Research and Technology - Forensic Science Communications - October 2004
October 2004 - Volume 6 - Number 4
Research and Technology
Development and Testing of a Solid Phase Microextraction Method for the Trace Analysis of Nitro Organic Explosives Using Gas Chromatography-Electron Capture Detector and Gas Chromatography-Mass Spectrometry
Jeffrey G. McDonald
Counterterrorism and Forensic Science Research Unit
Dennis E. Maslanka
Counterterrorism and Forensic Science Research Unit
Kelly H. Mount
Mark L. Miller
Counterterrorism and Forensic Science Research Unit
Federal Bureau of Investigation
Abstract | Introduction | Experiment | Results and Discussion | Summary | References
The determination of organic explosives residue on debris, in soil, and in water after bombings is critical to investigations that require the determination of whether or not an act of terrorism has been committed. Presented here is a rapid method for the trace-level analysis of explosives in soil, water, and swab samples of common areas and postblast debris using solid phase microextraction. Solid phase microextraction is a solvent-free technique capable of extraction, processing, concentration, and sample introduction of a wide variety of compounds present in a variety of matrices. This technique has been developed for use in the laboratory, but results indicate that it may also be used in the field. Soil, water, and swab samples were collected from an explosives range and common areas at the FBI Academy in Quantico, Virginia. For comparative purposes, the samples were processed using solid phase microextraction and acetone extraction techniques. The solid phase microextraction-based method yielded comparable (and sometimes superior) results to acetone extractions, except for soils, demonstrating that solid phase microextraction is an effective extraction technique for sampling actual postblast debris. The solid phase microextraction-based method provided better sensitivity for ethylene glycol dinitrate (EGDN), resulted in chromatograms with lower, less complex background signals when compared to acetone extractions, and does not rely on the use of organic solvents. The potential for solid phase microextraction to be used in the field for explosives sampling was also investigated and found to be adequate for a variety of matrices.
The rapid determination of organic explosives residue on debris, in soil, and in water from bombings is critical to investigations that require a determination of whether or not an act of terrorism has been committed. Sometimes large numbers of samples of various origins must be analyzed before a determination can be made about whether a criminal act has occurred. On-site screening methodology exists but is limited by sensitivity and selectivity and must be followed by other instrumental techniques in the laboratory for definitive confirmation. A solvent-free technique capable of extraction, processing, concentration, and sample introduction of a wide variety of compounds and samples is desirable for use in the laboratory or possibly the field.
Solid phase microextraction was developed in the early 1990s by Pawliszyn and coworkers (Zhang et al. 1994) and has evolved into a widely accepted technique for sampling a variety of compounds and materials. Solid phase microextraction uses a liquid polymer absorbent or a solid adsorbent coated on the outside of a fused silica fiber (typically 1cm in length and 0.1cm outer diameter) to extract organic compounds from aqueous media. The sorbent coating is selected based upon the physical characteristics of the compounds being extracted to maximize adsorption and provide selectivity (Scheppers Wercinski and Pawliszyn 1999). Similar to selecting a gas chromatography column, thickness and polarity of the stationary phase are important considerations when choosing an appropriate phase. An often-used description of solid phase microextraction is a short length of capillary gas chromatography column that has been turned inside out. Solid phase microextraction makes use of a selective affinity to preferentially remove the analyte(s) of interest from the matrix in aqueous or headspace samples and concentrate them onto the fiber. Sampling is conducted by immersing a solid phase microextraction fiber in an aqueous sample (either a water sample or a water extraction of a solid sample) or in the headspace above an aqueous or solid sample. Heating, stirring, and salting the samples can add to the extraction efficiency, depending on the sample, matrix, and analyte (Penton 1999; Scheppers Wercinski and Pawliszyn 1999). In this way, solid phase microextraction combines sample extraction and preparation into one step, eliminating lengthy and complex sample preparation, which often involves the use of organic solvents. Once concentrated onto a solid phase microextraction fiber, samples are ready for immediate analysis by desorption into inlets of gas or liquid chromatographic instruments with little or no modification required.
Solid phase microextraction methods have been developed for a wide variety of sampling applications. For example, solid phase microextraction has been used in the areas of pharmaceutical, environmental, food, and flavor analysis (Barshick and Griest 1998; Cam and Cagni 2001; Scypinski and Smith 1999; Yang and Peppard 1999). Solid phase microextraction techniques have also been explored for the forensic analysis of drugs, accelerants, and explosives (Furton et al. 2000A; Furton et al. 2000B; Furton et al. 2000C).
Recently, analysis of explosives using solid phase microextraction has garnered considerable attention (Furton et al. 2000A; Furton et al. 2000B; Furton et al. 2000C; Kirkbride et al. 1998). Researchers have developed methods for explosives detection with solid phase microextraction using explosives standards. These included methods for aqueous samples and for headspace sampling of debris spiked with explosives. However, these methods used organic solvents to extract the explosives from the debris, concentrated the extract, and then spiked it into water prior to sampling with solid phase microextraction (Furton et al. 2000B; Furton et al. 2000C). Although this method is effective, it negates the solvent-free benefit of solid phase microextraction. A rapid, efficient method is necessary to extract and sample a wide variety of matrices for explosives using solid phase microextraction without use of organic solvents.
Presented here is a rapid method for the trace-level analysis of explosives in soil, water, and swab samples of common areas and postblast debris using solid phase microextraction. Twenty-five samples were collected from common areas at the FBI Academy and from an explosives range following an explosives demonstration in Quantico, Virginia. These samples were analyzed with solid phase microextraction (without use of organic solvents) and with standard acetone methods for comparative purposes. The ability of solid phase microextraction to be used as a field sampling technique was also investigated.
Standards of eight nitro explosives were obtained from Cerilliant (Austin, Texas). These included ethylene glycol dinitrate (EGDN), trinitroglycerin (NG), 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), hexahydro-1,3,5-triazine (RDX), methyl-2,4,6-trinitrophenyl-nitramine (tetryl), and 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX). The standards were combined and diluted into a 10ppm stock solution with deionized water. From this stock, 1, 10, 25, and 100ppb standards were prepared as 25 percent weight/volume sodium chloride solutions.
Solid Phase Microextraction Fibers and Related Materials
Solid phase microextraction fibers, fiber holders, and a sampling stand were purchased from Supeloco (Bellefonte, Pennsylvania). Polydimethylsiloxane/divinylbenzene fibers (PDMS/DVB) were used for this study. The fiber coating was 65μm film thickness and bonded to a 1cm flexible fused silica substrate (StableFlex). Fibers were placed in 23-gauge, manual sampling fiber holders. The fiber assemblies were stored in field-test kits consisting of aluminum storage tubes (22.5cm long, 2.8cm diameter) placed in a foam-lined plastic case designed to protect the fibers during storage and transport. Storage tubes were fitted with Viton o-rings between the cap and body of the container resulting in an airtight seal. Access ports with Teflon-faced septa were incorporated into the bottom of the tubes. This feature allows preliminary screening of the contents in the storage tubes in a controlled manner, reducing potential exposure to hazardous materials.
Ten swab samples were collected at the FBI Academy using drugstore cotton balls precleaned with deionized water and drugstore isopropanol (90 percent), then dried at 70°C. Dry cotton balls were held with disposable forceps and rubbed across surfaces of common areas (e.g., mailboxes, lunch tables, vending machines) then placed in 20mL screw-cap glass vials. Fifteen swab, soil, and water samples were collected at an explosives-demonstration range on the U.S. Marine Base in Quantico, Virginia, following an explosives demonstration. Swab samples of explosives debris and hands of those who had recently handled explosives were collected using the technique described above. Soil and water samples were collected near recent blasts and placed directly into the 20mL vials.
Swab samples were cut in two—one half was used for solid phase microextraction, the other half for acetone extraction. Half-swabs were placed in the barrels of 5mL syringes, the plungers were inserted to the 2mL mark, and 4mL of deionized water was drawn into the syringes. Filters (Whatman Anotop Plus, Maidstone, England; 0.2µm cutoff) were mounted on the ends of the syringes, and the units were shaken vigorously for ten seconds and then allowed to sit for ten minutes. The extracts were subsequently filtered into vials (all vials were 4mL screw-cap septum vials) containing 1g sodium chloride, making the salt content approximately 25 percent (weight/volume). In this study, all syringes were 5mL disposable plastic; all filters were 0.2 µm cutoff; syringes and filters were prerinsed with 3 × 4mL of deionized water.
Soil samples were extracted with water by placing 2–2.5g of soil in vials, adding 4mL water, and stirring for 30 minutes. After the soil settled, the supernatants were pipetted into the barrels of syringes mounted to filters and filtered into vials containing 1g sodium chloride.
Water samples (4mL) were pipetted into the barrels of syringes mounted on filters and filtered into vials containing 1g sodium chloride.
The other halves of the swab samples were extracted following the acetone extraction procedure used by the FBI’s Explosives Unit. Enough acetone was added to the vials to cover the half-swabs (approximately 4–4.5mL). The vials were shaken vigorously for one minute and allowed to sit for five minutes. The acetone was transferred by pipette to the barrels of syringes mounted on filters (all syringes and filters used for acetone extraction were precleaned with 3 × 4mL acetone rinses) and expelled into vials.
Soil samples were extracted with acetone by placing 2–2.5g of soil in vials, adding 4mL acetone, and stirring for 30 minutes. After the soil settled, the supernatants were removed with pipettes and transferred to the barrels of syringes mounted to filters and filtered into vials.
Water samples (4mL) were placed in vials with 4mL of hexane and 1g of sodium chloride. The samples were shaken vigorously for ten minutes, and the two layers were allowed to separate. Due to the hydrophobicity of these compounds, which is further increased by the addition of salt, explosives in the water samples will partition into the hexane layer. The hexane layers were pipetted into the barrels of syringes mounted to filters and filtered into vials. All hexane extracts of water (and all acetone extracts of swabs and soil) were reduced in volume to about 100µL under a gentle stream of nitrogen while heating at about 35°C.
Solid Phase Microextraction
Solid phase microextraction fibers were conditioned for use as suggested by the manufacturer by placing them in an unused gas chromatograph’s injection port for 30 minutes at 260°C with a purge flow of 200mL/minute; and subsequently for one minute between samples. Aqueous extracts were sampled with solid phase microextraction fibers placed in the solution for five minutes while stirring at 1,500rpm. In this experiment, extracts were sampled without caps on the vials. However, subsequent experiments showed that better results were obtained by sampling extracts through a capped vial with a septum, which was adopted as part of the procedure. By extracting from a sealed vial, volatile components are retained; whereas, they were lost to volatilization out of the vial when stirred unsealed at a high rate of speed. After sampling, the fibers were thoroughly rinsed with deionized water to remove residual sodium chloride. Solid phase microextraction fibers were placed directly in the gas chromatograph’s injection port for one minute for desorption of the explosives from the fiber onto the column head.
An Agilent 6890 gas chromatograph (Agilent Technologies, Palo Alto, California) with a micro electron capture detector was used for screening samples. The inlet was operated in the splitless mode using a Siltek-treated, 1mm drilled Uniliner (Restek, Bellefonte, Pennsylvania) with a Press-Tight connector at 200°C and 225°C for liquid and solid phase microextraction injections, respectively. A Merlin Microseal (Agilent Technologies, Palo Alto, California) was used in place of a septum. The gas chromatograph-electron capture detector (GC-ECD) inlet was operated at a nominal head pressure of 9.5psi. A J&W DB-5MS (Palo Alto, California) column (6m × 0.32mm × 0.25µm) was used with nitrogen as the carrier gas. The temperature program for the GC-ECD started at 50°C for one minute, then 25°/minute to 250°C with a one-minute hold. A ramped flow program was used for GC-ECD analysis, which started at 10mL/minute for 1.5 minutes, then ramped down at 3mL/minute to a final flow of 4.7mL/minute. The micro electron capture detector was operated at 275°C, and nitrogen was used as the makeup gas with a combined flow of 60mL/minute (column plus makeup).
An Agilent 6890 gas chromatograph with a 5973 mass selective detector was used for confirming positive results on the GC-ECD. The inlet was operated in the splitless mode using a 4mm drilled Uniliner with a Press-Tight connector at 200°C for both solid phase microextraction and liquid injections. A Merlin Microseal was used in place of a septum. The inlet of the gas chromatograph-mass spectrometer (GC-MS) was operated at a nominal head pressure of 2.2psi with a constant flow of 13mL/minute, and a J&W DB-5MS column (6 m × 0.53mm × 1.5µm) was used with helium as the carrier gas. The temperature program for the GC-MS started at 50°C for one minute, then 25°/minute to 250°C with a one-minute hold. To decrease the gas flow into the mass spectrometer, the flow was split by sliding the column end over a 0.1mm fused silica transfer line (extending approximately 20cm beyond the transfer line nut) venting some of the flow into the oven. The transfer line was operated at 210°C. The mass spectrometer was operated in electron capture negative ionization mode with methane as the reagent gas at a source pressure of 2 × 10-4 torr. The source temperature was 150°C, the quadrupole temperature was 125°C, and the mass range was 40 to 300 daltons.
Semiquantitative analysis for solid phase microextraction samples was conducted on the GC-ECD by external calibration. Calibration curves for each explosive were generated using 1, 10, 25, and 100ppb eight-mix standards (25 percent sodium chloride weight/volume) sampled with a solid phase microextraction fiber for five minutes. Concentrations of explosives in the extracts were estimated by using peak areas and the regression equations from the calibration curves.
Concentrations of explosives in the reduced-volume acetone extracts (injected as liquid samples) were extrapolated by determining the concentrations for which solid phase microextraction and liquid injections resulted in equivalent chromatographic peak areas. A series of experiments were conducted to determine an “equivalency factor” to allow for comparison of the two injection techniques. It was determined that using a PDMS/DVB solid phase microextraction fiber to sample a 10ppb eight-mix aqueous standard (4mL) for five minutes and analyzed with a GC-ECD resulted in similar peak areas to a 0.2µL liquid injection of a 1.0ppm eight-mix standard in acetone. The liquid, and therefore solid phase microextraction injections, both result in approximately 0.2ng of each explosive on-column.
If a 4mL 10ppb eight-mix standard in acetone is reduced in volume to about 100µL, the concentration would be 0.4ppm (100µL is the approximate volume of the reduced-acetone extract). A 0.5µL injection of this concentrated 0.4ppm standard also results in approximately 0.2ng on each explosive on-column. It can, therefore, be extrapolated that an injection of 0.5µL of a 4mL acetone extract of a sample concentrated to about 100µL is equivalent to a five-minute aqueous solid phase microextraction sample. This comparison allows for a semiquantitative estimation for the acetone (and hexane) extracts.
Test mixtures of explosives were analyzed using the GC-ECD and GC-MS each day to verify instrument performance. Although an eight-mix explosive standard is used throughout this study, only seven of the components are detectable by either instrumental technique at the concentrations used in this study. HMX is not detected at 100ppb using solid phase microextraction injections; however, it is detectable at higher concentrations by liquid injection. Instrument and fiber blanks were conducted for quality control purposes each day to verify instrument performance. Negative controls were collected for all field samples, and all were below the detection limit of the method. All water used was deionized, and all solvents were high performance liquid chromatography or spectrophotometric grade.
Results and Discussion
Swab, Soil, and Water Samples
Twenty-five samples were collected from specific areas around the FBI Academy and from a nearby explosives-demonstration range. Samples included swabs, soil, and water, which were analyzed using solid phase microextraction and acetone methods (Table 1). Of the 15 samples collected on the explosives-demonstration range and extracted with acetone, ten showed responses at the appropriate retention time for one or more explosives using solid phase microextraction with GC-ECD, which were then confirmed using GC-MS. Example chromatograms are shown in Figure 1. In the demonstration range samples, 23 positives were identified, including one sample with HMX (not shown), although it was not confirmed with GC-MS. Explosives were not detected in the water samples, although nearby soil samples showed detectable levels for six of eight screened explosives. Two of the three hand swabs collected while at the range indicated the presence of TNT. Explosives were detected in the FBI Academy in the gun-vault area, where weapons and ammunition are handled, and the mailboxes. NG was observed in each of the three gun-vault samples, which is to be expected, as NG is a principal component of double-base smokeless powder. Both EGDN and DNT were observed in one of three samples from the gun-vault area.
Figure 1. Total ion chromatograms of extracts from the FBI Academy and the explosives-demonstration range shown with a 100ppb standard. A * indicates a match in retention time and mass spectra with the standard.
In the 15 range samples, 19 positives for five of the eight explosives were identified; EGDN, tetryl, and HMX were not detected and confirmed. In eight of the 19 positives, explosives were only detected in the acetone extracts and not in solid phase microextraction extracts, most of which were soil samples. Due to the hydrophobic nature of the explosives, water does not efficiently extract them from the organic-rich soil. Similar to the solid phase microextraction results, the acetone extracts from the FBI Academy showed positives for NG in the gun-vault swabs.
The results for the 25 samples show that solid phase microextraction is a viable alternative to acetone extractions except for soils, due to the limitations of water as a solvent, and may in fact be superior to acetone extraction in other cases. Solid phase microextraction excels in the extraction of EGDN from a variety of samples, which was detected three times using solid phase microextraction, and none with acetone extraction. EGDN has the highest vapor pressure of the eight explosives (2.8 × 10 -2 torr at 25°C) (Kirkbride et al. 1998) studied here and can be easily lost during the preconcentration step of the acetone extraction. With solid phase microextraction, this step is not necessary, thereby reducing the chance for loss of EGDN. Solid phase microextraction chromatograms showed much cleaner baselines and much lower background levels compared to chromatograms from samples extracted with acetone (Figure 2). Solid phase microextraction is much more selective in its extraction compared to acetone, which results in fewer interferences and contaminants being introduced into the instrument. Over time, these compounds can lead to a decrease in instrument performance. Solid phase microextraction also has the advantage of reduced analysis time (10 versus 30 minutes/sample) and reduces waste because it does not use organic solvents.
Figure 2. Comparison of Solid Phase Microextraction and Acetone Extracts
Solid phase microextraction is not an optimum technique for soil or other materials rich in organic material unless the explosives are present at relatively high levels. Water is not an effective solvent for removing the explosives from soil or other organic-rich material. Although these explosives have water solubilities on the order of mg/L, their preference for organic material over water is significant, requiring organic solvents for efficient extraction from soil.
Solid phase microextraction has been demonstrated here as a viable technique for processing and extracting samples in the laboratory. However, solid phase microextraction has the potential to be used in the field as a sample collection technique, where the fiber is then returned to the laboratory for analysis. This may have several benefits, including sample stability, rapid turn-around time, and reduced evidence handling and transport. Sampling kits could be prepared containing all the necessary components for field sampling. In order for solid phase microextraction to be used in the field, several parameters have to be established. These include long-term fiber stability, stability of the samples adsorbed to the fiber, and methods of manually agitating the sample/fiber in the field.
Long-term stability of the fiber and of a sample adsorbed onto the fiber is necessary if solid phase microextraction is to be performed in the field. Should solid phase microextraction be used for field sampling, the fibers will need to be returned to the laboratory, where a backlog may exist, so once the analytes are adsorbed to the fiber, they must be stable for days to weeks. It is desirable that fibers should be reliable for at least a month after conditioning when properly stored. Previous research has shown that after an initial conditioning for 30 minutes at 260°C, a one-minute conditioning of the PDMS/DVB fiber at 260°C between analyses is suitable (Miller and Maslanka 2001). To determine if these fibers are still suitable for use after days or weeks of storage, four identical fibers were conditioned for 30 minutes, analyzed using GC-ECD (Figure 3A) to verify the fibers were free of contamination, conditioned for one minute, and then placed in their aluminum transport tubes. Teflon caps were placed on the ends of two fiber holders, while the others were left open to determine if the caps reduced contamination or increased it due to the Teflon composition of the caps. It is reported that fibers can adsorb compounds from Teflon (caps); from the glue used to bond the substrate to the needle, silicone and Teflon septa; and from o-rings and vacuum grease (Penton 1999).
After 50 hours, two fibers were removed from their containers (one with Teflon cap, one without), and analyzed using GC-ECD. Figure 3A-C shows that there was a slight increase in the baseline for the chromatograms of both fibers, compared to the baseline immediately following conditioning. The complexity of the chromatograms also increased for both cases. Although numerous peaks were present in both chromatograms, none were greater than a peak height of 500 counts (baseline approximately 250), and most were small. Peaks of similar retention times were present in both chromatograms, but their relative intensities differed. No peaks were found at exact retention times of explosives measured in these studies. It should be noted that the relative scales of the blank chromatograms in this study are less than that of a real sample (see inset on Figure 4C for comparison). Furthermore, use of GC-MS would differentiate between an interferent and an explosive if there were a need to do so. The results show that the Teflon cap does not help nor hinder the cleanliness of the fiber while it is in the storage container.
The GC-ECD chromatograms of the fibers stored for one week are similar to the chromatograms of the 50-hour storage blanks, except some peak heights had increased as shown in Figure 3D. Baselines also increased by 100-200 counts, but the overall appearance of the chromatograms remained similiar. After 28 days, there is still no significant increase in baseline or overall peak height, but many more small peaks are present in the chromatogram. Figure 3E shows the baseline increased to a height of about 300, and several peaks are approaching peak heights of 600. The results indicate that storing the fibers does result in a slightly elevated background and some contaminant peaks. However, none of them eluted at retention times that interfere with the analysis of the explosives studied. It is important to point out that relative levels of the background and contaminant peaks observed after storage are minor in comparison to levels observed in actual samples at greater than 10ppb such as those in Figure 4A.
Figure 3. GC-ECD chromatograms of PDMS/DVB solid phase microextraction fiber blank (A) and of similar fibers after being stored for various lengths of time, with and without Teflon caps (B-E).
Stability of Explosives Adsorbed onto Fiber
Stability of the organic explosives once adsorbed onto the fiber is also a requirement for field use because it may be days or possibly weeks before a fiber can be analyzed. To investigate storage and stability, a conditioned PDMS/DVB fiber was analyzed as a blank and then analyzed (using GC-ECD) after sampling a 10ppb eight mix for five minutes for control purposes (Figure 4A). The fiber was then reconditioned, used to sample the same 10ppb eight mix, and stored in its aluminum container with a Teflon cap on the fiber holder. The fiber was removed from its container after 15 days and analyzed with GC-ECD. The chromatogram is shown in Figure 4B, and the results are shown graphically in Figure 5. With the exception of PETN and tetryl, the peak areas were equivalent to those obtained using a freshly conditioned fiber. Compared to when using a freshly conditioned fiber, PETN and tetryl were reduced in peak intensity by 40 and 70 percent, respectively. Some of this difference can be attributed to instrumental variation, but PETN and tetryl do show less stability on a stored fiber than the other explosives. Although PETN and tetryl show some instability on the fiber, they are generally stable when stored on a fiber for two weeks.
Viability of a Stored Fiber
This research has shown that fibers stored for up to one month remain reasonably free of contamination and that explosives adsorbed onto fibers remain stable for at least two weeks. However, the viability of a fiber stored for several weeks and its ability to effectively sample an extract must be determined. To examine this, one of the blank fibers stored for 27 days (in the aluminum transport tube) was selected and used to sample a 10ppb eight-mix standard (25 percent sodium chloride weight/volume) for five minutes. The GC-ECD chromatogram is shown in Figure 4C along with a 10ppb eight-mix performance test in Figure 4A for comparison. The results are shown graphically in Figure 5. The abundances of the compounds are similar to those obtained for the original 10ppb eight-mix performance test used as a baseline. Tetryl was the exception, which was 55 percent less abundant. Similar results were observed for tetryl in the 15-day sample stored on fiber. Based on previous discussion, it is logical to conclude that some of the tetryl may be lost on the fiber while stored. However, in this experiment all compounds except tetryl gave equivalent abundances, so it is possible some of the tetryl in the standards has been lost to degradation. Even with the reduced abundance of tetryl, the results show that a blank fiber stored for up to four weeks is still viable for sampling.
Figure 4. GC-ECD chromatograms of 10ppb eight-mix standard sampled with PDMS/DVB solid phase microextraction fiber for five minutes while stirring at 1500rpm (A), after being stored two weeks (B), and sampling with a conditioned fiber that had been stored for two weeks (C).
Currently, when swabs are processed for acetone extraction, microbial or thermal degradation or desorption into the headspace is of concern. Depending on the sample matrix and the time between sample collection and analysis, a considerable amount of analyte could be lost. Samples extracted with solid phase microextraction are stable on the fiber for up to two weeks (and the conditioned fibers are stable for at least four weeks). Because the explosives have been removed from their matrix, microbial degradation is not likely. Other possible loss mechanisms, such as desorption into the headspace, are negated because of the compound’s affinity for the fiber material versus the air at ambient temperatures. The solid phase microextraction test kit provides safe, rugged storage for the samples while waiting for analysis in the laboratory or in the field. With the extraction having taken place in the field (15-20 minutes/sample), fibers are ready for immediate analysis upon returning to the laboratory. Solid phase microextraction is easily adaptable to current laboratory instruments and field instruments, such as portable gas GC-ECD, which could provide rapid screening.
Figure 5. Abundances for seven explosives in a 10ppb eight-mix standard with a PDMS/DVB fiber, after being stored for 15 days, and sampling with a conditioned fiber that had been stored for four weeks at room temperature. Error bars represent 21 percent for each explosive. This was the average error of a previous experiment where n = 42 (six solid phase microextaction analyses with seven analytes each). Fibers were analyzed using gas chromatography/electron capture detector.
Sample Agitation During Solid Phase Microextraction
For solid phase microextraction to achieve adequate levels of sensitivity (ppb) in a reasonable amount of time (five minutes), the extract must be agitated. By keeping the extract well mixed, the area immediately surrounding the fiber does not become depleted of analyte as in the case of a static extraction. In comparison, static extractions rely solely on diffusive properties to move the analyte to the fiber. If solid phase microextraction is to be used in the field, a digital stir plate capable of 1500rpm (as is used in these studies) may not be available. One option is battery-powered stir plates, but this adds another level of complexity to the field sampling. A second, more field-useable method is to not stir the extract while sampling, although this may require extended extraction times. To determine if static extractions were practical, a 10ppb eight mix-standard was extracted without stirring for 5, 30, 60, 120, and 900 minutes, then analyzed using GC-ECD. It took approximately 120 minutes to reach peak areas equivalent to a five-minute stirred extraction for ethylene glycol dinitrate, NG, DNT, and RDX. PETN only reached half the peak area of a five-minute stirred extraction, while tetryl’s area doubled. There is competition between compounds for space on the fiber, and this may be why the results are not consistent for all compounds. The longer the extraction, the more time the system has to come to equilibrium. Regardless, extracting samples in the field for 120 minutes may not be practical.
An alternative to stir plates or static extractions is to manually shake the fiber and vial containing the extract. To determine if manually shaking was comparable to mechanical stirring, a fiber was placed in a vial containing 4mL of 10ppb eight-mix standard (25 percent sodium chloride weight/volume) through a septum and lowered it until the bottom of the fiber holder was resting on the cap. The vial and fiber were shaken by hand (about two shakes per second), for five and ten minutes. Results are shown in Figure 6. When the solid phase microextraction fiber/extract was shaken for five minutes, the peak areas were at least 50 percent of the peak areas for the five-minute stirred extraction, except for PETN, which was only about 25 percent as large. When the solid phase microextraction fiber/extract was shaken for ten minutes, peak areas for ethylene glycol dinitrate and NG were approximately 75 percent of the peak areas for the five-minute stirred extractions. For DNT, TNT, RDX, and tetryl, the peak areas either matched or exceeded the areas for the stirred sample, while the peak area for PETN was only about half of the stirred sample. Although sensitivity is slightly less for three of the explosives, shaking the extracts clearly strengthens solid phase microextraction’s ability to be used in the field.
Figure 6. Abundances measurements (gas chromatography/electron capture detector) for explosives in a 10ppb eight-mix standard, sampled with a PDMS/DVB fiber by stirring, shaking, and static extraction.
Solid phase microextraction compares favorably with acetone extractions for laboratory use, and its use as a field-sampling tool is promising. Solid phase microextraction has several unique advantages over current laboratory-based extraction techniques. These advantages include speed of analysis, selectivity for explosives over the sample matrix, stability of explosives adsorbed to a fiber, and increased performance for the analysis of ethylene glycol dinitrate. A disadvantage of solid phase microextraction is that solid phase microextraction fibers are delicate and easily damaged.
Although solid phase microextraction is a suitable field sampling technique based on the data and results presented here, solid phase microextraction is most effectively used as a complementary technique or screening method when needed. With current methodology used by the FBI’s Explosives Unit, the cotton ball is cut in half for acetone extraction (organic explosives) and for aqueous extraction (inorganic explosives). The aqueous extract is analyzed by high performance liquid chromatography, but only a small portion is used, leaving the remainder available for analysis by solid phase microextraction. The second half of the cotton ball could be used for further organic analysis if warranted. In this way, the sample preparation does not change, yet by using solid phase microextraction, a complementary or screening analysis can be performed that would reduce time and labor if the acetone extraction is not needed.
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