Determination of toxic metals in Traditional Chinese Medicine

Traditional Chinese Medicines(TCM) have gained increasing attention worldwide because of their effectiveness and small side effects^[1]. The excess of toxic metals in these medicines have become one of the biggest problems that affected their export and hindered their introduction to the world[2]. In recent years, many domestic and abroad testing organizations paid great attention to the toxic metals in traditional Chinese medicines. Generally, toxic metals in traditional Chinese medicines including Lead, Cadmium, Chromium, Mercury, Copper and so on[3], sometimes people also prefer including Arsenic as toxic heavy metal. Different types of heavy metals in human effect different parts or systems, and the symptoms of poisoning can be on the nervous, digestive, hematopoietic systems or people’s metabolism and other body damage. So to accurate detect and limit the content of toxic metals is the key to protect the people of medication safety, to promote the internationalization of traditional Chinese medicines. Recently, themost commonly used detection methods of detection of toxic metals include colorimetric assay and instrumental analysis[4]. With the increasing requirements for toxic metal limits, instrumental analysis method has become the primary means of detection of heavy metals. In this paper, the determination of toxic metals in traditional Chinese medicine samples by atomic emission spectrometry technology are reviewed.

The national limits for toxic metals in herbal medicines and product

In general, quantitative tests and limit tests accurately determinate the concentrations of toxic metals in the form of impurities and contaminants. The latter are unavoidably present in the samples being tested. Member states can elect to use either quantitative tests or limit tests and their choices will be influenced by the nature of the sample and the contaminants or residues, assessed on a case-by-case basis. Another factor would be that the methods identified, and chosen to be applied to control toxic metals, should be relevant and should meet the requirements at a regional and national level. Some examples of proposed national limits for arsenic and toxic metals in various types of herbal products are shown in Table 1^[5].

Table 1. Example of national limits for arsenic and toxic metals in herbal medicines and products

The sources of toxic metals in traditional Chinese medicine samples

The toxic metals in traditional Chinese medicines can be from the soil where is contaminated, in the processes of collection and production.

Atomic emission spectroscopy (AES)

Atomic emission spectroscopy (AES) is a method of chemical analysis, when the analyte atoms in solution are aspirated into the excitation region(flame, plasma, arc, or spark at a particular wavelength) and underwent desolvated, vaporized, atomized, these high-temperature sources provide sufficient energy to promote the atoms into high energy levels, after decaying back to lower energy levels by emitting light, the wavelength of the atomic spectral lines gives the identity of the element and the intensity of light proportional to the concentration of atoms, this can be used to determinate the quantity of elements in a sample. Since all atoms are excited simultaneously, they can be detected simultaneously.

The classification of AES

Flame atomic emission spectroscopy(FAES) The energy source is flame, a sample is brought into the flame by a nebulizer in the form of gas, sprayed solution. A flame provides a high-temperature source for desolvating and vaporizing a sample to obtain free atoms for spectroscopic analysis. For atomic emission spectroscopy the flame must also excites the atom to higher excited states. Then subsequently emit light when they returning to the ground electronic state. Each element emits light at a characteristic wavelength, which is dispersed by a grating or prism(monochromator) and detected by photo detector. Due to its low temperature(1700~3200°C), nitrous oxide-acetylene is the best flame as this gives the highest temperature. FAES used mostly for determination of alkali metals and occasionally calcium, and need internal standard to correct for variations flame^[6]. So FAES is always used with FAAS together to determinate the content of metals in a sample. Slavica Ražić used FAAS/FAES to determinate the elements of Cu, Zn, Mn, Fe, K, Ca, Mg in some of herbal drugs^[7].

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Inductively coupled plasma atomic emission spectroscopy(ICP-AES) Inductively coupled plasma atomic emission spectroscopy(ICP-AES) uses an inductively coupled plasma as thermal source to excite atoms and ions to their excited energy levels and emit electromagnetic radiation at wavelengths characteristic of a particular element^[8].

ICP-AES has a lot of advantages such as low inter-element interference, multi-element capability, high sensitivity, high concentration range, low chemical interference, with stable and reproducible signal and high degree of selectivity, precision and accuracy(0.3~3%), can use with gas, liquid or solids samples. Disadvantages are serious spectral interferences(too many emission lines), procedures complicated, consume large Ar gas and operating expense, not possible to determinate the elements of H, N, O, C or Ar in trace levels as they are used in solvents and plasma, and also not possible to determinate F, Cl and noble gases at trace levels as they require high excitation energy.

Spark and arc atomic emission spectroscopy For solid samples, spark or arc atomic emission spectroscopy is used for the analysis of metallic elements.

Determination of toxic metals in traditional Chinese medicine by ICP-AES

Because of its high sensitivity, low detection limit, high precision, wide linear range, multi-element analysis, small matrix effects and many other advantages for the detection of most of the metallic elements. ICP-AES has become the most effective method that was widely used in all kinds of traditional Chinese medicines in the determination of toxic metals[9]. Yan Qing-hua used ICP-AES to determinate 14 kinds of elements Cu, Fe, Zn, Mn, Na, K, Ca, Mg, Cr, Ni, Pb, Se, As, Cd in traditional Chinese medicines for clearing heat and detoxification. Showed the determining elements in traditional Chinese medicine by IPC-AES saving time and effort, reducing environmental pollution, good stability, high sensitivity and multi-element simultaneous analysis^[10]. Liu Er-dong used ICP-AES to determinate metallic elements for quality control of Chinese herbal medicines showed this method is accurate and rapid^[11].

Sampling process

With the development of separation science, sample preparation methods have also been substantially developed, especially the sample preparation method for complex material systems, which has gained more attention in recent years. The specific method requires special preparation methods or combination methods according to the specific nature and status of the sample preparation. The traditional Chinese medicines samples are very complicated[12], the preparation method usually includes conventional extraction methods such as steam distillation, water extraction, and alcohol precipitation, alcohol precipitation of water, organic solvent extraction, fractionation, absorption. For the determination of toxic metals in traditional Chinese medicine samples, people have made a lot of work in improving the equipment, good analysis methods to improve the experimental measurements. But always overlook one more important factor, sampling. So how to improve the accuracy of sampling is also one of the most important topics to be studied.

Digestion condition

Digestion is an important process in the determination of toxic metals in traditional Chinese medicines. The general methods of digestion including ashing, wet digestion, microwave digestion, combustion method and high pressure dissolving. But all of these methods have their limitations: (1) can’t be guaranteed fully adapted to all organic samples; (2) the waste of analyte; (3) perchloric acid is flammable and explosible. Li Yang compared the digestion methods of dry ashing, wet digestion and microwave digestion in the process of determination of elements in periostracum serpentis and periostracum cicadae[13]. The results showed the microwave digestion with the highest efficiency.

The samples of traditional Chinese medicine are always cleaned the sediment and washed with tap water, rewashed with de-ionized water, dried under 80℃ smashed and filtered through 40 mesh sieve, dried under 80℃ again for 1h, put into drier for using.

Ashing digestion

Ashing digestion is digest samples at high temperature, this is used to find the total mineral content of a sample. Kuziemska Beata used ashing digestion method to determinate the contents of toxic metallic elements in red clover biomass^[14].

Normal procedure as follows: Weighed dry sample 1g (wet 2~4 g), put it in porcelain crucible, charring with low heat , then ashing in furnaces at 450℃ for 3 hours, cooled and added a little distilled water, heated slightly to dry, then placed in a high temperature furnace ashing completely, cooled, add 1: 1 HNO3 1.0 ml, dissolved with heating, set the volume to 25ml.

Wet digestion

In the process of wet digestion, normally use the solvent of acid or oxidant to digest the samples. The commonly used acid and oxidant are as follows:

1. Nitric acid: The most commonly used acid in the process of digestion, it is also a strong oxidant which is widely used to dissolve trace elements in plant samples, get their soluble nitrate. Xue Yan used nitric acid as digestion reagent to detect chromium in medical materials showed the detection limit was 1.42 ng/mL, and the deviations were 1.43%~1.79%^[15].
2. Perchloric acid: Can decompose organic component completely, which is used when the other reagents can not digest. However, the heat concentrated perchloric acid is explosible when contact with organic components, so the operator must be very careful.
3. Aqua regia: It is a strong oxidant, can applied to digest inorganic components, such as gold and platinum. Tahar Kebir used aqua regia to digest food plants near a polluted site for the measurement of toxic metals(Fe, Pb, Zn, Ni, Cu, Cd, Mn, Cr and As) and got excellent results^[16].
4. Hydrofluoric acid: It is an efficient reagent to dissolve silicon-containing material. It can change silicate into SiF₄, which is used to digest silicon samples completely. When mixed with nitric acid, could digest TiO2, W, Zr et al.
5. Hydrogen peroxide: It is one of the most commonly used oxidants, if combined with HNO₃, HCIO₄, HCI or mixed acid, the efficiency could be better.
6. Sulfuric acid: Strong acid and oxidant.

The capacity of these acid and oxidant are different. In the practical applications, normally use the mixture of two or more inorganic acids with different proportion(HNO₃-H₂SO₄, HNO₃-HCIO₄, H₂SO₄-HNO₃-HCIO₄). It has proven that the optimal digestion reagents are the mixture of HNO₃-HCIO₄, could digest multiple components simultaneously. Tong Wen-jie used HNO₃-HCIO₄ to digest sunflower(Helianthus annuus L) and analyzed the mineral elements content^[17]. Cai Yanrong also used HNO₃-HCIO₄ as digestion reagents in the study of trace elements(Fe, Cu, Zn, Al, K, Na, Ba, Sb, Pb, As) in hair samples^[18].

Microwave digestion

Microwave digestion requires only a small amount of sample and can digest in short time. It is a widely used digestion method in the determination of elements in herbal drugs. Li Feng-xia used microwave digestion method to test and analyze of inorganic elements in 466 traditional Chinese medicines^[19] showed that the measurements of each element are within the reference range, and RSD of determination is less than 10% for most of detected elements. In Yan Qing-hua’s study, his experiments also used microwave digestion technology, the recovery of the element reach 96.79%~103.47% and RSD less than 5.0%^[10]. Zhang Sheng-bang used HNO₃-H₂O₂ as solvents, microwave digestion to study multi-elements in traditional Chinese medicine Ophiopogon japonicus and Lotus seeds by ICP-AES^[20].

Conclusion and outlook

In recent years, the toxic metals in traditional Chinese medicines are concerned increasing all over the world. The methods of determination of toxic metals are not yet fully unified and also lack of regulation, scientific and systemic research. In the Chinese Pharmacopoeia(2010 version), in addition to the classical colorimetric method, more and more inclined to the using of high sensitivity and precision instruments measurements. ICP-AES has proven to be a quick, high sensitive and multi-elements analysis method. But there are still a lot of problems to be solved, new methods of determination of toxic metals are required, we hope there is a even faster and cheaper method to satisfy the measurements of toxic metals for TCM’s quick check in the process of import and export. Of course, the determination of toxic metals in Chinese medicine samples is a long-term work, with the development of detection technology, the detectable levels of toxic metals in traditional Chinese medicines will be increased, thereby enhancing their safety.

References

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