Under the alkaline conditions, the electron cloud of the hydroxyl group moved to benzene ring, which made bond energy of O–H weak; H+ was easily ionised to show acidity; phenonium ion with strong hydrophilicity was created after ionisation; it was easily dissolved in water and was not propitious to extraction. But under the acidic conditions, ionisation of CPs was restrained, which made CPs exist in the form of neutral
molecule, and hydrophobicity was enhanced, which was beneficial Navitoclax manufacturer to extraction and separation (Dong et al., 2014). Effects of pH from 1 to 12 on the enrichment recoveries of CPs were investigated as shown in Fig. 3A. Relatively higher recoveries of CPs can be achieved at pH 3. Therefore, samples solutions were then adjusted to pH 3 before enrichment by adding H3PO4 into samples. Temperature has less impact on the in situ IL-DLLME procedure as seen in Fig. 3B. Relatively higher temperature can benefit the dispersion of IL and enhance the mass transfer of the analytes. However, considering the different volatilities of CPs, 50 °C was used for experiments. The direct analysis of CPs enriched in IL microdroplet by GC is impossible (an interface would be needed to remove the IL). Thus HPLC-DAD was employed in this work and 215 nm was used as the detection wavelength in order to improve the LODs of CPs. To remove the interferences coming from IL, a back-extraction procedure was inserted between the IL-DLLME and HPLC determination.
Based on the reference (Santana, Padrón, Ferrera, & Rodríguez, 2007), six kinds of surfactants including DNS-328, DNS-330, potassium laureth INK 128 supplier phosphate (EO 4 mol), POLE, AES-7 and SDS, and two alkaline Na2CO3 and
NaOH aqueous solution were investigated as back-extractants with each at the same molarity of 0.1 M according Baf-A1 nmr to the reference (Feng, Tan, & Liu, 2011). As seen in Fig. 4, the results revealed that NaOH was the best acceptor of CPs, which is reasonable as the six studied CPs are weak acids with pKa values in the range of 6.0–9.4. The back-extraction was then further optimised to use 40 μL 0.14 M aqueous NaOH extract twice with each time 5 min under vortex oscillation, and the supernatant was collected and subjected to HPLC analysis. Some characters of the proposed method such as linear range, correlation coefficients, limits of detection (LODs) and repeatability were all investigated by enriching 5 mL of CPs standard working solutions and the results were shown in Table 1. Each analyte exhibited good linearity with correlation coefficient r2 > 0.99 in the studied range. The limits of detection, calculated on the basis of signal-to-noise ratio of 3 (S/N = 3), were in the range of 0.8–3.2 μg/L. The detection limits of this proposed method are comparable with that of a relevant method reported in literature ( Guo, Liu, Shi, Wei, & Jiang, 2014), which were 0.5–2.0 μg/L. The recoveries of six CPs were determined by spiking the diluted honey samples with different level of standard CPs.