American Journal of Anal yt ical Chemistry, 2011, 2, 689-696
doi:10.4236/ajac.2011.26079 Published Online October 2011 (
Copyright © 2011 SciRes. AJAC
Single Drop Microextraction of Biphenyl and Biphenyl
Oxide in Aqueous Samples by Gas Chromatography-Flame
Ionization Detection
Maryam Sarkhosh1, Ali Mehdinia2*, Ali Jabbari1, Yadollah Yamini3
1Department of Chemistry, Faculty of Sciences, K.N. Toosi University of Technology, Tehran, Iran
2Department of Marine Living Resources, Iranian National In stitute for Oceanography, Tehran, Iran
3Department of Chemistry, Tarbiat Modares University, Tehran, Iran
E-mail: *
Received June 23, 2011; revised July 29, 2011; accepted August 7, 2011
In this work, biphenyl and biphenyl oxide were extracted by direct single drop microextraction (di-
rect-SDME) and analyzed by gas chromatography flame ionization detection. The extraction occurred by
suspending a 7 µL drop of toluene (as extracting solvent) containing acetonaphton (as internal standard) from
the tip of a microsyringe in direct-SDME, respectively. The effect of different parameters such as nature of
extraction solvent, microdrop and sample temperatures, stirring rate, microdrop and sample volumes, ionic
strength and extracting time on the extraction efficiency of the analytes were investigated and optimized.
Under optimized conditions the detection limits (S/N = 3) of the biphenyl and biphenyl oxide were 1.80 ±
0.03 and 1.10 ± 0.02 µg·mL–1, respectively. Good linearity was obtained for both analytes using extraction
techniques with the correlation coefficients at least 0.997 and the relative standard deviations (R.S.D.) were
in the range of 1% - 3%. The percent recoveries of the analytes from spiked water samples were near to
Keywords: Biphenyl, Biphenyl Oxide, Direct Single Drop Microextraction
1. Introduction
One of the most common heat transfer fluids, Dowtherm
A, is a eutectic mixture containing 26.5% biphenyl and
73.5% biphenyl oxide [1]. Biphenyl is used as dye car-
rier for polyesters, feedstock, especially in the production
of alkylbiphenyls, and a citrus fruit wrapping impreg-
nate to reduce spoilage. Biphenyl oxide (DPO) is used in
the production of emulsifiers, surfactants and textile dye
labelling as a chemically reacted intermediate. The main
application of this mixture is heat transfer in the distilla-
tion tower and fatty acid production liner. This liquid has
optimum heat coefficient with maximum service tem-
perature up to 400˚C. There are many problems in fatty
acid distillation tower that this liquid leaks from stocks to
environment. The mixture is very harmful for human and
ecological system. The concentration upper critical limit
causes damage in liver and neurotic systems [2]. Thresh-
olds limit value (TLV) in the surface waters is 0.2 g·mL–1
for both of them [3]. Conventional method for analyz-
ing of this liquid is liquid-liquid extraction and gas
chromatographic analysis, that is very time-consuming
and uses much amount of organic solvent. Reported
method for measurement of diphenyl and diphenyl oxide
are very limited and in all of them, solution directly in-
ject into the instruments: 1) Direct injection to gas chro-
matograph equipped with flame ionization detector
(GC/FID) with the accuracy of 9 µg·mL–1 [4]. 2) Direct
injection to gas chromatograph equipped with mass
spectrometer detector (GC/MS) with the accuracy of 12
pg·kg–1 [5]. 3) Direct injection to high performance liq-
uid chromatograph equipped with fluorescence detector
(HPLC-FLD) with the accuracy of 2 µg·mL–1 [6].
In general, liquid-liquid extraction (LLE) and solid-
phase extraction (SPE) are the most commonly used sam-
ple pretreatment methods for the isolation and/or en-
richment of organic pollutants [7-9]. Nonetheless, the
necessity of reducing overall sample preparation time
and quantities of organic solvents led to the development
of several new extraction approaches, including solid-
phase microex-traction (SPME) [9-11] and solvent mi-
croextraction [12-14] for the extraction of organic pol-
lutants from environmental samples. Recently efforts
have been placed on miniaturising the LLE extraction
procedure by greatly reducing the solvent to aqueous
phase ratio, leading to the development of solvent mi-
croextraction methodologies. Single-drop microextrac-
tion (SDME) is one of these methodologies and involves
extraction of organic contaminants from an aqueous do-
nor solution into a microdrop of an organic acceptor sol-
vent suspended to the tip of a microsyringe. After ex-
tracting for a prescribed period of time, the microdrop is
retracted back into the microsyringe and injected into the
instruments such as GC and HPLC for further analysis.
SDME has the advantages of high extraction speed and
extreme simplicity. It uses inexpensive apparatus and
vir- tually eliminates solvent consumption. SDME has
been successfully applied for the determination of alco-
hols [15], nitroaromatics [16,17], chlorobenzenes [18],
drugs [19,20], volatile organic compounds (VOCs) [21],
polycyclic aromatic hydrocarbons (PAHs) [22], aliphatic
amines [23], BTEX [24], and also for the screening of
pesticides [25-27], in water samples. To the best of our
knowledge, there are no reports that used SDME for the
extraction of biphenyl and biphenyl oxide, while, other
micr4oextraction methods such as dispersive liquid-liq-
uid microextraction [28] and solid-phase microextraction
[29] were previously used to analyze biphenyl and bi-
phenyl oxide.
In this work direct-SDME method was used for the ex-
traction of biphenyl and biphenyl oxide from aqueous
samples. The method provided a sensitive and easy-to-
use tool for the environmental monitoring of these con-
taminants. For obtaining the optimum extraction condi-
tions, different parameters affecting the extraction effi-
ciency were studied and optimised.
2. Experimental
2.1. Reagents and Materials
Biphenyl, biphenyl oxide, acetonaphton, 1-butanol, 1-
octanol, n-hexane, toluene, acetone, dodecanol, decanol,
cyclohexane, isopropyl alcohol, benzyl alcohol, dihexyl
ether, reagent grade sodium chloride and HPLC grade
methanol were purchased from Merck. The stock stan-
dard solutions of biphenyl, biphenyl oxide, acetonaphton
(1000 mg·L–1) were prepared in methanol. They were
stored and refrigerated at 4˚C. Biphenyl and biphenyl
oxide stock standard solutions were diluted with metha-
nol to prepare a mixed stock solution of analytes with
concentration of 100 mg·L–1. Then, working standard
solutions were freshly prepared by diluting the mixed
standard solution with distilled water to the required
concentrations. A solution of acetonaphton as internal
standard (IS) with 25 and 15 mg·L–1 concentration was
prepared in toluene. This solution was used as extracting
2.2. Apparatus
A 10-µL Hamilton microsyringe (Hamilton, Bonaduz,
Switzerland) model 701RN with a bevel needle tip
(length: 5.1 cm, ID: 0.013 cm, bevel 22) was used for the
extraction and injection procedures. Stirring the solution
was carried out with a magnetic stirrer (Heidolph MR
3001 K) and a 8 mm × 1.5 mm stirring bar. Two circu-
lating water bathes were used for adjusting the tempera-
tures of syringe needle and sample solutions with accu-
racy of ± 0.01˚C. Also, a home-made two compartment
recirculation glass cell was used for controlling the sam-
ple temperature.
Separation and quantification of biphenyl and biphe-
nyl oxide analytes were carried out by using a Shima-
dzu-14 B gas chromatograph equipped with a flame
ionization detector and a DB-5 (5% biphenyl + 95%
ploydimethylsiloxane) fused-silica capillary column with
a 25 m × 0.33 mm I.D and 0.5 µm film thickness (Shi-
madzu). The injection port and detector were operated at
275 and 285˚C, respectively. The GC split valve was
open and helium was used as carrier gas to give a 4
mL·min–1 column flow and 5 mL·min–1 split line flow.
The detector gases flow rates were 300 mL·min–1 of air
and 30 mL·min–1 of hydrogen. The column was held at
100˚C for 1 min, increased to final temperature of 260˚C
at a ramp of 10˚C·min–1 and then held for 5 min in this
2.3. SDME Procedure
A 10-µL Hamilton microsyringe, containing 2 µL (unless
otherwise stated within the text) of a water-immiscible
solvent (typically toluene) was clamped above the vial
(14 mL) containing 13.5 mL of donor aqueous sample
and stirred at 400 rpm. For all quantification experiments,
fixed concentration of acetonaphton (25 mg·L–1) as in-
ternal standard was prepared in toluene as extracting
solvent. The Hamilton microsyringe was completely
washed with methanol, and then with acetone. After
drying the syringe, it was rinsed and primed at least
seven times with the solvent/internal standard. Each time,
the needle of the microsyringe passed through the sample
vial septum and the tip of the needle was immersed in the
liquid phase. The plunger was depressed and the 2 µL
drop of the organic phase was exposed to the sample.
The analytes were then allowed to partition between the
Copyright © 2011 SciRes. AJAC
aqueous solution and the organic phase at room tem-
perature for 15 min (unless otherwise stated). After ex-
traction, the microdrop was retracted into the microsy-
ringe and transferred to the hot injector of the GC-FID
for analysis.
3. Results and Discussion
The initial objective was to optimize the direct-SDME
sampling conditions and to fix the parametric values for
the extraction of biphenyl and biphenyl oxide. The dy-
namic characteristics of the microextraction process are
closely related to the mass transfer of the analytes from
the aqueous to the organic phase. Intrinsically, the
SDME process is driven by the difference gradient of
concentration between aqueous and organic phases.
There were several parameters which determine the mass
transfer such as the extraction solvent, drop and sample
volume, temperature and ionic strength, stirring rate and
extraction time. These parameters were chosen to opti-
mize the performance in the current study via a univari-
ate optimization approach. Conditions for the both pro-
cedure of SDME were tested using water solution of 25
µg·L–1 for each analyte. To obtain optimized extraction
conditions, the relative peak area of analyte to the inter-
nal standard (acetonaphton) was used in GC-FID analy-
sis of the extracts.
3.1. Optimization of the SDME Procedures
3.1.1. Selecti on of Org ani c S olv ent
The first step for the direct-SDME method in order to
obtain optimized extraction is the selection of an appro-
priate extraction solvent. The selection of the extraction
solvent was based on the principle of “like dissolve like”.
The final choice was based on the following factors: ex-
traction efficiency, stability of the microdrop, low vola-
tility, polarity, dipole moment, viscosity, and the com-
patibility with direct injection into GC system, therefore
several solvents with different chemical characteristics
were tested. Three replicate tests were performed for
each type of organic solvent. The responses obtained for
each analyte with the five solvents are indicated in Fig-
ure 1. The results showed that diehexyl ether exhibited
the highest extraction efficiency for all the analytes and
is more stable when compared with the other solvents.
But their chromatographic peaks overlapped with some
of the analytes’ peaks (Figure 2). After a detailed com-
parison of the other solvents on the peak areas of the two
main compounds, toluene was found to be optimal and
finally chosen as the extraction solvent for the SDME
For all quantification experiments, fixed concentration
Dihexyl etherTolueneCyclohexane2-DecanolDecanol
peak area
Biphenyl B iphenyl oxi de
Figure 1. Effect of different extraction solvent on the ex-
traction efficiency of direct-SDME, Other experimental
conditions are as follows: Concentration level at 25 µg·L1;
400 rpm stirring rate; temperature 24˚C; 3.0 µL drop vol-
ume; 13.5 mL sample volume; 0% NaCl; 11.30 min extrac-
tion time.
Figure 2. Chromatograms obtained from extraction solu-
tions with diehexyl ether. Experimental conditions were as:
Concentration level at 100 µg·L–1; 300 rpm stirring rate; 5
µL drop volume; 10 mL sample volume; Temperature,
25˚C; 0% NaCl; 5 min extraction time). (*) is solvent impu-
rity peak.
of acetonaphton (25 mg·L–1) as internal standard was
prepared in toluene as extracting solvent.
3.1.2. Solvent Drop V olume
In SDME, the volume of extraction solvent is an impor-
tant variable in extraction efficiency. The amount of the
analytes extracted into an organic drop is linearly pro-
portional to the drop size at equilibrium, as depicted by
the following equation [15]:
N = K Vorg,eq Caq,in (1)
where N is the number of moles of analytes extracted by
the organic drop; K is the distribution coefficient of an
analyte between the aqueous phase and the organic drop;
Vorg,eq is the volume of organic drop at equilibrium; and
Caq,in is the initial concentration of the analyte in aqueous
solution. It was demonstrated that a linear increase in GC
signals occurs with the size of toluene in the range of 1 -
Copyright © 2011 SciRes. AJAC
4.0 µL, as predicted from Equation (1), (Figure 3).
However, larger drops are difficult to manipulate and
less reliable. In addition, the analyte get into the drop
through the diffusion process, the larger the drop volume,
the longer the time to reach the equilibrium. However,
when the microdrop volume was 4 µL, the extraction ef-
ficiency increases, but microdrops were difficult to ma-
nipulate and led to instability of the microdrop at the
needle tip. Therefore, 3.5 µL was used for all subsequent
3.1.3. Stirring Ra te
Sample agitation can reduce the time needed to reach
thermodynamic equilibrium, especially for the analyte
with higher molecular weight, and thus stirring rate is an
important parameter affecting SDME efficiency. A series
of stirring rates varying from 0 to 700 rpm was opti-
mized. As shown in Figure 4, an increase in stirring rate
led to an increase of analyte peak area between stirring
rates of 0 and 500 rpm. This is because agitation permits
continuous exposure of the extraction surface to fresh
aqueous sample and reduces the thickness of the static
layer. However, the analyte peak area decreased when
the stirring rate exceeded 500 rpm. Perhaps extraction
equilibrium was difficult to establish in the two phases at
higher stirring rate. Moreover, higher stirring rate will
also result in drop dislodgement and drop dissolution.
However, the volume of the sedimented phase decreases
from 3.5 to 1 µL when the stirring rate was reached
above 500 rpm. It is due to the increase in solubility of
extraction solvent in aqueous solution at high stirring
rate. Consequently, a stirring rate of 500 rpm was used in
subsequent experiments.
3.1.4. Ionic Strength
The increased ionic strength of the sample solution is
Drop volum e ( )
Pea k a rea
Biphenyl Biphenyl oxide
Figure 3. Effect of solvent drop volume on the extraction
efficiency of direct-SDME for biphenyl and biphenyl oxide.
Other experimental conditions are as follows: Concentra-
tion level at 25 µg·L–1; 400 rpm stirring rate; 13.5 mL sam-
ple volume; microsyringe needle temperature, 0˚C; sample
temperature, 24˚C; 0% NaCl; 11.5 min extraction time.
expected to decrease the water solubility of the analytes
(salting-out effect) and consequently to enhance the ex-
traction yield. Figure 5 shows the influence of salt addi-
tion (NaCl) on the extraction efficiency. It is obvious that
salt, at any concentration, deteriorated extraction effi-
ciency. The NaCl dissolved in water might have changed
the physical properties of the Nernst diffusion film and
reduced the rate of diffusion of the target analytes into
the drop, and decreased die electric constant of water
which result in increasing stability of analytes. This sig-
nifies that with increasing the salt concentration, diffu-
sion of analytes towards the organic drop becomes more
and more difficult. Similar observations concerning the
effect of salt on the SME analysis was also made by
other researchers [30,31]. The aforementioned behaviour
of the studied system negates the need for salt addition.
0200400 600 800
Stirring rate (rpm)
Relative peak area
Biphenyl Bip hen yl oxi d e
Figure 4. Effect of stirring rate on the extraction efficiency
of direct-SDME for biphenyl and biphenyl oxide. Other
experimental conditions are as follows: Concentration level
at 25 µg·L–1; 3.5 µL drop volume; 13.5 mL sample volume;
0% NaCl; sample temperature, 24˚C; 11.30 min extraction
0 0.5 1 1.5 2 2.5 3 3.5
NaCl concentration (M)
Relative peak area
Biphenyl Biphenyl oxide
Figure 5. Effect of ionic strength on the extraction effi-
ciency of direct-SDME for biphenyl and biphenyl oxide.
Other experimental conditions are as follows: Concentra-
tion level at 25 µg·L–1; 3.5 µL drop volume; stirring rate
500 rpm; 13.5 mL sample volume; sample temperature,
24˚C; 11.30 min extraction time.
Copyright © 2011 SciRes. AJAC
3.1.5. Sample Temperature
Temperature was found to be critical for the extraction of
all three analytes (Figure 6). In order to examine this
impact, extraction producers were done in different tem-
peratures such as 5˚C, 10˚C, 15˚C, 20˚C, 25˚C, 30˚C,
35˚C, 40˚C and 45˚C. The results obtained from these
tests show that, the temperature of 30˚C give better ex-
traction efficiency achieved. On the other hand, by in-
creasing the temperature from 35˚C to 45˚C, the volume
of the sedimented phase decreases, because of increase in
solubility of extraction solvent in aqueous solution, and
the solvent drop to be unstable due to bubble formation
in the bulk solution. Still, the inevitable evaporation of
toluene and depletion of drop are compensated for by the
high extraction efficiency, as a result of the increased
extraction yield. We were able to attain an unimpeded
SDME process at 30˚C following a step of vigorous agi-
tation prior to extraction.
3.1.6. Sample Volumes
Based on Equation (2), increase when the ratio of
VV decreases:
of aqf aqinioaq
CkC kCkVV (2)
In this step, different volumes of sample solution con-
taining constant concenrtration of anaytes were extracted.
By increasing of sample volume, the absolute value of
analyte increases in the extraction vial, and then the con-
centration of analyte in organic drop increases [32,33]. In
Figure 5, the profile of extraction efficiency vs. sample
volume was drawn. Three volumes of 13.5, 21 and 52
were selected. As can be seen in Figure 7, the extraction
efficiency increased up to 21 mL and then decreased in 52
mL. It can be attributed to the un-homogenous stirring of
sample solution in 52 mL with magnet used.
0 10203040
Tempreature ()
Relative peak area
Biphenyl Biphenyl oxide
Te mperature (˚C)
Figure 6. Effect of sample temperature on the extraction
efficiency of direct-SDME for biphenyl and biphenyl oxide.
Other experimental conditions are as follows: Concentra-
tion level at 25 µg·L–1; 3.5 µL drop volume; stirring rate
500 rpm; 13.5 mL sample volume; 0% NaCl; 11.30 min
extraction time.
13.5 2152
Sample Volume (mL)
Relative peak area
Biphenyl B ip henyl oxid e
Figure 7. Effect of sample volume on the extraction effi-
ciency of direct-SDME for biphenyl and biphenyl oxide.
Other experimental conditions are as follows: Concentra-
tion level at 25 µg·L–1; 3.5 µL drop volume; stirring rate
500 rpm; 0% NaCl; sample temperature, 30˚C; 11.30 min
extraction time.
3.1.7. Extraction Time
The SDME is a process dependent on equilibrium rather
than exhaustive extraction. In most SDME applications,
the efficiency of extraction increased with extraction
time. The extraction of the two analytes into the organic
drop and the dissolution of some of drop into the aque-
ous solution govern the concentration in the microdrop.
Again, the factor of toluene dissolution was introduced.
Loss was largely due to drop depletion at long contact
time. However, a certain period of time was needed for
the equilibrium between organic drop and aqueous phase
to be established. It was demonstrated that extraction
time exerts strong influence on the peak heights. The
amount of analytes extracted should increase with longer
extraction time until a maximum is attained at equilib-
rium. It was found from the curves visualized in Figure
8, that signal kept rising linearly in the first 15 min, after
which it roughly flattened out. It was indicated that the
equilibration conditions were reached at 30 min. In addi-
tion, it is not necessary to reach equilibrium provided
that the extraction conditions are reproduced. So an ex-
traction time of 30 min was selected in order.
3.2. Performance of the SDME Method
The optimised direct-SDME conditions were then used
to evaluate the linearity of the proposed method over the
concentration range 5 - 500 µg·L–1 for all target com-
pounds. As shown in Table 1, good linearities were ob-
served for two analytes, with correlation coefficients (R2)
0.9958 and 0.9969. Dynamic linear ranges (DLR) were 5 -
500 µg·L–1 for both target compounds. The limits of de-
tection (LOD) of both target compounds were calcu-
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Copyright © 2011 SciRes. AJAC
Extra ction Time (min)
Relative peak area
Biphenyl Bip henyl oxide
organic phase to the original concentration of biphenyl
and biphenyl oxide in the aqueous phase were 134.3 and
145.14. The performance data of proposed methods were
compared to the liquid-liquid extraction (LLE) and solid
phase extraction (SPE) for biphenyl in Table 2. As can
be seen, good linearity and lower detection limit were
obtained in the SDME method in compared with LLE
and SPE methods.
3.3. Matrix Effect Assessment and Application to
Real Sample
The validation of a method is a process to establish that
the analytical performance parameters are adequate for
their intended use. The percentage of extracted analytes
was then evaluated by analysing of the water sample
output from distillation tower, the water sample input to
the condenser with a distance of 300 meters from the
distillation tower, the water sample chamber to 350 me-
ters away from distillation and waste water of Paxan Co.
of Tehran. Samples spiked at a 25 and 100 µg·L–1 for
recovery evaluation. The optimized extraction protocol
was applied to this sample and the recoveries were cacu-
lated as the ratio of the concentrations found in natural
and deionized water samples, spiked with the same
amount of analytes. For the real sample, at each concen-
tration, the extraction was repeated three times. Relative
recoveries and precision were calculated and are listed in
Table 3. As can been seen, acceptable recoveries (94.8% -
Figure 8. Effect of extraction time on the extraction effi-
ciency of SDME for biphenyl and biphenyl oxide. Other
experimental conditions are as follows: Concentration level
at 25 µg·L–1; 500 rpm stirring rate; 3.5 µl drop volume;
Temperature, 30˚C; 0% NaCl; 21 mL sample volume.
Table 1. Limit of detections and quantitation, regression
equations, correlation coefficients, dynamic linear ranges,
R.S.D., and enrichment factors for SDME of biphenyl and
biphenyl oxide.
Analyte LOD
(μg·L1)R2 DLR (μg·L1)EF
Biphenyl 1.8 ± 0.03 6.1 ± 0.01 0.996 5 - 500 134.3
Biphenyl oxide 1.1 ± 0.02 3.7 ± 0.03 0.997 5 - 500 145.14
aEnrichment Factor.
lated by comparing the signal-to-noise (S/N) ratio of the
lowest detectable concentration to the S/N ratio of 3. The
limits of quantitation (LOQ) [calculated as concentra-
tions giving signal-to-noise ratio = 10] were sufficiently
low in samples. In order to calculate the enrichment fac-
tor of each analyte, three replicate extractions were per-
formed at optimal conditions from aqueous solution. The
enrichment factor, Ee, defined as the ratio of the equilib-
rium concentration of biphenyl and biphenyl oxide in the
Table 2. Comparision of performance data of SDME, LLE
and SPE methods.
MethodLOD (μg·g1)R
2 DLR (μg·mL1) Ref.
LLE 1 - 0 - 1000 [34]
SPE 0.01 >0.99 0.5 - 10 [35]
SDME0.0018 0997 0.005 - 0.5 This work
Table 3. Relative recoveries, relative standard deviation values (R.S.D.%) and precision of Direct-SDME-GC-FID.
Added values
Found values
Sample Analyte
Initial Concentration found
(µg·L1) (µg·L1) Recovery (%) R.S.D. % (n = 3)
Biphenyl 20. 8 100 124. 5 103. 1 1. 5
Output of
Tower (Hotwell) Biphenyl oxide 72. 5 100 165. 5 96. 0 0. 9
Biphenyl 29. 9 100 131. 8 101. 4 1.9
Biphenyl oxide 80. 6 100 168. 9 94. 8 2.4
Biphenyl 16. 9 50 64. 5 96. 4 3.9
Biphenyl oxide 51. 2 50 99. 1 97. 9 1.8
Biphenyl - 25 23.8 97.4 0.9
Waste water of
Paxan Co. of Tehran Biphenyl oxide 15.4 25 41.6 103.5 1.2
Copyright © 2011 SciRes. AJAC
103.5%) and R.S.D. values (0.9% - 3.9%) was obtained
for two analytes in the tested water samples. These ob-
servations also clearly indicated that the different water
samples had no significant matrix effect on the precision
of the direct-SDME determination method.
4. Conclusions
In general SDME a method of liquid phase microextrac-
tion is attractive in terms of simplicity and ease of use,
analytical precision and accuracy, overall sample prepa-
ration time, cost, possibility of automation and minimi-
zation of organic waste. The results showed that the di-
rect SDME were successfully applied to determine bi-
phenyl and biphenyl oxide in water sample.
5. Acknowledgements
The authors acknowledge support from the Iran National
Science Foundation (INSF), the Presidential Office of
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