To determine the trace amount of four benzoic acid esters in cosmetics, ionic dispersive liquid–liquid microextraction (DLLME) and magnetic solid-phase extraction were combined and optimized. After solvent optimization, 1-octyl-3-methylimidazolium hexafluorophosphate was selected as the extraction solvent to form hydrophobic droplets in the process of ionic DLLME, followed by removal of ions from the sample solution containing Fe3O4@GO nano-materials. The magnetic nano-materials were characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, and vibrating sample magnetometer. Some parameters affecting the efficiency of extraction were optimized using Box-Behnken design. Under optimized conditions, the limit of detection for all the preservatives was less than 0.135 mg/L and the accuracy ranged from 88.5% to 101%. This technology could determine the trace amount of preservatives in cosmetics with comparatively higher accuracy and sensitivity.
- [C 8MIM][PF 6]
analysis of variance
coefficient of variation
cloud point extraction
dispersive liquid–liquid microextraction
- Fe 3O 4@GO
magnetic graphene oxide
fast liquid chromatography
Fourier transform infrared spectroscopy
high-performance liquid chromatography
limit of detection
limit of quantification
magnetic solid-phase extraction
relative standard deviation
scanning electron microscope
transmission electron microscopy
ultra-high-performance liquid chromatography
vibrating sample magnetometer
Currently, most of the active ingredients in cosmetics are nutrients, which may degrade easily because of microbial contamination, failing product specification . Because of their comparatively wider antibacterial spectrum, strong antibacterial activity, and good solubility in water, and because they are colorless, odorless, and cheaper, benzoate preservatives are widely used in the production of cosmetics [2,3,4,5]. According to recent medical research results, excessive paraben may cause irritation to the human skin and respiratory and endocrine systems [6,7,8,9]. In addition, exposure to paraben in some cosmetic products has been proved to be associated with birth size . The safety issue of paraben in cosmetics has attracted increasing attention, and in many countries, the accepted limits of preservatives are strictly controlled. Taking p-hydroxybenzoic acid as an example, the authorized maximum content of single esters and ester mixtures should not exceed 0.4% (v/v) and 0.8% (v/v), respectively .
As the formulations of cosmetic products are relatively complicated and diverse, the reported methods showed comparatively low sensitivity, accuracy, and efficiency . Thus, sample preparation, including preconcentration and purification, are the key steps to improve accuracy and sensitivity when using high-performance liquid chromatography (HPLC) and ultra-high-performance liquid chromatography (UHPLC) based on the ultraviolet-visible detector and diode array detector [13,14,15,16].
Different extraction technologies for benzoate preservative have been reported, such as vortex extraction, supercritical fluid extraction, matrix solid-phase dispersion, and quick, easy, cheap, effective, rugged, and safe. To reduce energy cost and organic solvent usage without compromising extraction efficiency, dispersive liquid–liquid microextraction (DLLME) was optimized to concentrate the target analytes in extracting solvent . Furthermore, ionic liquids (ILs), known as green solvents, were used as extracting solvents in ILs-DLLME with their unique properties, such as high chemical and thermal stability, low flammability, and excellent solvation ability for a wide range of compounds [18,19,20]. Moreover, to avoid time-consuming centrifugation in ILs-DLLME, the magnetic nano-materials were applied to absorb the target analytes by hydrophobic interaction which was known as magnetic solid-phase extraction (MSPE). Magnetic graphene oxide (Fe3O4@GO) nano-materials, with high extraction ability and its magnetic properties, could improve efficiency in purification [21,22,23,16].
In this paper, IL-DLLME-MSPE, which was a method rarely reported in the literature, was developed for the extraction of four types of benzoic acid esters from cosmetics. IL-DLLME remarkably cut the energy and cost by reducing the usage of organic solvents and improved the ease of operation, and MSPE greatly improved the efficiency of separation and extraction, which solved the problem of long-time centrifugation, freezing, or manual collection of large amounts of organic solvents in DLLME. The optimized method was proved to be easy operating, cost-effective, and eco-friendly with acceptable accuracy and precision, which can be applied in the routine assay for preservatives in cosmetics [24,25,26].
HPLC analysis was performed using Agilent 1200 high performance liquid chromatograph (California, USA) equipped with a Diamonsil C18 column (4.6 mm × 150 mm, 5 μm) (Beijing, China). The column temperature was kept at 35°C. The injection volume was set at 10 μL. The analytes were separated using isocratic elution with a 70:30 (v/v) mixed solution of methanol and water at a flow rate of 0.8 mL/min. The detection wavelength was set at 254 nm. FT-IR spectra were recorded on a Bruker IFS-66 Fourier transform infrared spectroscopy (Karlsruhe, Germany). Magnetization measurements were performed on a Quantum Design Vibrating Sample Magnetometer (VSM, San Diego, USA). The structure and morphology of the prepared nano-materials were characterized by a Helio Nanolab 600i Scanning Electron Microscope (SEM, Hillsboro, USA). The transmission electron microscopy (TEM) images of the prepared nano-materials were obtained using a JEM-2100F electron microscopy (Tokyo, Japan).
Deionized water was obtained from Wahaha Group Co., Ltd. (Hangzhou, China). HCl, NaOH, and absolute alcohol (purity, 95%) were supplied by Hengxing Chemical Reagent Factory (Tianjin, China). Chromatographic pure methanol, acetonitrile, and acetone were purchased from Shandong Yuwang Industrial Co., Ltd. (Yucheng, China). FeCl3·6H2O (purity, 99%) and FeCl2·4H2O (purity, 99%) were provided by Ailan Chemical Technology Co., Ltd. (Shanghai, China). Graphene oxide (GO) was supplied by Nanjing Ji Cang Nano Technology Co., Ltd. (Nanjing, China). Methyl 4-hydroxybenzoate (MP), ethyl 4-hydroxybenzoate (EP), propyl p-hydroxybenzoate (PP), butyl-paraben (BP) standard, and 1-octyl-3-methylimidazolium hexafluorophosphate ([C8MIM][PF6]) were purchased from Shanghai Aladdin Chemistry Co., Ltd. (Shanghai, China). The standard solutions of MP, EP, PP, and BP with a concentration of 100 μg/mL were prepared in a volumetric flask and stored away from light at 4°C. The standard solutions were diluted with methanol and prepared every day.
2.3 Preparation of Fe3O4@GO nano-materials
According to the chemical co-precipitation method, Fe3O4@GO nano-materials were prepared . Briefly, 0.5 g of GO powder and 100 mL of distilled water were taken in a 250 mL three-neck round bottom flask, followed by mechanical stirring until the water bath temperature increased evenly from 20°C to 70°C in 3 min. About 2.16 g of FeCl3·6H2O and 0.80 g of FeCl2·4H2O were weighed and dissolved in 40 mL of distilled water with vigorous stirring at 70°C for 5 min. Then the pH of this suspension was adjusted to 12.0 with an ammonia solution (28%, g/mL) to conjugate the Fe3O4 nano-materials onto GO sheets. After mechanically stirring at 70°C for 60 min, the black magnetic material was separated from the suspension using a magnet and washed thrice with distilled water and ethanol. To avoid complete growth and oxidation of the nano-material crystals, the above reaction needs to be carried out in a nitrogen environment. Fe3O4@GO nano-materials were achieved after drying under vacuum at 60°C for 6 h and grounded using a mortar for further use.
2.4 Sample preparation
We have purchased five different brands of cosmetics from a local shopping mall (Shenyang, Liaoning), including lotion and facial cleanser. Weighed 5 g of lotion and 5 g of facial cleanser, and measured 1 mL of toning lotion and 1 mL of cleansing water to 100 mL centrifugal tube separately. This was mixed with 50 mL of distilled water and the matrix dispersed by vortexing for 2 min. After sonication for 10 min, the suspension was filtered through a 0.45 μm filter membrane and stored at 4°C for further use.
2.5 Pretreatment of IL-DLLME and MSPE
About 4.0 g of NaCl was weighed and dissolved completely in the above samples, and the pH was adjusted to 3.2 with 0.1 mol/L HCl. Then, 30 μL of [C8MIM][PF6] and acetonitrile were added to the suspension before shaking for 2 min. After that, 32.7 mg of Fe3O4@GO was added to the dispersion solution, and sonicated for 6 min. After separation by a magnet, magnetic nano-materials were resuspended in 2 mL of absolute ethanol and sonicated for 5 min. Finally, Fe3O4@GO were magnetically separated from the suspension again, and the supernatant was collected and filtered with a 0.22 μm filter membrane before injecting into HPLC for further analysis.
3 Results and discussion
3.1 Characterization of Fe3O4@GO nanoparticles
The dimension and surface morphologies of the nano-materials were characterized by SEM, TEM, FT-IR, and VSM.
The surface morphologies of GO and Fe3O4@GO from SEM are shown in Figure 1a and b. Figure 1a show a typically smooth and disorderly folded sheet-like structure of GO. In comparison, the SEM image of Fe3O4@GO in Figure 1b show a rougher surface because a large number of Fe3O4 nano-materials have been attached to the surface of GO. The structural characteristics of GO and Fe3O4@GO observed by TEM are shown in Figure 1c, and the GO was sheet-like in shape with high transparency and wrinkled edges. Fe3O4 nano-materials were located at the cross-section of Fe3O4@GO nano-materials (Figure 1d) and observed as dark spots that distributed homogeneously on the smooth surface of the GO sheet.
Figure 2 shows the FT-IR spectra of GO and Fe3O4@GO. The most distinct peak at 3436.6 cm−1 was attributed to the O–H stretching vibration. The characteristic peaks of GO that appeared at 1720.8, 1629.6, 1401.1, and 1052.1 cm−1 were corresponded to the carbonyl (C═O) stretching, sp2 carbon skeletal network, C–OH group stretching, and C–O–C in the epoxide group stretching vibration, respectively. According to the spectra of MGO, the new presence of absorption peak at 1396.4 cm−1 indicated an additional vibration band in samples, confirming the formation of the chemical bond between the carboxyl group and Fe. In addition, the peak at 584.5 cm−1 was attributed to the Fe–O–Fe bond vibration. Considering the above evidence, we could conclude that the covalent bonds between magnetic nanoparticles and GO were successfully synthesized.
The magnetic property of Fe3O4@GO at 20°C was investigated using VSM, and the hysteresis loop diagram is shown in Figure 3. The magnetization hysteresis loops of Fe3O4@GO were S-like curves, indicating a superparamagnetic capability of the synthesized nano-materials. The saturate magnetization value of Fe3O4@GO was 47.15 emu/g and enough to be separated with conventional magnets that were sufficient for magnetic separation with conventional magnets. Therefore, Fe3O4@GO was able to be separated rapidly from the aqueous matrix.
3.2 Optimization of IL-DLLME
To obtain optimum extraction efficiency of preservatives in cosmetics, a series of experiments were designed. Some parameters that may affect the extraction efficiency, such as vortexing time, ionic strength, the volume of extraction solvent, and disperser solvent, were optimized. The types of extraction solvent and disperser solvent have been selected according to the previous study with modifications .
3.2.1 Volume optimization of extraction solvent
The volume of ILs also played a critical role in improving the extraction efficiency in IL-DLLME. The volume of the extraction solvent varied from 10 to 50 μL. As shown in Figure 4a, the recoveries decreased slightly when the volume was more than 30 μL. This may be because the excessive volume of [C8MIM][PF6] was easier to adhere to the side-walls of tubes and could not effectively extract the target analytes in the form of tiny droplets. Therefore, the volume of 30 μL of [C8MIM][PF6] was selected.
3.2.2 Volume optimization of disperser solvent
The influence of the acetonitrile volume on the extraction efficiency of the four common preservatives in cosmetics was also investigated in the range of 10–50 μL. When the extraction solvent is less than 30 μL, the extraction solvent unable to be evenly dispersed in the water phase in the form of droplets, however, when the volume of disperser solvent is higher than 30 μL, the solubility of the analyte in the aqueous phase and decrease its concentration in the extraction solvent, both of which resulted in poor recovery. Therefore, given the considerations of cost and recovery in Figure 4b, we selected the volume of 30 μL for acetonitrile in further study.
3.2.3 Optimization of vortexing time
The vortexing time is an important factor in the efficiency of dispersing cosmetic samples, and the time ranged from 0 to 5 min was investigated. The results are shown in Figure 4c; when the vortex time was 2 min, the recoveries of the four common preservatives were more than 90%. However, with the increase in time, the recoveries decreased slightly or basically unchanged. Therefore, vortexing for 2 min was used to the next study.
3.2.4 Optimization of ionic strength
In IL-DLLME, the ionic strength can reduce the solubility of target analytes in the aqueous phase and increase the partition coefficient of the [C8MIM][PF6] phase and further affect the recovery of extraction. The influence of ionic strength in this experiment was assessed by the addition of NaCl in the range of 0–6.0 g in initial aqueous sample. Figure 4d shows that the recoveries reached the maximum value when 4.0 g of NaCl was used. However, excessive salt could also increase the solubility of ILs in the aqueous phase, resulting in a decrease in the recoveries.
3.3 Optimization of MSPE conditions
To understand the influence of experimental variables on the MSPE process, the interaction between the main parameters that affect the extraction efficiency of the analyte must be fully considered, and each parameter must be optimized. On the basis of preliminary experiments, the influence of six factors was studied from two levels of extraction process and elution process. The main parameters, such as pH of the sample solution, extraction time, and amount of sorbent, were evaluated using the Box-Behnken design (BBD) with a small number of trials, which is a widely used method to fit a second-order response surface. The other three parameters, such as type and volume of desorption solvent, and desorption time, were optimized by one-factor-at-a-time procedure.
The Design-Expert 11.0 statistical software was used for the experimental design and data analysis. Based on the results acquired from the analysis of variance (ANOVA) test (presented in Table 1), the significance of model is mainly calculated by R2 value. As presented in Table 1, the coefficients were very high about 0.9569, which indicated that 95.69% of total variance were explained by quadratic model and only 4.31% of total variables were not explained by the model. The adjusted R2 value was 0.9015. The R2 and adjusted R2 values were closed to 1.0000, which displayed a good correlation between experimental and theoretical results. The model had F values of 17.26 with lower p-values (0.0005), which also indicated the impacts of this model. If the lack of fit value in a model was nonsignificant, it suggested that the quadratic model was valid. Here, the lack of fit values were 0.1613, which specified that our model was significant. Another one was the coefficient of variation (C.V.%); if this value was less than 10%, then the model was highly reliable and reproducible for the experimental studies. The C.V.% value for clenbuterol was 8.89%, less than 10%. Therefore, the model was reliable for experimental studies for the determination of four preservatives in cosmetics.
|Source||Sum of squares||Degree of freedom||Mean square||F-value||p-value (Prob > F)|
|Lack of fit||198.90||3||66.30||2.95||0.1613 not significant|
|R2 = 95.69%||Adj R2 = 90.15%||C.V.% = 8.89|
*Significant at 0.01 level.
The pH of the sample solution has a negative effect on the recovery in MSPE process because it can determine the ionization of the target analytes and further affect their recoveries. It is seen from Figure 5 that as the pH increases, the recoveries of four benzoates become more unfavorable. When the pH value was higher than the pKa of benzoate, the target analytes existed in the ionized form could hardly be extracted using organic solvents. The extraction efficiency decreased with the hydrolysis of benzoate at a pH of 3.2, and the results in Figure 5 show that the amounts of Fe3O4@GO was positive to efficiencies of MSPE in the range of 10–32.7 mg. To ensure sufficient contact between the adsorbent and the target analytes, the relative recoveries that are shown in Figure 5 increased with the increase in the extraction time from 1 to 5 min and then remained plateau after 5 min.
To elute the adsorbed analytes off the adsorbent completely, eluting conditions, such as the type and volume of solvent, as well as the time for desorption, were evaluated to improve the recovery. Three organic solvents, methanol, acetonitrile, and ethanol, were investigated. It can be seen from Figure 6 that ethanol generated the highest recovery of analytes. The volume of the desorption solvent was also investigated in the range of 3.0–9.0 mL. As shown in Figure 6, the results indicated that 2.0 mL of ethanol was sufficient to elute all the target analytes with an acceptable recovery. In addition, the desorption time in the range of 1–11 min was compared. As shown in Figure 6, benzoate could be efficiently eluted in 5 min.
3.4 Validation of the method
Under the optimal experimental conditions, the linearity, precision, accuracy, sensitivity, and specificity of the method were validated.
Mixed reference solution, blank solution, and test solution were injected into HPLC for analysis. The results (Figure 7) show that no interference was observed.
Within the range of 0.5–100 μg/mL for the standard solution, 0.5, 1, 5, 10, 20, 50, and 100 μg/mL of the standard solution in the sequence was added to 0.5 g placebo (cosmetic milk) and introduced into HPLC for analysis after sample preparation. The criteria for the limit of detection (LOD) were set at the signal-to-noise ratio of 3, and the limit of quantification (LOQ) was set at 10.
The linearity of all analytes showed a good correlation in the specified range of 0.5–100 mg/L and the R2 ranged from 0.9980 to 0.9997 in Table 2. Under the optimized condition, the LOD of the four target analytes ranged from 0.064 to 0.135 μg/mL. The sensitivity of this method meets the analysis and detection requirements of four preservatives in cosmetics .
|Analytes||Retention time (min)||Calibration equation||Linear range (mg/L)||R2||LOQ (mg/L)||LOD (mg/L)|
|MP||3.290||Y = 72.29X + 63.97||0.5–100||0.9997||0.216||0.064|
|EP||3.995||Y = 67.73X + 52.63||0.5–100||0.9995||0.228||0.068|
|PP||5.233||Y = 64.05X + 47.70||0.5–100||0.9989||0.231||0.069|
|BP||7.296||Y = 58.55X + 41.88||0.5–100||0.9980||0.451||0.135|
Accuracy and precision were conducted by spiking the samples with mixed standards at three different levels (n = 3 at each level), and six replicates of each concentration were made in parallel for 3 days. The mixed reference solution was added to a 0.5 g blank matrix to make final concentrations of 1, 10, and 50 μg/mL. The recovery and relative standard deviations (RSDs) are presented in Table 3, the intra-day RSDs for all the analytes were no more than 4.3%, and the inter-day RSDs were no more than 5.6%. The above results showed that the accuracy and precision of this method are good.
|Analytes||Spiked (mg/L)||Inter-day (n = 3)||Intra-day (n = 6)|
|Relative recovery (%)||RSD (%)||Relative recovery (%)||RSD (%)|
3.5 Assay of products
To verify the applicability of the optimized and validated method, six different types of cosmetic products were analyzed, including paste, emulsion, and liquid formula. The results are presented in Table 4, and the chromatogram of the real sample is shown in Figure 8. The “Safety Technical Specifications for Cosmetics” (2015 edition) stipulates that the total amount of parabens in cosmetics shall not exceed 0.5%, and the amount of monomers added shall not exceed 0.4% . After analysis, most of the four benzoate compounds in the cosmetic samples did not exceed the specified limit, and only one sample of the makeup remover was detected to be out of the specification MP.
|Sample||Content of the analytes (mg/kg)|
|Cleansing water 1||4,290||ND||ND||ND|
|Cleansing water 2||6.422||ND||ND||ND|
ND: not detected.
3.6 Comparison of pretreatment methods
The extraction recoveries of DLLME-MSPE were compared with DLLME or MSPE alone, and the results are shown in Figure 9. It was found that Fe3O4@GO nanocomposites adsorbed and separated ILs in the sample solution, greatly improving the separation and extraction efficiency, which solved the problem of long-time consumption by centrifugation or manual collection. The newly developed method possessed a wide linearity range, high accuracy, and short sample preparation time, which provides a powerful means for monitoring the trace amount of preservative compounds in cosmetics.
3.7 Comparison to reported methods
The comparison between the previously reported method and our proposed method for analyzing MP, EP, PP, and BP in different products is listed in Table 5. Identification of the four parabens in cosmetics, human secretions, and environmental and food samples were reported but the sample preparation using IL-DLLME-MSPE has rarely been reported for the analysis of preservatives in cosmetics. Alshana et al. applied capillary electrophoresis (CE) to determine the four parabens in breast milk and food samples, with an LOD of 0.1–0.2 μg/mL and recovery of 86.7–103.3% . For the reported ultrasonic-assisted extraction (UAE) combining fast liquid chromatography (FLC) method, the LOD of the four parabens in cosmetics, cleaning agents, and pharmaceutical products ranged in 0.06–4.38 μg/mL . Currently, for the analysis of preservatives, there is no universal sample preparation technique and analytical method.
|Sample type||Extraction and determination method||Analysis time (min)||Linear range (µg/mL)||LOD (µg/mL)||Precision (RSD%)||Recovery (%)||Reference|
|Saliva and toothpaste||SPE-LC-UV||15||0.3–50||0.1–0.3||<4||86–113|||
|Cosmetics, cleaning agents, and pharmaceuticals||UAE-FLC-UV||27||0.25–10||0.06–4.38||<5||69–119|||
|Breast milk and food||DLLME–CE||5||0.7–6.0||0.1–0.2||<3.5||86.7–103.3|||
In this experiment, two-step extraction techniques of IL-DLLME and MSPE were used, combined with HPLC method to carry out selective extraction enrichment and trace determination of four benzoate preservatives in different types of cosmetic samples. The optimized method was efficient, accurate, and environment-friendly. It can be applied to analyze preservatives in cosmetics samples without matrix interference. Compared with the traditional DLLME method, [C8MIM][PF6] of ILs is a relatively green extractant that offered higher sensitivity and linearity range. This method possessed accepted accuracy, linearity, precision, and detection limit for the determination of benzoic esters in real samples. This newly developed two-step extraction technology may have a broad application prospect in the analysis of trace preservatives in cosmetics.
Funding information: This work was supported by the National Natural Science Foundation of China (No. 81503029) and Young and Middle-aged Backbone Personnel Training Program of Shenyang Pharmaceutical University (ZQN2016011).
Author contributions: Kai Xu: writing – review and editing, conceptualization, funding acquisition, methodology; Xun Gao: conceptualization, formal analysis, funding acquisition, resources, supervision; Miaomiao Chi: data curation, writing – original draft, project administration; Jiaojiao Li: supervision, software; Lingzhe Suo: data curation, validation; Lin Zhu: software, validation; Kexin Chen: visualization; Jingqing Mu: investigation.
Conflict of interest: The authors state no conflict of interest.
Data availability: All data generated or analysed during this study are included in this published article.
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