United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Research Triangle Park NC 2771
Research and Development
EPA/600/S4-87/040 Jan. 1988
Project Summary
Supercritical Fluid Extraction of
Paniculate and Adsorbent
Materials: Part II
Bob W. Wright and Richard D. Smith
Supercritical fluids have solvent
strengths similar to those of liquids, but
have higher diffusion coefficients,
lower viscosities and an extended
temperature range which provides the
potential for more rapid and efficient
extraction than can be achieved with
liquids. In this project, the applicability
and efficiency of analytical supercrit-
ical fluid extraction and related meth-
odologies were evaluated. These stu-
dies included development of
quantitative off-line supercritical fluid
extraction methodology and a compar-
ison to traditional Soxhlet extraction,
the development and evaluation of on-
line supercritical fluid extraction-gas
chromatography for combined sample
preparation and analysis, and direct
supercritical fluid extraction-mass
spectrometry for the monitoring of
specific extraction profiles as a func-
tion of time. The sample matrices
included an air particulate sample,
XAD-2 resin, polyurethanefoam (PUF),
and Spherocarb®* adsorbent, which
were spiked with various model com-
pounds. Carbon dioxide, isobutane,
and methanol-modified (18.2 mole %)
carbon dioxide were used as supercrit-
ical fluid systems. The quantitative
analysis capability of a fluorescence
detection supercritical fluid chroma-
tography method and solute focusing
methods for capillary supercritical fluid
chromatography were also developed
and evaluated.
•Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.
This Project Summary was devel-
oped by EPA's Environmental Monitor-
ing Systems Laboratory, Research
Triangle Park, NC. to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
Numerous methods and combinations
of methods with varying degrees of
complexity are currently employed to
prepare samples for analysis. Less
complex, more rapid, and more sensitive
preparation procedures would be desir-
able in many cases, particularly for small
samples or low concentrations of ana-
lytes. Supercritical fluid extraction (SFE)
has been successfully used for several
large-scale chemical processing applica-
tions and is currently attracting renewed
interest. However, only limited attention
has been given to SFE methods for
analytical applications. A number of
advantages including more rapid extrac-
tion rates, more efficient extractions,
increased selectivity, combined analyte
fractionation in conjunction with extrac-
tion, and direct continuous on-line
analysis, such as with mass spectrome-
try or periodic gas chromatographic
analysis, are possible with SFE compared
to conventional liquid extraction
methods. The goals of this study were
to determine the potential utility of SFE
for analytical sample preparation and to
develop improved methods of analysis
utilizing these and other supercritical
fluid analytical methodologies.
The potential advantages of SFE
accrue from the properties of a solvent
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at temperatures and pressures above its
critical point. At elevated pressure this
single phase will have properties that are
intermediate between those of the gas
and liquid phases and depend on the fluid
composition, pressure, and temperature.
The compressibility of supercritical fluids
is large just above the critical temper-
ature, and small changes in pressure
result in large changes in density of the
fluid. The density of a supercritical fluid
is typically 102 to 103 times greater than
that of the gas. Consequently, molecular
interactions increase due to shorter
mtermolecular distances. However, the
diffusion coefficients and viscosity of the
fluid, although density-dependent,
remain similar to those of a gas The
"liquid-like" behavior of a supercritical
fluid results in enhanced solubilizmg
capabilities compared to the subcritical
gas, and m higher diffusion coefficients,
lower viscosity, and an extended temper-
ature range compared to the correspond-
ing liquid. These properties allow solvent
strengths similar to those of liquids but
with greatly improved mass transfer
characteristics, which provide the poten-
tial for both more rapid extraction rates
and more efficient extraction due to
better penetration of the matrix.
Procedure
Studies were conducted to evaluate
the applicability and efficiency of analyt-
ical SFE and related methodologies.
These studies included off-line SFE of
adsorbents and particles, on-line super-
critical fluid extraction-capillary gas
chromatography (SFE-GC), and direct
supercritical fluid extraction-mass spec-
trometry (SFE-MS). The quantitative
analysis capability of the fluorescence
detection supercritical fluid chromatog-
raphy (SFC) system that was previously
developed in this project was also
evaluated. Preliminary studies aimed at
developing solute focusing methods for
capillary SFC similar to cryogenic trap-
ping in gas chromatography were also
conducted.
In the off-line SFE studies, various fluid
systems were evaluated and compared
to Soxhlet extraction for the recovery of
low concentrations (ppm) of model
compounds from selected adsorbents.
Higher molecular weight (228-450 dal-
tons) polycyclic aromatic hydrocarbons
(PAH) were extracted from XAD-2 resin,
polyurethane foam, and Spherocarb®
adsorbents. Air paniculate matter was
also used as a complex matrix for
evaluation of the SFE methods. In
addition to carbon dioxide, isobutane and
methanol-modified (18.2 mole percent)
carbon dioxide were used for SFE
solvents. Instrumentation was designed
to allow extraction of gram quantities of
the matrix at pressures up to 400 bar
and temperatures up to 235°C, with
collection of the effluent in a sealed
liquid-nitrogen-cooled flask.
A logical extension of SFE is to com-
bine the process with a chromatographic
analysis method. The variable solvating
power of a supercritical fluid furnishes
the mechanisms for the selective extrac-
tion of the components of interest from
the sample matrix and provides the basis
for an automated method where sample
preparation and analysis can be instru-
mentally linked. Instrumentation and
methodology were developed for the
automated SFE-GC system. The super-
critical fluid extracts were decompressed
through a restrictor to deposit and
concentrate the analytes at the inlet of
a standard capillary gas chromatography
column for subsequent analyses. This
methodology allows several modes of
operation including quantitative extrac-
tion of all analytes from a sample matrix,
quantitative extraction and concentra-
tion of trace analytes, selective extrac-
tions at various solvating powers to
obtain specific fractions, and multiple-
step extractions at various pressures for
qualitative characterizations. These
modes of operation were developed, and
the potential utility of this methodology
for sample extraction and selective
fractionation was demonstrated using a
standard polycyclic aromatic hydrocar-
bon mixture and two complex sample
matrices.
Direct SFE-MS was used to monitor
the individual extraction of each model
compound as it was extracted from XAD-
2 resin, PDF, and Spherocarb® adsorb-
ents with carbon dioxide, isobutane, and
methanol-modified (18.2 mole %) carbon
dioxide fluid systems. Individual extrac-
tion profiles of each of the spiked
compounds as a function of time were
obtained, and the different extraction
behaviors (threshold pressures, extrac-
tion rate, solubility, decay curves, etc.)
of the model compounds from the various
adsorbents were observed.
A fluorescence detection capillary SFC
system was developed to obtain high
sensitivity detection when using fluids
that are not compatible with flame
mnization detection, and its quantitative
analysis capability was evaluated. These
procedures included evaluation of a
multi-level detector calibration of several
compounds, documentation of the repro-
ducibility of multiple determinations of
specific mixtures, and comparison of the
quantitative values obtained from paral-
lel determinations using this methodol-
ogy and capillary gas chromatography.
Preliminary studies were conducted to
develop solute focusing methods for
capillary SFC similar to cryogenic trap-
ping in gas chromatography. One
approach that proved moderately suc-
cessful was to create density gradients
within a short length of the column by
altering the fluid temperature.
Results
Optimized off-line analytical SFE
instrumentation and methods were
developed that allowed high molecular
weight polycyclic aromatic compounds to
be successfully extracted and recovered
from the adsorbent matrices. A recipro-
cating pump provided the potential for
use of unlimited volumes of fluid in an
extraction, and collection of the extrac-
tion effluent in a sealed liquid-nitrogen-
cooled flask prevented analyte losses due
to aerosol formation during fluid decom-
pression. When an open collection
method was used under conditions
where aerosol formation was expected
to be inhibited (low temperature expan-
sion where a two-phase product was
expected or utilization of a polar liquid-
modified fluid), significantly higher
extraction recoveries were obtained. The
model compounds were more easily
extracted from PDF than from XAD-2
resin. Higher levels of background
components were also extracted from
PUF, and the PUF itself appeared to be
slightly soluble in methanol-modified
carbon dioxide. Except for PUF, the
various adsorbents did not appear to be
affected by SFE. Supercritical fluid
extraction of the highly adsorptive
Spherocarb® matrix with isobutane and
with methanol-modified carbon dioxide
fluids provided more efficient extraction
recoveries than those achieved by Soxh-
let extraction. However, only low levels
of the less polar analytes were recovered.
Supercritical fluid extraction also pro-
vided more rapid and improved extraction
efficiency of PAH from Urban Dust than
did Soxhlet extraction. The concentra-
tions of selected compounds obtained
from these analyses exhibited surpris-
ingly good agreement with the NBS
certified values.
On-line SFE-GC provides potential for
combined sample preparation and anal-
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ysis. In addition to completely automated
operation, rapid analyses and high
sensitivities can be achieved with this
methodology. The selectivity obtainable
with the wide range of solvent powers
available with SFE provides the potential
for fractionation of complex samples and
isolation of specific analytes from a
matrix. The quantitative extraction and
transfer of PAH analytes from an XAD-
2 resin to the gas chromatographic
column for analysis was demonstrated.
Quantitative extraction and transfer is
dependent on solute solubility which
requires using an adequate volume of the
supercritical fluid.
Direct SFE-MS provides a means of
simultaneously monitoring the extrac-
tion profiles of the individual model
compounds from the adsorbents. The
extraction behavior of the model com-
pounds from the various adsorbents with
the different fluid systems was consist-
ent with the off-line SFE studies. The
compounds were more soluble and better
extractability was achieved with isobu-
tane than carbon dioxide. More rapid
exhaustion of the compounds was also
obtained with isobutane. The methanol-
modified carbon dioxide mixture was
used under subcritical temperature
conditions which resulted in a two-phase
liquid-gas mixture rather than a super-
critical fluid. Consequently, the polar
compounds capable of hydrogen bonding
with liquid methanol were extracted
more rapidly and more efficiently than
the neutral compounds. The model
compounds were all extracted more
easily from PUF than from XAD-2 resin
with all the fluid systems studied. The
extraction profile intensities from PUF
correspond to the expected solubility
orders of the compounds. The model
compounds were so strongly adsorbed by
Spherocarb® that SFE-MS extraction
profiles could not be obtained.
Linear response of the neutral PAH
model compounds was achieved over the
narrow concentration range studied, and
sensitivities in the low picogram range
were obtained using fluorescence detec-
tion SFC. Calibration plots with less than
an 8% standard error were constructed
from replicate analyses at four concen-
tration levels. Slightly better calibration
was obtained from peak heights than
from peak areas. Replicate analyses of
standard mixtures and authentic sample
extracts agreed to within approximately
15%. Quantitative determinations of
authentic sample extracts from parallel
analyses using capillary gas chromatog-
raphy and_ fluorescence detection SFC
were generally consistent within a 20%
range. A significant portion of the
quantitative variance can be attributed
to problems associated with the injection
method.
A moderately successful solute focus-
ing technique for capillary SFC was
developed. The technique used a local
zone of lowered fluid density created by
heating a short length of the column
inlet. Preliminary evaluations of other
approaches using a retention gap and a
pressure gradient were also conducted.
Conclusions and
Recommendations
Essentially quantitative recoveries of
all the model compounds from the XAD-
2 and PUF adsorbents were achieved by
Soxhlet extraction. In general, the SFE
methods provided extraction efficiencies
comparable to traditional Soxhlet
methods, but with over an order of
magnitude increase in extraction speed.
Supercritical fluid extractions were
accomplished in 30-45 minutes com-
pared to 16 hours for Soxhlet extraction.
The more polar and higher molecular
weight analytes were more difficult to
extract from the adsorbents, and the
methanol-modified carbon dioxide fluid
and higher critical temperature isobu-
tane systems provided higher extraction
efficiencies than did carbon dioxide
alone. Limited solubility of the higher
molecular weight materials in carbon
dioxide limited their recoveries with the
relatively small volumes of fluid that
were used.
The utililty of on-line SFE-GC, and its
qualitative operation for sample extrac-
tion from adsorbent matrices, and its
potential for fractionation and isolation
of specific components from complex
matrices was demonstrated.
Direct SFE-MS allowed the individual
extraction profile of each model com-
pound to be monitored as it was extracted
from XAD-2 and PUF adsorbents with
carbon dioxide, isobutane, or methanol-
modified (18.2 mole %) carbon dioxide.
Extraction profiles for Spherocarb® could
not be constructed since only very low
signal levels were detected.
The fluorescence detection SFC sys-
tem was able to provide reliable quan-
titative determinations with a combined
uncertainty of approximatley 15%.
Several recommendations for off-line
SFE studies can be made. Since the
overall SFE rates appear to be limited by
the solubility of the analytes in the
extraction fluid rather than by desorption
or mass transfer rates, more rapid and
more efficient extractions should be
feasible by use of larger fluid volumes
at higher rates. The use of higher fluid
volumes may be best obtained by recom-
pression and recycling of the fluid.
Additional studies should be conducted
to develop collection methods that can
be routinely applied. A more rigorous
study of the chemical composition of the
supercritical fluid extracts of NBS Urban
Dust should be conducted. Detailed
fractionations and analyses of the
extracts would not only provide addi-
tional chemical composition information,
but would provide additional insights on
the extraction process.
The on-line SFE-GC and SFE-MS
extraction studies should be expanded.
More rigorous quantitative studies using
multiple analyses and a multi-level
calibration with a variety of model
compounds should be conducted to verify
the performance of the on-line SFE-GC
methodology. Methods of increasing the
extraction fluid flow rate to allow extrac-
tion of larger sample matrices in a short
time-frame should be investigated. The
methanol-modified carbon dioxide SFE-
MS studies should be repeated using an
extraction temperature above the critical
point and the results compared to those
obtained in this study using a subcritical
two-phase mixture. Appropriate
response factors and quantitative SFE-
MS extraction profiles should be
obtained. The SFE-MS approach should
be expandedto include a chromatograph-
ic separation step to allow concentration
of trace components in the extraction
effluent and to provide separation of the
components prior to MS analysis.
The linearity of the fluorescence
detector response should be evaluated
over a much wider range, e.g., >103.
Improved splitless injection methods
should be developed to improve the
accuracy and reproducibility of quantita-
tive SFC analyses. Such improvements
could include high-speed valve switching
to effect lower volume injections and use
of long retention gaps (>10 m) to aid
solute focusing on the analytical column.
The use of valves with 1732" zero-dead-
volume outlet fittings should also be
investigated.
The development of on-column solute
focusing methods for capillary SFC
should be continued. This development
should include use of long retention gaps
coupled with thermal focusing and use
of on-column solute concentration
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methods employing extraction-injection
and pressure focusing techniques
Bob W. Wright and Richard D. Smith are with Battelle, Pacific Northwest
Laboratory, Richland, WA 99352.
Nancy K. Wilson is the EPA Project Officer (see below).
The complete report, entitled "Supercritical Fluid Extraction of Paniculate and
Adsorbent Materials: Part II," (Order No. PB 88-133 699/AS; Cost: $14.95,
subject to change! will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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