Comparison of the Saponins in Three Processed American Ginseng Products by Ultra-High Performance Liquid Chromatography-Quadrupole Orbitrap Tandem Mass Spectrometry and Multivariate Statistical Analysis.

A method with ultrahigh performance liquid chromatography Quadrupole-Orbitrap tandem mass spectrometry (UHPLC-Q-Orbitrap-MS/MS) was applied for the quality evaluation of different processing and drying of American ginseng, including natural drying (ND), steam drying (SD), and vacuum freeze-drying (VFD). A total of 51 saponins were successfully identified in three processed products. Three processed American ginseng products were well-differentiated in orthogonal partial least-squares discriminant analysis (OPLS-DA). The S-plot also identified the marker compounds in each product, while the major ginsenosides of ND (malonyl (M)-Rd, M-Rb1, Rg1), SD (20 (S)-Rg3, 20 (S)-Rg2), and VFD (M-Rd, M-Rb1) were found. The results indicate that the method by vacuum freeze-drying can retain the content of rare ginsenosides and malonyl-ginsenosides. The marker compounds selected will benefit the holistic evaluation of related American ginseng products.

1. Introduction

American ginseng (Panax quinquefolium L.) is well-known for replenishing Qi in Chinese medicine [1, 2]. American ginseng contains several bioactive compounds, including polysaccharides, saponins, amino acids, volatile oil, and mineral elements, while ginsenoside is one of the important active ingredients [3-9].

Ginseng has three popular processes, including natural air drying, steaming drying, and vacuum freeze-drying, respectively [10-17]. The drying process affects the quality of ginseng products and changes the content of ginsenosides. The essence of the change in ginsenoside content is the transformation of ginsenosides during the drying process. Thermal processing (natural air drying and steaming drying) converts saponins of larger molecular weight into saponins of smaller molecular weight. Ginsenoside M-Rb1, M-Rb2, M-Rc, Re, Rg1, Rb1, Ginsenoside Rb2, Rc, Rd, Re, Rg1, and Ginsenoside Rd, Rk1, Rg5, Rg3 are the major ginsenosides of white ginseng, red ginseng, and black ginseng, respectively [15, 18, 19]. Nonthermal processing (vacuum freeze-drying) can keep the shape and color of ginseng consistent with its fresh state, containing more natural active ingredients. The characteristic components in vacuum freeze-drying ginseng were M-Re, M-Rb1, M-Rc, M-Rb1 isomer, M-Rb2, M-Rb3 and M-Rd isomer [11].

The LC-MS technique combines the high separation ability for complex samples with the high selectivity of high-resolution mass spectrometry and the ability to provide information (molecular weight and structural) and is widely used to control quality standards for traditional Chinese medicine [20-22]. The composition of Chinese herbal medicines has been rapidly analyzed by UHPLC-Q-Orbitrap-MS/MS and the changes in their chemical composition before and after processing as an effective tool for identifying active ingredients with improved sensitivity and accuracy. The fragment information of tandem mass spectrometry can be used to identify the structure of compounds [23-25].

Multivariate analysis methods were applied to analyze whether differences existed between test samples and to determine which compounds were altered for quality evaluation of herbal medicines of different origins, parts, and processing methods [11, 23, 24, 26, 27]. Recent studies demonstrated that the UPLC-QTOF/MS is an optimal application for holistic evaluation of ginseng [18, 28]. Principal component analysis (PCA) is distinguished from the compounds of different drying processes of Houttuyniae Herba [29].

The common processed products of American ginseng are dried American ginseng [30-32]. It is necessary to control the drying process in the processing of American ginseng [33-35]. In recent years, steamed American ginseng and vacuum freeze-dried American ginseng have appeared in the functional food market. However, the systematic comparison of ginsenoside conversions of natural drying (ND), steam drying (SD), and vacuum freeze-drying (VFD) has not yet been studied.

In this paper, UHPLC-Q-Orbitrap-MS/MS analysis combined with multivariate analysis approach was applied to evaluate the composition of ND, SD, and VFD. This study aims to explore the trends of transformation of ginsenosides and characterize and quantify the chemical ingredients in three processed American ginseng products to standardize the processing procedures reasonably.

2. Materials and Methods

2.1. Chemicals and Reagents

The chemicals and reagents used were as follows: methanol, acetonitrile, and formic acid (HPLC grade, Fisher Scientific); reference ginsenosides Rh1, Rg5, Rk1, Rg3, F2, Re, Rg1, Rb1, Rg2, Rc, Rd; and pseudoginsenoside F11 (Shanghai Yuanye Biotechnology Co., Ltd.) were obtained. The purity of the standards was no less than 98%.

2.2. Processed American Ginseng Samples

The samples of fresh American ginseng were first cleaned and then processed by the experiment (steaming drying, natural air drying, and vacuum freeze-drying). The ND samples were produced by drying at 50°C. The SD samples were produced in a ginseng steaming cabinet at 100°C for 3 h and allowed to dry in an oven at 50°C. The VFD samples were produced by first prefreezing at −20°C for 12 h and then placing them in a vacuum freeze-dryer (Ningbo Xinzhi Biotechnology Co., Ltd., Zhejiang, China) to freeze-dry for 72 h, as shown in Figure 1.

Figure 1

The scheme of American ginseng products: natural drying (ND), steam drying (SD), and vacuum freeze-drying (VFD).

2.3. Preparation of Standard and Sample Solutions

A series of reference mixtures with ginsenosides Rh1, Rg5, Rk1, Rg3, F2, Re, Rg1, Rb1, Rg2, Rc, Rd, and pseudoginsenoside F11 were dissolved in 70% methanol-water to a final concentration of 0.2 mg/mL. Each of the stock solutions was combined to obtain final concentrations.

A fine powder (0.1 g) was ultrasonically extracted with 70% methanol-water (5 mL) for 45 min. The extraction was filtered through a 0.22 μm syringe filter before analysis [36].

2.4. Instrument and Condition

A UPLC system (Ultimate 3000) was used for separations. A Supelco C18 column (3.0 × 50 mm, 2.7 μm) was used at 35°C for separation and eluted by the mobile phases (solvent A and B) were acetonitrile and water containing 0.1% formic acid, separately. The gradient elution program with a 0.5 mL/min flow rate was as follows: 15%-15% A (0–2 min); 15–30% A (2–15 min); 30–95% A (15–25 min); 95–15% A (25–27 min); and 15–15% A (27–35 min). The injection volume was 10 µL.

A Q-Orbitrap-MS/MS via an ESI source in the negative ion mode. For the ESI source, sheath gas flow of 35 Arb, aux gas flow of 10 Arb, sweep gas flow of 1 Arb, capillary voltage of −3.5 kV, and capillary temperature of 350°C. A full MS data were scanned with m/z 150–2000 Da, 70,000 resolution, automatic gain control (AGC) target, 1 × 106, and maximum injection time (IT), 100 ms. The dd-MS2 was scanned with 17,000 resolution, AGC target, 1 × 105, IT, 50 ms, loop count 5, isolation window 4.0 m/z, and NCE, 25–55. Then, a Full-MS/dd-MS2 mode was used for analysis.

2.5. Data Analysis

The data were processed by SIEVE 2.1 (Thermo Fisher, San Jose, CA, USA). Then, the datasets were multivariate analyzed by SIMCA-P software 11.5 (Umetrics, Umea, Sweden). The components with VIP values larger than 1 and p < 0.05 were selected as analytical markers in the OPLS-DA model and the t-test by SPSS 19.0 (Chicago, IL, USA), separately.

3. Result and Discussion

3.1. Analysis of Three Processed American Ginseng Products by UHPLC-Q-Orbitrap-MS/MS

The method of UHPLC-Q-Orbitrap-MS/MS was applied for determination [23, 37, 38]. The identification of the extracts of ND, SD, and VFD samples in the negative ion mode is shown in Figure 2. The compounds were separated distinctly in 30 min by UPLC. The distinct samples of several peaks demonstrated the different intensities at 15–25 min. The ratio of Rg1 and Re was changed with steaming treatment, and the content of Rg1 in SD samples was significantly decreased at 10 min. Meanwhile, the content of minor ginsenosides was increased at 18–22 min. A slice of studies performed showed that the major ginsenosides (Re and Rg1) were converted to minor ginsenosides (Rh4, Rk1, Rk3, 20 (S)-Rg3, Rg5, and 20 (R)-Rg3) by heat treatment [39]. The content of malonyl-ginsenosides in VFD samples was higher than in SD samples. These results demonstrated the change in ginsenoside composition of ND, SD, and VFD samples.

Figure 2

Base peak chromatogram of American ginseng of natural drying (ND), steam drying (SD), and vacuum freeze-drying (VFD) in the negative ion mode.

The Q-Orbitrap-MS accurately measured the compounds' mass values [24]. Meanwhile, the fragmentation pattern of standards was compared to identified ginsenosides from three processed American ginseng products.

The full-scan MS was used to confirm the molecular weight. Ginsenosides were easily ionized into [M − H]− and [M + HCOO]− ions in the negative ion mode. All molecular ions were unambiguously identified within the mass accuracy of 10 ppm. The different tandem MS spectra of the various aglycone types are shown in Figure 3. In Figure 3(a), fragment ion at m/z 475 indicated the successive losses of two glucose residues. In Figure 3(b), the successive losses of four glucose residues were observed from fragment ion at m/z 459. The losses of two glucose residues and a β-d-glucuronic acid (176 Da) were observed from fragment ion at m/z 455 in Figure 3(c). In Figure 3(d), the loss of a malonyl residue was observed from malonyl-ginsenoside at m/z 1107, and the fragment ion at m/z 459 was similarly produced by successive loss of four glucose residues. In Figure 3(e), the loss of a rhamnose residue (146 Da) was observed from fragment ion at m/z 653. In total, 51 ginsenosides were identified from ND, SD, and VFD by comparing standards and literature records of the tandem MS spectra (Table 1).

Figure 3

The MS/MS spectrum of ginsenosides in the negative ion mode: (a) Rg1; (b) Rb1; (c) Ro; (d) M-Rb1; (e) PF11.

Table 1

Compounds identified from American ginseng by natural drying, steam drying, and vacuum freeze-drying.

No.	Rt (min)	Identification	Formula	Detected mass (Da)	Mass error (ppm)
1	5.67	Vina-ginsenoside R4	C48H82O19	1007.5457a	2.5
2	9.04	Notoginsenoside R1	C47H80O18	977.5346a	1.9
3	10.03	Ginsenoside Rg1	C42H72O14	845.4914a	1.2
4	10.41	Ginsenoside Re	C48H82O18	991.5504a	2.1
5	10.73	24 (S)-pseudo-ginsenoside F11	C42H72O14	845.4922a	2.1
6	11.45	Malonyl-ginsenoside Rg1	C45H74O17	885.4867b	1.6
7	11.51	Malonyl-ginsenoside Re	C51H84O21	1031.5457b	2.4
8	12.69	20 (S)-notoginsenoside R2	C41H70O13	815.4819a	2.6
9	13.47	Ginsenoside F5	C41H70O13	815.4814a	2.0
10	13.85	Acetyl-Rg1	C44H74O15	887.5024a	1.6
11	14.80	Pseudo-RT2	C41H70O14	785.4661b	4.1
12	15.26	24 (R)-pseudo-ginsenoside F11	C42H72O14	845.4918a	1.6
13	15.72	Notoginsenoside R4/Ginsenoside Ra3	C59H100O27	1285.6464a	1.6
14	16.46	Ginsenoside Rh1	C36H62O9	683.4391a	2.2
15	16.61	20 (S)-ginsenoside Rg2	C42H72O13	829.4971a	1.9
16	16.89	20 (R)-ginsenoside Rg2	C42H72O13	829.4974a	2.3
17	17.01	Ginsenoside Rb1	C54H92O23	1153.603a	1.6
18	17.06	Malonyl-ginsenoside Rb1/Isomer	C57H94O26	1193.5983b	1.8
19	17.13	Ginsenoside Rc	C53H90O22	1123.5920a	1.2
20	17.20	Ginsenoside Ro	C48H76O19	955.4933b	2.6
21	17.27	Malonyl-ginsenoside Rc	C56H92O25	1163.5883b	2.4
22	17.29	Malonyl-ginsenoside Rb2	C56H92O25	1163.5892b	3.2
23	17.36	Ginsenoside Rb2	C53H90O22	1123.5923b	1.5
24	17.59	Ginsenoside Rb3	C53H90O22	1123.5924a	1.6
25	17.66	Malonyl-ginsenoside Rb3	C56H92O25	1163.5878b	2.0
26	17.73	Pseudo-RT1	C47H74O18	971.4800a	5.9
27	17.84	Ginsenoside Rd	C48H82O18	991.5506a	2.3
28	18.07	Chikusetsusaponin IVa	C42H66O14	793.4396b	2.0
29	18.10	Malonyl-ginsenoside Rd	C51H84O21	1031.5458b	2.5
30	18.14	Gypenoside XVII	C48H82O18	991.5502a	1.9
31	18.40	Pseudo-RC1	C50H48O19	1033.5521a	6.6
32	18.63	Quinquefolium III	C50H48O19	1033.5529a	5.8
33	18.80	Ginsenoside Rg6	C42H70O12	811.4810a	4.8
34	18.90	Ginsenoside Rg4	C42H70O12	811.4807a	5.2
35	19.24	Ginsenoside F2	C42H72O13	829.4971a	1.9
36	19.58	20 (S)-ginsenoside Rg3	C42H72O13	829.4976a	2.5
37	19.63	Ginsenoside Rk3	C36H60O8	665.4231a	5.9
38	19.79	Ginsenoside Rh4	C36H60O8	665.4233a	5.6
39	20.01	Zingibroside R1	C42H66O14	839.4382a	5.1
40	20.27	20 (R)-ginsenoside Rg3	C42H72O13	829.4982a	3.3
41	20.34	20 (S)-ginsenoside Rs3	C44H74O14	871.5015a	5.3
42	20.60	Calenduloside E	C36H56O9	677.3869a	5.5
43	20.68	20 (R)-ginsenoside Rs3	C44H74O14	871.5014a	5.3
44	20.88	Ginsenoside Rk1	C42H70O12	811.4859a	1.2
45	20.92	Ginsenoside Rg5	C42H70O12	811.4868a	2.3
46	21.19	20 (S)-ginsenoside Rh2	C36H62O8	667.4391a	5.4
47	21.42	20 (R)-ginsenoside Rh2	C36H62O8	667.4393a	5.1
48	21.62	Ginsenoside Rs5	C44H72O13	853.4913a	4.7
49	21.85	Ginsenoside Rs4	C44H72O13	853.4913a	4.7
50	23.18	Ginsenoside Rk2	C36H60O7	649.4276a	6.9
51	23.33	Ginsenoside Rh3	C36H60O7	649.4294a	4.2

3.2. Multivariate Statistical Analysis

The statistical methods were applied to display the differences in ginsenosides intuitively. After data preprocessing of the ND, SD, and VFD samples, the dataset was conducted to discover the marker compound by multivariate statistical analysis.

The distinct ginsenoside composition of samples from ND, SD, and VFD was characterized in detail. The OPLS-DA model was effectively used to observe three processed American ginseng products (Figure 4). An excellent prediction ability was considered to the parameters of SD vs. ND (R2 Y = 0.865, Q2 = 0.999), VFD vs. ND (R2 Y = 0.822, Q2 = 0.998), and VFD vs. SD (R2 Y = 0.848, Q2 = 1). The score plot was used to discriminate between the groups of two selected samples (ND vs. SD, ND vs. VFD, and SD vs. VFD). The samples of ND were compared to the other samples by OPLS-DA, separately. By comparing the S-plot of ND and SD, the components observed from the lower left quadrant and the upper right quadrant were elevated in SD and ND, respectively (Figure 4(b)). The marker components were elevated in SD (20(S)-Rg3, 20(S)-Rg2, and an unknown compound) and ND (malonyl (M)-Rb1, M-Rd). Four components (20(S)-Rg3, 20(S)-Rg2, M-Rb1, and M-Rd) were identified as marker compounds in SD and ND. By comparing the S-plot of ND and VFD, it was performed that four unknown compounds and two compounds (M-Rb1, Rg1) were elevated in VFD and ND, respectively (Figure 4(d)). The ginsenosides of M-Rb1 and Rg1 were identified as marker compounds in ND and VFD. By comparing SD and VFD, it was performed that two malonyl-ginsenosides (M-Rd, M-Rb1) and three compounds (20(S)-Rg3, 20(S)-Rg2, and an unknown compound) were elevated in VFD and SD, respectively (Figure 4(f)). Four ginsenosides of M-Rd, M-Rb1, 20(S)-Rg3, and 20(S)-Rg2 were identified as marker compounds in VFD and SD. The repeated emergence of marker compounds was in different groups, including the SD group (20(S)-Rg3 and 20(S)-Rg2) and the ND group (M-Rb1). Meanwhile, the marker compounds could display various pharmacological activities such as antitumor (20(S)-Rg3) [40], against cardiocerebrovascular diseases (20(S)-Rg2) and affect the central nervous system (M-Rb1) [41, 42].

Figure 4

The OPLS-DA/S-plot of ND vs. SD, ND vs. VFD, and SD vs. VFD.

The VIP value was used to select the marker compounds [29]. The features with VIP values larger than 1 were highlighted as marker candidates. The compounds with the statistically significant difference (p < 0.05) were screened for marker compounds by ANOVA. To visualize the tendency of marker compounds, the intensities of 22 marker compounds were constructed as a heat map. The composition change was shown in different colors by the ND group, the SD group, and the VFD group (Figure 5). Between the SD and ND groups, the contents of ginsenoside-Rb1, -Rh1, -Rg3, -Rg2, -Rg4, -Rh4, –F2, and -Rg5 in SD samples were increased, and the contents of malonyl-ginsenoside-Rg1, -Re, -Rc, -Rb1, -Rb2, -Rd, and ginsenoside-Re, -Rc, -Rg1, -Rb3, -Rd, -Ro, 20-(S)-PF11, and 20-(R)-PF11 in ND samples were significantly higher. Comparing the groups of ND and VFD, the contents of malonyl-ginsenoside-Rg1, -Re, -Rc, -Rb1, -Rb2, and -Rd in the VFD group were significantly increased. Meanwhile, the contents of ginsenoside-Re, -Rc, -Rg1, -Rb3, -Ro, -Rd, 20-(S)-PF11, and 20-(R)-PF11 in the ND samples were significantly increased. The malonyl residue of malonyl-ginsenoside in ND and VFD was well preserved without heating treatment. Similar to ginseng, it was observed that the content of malonyl-ginsenosides in American ginseng was increased by a vacuum freeze-drying approach [11]. The thermal sensitivities of malonyl-ginsenosides were less affected by the vacuum freeze-drying technology.

Figure 5

The heat map visualizes the intensities of the 22 ginsenosides datasets from ND, SD, and VFD (n = 10).

The malonyl-ginsenoside of chemical transformation is related to heat and pressure treatment [24]. Meanwhile, the alteration of three processed American ginseng products is possibly related to the degree of enzymatic activity, moisture, and microstructure [43]. The content of moisture can result in a rapid transformation by promoting corresponding microbiological growth, enzymatic and nonenzymatic reactions in American ginseng. The active compounds are cleaved by enzymes with steam and high temperatures [9, 44]. As a result, the content of ginsenoside Rb1 in SD samples was higher than in VFD and ND samples. The content of ginsenoside Rg1 in ND and VFD samples was higher than in SD samples; it probably converted into 20(S)-Rg3, 20(R)-Rg3, Rk1, and Rg5 by heat treatment [39]. Malonyl-ginsenosides in VFD and ND samples were higher than in SD samples, and they were probably converted into malonic acid through decarboxylation by heat treatment. In summary, these factors all lead to the transformation of ginsenosides in the three processed products. The composition and content of components in various processed products can affect their pharmacological activity, and it is necessary to elucidate the change of ginsenosides.

3.3. Quantification of Ginsenosides from Three Processed American Ginseng Products

The 12 compounds' EIC was identified by the reference compounds in Figure S1 and Table S1 (Supplementary material). The standard curve was used to determine the actual amount of the three processed American ginseng products in Figure S2 and Table S2 (Supplementary material). The quantification was based on 12 ginsenosides for products (n = 3) and the amount of ginsenosides was calculated in Table S3 (Supplementary material).

Previously, Huang et al. has reported the 59 ginsenosides of protopanaxadiol, the concentrations of ginsenosides Rk1, Rg5, Rh1, 20(R)-ginsenoside Rg2, and 20(R)-ginsenoside Rg3 are highest in red American Ginseng, these results have been matched to ours [10]. For ginsenosides analysis, the content of ginsenosides Rg1, Re, Rb1, Rc, Rd, and 24(R)-pseudo-ginsenoside F11 is the highest in ND. As our results, the concentrations of ginsenosides Rg1, Re, Rb1, Rc, Rd, and 24(R)-pseudo-ginsenoside F11 are highest in ND, and this is matched to another paper [18]. Ginsenoside Rh1 is not detected in ND and VFD.

4. Conclusions

A successful method was performed for the chemical components of three processed American ginseng products using UHPLC-Q-Orbitrap-MS/MS. The 5 ginsenosides as characteristic marker compounds could be applied to elucidate the composition of ND, SD, and VFD samples by multivariate statistical analysis. The results will be useful to visualize the tendency of marker compounds in manufacturing. Furthermore, this study effectively provided a means for assessing and controlling different processed American ginseng products.