Research Article, Issue 2
Analytical Methods in Environmental Chemistry Journal
Journal home page: www.amecj.com/ir
AMECJ
1. Introduction
The edible oil toxicity by cadmium (Cd) and nickel
(Ni) ions is very important problem in food industry
because of bioaccumulation in human body and
cause damage to organs such as kidney dysfunction,
hypertension, liver damage, lung damage, cancer
and adverse DNA modifications [1-3]. The
presence of high concentration of Cd (II) and Ni
(II) in edible oils may also have deleterious effects
on the quality of the product, causing changes to
their taste and smell [4]. Cd (II) and Ni (II) ions
are easily transferred from arable land to oil plants,
which are increasingly contaminated by cadmium
and nickel from phosphate-based fertilizers. These
toxic elements can also be present in edible oils, as
a result of contamination from the environment, the
refining process, the storage tank, and the packing
with different materials [5-7]. The U.S. Food and
Drug Administration (USFDA) reported that the
permissible limits of nickel and cadmium in the
vegetables oils are less than 1.0 mg/kg and 0.1 mg/
kg, respectively. For healthy people, the mean of
cadmium and nickel concentration in human serum
samples is less than 0.2 L
-1
. Therefore, the
accurate determination of trace cadmium and nickel
ions in edible oils is an important concern because
a
Department of Chemistry, Bilkent University , Ankara, Turkey
of nickel and cadmium from olive oil samples by thermal
ultrasound-assisted dispersive multiphasic microextraction
Aisan Khaligh
a
Corresponding Author: Aisan Khaligh
Email: akhalighv@gmail.com
https://doi.org/10.24200/amecj.v2.i2.64
A R T I C L E I N F O:
Received 17 March 2019
Revised form 4 May 2019
Accepted 25 May 2019
Available online 20 Jun 2019
------------------------
Keywords:
Olive
oil
Cadmium and nickel
Thermal ultrasound-assisted
dispersive multiphasic
microextraction
Task specific ionic liquid
Atom trap flame atomic
absorption spectrometry
A B S T R A C T
In this study, a novel task-specific ionic liquid (TSILs) was used for highly
sensitive extraction and separation of nickel and cadmium in olive oil by
thermal ultrasound-assisted dispersive multiphasic microextraction (TUSA-
[HEMIM]
TSILs was added to diluted olive oil with n-hexane containing Cd (II) and
Ni (II) that was already complexed by TSILs in 60
O
C at pH 6.0-7.5. After
optimized conditions, the enrichment factor (EF), Linear range (LR) and
L
-1
92 L
-1
) and (1.3 L
-1
L
-1
14.2), (7.5- 600 L
-1
L
-1
) and (2.2 ng L
-1
L
-1
) with
oil samples respectively. In
addition, the ions extraction with [CHCA] [DEA] is more efficient than
the validation of methodology was achieved by standard oil by microwave
digestion/ETAAS technique and spike samples with atom trap flame atomic
absorption spectrometry (AT-FAAS)
Food analysis by task specific ionic liquids Hamid Shirkhanloo et al
Analytical Methods in Environmental Chemistry Journal Vol 2 (2019) 55-64
56
Analytical Methods in Environmental Chemistry Journal; Vol. 2 (2019)
of their toxic role in human body and possibilities
for adulteration detection and oil characterization
oils were quantitatively determined by different
techniques such as flame atomic absorption
spectrometry (F AAS) [1, 3, 11], electro-thermal
atomic absorption spectrometry (ET AAS) [12],
inductively coupled plasma atomic emission
spectrometry (ICP AES) [12, 13], inductively
coupled plasma mass spectrometry (ICP MS)
[4, 14], and adsorptive stripping square wave
voltammetry (Ad SSWV) [15]. Sample preparation
is a critical step in the whole analytical procedure
due to low concentration levels of metal ions and
high organic matrix of oils [3, 16]. Moreover,
many of conventional methods employed for
the sample pretreatment such as acid digestion,
wet or dry ashing of oil matrix, closed-vessel,
and focused open-vessel microwave dissolution,
dilution as well as basic alcoholic solubilization
are not recommended because of associated safety
hazards, a potential risk of sample contamination,
analyte losses and being time-consuming [17].
In this regard, preconcentration and separation
steps are required prior to analyte determination.
Recently, dispersive liquid–liquid microextraction
gained increasing popularity for the trace metal ions
preconcentration-separation in food and biological
samples because of its simplicity, rapidity, low cost,
low consumption of toxic organic solvents as well
as the high preconcentration factor and extraction
efficiency that are achieved [19, 20]. DLLME uses
an extraction solvent, immiscible in the aqueous
phase, and a disperser solvent, miscible in both
the extraction solvent and in the aqueous sample
solution. In this method, the contact area between
the extraction solvent and the sample solution
is extremely large, so the extraction equilibrium
is reached rapidly. In the DLLME, the choice of
the appropriate mixture of the extraction and
dispersive solvents is critical for achieving high
enrichment factor [21]. Ionic liquids (ILs), as an
alternative to traditional organic solvents in sample
preparation, are one of the mostly considered
extraction solvents in DLLME due to their low
volatility, high viscosity, dual natural polarity,
good thermal stability, and miscibility with water
and organic solvents (TSIL1,chempaper) without
requiring a dispersive solvent [20, 22, 23]. It has
been demonstrated that organic analytes can be
partitioned to ILs based on the hydrophobicity
of the analytes and ILs, but metal ions cannot
be extracted in liquid/liquid systems owing to
negligible partitioning and the tendency of metal
cations to remain hydrated in the aqueous phase
[21]. Therefore, it is necessary to use chelating
agents which firstly form hydrophobic complex
with metal ions, and then the formed complex
extract into extraction solvent. In recent years,
task specific ionic liquid (TSIL), a new group of
ionic liquids, has been introduced in which thiol or
urea groups are covalently attached to the cationic
or ionic impart of the IL. TSIL can be used as a
dispersive and extraction solvent for metal ions in
DLLME method without any chelating agents [23].
In the TSIL-DLLME, extraction and complexation
of metal ions were done simultaneously and this
newly developed procedure is a very fast and easy
single-step method without needing chelating
agents compared to the traditional DLLME method
[24]. To date, different DLLME modes based on
the ILs have been developed such as temperature
controlled, ultrasonic-assisted, microwave-assisted
and vortex-assisted [25]. Zhou et al. made the
most preferred modifications in IL-DLLME which
are ultrasound-assisted (US) and temperature-
controlled (TC) techniques [26, 27]. In TC, IL-
dispersive liquid–liquid microextraction, the
extraction solvent is heated until it is completely
solubilized in the water matrix. The heating
improves the mass transfer of the analyte to
microextraction, ultrasound and dispersive solvents
are used to increase the extraction ability of ILs
that a great number of the TSIL-DLLME analytical
procedures reported to date have dealt with water
analysis or relatively simple matrices. For example,
Werner developed TSIL-USA-DLLME combined
57
Aisan Khaligh
with liquid chromatography for preconcentration
and determination of Cd (II), Co(II) and Pb (II) ions
in tea samples [22]. However, application of this
method in edible oil samples has not been studied
yet. Therefore,
the main aim of the current study is to
develop a simple, rapid, reliable and economic method
for the determination of trace Cd
2+
and Ni
2+
ions in
olive oil samples without chelating agents. This method
is based on the combination of the thermal ultrasound-
assisted-task specific ionic liquid-dispersive multiphasic
FAAS. In the present approach, two hydrophilic TSILs,
compared not only as an extraction solvent but also as
a selective chelating agent. Experimental parameters
including sample pH, amount of sorbent, sample
volume, eluent type and volume, and time of ultrasound,
etc., have been studied and optimized.
2. Experimental procedure
2.1. Apparatus
The measurements of Cd (II) and Ni(II) ions were
atomic absorption spectrophotometer equipped
with atom trap, air-acetylene flame and ultra-pulse
deuterium as a background correction (AT-FAAS,
was installed on an air-acetylene burner. A data
station (AT-compatible computer) with 906 AAS
operating software was utilized for collecting
and storing data. For the matter of interest, the
operating parameters were set as recommended
by the manufacturer. A Cd and Ni hollow cathode
lamps as the radiation sources were used at a
current of 10 mA and 3 mA, and a wavelength
of 279.5 nm and
with a
spectral bandwidth of 0.5 nm. The pH values of the
solutions were measured by a Metrohm pH-meter
(model 744, Herisau, Switzerland) supplied with
a glass-combined electrode. Phase separation was
12). A Kunshan ultrasonic bath with temperature
control (model KQ-100DE, Kunshan, China) was
used throughout this study.
2.2 Chemical Reagents and Material
A standard stock solutions (1000 mg L
-1
) of Ni
(II) and Cd (II), -hydroxycinnamic
acid diethylamine ([CHCA] [DEA]) ionic liquid,
4
), ethyl
acetate, chloroethanol, 1-methylimidazole and all
of the other chemical compounds and reagents
were of analytical grade and purchased from Merck
(Darmstadt, Germany).
Ultra-pure deionized
-1
) from Milli-Q plus water
was used for preparing all aqueous solutions.
The
standard and experimental solutions of Ni
2+
and
Cd
2+
were prepared daily by appropriate dilution of
the stock solutions with DW. The pH adjustments
of samples were made using appropriate buffer
solutions including sodium acetate (CH
3
COONa/
CH
3
COOH, 1-2 mol L
-1
phosphate (Na
2
HPO
4
/NaH
2
PO
4
, 0.2 mol L
) for
3
/
NH
4
Cl, 0.2 mol L
glassware and plastic tubes were cleaned by
and then rinsed several times with deionized water
and dried in a clean oven prior to use. In this study,
five olive oils (Cooking and skin) was selected and
used for investigation.
2.3. Synthesis of TSIL
[DEA] purchased from Merck and [HEMIM]
method [27]. The general procedure for synthesis
First, 13.9 g (0.169 mol) of 1-methylimidazole (99
a round-bottomed flask equipped with a magnetic
stirrer and a condenser, under nitrogen atmosphere.
The mixture was refluxed at 100 ºC for 4 h. After
cooling to 70 ºC, the reaction mixture was washed
four times with ethyl acetate, and then dried in vacuo
methylimidazolium chloride ([HEMIM][Cl]), was
Analytical Methods in Environmental Chemistry Journal; Vol. 2 (2019)
yield, 17.9 g). This compound was characterized
by 1H NMR (500 MHz, D
2
O, 25 °C, δ; ppm). The
(2H, t, NCH
2
CH
2
2
CH
2
OH),
S, H-2).
4
in
under nitrogen atmosphere. The reaction mixture
through a short column of Celite to remove NaCl.
The solvent was removed using a rotary evaporator,
and the residual chloride test (a concentrated
AgNO
3
solution test) performed on product was
negative. The product, -(-hydroxyethyl)--
methylimidazolium tetrafluoroborate ([HEMIM]
yield). Karl Fischer test showed < 100 ppm of
water and the absence of chloride ion confirmed
also characterized by 1H NMR (500 MHz, D
2
O,
25 °C, δ;
NCH
2
CH
2
OH),4.24 (2H, t, NCH
2
CH
2
H-2).
2.4. General procedure
In this procedure, [CHCA] [DEA] and [HEMIM]
liquids (TSILs) to complex the ultra-trace amounts
of Ni (II) and Cd (II) ions in Olive oil samples and
performed as follows: In a 100 mL glass centrifuge
tube with a conical bottom, 26.0 g (20 mL) of olive
oil sample containing Cd and Ni ions was diluted
with n-hexane up to 40 mL. Then, the mixture of
0.25 g of the mentioned above TSILs and 0.5 mL of
acetone (as a dispersive solvent) which was diluted
with DW up to 5.0 mL was prepared and then, the
pH adjusted between 2 to 10 by buffer solution.
Afterwards, the mixture was injected rapidly into 40
mL of edible oil samples with HPLC autosampler
syringe (5 mL, gastight, CTC-PAL, Trajan, AUS)
were then separated by centrifuging of the turbid
solution for 4 min at 4000 rpm (25
o
C). The Cd/
Ni complexes were settled down at the bottom of
the conical tube into droplets of TSILs. The upper
phase of the sample (edible oil) was removed with
a pipette, the metal complexes was back extracted
from TSIL with HNO
3
( 0.2 mL, 0.5 M) and finally
the concentration of Ni (II) and Cd (II) ions in the
resulting solution was determined by AT-FAAS
after dilution with deionized water up to 1 mL.
For validation, I mL of sample oils was digested
HNO
3
/H
2
O
2
, 220
o
C) and concentration of Ni and
Cd ions determined by F-AAS/ET-AAS/ICP-MS
(Figure 1).
Fig. 1.
59
Aisan Khaligh
3. Results and Discussion
3.1. Effect of sample pH
Complexation was strongly conditioned by the
pH of the solutions. In the present work, the
solution pH affected the extraction process through
the functional group moieties of the TSIL. The
influence of sample pH on the complex formation
and the extraction of Ni (II) and Cd (II) ions with
separately investigated
at different pH values from
2 to 10 for 40 mL buffered diluted olive oil samples,
according to the general procedure.
The results
were depicted in figures 2a and 2b.
Obviously, the
maximum extraction efficiencies for Ni (II) and Cd
(II) ions were obtained with [CHCA] [DEA] TSIL
decreased by increasing of pH (Fig. 2a). Whereas,
pH 7.5-10.
3.2. Effect of sample volume
The sample volume is one of the most important
since it determines the sensitivity and enhancement
of the technique.
The effect of sample volume was
studied in the range of 5–40 mL for 5 - 100 L
of standard solution of Ni and Cd (Fig. 3).
Fig. 2b.
Fig. 2a.
60
Analytical Methods in Environmental Chemistry Journal; Vol. 2 (2019)
3.3.Effect of TSIL amount on Ni and Cd extraction
The amounts of ILs on the extraction efficiencies of
studied and optimized. For this purposed, the range
of 0.01-0.1 g of [CHCA] [DEA] and [HEMIM]
L
of Ni (II) and Cd(II) (Fig. 3). The results
show that the quantitative recoveries were obtained
recovery. Therefore, in order to achieve a suitable
preconcentration, 50 mg of [CHCA] [DEA]
was chosen as optimum leading to a final IL for
blood/urine and standard solution analysis. Also,
the effect of IL used on the recovery efficiencies
results showed us, the maximum recoveries of
lower recovery as compared to [CHCA] [DEA].
Moreover, by increasing the ultra-sonication time
(Fig. 4). So, the recovery extraction of ions with
or amount of IL.
Fig. 3. .
Fig. 4.
61
Aisan Khaligh
3.4.Effect of ultra-sonication time for Ni and Cd
extraction
The ultra-sonication time during the extraction
process can improve the extraction recovery
since it enhances the complexation of Ni/Cd ions
the most important factors for sonication time by
sonication type, shaker tube or ultrasound bath, on
the recovery efficiencies of Ni/Cd ions in olive oils
was examined by [CHCA] [DEA] and [HEMIM]
recovery was obtained using sonication at 5 min.
The optimization of sonication time increased
the extraction recovery and decreased the time
and IL mass. This fact can be related to the large
contact area between dispersion of TSILs with
olive oil samples. In this study, ultrasound assisted
extraction times ranging from 1 to 25 min were
evaluated. Therefore, the ultrasonic time of 5 min
was selected as optimum time (Fig. 5).
3.5.Interference study
The determination of Ni/Cd ions in different olive
oil samples in the presence of interfering matrix
ions was examined. In order to get the acceptable
+
, K
+
, V
3+
,
Cu
2+
, Zn
2+
, Al
3+
, Co
2+
, Cr
3+
, Hg
2+
, As
3
+, Mn
2+
and anions, SO
3
2-
, NO3-, Cl
-
, F
-
, PO
4
3-
(1-3 mgL
-
1
) were added individually to real olive oil and
-1
of Ni
2+
and Cd
2+
in optimized conditions. Interfering ions
the recovery of Ni(II) and Cd(II) was considered
presence of interfering ions has no considerable
effect on the recovery efficiencies of Ni(II) and
Cd(II) in olive oil samples.
3.6.Effect of Temperature on Ni and Cd extraction
The temperature has a critical role in extraction
recovery of Ni(II) and Cd(II) in olive oil samples
by TSILs. As evaluation of TSILs, the effect of
temperature was studied and optimized between
O
C. The results showed us, the extraction
efficiency of Ni(II) and Cd(II) by [CHCA] [DEA]
The results showed us, the optimized temperature
was obtained between 40- 60
o
C. In optimized
temperature, the extraction ions in olive oils with
3.7.Effect of mineral acids on the back-extraction
ions
injection of ILs into AT-FAAS is not possible. So,
the Ni and Cd ions back extracted from TSILs with
Fig. 5.
62
Analytical Methods in Environmental Chemistry Journal; Vol. 2 (2019)
a mineral acidic solution. Decreasing the pH leads
to dissociation and releasing of Ni(II) ions into the
aqueous phase. Different mineral acids such as
HCl, HNO
3
, CH
3
COOH and H
2
SO
4
were studied for Ni/Cd back-extraction from the
TSILs. The research showed that 0.1 mL of HNO
3
(0.5 M) quantitatively back-extracted Ni/Cd ions
from the olive oil samples before diluted with DW
up to 1 mL .
3.8.Validation of Method
The analytical characteristics of the developed
were shown in table 1. After sample preparation, Ni/
Cd in olive oils and standard samples was determined
by proposed method. The olive oils samples as a
real sample was used for determination of Ni/Cd as
the average of three separate determinations. The
accuracy of the results was verified by analyzing the
spiked samples with known concentration of Ni/Cd
obtained results, a good agreement was obtained
between the added and measured Ni/Cd amount,
which confirms the accuracy of the procedure
and its independence from the matrix effects. The
recoveries of spiked samples demonstrated that
the developed method was satisfactory for Ni/Cd
analysis. In order to validate the method described,
certified reference materials in olive oils (CRM,
A-D) which was determined with ET-AAS after
microwave digestion with pure acid and H
2
O
2
, were
analyzed by proposed method (Table 3). Analytical
results of the CRM samples were satisfactorily in
agreement with the certified values. Moreover, a
good agreement was obtained between added and
found values of the Ni/Cd ions in spiked samples
by CRM samples.
4. Conclusions
A novel, reliable and efficient method was used for
preconcentration, separation and determination
of trace Ni/Cd in olive oil samples by TUSA-
[CHCA] [DEA] and 1-(2-Hydroxyethyl)-3-
which was coupled by AT-FAAS technique. Finally,
the newly developed method was low interference,
Table 1.
[DEA].
Metal ions Sample volume
(mL)
Linear range
(𝜇g L
)
Regression
coefficient (R
2
)
LOD
a
(n = 10) (𝜇g L
)
RSD
b
PF
c
Cd
2+
20 2.7- 92 0.9996 0.6 19.6
Ni
2+
20 5.0- 415 1.3 19.3
a
Limit of detection ,
b
Relative standard deviation,
c
Preconcentration factor
Table 2. Analytical results for determination of analytes in spiked olive oil samples.
Recovery (%)RSD
b
(%)Found
a
(μg L
-1
)Added (μg L
-1
)Sample
Ni
2+
Cd
2+
Ni
2+
Cd
2+
Ni
2+
Cd
2+
Ni
2+
Cd
2+
----------5.74.265.73.6----------Olive 1
0.964.75.350.05.0
----------3.9----------Olive 2
99.31035.14.5197.739.1100.020.0
----------5.34.932.22.5----------Olive 3
97.495.05.45.761.44.430.02.0
----------5.35.422.6----------Olive 4
10296.04.74.495.950.020.0
a
Mean of three determinations ± confidence interval (P = 0.95, n =5),
b
Relative standard deviation.
63
Aisan Khaligh
Table 3. .
sample
Certified
a
(μg L
-1
)
Added
(μg L
-1
)
Found
b
(μg L
-1
)
Recovery
(%)
Cd(II) Ni(II) Cd(II) Ni(II) Cd(II) Ni(II) Cd(II) Ni(II)
A 4.5 10.5 5 10 9.3 19.9 97.9 97.1
45.6 10 50 22.1 94.4 96.9
C 55.2 117.9 50 100 99.9 209.7 94.9 96.2
D 267.9 100 200 191.1 460.2 101.0
a
Certified by ETAAS after digestion with micro wave of oils
b
Mean of three determinations ± confidence interval (P = 0.95, n =5),
easy usage for sample preparation in olive oil
samples and also provides low LOD, and RSD
values as well as good PF values and quantitative
recoveries for Ni/Cd extraction in difficulty olive
Therefore, the proposed method
can be considered as simple and applied sample
preparation techniques with TSILs for Ni/Cd
separation / determination in olive oil samples by
AT-FAAS.
5. Acknowledgements
The authors wish to thank the Iranian Research
Institute of Petroleum Industry (RIPI) for this work
(No grant or fund by RIPI).
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