Solid-Phase Microextraction John V. Hinshaw, Serveron Corp., Hillsboro, Oregon, USA. In this month’s “GC Connections,” John Hinshaw discusses solid-phase microextraction…
sensitive and selective separation technique
all by itself. Capable of resolving hundreds
similar applications. The principal difference
inlet system that desorbs the solutes into a
part-per-million (ppm, one part in 106) or
is that SPE is performed with a relatively
better sensitivity, GC has been applied to
large sorptive surface the size of a small
filter paper, and it requires liquid-phase
extraction of analytes; SPME, however, is
SPME absorptive layer as much as possible
coated with sorptive material and primarily
matrix effects. Non-volatile constituents,
completely as possible for chromatographic
large sample volumes as required for lower
detection limits and less-than-ideal chemical
favour solute release from the absorptive
detection. Classic liquid–liquid extraction,
preconcentration — as well as headspace,
permit full use of column resolving power.
thermal-desorption and large-volume sample
injection techniques — increase analyte
accomplished using a discrete thermal trap
applied to the analysis of organic matrices
Connections” column discusses absorptive
also apply to adsorptive SPME onto active
matrix side effects after injection but are
often limited by sample residue build-up. Figure 1: Cross-sectional diagram of
within the past few months in the Journal
relatively new sample extraction technique
of Separation Science and the Journal of
(a) a fibre device with external sorptive
that brings some unique capabilities to the
Chromatography include flower scents,1
solutions in difficult matrices, both liquid
process impurities,3 organochlorine pesticides
discrete steps: solute absorption from the
sample matrix into a thick — relative to
phenols in wine,7 environmental pollutants
conventional capillary GC columns — layer
in water samples,8 chloroanisoles in cork
of silicone or related adsorptive material and
stoppers,9 volatile aliphatic amines in air10
transfer of the analytes into a chromatography
inlet system by gaseous or liquid means.
samples.11 This list delineates the breadth of
SPME has significant potential to greatly
applications to which SPME can be applied.
and the concomitant issues of used solvent
disposal as part of sample preparation.
SPME relies upon the extraction of solutes
liquid chromatography (LC) separations.
layer. After a sampling period — during
LC•GC Europe December 2003 GC Connections
For analysing bulk samples contained in vials
a limiting factor for the rate required to
or otherwise easily accessed samples, SPME
same thickness coated inside a tube [Figure
can be performed, as shown in Figure 1(a),
1(b)] with inner radius (r2) would be the
with a short, absorptive film–coated fibre.
same. The assumption in Equation 1 is valid
instead of the liquid in a two-phase sample
A short tube coated on the inside with an
absorptive layer [Figure 1(b)] can also be
liquid–gas interface before encountering
the SPME layer. Interestingly, it makes littledifference to the ultimate equilibriumsolute amounts in the SPME layer whether
The presence of a gaseous headspace over a liquid
the sample is obtained from the liquid orthe gas. However, the time to reach
sample causes a portion of each solute to partition into the headspace in competition with the extraction
the choice of the sampling phase. Non-polar
process into the SPME layer.
and volatile solutes that strongly favour theheadspace phase will come to equilibrium
more rapidly if the SPME layer is exposed
layer chemistry and film thickness strongly
the subsequent efficiency of desorption.
more rapidly directly from a liquid phase. Step 1 — extraction: For the extraction Influence of the headspace: The
characterize the equilibrium times for each
absorptive layer, the layer is exposed to a
solute of interest. If equilibrium is reached
solute to partition into the headspace in
in a reasonable time period — perhaps less
into the SPME layer. This effect results in a
sampling time at least that long. However,
reduction of solute mass in the SPME layer
if an unreasonably long time is required —
stopped-flow sampling is also possible. The
— relative to having no headspace present
in terms of the time available for sampling
gradually reach an equilibrium level with
headspace volume and the partition ratios
or the liquid sample and the SPME layer.
sampling time is used for each sample and
These relationships are somewhat complex,
solute, i ,in the SPME layer at equilibrium
extracted solute mass on the relative liquid
partition coefficients — both between the
volumes throughout multiple samples to keep
the SPME layer and the liquid — must also
such multiple-phase influences consistent. Time to equilibrium: A finite time span is
sample-to-sample consistency. Adding salt
absorptive layer and the sample, VSPME is
to an aqueous sample will often shift the
the volume of the SPME layer, and Ci is the
solute concentration in the sample before
equilibrium will ideally occur before the
Figure 2: SPME extraction from a sealed
volume is much greater than the volume of
10–100 µm — roughly 10-fold the film
places at the SPME–sample interface. The
process of absorption is limited by the rate
capillary GC. The volume (VSPME) of a 1 cm
at which solute molecules can replenish the
long by 100 µm thick annular coating on a
transition layer near the SPME interface.
Thorough stirring of the liquid phase helps
influence of solute liquid-diffusion rates
liquid sample does nothing to increase the
diffusion rate of absorbed solutes inside
the SPME layer itself, which then becomes
www.lcgceurope.com GC Connections
partition coefficients for non-polar solutes
The chemistry of the sorptive SPME layer plays a
in favour of the SPME layer and decreasethe time required to attain equilibrium. significant role in enhancing or discriminating against SPME-layer chemistry: The chemistry of classes of compounds.
the sorptive SPME layer plays a significantrole in enhancing or discriminating againstclasses of compounds. For the most part,
absorbed solutes to desorb with as close to
prevent some degree of peak tailing.
stationary phases: polar SPME layers such
time that is short enough to be compatible
inlet systems are also well suited to SPME
as those that contain polyesters or acrylates
desorption because of their smaller internal
the instance of an SPME layer coated inside
discriminate against non-polar materials.
arrangement can switch from sample liquid
temperatures as conventional split–splitless
constituents will retain volatile components
flow to the mobile phase for LC analysis.
vaporization heat-up rates — on the order
desorption to come to equilibrium between
to desorption as well — a very strongly
the SPME layer and the liquid mobile phase
without some form of additional stationary-
held solute might be too difficult to pry off
Step 2 — transfer: The next step after Why SPME?
sampling is to transfer the SPME layer and
GC, primarily because of the instrumental
phase. Analysts have no need to physically
chromatographic separation; its simplicity and
ease of use; and its reduced or non-existent
solvent consumption. These characteristics
device, however, will be removed from the
the desorbed solutes into the LC injection
extraction and classic liquid–liquid extraction.
where the solutes are to be desorbed.
preparation and injection techniques such
Several trade-offs arise in the course of
SPME lends itself well to handling difficult
in-sample values to lower levels as solutes
high enough so that the solutes leave the
benefits of low cost and simplicity. SPME
naturally desorb into their surroundings.
doesn’t require elaborate and expensive
instrument accessories for occasional use,
and yet it seems to be capable of delivering
molecules can experience significant losses.
for trapping solutes at the beginning of a
In a laboratory situation, the transfer time
necessarily be said of manual headspace or
short enough that losses are insignificant.
thermal-desorption sampling. Autosamplers
transport and storage by sealing the SPME
bleed and from the SPME layer itself.
consistent operating conditions for success,
fibre into a split–splitless inlet, the inlet split
but this statement is true of the related
desorption. In addition to volatile sample
flow should be turned off so that all of the
splitting. It is unlikely that enough sample
valid use in analytical laboratories, and the
transportation to and from remote sites.
necessitate sample splitting. A narrow-bore
Enclosing the SPME layer will also prevent
inlet liner — often called a splitless liner —
for any of the other techniques. SPME has
a significant place in analysts’ arrays of
limiting the volume into which the solutes
incorporate sealing systems such as these. Step 3 — desorption: Once in place at a
been withdrawn from an inlet splitter, the
P. Barták et al., J. Sep. Sci., 26(8), 715–721 (2003). LC•GC Europe December 2003 GC Connections
G.L. Hook et al., J. Sep. Sci., 26(12–13), 1091–1096 (2003).
R.P. Frost, M.S. Hussain and A.R Raghani, J. Sep. Sci., 26(12–13), 1097–1103 (2003).
L. Cai et al., J. Chromatogr. A, 1015(1–2), 11–21 (2003).
W.A. Araújo et al., J. Sep. Sci., 26(6/7), 624–628 (2003).
O. Pinho, C. Peres and I.M.P.L.V.O. Ferreira, J. Chromatogr. A, 1015(1–2), 23–30 (2003).
R. Castro Mejas et al., J. Chromatogr. A, 995(1–2), 11–20 (2003).
H. Bagheri and A. Mohammadi, J. Chromatogr. A, 1011(1–2), 1–9 (2003).
F. Bianchi et al., J. Sep. Sci., 26(5), 369–375 (2003).
J. Namiesnik, A. Jastrzebska and B. Zygmunt, J. Chromatogr. A, 1016(1), 1–9 (2003).
H.-H. Lin, Y.-H. Sung and S.-D. Huang, J. Chromatogr. A, 1012(1), 57–66 (2003).
“GC Connections” editor John V. Hinshaw is senior staff engineer at Serveron Corp., Hillsboro, Oregon, USA, and a member of the Editorial Advisory Board of LC•GC Europe.
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