Detection of heavy metal ions technology, what are the instruments

Conventional methods include atomic absorption spectroscopy, atomic emission spectroscopy, etc., but can only measure ppm level, while femtosecond detection methods can accurately determine ppb and lower concentrations of metal ions.

From the environmental pollution said heavy metals, in fact, mainly refers to mercury, cadmium, lead, chromium, arsenic and other metals or analogous metals, but also refers to a certain degree of toxicity of the general heavy metals, such as copper, zinc, nickel, cobalt, tin and so on. We slightly discuss the hazards of heavy metals from several aspects such as naturalness, toxicity, activity and persistence, biodegradability, bioaccumulation, and additivity of action on organisms.

The commonly recognized methods for analyzing heavy metals are: ultraviolet spectrophotometry (UV), atomic absorption (AAS), atomic fluorescence (AFS), inductively coupled plasma (ICP), X-ray fluorescence spectrometry (XRF), inductively coupled plasma mass spectrometry (ICP-MS). In addition to the above methods, spectrometry has been introduced for testing with higher precision and accuracy!

Japan and the European Union countries have adopted inductively coupled plasma mass spectrometry (ICP-MS) analysis, but for domestic users, the instrument cost is high. Some also use X-ray fluorescence spectroscopy (XRF) analysis, the advantage of non-destructive testing, can directly analyze the finished product, but the detection accuracy and repeatability is not as good as spectroscopy. The latest popular detection method - anodic dissolution method, fast detection speed, accurate values, can be used in the field and other environmental emergency detection.

(I) Atomic Absorption Spectrometry (AAS)

Atomic Absorption Spectrometry (AAS) is a new type of instrumental analytical method created in the 1950's, which complements Atomic Emission Spectrometry (AES) which is mainly used for the qualitative analysis of inorganic elements, and has become the main means of quantitative elemental analysis of inorganic compounds.

Atomic absorption analysis process is as follows: 1, the sample is made into a solution (at the same time as the blank); 2, the preparation of a series of known concentrations of the analyzed elements of the calibration solution (standard sample); 3, in turn, measured the blank and the corresponding value of the standard sample; 4, based on the corresponding value of the above calibration curve; 5, the corresponding value of the unknown samples; 6, based on the calibration curve and the corresponding value of the unknown samples out of the sample's concentration. The value of the sample is calculated according to the calibration curve and the corresponding value of the unknown sample.

Now due to the development of computer technology, chemometrics and a variety of new components, so that the precision, accuracy and automation of the atomic absorption spectrometer greatly improved. Atomic absorption spectrometer controlled by microprocessor simplifies the operation procedure and saves the analysis time. Now has developed a gas chromatography - atomic absorption spectroscopy (GC-AAS) of the joint instrument, further expanding the application of atomic absorption spectrometry.

(B) ultraviolet-visible spectrophotometry (UV)

The detection principle is: heavy metals and coloring agents - usually organic compounds, can be complexed with heavy metals, generating a colored molecular group, the color of the solution is directly proportional to the concentration of the color shades. In a specific wavelength, colorimetric detection.

There are two types of spectrophotometric analysis, one is the use of the substance itself on the absorption of ultraviolet and visible light for determination; the other is to generate colored compounds, that is, "color", and then measured. Although many inorganic ions in the ultraviolet and visible light absorption, but because of the general intensity of weak, so less directly for quantitative analysis. Adding a color developer to make the substance to be tested into a compound with absorption in the ultraviolet and visible light region for photometric determination, which is currently the most widely used means of testing. Color developer is divided into inorganic color developer and organic color developer, and organic color developer used more. Most when the number of organic color developer itself for the colored compounds, and metal ions react with the compounds generated are generally stable chelates. The selectivity and sensitivity of the color reaction are higher. Some colored chelates are easily soluble in organic solvents, can be extracted and leached colorimetric detection. In recent years the formation of multi-coordinate color development systems has received attention. Multi-compound refers to the formation of three or more components of the complex. The formation of multiple complexes can be used to increase the sensitivity of spectrophotometric determination and improve the analytical properties. The selection and use of chromogenic agents in pre-treatment extraction and detection of colorimetry is an important research topic in spectrophotometry in recent years.

(C) atomic fluorescence method (AFS)

Atomic fluorescence spectrometry is through the measurement of the element to be measured in the atomic vapor at a specific frequency of radiation energy excitation below the fluorescence emission intensity generated by the determination of the element to be measured.

Atomic fluorescence spectrometry is a kind of emission spectrometry, but it is closely related to atomic absorption spectrometry, both atomic emission and atomic absorption of the advantages of the two analytical methods, but also to overcome the shortcomings of the two methods. Atomic fluorescence spectroscopy has a simple emission spectral line, sensitivity is higher than atomic absorption spectroscopy, the linear range is wider than the interference is less characterized by the ability to carry out multi-element simultaneous determination. Atomic fluorescence spectrometer can be used to analyze mercury, arsenic, antimony, bismuth, selenium, tellurium, lead, tin, germanium, cadmium, zinc and other 11 elements. It is now widely used in environmental monitoring, medicine, geology, agriculture, drinking water and other fields. In the national standard, food in the determination of arsenic, mercury and other elements of the standard has been atomic fluorescence spectrometry as the first method.

After absorbing the characteristic wavelength radiation, gaseous free atoms, the outer electrons of the atoms from the ground state or low-energy state will jump to the high-energy state, and at the same time emit the same or different energy radiation with the original excitation wavelength, i.e., atomic fluorescence. The emission intensity If of atomic fluorescence is proportional to the number of atoms N in the ground state of the element per unit volume in the atomizer. When the atomization efficiency and fluorescence quantum efficiency are fixed, the intensity of atomic fluorescence is proportional to the concentration of the specimen.

Atomic fluorescence spectrometer has been developed for the simultaneous determination of multiple elements, which uses multiple high-intensity hollow cathode lamps as the light source, and inductively coupled plasma (ICP) with high temperature as the atomizer, which can make a variety of elements realize atomization at the same time. The multi-element analysis system takes the ICP atomizer as the center and installs several detection units around it, which correspond to the hollow cathode lamps one by one at right angles, and the fluorescence generated is detected by photomultiplier tubes. The electrical signal after photoelectric conversion is amplified and then processed by the computer to obtain the analytical results of each element.

(D) electrochemical method - anodic dissolution voltammetry

Electrochemical method is a fast developing method in recent years, it is based on the classical polarographic method, on the basis of which it is derived from the waveform polarography, anodic dissolution voltammetry and other methods. The detection limit of electrochemical method is lower and the test sensitivity is higher, so it is worth to popularize the application. For example, the fifth method for the determination of lead and the second method for the determination of chromium in the national standard are oscillometric polarography.

Anodic dissolution voltammetry is a kind of electrochemical analysis method combining constant potential electrolytic enrichment and voltammetric determination. This method can continuously determine a variety of metal ions at a time, and the sensitivity is very high, can determine 10-7-10-9mol/L of metal ions. The instrument used in this method is relatively simple and easy to operate, which is a good means of trace analysis. China has promulgated the national standard of anodic dissolution voltammetry applicable to the determination of metal impurities in chemical reagents.

Anodic dissolution voltammetry is a two-step process. The first step is "electrodialysis", i.e., electrolytic deposition of the measured ion at a constant potential, which is enriched on the working electrode to produce an amalgam with mercury on the electrode. For a given metal ion, if the stirring rate is constant and the pre-electrolysis time is fixed, m = Kc, i.e., the amount of metal electrodeposited is directly proportional to the concentration of the metal under test. The second step is "dissolution", i.e., at the end of enrichment, generally after 30s or 60s of rest, a reverse voltage is applied to the working electrode, scanning from negative to positive, to reoxidize the metal in the amalgam into ions back into solution, generating an oxidation current, and recording the voltage-current curve, i.e., the voltammetric curve. The curve is peaked, the peak current is proportional to the concentration of the measured ion 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.

Oscillometric polarography, also known as "single-scan polarography". A polarographic analysis of the new force a method. It is a rapid addition of electrolytic voltage polarographic method. Often in the drop of mercury electrode each drop of mercury grows late in the electrolytic cell on both poles, quickly add a sawtooth pulse voltage, in a few seconds to get a polarographic map, in order to quickly record the polarographic map, usually with the fluorescent screen of the oscilloscope as a display tool, and therefore is called oscillometric polarographic method. Advantages: fast, sensitive.

(E) X-ray fluorescence spectrometry (XRF)

X-ray fluorescence spectrometry is the use of the sample absorption of x-rays with the composition of the sample and how much change to qualitatively or quantitatively determine the composition of the sample is a method. It is characterized by rapid analysis, simple sample pretreatment, wide range of elements that can be analyzed, simple spectral lines, less spectral interference, morphological diversity of the sample and non-destructive properties of the determination. It is not only used for qualitative and quantitative analysis of macronutrients, but also for the determination of trace elements, with detection limits of up to 10-6 in most cases, and up to 10-8 in combination with separation and enrichment, etc. The range of elements measured includes all elements in the Periodic Table from F to U. The analyzer can be used to analyze a wide range of elements in a few minutes. Multi-channel analyzer for simultaneous determination of more than 20 elements in a few minutes.

The x-ray fluorescence method not only analyzes bulk samples, but also analyzes the composition and film thickness of each layer of a multilayer coating separately.

When the sample is irradiated by x-rays, high-energy particle beams, ultraviolet light, etc., due to the collision of high-energy particles or photons and sample atoms, the inner layer of the atom electrons out of the formation of holes, so that the atom is in the excited state, the excited state of the ion life span is very short, when the outer electron to the inner layer of the hole jump, the excess energy that is in the form of x-rays released, and the teaching of the outer layer of the creation of new holes and the creation of new x-ray emission, which produces a series of x-ray fluorescence, which can be used to analyze the composition and film thickness of multi-layer coatings. ray emission, which produces a series of characteristic x-rays. Characteristic x-rays are inherent in various elements and are related to the atomic coefficients of the elements. So as long as the wavelength λ of the characteristic x-rays is measured, the element that produces the wavelength can be found. Qualitative analysis can be done. In the sample composition is uniform, the surface is smooth and flat, there is no mutual excitation between the elements under the conditions, when using x-rays (primary x-rays) as an excitation source irradiation of the specimen, so that the elements in the specimen to produce the characteristic x-rays (fluorescent x-rays), if the elements and the experimental conditions are the same, the intensity of fluorescent x-rays and the analysis of the elemental content of the existence of a linear relationship between the content of the elements. According to the intensity of the spectral lines can be quantitatively analyzed

(F) Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

ICP-MS detection limit is extremely impressive, the detection limit of its solution is mostly ppt level, and the actual detection limit is unlikely to be better than the clean conditions of your laboratory. It must be pointed out that the ppt level detection limit of ICP-MS is for the simple solution with few dissolved substances in the solution, if it involves the detection limit of the concentration in the solid, the merit of ICP-MS detection limit will be deteriorated by up to 50 times due to the poor salt tolerance of ICP-MS, and some common light elements (e.g., S,

Ca, Fe, K, Se) will have serious interferences in ICP-MS, which will also deteriorate its detection. Some common light elements (e.g. S, Fe, K, Se) have serious interferences in ICP-MS, which will also deteriorate its detection limit.

ICP-MS consists of an ICP torch as the ion source, an interface device and a mass spectrometer as the detector.

ICP-MS ionization source used is an inductively coupled plasma (ICP), the main body of which is a three-layer quartz casing torch tube, the upper end of the torch tube is wound with a loading coil, the three-layer tube from the inside to the outside of the carrier gas, respectively, through the gas, the auxiliary gas and the cooling gas, the loading coil by the high-frequency power supply coupling power supply to generate a magnetic field perpendicular to the coil plane. If the argon gas is ionized by the high-frequency device, the argon ions and electrons will collide with other argon atoms to produce more ions and electrons under the action of the electromagnetic field, forming an eddy current. The powerful current generates high temperature, instantly making the argon gas to form a plasma torch with a temperature of up to 10,000k. The sample being analyzed is usually in the form of an aerosol of aqueous solution introduced into the argon gas stream, and then into the center of the argon plasma at atmospheric pressure excited by the radio frequency energy, the high temperature of the plasma to make the sample desolventization, vaporization dissociation and ionization. A portion of the plasma passes through different pressure zones into a vacuum system where positive ions are pulled out and separated according to their mass-to-charge ratio. About 10 mm above the load coil, the torch temperature is about 8000 K. At such high temperatures, elements with ionization energies lower than 7 eV are completely ionized, and elements with ionization energies lower than 10.5 ev are ionized by more than 20%. Since most of the important elements have ionization energies below 10.5eV, they all have high sensitivity. A few elements with higher ionization energies, such as C, O, Cl, Br, etc., can also be detected, only with lower sensitivity.

(VII) Femtosecond Detection Methods

Femtosecond detection mainly uses femtosecond laser to study a variety of chemical processes and material composition, including chemical bond breaking, new bond formation, proton transfer and electron transfer, compound isomerization, molecular dissociation, reaction intermediates and final products of the velocity, angle, and state distribution, chemical reactions in solution and the role of the solvent, the vibration and rotation of molecules on chemical reactions. rotation of molecules on chemical reactions, etc. Femtosecond detection is an advanced detection technology, through the observation of molecules, atoms, electrons, nuclei, functional groups and other particles of the femtosecond level (one trillionth of a second, that is, 10-15s) of vibration, energy level jumps, can easily determine the composition of the material and content. Femtosecond detection technology can be used for unknowns analysis, formula analysis reduction, industrial diagnostics, satellite remote sensing, supercomputing, trace detection and analysis.