2.1 Reagents and materials
Eight batches of ANP (No. S1 – S8) were purchased from Beijing TongRenTang Technologies Co., Ltd and their information was listed in Additional file 1: Table S1. The reference standards of muscone, α-pinene, limonene, eucalyptol, isoborneol, and d-borneol were obtained from National Institutes for Food and Drug Control (Beijing, China). Acetophenone, fenchol and β-caryophyllene were bought from Shanghai Yuanye Bio-Technology Co., Ltd (Shanghai, China). Camphene, benzaldehyde, α-terpinene, benzeneacetaldehyde, terpinolene, camphor, tridecane, 4-methyl-4-phenyl-2-pentanone, and humulene were acquired from Chengdu Push Bio-Technology Co., Ltd (Chengdu, China). ar-Tumerone was purchased from BioBioPha Co., Ltd. (Kunming, China). The standard mix of n-alkanes (C7–C40) was obtained from Sigma-Aldrich (St. Louis, MO, USA). (+)-3-Carene was bought from Toronto Research Chemicals (Toronto, ON, Canada). β-Pinene was acquired from Dr. Ehrensdorfer GmbH (Augsburg, Germany). α-Curcumene was purchased from Extrasynthese (Lyon Nord, France). The purities of all used reference standards were higher than 95%.
Anhydrous ethanol and ethyl acetate (HPLC grade) were purchased from Yonghua Chemical Technology Co., Ltd (Suzhou, China). Distilled water was prepared using a Milli-Q Integral water purification system (Millipore, Bedford, MA, USA). Other reagents were analytical grade.
2.2 Preparation of standard and sample solutions
The 21 reference standards were dissolved with anhydrous ethanol respectively to prepare the corresponding stock solutions at the concentration of 1 mg/mL. An appropriate amount of individual standard stock solutions was mixed into a 10 mL volumetric flask to prepare a mixed standard stock solution, which was diluted with anhydrous ethanol to obtain a series of working solutions at proper concentrations for calibration curves. All working solutions were stored at -20 ℃ until analysis.
After ground into fine powder, 1.5 g ANP was accurately weighed and extracted using steam distillation for 4 h with 100 mL distilled water. The volatile oils were collected and residual water was removed with anhydrous sodium sulfate, finally were dissolved in 2 mL ethyl acetate. The components of high contents were diluted 40 times and determined, and the components of low contents were directly analyzed.
2.3 Collection and precipitation procedure of plasma samples
Male Wistar rats (200 ± 20 g) were purchased from Shanghai Lab. Animal Research Center (Shanghai, China). All rats were housed at 24 ± 2°C on a 12 h light/dark cycle, and fed a standard diet and water for one week before the experiment. Then all rats were randomly divided into two groups: ANP group and control group. The rats were fasted for 12 h prior to experiments but water was provided ad libitum. ANP suspension dissolved in physiological saline was orally administrated to nine rats (ANP group) at a dosage of 8.1 g/kg, and the rest two rats (control group) were orally administered with the same dose of saline respectively. All procedures were carried out in accordance with Guide for the Care and Use of Laboratory Animals (National Institutes of Health).
The blood samples were collected in heparinized 1.5-mL polythene tubes at 0, 0.25, 0.5, 0.75, 1, 1.5, 2 and 4 h after oral administration. Then, the blood samples were centrifuged at 4500 rpm for 10 min at 4°C, respectively. The supernatant was separated and stored at 80°C until analysis. An aliquot of 100 µL plasma samples was extracted using 100 µL of ethyl acetate for 1 min by vortex to precipitate protein. The extracted solution was centrifuged at 13000 rpm for 10 min and preserved in -80°C refrigerator before GC-MS analysis.
2.4 Chromatographic and mass spectrometric conditions
The qualitative analysis was performed on an Agilent 7890B gas chromatography system coupled to an Agilent 5977A quadrupole mass spectrometer (GC-MS, Agilent Technologies, USA). The GC separation was achieved on an Agilent DB-5 MS capillary column (60 m × 0.25 mm i.d.) coated with a 0.25 µm film of 5% phenyl polymethyl siloxane. High-purity helium gas was used as carrier gas, with a flow rate of 1.0 mL/min. The injection and interface temperatures were set to 280 ℃ and 250 ℃, respectively. The column temperature was programmed as follows, the oven was initially maintained at 60 ℃ for 3 min, increased to 100 ℃ at 10 ℃/min, increased to 135 ℃ at 2 ℃/min, then increased to 165 ℃ at 5 ℃/min, increased to 168 ℃ at 1 ℃/min, increased to 200 ℃ at 5 ℃/min, held for 2 min, and then increased to 295 ℃ at 10 ℃/min, held constant for 5 min. The split ratio was set to 10:1. The injection volume was 1 µL. The electron energy was set to 70 eV. The source temperature was 230 ℃ and the quadrupole temperature was 150 ℃. The mass data was acquired using full scan mode with a mass range of m/z 50–600 after a solvent delay of 9.1 min. Data acquisition was obtained by Agilent MassHunter GC-MS Acquisition Software Version B.07.03.2129.
For quantitative analysis, the Agilent 7890B GC system equipped with an Agilent 7000D triple quadrupole mass spectrometer (GC-MS/MS, Agilent Technologies, USA) was performed. The Agilent DB-5 MS UI capillary column (30 m × 0.25 mm i.d., 0.25 µm) were used for separation, the initial oven temperature was 60 ℃, and raised to 76 ℃ with 8 ℃/min, then increased to 82 ℃ at the rate of 2 ℃/min, and ramped with 15 ℃/min to 130 ℃, further rose at 20 ℃/min to 230 ℃ and held for 2 min. The injection and interface temperatures were both kept at 250 ℃. The carrier gas (helium, > 99.999%) flow rate was set at 1.0 mL/min. The electron energy and source temperature were set to 70 eV and 230 ℃, respectively. The flow rate of quenching gas (helium, > 99.999%) and collision gas (nitrogen, > 99.999%) was 2.25 mL/min and 1.5 mL/min, separately. Data acquisition was achieved on Agilent MassHunter Workstation GC/MS Data Acquisition Software Version 10.0.368.
2.5 Validation of quantitative method
2.5.1 Calibration curves, LODs and LOQs
After added with an equal amount of internal standard (IS) n-tridecane (final concentration: 14.20 µg/mL), a series of working solutions at multiple concentrations were analyzed. The calibration curves were constructed by plotting the relationships between peak area and concentration of the analytes and IS.
The mixed standard stock solution was further diluted with anhydrous ethanol and analyzed by GC- MS/MS. The concentration of the analyte with signal-to-noise ratio (S/N) of 10 was defined as the limit of quantification (LOQ), and the concentration of the analyte with S/N of 3 was assigned as the limit of detection (LOD).
2.5.2 Precision, repeatability, stability and recovery
The precision was evaluated by the determination of intra- and inter-day variances. The mixed standard solutions at three different concentration levels (low, medium, high) were analyzed for consecutive three days and continuous six times per day, and the peak area ratio of each compound and IS was recorded to calculate the relative standard deviation (RSD) value, respectively. Six parallel ANP samples from the same batch were prepared and analyzed for the repeatability test. To confirm the stability, a single sample solution was stored in sample chamber and analyzed at 0, 2, 4, 8, 12 and 24 h, respectively. Recovery test was used to verify the accuracy of the established method. A known amount of mixed reference solution at middle concentration was added in the same sample for six parallel extraction and analysis.
2.6 Data analysis
The qualitative analysis was realized on Agilent MassHunter Workstation Qualitative Analysis Software 10.0. The retention indices of all chromatographic peaks were calculated based on the data of C7 – C40 n-alkanes acquired with the same GC-MS method. The acquired components were tentatively identified by comparison with mass spectra and retention indices in the National Institute of Standards and Technology (NIST) 2017 library, and some were unambiguously determined by direct comparison with reference standards. Agilent MassHunter Workstation Quantitative Analysis Software Version 10.0 was used for quantitative analysis. Related graphical analysis was conducted on Graphpad Prism 8.02 (San Diego, USA).
The pairwise PCC between batches based on the vectors of concentrations from each batch as Formula (1).
$${PCC}_{\text{1,2}}=\frac{{\sum }_{i=1}^{n}({B1}_{i}-\stackrel{`}{B1})({B2}_{i}-\stackrel{`}{B2})}{\sqrt{{\sum }_{i=1}^{n}{({B1}_{i}-\stackrel{`}{B1})}^{2}}\sqrt{{\sum }_{i=1}^{n}{({B2}_{i}-\stackrel{`}{B2})}^{2}}} \left(1\right)$$
Here, B1 and B2 represent the vectors of ingredients’ concentration of the first and second batches respectively. Finally, the PCCs are displayed through the correlation heat map, the batch-to-batch similarity was evaluated according to the average value of PCCs among all pairs from batches.