Brazilian Journal of Analytical Chemistry - BRJAC - Brazilian Journal ...

Eberlin et al.
surface is very irregular, with the appearance of an agglomerate
of globular and/or cylindrical elements of ~ 5
µm or less. Accurate measurement of the coating thickness
is not feasible, due to this rough and irregular surface.
Comparing the coated fiber (Figure 2B, fiber thickness
of 194 ± 4 µm) with an uncoated fused silica fiber
(Figure 2A, fiber thickness of 130 µm), the film thickness
is calculated to be 32 ± 2 µm. Apart from preventing an
accurate estimation of thickness, this irregular surface is
an attractive characteristic for a SPME fiber since it leads
to an increase in the surface area of the fiber exposed
to analytes or to the vaccuum inside the MS ionization
chamber, allowing faster mass transfer of analytes to and
from the fiber. The microstructure of the coating can be
inspected after 10k× magnification (Figure 2C), and the
APTMS / PDMS surface appears here as being compact
and non-porous. The material seems to be a cluster of
scale-like pieces, and no macro- or mesoporous structures
are visible. The lack of a pore system can be considered
a drawback since irregular surfaces improve the
performance of SPME fibers. But, apparently, the analytical
performance of the fiber is not affected by this feature,
as demonstrated elsewhere [16]. The coating also
seems to maintain its integrity even after 200 extraction
and desorption cycles. This endurance is relevant,
since both processes occur under conditions which are
particularly aggressive to SPME fibers: the extraction is
performed by its immersion in acidified media, and the
fiber is directly exposed to heating from the EI filaments.
This robustness, not so typical for conventional fibers, is
typical of sol-gel coated SPME fibers, probably due to the
chemical bonding between the coating and the fused
silica support.
fi g u rE 3. sPmE ExtraCtion P r o f i l E o f CP f ro m a q u E o u s s o l u t i o n s (100
µg l -1 ) in Ph 1.
Extraction and MS Signal Profiles. Figure 3 shows FIMS
(APTMS/PDMS) extraction profiles for all CP investigated.
Equilibration is reached for periods ranging from
~6 min (PCP, 43CP) to ~13 min (246CP). These results
show that the equilibration is extremely fast, compared
to typical values of 60 min or more obtained for CP
with commercial SPME fibers. Such fast equilibration is
characteristic of sol-gel coated SPME fibers. Since the
coatings provided by the sol-gel process are thinner,
equilibration with sample is more rapidly achieved, in
comparison with thicker coatings from commercial fibers.
For these results, a time of 7.5 min was chosen
as ideal for direct extraction, with the solution pH adjusted
to 1 and with no CP derivatization. The standard
relative deviation presented in CP ranged from 1.4%
to 25%. Considering the extreme extraction and desorption
conditions, and the considerable gains in speed
and simplicity, these values seem quite acceptable.
fi g u r E 4. fims-aPs/Pdms P r o f i l E f o r sim o f t h E i o n o f m/z 128 f ro m
2CP. a: fiBEr B l a n k; B: dEionizEd w a t E r B l a n k; C: 200 µgl -1 a q u E o u s
tEst s a m P l E; d: na t u ra l w a t E r sPikEd w i t h 200 µgl-1 o f E a C h CP; E:
mEoh/w a t E r E x t ra C t o f w o o d E n ra i l w a y slEEPEr sPikEd w i t h 200 µgl-1
o f E a C h CP.
Figure 4 shows typical FIMS signal profiles obtained
by selective ion monitoring (SIM) of the diagnostic ion
of m/z 128 for 2CP. Similar profiles were observed for
the other CP investigated. Curve A is the fiber blank,
presenting no relevant signals that could interfere with
the analyte signal. Curve B is the extraction signal of a
deionized water sample: in this case, little signal perturbation
is observed likely due to water adsorption by
and subsequent desorption from the polar fiber coating.
Curves C and D, intense and quite similar peaks
for the ion of m/z 128 from 2CP obtained from a 200
µg L -1 aqueous solution are seen, with intensities ca.
ten times greater than the blank. Curve E is for a wood
sample spiked with 200 µg L -1 of 2CP; note the peak
with intensity ca. fourteen times greater than the blank.
After analysis, the fiber was re-inserted into the ionization
chamber for cleaning. No substantial carryover effects
were observed.
172 Br J Anal Chem