United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S4-86/017 June 1986
;¥
Project Summary
Supercritical Fluid Extraction of
Paniculate and Adsorbent
Materials
Bob W. Wright and Richard D. Smith
The applicability of supercritical fluid
extraction of semivolatile and heavier
molecular weight compounds from adsor-
bents and particulate materials was in-
vestigated. These studies included off-line
supercritical fluid extraction, direct super-
critical fluid extraction-mass spectrometry,
and on-line supercritical fluid extraction-
gas chromatography. A related study on
the development of fluorescence detection
for capillary supercritical fluid chromato-
graphy (SFC) was also conducted.
Gram quantities of spiked XAD-2 resin,
SK-4 carbon, polyurethane foam (PUF),
and NBS Urban Dust particles were sub-
jected to off-line extraction, using carbon
dioxide, pentane, 20 mole percent meth-
anol in carbon dioxide, and 20 mole per-
cent ethanol in pentane as supercritical
fluid extracting solvents under a variety of
conditions. The supercritical fluid extrac-
tions were more rapid than Soxhlet extrac-
tions, but similar extraction efficiencies
were obtained. Different extraction re-
coveries were obtained with the various
fluids. Evidence suggests that much bet-
ter recoveries could be obtained if the col-
lection procedures were modified to eli-
minate volatilization and aerosol formation
after extraction. More rapid extraction pro-
cedures also appear feasible.
Direct supercritical fluid extraction-mass
spectrometry allowed the extraction pro-
files for each model compound from the
XAD-2 resin to be obtained as a function
of time. The compounds displayed dif-
ferent threshold pressures at which
extractability was detected and widely
varying decay curves as the material was
removed.
To develop methods to eliminate or
minimize sample preparation procedures.
studies using on-line supercritical fluid
extraction-gas chromatography were con-
ducted. With this methodology, milligram
quantities of samples such as particles or
adsorbents were selectively extracted
using supercritical carbon dioxide at
various pressures, followed by capillary
gas chromatographic analysis of each
extract. The technique was applied to the
NBS Urban Dust particles to demonstrate
that selective and direct analyses could be
obtained.
To obtain high detection sensitivity for
SFC using fluids that are incompatible with
flame ionization detection, development of
reliable capillary SFC fluoresence detec-
tion was also undertaken. The necessary
modifications of a commercial HPLC fluor-
escence detector were completed and
successfully interfaced to an SFC instru-
ment using supercritical pentane as a
mobile phase. The model compounds from
the extraction studies were used to de-
monstrate successful chromatographic
separation and fluorescence detection.
This Project Summary was developed
by EPA's Environmental Monitoring
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 com-
plexity are currently employed to prepare
samples for analysis. Less complex, more
rapid, and more sensitive preparation pro-
cedures would be desirable in many cases,
particularly for small samples or low con-
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centrations of analytes. Supercritical fluid
extraction procedures are attracting atten-
tion for use in selective large-scale process
applications, but only limited attention has
been given to using supercritical fluid ex-
traction methods in analytical applications.
The goals of this study were to determine
the potential utility of supercritical fluid ex-
traction for analytical sample preparation
and to develop improved methods of
analysis utilizing these procedures.
The potential advantages of super-
critical fluid extraction accrue from the
properties of a solvent at temperatures
and pressures above its critical point. At
elevated pressure this single phase will
have properties that are intermediate be-
tween those of the gas and liquid phases
and are dependent upon the fluid composi-
tion, temperature, and pressure. The com-
pressibility of supercritical fluids is large
just above the critical temperature; at this
point small changes in pressure result in
large changes in the density of the fluid.
Figure 1 shows a typical pressure-density
relationship in terms of reduced para-
meters (e.g., pressure, temperature, or
density divided by its respective critical
parameter). Isotherms for various reduced
temperatures show the variations in den-
sity that can be expected with changes in
pressure. Thus, the density of a super-
critical fluid will be typically 102 to 103
times greater than that of the gas. Con-
sequently, molecular interactions increase
due to the shorter intermolecular dis-
tances. However, the diffusion coefficients
and viscosity of the fluid remain similar to
that of the gas. The "liquid-like" behavior
of a supercritical fluid results in greatly
enhanced solubilizing capabilities com-
pared to a subcritical gas, but with higher
diffusion coefficients, lower viscosity, and
an extended temperature range compared
to the corresponding liquid. These proper-
ties allow solvent strengths similar to
those of liquids and the potential for more
rapid extraction rates than are possible
with liquids.
Procedure
Various studies were conducted to
evaluate the applicability and efficiency of
analytical supercritical fluid extraction
methods. These investigations were
among the very first to specifically address
the use of supercritical fluid extraction for
analytical purposes. Consequently, little
related information exists. The studies in-
volved in this project included offline
supercritical fluid extraction of adsorbents
and particles, direct supercritical fluid
extraction-mass spectrometry, and on-line
1.0 2.0
Reduced Density
3.0
T, = Reduced Temperature
Figure 1. Typical pressure-density behavior of a pure supercritical fluid.
supercritical fluid extraction-gas chro-
matography. A fluorescence detection
supercritical fluid chromatography (SFC)
system was also developed. Gram quan-
tities of XAD-2 resin, SK-4 carbon and
polyurethane foam (PUF) were spiked with
selected large polycyclic aromatic com-
pounds and subjected to supercritical fluid
extraction using either carbon dioxide, a
carbon dioxide-methanol mixture, pentane,
or a pentane-ethanol mixture as extracting
fluids. NBS Urban Dust (SRM 1649) was
also subjected to the same extraction
conditions. The extraction efficiency as a
function of the extracting fluid was
evaluated. Traditional liquid Soxhlet extrac-
tions of the same matrices were con-
ducted and compared to the supercritical
fluid extractions. Preliminary extractions
using direct supercritical fluid extraction-
mass spectrometry were also conducted
to obtain individual extraction profiles as
a function of time for each of the spiked
compounds. Direct analysis methods, in
which sample preparation and analysis are
combined, were investigated using on-line
supercritical fluid extraction-gas chroma-
tography with the NBS Urban Dust as a
sample matrix. Fluorescence SFC detec-
tion was developed to provide high detec-
tion sensitivity using supercritical fluid
systems that are not compatible with
flame ionization detection.
Results
High molecular weight polycyclic aro-
matic compounds at ppm concentrations
were extracted from typical adsorbent
matrices using supercritical fluid extrac-
tion methods. The various fluids displayed
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different extraction strengths, with carbon
dioxide generally being the poorest and the
alcohol-modified mixtures the best extrac-
ting fluid systems. Supercritical fluid ex-
traction provided relatively fast extractions
(1-2 hr or less) compared to traditional
Soxhlet extraction.
Of the fluids studied, the carbon di-
oxide-methanol mixture and pentane gave
the best extraction recoveries from the
XAD resin. Carbon dioxide alone at 50 °C
was a poor extraction solvent. Under the
experimental conditions used, Soxhlet
extraction with methylene chloride gave
better extraction recoveries than did car-
bon dioxide.
All the supercritical fluid and Soxhlet
extractions gave poor recoveries from the
activated carbon. Of the supercritical
fluids used with this matrix, the pentane-
ethanol fluid system gave the best overall
results. Soxhlet extraction after super-
critical fluid extraction gave higher extrac-
tion recoveries than Soxhlet extraction
before supercritical fluid extraction.
Difficulties were encountered with poly-
urethane foam, since it was soluble in the
supercritical fluids. However, comparable
extraction efficiencies were achieved with
both carbon dioxide-methanol and pen-
tane fluid systems compared to Soxhlet
extraction with methylene chloride. Car-
bon dioxide at 50 °C was a poor extrac-
tion solvent for use with PUF.
The pentane-ethanol fluid system gave
the highest recovery of material with the
best extraction efficiency from the Urban
Dust. Carbon dioxide at 50 °C and pentane
at 210 °C gave poor extraction recoveries.
Additional material was extracted by Sox-
hlet extraction with methylene chloride
following supercritical fluid extraction.
In most cases higher extraction efficien-
cies (recoveries) were obtained with Sox-
hlet extraction than with supercritical fluid
extraction. However, there was a problem
with the collection of the supercritical fluid
extracts, and a large percentage of maferr
ial was lost due to aerosol formation.
Unfortunately, this problem was not
discovered until after completion of the
work discussed above. Preliminary results
using a collection system in which the
total extraction effluent was trapped by
freezing in a sealed vessel cooled in liquid
nitrogen indicate that comparable extrac-
tion efficiencies can be obtained with both
supercritical fluid and Soxhlet extraction.
Conclusions and Recommendations
Direct supercritical fluid extraction-mass
spectrometry provides a valuable means
of simultaneously monitoring the extrac-
tion kinetics and threshold pressures for
a number of compounds. An understand-
ing of these parameters is necessary to
obtain selective fractionation of complex
samples.
On-line supercritical fluid extraction-gas
chromatography provides a means of
eliminating sample preparation and allows
automated extraction and analysis of
organic matrices. By utilizing the variable
solvating power of supercritical fluids,
selective extractions can be achieved.
Several recommendations can be made.
The off-line supercritical fluid extraction
studies should be repeated using the
improved collection system (e.g., freezing
the extraction effluent in a sealed flask
cooled in liquid nitrogen) to prevent
recovery losses. These results would be
more meaningful than the existing data in
terms of comparing extraction efficiencies
between the various fluid systems and to
Soxhlet extractions.
Spiked adsorbents and particulate
samples should be subjected to super-
critical fluid extraction after Soxhlet ex-
traction to determine if additional material
can be recovered. Larger volumes of the
supercritical extraction fluids (>250 ml li-
quid volume) should be utilized to deter-
mine if more complete extraction can be
obtained. Methods of enhancing extrac-
tability by recycling the fluid or improving
fluid contact with the sample matrix
should be investigated. Work on direct
supercritical fluid extraction-mass spec-
trometry should be continued using
isobaric conditions to obtain extraction
kinetics data. These data would be useful
for defining the optimum conditions to
selectively extract specific compounds
from a complex matrix.
The on-line supercritical fluid extraction-
gas chromatography methodology should
be applied to additional sample matrices,
and investigations should be conducted to
answer questions concerning quantitative
methods. Extraction kinetics could also be
studied using the instrumentation.
The fluorescence detection supercritical
fluid chromatography instrumentation
should be evaluated for quantitative analy-
sis and applied to the nonvolatile materials
in the model compound mixture. The ap-
plicability of supercritical fluid chromato-
graphic analysis to other nonvolatile
materials with fluorescent properties
should also be investigated.
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Bob W. Wright and Richard D. Smith are with Battelle Pacific Northwest
Laboratory, Rich/and, WA 99352.
Nancy K. Wilson is the EPA Project Officer (see below).
The complete report, entitled "Supercritical Fluid Extraction of Paniculate and
Adsorbent Materials," (Order No. PB 86-175 999/AS; Cost: $11.95, 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
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