Abstract
A sweeping micellar electrokinetic chromatography (sweeping-MEKC) enrichment model was established for the determination of three chlorophenols (CPs) in cosmetics, namely, bithionol, pentachlorophenol (PCP), and 2,4,6-trichlorophenol (2,4,6-TCP). The optimum electrophoretic conditions were 20 mM NaH2PO4-80 mM sodium dodecyl sulfate (SDS) and 30% (v/v) acetonitrile (pH 2.3). The optimum on-line concentration conditions were as follows: sample matrix, 100 mM NaH2PO4; pressure injection at 20.67 kPa (3 psi) for 25 s. The linear range of bithionol, PCP, and 2,4,6-TCP are 0.20–4.00 μg mL−1, 0.10–2.00 μg mL−1, and 0.05–2.00 μg mL−1 respectively, with correlation coefficient (r) over 0.9972. The limits of detection (LOD) based on three times the signal-to-noise ratio (S/N = 3) are in the range of 0.0061–0.024 μg mL−1. Recoveries for the three CPs in powder and lotion samples are between 79.7 and 110.2% with relative standard deviation (RSD) of 1.38–5.54% and 92.2–121.3% with RSD of 0.72–6.09%, respectively. The proposed method can provide reference for the determination of trace CPs in different sample matrix.
Introduction
Halogenated phenols have strong bactericidal action against gram-positive bacteria. They are generally used as preservative in toothpaste, cream, and lotion cosmetics, and the most commonly used are chlorophenols (CPs) [1]. Cosmetics are a common component of our daily lives and usually contain preservatives that are added to prevent microbial growth [2]. Bithionol has been used as preservatives in cosmetics production, but it has been banned by the United States (US) Food and Drug Administration due to photosensitivity and mild urticaria reaction [3]. Studies show that the toxicity of halogenated phenol is higher than that of phenol, and it increases with the increase of the number of halogen atoms. Long-term exposure can cause damage to multiple organs of the human body, and its high toxicity and non-degradability have been widely concerned [4–7]. China's cosmetics hygiene regulations (2007 version) clearly stipulate that bithionol, pentachlorophenol (PCP), 2,4,6-trichlorophenol (2,4,6-TCP) are banned substance in cosmetics, and should not be used as a cosmetic raw material [8]. Therefore, the detection of bithionol, PCP, and 2,4,6-TCP residues in cosmetics is of great significance to ensure the quality of cosmetics products, which can protect the health of consumers and improve the cosmetics inspection technology system.
Several methods have been developed to detect and quantify CPs in different samples. CPs in water samples and cosmetics have been analyzed by high performance liquid chromatography with ultraviolet detection (HPLC-UV) [9, 10] or photodiode array detection (HPLC-DAD) [11, 12]. Gas chromatography (GC) with either flame ionization detection (FID) [13], or mass spectrometric (MS) [14] detection also are used to detect phenolic compounds in urine samples. However, it is difficult to analyze phenolic compounds by GC due to their polar nature. To overcome these difficulties, it is usually necessary to derivatize them before GC analysis. The sample derivation process is cumbersome and time consuming. Comparatively, HPLC is known as the most convenient technique for phenolic compounds [12, 15, 16]. In China, HPLC method was specified as the standard method for the determination of bithionol, PCP, and 2,4,6-TCP in cosmetics [17].
Capillary electrophoresis (CE) is an efficient and economical method for the analysis of small molecules and biopolymers. Compared with HPLC, it usually has the advantages of small sample and reagent consumption, fast separation speed and high theoretical plate number [18–21]. Unfortunately, due to the small inner diameter of the capillary and the small amount of sample injection, the detection sensitivity becomes a problem that restricts its development [22, 23]. How to improve the detection sensitivity to meet the requirements to detect trace and ultra-trace components in complex samples is still the focus and hotspot in the field of electrophoresis research. Online enrichment technology is a method to improve sensitivity during sample injection or separation. Commonly used are field-amplified sample [24], dynamic pH junction [25], sweeping [26], and transient isotachophoresis [27]. The sweeping technique is used in micellar electrokinetic chromatography (MEKC) and is the picking and accumulation of analyte molecules by the pseudostationary phase (PSP) that penetrates the sample zone during application of voltage, the injected length of an analyte can be narrowed by a factor of (1 + k), where k is the retention factor. A concentration of over 5,000-fold has been achieved with this technique [28, 29]. Some research groups have applied these on-line preconcentration techniques in small molecules and biopolymers analysis field. For example, Tsai et al. [30] established rapid analysis of melamine in infant formula by sweeping-MEKC, and melamine content in infant formulas could be determined within 6 min. Zhang et al. [31] established detection of some aromatic amines in water samples by using sweeping-MEKC. The limits of detection (LOD) for four aromatic amines was about 80–1,090 folds lower than those of conventional sample injections. The method has the advantages of high enrichment multiple, good reproducibility and high recovery rate.
Based on the above advantages of sweeping-MEKC, a sweeping-MEKC method for the separation and determination of bithionol, PCP, and 2,4,6-TCP was established. Key parameters were optimized to achieve baseline separation and high sensitivity. The method has been applied to the determination of the three CPs in cosmetic samples. To the best of our knowledge, this is the first study using CE on-line preconcentration technique to analyze CPs in cosmetic samples.
Experimental
Apparatus
A SCIEX P/ACE™ MDQ plus CE system (Fullerton, CA, USA) equipped with a diode-array detector (DAD) was used for the experiments. Data collection, processing, and analysis were performed using a system 32 karat software (Beckman) and recorded on a Lenovo-compatible personal computer. Baer fused-silica capillary (Yongnian Photoconductive Fiber Factory, Hebei, China) with 75 μm id, total length of 50.2 cm, and effective length of 40 cm was utilized in all the experiments. The capillary was thermostated at 25 °C. STARTER 3100 (Shanghai OHAUS Instrument Corporation, Shanghai, China) was used to measure the pH value.
Reagents and samples
Bithionol with purity of greater than 98% and 2,4,6-TCP with purity of greater than 99% both were purchased from Sigma-Aldrich (Shanghai, China). PCP having a purity of 99.1% was purchased from Manhag Biotech Co., Ltd. (Beijing, China). The standard stock solutions were prepared by dissolving them in methanol (MeOH) at a concentration of 1 mg mL−1 for each. The structures of the three CPs are shown in Fig. 1. All the above solutions were stored in a refrigerator at 4 °C. Sodium dihydrogen phosphate (NaH2PO4), hydrochloric acid (HCl), phosphoric acid (H3PO4), and sodium hydroxide (NaOH) are all of analytically pure and purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Chromatographic grade acetonitrile (ACN) and MeOH were purchased from Komio Chemical Reagent Co., Ltd. (Beijing, China) and J&K Chemical (Beijing, China), respectively. Sodium dodecyl sulfate (SDS) with purity of 97% was purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). Ultrapure water with the specific resistances of 18.2 MΩ·cm was produced by a Milli-Q ultrapure water system (Millipore, Bedford, MA, USA) for aqueous solution preparation throughout the study.
Chemical structure of bithionol, PCP, and 2,4,6-TCP
Citation: Acta Chromatographica 33, 4; 10.1556/1326.2020.00812
Samples for the experiment, loose powder (Guangzhou, China, expiry data: 20210902) and body lotion (Shanghai, China, expiry data: 20220306) were purchased from local supermarkets. Sample preparation was carried out referencing to the National Standards of People's Republic of China [17]. 0.5 g (accurate to 0.1 mg) of the cosmetic samples were weighed and placed into a 10 mL polypropylene centrifuge tube and then 10 mL MeOH was added to the weighed sample and mixed well. The sample was ultrasonically extracted for 20 min and centrifuged at 6,000 r/min for 10 min. Then the supernatant was removed and dried under a gentle flow of nitrogen. At last, the evaporation residue was redissolved using 1 mL of 100 mM Na2HPO4 and filtered through a 0.45 μm filter for further CE analysis.
CE conditions
Before the first usage, new capillary was conditioned by the procedure referring to our previous work [7], and the rinsing pressure was 20 psi (1 psi = 6894.76 Pa). The background solution (BGS) consisted of 20 mM NaH2PO4-80 mM SDS and 30% (v/v) ACN, and the solution pH was adjusted with 3 mol L−1 H3PO4 after addition of ACN to pH 2.3. Sample matrix was 100 mM Na2HPO4, and samples were injected by pressure at 3 psi × 25 s. The applied separation voltage was −15 kV and the detection wavelength was set at 214 nm. BGS with different concentrations was prepared from 200 mM Na2HPO4 and 400 mM SDS stock solutions. All electrolytes and samples were filtered through 0.45 μm membrane filters and degassed by ultrasonic prior to analysis.
Sweeping-MEKC
When preparing the sample, the working stock solution was diluted with sample matrix containing no SDS (non-micelle BGS) and a conductivity value similar to that of the micellar BGS. In sweeping-MEKC, in order to suppress the electroosmotic flow (EOF) and make the electrophoretic speed of SDS larger than the EOF rate, an acidic phosphate buffer solution (pH 2.3) was selected. Much longer than the normal sample plug sample was then hydrodynamically injected into a capillary. The PSP with negative charge has a higher electrophoretic velocity than that of the CPs without charge under low pH conditions. It made sure that the PSP penetrates into the sample plug and the analytes in the sample zone are concentrated into a relatively narrow zone under the sweeping of the micelle. The introduction of a large sample zone is integrated into a narrow sample zone to improve the sensitivity of the test and then the voltage was applied at negative polarity (positive at outlet) until all peaks were detected.
Results and discussion
Optimization of sweeping-MEKC
Fig. 2 gives a brief description for the sweeping-MEKC process of CPs in the presence of very low (nearly zero) EOF. In the beginning (Fig. 2A), CPs solution has similar conductivity as that of the injected BGS. The length of the CPs zone injected is given as Linj, which is much longer than that of normal injection. Because the pKa values of bithionol, PCP, and 2,4,6-TCP are 4.82, 4.93, and 5.99, respectively, the CPs are nearly uncharged molecules in the BGS with pH value lower than their pKa, upon application of voltage at negative polarity (Fig. 2B), the electrophoretic velocity are nearly zero. The PSP with negative charge has a higher electrophoretic velocity thus sweep the sample plug toward the anode (detection window). After a certain period of time, all CPs molecules are picked and accumulated by PSP (Fig. 2C), the accumulated CPs are then separated by MEKC.
Schematic illustration of sweeping-MEKC. (A) Starting situation, a longer than a typical injection of CPs prepared in a matrix having a conductivity similar to that of the BGS; (B) application of voltage at negative polarity, micelles emanating from the cathodic side sweeping CPs; (C) the injected CPs are assumed to be completely swept
Citation: Acta Chromatographica 33, 4; 10.1556/1326.2020.00812
SDS concentration
According to the equation above, narrower bands of the swept zone are obtained with larger k values, which in turn can be obtained with higher concentrations of SDS. The concentration of SDS from 60 to 100 mM was investigated. The peak grew higher from 60 to 80 mM (Fig. 3A). The migration time of analytes decreased with the increase of SDS concentration because more and more of the analytes were incorporated into the micellar phase. However, a higher concentration at 100 mM did not further increase the peak height and the stability of the peak is deteriorated due to Joule heating. From the standpoint of higher sweeping efficiency and shorter analytical time 80 mM SDS was selected and used in the micellar BGE for sweeping-MEKC.
Effect of SDS concentration (A) and ACN concentration (B) on peak area of the three CPs in sweeping-MEKC system. Other sweeping-MEKC conditions: BGE, 25 mM NaH2PO4 with SDS, and ACN concentration indicated in the figure (pH 2.3); samples: bithionol, PCP, and 2,4,6-TCP; 2, 1, 1 μg mL−1 dissolved in 80 mM NaH2PO4; injection, 3 psi × 25 s; separation voltage, −15 kV
Citation: Acta Chromatographica 33, 4; 10.1556/1326.2020.00812
Kinds and content of organic solvent
The addition of organic solvent in the buffer solution could adjust the distribution capacity of the analytes in the two phases of the micelle and buffer system to improve the separation degree and peak shape of the analytes. In the experiment, 25 mM NaH2PO4 and 80 mM SDS were used as buffer, and 30% MeOH and ACN were added respectively to investigate the influence on the peak of the analytes. The results showed that both of the two kinds of organic solvent could improve the peak shape of the analytes. Compared with MeOH, ACN as an additive can get better peak shape and larger peak area. Therefore the content of ACN (20, 25, 30, 35, 40% (v/v)) was investigated and the results were shown in Fig. 3B. As the ACN content increased, the separation time of the analytes were prolonged. Considering both peak shape and peak area, 30% ACN was selected.
Running buffer concentration and sample matrix composition
The effect of the concentration of running buffer on the separation of the three CPs in the range of 10–40 mM is illustrated in Fig. 4A. With the increase of NaH2PO4 concentration, the separation time of the analytes was prolonged, but the peak shape and resolution did not change significantly. When the concentration of NaH2PO4 was 20 mM, the sensitivity enhancement of the three CPs reaches the maximum. Therefore, 20 mM NaH2PO4 was used. Sample matrix is important for sweeping-MEKC since it could decide the dissociation of the analytes, which affect the degree of sample stacking. The effect of sodium dihydrogen phosphate solution with concentrations of 60–120 mM on sweeping was investigated. The results in Fig. 4B indicate that the highest peak intensity of CPs was achieved using the sample matrix solution of 100 mM NaH2PO4.
Effect of buffer concentration (A) and sample matrix composition (B) on the sweeping efficiency for the three CPs. Peak identification: (1) bithionol, (2) PCP, and (3) 2,4,6-TCP; BGE: the concentration of NaH2PO4 indicated in Fig. 4A with 80 mM SDS and 30% ACN (pH = 2.3); Other conditions same to Fig. 3
Citation: Acta Chromatographica 33, 4; 10.1556/1326.2020.00812
Sample injection time and separation voltage selection
For sweeping technique, the injected length of an analyte zone could be limited by the sweeping capacity of the micelles, which is dependent on the retention factor (k) of the analytes with the micelles. When k goes to infinity, only the column length restricts the injection lengths. If the k value of the analyte is not high enough, it is suggested that the length of the injected sample must be optimized. In this study, the sample solution was injected at 3 psi for 10, 20, 25, and 30 s into the capillary column. From Fig. 5 we can see that the peak height of the analytes increased when the injection time increased from 10 to 30 s. When the injection time was increased to 25 s, the three CPs can get better resolution and detection sensitivity. When the injection time is 30 s, although the sensitivity of the analyte is higher, the resolution is decreased. Hence, 25 s was used as the optimal injection time. The effect of separation voltage (−15, −17, and −20 kV) on separation was also investigated. The results show that the peak area of the analytes decreases with the increasing voltage. Further increase of separation voltage resulted in current disruption due to Joule heating. Therefore, −15 kV was chosen as the separation voltage.
Effect of injection time on the sweeping-MEKC efficiency for the three CPs. BGE: 20 mM NaH2PO4, 80 mM SDS, and 30% (v/v) ACN (pH 2.3); Sample matrix: 100 mM NaH2PO4; Other conditions the same as Fig. 3. Peak identification: the same as Fig. 4
Citation: Acta Chromatographica 33, 4; 10.1556/1326.2020.00812
To sum up, the optimum conditions are as follows: BGS composed of 20 mM NaH2PO4, 80 mM SDS, and 30% (v/v) ACN (pH 2.3); sample matrix composed of 100 mM NaH2PO4; sample was injected by pressure injection (3 psi) for 25 s.
Method performance
Under the optimal conditions, Fig. 6 shows the electropherograms of CPs obtained by sweeping-MEKC (a) and normal CZE (b) The EFs for the three CPs were 57 (bithionol), 76 (PCP) and 57 (2,4,6-TCP), respectively. The detection limits, linearity, and repeatability of the three CPs under optimal conditions are shown in Table 1. The linearity of the sweeping-MEKC method was evaluated by calibration curves in the range of 0.20–4.00 µg mL−1 for bithionol, 0.10–2.00 μg mL−1 for PCP, and 0.05–2.00 μg mL−1 for 2,4,6-TCP at five concentration levels. Linear relationship was found between peak area and the concentration of the analytes with correlation coefficient >0.99, indicating a good linearity of this method. The LOD were obtained based on the peak height as three times of background noise (S/N = 3), in the range of 0.0061–0.024 μg mL−1 and the limits of quantification (LOQ) were obtained based on the peak height as ten times of background noise (S/N = 10), in the range of 0.020–0.080 μg mL−1, which meet the requirements of trace analysis. From Table 2 we can see that the intra-day precision of migration time and peak area were 0.17–1.64 and 2.61–3.90%, respectively. The inter-day precision of migration time and peak area were 2.44–3.57 and 3.54–6.55%, respectively. The repeatability of this method is satisfactory. All these results indicated that the developed sweeping-MEKC method could be a reliable approach for the analysis of CPs with high sensitivity.
Electrophoregrams of the sweeping-MEKC (a) and CZE (b). (a) sweeping-MEKC: BGE: 20 mM NaH2PO4, 80 mM SDS, and 30% (v/v) ACN (pH 2.3); sample matrix: 100 mM NaH2PO4; pressure injection: 3 psi × 25 s; separation voltage: −15 kV; 214 nm. bithionol (2 μg mL−1), PCP(1 μg mL−1), 2,4,6-TCP(1 μg mL−1). (b) CZE: BGE: 15 mM Na2B4O7 and 15% (v/v) MeOH (pH 9.8); pressure injection: 0.5 psi × 5 s; separation voltage: +15 kV; 214 nm. Bithionol (20 μg mL−1), PCP(10 μg mL−1), 2,4,6-TCP(10 μg mL−1). Peak identification: (1) bithionol, (2) PCP, and (3) 2,4,6-TCP
Citation: Acta Chromatographica 33, 4; 10.1556/1326.2020.00812
Parameters of the analytical performance of sweeping-MEKC for the three CPs
| CPs | Calibration curvea | Correlation coefficient (r) | Linear range (µg mL−1) | LOD (µg mL−1) | LOQ (µg mL−1) | EF |
| bithionol | y = 120350x − 29797 | 0.9975 | 0.20–4.00 | 0.024 | 0.080 | 57 |
| PCP | y = 186590x − 9670.9 | 0.9972 | 0.10–2.00 | 0.015 | 0.048 | 76 |
| 2,4,6-TCP | y = 180935x − 3677.8 | 0.9998 | 0.05–2.00 | 0.0061 | 0.020 | 57 |
y and x stand for the peak area and the concentration (µg mL−1) of the there CPs, respectively.
Method precision of sweeping-MEKC for three CPs
| CPs | RSDa (%, n = 5) | |||
| Intra-day | Inter-day | |||
| Migration time | Peak area | Migration time | Peak area | |
| Bithionol | 1.64 | 3.90 | 3.57 | 6.55 |
| PCP | 1.41 | 2.61 | 3.54 | 3.54 |
| 2,4,6-TCP | 0.17 | 2.72 | 2.44 | 5.86 |
The concentrations of 3 CPs are 2, 1, 1 μg mL−1 for bithionol, PCP and 2,4,6-TCP, respectively were chosen for the assays of intra-day precision and inter-day precision.
Sample analysis
To demonstrate how the sweeping-MEKC method can be applied for routine analysis of real cosmetic samples, the developed sweeping-MEKC method was applied for analyzing the three CPs levels in two cosmetic samples including loose powder and body milk. Fig. 7 (curve a) showed the electropherograms of blank cosmetic samples. It can be seen that none of the three CPs were found in blank cosmetic samples. As mentioned in the introduction, China's cosmetics hygiene regulations (2007 version) clearly stipulate that bithionol, PCP, 2,4,6-TCP are banned substance in cosmetics, and should not be used as a cosmetic raw material [8]. To further evaluate the performance of the sweeping-MEKC method in complex matrices and verify the specificity and accuracy of the proposed method, three replicates of the cosmetic samples spiked with 0.50 (Fig. 7 curve b) and 1.00 μg mL−1 (Fig. 7 curve c) of the three CPs were analyzed. From Fig. 7 we can see that the three CPs can be detected and well separated with other components in the samples. As listed in Table 3, recoveries for bithionol, PCP, 2,4,6-TCP in loose powder and body lotion were between 79.7 and 110.2% with relative standard deviations (RSDs) of 1.38–5.54% and 92.2–121.3% with RSDs of 0.72–6.09%, respectively. All of these results demonstrate that the sweeping-MEKC method developed in this work is an alternative approach for analyzing CPs levels in cosmetic samples.
Sweeping-MEKC analysis of three CPs in blank and spiked samples of loose powder (A) and body lotion (B). (a) without spiking; (b) 3CPs, 0.50 μg mL−1, (c) 3CPs, 1.00 μg mL−1. Peak identification: (1) bithionol, (2) PCP, and (3) 2,4,6-TCP; BGE: 20 mM NaH2PO4, 80 mM SDS, and 30% (v/v) ACN (pH 2.3); Sample matrix: 100 mM NaH2PO4; Other conditions the same as Fig. 3
Citation: Acta Chromatographica 33, 4; 10.1556/1326.2020.00812
Recoveries of the three CPs in cosmetic samples by sweeping-MEKC (n = 3)
| CPs | Background (μg mL−1) | Added (μg mL−1) | Loose powder | Body lotion | ||||
| Found (μg mL−1) | Recovery (%) | RSD (%) | Found (μg mL−1) | Recovery (%) | RSD (%) | |||
| Bithionol | ND | 0.50 | 0.55 ± 0.025 | 110.2 | 4.64 | 0.61 ± 0.035 | 121.3 | 5.74 |
| 1.00 | 0.80 ± 0.020 | 79.7 | 2.51 | 0.92 ± 0.056 | 92.2 | 6.09 | ||
| PCP | ND | 0.50 | 0.45 ± 0.011 | 89.2 | 2.46 | 0.52 ± 0.021 | 104.6 | 4.04 |
| 1.00 | 1.01 ± 0.014 | 101.2 | 1.38 | 0.99 ± 0.007 | 100.0 | 0.72 | ||
| 2,4,6-TCP | ND | 0.50 | 0.48 ± 0.025 | 96.0 | 5.21 | 0.55 ± 0.045 | 110.0 | 5.18 |
| 1.00 | 0.81 ± 0.061 | 80.9 | 5.54 | 1.13 ± 0.042 | 99.9 | 3.71 | ||
ND: not detected.
Conclusion
In this experiment, the electrophoretic separation mode of MEKC, combined with its unique sweeping technology, was selected to establish a method for simultaneous enrichment, separation and detection of bithionol, PCP, and 2,4,6-TCP. The influencing factors for separation and enrichment in the sweeping-MEKC were systematically investigated. The method has the advantages of high enrichment factor, high sensitivity, simple operation, good repeatability and low cost, and has been successfully applied to the determination of the three CPs in cosmetic samples. It provides an effective way for the detection of CP fungicides in cosmetics.
Conflict of interest
There are no conflicts to declare.
Acknowledgments
We are grateful for financial support from the National Natural Science Foundation of China (No. 51573155).
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