Electronic Supplementary Material
Ultrasensitive determination of mercury in human saliva by atomic
fluorescence spectrometry based on solidified floating organic drop
microextraction
Chun-Gang Yuan*, Jincong Wang and Yi Jin
School of Environmental Science & Engineering, North China Electric Power
University, Baoding 071003, Hebei Province, P. R. China
*Corresponding author: Chun-Gang Yuan, Email: chungangyuan@hotmail.com, Tel.:
+86-312-7522516, Fax: +86-312-7522201
Effect of extraction temperature and time
The extraction temperature influences the extraction kinetics. The mass transfer
performance of analyte between hydrophilic and hydrophobic phases is related with
the temperature. In order to achieve high extraction yields, effect of different
extraction temperature was investigated in our experiment. The dependency of
extraction recovery on the extraction temperature was investigated in the range of
15 °C to 55 °C. Effects of temperature on the extraction efficiency were illustrated in
Figure S1. The results demonstrated that the extraction efficiency increased with
temperature increasing from 15 °C to 35 °C and reached its maximum at 35 °C.
Further increase in temperature caused a decrease in recovery, which could be
interpreted by that higher temperature increased solubility of organic phase and
degradation of complex. In our study, 35 °C was adequate to achieve quantitative
1
extraction and was chosen as the optimized equilibration temperature.
The equilibration time is also regarded as an important parameter affecting on the
extraction efficiency during the extraction procedure. It is desirable to complete
extraction with high efficiency and speed by using shorter extraction time. However,
adequate extraction time should be employed to make sure the equilibrium between
the aqueous and the organic phase is reached. To evaluate the effect of equilibration
time on extraction efficiency, the time range of 0.5-20 min was employed for
investigation. The influence of equilibration time on extraction efficiency was
illustrated in Figure S2. The results indicated that the extraction efficiency was
dependent upon the extraction time obviously. The extraction efficiency increased
with the increase in time of first 2 min and remained constant up to 5 min. The results
demonstrated that the extraction equilibrium could be reached very fast, which was
very helpful to shorten the extraction time. The extraction efficiency decreased with
the increase in time after 5 min. Thus the time of 5 min was chosen for extraction.
Effect of sample volume
Another important parameter which may influence the enrichment factors is
sample volume. During extraction procedure, the increase in the volume ratio of
aqueous phase to organic phase results in the increase in the enrichment factor. In
order to evaluate the effects of sample volume on the extraction efficiency, different
sample volume was applied during the extraction procedure. The sample volume
varied from 20 mL to 400 mL containing 5.0 ng of Hg2+ and 100 µL of 0.06% (m/v)
diethyldithiocarbamate with 60 µL of 1-undecanol. The results indicated that the
extraction efficiency and recovery were quantitative in the range of 20 mL to 400 mL.
Based on the maximum sample volume (400 mL) and organic phase volume (60 µL)
a very high preconcentration factor (6200) could be achieved. The preconcentration
factor was calculated by the ratio of volumes and extraction efficiency (93%).
Considering the high preconcentration factor and the requirements of real sample
analysis, 25 mL of sample volume was used in our study for convenience.
2
Effect of salt and potentially interfering ions
It was reported that the physical properties of aqueous solution can be changed by
the presence of dissolved salt in it [33]. The diffusion rate of the targets into the
organic drop and consequently the enrichment factor can be influenced by dissolved
salt in solution. In order to evaluate the possible effects of ionic strength on the
extraction efficiency, the dissolved NaCl with different concentration in the range of
0.1%-5% (m/v) was investigated in our experiment. From the results, the salt addition
had no significant influence on the extraction efficiency from 0.1% to 2% (m/v) NaCl
concentrations. However, higher NaCl concentration than 2% (m/v) resulted in
obvious decrease in recovery. The results from our experiment demonstrated that the
quantitative extraction could only be obtained with the concentration of NaCl lower
than 2% (m/v) and the salt addition did not benefit the extraction. Therefore, the
subsequent experiments were carried out without salt addition.
The effect of representative potential interfering ions was tested in our study.
Different amounts of common ions were added to the tested solution containing 0.5
ng mL-1 of Hg2+. The same extraction procedure was performed. The results were
summarized in Table S1 and proved that the recovery was quantitative and
satisfactory in the presence of excess amount of potential interfering ions under the
selected conditions.
Comparison with other methods
Determination of mercury in saliva samples by solidified floating organic drop
microextraction in this paper was compared with the other reported methods for Hg2+
preconcentration (Table S2). The preconcentration method developed in our
experiment showed a comparatively low detection limit and high enrichment factor.
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4
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Table S1 Influence of coexisting ions on the preconcentration and determination of
Hg2+ a.
Coexisting ion
ion Recovery (%) b
Coexisting
concentration/μg mL-1
NO3-
1000
92.8±0.1
SO42-
100
91.7±0.2
K+
1000
92.0±0.1
Fe3+
500
91.7±0.1
Ca2+
500
90.8±0.2
Mg2+
500
91.8±0.7
Cu2+
500
86.3±0.2
Zn2+
500
90.1±0.4
Mn2+
500
89.6±0.3
Co2+
500
90±3
Al3+
10
92.9±0.3
Ni2+
10
89.6±0.4
Pb2+
10
87.6±0.2
a
Preconcentration step: 25 mL of 0.5 ng mL-1 Hg2+, pH 2.0, 100
DDTC, 60 μL of 1-undecanol at 35 ºC, 5 min of extraction time.
b
Mean±standard deviation(n=3).
5
μL of 0.6% (m/v)
Table S2 Comparison of the present method with other methods for Hg2+ preconcentration
Preconcentration Method
Detection limit (ng mL-1)
RSDb (%)
Sample volume (mL)
Enrichment factor
Ref.
SFODMEa-ETAAS
0.07
3.5
25
430
[20]
SPEb-CVAAS
0.16
2.2
--
--
[19]
SPE-ICP-AES
0.09
3.0
--
175
[23]
SDMEc-ETAAS
0.01
4.6
--
75
[21]
LLMEd-CV-AAS
0.0023
2.8
20
36
[10]
SPE-CVAFS
0.18
3.2
7
--
[17]
SPE-CVAAS
0.0038
3.1
1500
500
[24]
LLME-CVAAS
0.01
--
1000
--
[25]
SPE-CVAAS
0.43
2.4
100
50
[26]
SFODME-CVAFS
0.0025
4.1
25
182
This work
a
SFODME means solid floating organic drop microextraction; b SPE means solid phase extraction; c SDME means single-drop microextraction; d
LLME means liquid-liquid miroextraction;
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1
0.8
Recovery
0.6
0.4
0.2
0
0
10
20
30
40
Temperature/ºC
50
60
Figure S1 Effect of equilibration temperature on recovery of Hg2+
(Conditions: pH 2.0, 25 mL of 0.5 ng L-1 Hg2+ solution, 100 μL of 0.6% (m/v) DDTC solution, 60 μL
of 1-undecanol, 5 min of extraction time, n=3, Y-axis value “1” means “100%”)
1
0.8
Recovery
0.6
0.4
0.2
0
0
5
10
15
20
25
Time/min
Figure S2 Effect of equilibration time on recovery of Hg2+
(Conditions: pH 2.0, 25 mL of 0.5 ng L-1 Hg2+ solution, 100 μL of 0.6% (m/v) DDTC solution, 60 μL
of 1-undecanol, extraction at 35 ºC, n=3, Y-axis value “1” means “100%”)
7