Journal of Chromatographic Science 2015;53:1333– 1340
doi:10.1093/chromsci/bmv017 Advance Access publication April 1, 2015
Article
Determination of Sudan Residues in Sausage by Matrix Solid-Phase Dispersion
and High-Performance Liquid Chromatography
Yujuan Zhai1,2* and Jianhua Cheng3
1
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, Jilin 130022, PR China, 2Jilin Province Food Testing Institute, Changchun, Jilin 130103, PR China, and 3National Center of
Biomedical Analysis, Beijing 100850, PR China
*Author to whom correspondence should be addressed. Email: [email protected]
Received 8 November 2013; revised 28 January 2015
A method based on matrix solid-phase dispersion and highperformance liquid chromatography was applied to the determination
of four Sudan red residues in sausage. The proposed method required
only 0.5 g sample. The neutral alumina was used as the dispersant
sorbent while n-hexane containing 10% (v/v) acetone was used as
the eluting solvent. The recoveries in samples ranged from 76.4 to
111.0% and relative standard deviations were <10.2% with different
spiked levels of 0.05–1.50 mg kg21. Detection and quantification limits were 5 –19 and 18– 39 mg kg21, respectively, with linear calibration curves extending up to 2.50 mg g21. The results demonstrate
that the method can be successfully applied with acceptable recoveries to a broad range of target analytes within a diverse range of
sausage.
Introduction
Sudan red has four categories, such as Sudan I, II, III and IV. The
structures are shown in Supplementary Material. They are nonionic fat-soluble dyes and widely used all over the world in
household commodities, textile, leather, wood industries, gasoline, grease, oils and plastics for their low price, high effectiveness and excellent stability. Some investigations indicated that
Sudan red and their metabolites have potential effects of causing
cancer on humans and have been classified as category 3 carcinogens by International Agency for Research on Cancer (1).
Because of this, the use of Sudan red as colorants, at any level, in
food products for human consumption is banned in China, the
European Union and many other countries (2). However, Sudan
I, II, III and IV are unauthorized and illegally used in the food industry to enhance and maintain its visual aesthetics and to promote sales (3). What is more is that in some countries, Sudan
red has been found as a contaminant in foodstuffs (4), and this illegal use has serious impact on public health. For this reason, sensitive, selective and accurate analytical methods are to be
developed to identify and quantify Sudan red in complex matrices.
Several methods for the determination of Sudan red in food
products, such as chilli powder, egg, flavoring and fast food, including thin-layer chromatography, high-performance liquid
chromatography (HPLC), HPLC-mass spectrometry, gas chromatography –mass spectrometry, and pressurized capillary electrochromatography with amperometric detection have been
applied (3, 5 –8). The methods of sample preparation for extraction of Sudan red have been described, such as liquid –liquid extraction (4), ultrasonic extraction (6, 9, 10), cloud point
extraction (3), solid-phase extraction clean-up (11), and gel
permeation chromatograph clean-up (12). Good linearities and
recoveries of four Sudan red compounds were obtained using
molecularly imprinted solid-phase extraction combined with
ultrasound-assisted dispersive liquid – liquid microextraction
method (13).
Matrix solid-phase dispersion (MSPD) is a relative simple sample preparation technique based on solid-phase extraction. In
this technique, a simultaneous extraction and clean-up of the extracts are allowed in one step. When the MSPD technique was applied, the analytical time and organic solvent consumption were
substantially reduced and the expensive instruments were not
required. It has been widely used to detect various drugs in
foods of animal origin or some other biological samples (14).
The working procedures of MSPD include sample homogenization, transferring mixture to a column, seamlessly compressing,
eluting and purification in a simple column. Different solid phases such as C18, C8, CN, Florisil, alumina and silica can be used as
non-polar or polar dispersion phases to blend homogeneously
with solid or semi-solid samples.
In this paper, an extraction procedure is presented for Sudan I,
II, III and IV in sausage based on MSPD, in which neutral alumina
was used as the dispersant sorbent and n-hexane containing acetone with a suitable ratio as the eluting solvent. The determination of analytes was carried out using HPLC-UV. This procedure
was optimized in order to obtain quantitative recovery, enhance
selectivity and apply easily. The performance of proposed method
was compared with that of the national standard method of China.
Experimental
Instrumentation and reagents
Instrumentation
An Agilent Technologies HPLC 1100 system (Agilent Technologies, USA) equipped with a vacuum degasser, a quaternary gradient pump, a heated column compartment, a UV detector and a
20-mL injection loop was used. The chromatographic separation
was carried out on a Venusil C18 column (250 4.6 mm i.d.,
5 mm, Agela Technologies, USA). Sample injection quantity was
10 mL and the temperature of column was controlled at 308C.
The mobile phase consists of acetonitrile–methanol (20:80, v/v).
The absorbance was measured at the wavelength of 505 nm
and the flow rate was kept at 1.0 mL min21. The glass column
for MSPD extraction made in the laboratory has an internal diameter of 10 mm and a length of 30 cm.
# The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Reagents
Analytical reagent grade n-hexane, dichloromethane, petroleum
ether, acetonitrile and acetone were purchased from Beijing
Chemical Factory (Beijing, China). HPLC grade acetonitrile and
methanol were obtained from Fisher Scientific (Pittsburgh,
USA). Sudan I (1-[(2,4-dimethylphenyl) azo]-2-naphthalenol),
content 90%; Sudan II (1-( phenylazo)-2-naphthol), content
88%; Sudan III (1-(4-phenylazophenylazo)-2-naphthol), content
96%. Sudan IV (o-tolyazo-o-tolylazo-betanaphthol), content
92%, was obtained from Dr Ehrenstorfer (Augsburg, Germany).
Standard calibration mixtures in different concentrations were
prepared by diluting a stock standard solution in acetonitrile
and stored at 4 –58C in the dark. Diatomaceous earth (400 meshes) was purchased from Wako Pure Chemical Industries (Osaka,
Japan). Silica gel (60–325 meshes), active carbon (60– 80 meshes) and neutral alumina (200 meshes) were obtained from
Chinese Medical and Biological Products Institute (Beijing,
China). Before use, diatomaceous earth and neutral alumina
were baked at 6508C for 4 h and dried at 1108C for 2 h, silica
gel was baked at 1308C for 2 h, and then, these reagents were
stored in the desiccator for coming employment. Neutral alumina was deactivated by a suitable amount of Milli-Q water.
Methods
Sample
The blank and spiked sausage samples were obtained from a local
market. The samples were cut out and put into the food chopper.
The chopper was operated for several minutes to shiver the samples thoroughly. The sample homogeneously chopped was stored
in the refrigerator at a low temperature for coming analysis.
Procedure
A 0.5 g of sausage homogeneously chopped was weighed accurately and placed into a glass mortar. Then, a suitable amount
of standard solution of Sudan I – IV was added in the sample
and mixed together. After 10 min, 1.0 g of neutral alumina was
mixed with the sample and whisked to obtain a homogenous material. Three centimeters thickness of neutral alumina were filled
the glass column pre-plugged with a little of pledget and washed
by 10 mL of n-hexane for gravitational flow. Subsequently, the
homogeneous mixture was transferred into the column and filled
with a little of pledget at the top of it. Then, the mixture was
softly and seamlessly compressed with a plastic stick. The target
compounds in the mixture were eluted with 30 mL of n-hexane
solution containing 10% acetone by gravitational flow. The eluate
was evaporated to dry at low pressure using a rotary evaporator
under 508C. The residue was dissolved with dichloromethane
,5 mL and transferred into a glass tube. Common extraction solvents include acetone, n-hexane, dichloromethane and ethyl acetate. In this experiment, n-hexane is non-polar solvent and the
few residues were dissolved while acetone and ethyl acetate have
strong polarity and more impurities were dissolved. Thus,
dichloromethane was used as the solvent to dissolve the residues.
Then the solution was volatilized to dry and 0.5 mL of the HPLC
grade acetonitrile was added in it to dissolve the residue. The obtained solution was filtered through a 0.45-mm filter membrane.
The filtrate was injected into the HPLC system.
1334 Zhai and Cheng
Evaluation of the method
Calibration curve
The chromatographic identification of the analyte Sudan I, II, III
and IV were realized by their relative retention time. Calibration
curves were obtained by plotting the peak area of the studied analytes versus the theoretical concentration of the analytes. The
curves were constructed from replicate measurements of six
concentrations over a range of 0.05 – 2.50 mg mL21. The data
were employed to provide information on linearity of the method by least squares regression analysis.
Sensitivity
The limit of detection (LOD) in spiked-free sausage was defined
as the concentration providing a signal-to-noise ratio of 3. The
limit of quantification (LOQ) was the minimum injected amount
that demonstrates precise measurements and is defined as the
concentration providing a signal-to-noise ratio of 10.
Precision
For the intraday assay precision, three replicates of quality control samples were analyzed for the same day on three different
spiked levels. For the interday assay precision, control samples
were analyzed once a day for three sequence days on threespiked levels, too.
Recovery studies
Recovery studies were carried out on sausage samples (0.5 g)
spiked with suitable volume of the working standard solutions
and maintained at room temperature for 3 h. Samples were spiked
with Sudan I, II, III and IV at 0.05, 0.10, 0.50 and 1.50 mg g21. Three
replicate samples were analyzed for each concentration level to
evaluate the relative standard deviation (RSD).
National standard method
In order to evaluate the proposed method, the national standard
method of China was used for extraction of Sudan I – IV in the
sausage. Ten grams of sausage sample were accurately weighted
and placed in a triangle flask, in which 60 mL of n-hexane was
added, and homogenized with sausage for 5 min. The extractant
was filtered and rinsed with 20 mL of n-hexane twice. All extractants were collected and evaporated to not .5 mL at low pressure under 508C. The obtained extractants were added slowly to
a glass column, in which 3 cm of thickness neutral alumina was
pre-added and washed with 10 mL of n-hexane, before 2 mm
height of n-hexane was remained on the surface. The flask was
washed for several times by n-hexane and the obtained washing
solution was added into the column too. After all extractants
flowed out, 10 mL of n-hexane was added to wipe off grease.
And then, 60 mL of n-hexane containing 5% acetone was
added to elute the analytes from the column of neutral alumina.
The eluate was collected and evaporated to dry under 508C. The
residues were dissolved with dichloromethane ,5 mL and transferred into a glass tube. Then the solution was volatilized to dry
and 0.5 mL of the HPLC grade acetonitrile was added in it to dissolve the residues. The solution was filtered through a 0.45-mm
filter membrane. The filtrate was injected into the HPLC system.
Discussion
Optimization of the chromatographic conditions
The HPLC method proposed provided a simple procedure for the
determination of Sudan I –IV in sausage samples. In order to optimize the separation and get the maximal signal of peaks, the
mixture of methanol and acetonitrile in different ratio was
used as mobile phase. When the content of methanol increases,
the retention times of the four analytes, especially Sudan III and
IV, increase. When the mobile phase was composed of methanol
and water (15) or gradient elution was applied (3), the retention
times of Sudan III and IV decrease very much. The results are
Figure 1. The effect of mobile phase for HPLC. [1]*: the mobile phase is 95% methanol
aqueous solution; [2]*: mobile phase of acetonitrile – water (90 : 10, v/v) for 4 min,
then a gradient until 6 min to acetonitrile –water (100 : 0, v/v), isocratic until 25 min
and maintaining the proportion until the end of the run.
shown in Figure 1. In this work, taking the toxicity and price
of the mobile phase and the retention time of the analytes into
overall consideration, the solvent (methanol : acetonitrile, 80:20;
v/v) was selected as the optimal mobile phase. The influence of
temperature of column was studied at two different levels 25 and
308C, and the best separation was obtained at the temperature of
308C in this condition the retention times of four analytes
decrease. As can be seen from Figure 2A, the separation process
can be completed in 18 min.
Method development for sample extraction
Optimization of dispersion sorbent
Several different sorbents were used as dispersion sorbent, including diatomaceous earth (400 meshes), silica gel (60 – 325
meshes), active carbon (60 – 80 meshes) and neutral alumina
(200 meshes) in order to obtain an efficient sorbent of the sample matrices to extract the analytes. 1 g of every sorbent was
weighed accurately and mixed homogenously with 0.5 g spiked
sausage, respectively, and then the operation described in
Section 2.4 was carried out. The results indicated that when diatomaceous earth was used as dispersion sorbent the eluting
speed was very slow (over 60 min); when active carbon was
used as dispersion sorbent, four analytes were hardly eluted by
the eluting solvent; when silica gel was used as dispersion sorbent,
four analytes were almost eluted by the washing solvent and it got
very low recoveries of analytes and when neutral alumina was used
as dispersion sorbent, it could get considerable recoveries for four
analytes (87–98%). The neutral alumina was selected as an optimal
dispersion sorbent used among these sorbents in this experiment.
Before used, neutral alumina was baked at 6508C for 4 h and
dried at 1108C for 2 h. The activity of neutral alumina was
Figure 2. The chromatograms of standard solution (A), the blank sample (B) and spiked sample of sausage (C). The concentration of Sudan I, II, III and IV in standard solution is
2.5 mg mL21 and that in spiked sample is 0.5 mg mL21.
Sudan Residues in Sausage 1335
different for varying original areas, so a suitable amount of Milli-Q
water was added into neutral alumina to be deactivated. The effect of moisture contents in neutral alumina at six levels of 0, 2, 4,
6, 8 and 10% (w/w) was investigated. The results are shown in
Figure 3. The results indicate that the optimal moisture content
in neutral alumina was at the level of 4%.
The effect of ratio of dispersion sorbent on sample matrices
was examined. The results shown in Figure 4 indicated that the
difference of recoveries of four analytes was not very significant.
The optimal ratio of dispersion sorbent to sample matrix was 2: 1.
Eluting solvent
In this experiment, n-hexane containing a suitable amount of acetone was used as the eluting solvent. The results are given in
Figure 5. The recoveries of four analytes increase when the content of acetone in the mixed solvent increases from 1 to 3 mL,
and then, do not significantly change with the increase of
amount of acetone from 3 to 7 mL. The experimental results
indicated that the eluting intensity of n-hexane containing 10%
(v/v) acetone was enough to elute four analytes from the mixture in the column.
The effect of volume for eluting solvent is shown in Figure 6.
When the volume of eluting solvent increases from 10 to 30 mL
the recoveries for Sudan I increase from 79 to 109%, for Sudan II
from 68 to 89%, for Sudan III from 71 to 90%, for Sudan IV from
69 to 93%. The recoveries did not increase obviously when the
volume of eluting solvent increases from 30 to 60 mL. The recovery has a little decrease when the volume is .70 mL and the interferences are eluted by superfluous volume of solvent, which
can interfere with the determination of analytes.
Clean-up sorbent
In order to wipe off some interference from eluent, neutral alumina (de-activated by 4% moisture, w/w) was used as clean-up
Figure 3. The effect of moisture content in neutral alumina.
1336 Zhai and Cheng
sorbent while the quantity of neutral alumina added into the column under the layer of dispersion sorbent was investigated. The
effect of layer of neutral alumina is shown in Figure 7. It indicated
that it obtained preferable recoveries of four analytes when the
layer of neutral alumina has the thickness of 3 cm. The HPLC
chromatogram was given in Figure 2.
Washing solvent
In order to remove grease from the sample before elution, an
amount of n-hexane or petroleum ether was used as the washing
solvent. Figure 8 reveals the effect of washing solvent volume on
analytes recoveries when n-hexane was used as the washing solvent. The recoveries of four analytes were not significantly
changed when the volume of n-hexane increased from 0 to
20 mL, but the recoveries decreased slightly when the volume increased from 20 to 50 mL. Plenty of n-hexane could remove a little of analytes under these conditions. The effect was not
significant when petroleum ether was used as the washing solvent. As the recoveries of four analytes were not affected evidently, the washing solvent was not used before the analytes were
eluted in order to make the preparative step simple and decrease
more employment of organic solvents.
Performance analysis
Linearity and repeatability
The calibration curves for four analytes were established. The relationships between the analyte concentration (X) and peak area
of measured signal (Y) are noted as regression equations given in
Table I for four analytes. Calibration curves show good linear relationship (r . 0.9997) between 0.05 and 2.50 mg mL21.
The calibration standards were used to determine the intraday (three replicates at each concentration, 1 day) and interday
Figure 4. The effect of ratio of neutral alumina to sample.
Figure 5. The effect of volume of acetone in 30 mL of eluting solvent.
(three replicate at each concentration, 3 days) repeatability.
The results of intra- and interday repeatability (for three levels)
are shown in Table II. RSD values are between 1.03 and 12.50%
for the intraday and between 0.70 and 19.35% for interday
assays on three-spiked concentrations, high (25 ng, absolute injection amount), medium (5 ng) and low (1 ng) level. These results indicated that the developed method has acceptable
precision.
Sudan Residues in Sausage 1337
Figure 6. The effect of volume for eluting solvent.
Figure 7. The effect of the thickness for clean-up sorbent.
Limit of detection and quantification
LOD is the lowest concentration of an analyte to bring an analytical signal that can reliably differentiate from background level.
LOQ is the lowest amount of an analyte that can be quantitatively
1338 Zhai and Cheng
detected with defined precision and accuracy under the given
experimental conditions. LOD and LOQ were determined using
a blank sausage sample and were defined as the minimum
amount of target analyte providing a signal-to-noise ratio of 3
Figure 8. The effect of volume of washing solvent.
Table I
Analytical Performance of MSPD-HPLC-UV
Compound
Retention time (min)
Calibration cure
Linear range (mg mL21)
Linear coefficient
LOD (mg g21)
LOQ (mg g21)
Sudan
Sudan
Sudan
Sudan
5.4
8.3
11.0
17.3
Y ¼ 0.0246 þ 29.41X
Y ¼ 20.1742 þ 26.31X
Y ¼ 20.6454 þ 35.89X
Y ¼ 20.9133 þ 37.05X
0.05 –2.50
0.05 –2.50
0.05 –2.50
0.05 –2.50
0.9999
0.9999
0.9997
0.9998
0.005
0.010
0.013
0.019
0.018
0.028
0.033
0.039
I
II
III
IV
and 10, respectively. LODs and LOQs are shown in Table I. LODs
of Sudan are 5 –19 mg kg21 and LOQs are 1839 mg kg21.
Recovery
The sausage samples were spiked at the levels of 0.05, 0.10, 0.15,
0.50 and 1.50 mg g21 for four Sudan red residues and subjected
to the sample preparation procedure mentioned above (n ¼ 3).
The results of percentage recoveries of four analytes using MSPD
from sausage samples are shown in Table III. The method validation studies for spiked samples indicated that the present method provides good recoveries and reasonable precision for
analytes in the range 0.05 – 1.50 mg g21. As can be seen from
Table III, the recoveries of analytes using MSPD were 76.4 –
111.0% with good precision (10.2%).
Comparison of different preparation techniques
The recoveries of four Sudan residues obtained with different
preparation techniques are shown in Table IV. The recoveries
of four analytes and RSD obtained by the proposed method and
by the national standard method of China are similar. The
Table II
Intra- and Interday Repeatability
Amount
injected (ng)
Compound
Intraday repeatability RSD
(n ¼ 3) (%)
Interday repeatability RSD
(n ¼ 9) (%)
25
Sudan
Sudan
Sudan
Sudan
Sudan
Sudan
Sudan
Sudan
Sudan
Sudan
Sudan
Sudan
1.73
1.03
1.64
3.64
1.06
5.56
1.03
6.11
4.03
2.37
12.50
6.25
1.62
0.70
2.62
2.96
5.16
2.14
2.98
6.12
7.63
2.62
18.70
19.35
5
1
I
II
III
IV
I
II
III
IV
I
II
III
IV
recoveries of four Sudan residues obtained by the proposed method are in the range from 84.8 to 98.2% with good RSD from 1.0 to
3.6%, and the recoveries of four Sudan resides obtained by the national standard method of China are 84.1–97.0% with RSD, 0.1–
8.9%. Compared with national standard method of China, the consumption of sample and organic solvent is evidently lower and the
preparation is simpler when the MSPD was applied.
Sudan Residues in Sausage 1339
Table III
Recoveries of Sudan I2IV
Spiked concentration (mg g21)
0.05
0.10
0.15
0.50
1.50
Sudan I
Sudan II
Sudan III
Sudan IV
Recovery (%)
RSD (%)
Recovery (%)
RSD (%)
Recovery (%)
RSD (%)
Recovery (%)
RSD (%)
103.7
109.7
104.7
91.0
82.8
4.6
2.2
1.4
1.0
0.5
100.7
94.1
86.8
89.5
90.5
6.1
6.1
2.2
4.9
3.3
100.1
98.8
91.9
88.6
76.4
6.1
4.9
3.3
3.2
1.0
106
111.0
96.5
86.5
87.8
6.7
10.2
4.8
2.5
2.9
Table IV
Comparison of Different Extraction Techniques (n ¼ 3)
Compound
Sudan
Sudan
Sudan
Sudan
I
II
III
IV
The proposed method
The national standard method of China
Added (mg g21)
Found (mg g21)
Average recovery (%)
RSD (%)
Added (mg g21)
Found (mg g21)
Average recovery (%)
RSD (%)
1.000
1.000
1.000
1.000
0.932
0.982
0.848
0.895
93.2
98.2
84.8
89.5
1.7
1.0
1.6
3.6
1.000
1.000
1.000
1.000
0.970
0.898
0.841
0.872
97.0
89.8
84.1
87.2
0.8
8.9
1.4
0.1
Conclusions
The proposed MSPD method can be readily applied to the extraction of Sudan I –IV in 0.5 g of sausage sample. Neutral alumina deactivated by 4% (w/w) moisture was used as MSPD dispersion adsorbent and additional neutral alumina was used as clean-up adsorbent, n-hexane containing 10% (v/v) acetone was used as the
eluting solvent. In the standard method, a large amount of sample
and the washing solvent were used; the extraction process was
complex and time-consuming. Compared with the proposed
method, the standard method was wasted and can pose an environmental hazard. The proposed procedure is simple and rapid,
only small amount of sample and solvent are required. The
HPLC-UV detection provides excellent selectivity and sensitivity
for the quantitative analysis of those target compounds in sausage samples.
6.
7.
8.
9.
10.
Supplementary Material
Supplementary Material are available at Journal of Chromatographic Science online.
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