Introduction

Guangxi, with its unique geographical conditions and humid and warm climate, is very suitable for the growth of Yao medicine and the planting of traditional Chinese medicine, making Guangxi one of the producing areas of traditional Chinese medicine in China, known as “genuine medicine”. In Laibin, Guangxi, the mountains and rivers are beautiful and the climate is pleasant, which is very suitable for the cultivation of Chinese medicinal materials1,2. Diding has the functions of clearing heat and dampness, detoxifying and reducing swelling; curing boils, carbuncle, scrofula, jaundice, dysentery, diarrhea, red eyes, throat numbness, snake bites and other effects3. Straw sandal board has the effects of clearing heat, dispersing blood stasis, promoting water and removing dampness4,5. Purslane has clearing heat and detoxification, cooling blood to stop bleeding, and stop dysentery. June snow has the effects of dispersing wind, relieving exterior, clearing heat and dampness, relaxing muscles and activating collaterals6,7. Pine vine rattan has the function of relaxing tendons and activating blood circulation, and has the effects of treating rheumatic joint pain, traumatic injury, muscle and bone pain and so on.

Modern research has proved that the curative effect of traditional Chinese medicine is not only related to organic ingredients, but also closely related to the types and contents of inorganic elements8. In addition to being essential elements for the body to participate in and regulate metabolism, inorganic elements also have a strong ability to form complexes, it is easy to form coordination bonds with ligands containing nitrogen, oxygen and sulfur in organisms, coordinate the balance of substances in the body, and form effective ingredients. Therefore, the synergistic effect of inorganic elements on the efficacy of the drug cannot be ignored.

The thermal analysis is simple, rapid, without pre-preparation, reproducible and easy analysis. In recent years, thermal analysis has a good application prospect in the research of traditional Chinese medicine. By reference to relevant domestic and foreign literatures, this essay summarizes the advance in studies on thermal analysis in terms of identification of traditional Chinese medicine and Chinese medicine preparation, to provide a reference for the application and extension of thermal analysis in the field of traditional Chinese medicine9.

Traditional Chinese medicine is a complex system of mixed chemical components, relying on a variety of chemical components to play a comprehensive therapeutic effect. The compatibility and therapeutic effects of traditional Chinese medicines are closely related to the energy effects of traditional Chinese medicines. When the medicine enters the body under the action of metabolic enzymes in the body, it will inevitably be accompanied by the release of energy of Chinese medicine chemicals. From a holistic perspective, it is necessary to conduct a macroscopic and comprehensive analysis of traditional Chinese medicine to control the quality of traditional Chinese medicine10.

The effective ingredients of traditional Chinese medicines and their preparations are complex, and it is difficult for a single-component quality control method to comprehensively and effectively control the overall quality of the product. Multi-index component quantitative control mode has been gradually applied to quality control of traditional Chinese medicine11.

The above studies are basically based on the quality evaluation or cluster analysis of a single index or active ingredient or some organic components on genuine medicinal materials. There are few studies on grey pattern recognition and cluster analysis from the multi-indexes such as trace elements, combustion heat, thermogravimetric analysis, fat, calcium, and energy of genuine medicinal materials. However, there are no relevant reports on the study of genuine medicinal materials stability by thermogravimetric method. Therefore, according to the thermogravimetric parameters, the entropy method is used to construct the combustibility (combustion stability of genuine medicinal materials) of Diding, Purslane, straw sandal board, June snow, pine vine rattan in different regions of China. The research on the combustion heat and thermogravimetric analysis of genuine medicinal materials has important theoretical and practical significance;the combustion heat, differential thermal gravimetric analysis, fat content, calcium content, trace elements content, and ash content of 5 kinds of genuine medicinal materials were used to build a systematic multi-index evaluation system by gray pattern recognition and grey correlation coefficient cluster analysis, which can make up for the gaps in this area and provide scientific basis and research significance for the study of genuine medicinal materials quality.

5 kinds of genuine medicinal materials, including Diding, Purslane, straw sandal board, June snow, pine vine rattan, were selected as the research objects. The combustion heat12,13,14,15,16,17 was measured by oxygen bomb calorimeter, the combustion stability18,19,20 of genuine medicinal materials was analyzed by thermogravimetric analysis, fat content was measured by fat analyzer21,22,23,24. According to the combustion heat, thermogravimetric parameters, and fat content, calcium content, trace element content, ash content, the multi-index comprehensive evaluation system25,26 of five kinds of genuine medicinal materials was established, and the entropy method and cluster analysis method were used to evaluate multiple indicators. The quality of genuine medicinal materials was evaluated by stoichiometric method from the aspect of multiple indicators, which provided a strong scientific basis for the large-scale development of genuine medicinal materials resources and the research of genuine medicinal materials classification.

Methods

5 kinds of materials, including wild Diding (Latin name: Corydalis bungeana Turcz), Purslane (Latin name: Portulaca oleracea L.), straw sandal board (Latin name: Hoya carnosa (L.f.) R. Br), June snow (Latin name: Serissa japonica (Thunb.) Thunb.), pine vine rattan (Latin name: Lycopodiastrum casuarinoides (Spring) Holub. [Lycopodium casuarinoides Spring]), were purchased from Laibin traditional Chinese medicine market in June 2020, in Guangxi, China, and were selected as the research objects and the analysis samples, all samples were sieved through 40 mesh pharmacopoeia. Repeat the test multiple times for each sample to reduce errors. The Diding plant was identified as dry whole grass of Papaveraceae (Fig. S1), and Portulaca oleracea plant was identified as dry whole herb of Portulaca oleracea L. (Fig. S2), and straw sandal board was the dry leaf of dicotyledonous plant medicine bulbus of Asclepiadaceae (Fig. S3), and June snow was a dry whole herb of Serissa serissoides (DC.) Druce of Rubiaceae (Fig. S4), and pine vine rattan was the dry stem of a plant of the Convolvulaceae fangchiaceae (Fig. S5). The above materials GKS20190600010, GKS20190600011, GKS20190600012, GKS20190600013 and GKS20190600014 were respectively identified from the source, eye observation, hand touch, nose smell, mouth taste, comparison, etc. by Prof. Caiyun Jiang of Guangxi Science & Technology Normal University and were stored in Guangxi Science & Technology Normal University. Figures S1–S5 are in Supplementary Materials.

Determination method of genuine medicinal material combustion heat

In this experiment, bomb calorimeter was used to determine the heat values under constant volume. The data processing formula is:

$$ {\text{mQv}} = {\text{W}}_{{{\text{cal}}}} \Delta {\text{T}} - {\text{Q}}_{{\text{ignition wire}}} \Delta {\text{m}} - {\text{ Q}}_{{{\text{capsule}}}} \,{\text{m}}_{{{\text{capsule}}}} . $$

In the formula: m, Qv, ∆T, Wcal, Qignition wire, ∆m respectively are the quality of the sample to be tested, the constant volume combustion heat, the temperature change before and after combustion, the calorimeter water equivalent, and the combustion heat of the ignition wire (Qignition wire = 1400.8 J/g), the actual mass of the ignition wire participating in the combustion reaction27.

Instruments and reagents

BH series combustion heat measurement experimental device, oxygen cylinder, oxygen meter, grinder, sheet press, ignition wire (nickel–chromium wire, Changsha Changxing Higher Education Instrument Equipment Co., Ltd.); electronic balance(model FA2004, Shanghai Shunyu Hengping Scientific Instrument Co., Ltd.), benzoic acid (AR, Tianjin KERMEL Chemical Reagent Co., Ltd.), medicinal capsules.

Thermogravimetric analysis method28,29,30

Thermogravimetric analysis is a technology to measure the relationship between the mass change of samples and temperature or time under a programmed temperature and a certain atmosphere. The temperature values on the curves are often used to compare the thermal stability of samples, which is the data basis for evaluating the combustibility of combustibles.

An appropriate amount of 2–10 mg samples were placed into an alumina crucible with a heating rate of 10 °C min−1, a reference compound of ɑ-Al2O3, N2 atmosphere (flow rate of 100 mL min−1), and a temperature range of 30–600 °C. TG, DTG and DTA analysis were performed at the same time. In order to reduce the experimental error, the experiment was repeated for each sample31.

Instruments and reagents

Thermogravimetric analyzer (Germany NETZSCH STA 2500), crucible.

Fat determination method

Soxhlet extraction, the fat content was determined by gravimetric method, that is, after the solvent of the extraction sample was extracted, the fat of the measured substance was extracted from the sample, dried and weighed, and calculated32,33,34,35. Wfat = m1/m2 × 100%, m1 is the fat mass (g), m2 is the sample mass (g).

Instruments and reagents

SE206 fat tester, analytical balance, filter paper, 100 mL beaker, drying oven, petroleum ether.

Determination method of calcium content

Calcium and aminocarboxylate can form metal complexes quantitatively, and its stability is stronger than that of calcium and indicator36,37,38,39,40,41,42,43. In the appropriate pH range, EDTA was titrated with ammonia-carboxylate complexing agent. When the measurement was reached, EDTA captured calcium ions in the indicator complex, making the solution present the color of free indicator (end point). According to the amount of EDTA complexing agent, calcium content can be calculated.

Instruments and reagents

Muffler furnace, electric furnace, crucible, electronic balance, alkaline titration tube, conical bottle (250 mL), capacity bottle (100 mL), 20% sodium hydroxide solution, 1:1 triethanolamine aqueous solution, calcium red indicator (1 g mixed with 99 g sodium chloride grinding), hydroxylamine hydrochloride (analytical purity), malachite green indicator, ethylenediamine tetraacetic acid disodium (EDTA, referred to as a certain mass plus distilled water, and then 25 mL 0.1 mol/mL calcium standard solution for titration).

Ash determination method

The residual inorganic matter in genuine medicinal materials after burning is called ash44,45,46,47,48.

Instruments and reagents

Muffle furnace, electric furnace, crucible, electronic balance.

Determination method of trace elements in 5 kinds of genuine medicinal materials

Determination method of trace elements content

In this experiment, 5 kinds of genuine medicinal materials, including Diding, Purslane, straw sandal board, June snow, pine vine rattan, in Guangxi, China, are all medicinal materials produced49,50,51,52,53,54,55. After sampling the uniform sample has been crushed with a crushing grinder, accurately weigh the sample 0.3 g (accurate to 0.0001 g) to 50 mL digestion container. Add aqua regia 8 mL and hydrogen peroxide 3 mL to the container. After standing overnight, put it into microwave digestion until the sample is completely digested, and the acid is removed. After cooling, it is filtered and the volume is fixed to a 50 mL volumetric flask, and the instrument is tested. The measurement of 12 kinds of trace elements is in accordance with GB/T 30903-2014. At the same time, the reagent blank experiment was performed, and the number of collection points and the number of repetitions were both 6 times.

Instruments and reagents

Agilent7700 ICP-MS (Agilent, USA), CEM MARS-6 microwave digestion instrument (CEM, USA), Milli-Q ultrapure water preparation system (Millipore, USA), AUY120 millionth electronic analytical balance (Shimadzu Company). Nitric acid, hydrogen peroxide (super pure, Guangzhou Chemical Reagent Co., Ltd.), self-made ultra-pure water.

Multi-index comprehensive evaluation method

According to the combustion heat, thermogravimetric parameters, and fat content, calcium content, trace element content, ash content, the multi-index entropy method and grey correlation coefficient cluster analysis of five kinds of genuine medicinal materials were constructed in Guangxi, China.

Statement

5 kinds of materials, including wild Diding (Latin name: Corydalis bungeana Turcz), Purslane (Latin name: Portulaca oleracea L.), straw sandal board (Latin name: Hoya carnosa (L.f.) R. Br), June snow (Latin name: Serissa japonica (Thunb.) Thunb.), pine vine rattan (Latin name: Lycopodiastrum casuarinoides (Spring) Holub. [Lycopodium casuarinoides Spring]), were purchased from Laibin traditional Chinese medicine market in June 2020, in Guangxi, China, and were selected as the research objects and the analysis samples, all samples were sieved through 40 mesh pharmacopoeia. Repeat the test multiple times for each sample to reduce errors. The Diding plant was identified as dry whole grass of Papaveraceae, and Portulaca oleracea plant was identified as dry whole herb of Portulaca oleracea L., and straw sandal board was the dry leaf of dicotyledonous plant medicine bulbus of Asclepiadaceae, and June snow was a dry whole herb of Serissa serissoides (DC.) Druce of Rubiaceae, and pine vine rattan was the dry stem of a plant of the Convolvulaceae fangchiaceae. The above specimens GKS20190600010, GKS20190600011, GKS20190600012, GKS20190600013 and GKS20190600014 were respectively identified by Prof. Caiyun Jiang of Guangxi Science & Technology Normal University, and were stored in Guangxi Science & Technology Normal University.

Studies comply with relevant institutional, national, and international guidelines and legislation, local and national regulations.

Results

Calculation of combustion heat of five kinds of genuine medicinal materials

  1. (1)

    Sample name: the first group of experimental samples of Diding. According to the experimental data, ∆T Curve of Reynolds temperature of Diding is shown in Fig. 1. The experiment is repeated three times. Figure 1 shows ΔT Curve of Reynolds temperature. According to the calculation,

    $$ \Delta {\text{m}}_{\text{Diding}} = 0.0768\;{\text{g}},\;{\text{W}}_{\text{cal}} = 35054.24\;{\text{J}}/^\circ {\text{C}},\;Q_{\text{capsule}} = 60406.893\;{\text{J/g}},\;{\text{m}}_{\text{capsule}} = 0.0983\;g,\;\Delta {\text{T}} = 0.298\;^\circ {\text{C}}, $$

    ∆mignition wire = 0.0095 g, according to, ∆mDiding Qv = Wcal ∆T−Qignition wire ∆mignition wire−Qcapsule mcapsule, QDiding = 58526.79 J/g. The average of QDiding is 56915.503 J/g.

  2. (2)

    In the same way, determine the combustion heat of Purslane, straw sandal board, June snow, pine vine rattan, and repeat the test for three times.The heat of combustion of 5 kinds of genuine medicinal materials, is shown in Table 1. According to Table 1, the order of combustion heat of 5 kinds of genuine medicinal materials, including Diding, Purslane, straw sandal board, June snow, pine vine rattan, was Diding > June snow > straw sandal board > Purslane > pine vine rattan. The combustion heat of the 5 kinds of genuine medicinal material test samples has ranged from 49,779.54 to 56,915.503 J/g, CV% < 3.65%.The combustion heat of Diding is 56,915.503 J/g, and the energy is the highest. The combustion heat of pine vine rattan is 49,779.548 J/g, and the energy is relatively small. Combustion heat is regarded as an important physical data to measure genuine medicinal material energy.

Figure 1
figure 1

∆T curve of Reynolds temperature of Diding.

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Table 1 Combustion heat of 5 genuine medicinal materials (n = 3).
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Thermo gravimetric analysis

Thermo gravimetric analysis results

Thermogravimetric analysis of Diding

The thermal gravimetric data of Diding are shown in Fig. 2 and Table 2. It can be seen from Fig. 2 that the sample begins to decompose at 34.8 °C, which may be due to the thermal desorption of the residual small molecular substances in the sample, resulting in a small amount of mass loss of the sample, with a loss rate of 7.07%; After heating for a period of time, the temperature reaches 184.1 °C and enters the second stage of decomposition. A large amount of mass loss begins to appear in the sample until 404.05 °C, and the loss rate is 37.64%; then, with the continuous increase of temperature, the sample is further decomposed, and the mass of the remaining sample is 43.83%.

Figure 2
figure 2

Thermogravimetric (TG %) curve, derivative thermogravimetric (DTG) curve and differential thermal analysis (DTA) curve of Diding.

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Table 2 Thermal analysis data of Diding.
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With the increase of temperature, the DTG curve shows two peak shapes, and the inflection points of the peak shapes are 76.2 °C and 316.8 °C respectively. In addition, the DTA curve of Diding has an exothermic peak, with a peak value of 119.6 °C, a temperature range of 91.5–153.1 °C, and a peak area of 32 J/g; there is a smaller endothermic peak, with a peak value of 282.9 °C, and the temperature range is 252.8–308.6 °C, the peak area is 15.58 J/g.

Thermogravimetric analysis of Purslane

The thermal gravimetric data of Purslane are shown in Fig. 3 and Table 3. It can be seen from Fig. 3 that the sample begins to decompose at 31.3 °C, which may be due to the thermal desorption of the residual small molecular substances in the sample, resulting in a small amount of mass loss of the sample, with a loss rate of 4.25%; After heating for a period of time, the temperature reaches 146.9 °C, entering the second stage of decomposition, the sample begins to have a large mass loss until 279.7 °C, and the loss rate is 23.55%; As the temperature continues to rise, it enters the third stage of decomposition until 414.9 °C, the loss rate is 27.48%, the temperature continues to rise, the sample continues to decompose, and finally the mass of the remaining sample is 34.69%.

Figure 3
figure 3

Thermogravimetric (TG %) curve, derivative thermogravimetric (DTG) curve and differential thermal analysis (DTA) curve of Purslane.

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Table 3 Thermal analysis data of Purslane.
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With the increase of temperature, the DTG curve shows three peaks, and the inflection points of the peaks are 94.1 °C, 243.5 °C and 312.7 °C respectively. In addition, the DTA curve of Purslane has three exothermic peaks, the first peak is 107.6 °C, the temperature range is 83.1–145.0 °C, and the peak area is 23.87 J/g; The second peak is 186.4 °C, the temperature range is 171.4–207.3 °C, and the peak area is 8.054 J/g; The third peak is 228.6 °C, the temperature range is 211.2–242.8 °C, and the peak area is 19.21 J/g.

Thermogravimetric analysis of straw sandal board

The thermal gravimetric data of straw sandal board are shown in Fig. 4 and Table 4. It can be seen from Fig. 4 that the sample begins to decompose at 60.8 °C. This may be due to the thermal desorption of the remaining small molecules in the sample, causing a small amount of mass loss in the sample, with a loss rate of 4.77%; after a period of time, When the temperature is increased, the temperature reaches 192.8 °C and enters the second stage of decomposition. The sample begins to show a large mass loss until 281.4 °C, and the loss rate is 14.65%; as the temperature continues to rise, it enters the third stage of decomposition until 410.6 °C, the loss rate is 28.35%, the temperature continues to rise, the sample continues to decompose, and finally the remaining sample mass is 43.18%.

Figure 4
figure 4

Thermogravimetric (TG %) curve, derivative thermogravimetric (DTG) curve and differential thermal analysis (DTA) curve of straw sandal board.

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Table 4 Thermal analysis data of straw sandal board.
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With the increase of temperature, the DTG curve of straw sandal board shows three peak shapes, and the inflection points of the peak shapes are 98.9 °C, 243.5 °C and 329.6 °C respectively. In addition, the DTA curve has an exothermic peak, with a peak value of 125.3 °C, a temperature range of 100.3–152.3 °C, and a peak area of 21.25 J/g; there is an endothermic peak with a peak value of 265.7 °C and a temperature range of 225.3–302.0 °C, the peak area is 30.28 J/g.

Thermo gravimetric analysis of June snow

The thermal gravimetric data of June snow are shown in Fig. 5, and Table 5. It can be seen from Fig. 5 that the sample begins to decompose at 42.6 °C, which may be due to the thermal desorption of the residual small molecular substances in the sample, resulting in a small amount of mass loss of the sample, with a loss rate of 5.21%; After heating for a period of time, the temperature reaches 170.5 °C and enters the second stage of decomposition until 278.7 °C, and the loss rate is 21.91%; As the temperature continues to rise and enters the third stage of decomposition, the sample begins to have a large mass loss until 387.5 °C, the loss rate is 33.23%, the temperature continues to rise, the sample continues to decompose, and finally the mass of the remaining sample is 26.25%.

Figure 5
figure 5

Thermogravimetric (TG %) curve, derivative thermogravimetric (DTG) curve and differential thermal analysis (DTA) curve of June snow.

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Table 5 Thermal analysis data of June snow.
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With the increase of temperature, the DTG curve of June snow presents three peaks, and the inflection points of the peaks are 105.0 °C, 243.9 °C and 335 °C respectively. In addition, the DTA curve has three exothermic peaks, the first peak is 122.3 °C, the temperature range is 92.2–167.2 °C, and the peak area is 54.72 J/g; The second peak value is 218.4 °C, the temperature range is 188.3–256.2 °C, and the peak area is 30.92 J/g; The third peak is 312.9 °C, the temperature range is 286.4–340.3 °C, and the peak area is 8.942 J/g.

Thermo gravimetric analysis of pine vine rattan

The thermal gravimetric data of pine vine rattan are shown in Fig. 6 and Table 6. It can be seen from Fig. 6 that the sample begins to decompose at 46.9 °C. This may be due to the thermal desorption of the remaining small molecules in the sample, causing a small amount of mass loss in the sample, with a loss rate of 6.85%; after a period of heating up, the temperature reaches 180.3 °C, and enter the second stage of decomposition, the sample begins to show a large amount of mass loss, until 422.4 °C, the loss rate is 50.81%; the temperature continues to rise, the sample further decomposes, and the remaining sample mass is 30.59%.

Figure 6
figure 6

Thermo gravimetric (TG %) curve, derivative thermo gravimetric (DTG) curve and differential thermal analysis (DTA) curve of pine vine rattan.

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Table 6 Thermal analysis data of pine vine rattan.
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With the increase of temperature, the DTG curve of pine vine rattan shows two peak shapes, and the inflection points of the peak shapes are 89.6 °C and 308.8 °C respectively. In addition, the DTA curve has an exothermic peak, with a peak value of 116.8 °C, a temperature range of 90.9–161.7 °C, and a peak area of 31.43 J/g; there is a larger endothermic peak with the peak value of 302.6 °C and a temperature range of 259.8–364.5 °C, and the peak area is 82.84 J/g.

Combustion stability analysis of 5 kinds of genuine medicinal materials

The thermogravimetric parameters, including index X1 the first stage weight loss percentage, index X2 the fastest temperature in the first stage of weight loss, index X3 the weight loss percentage in the second stage, index X4 the fastest temperature in the second stage of weight loss, index X5 the percentage of weight loss in the third stage, index X6 the remaining mass percentage, index X7 the peak area of the first stage and index X8 the second stage peak area, were used to build combustion stability of genuine medicinal materials through gray pattern recognition. The thermal gravimetric parameter data of 5 kinds of genuine medicinal materials were shown in Table S1. Table S1 is in Supplementary Materials. Based on the thermogravimetric parameter data of five genuine medicinal materials, a multi-index evaluation system for combustion stability was established. Thermo gravimetry is to study the combustion characteristic index of genuine medicinal particles at different heating rates by thermo gravimetry analyzer to judge the combustion stability of genuine medicinal materials. According to the method of grey pattern recognition56, this subject calculates the correlation coefficient between each scheme and the ideal scheme composed of the best indicators, obtains the correlation degree from the correlation coefficient, and then sorts and analyzes it according to the correlation degree to draw a conclusion. The greater the correlation degree Z is, the better the sample effect is. Finally, compare all the Z values to draw the evaluation conclusion. Calculated by EXCEL, the Z values of 5 kinds of genuine medicinal materials, including Diding, Purslane, straw sandal board, June snow and pine vine rattan, are 0.7199, 0.7050, 0.7373, 0.8014 and 0.7749 respectively. From the analysis of thermogravimetric analysis results and thermogravimetric combustion stability, the order of combustion stability of 5 kinds of genuine medicinal materials was June snow > pine Vine rattan > straw sandal board > Diding > Portulaca oleracea.

Determination of fat, calcium, ash content

The determination results of fat, calcium content, ash content of 5 kinds of genuine medicinal materials were shown in Table 7. Table 7 showed, the order of fat content (%) of 5 kinds of genuine medicinal materials, including Diding, Purslane, straw sandal board, June snow, pine vine rattan, was straw sandal board > Diding > pine vine rattan > June snow > Purslane, the order of calcium content (%) was pine vine rattan > June snow > Purslane > straw sandal board > Diding, the order of ash content was June snow > Purslane > straw sandal board > pine vine rattan > Diding. The energy value of genuine medicinal materials can also be reflected by the combustion heat, fat content to a certain extent. The contents of ash, fat, and calcium are regarded as important physical data to measure the quality of genuine medicinal materials. The quality of genuine medicinal materials is evaluated from the aspect of energy, which provides a strong scientific basis for the classification of genuine medicinal materials.

Table 7 Determination results of fat, calcium, ash content in 5 kinds of genuine medicinal materials (n = 3, CV% < 2.0%).
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Determination of trace elements

Determination results of trace elements

A method for the determination of 12 trace elements, including Mn, Mg, Fe, Co, Zn, Cu, Ni, Se, Sn, As, Li and Mo in 5 genuine medicinal materials, was established by inductively coupled plasma mass spectrometry (ICP-MS) based on microwave digestion57,58,59,60,61. Average value and standard deviation of 12 trace elements in 5 kinds of genuine medicinal materials, including Diding, Purslane, straw sandal board, June snow, pine vine rattan, were shown in Table 8.

Table 8 Average values ± standard deviation of 12 trace elements in 5 kinds of genuine medicinal materials (μg/g, n = 6).
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Grey factor analysis of 12 trace elements in 5 kinds of genuine medicinal materials62,63,64,65,66

Through grey factor analysis, the characteristic roots of grey factor correlation coefficient matrix and variance contribution rate of trace elements, including Mn, Mg, Fe, Co, Zn, Cu, Ni, Se, Sn, as, Li and Mo, are obtained, as shown in Table 9. According to Table 9, the cumulative contribution rate of the main factors of the first three grey factors reaches 93.403%, and the eigenvalues of the main factors of the first three grey correlation coefficient factors (λ > 1) are larger, that is, the main factors of the first three grey factors contribute the most to the explanatory variables. It is most appropriate to extract the main factors of the first three grey correlation coefficient factors, which represents 93.403% of the information of 12 trace elements in the five genuine medicinal materials in Guangxi, China.

Table 9 Grey factor coefficient characteristic root and variance contribution rate of 12 trace elements in 5 kinds of genuine medicinal materials.
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The gray correlation coefficient factor load matrix after rotation is shown in Table 10. It can be seen from Table 10 that the first main factor F1 of the gray correlation coefficient mainly contains the original variables As, Co, Cu, Fe, Mn, Ni, Se trace element information necessary for the human body. The second main factor F2 of the gray correlation coefficient mainly contains the information of the original variables Li, Mg and Mo. The third main factor F of the gray correlation coefficient mainly contains the information of the original variables Zn and Sn.

Table 10 The rotated grey correlation coefficient factor loading matrix of 12 trace elements in 5 kinds of genuine medicinal materials.
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The gray correlation coefficient factor score and the comprehensive gray correlation coefficient factor score are shown in Table 11. As can be seen from Table 11, the order of the content of 12 trace elements in 5 kinds of genuine medicinal materials, including Diding, Purslane, straw sandal board, June snow, pine vine rattan, is June snow > straw sandal board > Diding > Purslane > pine vine rattan. In terms of trace element content, June snow contains the highest trace elements in all samples.

Table 11 Grey correlation coefficient factors and comprehensive factor scores of 12 trace elements in 5 kinds of genuine medicinal materials.
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Discussion

Construction of multi-index comprehensive evaluation system for 5 kinds of genuine medicinal materials

Construction of multi-index analysis and comprehensive evaluation system by entropy analysis

According to the combustion heat, thermogravimetric parameters, fat content, calcium content, trace element content and ash content, the multi-index comprehensive evaluation systems of five kinds of genuine medicinal materials including Diding, Purslane, straw sandal board, June snow, pine vine rattan were established by entropy method67,68,69,70,71,72.

According to the characteristics of entropy, this paper judges the randomness and disorder degree of an event by calculating the entropy value, and judges the dispersion degree of an index by using the entropy value73,74,75,76. The greater the dispersion degree of the index is, the greater the influence (weight) of the index on the comprehensive evaluation is, and the smaller the entropy value is. Using entropy method, 5 kinds of genuine medicinal materials were weighted to calculate the comprehensive score S.

  1. (1)

    Standardized treatment,\({\text{y}}^{\prime}_{{{\text{ij}}}} { = }\frac{{x_{{{\text{ij}}}} - x_{j\min } }}{{x_{j\max } - x_{j\min } }}\), where \(y^{\prime}_{ij}\)( i = 1,2,…, n ; j = 1,2,…, m ) is the j index value of the i sample after dimensionless treatment, the original data of the j index of the i sample is the maximum value of the j index and the minimum value of the j index.

  2. (2)

    Calculation of the proportion of sample i under indicator j \({\text{P}}_{ij}\)\((0 \le P_{ij} \le 1)\)\(P_{ij} = \frac{{y^{\prime}_{ij} }}{{\sum\nolimits_{i = 1}^{{\text{n}}} {y^{\prime}_{ij} } }}.\)

  3. (3)

    Information entropy value e and information utility value d, information entropy value of item j is \(e_{j} = - \frac{1}{\ln m}\sum\nolimits_{i = 1}^{{\text{n}}} {P_{ij} } {\text{lnP}}_{ij}\) Information utility value \(d_{j} = 1 - e_{j} .\)

  4. (4)

    Weight of evaluation indicators. The greater the information utility value is, indicating that the more important the indicators, the greater the importance of evaluation is. Finally, the weight of the j index is \(W_{j} = \frac{{d_{j} }}{{\sum\nolimits_{j = 1}^{{\text{m}}} {d_{j} } }}\)

  5. (5)

    Comprehensive evaluation \(S = \sum\nolimits_{j = 1}^{m} {(W_{j} P_{ij} } ).\)

The weighted summation formula is used to calculate the evaluation value of the sample. The larger the comprehensive score S is, the better the sample effect is. Finally, compare all S values, that is, draw the evaluation conclusion. Using EXCEL calculation, the S values of Diding, Purslane, straw sandal board, June snow, pine vine rattan were 0.3764, 0.2777, 0.4876, 0.5744 and 0.2688 respectively.

According to combustion heat, combustibility (combustion stability of genuine medicinal materials), fat, calcium, ash, trace element content, the comprehensive evaluation results of multi-index analysis constructed by entropy analysis showed that the comprehensive evaluation multi-index order of 5 genuine medicinal materials, including Diding, Purslane, straw sandal board, June snow and pine vine rattan, was June snow > straw sandal board > Diding > Purslane > pine vine rattan. Therefore, the comprehensive evaluation results of the quality of genuine medicinal materials selected in this study were June snow the best, followed by straw sandal board.

Construction of multi-index analysis and comprehensive evaluation system by grey correlation coefficient cluster analysis

The gray correlation coefficient cluster analysis is based on the many properties of the sample, and the cluster analysis diagram is obtained from the correlation coefficient77,78,79,80. According to the literature81,82, the classification is carried out according to the degree of affinity of the nature of the sample. All cases are classified into different classes, making the same class Individuals in different classes have greater similarities, and individuals in different classes have greater differences. The multi-index comprehensive cluster analysis system of combustion heat, combustibility (combustion stability of genuine medicinal materials), fat content, calcium, ash and trace element content of 5 genuine medicinal materials in Guangxi was established. The gray correlation coefficient cluster analysis tree diagram was shown in Fig. 7. As can be seen from Fig. 7, 5 kinds of genuine medicinal materials from different producing areas, namely Diding, Purslane, straw sandal board, June snow and pine vine rattan, were divided into three categories according to the results of grey correlation coefficient cluster analysis. Straw sandal board, June snow were a class, Diding for one class, and Purslane and pine vine rattan for a class. Through the grey correlation coefficient cluster analysis, we found the similarity degree and genetic relationship between the properties of genuine medicinal materials from different origins, which can help better study the classification of genuine medicinal materials.

Figure 7
figure 7

Tree diagram of grey correlation coefficient cluster analysis of multiple indexes of 5 kinds of genuine medicinal materials.

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Conclusion

Thermogravimetric parameters were applied to the evaluation of the combustion stability of genuine medicinal materials through gray pattern recognition, which provided a strong scientific basis for the evaluation and research of the combustion stability of genuine medicinal materials by thermogravimetric analysis.

A method for the determination of 12 trace elements, including Mn, Mg, Fe, Co, Zn, Cu, Ni, Se, Sn, As, Li and Mo in 5 genuine medicinal materials, was established by inductively coupled plasma mass spectrometry (ICP-MS) based on microwave digestion, and the data were comprehensively analyzed by the grey factor analysis method. From the content of trace elements, the trace elements contained in Junxue were the highest in all samples. These studies are for scientific research on trace elements and biological activities in medicinal materials, for the rational development of the quality control of Chinese medicinal materials in the standardized planting base of Chinese medicinal materials in Guangxi, and to provide reference for the medical and health care of Chinese medicinal materials.

In this paper, according to combustion heat, differential thermal-thermogravimetric analysis, fat content, calcium and ash content, trace element content data of genuine medicinal materials, systematic multi-index comprehensive evaluation systems were constructed through gray pattern recognition, gray factor analysis, entropy method and gray correlation coefficient cluster analysis. As long as the various parameters like combustion heat, thermogravimetric parameters, and fat content, calcium content, trace element content, ash content, etc. are measured, systematic multi-index comprehensive evaluation systems are constructed through gray pattern recognition, gray factor analysis, entropy method and gray correlation coefficient cluster analysis, and then can be utilized for the identification of genuine materials. This research has important theoretical and practical significance for the multi-index measurement and comprehensive evaluation of genuine medicinal materials, and can provide scientific basis and research significance for the research of multi-index quality control of genuine medicinal material.

The multi-index comprehensive evaluation system established in this study provides a new idea for the quantitative control of the quality of genuine medicinal materials, and provides a powerful way for the large-scale development and classification research of genuine medicinal materials and provides basic support for the selection of raw materials of genuine medicinal materials and the application of quantitative control mode of multi-index ingredients to the quality control of genuine medicinal materials.