Detection methods and applications of heavy metals \x0d\ 1. Hazard characteristics of heavy metals \x0d\ (1) naturalness: \x0d\ Humans who have lived in the natural environment for a long time have strong adaptability to natural substances. Some people have analyzed the distribution law of more than 60 common elements in human body and found that the percentage content of most of them in human blood is very similar to that in the earth's crust. However, human beings are much less tolerant of synthetic chemicals. Therefore, it is helpful to estimate the harm of pollutants to human beings by distinguishing their natural or man-made attributes. Due to the development of industrial activities, heavy metals such as lead, cadmium, mercury and arsenic are enriched in the surrounding environment of human beings and enter the human body through the atmosphere, water and food. And accumulate in some organs of human body, causing chronic poisoning and endangering human health. \x0d\ (II) Toxicity: \x0d\ The main factors that determine the toxicity of pollutants are their material properties, contents and existing forms. For example, chromium has three forms: bivalent, trivalent and hexavalent, among which hexavalent chromium is very toxic, and trivalent chromium is one of the important elements in human metabolism. In natural water, the toxic range of heavy metals is about 1 ~ 10 mg/L, while the toxic range of mercury and cadmium is 0.01~ 0.001mg/l \x0d\ (3) Temporal and spatial distribution: \ x0d \ pollutants enter the environment. The pollutants with high activity are prone to chemical reactions in the environment or during treatment, and their toxicity is reduced, but they may also generate pollutants with stronger toxicity than the original ones, which constitutes secondary pollution. If mercury can be converted into methylmercury, it is more toxic. Contrary to activity, persistence means that some pollutants can maintain their harmfulness for a long time, such as heavy metals such as lead and cadmium, which are toxic and difficult to degrade in nature, and can produce bioaccumulation, threatening human health and survival for a long time. \x0d\ (5) Biodegradability: \x0d\ Some pollutants can be absorbed, utilized and decomposed by organisms, and finally produce harmless and stable substances. Most organic substances are likely to be biodegradable, while most heavy metals are not easily biodegradable, so once heavy metal pollution occurs, it is more difficult to control and more harmful. \x0d\ (6) Bio-accumulation: \x0d\ Bio-accumulation includes two aspects: First, pollutants accumulate in the environment through food chain and chemical and physical effects. The second is the accumulation of pollutants in some organs and tissues of the human body due to long-term intake. For example, cadmium can accumulate in organs and tissues such as liver and kidney, causing damage to various organs and tissues. Another example is the Minamata disease in Japan from 1953 to 196 1. Inorganic mercury is converted into methylmercury in seawater, which is ingested and accumulated by fish and shellfish. After biomagnification in the food chain, local residents were poisoned after eating. \x0d\ (7) Additive effect on organisms: \x0d\ Various pollutants coexist and interact with organisms. There are two kinds of superposition effects of pollutants on organisms: one is synergistic effect, and mixed pollutants make it more harmful to the environment than simple superposition of pollutants; The other is antagonism, that is, the persistence of pollutants weakens the harm. \x0d\ II。 Quantitative detection technology of heavy metals \x0d\ Generally recognized methods for heavy metal analysis include ultraviolet spectrophotometry (UV), atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), inductively coupled plasma spectrometry (ICP), X-ray fluorescence spectrometry (XRF) and inductively coupled plasma mass spectrometry (ICP-MS). In addition to the above methods, spectroscopy is also introduced for detection, which is more precise and accurate! \x0d\ Inductively coupled plasma mass spectrometry (ICP-MS) is used for analysis in Japan and some European Union countries, but for domestic users, the instrument cost is high. Some also use X-ray fluorescence spectrometry (XRF) analysis, which has the advantages of nondestructive testing and direct analysis of finished products, but the detection accuracy and repeatability are not as good as those of spectrometry. The latest popular detection method, anodic stripping method, has fast detection speed and accurate value, and can be used for on-site environmental emergency detection. \x0d\ (1) atomic absorption spectrometry (AAS)\x0d\ atomic absorption spectrometry is a new instrumental analysis method founded in 1950s. It supplements atomic emission spectrometry, which is mainly used for qualitative analysis of inorganic elements, and becomes the main means for quantitative analysis of inorganic compounds. \x0d\ The flow of atomic absorption analysis is as follows: 1. Make the sample into solution (blank at the same time); 2. Prepare a series of analytical element calibration solutions (standard samples) with known concentrations; 3. Measure the corresponding values of blank and standard samples in turn; 4. Draw a calibration curve according to the corresponding values; 5. Measure the corresponding value of the unknown sample; 6. Calculate the concentration value of the sample according to the calibration curve and the corresponding value of the unknown sample. Now, due to the development of computer technology and chemometrics and the appearance of many new components, the precision, accuracy and automation of atomic absorption spectrometer have been greatly improved. The atomic absorption spectrometer controlled by microprocessor simplifies the operation procedure and saves the analysis time. At present, gas chromatography and atomic absorption spectrometry have been successfully developed, which further expands the application field of atomic absorption spectrometry. \x0d\ (II) Ultraviolet-visible spectrophotometry (UV)\x0d\ Its detection principle is that heavy metals and chromogenic agents-usually organic compounds-can react with heavy metals to form colored molecular groups, and the color depth of the solution is directly proportional to the concentration. At a specific wavelength, colorimetric detection is carried out. \x0d\ There are two kinds of spectrophotometric analysis, one is to determine the absorption of ultraviolet and visible light by the substance itself; The other is to generate colored compounds, that is, "color development", and then determine. Although many inorganic ions have absorption in ultraviolet and visible light regions, they are rarely directly used for quantitative analysis because of their weak strength. Adding chromogenic agent to convert the substance to be detected into a compound that can absorb under ultraviolet and visible light for photometric determination, which is the most widely used testing method at present. Color developers are divided into inorganic color developers and organic color developers, and organic color developers are used more. Most organic color developers are colored compounds themselves, and the compounds generated by the reaction with metal ions are generally stable chelates. The color reaction has high selectivity and sensitivity. Some colored chelates are easily soluble in organic solvents and can be detected by colorimetry after extraction and leaching. In recent years, the color system of multicomponent complexes has attracted extensive attention. Multicomponent complex refers to a complex formed by three or more components. The formation of multicomponent complexes can improve the sensitivity of spectrophotometric determination and improve the analytical characteristics. The selection and use of chromogenic reagents in pretreatment, extraction and colorimetric detection are important research topics of spectrophotometry in recent years. \x0d\ (3) Atomic fluorescence spectrometry (AFS)\x0d\ Atomic fluorescence spectrometry is a method to determine the content of elements to be measured by measuring the fluorescence emission intensity of atomic vapor of the elements to be measured under the excitation of radiation energy at a specific frequency. Although \x0d\ atomic fluorescence spectrometry is emission spectrometry, it is closely related to atomic absorption spectrometry, which has the advantages of both atomic emission spectrometry and atomic absorption spectrometry, and overcomes the disadvantages of the two methods. Atomic fluorescence spectrometry has the characteristics of simple emission line, higher sensitivity than atomic absorption spectrometry, wide linear range and less interference, and can be used for simultaneous determination of many elements. Atomic fluorescence spectrometer can be used to analyze 1 1 elements, such as mercury, arsenic, antimony, bismuth, selenium, tellurium, lead, tin, germanium, cadmium and zinc. Now it has been widely used in environmental monitoring, medicine, geology, agriculture, drinking water and other fields. Among the national standards, atomic fluorescence spectrometry has been regarded as the first choice for the determination of arsenic, mercury and other elements in food. \x0d\ After the gaseous free atom absorbs the radiation with characteristic wavelength, the electrons in the outer layer of the atom will jump from the ground state or low energy state to the high energy state, and at the same time emit energy radiation with the same or different excitation wavelength, that is, atomic fluorescence. The emission intensity If of atomic fluorescence is proportional to the number n of atoms in the ground state per unit volume of the element in the atomizer. When the atomization efficiency and fluorescence quantum efficiency are fixed, the atomic fluorescence intensity is proportional to the sample concentration. \x0d\ An atomic fluorescence spectrometer for simultaneous determination of multiple elements is developed, which uses several high-intensity hollow cathode lamps as light sources and high-temperature inductively coupled plasma as atomizer to atomize multiple elements simultaneously. Multi-element analysis system takes ICP atomizer as the center, and multiple detection units are installed around it, which are at right angles to the hollow cathode lamp, and the generated fluorescence is detected by photomultiplier tube. The electrical signal after photoelectric conversion is amplified by computer, and the analysis results of each element are obtained. \x0d\ (4) Electrochemical method-anodic stripping voltammetry \x0d\ electrochemical method is a rapid development method in recent years. It relies on classical polarography, and on this basis, it derives polarography, anodic stripping voltammetry and other methods. Electrochemical method has low detection limit and high sensitivity, which is worth popularizing and applying. For example, the fifth method for determining lead and the second method for determining chromium in the national standard are oscillopolarography. Anodic stripping voltammetry is an electrochemical analysis method which combines potentiostatic electrolytic enrichment with voltammetric determination. This method can continuously determine various metal ions at one time with high sensitivity, and can determine the metal ions of10-7-10-9 mol/L. The instruments used in this method are simple and easy to operate, and it is a good trace analysis method. China has promulgated the national standard of anodic stripping voltammetry for the determination of metal impurities in chemical reagents. \x0d\ anodic stripping voltammetry is divided into two steps. The first step is "electrodeposition", that is, at a constant potential, the ions to be detected are electrodeposited and enriched on the working electrode to form amalgam with mercury on the electrode. For a given metal ion, if the stirring speed is constant and the pre-electrolysis time is fixed, m=Kc, that is, the amount of electrodeposited metal is proportional to the concentration of the metal to be measured. The second step is "dissolution", that is, after enrichment, usually after resting for 30s or 60s, reverse voltage is applied to the working electrode, and the metal in amalgam is re-oxidized into ion regression solution from negative positive scanning to generate oxidation current, and the voltage-current curve, that is, volt-ampere curve, is recorded. The curve is peak-shaped, and the peak current is proportional to the measured concentration in the solution, which can be used as the basis for quantitative analysis and the peak potential can be used as the basis for qualitative analysis. \x0d\ Oscillopolarography is also called "single sweep polarography". A new polarographic analysis method. This is a polarographic method for rapidly increasing electrolytic voltage. At the late growth stage of each mercury drop on the mercury drop electrode, a sawtooth pulse voltage is rapidly applied to the two poles of the electrolytic cell, and a polarogram is obtained in a few seconds. In order to quickly record polarogram, the screen of oscilloscope tube is usually used as a display tool, so it is called oscillopolarography. Its advantages are high speed and high sensitivity. \x0d\ (5) X-ray fluorescence spectrometry (XRF)\x0d\X-ray fluorescence spectrometry is a method for qualitative or quantitative determination of components in a sample by using the X-ray absorption of the sample to change with the change and amount of components in the sample. It has the characteristics of fast analysis speed, simple sample pretreatment, wide range of analyzable elements, simple spectral lines, less spectral interference, diverse sample forms and nondestructive determination. It can be used not only for qualitative and quantitative analysis of major elements, but also for the determination of trace elements. The detection limit is 10-6. Combined with separation and enrichment, it can reach 10-8. The range of elements measured includes all elements of F-U in the periodic table. Multichannel analyzer can simultaneously determine the content of more than 20 elements in a few minutes. \x0d\x-ray fluorescence method can not only analyze massive samples, but also analyze the composition and film thickness of each layer of multilayer coatings. \x0d\ When the sample is irradiated by X-rays, high-energy particle beams, ultraviolet rays, etc. Due to the collision of high-energy particles or photons with sample atoms, electrons in the inner layer of the atoms are driven out to form holes, which makes the atoms in excited states. The lifetime of this excited ion is very short. When the electrons in the outer layer jump to the holes in the inner layer, the excess energy is released in the form of X-rays, and the outer layer produces new holes and new X-ray emission, thus producing a series of characteristic X-rays. Characteristic X-ray is inherent in various elements, and it is related to the atomic coefficient of elements. So as long as the wavelength λ of the characteristic X-ray is measured, the element that produces this wavelength can be found out. You can do qualitative analysis. Under the conditions of uniform sample composition, smooth surface and no mutual excitation between elements, when the elements in the sample are irradiated with X-rays (primary X-rays) to generate characteristic X-rays (fluorescence X-rays), if the elements are the same as the experimental conditions, the intensity of fluorescence X-rays is linearly related to the content of analytical elements. Quantitative analysis can be made according to the intensity of spectral lines \x0d\ (6) The detection limit of inductively coupled plasma mass spectrometry (ICP-MS)\x0d\ICP-MS is very considerable, and the detection limit of its solution is mostly ppt level, so the actual detection limit cannot be better than the clean conditions in your laboratory. It must be pointed out that the ppt detection limit of ICP-MS is for simple solutions with few dissolved substances in the solution. When it comes to the detection limit of solid concentration, due to the poor salt tolerance of ICP-MS, the advantage of ICP-MS detection limit will be as much as 50 times worse, and some common light elements (such as S, Ca, Fe, K, se) will seriously interfere with ICP-MS and worsen its detection limit. \x0d\ICP-MS consists of an ICP torch as an ion source, an interface device and a mass spectrometer as a detector. \ x0d \ The ionization source used in ICP-MS is inductively coupled plasma, and its main body is a torch consisting of three layers of time sleeves. The upper end of the torch tube is wound with a load coil, and the three-layer tube supplies carrier gas, auxiliary gas and cooling gas respectively from the inside to the outside. The load coil is coupled by a high frequency power supply to generate a magnetic field perpendicular to the plane of the coil. If argon is ionized by a high-frequency device, argon ions and electrons will collide with other argon atoms under the action of electromagnetic field to produce more ions and electrons, forming eddy currents. Strong current generates high temperature, which instantly makes argon gas form a plasma torch with the temperature of10000 k k. The sample to be analyzed is usually introduced into argon gas flow in the form of aqueous aerosol, and then enters the central area of argon plasma at atmospheric pressure under the excitation of radio frequency energy. The high temperature of plasma makes the sample desolvate, evaporate and ionize. Part of the plasma enters the vacuum system through different pressure zones. In a vacuum system, positive ions are pulled out and separated according to the mass-to-charge ratio. At about 10mm above the load coil, the torch temperature is about 8000 k. At this high temperature, the elements with ionization energy lower than 7eV are completely ionized, and the ionization degree of the elements with ionization energy lower than 10.5ev is greater than 20%. Because the ionization energy of most important elements is lower than 10.5eV, they all have high sensitivity. A few elements with high ionization energy, such as C, O, Cl and Br, can also be detected, but the sensitivity is low.