One-step ionic liquid-based ultrasound-assisted dispersive liquid–liquid microextraction coupled with high-performance liquid chromatography for the determination of pyrethroids in traditional Chinese medicine oral liquid preparations

In this study, a simple one-step ionic liquid-based ultrasound-assisted dispersive liquid–liquid microextraction technique was coupled with high-performance liquid chromatography for the analysis of four pyrethroids in three kinds of traditional Chinese medicine oral liquid preparations: simotang oral liquid, kangbingdu oral liquid, and huaji oral liquid. The extraction parameters were examined to improve extraction efficiency. The optimum extraction conditions were 50 μL of 1-octyl-3-methylimidazolium hexafluorophosphate utilized as the extraction solvent and 800 μL of acetonitrile applied as the dispersive solvent. The extraction was assisted by ultrasonication for 8 min. The limits of detection for the four pyrethroids were within 0.007–0.024 mg L−1, and the limits of quantitation ranged between 0.023 and 0.080 mg L−1. The accuracy of the pyrethroid determination ranged from 80.1 to 106.4%. It was indicated that the proposed ionic liquid-based ultrasound-assisted dispersive liquid–liquid microextraction method had an easy operation and was accurate and environmentally friendly. This approach has potential for the analysis of pyrethroids in traditional Chinese medicine oral liquid preparations.


Introduction
Traditional Chinese medicine (TCM) is widely employed in the treatment of a variety of diseases, including cough, hyperlipidemia, hypertension and infectious diseases [1,2]. During the cultivation of Chinese herbal medicine, pesticides are commonly used to control pests and diseases. Currently, synthetic pyrethroid insecticides are more frequently used than traditional organophosphate [3], organonitrogen [4], organochlorine [5], and carbamate pesticides [6] because of their strong insecticidal activity and good stability upon exposure to light and air [7]. However, numerous studies have indicated that these pyrethroid pesticides are toxic to the nervous, reproductive, immune and cardiovascular systems [8]. Oral liquid is one of the most commonly used TCM preparations, and residues of pyrethroid pesticides in TCM oral liquid preparations greatly affect the patients' health and course of therapy. Because TCM preparation contains a great many of herbal components, the interference of complex matrix to the pyrethroid residues and the limitation of Open Access BMC Chemistry *Correspondence: songyt@hainanu.edu.cn 1 Key Laboratory of Tropical Biological Resources of Ministry of Education; Department of Pharmaceutical Sciences, School of Life and Pharmaceutical Sciences, Hainan University, Haikou 570228, China Full list of author information is available at the end of the article current analytical methods will make the residue analysis of pyrethroid pesticides very difficult. Therefore, an accurate measurement method for the pyrethroid pesticides in TCM oral liquid preparations is urgently required.
Varieties of methods have been exploited for the measurement of pyrethroid residues, and the potential analytical methods include gas chromatography with electron capture detection [9] or mass spectrometry (MS) [10] and high-performance liquid chromatography (HPLC) with ultraviolet (UV) detection [11], diode array detection [12], or MS [13]. MS significantly improved the analysis of pyrethroid residues owing to its high sensitivity; however, it has stricter instrumentation requirements and is not suitable for some typical analytical laboratories. Among these techniques, HPLC-UV has been frequently employed in the analysis of pyrethroid residues [14][15][16]. However, the analysis of pyrethroid residues in TCM oral liquid preparations is difficult because of the extremely low pyrethroid concentrations and the complexity of the TCM sample. Therefore, pretreatment of the sample before HPLC analysis is crucial for the whole analysis process. Several approaches have been employed for the extraction of pyrethroids from samples, and these methods include the Soxhlet extraction [17], ultrasonic extraction [18], liquid-liquid extraction [19], and solid-phase extraction [20]. However, the extraction approaches have certain limitations, including large organic solvent consumption and a time-consuming extraction procedure.
Recently, ionic liquids (ILs)-semi-organic molten salts with an organic or inorganic anion and an organic cation-have emerged as alternative extraction solvents for sample treatment because of their advantages of strong thermal stability, good miscibility with organic and aqueous solvents, low vapor pressure, and good solubility for both organic and inorganic compounds. ILs have been utilized for the analysis of several kinds of organic compounds, such as benzoylurea insecticides, neonicotinoid insecticides, and endocrine-disrupting compounds [21][22][23]. Compared with conventional extraction methods, less organic solvent was consumed during IL dispersive liquid-liquid microextraction, and a higher extraction efficiency was achieved within a shorter extraction time.
The current study was performed to exploit a one-step ionic liquid dispersive liquid-liquid microextraction (IL-DLLME) for the sensitive measurement of the pyrethroid insecticide in TCM oral liquid preparations. In the current research, ultrasound technology was utilized to cause the ILs to disperse into the aqueous phase as well as to enrich the efficiency. The extraction conditions were examined to improve extraction efficiency. The current approach was then employed in the trace measurement of four pyrethroid insecticides in TCM oral liquid preparations.

Chromatographic conditions
The determination of pyrethroids was implemented on an HPLC system (Waters Corporation, USA) with a 1525 HPLC pump and a 2489 UV/visible detector. A Diamonsil C18 column (5 μm, 4.6 mm id × 150 mm) from Dikma Technologies Inc. (Beijing, China) was used. Eluent A was water/acetonitrile (95/5, v/v), and eluent B was water/acetonitrile (5/95, v/v). The mobile phases were eluted according to the following program: 12% (A) from 0 to 9.0 min, followed by 12-0% (A) from 9.0 to 35.0 min. The flow rate was 0.6 mL min −1 , and the column temperature was 30 °C. The detection was monitored at 210 nm.

Optimization of IL-DLLME extraction method
The effects of extraction conditions, including the type of IL, IL volume, type of dispersive solvent, dispersive solvent amount, and ultrasonic extraction time, on the recoveries were optimized by single-factor experiments. The experiments were all carried out in triplicate.

IL-DLLME procedure
The TCM oral liquid preparations were centrifuged at 8000 rpm for 30 min, and the supernatant was filtered by a membrane filter (0.22 μm) before the IL-DLLME procedure. Then, 50 μL of [C 8 MIM][PF 6 ] and 800 μL of acetonitrile were measured by microsampler and pipette respectively, and added to 5 mL of the filtered sample solutions in a conical tube. The ultrasound-assisted extraction (output power of 100 W and frequency of 40 kHz) was carried out for 8 min. Subsequently, 8 mL of the sample was centrifuged at 6153×g for 5 min. The pyrethroids were extracted into a droplet of IL settled at the bottom of the tube. A syringe was used to remove the upper aqueous phase. The IL phase containing the analytes was diluted with 70 μL of acetonitrile. Ultimately, 10 μL of the resultant solution was delivered into the chromatographic system for analysis.

Calculations
The enrichment factor (EF), defined as the ratio of the final concentration in the sediment phase (C fin ) to the initial target component concentration in the TCM oral liquid preparation (C ini ), was calculated as: The extraction recovery (ER), which was utilized to estimate the pretreatment procedure under various experimental conditions, was calculated as follows: where V fin is the final target component concentration in the sediment phase and V ini is the initial target component concentration in the TCM oral liquid preparation [23].

Preparation of spiked samples
Spiked samples were prepared by spiking appropriate amount of the standard solutions in the TCM oral liquid preparations to yield final concentrations of 20, 50 and 100 μg L −1 for four pyrethroids, respectively. Then the samples were subsequently prepared according to the upper IL-DLLME procedure.

Type of IL
The proper extraction solvent is vital for the success of the IL-DLLME process. The proper extraction solvent should have several key characteristics, including good chromatographic behavior, higher density than water, and lower water solubility [22,24].

IL volume
When a smaller amount of IL was used, a small amount of precipitation formed, which indicated that the target compound could not be extracted efficiently and that repeatability was poor. In contrast, excess IL may decrease the enrichment factor and sensitivity of the analytical method.  6 ] were used (Fig. 2b). Considering the enrichment factor and sensitivity of the analytical method, a lower volume of IL was preferred, and 50 μL of [C 8 MIM][PF 6 ] was employed during the extraction.

Type of dispersive solvent
To reduce interfacial tension and increase surface area between the two phases, a proper dispersive solvent with excellent miscibility during the one-step ionic liquid-based ultrasound-assisted dispersive liquid-liquid microextraction (IL-UA-DLLME) process is necessary.
Methanol, acetonitrile and acetone were investigated (Fig. 2c). A higher extraction efficiency was achieved when the dispersive solvent was acetonitrile. Thus, acetonitrile was applied for the dispersive solvent.

Dispersive solvent amount
The effect of the acetonitrile amount was examined by varying the amount from 700 to 900 μL (Fig. 2d). The highest extraction recovery was achieved with 800 μL of acetonitrile. Therefore, 800 μL of acetonitrile was added during the extraction procedure.

Ultrasonic extraction time
The ultrasonic extraction time was investigated from 6 to 9 min (Fig. 2e). When the ultrasonic extraction time was changed from 6 to 8 min, the recovery increased. However, with further extension of the extraction time from 8 to 9 min, the extraction recovery did not greatly vary. As a consequence, 8 min was selected as the ultrasonic extraction time.

Analysis of real TCM oral liquid preparations
Pyrethroid-free TCM samples were employed as blanks for the analytical method validation. As shown in Table 1, the limits of detection (LODs, signal/noise = 3) for the four pyrethroids were within 0.007-0.024 mg L −1 , and the limits of quantitations (LOQs, signal/noise = 10) were within 0.023-0.080 mg L −1 . The linearity was investigated within 0.1-10 mg L −1 . The peak areas versus the injection amount was plotted, and linear regression equations were obtained. The correlation coefficients of the four pyrethroids were greater than 0.999. As presented in Table 1, the relative standard deviation (RSD) values were 0.8-2.9% for the intra-day precision and 0.7-3.9% for the inter-day precision. The pyrethroids in spiked samples were determined at three concentrations (20, 50 and 100 μg L −1 ). As shown in Table 2, the average recoveries were within 80.1-106.4%.
Three kinds of TCM oral liquid preparations (simotang, kangbingdu, and huaji) were obtained from a local community pharmacy and were analyzed with HPLC-UV after the IL-UA-DLLME procedure. A typical highperformance liquid chromatogram under the conditions described in "Chromatographic conditions" section is shown in Fig. 3. The pyrethroids were separated successfully in the spiked oral liquid (0, 20 and 100 μg L −1 ) and spiked blank (50 μg L −1 ), indicating that the components in the TCM oral liquid preparations did not interfere with the analysis of the pyrethroids. Thus, the IL-DMLLE pretreatment approach is applicable for the measurement of pyrethroids in TCM oral liquid preparations. Table 3 lists a comparison of the current analytical approach with the published approaches for the measurement of pyrethroids in liquid samples [25][26][27][28]. Compared with other techniques, the current method consumed fewer types and lower amounts of organic solvents. In addition, although the sample matrix of a TCM oral liquid is very complex, the proposed method can provide LODs and enrichment factors comparable to other methods. Because this method has no special instrumentation requirements, it is highly applicable for the routine analysis of pyrethroids in TCM oral liquid preparations.

Conclusions
In the current work, a sensitive analytical approach was investigated for the measurement of four pyrethroids in TCM oral liquid preparations by the utilization of IL-UA-DLLME coupled with HPLC. The extraction parameters were investigated to improve the extraction efficiency, and excellent enrichment performance was achieved.
The chromatographic conditions were also tested, and the chromatographic determination was achieved within 35 min. Compared with previous studies, although the sample matrix is more complex on account of the various of herbal component in TCM oral liquid, the proposed method achieved similar LODs with the utilization of less types and lower volume of toxic organic solvents during the microextraction procedure. The results reveal that the method is an accurate, simple, and environmentally  This work