ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2015, том 70, № 9, с. 992-997
ОРИГИНАЛЬНЫЕ СТАТЬИ
УДК 543
DETERMINATION OF FIVE PHTHALATE ESTERS IN RUNNING WATER AND MILK BY MICELLAR ELECTROKINETIC CAPILLARY
CHROMATOGRAPHY © 2015 Mei-E Yue1, Jie Xu, Wan-Guo Hou
State Key Laboratory Base of Eco-chemical Engineering, Lab of Colloids and Interfaces, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology
Qingdao 266042, China 1E-mail: yuemeie@qust.edu.cn Received 02.01.2014; in final form 23.09.2014
A simple, reproducible and sensitive micellar electrokinetic chromatography method was developed for the separation and determination of dimethyl phthalate (DMP), diethyl phthalate (DEP), dibuthyl phthalate (DBP), dihexyl phthalate (DHP) and bis(2-ethylhexyl) phthalate (DEHP) in running water and milk. Field-enhanced sample injection with reverse migrating micelles was used for on-line preconcentration of the an-alytes. The buffer contained 50 mM H3PO4-NaOH, 160 mM sodium dodecyl sulfate (SDS), 15% acetoni-trile and 20% 2-propanol, at the pH of 2.0. The sample solution was diluted with water containing 5 mM SDS and injected for 5 s with —20 kV after gravity injection of 5 s water plug at 20 cm high. Under the optimum conditions, the analytes were well separated and by optimizing the stacking conditions, about 51, 62, 57, 48 and 43 fold improvement in the detection sensitivities were obtained for DMP, DEP, DBP, DHP and DEHP, respectively. The instrument detection limits (S/N = 3) of DMP, DEP, DBP, DHP and DEHP were 0.1, 0.05, 0.05, 0.02 and 0.05 ^g/mL, respectively. The recoveries of DMP, DEP, DBP, DHP and DEHP in water were 86.9-98.7, 88.9-105.2, 91.7-109.4, 93.0-108.2 and 91.0-99.2%, respectively. The phthalates were successfully determined in running water and milk with satisfactory repeatability and recovery. The contents of DEHP and DHP in the running water were 0.45 and 1.23 p.g/mL, respectively. The other analytes were not detected in the analyzed running water and milk.
Keywords: micellar electrokinetic chromatography, field-enhanced sample injection, reverse migrating micelles, phthalate esters.
DOI: 10.7868/S0044450215090091
Phthalate esters (PAEs) are widely used as plasti-cizers in polyvinyl chloride, polyvinyl acetates, cellu-losics, and polyurethanes and as nonplasticizers in products such as lubricating oils, automobile parts, paints, glues, insect repellents, photographic films, perfumes, and food packaging materials (e.g., paper-board and cardboard) [1—3]. As a result of their extensive use with global production of approximately 4.0 million ton per year, PAEs are detected throughout the world in various media, including food, water, soil, and marine ecosystems [4, 5]. Their concentration in aquatic environment is about 0.1—1000 ppb. Some recent studies show that they may cause hormone-disrupting activities [6—8]. The toxicological evaluation of PAEs has revealed that low molecular weight phthalates, such as DEP, can cause irritation of eyes, nose, and throat. However, several larger phthalate molecules, such as benzyl butyl phthalate, dioctyl phthalate, and diisooc-tyl phthalate are suspected as human carcinogens; they could damage liver, kidneys, and reproductive or-
gans, and might interfere with growth by acting as a mimic of the sex hormone [9—11]. Numerous in vivo screens and tests have demonstrated that PAEs mediated their effects through binding to the estrogen receptor [8, 12]. Therefore, a simple, rapid, and selective analytical method for determination of PAEs in the natural water is desired.
Until now, different methods such as solid phase extraction [13], solid phase microextraction [14, 15] and liquid—liquid microextraction [16] followed by high-performance liquid chromatography [17], gas chromatography [18], or mass spectrometry detection have been developed for the determination of PAEs in different matrices. However, those methods require large amounts of organic reagents and tedious operation steps. Recently, owing to its high resolving power, low solvent consumption and simple sample pretreat-ment, capillary electrophoresis (CE) has been used as an attractive method for environmental analysis [19—23]. The detection of three phthalate esters including DMP,
DEP and DBP by non-aqueous micellar electrokinetic chromatography (MEKC) was investigated by electro-kinetic injection [24]. Various phthalate esters, including benzyl butyl phthalate and DEHP, were studied by microemulsion electrokinetic chromatography [25]. However, these capillary electrophoresis methods have not been applied to milk and running water samples.
Due to its relatively low sensitivity, CE is not the most representative method among analytical separation techniques. On-line solid phase extraction, membrane pre-concentration [26], powerful detectors [27], sample stacking [28—31] have been attempted to increase the detection sencitivity and extend the use of CE for the determination of analytes in trace amounts. In 1998, Quirino and Terabe [32] developed an on-line concentration technique using field-enhanced sample injection with reverse migrating micelles (FESI-RMM). Low pH buffer was used to reduce the electro-osmotic flow. The sample is dissolved with the aid of micelles in solution and injected into the capillary using voltage. This
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preconcentration technique has been shown to provide more than 100-fold increase in UV detector response with very high plate numbers. Sample stacking with a dynamic pH junction depends on the change in elec-trophoretic mobility as the charged analytes encounter the pH junction between the sample zone and the background electrolyte zone upon application of voltage. Due to the good result of the sample stacking, the FESI-RMM has been used as an attractive method [33-36].
In this paper, we developed a FESI-RMM method for the simultaneous determination of DMP, DEP, DBP, DHP and DEHP (Scheme). The optimum separation and stacking conditions were achieved by optimizing the concentrations of SDS and organic modifier, the sample matrix, the injection time of water plug, the injection voltage and injection time of sample. The DMP, DEP, DBP, DHP and DEHP were successfully determined in running water and milk with satisfactory repeatability and recovery.
O
^-O
Dimethyl phthalate (DMP)
Diethyl phthalate (DEP)
O
Dibutyl phthalate (DBP)
O
Bis(2-ethylhexyl)phthalate (DEHP)
O
Dihexyl phthalate (DHP)
Chemical structures of phthalate esters.
EXPERIMENTAL
Apparatus and conditions. A CL1030 capillary elec-trophoresis system (Huayanlimin, China) was used. The applied voltage was held constant at -20 kV. The column was an uncoated 50 ^m ID fused-silica capillary of 80 cm and an effective length of 45 cm (Yongnian, Hebei Province, China). UV detection was set at 200 nm. Before each use, the capillary was rinsed with 1 M NaOH for 10 min and water for 10 min; then it was conditioned with running electrolyte for 10 min. Between each run, the capillary was rinsed with water and electrolyte for 5 min each. Samples were injected
for 5 s with —20 kV after gravity injection of 5 s water plugs at 20 cm high.
Materials and reagents. The running water was obtained from Laboratory ofcolloids and interfaces, College of chemistry and molecular engineering, Qingdao University of science and technology, Qingdao, Shandong province, China. The Mengniu Milk was purchased from Liqun supermarket ofQingdao, Shandong province, China. Standards of DMP DEI, DBP DHP and DEHP were obtained from Aladdin (Shanghai, PR. China). H3PO4, NaOH, SDS, acetonitrile, 2-propanol, neutral alumina and ammonium acetate were of analytical re-
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MEI-E YUE и др.
mAU
40 35 30 25 20 15 10 5 0
1 2 3
10 15 20 25 Migration time, min
30
35
Fig. 1. Effect of the acetonitrile concentration on the migration time: 1 - DEHP, 2 —DHP, 3 - DBP, 4 - DEP, 5 -DMP. Analytical conditions: 50 mM H3PO4, 160 mM SDS, and 20% 2-propanol at pH 2.0.
mAU
40 35 30 25 20 15 10 5 0
1 23
1 + 2 + 3
1 + 2
I
+ 3 4
-A._
_L
m
4 5
^ I H 15%
-5%
.0%
20%
10%
0 5 10 15 20 25
Migration time, min
Fig. 2. Effect of the 2-propanol concentration on the migration time: 1 - DEHP, 2 - DHP, 3 - DBP, 4 - DEP, 5 -DMP. Analytical conditions: 160 mM SDS, 15% acetonitrile. Other conditions were similar to Fig. 1.
4
1
+
4
+
3
0
5
agent grade from Beijing Chemical Factory (Beijing, PR. China). Deionized water was used throughout.
The buffer solutions containing H3PO4, SDS and organic solvents adjusted to the desired pH with 0.1 M NaOH, were all filtered through a 0.45 ^m membrane filter and degassed prior to use. A standard solution of 1500 ^g/mL of each analyte was prepared in methanol, filtered, and degassed by the same procedure as used for buffer solutions. The various concentrations of the sample solutions were prepared by appropriate dilution from the stock solution with water containing 5 mM SDS when needed.
Sample preparation. The running water was got and filtered using a 0.45 ^m cellulose membrane to obtain the sample solution. The milk sample was prepared as described with some modifications: 1.00 mL of milk was added into a 20 mL beaker. A 5 mL acetonitrile acidified by HCl (pH 2) was then added. The mixture was ultrasonicated for 10 min and transferred to a 25 mL centrifuge tube, and then centrifuged at 8000 rpm for 10 min. The supernatant was moved to a 125 mL separatory funnel and 20 mL of hexane was then added. After vigorous shaking and standing, the subnatant was got and filtered using a 0.45 ^m cellulose acetate membrane to obtain the sample solution.
RESULTS AND DISCUSSION
Optimizing the separation conditions. Effect of SDS concentration. The experiments were performed with the 50 mM H3PO4 (pH 2.0). In FESI-RMM method, the effect of SDS concentration on the separation and stacking of analytes was important. Different SDS concentrations (80, 100, 120, 140, 160 and 180 mM)
of the electrolyte
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