β-cyclodextrin-bonded silica particles as novel sorbent for stir bar sorptive extraction of phenolic compounds.
ABSTRACT A stir bar coated with β-cyclodextrin-bonded-silica (CDS) as novel sorbent has been developed and used to analyze seven phenolic compounds in aqueous samples, followed by thermal desorption and gas chromatography-mass spectrometric detection. Significant parameters affecting sorption process such as the time and temperature of sorption and desorption, ionic strength, pH and stirring rate have been optimized and discussed. The coating has a high thermal stability up to 300°C and long application lifetime (80 times). The porous structure of CDS coating provides high surface area and allows high extraction efficiency. Under the selected conditions, linearity range of 0.1-400 μg/L, limit of quantifications of 0.08-3.3 μg/L and method detection limits of 0.02-1.00 μg/L have been obtained. A satisfactory repeatability (RSD ≤ 6.5, n = 7) with good linearity (0.9975 ≤ r(2) ≤ 0.9996) of results illustrated a good performance of the present method. The recovery of different natural water samples was higher than 81.5%.
A stir bar coated with β-cyclodextrin-bonded-silica (CDS) as novel
sorbent has been developed and used to analyze seven phenolic
compounds in aqueous samples, followed by thermal desorption
and gas chromatography-mass spectrometric detection. Significant
parameters affecting sorption process such as the time and
temperature of sorption and desorption, ionic strength, pH and
stirring rate have been optimized and discussed. The coating has a
high thermal stability up to 300°C and long application lifetime (80
times). The porous structure of CDS coating provides high surface
area and allows high extraction efficiency. Under the selected
conditions, linearity range of 0.1–400 µg/L, limit of quantifications
of 0.08–3.3 µg/Land method detection limits of 0.02–1.00 µg/L
have been obtained. A satisfactory repeatability (RSD ≤ 6.5%, n =
7) with good linearity (0.9975 ≤ r2≤ 0.9996) of results illustrated a
good performance of the present method. The recovery of different
natural water samples was higher than 81.5%.
Stir bar sorptive extraction (SBSE) has been reported by
which provides many advantages over conventional sample
tion and clean up in one step. In this technique a magnetic rod
ronmental samples (2,3), essential oils (4), food (5,6), polar phe-
nols (7), and utilized to measure organic compounds in
biological matrixes (8,9). These applications have been accom-
plished using only commercial stir bar under the name of
Twister (Gerstel GmbH), and its sorbent polydimethylsiloxane
(PDMS), which is apolar and less accordant for the polar com-
pounds. Bicchi et al. (10) have reported a dual-phase stir bar
which consisted of a short PDMS tube closed at both ends with
two magnets, and packed with different carbons. It has been
employed for the extraction of polar compounds. Sol-gel tech-
nology in stir bar has also been used for selective extraction of
poly aromatic hydrocarbons (PAHs), organophosphorous pesti-
cides (OPPs), bisphenol A and organic sulfur compounds
(11–13). A novel approach that applied molecular imprinted
polymers (MIP) to the coating material for stir bar has also been
recently reported by Zhu and coworkers (14). Huang et al. (15)
have reported the potential of a stir bar coated with monolithic
(16). Also, a novel multi residue method for screening organic
compounds by multi-stir bar sorptive extraction has been pre-
β-Cyclodextrin is a cyclic oligosaccharide with seven glucose
units, with a cavity structure, and can create an inclusion com-
plex with certain molecules through a host-guest interaction.
Thus, it has been used as HPLC stationary phase for the separa-
tion of various compounds (19,20). β-Cyclodextrin bonded silica
as a stationary sorbent for solid phase extraction of phenol com-
pounds has been expanded (21,22). Also, the application of
β-cyclodextrin in SPME (23,24) and membrane (25) has
enhanced enrichment factor. In this study, the stir bar coated
with β-cyclodextrin bonded silica stationary phase (CDS) has
been developed and used for selective adsorption and separation
of phenolic compounds in water samples, followed by thermal
desorption and gas chromatography–mass spectrometry detec-
tion (GC–MS), with improved efficiency.
β-Cyclodextrin and irregular silica gel were obtained from
Merck (Darmstadt, Germany). 3-Glycidoxy-propyltrimethoxysi-
lane (KH-560) and high-temperature epoxy resin of type 5203
were acquired from Huili Company (Jiangsu, China). Phenol
(PN), 2,4-dimethylphenol (24DMP), 2,4-dinitrophenol (24DNP),
chlorophenol (3CP) and 4-methylphenol (4MP) were obtained
from Merck (Darmstadt, Germany). Standard solutions (2000
mg/L) for each individual compounds were prepared in
weekly by diluting the standard solution with methanol, and
β β-Cyclodextrin-Bonded Silica Particles as Novel Sorbent
for Stir Bar Sorptive Extraction of Phenolic Compounds
Hakim Faraji1,*, Syed Waqif Husain2, and Masoumeh Helalizadeh3
1Department of Chemistry, Varamin Branch, Islamic Azad University, Varamin; 2Department of Chemistry, Faculty of Science, Science &
Research Branch, Islamic Azad University; 3Young Researchers Club, Varamin Branch, Islamic Azad University, Varamin
Reproduction (photocopying) of editorial content of this journal is prohibited without publisher’s permission.
Journal of Chromatographic Science, Vol. 49, July 2011
*Author to whom correspondence should be addressed: email firstname.lastname@example.org.
Journal of Chromatographic Science, Vol. 49, July 2011
more diluted working solutions were prepared daily by diluting
these solutions with Milli-Q water or sample water to give the
corresponding solutions for calibration, limits of detection and
Well water was collected from a well in our university, river
water was obtained from Jajrood River (northeast of Tehran,
Iran), and drinking mineral water sample available at the super-
market packed in polymeric containers. The river, well and min-
eral water samples were collected in glass bottles. The river and
well water sample were filtered before the analysis by a 0.45 µm
membrane filter (MSI, Westboro, MA. The water samples were
stored in refrigerator at 4°C.
Method detection limits (MDL) and limit of quantifications
(LOQ) for each analyte were determined by the accepted proce-
dure of the U.S. Environmental Protection Agency (26). A series
of eight replicate samples at 10 µg/L were analyzed. Precision was
assessed by determination of repeatability, which expresses the
precision of the method under the same operating conditions
over a short interval of time. It is also identified intraday precision
and is explained as %RSD. For repeatability, seven samples of
same concentration (15 µg/L) were analyzed by the CDS-coated
stir bar to examine variation arising, expressed as %RSD. The
accuracy of the analytical procedure was appraised using the
recovery test. The recoveries were examined in fortified water
samples at two levels of mixed standard solutions. The recovery
(R%) was expressed as observed concentration ×100 / theoretical
A Hewlett-Packard (HP, Palo Alta, CA) HP 6890 series gas chro-
matograph equipped with a split/split less injector and a HP 5973
mass-selective detector was also used. The analytical column was
a HP-5 MS 0.25 µm of 30 m × 250 µm i.d. The column tempera-
ture was programmed as follows: 80°C for 3 min then was heated
with a rate of 20°C/min up to 260°C and then with a rate of
30°C/min up to 290°C. The mass spectrometry was run at elec-
tron energy of 70 eV. The injection and GC–MS interface temper-
atures were set at 260°C and 280°C, respectively. The ion source
temperature was set at 250°C, and quadrupole temperature was
set at 200°C. The mass range scanned was 40–250 amu, quantita-
tive assessment of phenolic compounds were carried out in the
selected ion monitoring mode (SIM) in order to enhance the limit
of detection. Helium and nitrogen (99.999%) were utilized as car-
rier and make-up gases, respectively. PDMS-coated stir bars (10
mm length, 0.5 mm film thickness, Twister, Gerstel Gmbh and
Mulheim, Germany) were preconditioned for 1 h at 250°C under
a stream of high-purity N2. A home-made thermal desorption
system was assembled according to a procedure reported by Liu
et al. (11). A JSM-6330F scanning electron micro analyzer (Japan
Electronic Company) was used to measure the CDS surface.
Preparation of CDS sorptive bars
The CDS was prepared according to previous study reported
elsewhere with some modification (22). 1.718 g of β-cyclodextrin
was dissolved in 37.5 mL of dry dimethylformamide (DMF), to
which 0.15 g of metal sodium was added. It was stirred at room
temperature, for ~ 30 min to cause a reaction. After filtration, to
remove no reacted sodium hydride 0.68 mL of 3-glycidoxypropy-
ltriethoxysilane was added to the filtrate, which was allowed to
react at 90°C for 5 h under a nitrogen atmosphere. Then, 7.5 g of
silica gel was added, and the mixture was allowed to react for 10
h at 80–100°C. The CDS was filtered, and washed with DMF,
methanol, distilled water and acetone in sequence.
Subsequently, the CDS was dried at 120°C for 3 h, and kept in a
desiccator before use.
The glass tubes (15 mm ×1 mm O.D.) with magnetic bar were
sealed by the alcohol flame and sequentially cleaned by water and
acetone, followed by 1 mol/L NaOH and 1 mol/L HCl for 3 h,
respectively. After washing by distilled water, the bars were kept
at room temperature inside desiccator. The CDS was then fixed
on the glass bars utilizing a high temperature epoxy resin. After
that the bars were dried at room temperature for 4 h, and then
they were heated at 280°C under nitrogen protection for 2 h.
Finally, they were allowed to cool to room temperature inside
desiccator. The procedure was repeated three times.
The spiking standard was added to a 10 mL sample of pH 2
buffer solution, which had been saturated with sodium chloride.
The stir bar was immersed in the vial and it was stirred in the
sample for 50 min at 50°C with 800 rpm. Then, the stir bar was
removed from the vial and the water remaining on the surface
was desiccated with lint-free tissue, placed in glass thermal des-
orption tube and thermally desorbed in the laboratory-made des-
orption unit for 5 min at 250°C.
Results and Discussion
Sorptive enrichment in aqueous media is an equilibrium pro-
cess; therefore, extraction efficiency is significantly influenced by
different parameters such as sample pH, ionic strength, stirring
rate, sample solution temperature and extraction time, and have
Optimized SBSE conditions
The addition of acid and salt, singularly and in combination,
was studied as a means of enhancing the amount extracted by
Figure 1. Extraction enhancement under salt saturated and pH 2 conditions
(RSDs ≤ 7.2%).
the sorbent. Figure 1 shows the amounts of different phenolic
compounds extracted from the pH 2 buffer-saturated salt solu-
tion and the control sample at neutral pH and with no salt added.
Generally, in order to increase the extraction efficiencies of the
phenolic compounds in aqueous solution, the solution is acidi-
fied to preclude them from dissociating during extraction (27).
At low pH the acid-base equilibrium for the phenolic compounds
transfers significantly towards the neutral forms, which have
greater affinities for the sorbent, and the extraction efficiencies
are, therefore, enhanced. The effect of neutral molecules
becoming insoluble as the water molecules prefer to solvate the
salt ions is prevalently demonstrated as “salting out” (28). The
presence of the salt prevents their solubility in the water and
forces more of these analytes into the sorbent. The positive
effects of both acid and salt can be conceived when they are uti-
lized in combination. With pH 2 and saturated salt circum-
stances, the amount extracted for every analyte in the mixture
was higher than the control sample at pH 7 and with no salt
added. Under these conditions, all phenolic compounds are in
their neutral form and are salted out of solution into the sorbent.
Stirring intensity is one of the important parameter that
increases the extraction efficiency and lowers extraction time.
The agitation can renovate a new sample solution surface, there-
fore accelerating the mass transfer from the aqueous phase to
the sorbent phase. The optimum stirring rate was evaluated by
testing different stirring rates between 400 and 1000 rpm. The
results demonstrated that the equilibrium was attained at 800
rpm and no significant enhancement after that was observed.
Hence, 800 rpm was applied in the following experiments.
Temperature greatly influences the kinetics of extraction. By
increasing the temperature of the sample solution, molecules of
the analytes become more active; therefore, this process expe-
dites the mass transfer of the analyte from the sample solution to
sorbent phase. The effect of temperature on the extraction effi-
ciency was also studied for 10 mL of a saturated salt solution at
pH 2 containing 10.0 µg/L of each phenolic compound in the
range of 20–70°C by the CDS stir bar. The stirred solution (800
rpm) was kept for 50 min. Figure 2 shows that the extraction effi-
ciencies of the analytes were first increased slowly with
increasing extraction temperature from 25°C to 50°C, but after
that they were declined. However, because the adsorption pro-
cess was exothermic, the stir bar coating/sample partition coeffi-
cient (K) reduced with increased temperature, and affinity of the
analytes for the stir bar coating decreased at high temperature,
which resulted in lower equilibrium amounts of analytes that the
coating was able to extract (29). Thus, 50°C was selected as
The SBSE process depends on equilibrium. Therefore, the
equilibration time was determined by exposing the stir bar to
sample matrix containing the studied analytes for a variety of
times, from 10 to 70 min, until the amounts extracted remained
constant. The experimental results illustrated that the equilib-
rium of phenolic compounds was reached within 50 min.
The amounts of analytes desorbed from the loaded stir bar will
modify detection sensitivity. These quantities depended on des-
orption temperature and the time, while the stir bar was in the
thermal desorption unit. The optimum desorption conditions
were also examined and on the basis of the results obtained, it
was desorbed at 250°C for 5 min in all experiments.
Characterization of CDS stir bar
To investigate the thermal stability of the CDS-coated stir bar
it was conditioned at 230, 250, 270, 290,and 300°C for 1 h under
the protection of a nitrogen atmosphere. After thermal condi-
tioning at each temperature it was utilized to extract 10 mL an
aqueous sample containing 10.0 µg/L–1of each analyte under
optimum conditions. It is clear from Figure 3 that the extraction
capacity of sorbent was not significantly affected by the temper-
ature applied for thermal condition.
The use of capillary glass bars as the carrier of coating can
extend the amount of sorbent on the bar. Figure 4 shows the scan-
ning electron micrograph of the CDS-coated stir bar. It is clear
Journal of Chromatographic Science, Vol. 49, July 2011
Figure 2.The effect of temperature on the extraction efficiency of CDS-coated
stir bar (RSDs ≤ 6.5%).
Figure 3. Thermal stability profile of the CDS-coated stir bar (RSDs ≤ 6.2%).
Figure 4. Scanning electron micrograph of the CDS-coated stir bar.
that the CDS bar has a porous structure. Such a porous structure
should significantly enhance the extraction capacity. Figure 5
shows the extraction efficiency of the CDS-coated stir bar in
extracting target analytes from the aqueous solution under
optimum conditions after being used for 20, 40, 60,and 80 times.
To ensure the reproducibility, the data utilized for the y-axis are
the comparison of the areas in the chromatogram measured
using the CDS-coated stir bar (to extract 10 mL aqueous sample
containing 10.0 µg/L of each analyte under optimum conditions)
with the corresponding peak areas obtained by direct injection of
a solution with the same concentration. The ratios of areas
achieved after the stir bars were employed for different times are
convenient in order to compute for change of factors in SBSE-GC
conditions. The ratios are displayed in Figure 5 and indicate that
there is nearly no obvious decrease after being used for 80 times.
The chromatograms of phenolic compounds using CDS and
commercial PDMS coating are shown in Figure 6D and 6C,
respectively. Examination of the chromatograms reveals that the
response signals of the CDS coating were significantly stronger
than those of PDMS coating. The polar compounds have higher
affinities for the polar coating than for the nonpolar coating;
therefore, the amount of phenolic compounds extracted by polar
CDS coating was more than with the nonpolar PDMS coating.
This is because the surface area of the CDS coating is much
higher due to the porosity of the bonded silica particles.
Moreover, the unique molecular structure of β-cyclodextrin on
the surface of coating performs a role in the extraction of phe-
nolic compounds. The β-cyclodextrin molecule has the shape of
a hollow truncated cone. The interior of the cavity, which con-
tains two rings of C-H groups with a ring of glycoside oxygen in
between, is relatively hydrophobic, and the external faces with
hydroxyl groups are also hydrophobic (19). On account of this
special structure, they can selectively embrace guest molecules
into their hydrophobic cavity to create inclusion compounds
with different stabilities.
The coefficient of determinations (r2), linearity, repro-
ducibility, repeatability, limit of quantifications (LOQ) and MDL
were calculated and are recorded in Table I. The coefficient of
determinations and linearity values for the tested phenolic com-
pounds were in the range 0.9975–0.9996 and 0.1–400 µg/L,
respectively. The bar-to-bar and batch-to-batch reproducibility of
the CDS coating procedure were also studied. Five
different bars were coated under the same condi-
tions and three identically prepared bars among
three different batches were examined for the
extraction target analytes from aqueous solution.
The relative standard deviations (RSDs) for dif-
ferent bars and different batches were 6.9–10.2%
and 8.5–12.3%, respectively. These results show a
good reproducibility for preparation of the CDS-
coated stir bars. The repeatability of the CDS-
coated stir bar attained at 15.0 µg/L for each studied
analyte by calculating the RSDs of seven replicates
were 3.1–6.5%. The obtained LOQs and MDLs were
in the range of 0.08–3.3 µg/L and 0.02–1.00 µg/L,
Journal of Chromatographic Science, Vol. 49, July 2011
Figure 5.Lifetime profile of the CDS-coated stir bar for different times of their
use (RSDs ≤ 5.7%).
Figure 6. SBSE-GC-MS chromatograms of mineral water sample: (A)
unspiked, (B) spiked at 10.0 µg/L and (D) spiked at 25.0 µg/L of phenol com-
pounds utilizing CDS coating, and (C) spiked at 25.0 µg/L of analytes using
PDMS-coated stir bar. (1) PN, (2) 4MP, (3) 24DMP, (4) 3CP, (5) 24DNP, (6)
4NP, and (7) 2M46DNP.
Table I. Figures of Merit of the Proposed Method for SBSE of Phenolic Compounds
Reproducibility (RSD %)
(RSD%, n = 7)Analytesr2
* MDL = Method detection limit. LOQ = Limit of quantification. LR = Linearity range.
†(n = 5).
‡(n = 3).
Journal of Chromatographic Science, Vol. 49, July 2011
Application to real samples
Finally, the applicability of the extraction method was evalu-
ated by analysis of the real samples including well water, river
water, and mineral water. The results indicated that analyzed
samples had not been contaminated by phenolic compounds. All
the real water samples were spiked at two different concentration
levels (10.0 and 25 µg/L) to assess the matrix effect. The relative
recovery defined as the peak area ratio of a natural water sample
and ultrapure water sample spiked with analytes at the same
level, was applied (30). The relative recoveries of the analytes are
given in Table II which varies from 81.5% to 101.7%, it shows
that the influence of matrix is not significant on the extraction
recoveries. The chromatograms obtained by GC–MS of unspiked
mineral water and that spiked at two concentrations of each ana-
lyte after the developed method at optimum conditions is shown
in Figure 6.
A novel CDS-coated stir bar is developed and has been investi-
gated with seven phenolic compounds. The porous structure
provided large adsorption capacity, high adsorption rate and
strong analyte interaction. The developed stir bar indicated good
thermal stability and can be reused for at least 80 times. It also
demonstrates better selectivity to polar compounds compared to
the PDMS coated bars. The stir bar is employed for the determi-
nation of studied analytes in aqueous samples, with good and
The authors gratefully acknowledge the financial support pro-
vided by the Research Council of the Islamic Azad University at
Varamin and Science & Research Branch to carry out this work.
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revision received June 24, 2010.