Multivariate analyses of major and trace elements in 19 species of herbs consumed in Yunnan, China

ABSTRACT

This study aimed to analyze the concentrations of 10 elements in 19 species of herbs related to medical and edible purpose in Yunnan, China. Microwave-assisted acid digestion was used for all of the dried herbs and element contents were determined by inductively coupled plasma atomic emission spectrometry. The accuracy of this method was validated by analyzing GBW07605 certified reference material. The results indicated that the distributions of element contents were varied over a wide range in the specimens tested. The decreasing sequence of average element content expressed as dry weight was presented as follows: calcium (1740–22,246 µg/g dry weight), magnesium (634–6367 µg/g dry weight), iron (52.8–5707 µg/g dry weight), barium (9.19–465 µg/g dry weight), zinc (10.7–82.1 µg/g dry weight), strontium (8.25–69.8 µg/g dry weight), copper (4.10–36.6 µg/g dry weight), chromium (0.26–13.3 µg/g dry weight), nickel (0.57–14.7 µg/g dry weight), and cadmium (0.11–2.66 µg/g dry weight). The element contents of samples were different depending on several species. However, the accumulations of toxic elements (cadmium, chromium, and nickel) were above the international safety standards limit in most samples. Principal component analysis generated three principal components that explained 77% of the total variance in the data. Similar samples may get together by cluster analysis and could correspond to the result of principal component analysis.

KEYWORDS:

• Microwave digestion
• Elemental content
• Principal component analysis
• Cluster analysis
• Correlation analysis

Introduction

Herbs are inextricably intertwined with the abundant history, culture, and cuisine. They are used to relieve and treat many human diseases in China and in fact worldwide. The effect of herbs include simple diseases like insomnia, dizziness and headache (Gastrodia elata), cough and fever (Polygonatum odoratum), and more complicated problems like diabetes (Polygonatum curvistylim), bronchitis, diarrhea, enteritis, and hypertension (Plantago major).^[1] In addition, herbs are being consumed in daily diets to give flavor to a range of dishes. For example, Fallopia multiflora (100 g), black soybean (100 g), and spare ribs (1000 g) are cooked with boiling water and seasonings (salt, cooking wine) in a casserole. The root and leaf of Houttuynia cordata can be used for cooking (sauteed eggs or shredded meat using wok which is a Chinese type pan) or making cool dishes (only mixed with salt, vinegar, sugar, chicken powder, chili oil, chili, and pepper). Therefore, it is meaningful to determine their element compositions so that their effect on human health can be clearly understood.

Different parts of herbs contain wide range of elemental contents.^[2] Elements play significant positive or negative roles in human.^[3] More than 20 elements have known important physiological activities in humans and other mammals. Some of these elements, including calcium and magnesium are major elements which are required for the body. Elements such as chromium, copper, and zinc are trace elements belonging to the category of micro-nutrients, which are needed by the human body in small quantities, generally less than 100 mg/day.^[4] Some trace elements are essential components of enzyme systems and biological structures, but can also be toxic at concentrations beyond the limits necessary for their functions.^[5,6] So it is meaningful to study content of major and trace elements in Chinese herbal medicine for understanding their nutritive importance.

The major and trace element contents have been determined in spices, cereals, and vegetables and also in herbs in several previous publications.^[7–9] Some of these studies compared the concentration of one element with another or compared one herb with another. Furthermore, multivariate analyses are used in many studies. Gong et al.^[10] classified 32 propolis samples into five groups by principal component analysis (PCA). Kolasani et al.^[11] disclosed two significant groups of 50 Chinese medicinal herbs based on their elemental concentrations with chemometric techniques. Up to now, the studies of the elements in herbs in Yunnan province, Southwest China are limited. Therefore, it is meaningful to use multivariate analyses to reveal the relationships of elemental concentration of medicinal herbs in Yunnan.

In this study, we have analyzed 19 herbal species commonly used in indigenous systems of medicine and Chinese diet. These herbs were analyzed for 10 elements namely barium, calcium, cadmium, chromium, copper, iron, magnesium, nickel, strontium, and zinc by inductively coupled plasma atomic emission spectrometry (ICP-AES). ICP-AES method was used because of its low cost, rapid analysis, wide linear range, low detection limit, and simultaneous determination of multi-element. Data analysis was done by common chemometrics such as correlation analysis, PCA, and cluster analysis.

Materials and methods

Reagents and standards

For the experimental work, nitric acid, hydrogen peroxide, ultra-pure water, and a mixed calibration curve were used. Nitric acid was used as guaranteed reagent and hydrogen peroxide was of analytical reagent grade. Barium, calcium, cadmium, chromium, copper, iron, magnesium, nickel, strontium, and zinc (Standard Material Center of China) were used for the preparation of standard solution and the stock solution was diluted to 0–400 µg/mL. All vessels and glassware were soaked with 30% HNO₃ at least 24 h and then rinsed with ultra-pure water prior.

Apparatus

Microwave dissolver (Ethos One, Milestone, Italy) with teflon reaction vessels was used in this work. Barium, calcium, cadmium, chromium, copper, iron, magnesium, nickel, strontium, and zinc contents in herbs were determined by ICP-AES (ICPE-9000, Shimazu, Japan) which generates a high-temperature plasma by ionizing argon using a high frequency (27.12 MHz). The instrumental conditions are listed in Table 1.

Table 1. Operating conditions of ICP-AES for herbal samples.

Instrument	ICPE-9000
Spectrometer	Echelle grating
	CCD detector
Power	1.20 kW
Output power	1.2 kW
Argon flow	Cooling gas: 10 L/min
	Auxiliary: 0.6 L/min
	Carrier gas: 0.8 L/min
Nebuliser	Pneumatic (glass concentric)
Spray chamber	Glass cyclonic
Plasma viewing	Axial
Replicates for each analysis run	3
Sample uptake delay	30 s
Elements monitored (wavelength, nm)	barium (230.424), calcium (220.861), cadmium (214.438), chromium (267.716), copper (224.700), iron (234.349), magnesium (383.826), nickel (231.604), strontium (216.596), and zinc (213.856)

Sample preparation

In this study, 19 species of medicinal herbs were collected from Yunnan, China during 2012. The information of herbal samples, part used and their medicinal uses are given in Table 2. These herbs were washed in tap water in the field and rinsed with deionized water thoroughly in the laboratory. Samples are picked from each medicinal part. Each sample consisted of 10 randomly selected individual herbs. All samples were dried at 60°C to a constant weight and ground into powder using an agate mortar.

Table 2 List of herbal samples and part used and their uses.

Sample code	Pharmaceutical name	Local name	Part use	Medicinal uses
No. 1	Panax ginseng C. A. Mey.	Ren Shen	Root	Strengthen the immunity and tranquillization
No. 2	Panax stipuleanatus Tsai et Feng	Ping Bian San Qi	Rhizome	Eliminate blood stasis and relieve pain
No. 3	Epimedium acuminatum Franch.	Yin Yang Huo	Whole plant	Rheumatic arthritis, enhance libido and sexual performance
No. 4	Codonopsis pilosula (Franch.) Nannf.	Dang Shen	Root	Heart disease and cough
No. 5	Lonicera japonica Thunb	Jin Yin Hua	Flower	Infection diseases, epidemic febrile diseases, sores, and carbuncles
No. 6	Taraxacum mongolicum Hand.-Mazz.	Pu Gong Ying	Whole plant	Clearing heat-toxicity, cold, gastritis, hepatitis, and cholecystitis
No. 7	Lepidium meyenii Walp	Ma Ka	Rhizome	Anti fatigue, enhance memory, improve sleep and sexual performances
No. 8	Gynostemma pentaphyllum (Thunb.) Makino	Jiao Gu Lan	Whole plant	Antitumor and antioxidant activities
No. 9	Perilla frutescens (L.) Britt.	Zi Su	Leaf	Moistening lntestinal, tocolysis, and relieve pain
No. 10	Salvia miltiorrhiza Bunge	Dan Shen	Root	Irregular menstruation, analgesic, sedative, and cirrhosis
No. 11	Lilium brownie var. viridulum	Bai He	Bulb	Dizzy and moisten lung for arresting cough
No. 12	Polygonatum curvistylim Hua	Huang Jing	Tuber	Moisten lung, diabetes, and weakness of the spleen and the stomach
No. 13	Polygonatum odoratum (Mill.) Druce	Yu Zhu	Rhizome	Clear heat and cough
No. 14	Gastrodia elata Bl.	Tian Ma	Tuber	Hypnotic, anti-inflammatory, dizzy, and headache
No. 15	Plantago major L.	Da Che Qian	Whole plant	Cough, cold, bronchitis, enteritis, diarrhea, and hypertension
No. 16	Fagopyrum dibotrys (D. Don) Hara	Jin Qiao Mai	Root tuber	Sore throat, pneumonia, hepatitis, indigestion, and amenorrhea
No. 17	Fallopia multiflora (Thunb.) Harald.	He Shou Wu	Tuber	Acetanilide, hemostatic, hepatitis, insomnia, and soreness and weakness of waist and knees
No. 18	Houttuynia cordata Thunb.	Yu Xing Cao	Leaf	Relieve fever, resolve toxin, reduce swell, drain pus, and promote urination
No. 19	Angelica sinensis (Oliv.) Diels	Dang Gui	Root	Anemia, heart and liver disease, cough, insomnia, and analgesic

Powdered samples (0.5000 ± 0.0005 g) were weighed accurately into Polytetrafluoroethylene (PTFE) vessels, and then 2 mL H₂O₂, 8.0 mL of concentrated HNO₃ and 1 mL ultra-pure were added into the vessels. The vessels were closed and placed on the rotating turntable of the microwave oven, and then the digestion process was started. After microwave digestion, the digested solutions were diluted to 25 mL with ultra-pure water.

Quality assurance

In order to validate the accuracy and precision of the method, GBW07605 tea certified reference material was used as reference materials to assess the experimental procedures (Table 3). The certified reference material analysis and blank experiments were prepared by using the method mentioned above and measured in triplicate. The results obtained were in good agreement with certified values. The relative standard deviation (RSD%) values for the studied elements were found to be <13% and the obtained recovery values range from 86 to 118%.

Table 3. Assessment of the accuracy and precision by certified reference material, GBW07605 (n = 3).

	Concentration, Mean ± SD (µg/g)
Element	Determined value	Certified value	Recovery (%)
Barium	52 ± 0.0054	58 ± 6	90
Calcium	4081 ± 0.293	4300 ± 400	95
Cadmium	0.05 ± 0.0003	0.057 ± 0.01	88
Chromium	0.8 ± 0.0004	0.8 ± 0.03	100
Copper	16.4 ± 0.0039	17.3 ± 1.8	95
Iron	265 ± 0.0217	264 ± 15	101
Magnesium	1608 ± 0.164	1700 ± 200	95
Nickel	4.2 ± 0.0003	4.6 ± 0.5	92
Strontium	14.8 ± 0.0008	15.2 ± 0.7	97
Zinc	27.6 ± 0.0034	26.3 ± 2	105

Results and discussion

The mean concentrations of elements in analyzed samples are given in Table 4. According to the results, the major part of elements in herbs consists of calcium, followed by magnesium and other elements such as iron, zinc, copper, and nickel. The concentrations of elements were varied over a wide range in the samples studied. This might be due to the composition of the herbal medicine such as chemical form of the element, species of herbs, places of origin, and preservation.

Table 4. Statistics of average element concentrations (n = 3) of herbal samples (mean ± SD µg/g).

Sample code	Calcium	Iron	Magnesium	Nickel	Zinc	Barium	Cadmium	Chromium	Copper	Strontium
No. 1	4836 ± 0.18	2273 ± 0.05	2999 ± 0.09	3.05 ± 0.0004	19.0 ± 0.0010	32.5 ± 0.000	0.38 ± 0.0011	5.72 ± 0.0007	7.32 ± 0.003	31.8 ± 0.002
No. 2	5539 ± 0.26	5707 ± 0.22	1998 ± 0.15	14.7 ± 0.0022	36.6 ± 0.0035	148 ± 0.011	0.69 ± 0.0013	5.81 ± 0.0009	36.6 ± 0.004	45.7 ± 0.005
No. 3	7142 ± 1.56	1451 ± 0.30	1564 ± 0.42	2.60 ± 0.0004	82.1 ± 0.0187	17.0 ± 0.004	2.23 ± 0.0006	5.60 ± 0.0015	27.3 ± 0.009	12.2 ± 0.002
No. 4	3359 ± 0.13	1663 ± 0.06	2310 ± 0.32	3.27 ± 0.0006	35.1 ± 0.0013	465 ± 0.044	0.39 ± 0.0006	4.74 ± 0.0007	7.03 ± 0.006	69.8 ± 0.003
No. 5	4689 ± 0.20	775 ± 0.03	2692 ± 0.28	3.11 ± 0.0003	24.1 ± 0.0018	18.2 ± 0.001	0.83 ± 0.0006	3.95 ± 0.0005	9.4 ± 0.004	16.2 ± 0.001
No. 6	22246 ± 0.99	3418 ± 0.11	3634 ± 0.25	6.90 ± 0.0006	44.1 ± 0.0035	63.2 ± 0.002	1.39 ± 0.0013	13.3 ± 0.0005	15.9 ± 0.004	42.1 ± 0.001
No. 7	2539 ± 0.63	183 ± 0.05	823 ± 0.32	0.63 ± 0.0009	18.6 ± 0.0051	9.19 ± 0.003	0.35 ± 0.0007	nd	15.0 ± 0.007	8.25 ± 0.001
No. 8	14693 ± 0.93	2862 ± 0.19	4483 ± 0.33	5.58 ± 0.0011	41.7 ± 0.0047	177 ± 0.010	2.66 ± 0.0010	8.45 ± 0.0005	10.7 ± 0.003	60.7 ± 0.005
No. 9	14474 ± 1.18	1537 ± 0.12	5160 ± 0.13	4.81 ± 0.0005	38.9 ± 0.0014	45.4 ± 0.003	0.49 ± 0.0004	11.0 ± 0.0001	12.1 ± 0.002	53.8 ± 0.003
No. 10	3617 ± 0.15	1078 ± 0.07	6367 ± 0.08	1.24 ± 0.0007	30.6 ± 0.0014	17.0 ± 0.002	0.49 ± 0.0011	3.35 ± 0.0011	13.3 ± 0.004	12.8 ± 0.001
No. 11	3626 ± 0.11	510 ± 0.01	961 ± 0.06	1.73 ± 0.0005	40.3 ± 0.0007	16.6 ± 0.001	0.75 ± 0.0007	1.75 ± 0.0008	17.6 ± 0.002	8.57 ± 0.001
No. 12	3844 ± 0.33	673 ± 0.03	989 ± 0.06	1.23 ± 0.0009	20.3 ± 0.0013	14.0 ± 0.002	1.05 ± 0.0003	1.65 ± 0.0004	11.3 ± 0.007	11.2 ± 0.001
No. 13	6992 ± 0.39	701 ± 0.03	2769 ± 0.16	1.9 ± 0.0007	46.0 ± 0.0002	23.2 ± 0.001	0.15 ± 0.0007	nd	13.3 ± 0.005	12.6 ± 0.002
No. 14	1740 ± 0.22	52.8 ± 0.01	634 ± 0.03	2.00 ± 0.0007	25.8 ± 0.0004	15.4 ± 0.001	0.93 ± 0.0010	0.26 ± 0.0025	6.70 ± 0.000	8.86 ± 0.001
No. 15	21382 ± 0.91	3184 ± 0.09	3336 ± 0.08	4.39 ± 0.0015	36.7 ± 0.0018	218 ± 0.004	0.59 ± 0.0003	8.45 ± 0.0005	11.9 ± 0.005	42.2 ± 0.003
No. 16	6090 ± 0.40	2280 ± 0.07	1924 ± 0.08	6.90 ± 0.0006	17.1 ± 0.0004	36.7 ± 0.001	0.48 ± 0.0010	1.35 ± 0.0006	28.5 ± 0.011	20.2 ± 0.001
No. 17	2857 ± 0.13	460 ± 0.03	1651 ± 0.08	0.97 ± 0.0006	10.7 ± 0.0007	23.1 ± 0.002	0.18 ± 0.0014	0.69 ± 0.0001	4.10 ± 0.003	12.3 ± 0.001
No. 18	10586 ± 0.37	1576 ± 0.05	3862 ± 0.05	6.8 ± 0.0001	37.8 ± 0.0013	53.2 ± 0.002	1.28 ± 0.0003	6.99 ± 0.0007	11.4 ± 0.003	34.7 ± 0.005
No. 19	3744 ± 0.21	428 ± 0.02	3430 ± 0.36	0.57 ± 0.0004	26.0 ± 0.0007	15.7 ± 0.001	0.11 ± 0.0012	1.15 ± 0.0007	6.6 ± 0.001	33.6 ± 0.006
Mean	7579 ± 6214	1622 ± 1416	2715 ± 1551	3.8 ± 3.4	33.2 ± 15.7	74.1 ± 112	0.81 ± 0.68	4.95 ± 3.78	14 ± 8.4	28.3 ± 19.5

nd: not detected.

Calcium and magnesium are essential for all living organisms. Calcium plays an essential role in blood clotting, muscle contraction, and bone and tooth formation. Magnesium may be necessary to maintain renal function and protect the kidneys from damage. Our results showed that high concentrations of calcium and magnesium are accumulated in herbal samples, and their contents change of maximum range (Table 4). The concentrations of calcium and magnesium were in the range of 1740–22,246 and 634–6367 µg/g dry weight (dw), respectively. The abundance of calcium and magnesium in the present study was in agreement with the previous literatures, which indicated that these two elements were the most abundant elements in many medicinal plants and food.^[12–15]

Iron toxicity is associated with several pathological conditions, including certain cancers, liver and heart disease, diabetes, hormonal abnormalities, and immune system dysfunctions.^[16] Excess doses or prolonged intake of zinc could lead to altered iron function, reduced immune function, and reduced levels of the high-density lipoprotein.^[17] In this work, iron concentration was in the range of 52.8–5707 µg/g dw. Sample No. 2 had the highest concentration of iron. The minimum and maximum levels of zinc were found to be 10.7 and 82.1 µg/g dw, respectively. The average concentration of zinc was comparable with previously reported concentration for 50 Chinese medicinal herbs (72 µg/g dw).^[11]

High intakes of copper significantly reduced the percentage of circulating neutrophils.^[18] The proliferation of lymphocytes cultured with mitogens and their secretion of IL-2R reduced significantly with the feeding of a low-copper (380 µg/day) diet, while the percentage of circulating B cells increased.^[19] As can be seen in Table 4, the concentration of copper varied between 4.10 and 36.6 µg/g dw. It is lower than the range of copper in 50 medicinally important leafy materials (17.6-57.3 µg/g dw) in India.^[20] Recommended dietary allowance and adequate intake of copper concentration is 340–440 µg/day in children and 700–900 µg/day in adults.^[21] Table 5 showed the national limit for copper (150 µg/g dw) in finished herbal products in Singapore.^[22] Each herb analyzed in this study is relative safe as appropriate intake of copper for human.

Table 5. National limits for toxic metals in herbal medicines.

		Cadmium	Chromium	Copper
Canada	Raw herbal materials	0.3 µg/g	2 µg/g
	Finished herbal products	6 µg/day	20 µg/day
China	Herbal materials	1 µg/g
Singapore	Finished herbal products			150 µg/g
Thailand	Herbal material and Finished herbal products	0.3 µg/g

Cadmium is one of the relatively common toxic heavy metal contaminants even in trace amounts. Cadmium and Cadmium compounds may cause cancer in humans. It is apparent that there is a risk of adverse health effects resulting from overdose of cadmium. The level of cadmium ranged from 0.11 to 2.66 µg/g dw in the samples, and most of them varied between 0.38 and 2.66 µg/g dw (Table 4). According to the World Health Organization (WHO), the maximum amount in dried plant materials for cadmium was 0.3 µg/g dw based on the acceptable daily intake.^[23] The total daily intake of cadmium by the inhabitants of Tarragona, Spain was 56.31 µg/day.^[24] In this study, cadmium concentration in most of the samples did not meet the national limit as we can see from Table 5. Taraxacum mongolicum (No. 6) is a newly reported cadmium accumulator.^[25] However, the average concentration of cadmium (1.39 µg/g dw) in No. 6 was not the highest data in this work. The variation of element concentration may be caused by geographical origin and growing environment.^[26,27]

Acute barium poisoning is descent of blood pressure, cerebral hemorrhage, and serious disorder of heart rate, eventually leading to death. Strontium can increase the mortality of cardiovascular disease. For all of the herbs, No. 7 contained significantly lower barium and strontium concentration than that of No. 4, which ranged from 9.19 to 465 and 8.25 to 69.8 µg/g dw, respectively. The average concentration of barium (74.1 µg/g dw) was higher than the reported in tender leaves (42.2 µg/g dw) collected from 28 cities/towns spread over 19 Indian states, which had higher strontium concentration than the content in this study.^[28]

Nickel is cofactor for an enzyme functioning in nitrogen metabolism. Chromium is a highly toxic heavy metal, causing damage to the liver, kidney, and other organs. The concentration of nickel and chromium in the herbs indicated similar range (Table 4). Nickel was determined in the range of 0.57–14.7 µg/g dw. The highest nickel was determined in No. 2, whereas the lowest level was in No. 19. The highest concentration of chromium was 13.3 µg/g dw in No. 6 while the lowest value was 0.26 µg/g dw in No. 14. The concentrations of nickel and chromium were lower than the values of them determined by Yan et al.^[29] who reported six herbal drugs had 5.48–49.04 µg/g dw nickel and 1.13–18.19 µg/g dw chromium, respectively. According to Food and Nutrition Board,^[21] the tolerable upper intake level for nickel was 0.2–1.0 µg/g dw. Only three samples (No. 7, No. 17, and No. 19) in this work meet this standard. The average concentration of chromium (4.95 µg/g dw) was higher than that of the permissible limits in raw herbal materials set by Canada (2 µg/g dw).^[22] In terms of nickel and chromium contents, only No. 19 meets the safety standard. Heavy metal contamination may be increased due to industrial emissions and the application of fertilizer and sewage sludge to farm land.

Correlation analysis

Correlation analysis of the total element content quantifies the relationship between the two variables and is measured by correlation coefficient (Table 6). The correlation coefficient ranges from –1 to 1. A value near 0 indicates weak or no correlation exists between the variables, whereas a value near –1 or 1 indicates the two variables are more similar. It can be seen that there were highly correlation between calcium–iron (0.533), calcium–chromium (0.849), calcium–strontium (0.520), iron–nickel (0.899), iron–chromium (0.631), iron–copper (0.584), iron–strontium (0.602), magnesium–chromium (0.562), nickel–chromium (0.510), nickel–copper (0.664), nickel–strontium (0.505), zinc–cadmium (0.599), barium–strontium (0.770), and chromium–strontium (0.690), whereas there was no significant correlation with other elements. The correlations given in Table 6 indicated that the relationships between the elements are complex and are difficult to explain individually. More interpretations between elements and herbs may be obtained using more powerful chemometric technique such as PCA.

Table 6. Correlation matrix for the element contents in the herbal samples.

	Calcium	Iron	Magnesium	Nickel	Zinc	Barium	Cadmium	Chromium	Copper	Strontium
Calcium	1
Iron	0.533	1
Magnesium	0.472	0.238	1
Nickel	0.378	0.899	0.132	1
Zinc	0.370	0.225	0.134	0.170	1
Barium	0.215	0.410	0.093	0.281	0.117	1
Cadmium	0.382	0.253	0.077	0.226	0.599	0.046	1
Chromium	0.849	0.631	0.562	0.510	0.417	0.308	0.452	1
Copper	0.057	0.584	–0.173	0.664	0.400	–0.061	0.212	0.099	1
Strontium	0.520	0.602	0.460	0.505	0.158	0.770	0.184	0.690	–0.045	1

PCA

By extracting the principal components which have Eigenvalues greater than 1, three principal components were formed. The first, second, and third components reported 43.9, 18.2, and 14.9% of the variance, respectively. The first three components described 77% variances for all the data. The loadings for calcium, iron, nickel, barium, chromium, and strontium on the first component were large and so were the copper and magnesium on the second component. The third component was positively correlated with zinc and cadmium. On the second component, copper had opposite properties with magnesium because copper had highest positive loadings while magnesium had the highest negative loading value. From the scores of the first principal component, it can be interpreted that the concentrations of calcium, iron, nickel, barium, chromium, and strontium on the first principal component loadings were higher for sample No. 6, 2, 8, 15, 9, and 18 than sample No. 3 and 4, and were lower for all the other samples. When the second principal component was interpreted, copper concentrations on the second principal components loadings were higher for sample No. 2, 3, 16, and 11 than sample No. 7, 12, 14, and 13, and were lower for all the other samples. On the third principal component, the concentrations of zinc and cadmium were high in No. 3, 8, 6, and 10 than sample No. 18, 9, 11, 13, 5, 14, and 12 and were lower for all the other samples.

A 2D plot of the first and the second PCA loadings was illustrated in Fig. 1. The associations between calcium, iron, nickel, barium, chromium, and strontium on the first principal component and copper on the second principal component were readily seen. The loadings indicated the association between the elements. The elements with bigger loadings affect the separation of the samples. The score values of the herbs are interpreted with the larger loadings obtained for elements for both the first and the second principal components. The scores of the first two principal components were shown in Fig. 2. Individualized behavior between the elements appeared to be in order when interpreted with the loadings of the scores. Figure 2 showed that the herbal samples were classified into three groups. Group 1 contains No. 2, 3, and 16 samples. Group 2 contains No. 6, 8, 15, 18, 9, and 4 samples. Group 3 contains No. 11, 13, 14, 7, 17, 12, 5, 19, 10, and 1. The results showed that the concentrations of copper were high in the samples of group 1. Samples of group 2 had high concentrations of calcium, iron, nickel, barium, chromium, and strontium, while the concentrations of calcium, iron, nickel, barium, chromium, and strontium were lower in samples of group 3. These results showed that the samples with similar element concentrations were grouped into the same groups.

Figure 1. The principal component loadings of the first two principal components. Principal component 1 (PC1) versus principal component 2 (PC2).

Figure 2. The scores of the first two principal components. Principal component 1 (PC1) versus principal component 2 (PC2)

Cluster analysis

In this study, a cluster analysis by applying the between-groups linkage method and cosine measurement was performed with the standardized data set. Cluster analysis generated similar groupings of the herbal samples to those derived from the principal components. Three groups were obtained (Fig. 3). Group 1 contained No. 2, 3, and 16 samples (high in copper from PCA). Group 2 contained No. 8, 18, 6, 15, 9, and 4 samples (high in calcium, iron, nickel, barium, chromium, and strontium from PCA). Group 3 contained No. 12, 14, 7, 5, 17, 19, 11, 13, 1, and 10 samples (low in calcium, iron, nickel, barium, chromium, and strontium from PCA).

Figure 3. Dendrogram obtained by the between-groups linkage method and cosine of cluster analysis for the 19 variables and 10 elements (the distances reflect the degree of correlation between different herbs). Number 1-19 is representative of different sample names. See Table 2 for sample names.

The results gave three significant groups of samples. The cluster analysis made it clear that the samples with similar element concentrations formed independent groups, which implied that multi-element information could be suitably utilized to classify herbal samples consumed in Yunnan, China. No. 2, 3, and 16 samples were high in copper. No. 8, 18, 6, 15, 9, and 4 samples were high in calcium, iron, nickel, barium, chromium, and strontium. No. 12, 14, 7, 5, 17, 19, 11, 13, 1, and 10 samples were low in calcium, iron, nickel, barium, chromium, and strontium. In Chinese herbal medicine practice, herbs are commonly used together as a formula to achieve better treatment outcomes in balancing or improving body’s health. In the human diet, they always used to give flavor to a range of dishes. Significant amounts of harmful heavy metals are accumulated in the body if people live on them. Fortunately, human in health do not have to take them everyday, therefore the possibility of daily consumption are limited, and thus the risk of an excess of detrimental metals is low. The elemental concentrations of the herbs related to medical and edible purpose in Yunnan, China obtained in this work can further assist in the development of herbal formulae to improve body’s health and the flavor of dishes.

Conclusions

Toxic elements (cadmium, chromium, and nickel) contents were much higher than the maximum permissible levels in many samples. The elemental concentrations were varied over a wide range among the herbs. PCA generated three principal components that explained 77% of the total variance in the data. Similar samples may get together by cluster analysis and could correspond to the result of PCA. The result clearly showed that the groups of the herbs based on their elemental concentrations and the elemental concentrations in medicinal plants varied depending on the species of herb, places of origin, and harvest period as well as other factors that can influence the elemental concentrations. Furthermore, medicinal herbs may be easily contaminated during growing and processing. Therefore, it is important to throw more light on the good quality control for medicinal herbs.

Funding

This study has been supported by the National Natural Science Foundation of China (31460538,81260610, 81260608) and the Yunnan Provincial Natural Science Foundation (2013FZ150).

Additional information

Funding

This study has been supported by the National Natural Science Foundation of China (31460538,81260610, 81260608) and the Yunnan Provincial Natural Science Foundation (2013FZ150).