Identification and Quantification of Four Anthraquinones in Rhubarb and its Preparations by Gas Chromatography–Mass Spectrometry

Identification and Quantification of Four Anthraquinones in Rhubarb and its Preparations by Gas... Abstract A gas chromatographic–mass spectrometric (GC–MS) method was developed for the determination of four anthraquinones found in rhubarb. Chrysophanol, physcion, aloe-emodin and emodin were confirmed by GC–MS and the possible main cleavage pathways of fragment ions are discussed in this study. Rhubarb is a traditional Chinese medicinal herb which required an effective evaluation method to quantitate the four major active anthraquinone compounds described. The determinations of analytes were accomplished by GC–MS using osthole as an internal standard. MS detection was performed in selected ion monitoring mode to increase the sensitivity. The method was evaluated by a number of validation characteristics (precision, limit of detection, calibration range and recovery). The calibration ranges were all 3.2–30.0 μg/mL. This method was fully validated and showed good performances in terms of recovery (96.9–102.9%) and precision (1.4–2.9%). Finally, the method was applied to the analysis of four anthraquinones in rhubarb and its preparations in the first time. Introduction Rhubarb is one of the most popular traditional Chinese medicinal herbs and has been widely used for a long time. It has been frequently used for the treatment of obstipation, gastrointestinal indigestion, diarrhea and jaundice in Chinese clinics (1, 2). This crude drug is not only officially listed in the Chinese Pharmacopoeia but also appears in the British Pharmacopoeia and European Pharmacopoeia (3). Rhubarb is known to contain a large number of anthraquinones including chrysophanol, physcion, aloe-emodin and emodin. Their structures are shown in Figure 1. These are the basis for the quality control of rhubarb and other plant-derived drugs from Rheum, Cassia and Polygonum genera (4, 5). Figure 1. View largeDownload slide Chemical structures of four anthraquinones and osthole (IS). Figure 1. View largeDownload slide Chemical structures of four anthraquinones and osthole (IS). In order to evaluate or control the quality of rhubarb and its preparations, it is necessary to develop analytical methods to effectively separate and detect these anthraquinones. A number of studies have been performed on the isolation and identification of the constituents of these anthraquinones. Analytical methods reported mainly include thin layer chromatography (TLC) (6), high-speed counter current chromatography (HSCCC) (7, 8), micellar electro kinetic chromatography (MEKC) (9), capillary zone electrophoresis (CZE) (10), high-performance liquid chromatography (HPLC) (11–13) and ultra-performance liquid chromatography (UPLC) (14). Among them, methods based on liquid-phase separation techniques were often used for analysis of these anthraquinones. LC–MS technique has demonstrated its value in analyzing complex mixtures (15, 16). Besides, gas chromatographic-flame ionization detector and gas chromatographic–mass spectrometric (GC–MS) have also gained general acceptance for analysis of Chinese herbs, owing to high sensitivity combined with the possibility of achieving efficient separations of complex mixtures (17, 18). And GC–MS-selected ion monitoring (SIM) is a significant analytical tool for MS detection performed in SIM mode which can increase the sensitivity. Electron ionization–mass spectrometry (EI–MS) has been used to analyze different kinds of compounds, but it is more suitable for single analyte. The application of EI–MS in the structure elucidation, fragmentation behaviors and pathways of compound is well established in our group (19, 20). By EI–MS, it can give much more confidence for compound identification and much lower noise level for quantification. The fragmentation map of a target analytes is useful in performing EI–MS for either qualitative or quantitative analysis. Therefore, it is significant to investigate the fragmentation pathways of the compounds of interest. The choice of the internal standard (IS) is of crucial importance since it affects precision and accuracy of the method. Physicochemical properties may vary to some degree and cause high variability during sample pretreatment, therefore, differences in detector response will appear. Ideally, isotope-labeled analogs should provide the best results, but structurally closely related compounds may also be of similar usability (21). In this work, we have successfully chosen a suitable IS (osthole, structure shown in Figure 1) for the measurement of analytes by GC–MS-SIM. To date, however, there is no comprehensive GC–MS qualitative and quantitative analysis of these anthraquinones. In this work, a new method of structure elucidation based on MS was developed. The possible main fragment ions cleavage pathway was supplied. Meanwhile, we developed a simple and sensitive GC–MS method for simultaneous determination of chrysophanol, physcion, aloe-emodin and emodin in rhubarb using SIM in the first time. Experimental Chemicals and materials Chrysophanol, physcion, aloe-emodin, emodin and osthole were obtained from National Institute for the Control of Pharmaceutical and Biological Products of China. Ethyl acetate was obtained from Shanghai Analytical Reagent Company (Shanghai, China). Helium (purity, 99.999%) was from Hubei Heyuan Gases Co. Ltd (Wuhan, China). Other reagents used in the experiment were of analytical grade and from commercial sources. Dried rhubarb (Rheum palmatum L.) was identified by Prof. Li of Hubei University of Medicine and collected from the local herb stores in Hubei, China. These were ground to powder (about 60 meshes). A voucher specimen was deposited in the herbarium of the department of pharmacy, Hubei University of Medicine (Shiyan, China). Ruyi Jinhuang powder (Beijing Tongrentang Co., Ltd, China). Standard solutions and sample preparation The stock solutions were prepared by dissolving 10 mg of chrysophanol, physcion, aloe-emodin and emodin in 25 mL acetic ether, respectively. The aloe-emodin, emodin, chrysophanol, and physcion stock solutions were diluted with ethyl acetate to obtain calibration solutions. The solutions were kept at 4°C before use. Osthole was prepared by diluting the stock solutions to a concentration of 1 μg/mL. The powders of dried rhubarb (300.00 mg), dried at 60°C, and were accurately weighed. The sample was introduced into 10 mL volumetric flask, adding appropriate osthole (IS), diluting with ethyl acetate to volume. The weight of vial was recorded and the vial solution was kept until at room temperature and was filtered with quantitative filter paper. Next, 1 mL of the solution was sealed and sonicated at room temperature for 15 min. The original solution weight was restored. Then, the solution was diluted to 10 mL solution by ethyl acetate. Lastly, the solution was filtered through a 0.45 μm membrane before injected into the GC–MS system. The powders of Ruyi Jinhuang powder (500.00 mg), dried at 60°C, and were accurately weighed. Simultaneously, the sample was introduced into 10 mL volumetric flask, adding appropriate osthole (IS), diluting with ethyl acetate to volume. The weight of vial was recorded and the vial was sealed and sonicated at room temperature for 15 min. Then, the solution was filtered through a 0.45 μm membrane before injected into the GC–MS system. GC–MS conditions and instrumentation A GC–MS (GC-MS-Trace1300-ISQ Thermo Scientific, USA) with a TG-5MS capillary column (30 m × 0.25 mm I.D, 0.25 μm film thicknesses, Thermo Scientific, USA) was used. The inlet temperature was maintained at 280°C. The oven temperature was initially at 100°C, programmed to 250°C at 20°C/min and then programmed to 300°C at 10°C/min holding for 4 min. Helium was used as carrier gas at a constant flow rate of 1.0 mL/min. Injection volume is 1 μL. The samples were analyzed by GC–MS with the pulsed splitless injection mode. The ion source was set to 280°C and the MS transfer line was set to 300°C. Ionization was carried out in electron impact ionization (EI) mode at 70 eV. The mass spectra were recorded within 50–300 amu in full scan mode to collect the total ion current (TIC). Single ion monitoring (SIM) chromatograms were reconstructed at the base peak of the studied analyses shown in Table I. Table I. Instrumental Setting for Detection of Selected Metabolites Using GC–MS: Productions and Quantitation Ion Obtained on Distinct Time Segments (Boldface italic values represent quantitation ions) Analyte  GC–MS segments (min)  Product ions and relative abundances, (%)  Quantitation ion (m/z)  Osthole  8.00–9.00  244(100), 189(86.2), 229(79.4), 201(76.5), 131(72.4), 159(58.4), 213(56.2), 187(55.4), 115(44.4), 186(44.3)  244  Chrysophanol  9.90–10.40  254(100), 226(24.8), 152(20.4), 115(17.6), 197(16.5), 198(16.4), 76(15.9), 255(14.3), 63(13.6), 141(12.4)  254  Physcion  11.730–12.10  284(100), 128(37.6), 255(24.5), 139(17.6), 254(16.2), 285(15.8), 241(15.5), 77(15.0), 198(15.0), 226(14.3)  284  Aloe-emodin  12.11–12.40  241(100), 270(77.7), 121(42), 139(37.9), 242(21.4) 77(21), 63(20.2), 128(19.7), 127(18), 168(17.6)  270, 241  Emodin  12.50–12.80  270(100), 69(24.3), 139(23.2), 242(20.8), 213(18.7), 271(16.28), 115(15.3), 77(14.8), 168(14), 128(13.9)  270  Analyte  GC–MS segments (min)  Product ions and relative abundances, (%)  Quantitation ion (m/z)  Osthole  8.00–9.00  244(100), 189(86.2), 229(79.4), 201(76.5), 131(72.4), 159(58.4), 213(56.2), 187(55.4), 115(44.4), 186(44.3)  244  Chrysophanol  9.90–10.40  254(100), 226(24.8), 152(20.4), 115(17.6), 197(16.5), 198(16.4), 76(15.9), 255(14.3), 63(13.6), 141(12.4)  254  Physcion  11.730–12.10  284(100), 128(37.6), 255(24.5), 139(17.6), 254(16.2), 285(15.8), 241(15.5), 77(15.0), 198(15.0), 226(14.3)  284  Aloe-emodin  12.11–12.40  241(100), 270(77.7), 121(42), 139(37.9), 242(21.4) 77(21), 63(20.2), 128(19.7), 127(18), 168(17.6)  270, 241  Emodin  12.50–12.80  270(100), 69(24.3), 139(23.2), 242(20.8), 213(18.7), 271(16.28), 115(15.3), 77(14.8), 168(14), 128(13.9)  270  Results Characterization of the reference compounds by GC–MS Chrysophanol, physcion, aloe-emodin and emodin are the major anthraquinones in rhubarb that could be analyzed by GC–MS. In present work, chrysophanol, physcion, aloe-emodin, emodin and osthole reference compounds were dissolved in acetic ether and analyzed by GC–MS within 50–300 amu in full scan mode. A typical gas chromatography-total ion current (GC-TIC) mass spectrogram obtained for IS, chrysophanol, physcion, aloe-emodin and emodin standards is shown in Figure 2A by GC–MS in the electron impact (EI) mode. It is obvious that the GC-TIC method improves the chromatogram efficiently and provides a single peak for identification. The retention times of osthole, chrysophanol, physcion, aloe-emodin and emodin were 8.81 min, 10.07 min, 11.89 min, 12.23 min and 12.59 min, respectively. The molecule ion of [M]+ and major fragments ions of chrysophanol, physcion, aloe-emodin and emodin were observed in the full scan MS spectra in Figure 3A–D. Figure 2. View largeDownload slide Total ion current chromatogram (A) and SIM chromatograms of mixed standards (B), rhubarb (C) and its preparation (D). Peak identification:1 = osthole; 2 = chrysophanol; 3 = physcion; 4 = aloe-emodin and 5 = emodin. Figure 2. View largeDownload slide Total ion current chromatogram (A) and SIM chromatograms of mixed standards (B), rhubarb (C) and its preparation (D). Peak identification:1 = osthole; 2 = chrysophanol; 3 = physcion; 4 = aloe-emodin and 5 = emodin. Figure 3. View largeDownload slide Mass spectra and proposed fragmentation pathways of main fragment ions for chrysophanol (A,a), physcion (B,b), aloe-emodin (C,c) and emodin (D,d) by GC–MS. Figure 3. View largeDownload slide Mass spectra and proposed fragmentation pathways of main fragment ions for chrysophanol (A,a), physcion (B,b), aloe-emodin (C,c) and emodin (D,d) by GC–MS. The MS spectra from [M]+ of chrysophanol contain the molecular ion, which is m/z 254. The abundance of molecule ion [M]+ is base peak fragment ion. The other major fragment ions were m/z 226, m/z225, m/z 198, m/z 197, m/z 180, m/z 169 and m/z 152. The fragment ion [M]+ m/z 254 lost carbon monoxide and hydrogen (CHO) form the fragment ion at m/z 225, and lost carbon monoxide (CO) form the fragment ion at m/z 226. The fragment ion at m/z 226 would be further losing CO or CHO to produce the fragment ion m/z 197 or m/z 198.The fragment ions at m/z 169 and m/z 152 were formed by the loss of CO and OH step-by-step from the fragment ion at m/z 197. Moreover, the fragment ion m/z 198 could loss H2O to be the fragment ion at m/z 180. The possible main cleavage pathways and some possible chemical structures of fragment ions have been supplied in the Figures 3a and 4A. Figure 4. View largeDownload slide The possible chemical structure and the possible main cleavage pathway of fragment ions of chrysophanol (A), physcion (B), aloe-emodin (C) and emodin (D). Figure 4. View largeDownload slide The possible chemical structure and the possible main cleavage pathway of fragment ions of chrysophanol (A), physcion (B), aloe-emodin (C) and emodin (D). Figure 3b shows the possible fragmentation pathways of main fragment ions for physcion in EI–MS. The MS spectra from [M]+ m/z 284 separately lost CH2O, CHO, CO and C2H3O, corresponding to be yielded product ions at m/z 254, m/z 255, m/z 256 and m/z 241. The other main fragmentation ions at m/z 227, m/z 226, m/z 213.198 and m/z 185 were formed by the loss of one or two CO2 from the first cleavage fragment ions respectively. The possible structure of fragment ions was shown in Figure 4B. The major fragment ions of aloe-emodin were m/z 270, m/z 241, m/z 224, m/z 213, m/z 196, m/z 185, m/z 168 and m/z 157 which were shown in Figure 3C. The base peak fragment ion was the fragment ion at m/z 241, which was formed by the loss of CHO from the molecular ion [M]+ m/z 270. Meanwhile, the molecular ion [M]+ m/z 270 could loss CH2O2 and CO to yield fragment ions at m/z 224 and m/z 242, respectively. Moreover, the fragment ion at m/z 224 could lose CO group step-by-step to produce the fragment ions at m/z 196 and m/z 168. On the one hand, the base peak fragment ion [M-CHO]+ m/z 241 lost CO group gradually to produce the fragment ions at m/z 213, m/z 185 and m/z 157, respectively. The possible main cleavage pathway of fragment ions of aloe-emodin had been shown in Figure 3c and the possible structure of the fragment ions has been shown in Figure 4C. Figure 3D shows the main fragment ions of emodin at m/z 270, m/z 242, m/z 214, m/z 213, m/z 196, m/z 185 and m/z 168. The molecular ion [M]+ m/z 270 losing CHO and CO form the fragment ions at m/z 241 and m/z 242. The two main fragmentation pathways of the characteristic fragment ion at m/z 242 could be as follow. First, the fragment ion at m/z 242 lost CHO and CO formed product ions at m/z 213 and m/z 185. Second, the characteristic fragment ion at m/z 242 successively lost CO, H2O and CO in turn formed the fragment ions at m/z 214, m/z 196 and m/z 168. The fragment ion at m/z 214 could also produce the ion at m/z 185 by losing CHO. The possible main cleavage pathway of fragment ions of emodin had been shown in the Figure 3d, corresponding with the possible structures as shown below the Figure 4D. The relative abundances of major fragment ions were shown in Table I. The base peak fragment ions of four compounds were [M]+ m/z 254, [M]+ m/z 284, [M-CHO]+ m/z 241 and [M]+ m/z 270. The main cleavage pathway of fragment ions of four anthraquinones was losing the CO group, CHO groups and the functional groups of these anthraquinones. Various structures such as R–OH, R–C = O and R-O-R related to the behavior of mass spectrometric notably. Quantification of four anthraquinones by GC–MS To reach higher sensitivity and selectivity, routine separations of chrysophanol, physcion, aloe-emodin, emodin and IS were performed by GC–MS in the SIM mode. Selection of fragments (m/z) for detection from the mass spectra of compounds was performed following abundance and specificity criteria. The product ions and quantification ions are listed in Table I. Instrumental parameters were studied to give the best sensitivity and mass spectra quality and are presented in Table I along with respective product ions used for quantification. The base peak fragment ions were m/z 254 for chrysophanol, m/z 284 for physcion, m/z 241 for aloe-emodin and m/z 270 for emodin. The relative abundances about the molecular ion of the aloe-emodin were particularly high, so we chose the fragment ions m/z 270 and m/z 241 as the monitored ions of it. The determination of analytes was performed in SIM mode shown in Table I. Selectivity of GC–MS method was evaluated by comparison of the migration time and mass spectrum of standard reference osthole (IS) and four anthraquinones with those of the peak obtained in the analysis of real extracts from the rhubarb. In Figure 2, it is shown a typical gas chromatography-total ion current (GC-TIC) mass spectrogram by GC–MS in the electron impact (EI) mode; GC-SIM chromatograms obtained for analyte and IS in standard, samples and preparation. It is obvious that the GC-SIM method improves the chromatograms very efficiently and provides a single peak for identification. Method validation The calibration curves were established by injecting six concentration levels of chrysophanol, physcion, aloe-emodin and emodin. The concentration range is 3.2–30.0 μg/mL for all. The characteristics of the calibration plots are summarized in Table II. As can be seen, the proposed method exhibited excellent linearity and sensitivity. Table II. Calibration Curves and LOD of Aloe-emodin, Emodin, Chrysophanol and Physcion by GC–MS Analyte  Calibration curve  R2  Line arrange (μg/mL)  LOD (ng/mL)  Chrysophanol  Y = 3.36244 + 93.6657X  0.9999  3.2–30  0.1  Physcion  Y = −22.0467 + 55.2551X  0.9999  3.2–30  0.5  Aloe-emodin  Y = −72.4639 + 61.5269X  0.9999  3.2–30  10  Emodin  Y = −64.9676 + 24.3869X  0.9998  3.2–30  20  Analyte  Calibration curve  R2  Line arrange (μg/mL)  LOD (ng/mL)  Chrysophanol  Y = 3.36244 + 93.6657X  0.9999  3.2–30  0.1  Physcion  Y = −22.0467 + 55.2551X  0.9999  3.2–30  0.5  Aloe-emodin  Y = −72.4639 + 61.5269X  0.9999  3.2–30  10  Emodin  Y = −64.9676 + 24.3869X  0.9998  3.2–30  20  The limits of detection (LODs) were determined by the signal-to-noise (S/N) ratio = 3. The LODs were determined to be 0.1 ng/mL for chrysophanol, 0.5 ng/mL for physcion, 10.0 ng/mL for aloe-emodin and 20.0 ng/mL for emodin, respectively. The precision of injection was evaluated by repeat injection of the standard solutions for six times. Intra and interday variabilities were determined by the analysis of average amount of standards in quality control samples prepared by standard solutions at low, medium and high concentrations on three different days. The quality control samples were prepared as a single batch on the same day at each concentration, and then divided into aliquots and stored at 4°C until required for analysis. The calculated RSDs from repeated measurements are summarized in Table III. As shown in Table III, intraday precisions were from 0.8% to 1.6% for chrysophanol, 0.5% to 1.4% for physcion, 0.8% to 1.0% for aloe-emodin and 0.5% to 2.0% for emodin by GC–MS, respectively. And interday precisions were from 1.9% to 2.9% for chrysophanol, 1.7% to 2.4% for physcion, 1.4% to 2.8% for aloe-emodin and 1.7% to 2.5% for emodin by GC–MS, respectively. Table III. Intra- and Interday Precision, Accuracy and Recovery of Aloe-emodin, Emodin, Chrysophanol and Physcion (n = 6) Analyte  Actual concentration (μg/mL)  Intraday  Interday  Recovery  RSD (%)  Recovery  RSD (%)  Recovery  Mean (%)  RSD (%)  Chrysophanol  3.2  0.8  98.1  1.9  97.4  98.8  2.5  12.0  1.6  100.2  2.9  102.9  20.0  1.2  100.6  2.6  103.9  Physcion  3.2  0.5  102.2  1.8  100.9  97.9  2.4  12.0  1.4  103.7  2.4  102.5  20.0  1.4  102.9  1.7  103.6  Aloe-emodin  3.2  0.8  98.8  1.4  97.2  102.9  2.4  12.0  0.8  97.8  1.8  97.3  20.0  1.0  101.7  2.8  103.9  Emodin  3.2  0.5  102.0  1.7  103.3  96.9  2.8  12.0  2.0  100.5  2.0  101.1  20.0  1.5  103.8  2.5  104.8  Analyte  Actual concentration (μg/mL)  Intraday  Interday  Recovery  RSD (%)  Recovery  RSD (%)  Recovery  Mean (%)  RSD (%)  Chrysophanol  3.2  0.8  98.1  1.9  97.4  98.8  2.5  12.0  1.6  100.2  2.9  102.9  20.0  1.2  100.6  2.6  103.9  Physcion  3.2  0.5  102.2  1.8  100.9  97.9  2.4  12.0  1.4  103.7  2.4  102.5  20.0  1.4  102.9  1.7  103.6  Aloe-emodin  3.2  0.8  98.8  1.4  97.2  102.9  2.4  12.0  0.8  97.8  1.8  97.3  20.0  1.0  101.7  2.8  103.9  Emodin  3.2  0.5  102.0  1.7  103.3  96.9  2.8  12.0  2.0  100.5  2.0  101.1  20.0  1.5  103.8  2.5  104.8  The recovery experiment was performed to verify the reliability of the proposed method. Four anthraquinones standards were added to the rhubarb containing known quantities of four anthraquinones and the mixture was treated as described above. The recoveries were calculated by spiking six samples of rhubarb solution, which was added with a certain amount of the four standards respectively. The recovery was calculated by comparing the found amount of standards with those added. Three injections of each preparation were made and the theoretical amount of analyte in the sample preparations and the average percentage analyte recovered in the spiked solutions were calculated. The mean recoveries for chrysophanol, physcion, aloe-emodin and emodin were 96.9–102.9% with RSDs <2.8%. Results are shown in Table III, which indicate that the method using osthole as IS obtains better accuracy and is suitable for the sample analysis. The reproducibility of the method was proved by analyzing samples of four analytes in rhubarb. RSDs (n = 6) were 2.3–6.1%. Applications Analyses of real samples (300.00 mg dried rhubarb and 500.00 mg Ruyi Jinhuang powder) were performed by GC–MS with the condition of sample preparation. The quantification of four anthraquinones was performed by GC–MS and the contents of four anthraquinones in rhubarb and its preparation are shown in Table IV. Rhubarb had higher contents of analytes than those in Ruyi Jinhuang powder. But the contents sequence of analytes in the rhubarb was the same in the Ruyi Jinhuang powder. The concentration sequences were as follows: chrysophanol > emodin > physcion > aloe-emodin. Table IV. The Contents of Aloe-emodin, Emodin, Chrysophanol and Physcion in Rhubarb and Its Preparation by GC–MS Sample  Chrysophanol (μg/mg)  Physcion (μg/mg)  Aloe-emodin (μg/mg)  Emodin (μg/mg)  Rhubarb  8.2756  1.4468  1.3048  2.5668  Ruyi Jinhuang powder  0.4260  0.0700  0.0643  0.1270  Sample  Chrysophanol (μg/mg)  Physcion (μg/mg)  Aloe-emodin (μg/mg)  Emodin (μg/mg)  Rhubarb  8.2756  1.4468  1.3048  2.5668  Ruyi Jinhuang powder  0.4260  0.0700  0.0643  0.1270  Discussion In this work, in order to compare the precision and accuracy, GC–MS was developed for the simultaneous quantification of four anthraquinones with IS. The choice of the IS is of crucial importance since it affects precision and accuracy of the method. For a proper IS, it should be structurally or chemically similar to the analyte, have similar retention behavior to the analyte, and be well resolved from the analyte and other peaks. Thus, osthole was chosen as the IS for the analysis because of its similarity of structure, retention and polarity to the chrysophanol, physcion, aloe-emodin and emodin. In order to increase the sensitivity of the developed method, quantitative determination was performed by SIM of base peak ion at 254, 270, 284 and 241 for analytes, and molecule ion at m/z 244 for IS, respectively. LODs of the four analytes by GC–MS in this study provides a better sensitivity than HPLC (11–13), UPLC (14), HPLC-DAD (15) and HPLC–MS (15). Moreover, LODs of chrysophanol and physcion showed in Table II were more sensitivity than HPLC-FLD (13). Therefore, GC–MS provides a high selectivity and sensitivity for determination of four anthraquinones in rhubarb and its preparations. Conclusions A novel GC–MS method was developed to simultaneously identify and quantify four anthraquinones in rhubarb and its preparations using osthole as IS. The method provides acceptable figures of merit. It has been demonstrated by EI–MS spectra. The possible main fragment ions cleavage pathways were discussed. According to the ion peak intensity, the fragment ions at m/z 254, m/z 270, m/z 284 and m/z 241 were selected as monitoring ions for analytes by SIM. Subsequently, we develop a simple and sensitive GC–MS method for simultaneous determination of four anthraquinones. The LODs were up to 0.1 ng/mL, 0.5 ng/mL, 10 ng/mL and 20 ng/mL for four anthraquinones, respectively. The method exhibits good linearity, reproducibility, precision, accuracy and recovery and could be used for quantitative analysis and pharmacokinetics of four anthraquinones in the rhubarb and its preparations. Funding This work was supported by the Natural Science Foundation of Hubei Provincial Department of Education (No. D20172101), the Foundation for Innovative Research Team of Hubei University of Medicine (2014CXG04), the Science and Technology Key Program of Shiyan (Nos. 16Y70, 17K74, 17Y46, 17Y53 and 17Y54), the School Foundation for Hubei University of Medicine (No. FDFR201614), the Foundation of Health and Family Planning (No. WJ2017F082) and the Key Discipline Project of Hubei University of Medicine. References 1 Gao, J.W., Shi, Z., Zhu, S.Z., Li, G.Q., Yan, R., Yao, M.; Influences of processed rhubarbs on the activities of four cyp isozymes and the metabolism of saxagliptin in rats based on probe cocktail and pharmacokinetics approaches; Journal of Ethnopharmacology , ( 2013); 145: 566– 572. 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Identification and Quantification of Four Anthraquinones in Rhubarb and its Preparations by Gas Chromatography–Mass Spectrometry

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Oxford University Press
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© The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com
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0021-9665
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1945-239X
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10.1093/chromsci/bmx103
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Abstract

Abstract A gas chromatographic–mass spectrometric (GC–MS) method was developed for the determination of four anthraquinones found in rhubarb. Chrysophanol, physcion, aloe-emodin and emodin were confirmed by GC–MS and the possible main cleavage pathways of fragment ions are discussed in this study. Rhubarb is a traditional Chinese medicinal herb which required an effective evaluation method to quantitate the four major active anthraquinone compounds described. The determinations of analytes were accomplished by GC–MS using osthole as an internal standard. MS detection was performed in selected ion monitoring mode to increase the sensitivity. The method was evaluated by a number of validation characteristics (precision, limit of detection, calibration range and recovery). The calibration ranges were all 3.2–30.0 μg/mL. This method was fully validated and showed good performances in terms of recovery (96.9–102.9%) and precision (1.4–2.9%). Finally, the method was applied to the analysis of four anthraquinones in rhubarb and its preparations in the first time. Introduction Rhubarb is one of the most popular traditional Chinese medicinal herbs and has been widely used for a long time. It has been frequently used for the treatment of obstipation, gastrointestinal indigestion, diarrhea and jaundice in Chinese clinics (1, 2). This crude drug is not only officially listed in the Chinese Pharmacopoeia but also appears in the British Pharmacopoeia and European Pharmacopoeia (3). Rhubarb is known to contain a large number of anthraquinones including chrysophanol, physcion, aloe-emodin and emodin. Their structures are shown in Figure 1. These are the basis for the quality control of rhubarb and other plant-derived drugs from Rheum, Cassia and Polygonum genera (4, 5). Figure 1. View largeDownload slide Chemical structures of four anthraquinones and osthole (IS). Figure 1. View largeDownload slide Chemical structures of four anthraquinones and osthole (IS). In order to evaluate or control the quality of rhubarb and its preparations, it is necessary to develop analytical methods to effectively separate and detect these anthraquinones. A number of studies have been performed on the isolation and identification of the constituents of these anthraquinones. Analytical methods reported mainly include thin layer chromatography (TLC) (6), high-speed counter current chromatography (HSCCC) (7, 8), micellar electro kinetic chromatography (MEKC) (9), capillary zone electrophoresis (CZE) (10), high-performance liquid chromatography (HPLC) (11–13) and ultra-performance liquid chromatography (UPLC) (14). Among them, methods based on liquid-phase separation techniques were often used for analysis of these anthraquinones. LC–MS technique has demonstrated its value in analyzing complex mixtures (15, 16). Besides, gas chromatographic-flame ionization detector and gas chromatographic–mass spectrometric (GC–MS) have also gained general acceptance for analysis of Chinese herbs, owing to high sensitivity combined with the possibility of achieving efficient separations of complex mixtures (17, 18). And GC–MS-selected ion monitoring (SIM) is a significant analytical tool for MS detection performed in SIM mode which can increase the sensitivity. Electron ionization–mass spectrometry (EI–MS) has been used to analyze different kinds of compounds, but it is more suitable for single analyte. The application of EI–MS in the structure elucidation, fragmentation behaviors and pathways of compound is well established in our group (19, 20). By EI–MS, it can give much more confidence for compound identification and much lower noise level for quantification. The fragmentation map of a target analytes is useful in performing EI–MS for either qualitative or quantitative analysis. Therefore, it is significant to investigate the fragmentation pathways of the compounds of interest. The choice of the internal standard (IS) is of crucial importance since it affects precision and accuracy of the method. Physicochemical properties may vary to some degree and cause high variability during sample pretreatment, therefore, differences in detector response will appear. Ideally, isotope-labeled analogs should provide the best results, but structurally closely related compounds may also be of similar usability (21). In this work, we have successfully chosen a suitable IS (osthole, structure shown in Figure 1) for the measurement of analytes by GC–MS-SIM. To date, however, there is no comprehensive GC–MS qualitative and quantitative analysis of these anthraquinones. In this work, a new method of structure elucidation based on MS was developed. The possible main fragment ions cleavage pathway was supplied. Meanwhile, we developed a simple and sensitive GC–MS method for simultaneous determination of chrysophanol, physcion, aloe-emodin and emodin in rhubarb using SIM in the first time. Experimental Chemicals and materials Chrysophanol, physcion, aloe-emodin, emodin and osthole were obtained from National Institute for the Control of Pharmaceutical and Biological Products of China. Ethyl acetate was obtained from Shanghai Analytical Reagent Company (Shanghai, China). Helium (purity, 99.999%) was from Hubei Heyuan Gases Co. Ltd (Wuhan, China). Other reagents used in the experiment were of analytical grade and from commercial sources. Dried rhubarb (Rheum palmatum L.) was identified by Prof. Li of Hubei University of Medicine and collected from the local herb stores in Hubei, China. These were ground to powder (about 60 meshes). A voucher specimen was deposited in the herbarium of the department of pharmacy, Hubei University of Medicine (Shiyan, China). Ruyi Jinhuang powder (Beijing Tongrentang Co., Ltd, China). Standard solutions and sample preparation The stock solutions were prepared by dissolving 10 mg of chrysophanol, physcion, aloe-emodin and emodin in 25 mL acetic ether, respectively. The aloe-emodin, emodin, chrysophanol, and physcion stock solutions were diluted with ethyl acetate to obtain calibration solutions. The solutions were kept at 4°C before use. Osthole was prepared by diluting the stock solutions to a concentration of 1 μg/mL. The powders of dried rhubarb (300.00 mg), dried at 60°C, and were accurately weighed. The sample was introduced into 10 mL volumetric flask, adding appropriate osthole (IS), diluting with ethyl acetate to volume. The weight of vial was recorded and the vial solution was kept until at room temperature and was filtered with quantitative filter paper. Next, 1 mL of the solution was sealed and sonicated at room temperature for 15 min. The original solution weight was restored. Then, the solution was diluted to 10 mL solution by ethyl acetate. Lastly, the solution was filtered through a 0.45 μm membrane before injected into the GC–MS system. The powders of Ruyi Jinhuang powder (500.00 mg), dried at 60°C, and were accurately weighed. Simultaneously, the sample was introduced into 10 mL volumetric flask, adding appropriate osthole (IS), diluting with ethyl acetate to volume. The weight of vial was recorded and the vial was sealed and sonicated at room temperature for 15 min. Then, the solution was filtered through a 0.45 μm membrane before injected into the GC–MS system. GC–MS conditions and instrumentation A GC–MS (GC-MS-Trace1300-ISQ Thermo Scientific, USA) with a TG-5MS capillary column (30 m × 0.25 mm I.D, 0.25 μm film thicknesses, Thermo Scientific, USA) was used. The inlet temperature was maintained at 280°C. The oven temperature was initially at 100°C, programmed to 250°C at 20°C/min and then programmed to 300°C at 10°C/min holding for 4 min. Helium was used as carrier gas at a constant flow rate of 1.0 mL/min. Injection volume is 1 μL. The samples were analyzed by GC–MS with the pulsed splitless injection mode. The ion source was set to 280°C and the MS transfer line was set to 300°C. Ionization was carried out in electron impact ionization (EI) mode at 70 eV. The mass spectra were recorded within 50–300 amu in full scan mode to collect the total ion current (TIC). Single ion monitoring (SIM) chromatograms were reconstructed at the base peak of the studied analyses shown in Table I. Table I. Instrumental Setting for Detection of Selected Metabolites Using GC–MS: Productions and Quantitation Ion Obtained on Distinct Time Segments (Boldface italic values represent quantitation ions) Analyte  GC–MS segments (min)  Product ions and relative abundances, (%)  Quantitation ion (m/z)  Osthole  8.00–9.00  244(100), 189(86.2), 229(79.4), 201(76.5), 131(72.4), 159(58.4), 213(56.2), 187(55.4), 115(44.4), 186(44.3)  244  Chrysophanol  9.90–10.40  254(100), 226(24.8), 152(20.4), 115(17.6), 197(16.5), 198(16.4), 76(15.9), 255(14.3), 63(13.6), 141(12.4)  254  Physcion  11.730–12.10  284(100), 128(37.6), 255(24.5), 139(17.6), 254(16.2), 285(15.8), 241(15.5), 77(15.0), 198(15.0), 226(14.3)  284  Aloe-emodin  12.11–12.40  241(100), 270(77.7), 121(42), 139(37.9), 242(21.4) 77(21), 63(20.2), 128(19.7), 127(18), 168(17.6)  270, 241  Emodin  12.50–12.80  270(100), 69(24.3), 139(23.2), 242(20.8), 213(18.7), 271(16.28), 115(15.3), 77(14.8), 168(14), 128(13.9)  270  Analyte  GC–MS segments (min)  Product ions and relative abundances, (%)  Quantitation ion (m/z)  Osthole  8.00–9.00  244(100), 189(86.2), 229(79.4), 201(76.5), 131(72.4), 159(58.4), 213(56.2), 187(55.4), 115(44.4), 186(44.3)  244  Chrysophanol  9.90–10.40  254(100), 226(24.8), 152(20.4), 115(17.6), 197(16.5), 198(16.4), 76(15.9), 255(14.3), 63(13.6), 141(12.4)  254  Physcion  11.730–12.10  284(100), 128(37.6), 255(24.5), 139(17.6), 254(16.2), 285(15.8), 241(15.5), 77(15.0), 198(15.0), 226(14.3)  284  Aloe-emodin  12.11–12.40  241(100), 270(77.7), 121(42), 139(37.9), 242(21.4) 77(21), 63(20.2), 128(19.7), 127(18), 168(17.6)  270, 241  Emodin  12.50–12.80  270(100), 69(24.3), 139(23.2), 242(20.8), 213(18.7), 271(16.28), 115(15.3), 77(14.8), 168(14), 128(13.9)  270  Results Characterization of the reference compounds by GC–MS Chrysophanol, physcion, aloe-emodin and emodin are the major anthraquinones in rhubarb that could be analyzed by GC–MS. In present work, chrysophanol, physcion, aloe-emodin, emodin and osthole reference compounds were dissolved in acetic ether and analyzed by GC–MS within 50–300 amu in full scan mode. A typical gas chromatography-total ion current (GC-TIC) mass spectrogram obtained for IS, chrysophanol, physcion, aloe-emodin and emodin standards is shown in Figure 2A by GC–MS in the electron impact (EI) mode. It is obvious that the GC-TIC method improves the chromatogram efficiently and provides a single peak for identification. The retention times of osthole, chrysophanol, physcion, aloe-emodin and emodin were 8.81 min, 10.07 min, 11.89 min, 12.23 min and 12.59 min, respectively. The molecule ion of [M]+ and major fragments ions of chrysophanol, physcion, aloe-emodin and emodin were observed in the full scan MS spectra in Figure 3A–D. Figure 2. View largeDownload slide Total ion current chromatogram (A) and SIM chromatograms of mixed standards (B), rhubarb (C) and its preparation (D). Peak identification:1 = osthole; 2 = chrysophanol; 3 = physcion; 4 = aloe-emodin and 5 = emodin. Figure 2. View largeDownload slide Total ion current chromatogram (A) and SIM chromatograms of mixed standards (B), rhubarb (C) and its preparation (D). Peak identification:1 = osthole; 2 = chrysophanol; 3 = physcion; 4 = aloe-emodin and 5 = emodin. Figure 3. View largeDownload slide Mass spectra and proposed fragmentation pathways of main fragment ions for chrysophanol (A,a), physcion (B,b), aloe-emodin (C,c) and emodin (D,d) by GC–MS. Figure 3. View largeDownload slide Mass spectra and proposed fragmentation pathways of main fragment ions for chrysophanol (A,a), physcion (B,b), aloe-emodin (C,c) and emodin (D,d) by GC–MS. The MS spectra from [M]+ of chrysophanol contain the molecular ion, which is m/z 254. The abundance of molecule ion [M]+ is base peak fragment ion. The other major fragment ions were m/z 226, m/z225, m/z 198, m/z 197, m/z 180, m/z 169 and m/z 152. The fragment ion [M]+ m/z 254 lost carbon monoxide and hydrogen (CHO) form the fragment ion at m/z 225, and lost carbon monoxide (CO) form the fragment ion at m/z 226. The fragment ion at m/z 226 would be further losing CO or CHO to produce the fragment ion m/z 197 or m/z 198.The fragment ions at m/z 169 and m/z 152 were formed by the loss of CO and OH step-by-step from the fragment ion at m/z 197. Moreover, the fragment ion m/z 198 could loss H2O to be the fragment ion at m/z 180. The possible main cleavage pathways and some possible chemical structures of fragment ions have been supplied in the Figures 3a and 4A. Figure 4. View largeDownload slide The possible chemical structure and the possible main cleavage pathway of fragment ions of chrysophanol (A), physcion (B), aloe-emodin (C) and emodin (D). Figure 4. View largeDownload slide The possible chemical structure and the possible main cleavage pathway of fragment ions of chrysophanol (A), physcion (B), aloe-emodin (C) and emodin (D). Figure 3b shows the possible fragmentation pathways of main fragment ions for physcion in EI–MS. The MS spectra from [M]+ m/z 284 separately lost CH2O, CHO, CO and C2H3O, corresponding to be yielded product ions at m/z 254, m/z 255, m/z 256 and m/z 241. The other main fragmentation ions at m/z 227, m/z 226, m/z 213.198 and m/z 185 were formed by the loss of one or two CO2 from the first cleavage fragment ions respectively. The possible structure of fragment ions was shown in Figure 4B. The major fragment ions of aloe-emodin were m/z 270, m/z 241, m/z 224, m/z 213, m/z 196, m/z 185, m/z 168 and m/z 157 which were shown in Figure 3C. The base peak fragment ion was the fragment ion at m/z 241, which was formed by the loss of CHO from the molecular ion [M]+ m/z 270. Meanwhile, the molecular ion [M]+ m/z 270 could loss CH2O2 and CO to yield fragment ions at m/z 224 and m/z 242, respectively. Moreover, the fragment ion at m/z 224 could lose CO group step-by-step to produce the fragment ions at m/z 196 and m/z 168. On the one hand, the base peak fragment ion [M-CHO]+ m/z 241 lost CO group gradually to produce the fragment ions at m/z 213, m/z 185 and m/z 157, respectively. The possible main cleavage pathway of fragment ions of aloe-emodin had been shown in Figure 3c and the possible structure of the fragment ions has been shown in Figure 4C. Figure 3D shows the main fragment ions of emodin at m/z 270, m/z 242, m/z 214, m/z 213, m/z 196, m/z 185 and m/z 168. The molecular ion [M]+ m/z 270 losing CHO and CO form the fragment ions at m/z 241 and m/z 242. The two main fragmentation pathways of the characteristic fragment ion at m/z 242 could be as follow. First, the fragment ion at m/z 242 lost CHO and CO formed product ions at m/z 213 and m/z 185. Second, the characteristic fragment ion at m/z 242 successively lost CO, H2O and CO in turn formed the fragment ions at m/z 214, m/z 196 and m/z 168. The fragment ion at m/z 214 could also produce the ion at m/z 185 by losing CHO. The possible main cleavage pathway of fragment ions of emodin had been shown in the Figure 3d, corresponding with the possible structures as shown below the Figure 4D. The relative abundances of major fragment ions were shown in Table I. The base peak fragment ions of four compounds were [M]+ m/z 254, [M]+ m/z 284, [M-CHO]+ m/z 241 and [M]+ m/z 270. The main cleavage pathway of fragment ions of four anthraquinones was losing the CO group, CHO groups and the functional groups of these anthraquinones. Various structures such as R–OH, R–C = O and R-O-R related to the behavior of mass spectrometric notably. Quantification of four anthraquinones by GC–MS To reach higher sensitivity and selectivity, routine separations of chrysophanol, physcion, aloe-emodin, emodin and IS were performed by GC–MS in the SIM mode. Selection of fragments (m/z) for detection from the mass spectra of compounds was performed following abundance and specificity criteria. The product ions and quantification ions are listed in Table I. Instrumental parameters were studied to give the best sensitivity and mass spectra quality and are presented in Table I along with respective product ions used for quantification. The base peak fragment ions were m/z 254 for chrysophanol, m/z 284 for physcion, m/z 241 for aloe-emodin and m/z 270 for emodin. The relative abundances about the molecular ion of the aloe-emodin were particularly high, so we chose the fragment ions m/z 270 and m/z 241 as the monitored ions of it. The determination of analytes was performed in SIM mode shown in Table I. Selectivity of GC–MS method was evaluated by comparison of the migration time and mass spectrum of standard reference osthole (IS) and four anthraquinones with those of the peak obtained in the analysis of real extracts from the rhubarb. In Figure 2, it is shown a typical gas chromatography-total ion current (GC-TIC) mass spectrogram by GC–MS in the electron impact (EI) mode; GC-SIM chromatograms obtained for analyte and IS in standard, samples and preparation. It is obvious that the GC-SIM method improves the chromatograms very efficiently and provides a single peak for identification. Method validation The calibration curves were established by injecting six concentration levels of chrysophanol, physcion, aloe-emodin and emodin. The concentration range is 3.2–30.0 μg/mL for all. The characteristics of the calibration plots are summarized in Table II. As can be seen, the proposed method exhibited excellent linearity and sensitivity. Table II. Calibration Curves and LOD of Aloe-emodin, Emodin, Chrysophanol and Physcion by GC–MS Analyte  Calibration curve  R2  Line arrange (μg/mL)  LOD (ng/mL)  Chrysophanol  Y = 3.36244 + 93.6657X  0.9999  3.2–30  0.1  Physcion  Y = −22.0467 + 55.2551X  0.9999  3.2–30  0.5  Aloe-emodin  Y = −72.4639 + 61.5269X  0.9999  3.2–30  10  Emodin  Y = −64.9676 + 24.3869X  0.9998  3.2–30  20  Analyte  Calibration curve  R2  Line arrange (μg/mL)  LOD (ng/mL)  Chrysophanol  Y = 3.36244 + 93.6657X  0.9999  3.2–30  0.1  Physcion  Y = −22.0467 + 55.2551X  0.9999  3.2–30  0.5  Aloe-emodin  Y = −72.4639 + 61.5269X  0.9999  3.2–30  10  Emodin  Y = −64.9676 + 24.3869X  0.9998  3.2–30  20  The limits of detection (LODs) were determined by the signal-to-noise (S/N) ratio = 3. The LODs were determined to be 0.1 ng/mL for chrysophanol, 0.5 ng/mL for physcion, 10.0 ng/mL for aloe-emodin and 20.0 ng/mL for emodin, respectively. The precision of injection was evaluated by repeat injection of the standard solutions for six times. Intra and interday variabilities were determined by the analysis of average amount of standards in quality control samples prepared by standard solutions at low, medium and high concentrations on three different days. The quality control samples were prepared as a single batch on the same day at each concentration, and then divided into aliquots and stored at 4°C until required for analysis. The calculated RSDs from repeated measurements are summarized in Table III. As shown in Table III, intraday precisions were from 0.8% to 1.6% for chrysophanol, 0.5% to 1.4% for physcion, 0.8% to 1.0% for aloe-emodin and 0.5% to 2.0% for emodin by GC–MS, respectively. And interday precisions were from 1.9% to 2.9% for chrysophanol, 1.7% to 2.4% for physcion, 1.4% to 2.8% for aloe-emodin and 1.7% to 2.5% for emodin by GC–MS, respectively. Table III. Intra- and Interday Precision, Accuracy and Recovery of Aloe-emodin, Emodin, Chrysophanol and Physcion (n = 6) Analyte  Actual concentration (μg/mL)  Intraday  Interday  Recovery  RSD (%)  Recovery  RSD (%)  Recovery  Mean (%)  RSD (%)  Chrysophanol  3.2  0.8  98.1  1.9  97.4  98.8  2.5  12.0  1.6  100.2  2.9  102.9  20.0  1.2  100.6  2.6  103.9  Physcion  3.2  0.5  102.2  1.8  100.9  97.9  2.4  12.0  1.4  103.7  2.4  102.5  20.0  1.4  102.9  1.7  103.6  Aloe-emodin  3.2  0.8  98.8  1.4  97.2  102.9  2.4  12.0  0.8  97.8  1.8  97.3  20.0  1.0  101.7  2.8  103.9  Emodin  3.2  0.5  102.0  1.7  103.3  96.9  2.8  12.0  2.0  100.5  2.0  101.1  20.0  1.5  103.8  2.5  104.8  Analyte  Actual concentration (μg/mL)  Intraday  Interday  Recovery  RSD (%)  Recovery  RSD (%)  Recovery  Mean (%)  RSD (%)  Chrysophanol  3.2  0.8  98.1  1.9  97.4  98.8  2.5  12.0  1.6  100.2  2.9  102.9  20.0  1.2  100.6  2.6  103.9  Physcion  3.2  0.5  102.2  1.8  100.9  97.9  2.4  12.0  1.4  103.7  2.4  102.5  20.0  1.4  102.9  1.7  103.6  Aloe-emodin  3.2  0.8  98.8  1.4  97.2  102.9  2.4  12.0  0.8  97.8  1.8  97.3  20.0  1.0  101.7  2.8  103.9  Emodin  3.2  0.5  102.0  1.7  103.3  96.9  2.8  12.0  2.0  100.5  2.0  101.1  20.0  1.5  103.8  2.5  104.8  The recovery experiment was performed to verify the reliability of the proposed method. Four anthraquinones standards were added to the rhubarb containing known quantities of four anthraquinones and the mixture was treated as described above. The recoveries were calculated by spiking six samples of rhubarb solution, which was added with a certain amount of the four standards respectively. The recovery was calculated by comparing the found amount of standards with those added. Three injections of each preparation were made and the theoretical amount of analyte in the sample preparations and the average percentage analyte recovered in the spiked solutions were calculated. The mean recoveries for chrysophanol, physcion, aloe-emodin and emodin were 96.9–102.9% with RSDs <2.8%. Results are shown in Table III, which indicate that the method using osthole as IS obtains better accuracy and is suitable for the sample analysis. The reproducibility of the method was proved by analyzing samples of four analytes in rhubarb. RSDs (n = 6) were 2.3–6.1%. Applications Analyses of real samples (300.00 mg dried rhubarb and 500.00 mg Ruyi Jinhuang powder) were performed by GC–MS with the condition of sample preparation. The quantification of four anthraquinones was performed by GC–MS and the contents of four anthraquinones in rhubarb and its preparation are shown in Table IV. Rhubarb had higher contents of analytes than those in Ruyi Jinhuang powder. But the contents sequence of analytes in the rhubarb was the same in the Ruyi Jinhuang powder. The concentration sequences were as follows: chrysophanol > emodin > physcion > aloe-emodin. Table IV. The Contents of Aloe-emodin, Emodin, Chrysophanol and Physcion in Rhubarb and Its Preparation by GC–MS Sample  Chrysophanol (μg/mg)  Physcion (μg/mg)  Aloe-emodin (μg/mg)  Emodin (μg/mg)  Rhubarb  8.2756  1.4468  1.3048  2.5668  Ruyi Jinhuang powder  0.4260  0.0700  0.0643  0.1270  Sample  Chrysophanol (μg/mg)  Physcion (μg/mg)  Aloe-emodin (μg/mg)  Emodin (μg/mg)  Rhubarb  8.2756  1.4468  1.3048  2.5668  Ruyi Jinhuang powder  0.4260  0.0700  0.0643  0.1270  Discussion In this work, in order to compare the precision and accuracy, GC–MS was developed for the simultaneous quantification of four anthraquinones with IS. The choice of the IS is of crucial importance since it affects precision and accuracy of the method. For a proper IS, it should be structurally or chemically similar to the analyte, have similar retention behavior to the analyte, and be well resolved from the analyte and other peaks. Thus, osthole was chosen as the IS for the analysis because of its similarity of structure, retention and polarity to the chrysophanol, physcion, aloe-emodin and emodin. In order to increase the sensitivity of the developed method, quantitative determination was performed by SIM of base peak ion at 254, 270, 284 and 241 for analytes, and molecule ion at m/z 244 for IS, respectively. LODs of the four analytes by GC–MS in this study provides a better sensitivity than HPLC (11–13), UPLC (14), HPLC-DAD (15) and HPLC–MS (15). Moreover, LODs of chrysophanol and physcion showed in Table II were more sensitivity than HPLC-FLD (13). Therefore, GC–MS provides a high selectivity and sensitivity for determination of four anthraquinones in rhubarb and its preparations. Conclusions A novel GC–MS method was developed to simultaneously identify and quantify four anthraquinones in rhubarb and its preparations using osthole as IS. The method provides acceptable figures of merit. It has been demonstrated by EI–MS spectra. The possible main fragment ions cleavage pathways were discussed. According to the ion peak intensity, the fragment ions at m/z 254, m/z 270, m/z 284 and m/z 241 were selected as monitoring ions for analytes by SIM. Subsequently, we develop a simple and sensitive GC–MS method for simultaneous determination of four anthraquinones. The LODs were up to 0.1 ng/mL, 0.5 ng/mL, 10 ng/mL and 20 ng/mL for four anthraquinones, respectively. The method exhibits good linearity, reproducibility, precision, accuracy and recovery and could be used for quantitative analysis and pharmacokinetics of four anthraquinones in the rhubarb and its preparations. 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Journal of Chromatographic ScienceOxford University Press

Published: Mar 1, 2018

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