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
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
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 twoone 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
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.
Solid Phase Microextraction
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 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
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.
Follow this link to
view Table 1.
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
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
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
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
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
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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