine the potential for analyte loss. The successful drying agents eliminated restrictor plugging when used with moderately wet (approximately 20% (w/w) water at a 1:1 reagent-to-sample ratio) and very wet (approximately 90% (w/w) water a
31、t 4:1 reagent-to-sample ratio) sam- ples without the need to heat the restrictor or the collection solvent. Fused-silica restrictors used for off-line SFE frequently break when extrac- tions are performed with polar supercritical fluids (e.g., Freon-22) or C02 containing polar modifiers (e.g., metha
32、nol) (Burford et al., 1993c). Securing the fused-silica restrictor inside a 1/16 in. (1.6 mm) 0.0. stain- less-steel tube with an epoxy resin eliminated the restrictor breakage and allowed restrictors to be connected to the extraction cell with conventional stainless-steel fittings. The stainless-st
33、eel clad fused-silica restrictor was 16 Recommendations simple and inexpensive to construct, physically robust, and proved ideal for SFE applications, since no artifacts from the clad restrictor were detected in the collection solvent. A simple correlation to predict the flow using lin- ear restrict
34、ors was also developed. The correlation accounts for pressure, temperature, restrictor i.d., and restrictor length (Yang et al., 1994c). The correlation allows the proper size of restrictor to be selected for the desired flow rate under different extraction conditions. RECOMMENDATIONS The results of
35、 this study clearly demonstrate that analytical-scale SFE can successfully compete with conventional (e.g., Soxhlet) extraction for the extraction of hydrocarbons from soils and sludges. However, it must be remembered that SFE is not yet as simple to perform as a Soxhlet extrac- tion and, therefore,
36、 it is likely that a more highly-trained analyst will be required to obtain good results. SFE methods for gasoline- and diesel range organics (e.g., the proposed EPA method) are well-developed and easily implemented using commercial SFE instrumentation. The only rea- son for slow adoption of SFE for
37、 routine TPH determinations appears to be based on the slow promulgation of the EPA method. For higher boiling species, SFE conditions utilizing elevated temperatures (e.g., 150C) and/or the addition of organic modifiers are often required to obtain quan- titative recoveries (e.g., for alkanes C30 a
38、nd PAHs), but such techniques are relatively simple to perform with most commercial instrumentation (and properly-designed “home-built“ systems). On the negative side, some problems still exist with commercial instru- mentation, particularly in the areas of restrictor plugging (with particularly “di
39、rty“ matrices, e.g., wet sludges with very high extractable organic con- tent) and collection efficiencies of more volatile analytes. The primary dif- Recommendations 17 ferences in commercial instruments occur in these two areas, i.e., the method used to control the C02 flow rate (the restrictor sy
40、stem), and the method used to collect extracted analytes. Often, seemingly insignificant differences in commercial instrumentation can greatly affect whether a complex sample can be extracted (e.g., whether the restrictor plugs) and whether a particular analyte can be efficiently trapped. While it i
41、s unfair to require an instrument supplier to develop a method for a particular appli- cation, investigators who wish to evaluate various SFE instrumentation should request two test evaluations. First, the restrictor system should be capable of controlling flow and not plugging while extracting the
42、most com- plex (e.g., highest water content and highest extractable matrix content) samples that are expected to be encountered by the purchaser. Second, the trapping system (e.g., sorbent, solvent, or cryogenic trapping) should be demonstrated to quantitatively collect the analytes of interest (par
43、ticu- larly the more volatile species) before purchase should be considered. In addition, the majority (but not all) of analytical SFE instruments use sam- ple sizes of 10-ml or less, because of increased reliability in high pressure systems. It should be noted that, since analytical SFE instrumenta
44、tion has been commercially available for only a few years, and since substantial develop- mental efforts (particularly in automated operation) are just coming to fruition, this report will not attempt to offer purchasing advice. However, the investigator who wishes to utilize the methods described i
45、n this report should consider the following results that relate to instrumentation: 1. Both solvent trapping and sorbent trapping were successful with species as volatile as benzene if properly performed. However, both methods can yield very poor collection efficiencies if not prop- erly performed (
46、thus the importance of the collection efficiency evaluation for different vendors instruments as discussed above). 18 Recommendations 2. A single addition of modifier to the sample was generally sufficient to increase recoveries of analytes in these investigations where pure C02 was not successful.
47、Therefore, an SFE instrument should be able to perform extractions in the static (non-flowing) followed by dynamic (flowing) mode. 3. Since a single addition of modifier directly to the sample was gen- erally sufficient (using static followed by dynamic extraction), dual pump systems (which add to i
48、nstrument costs) to add modifiers were not necessary. However, since future applications may require constant addition of modifier, an instrument should have provisions to add a modifier pump at a future date. 4. We were able to extract from all of the sludge and soil matrices, but approximately 1/3
49、 of the samples caused plugging of the restrictors severe enough necessitate premixing the sample with a dispersant (e.g., 40 m “Empore“ glass beads). This problem was most severe with samples containing high concentrations of heavy hydrocarbons, and samples with high concentrations of ele- mental sulfur. In general, soils contaminated with gasoline- and diesel-range organics showed no significant restrictor plugging, even though water contents were as high as 20 wt%. Therefore, an SFE system should be evaluated with the range of “real-world“ samples that will be extrac
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