Solid
phase
microextraction
(SPME)
is
one
of
several
important
sample
preparation
techniques.
SPME,
invented
in
early
nineties
by
Prof.
Janusz
Pawliszyn
from
the
University
of
Waterloo
in
Ontario,
Canada,
integrates
sampling,
extraction,
concentration
and
sample
introduction
into
a
single
step.
It
enables
solventless
extraction
via
a
fused
silica
or
stainless
steel
fibre
coated
with
a
thin
film
polymer,
which
acts
as
the
solvent
during
the
extraction
of
compounds.
The
fibre
is
mounted
on
syringe-like
device
for
extraction
of
analytes
from
various
matrices
and
introduction
to
a
chromatographic
system.
The
extraction
principle
can
be
described
as
an
equilibrium
process
in
which
the
analyte
partitions
between
the
fibre
and
the
aqueous
phase.
This
technology
has
been
patented,
US
patent
number
5691206,
European
patent
number
523092,
and
PCT
international
patent
application
#W091/15745,
filed
with
the
University
of
Waterloo.
SPME
has
been
commercialized
by
Supelco,
Varian,
Leap
Technologies,
and
Gerstel.
Volatile
or
semi-volatile
organic
compounds
may
be
extracted
by
either
immersing
the
fibres
in
a
liquid
sample
or
exposing
them
to
the
headspace
of
a
solid,
liquid
or
gas
sample.
The
analytes
partition
or
adsorb
to
the
polymer
coating
on
the
fibres.
The
absorbed
analytes
may
be
thermally
desorbed
in
the
injector
of
a
gas
chromatograph
(GC)
for
separation
and
further
quantification.
Intermediate
to
low
volatility
compounds
may
be
extracted
by
fibres
designed
for
use
with
high
performance
liquid
chromatography
(HPLC)
,
and
either
desorbed
in
a
SPME/HPLC
interface
or
offline
into
a
desorption
solvent,
which
is
subsequently
injected
with
a
conventional
injection
interface
or
autosampler.
Aside
from
the
obvious
advantages
of
a
solventless
sample
preparation
technique,
SPME
is
fast,
amiable
to
automation
and
easy
to
use.
Like
other
sample
preparation
techniques,
however,
users
still
need
to
understand
the
nature
of
the
target
analytes
and
the
complexity
of
the
matrix.
The
development
of
new
applications
and
methods
has
progressed
rapidly
over
the
years,
providing
the
thrust
behind
technological
advancements
and
overall
market
growth.
Many
applications
in
environmental,
food,
forensics,
clinical
and
pharmaceutical
laboratories
have
become
commonplace
for
analysis
of
semi-volatile
and
volatile
compounds
in
air,
aqueous
matrices
including
complex
slurry
and
soil
samples.
Theory of SPME
The
most
widely
used
form
of
SPME
sampling
consists
of
exposing
a
thin
polymeric
coating
directly
to
the
sample
matrix
for
a
predetermined
amount
of
time
as
shown
in
figure
below.
Once
the
fibre
is
exposed
to
the
sample
matrix,
the
transport
of
analytes
from
the
matrix
to
the
coating
begins
immediately.
SPME
extraction
is
considered
to
be
complete
when
the
analyte
concentration
has
reached
distribution
equilibrium
between
the
sample
matrix
and
the
fibre
coating.
The
equilibrium
conditions
can
be
described
by
equation
(1)
according
to
the
law
of
mass
conservation
for
a
two-phase
system
(for
example,
the
sample
matrix
and
the
fibre
coating).
CoVs = C∞sVs + C∞fVf (1)
where Co : initial concentration of analyte in the sample
Vs : volume of sample
Vf : volume of fibre coating
C∞f
:
equilibrium
concentration
on
the
fibre
C∞s
:
equilibrium
concentration
in
the
sample
The
distribution
constant
of
the
analyte
between
the
fibre
coating
and
the
sample
matrix
is
defined
by
equation
(2):
Kfs
=
C∞fVf
/
C∞sVs
(2)
The
number
of
moles
of
analyte
extracted
n
by
the
coating
at
equilibrium
can
be
expressed
in
equation
(3)
by
combining
equations
(1)
and
(2):
n
=
C∞fVf
=
(
KfsVf
Vs
Co)
/
(KfsVf
+Vs)
(3)
where
Kfs
is
the
distribution
coefficient
between
fibre
coating
and
sample
matrix.
For
a
three-phase
system
(e.g.
where
headspace
is
included),
equation
(4)
can
be
used
for
equilibrium
conditions:
n
=
C∞fVf
=
(
KfsVf
Vs
Co)
/
(KfsVf
+KhsVh
+Vs)
(4)
where
Khs
is
the
distribution
coefficient
between
fibre
coating
and
headspace.
This
equation
(4)
states
that
the
amount
of
analyte
extracted
is
independent
of
the
location
of
the
fibre
in
the
system.
The
fibre
may
be
placed
in
the
headspace
or
directly
in
the
sample,
as
long
as
the
volumes
of
the
fibre
coating,
headspace,
and
sample
are
kept
constant.
Commercially Available Fibre Coatings
PDMS - polydimethylsiloxane
PA - polyacrylate
PDMS/DVB - polydimethylsiloxane / divinylbenzene
PEG - carbowax-polyethylene glycol
CW/TPR - carbowax / templated resin
CAR/PDMS - carboxen / polydimethylsiloxane
DVB/CAR/PDMS - divinylbenzene / carboxen / polydimethylsiloxane
C18 - C18 silica particles
Alternative
Implementations
Thin Film Membrane Extraction
Needle Trap
In-tube
MESI (Membrane Extraction with a Sorbent Interface)
Cold Fibre
SPME
Couplings
in
use
SPME-GC
SPME-LC
SPME-MALDI