1. Combustion characteristics of flame
The ignition limit, ignition temperature and burning speed are the burning characteristics of the flame, and are commonly referred to as the three elements of flame. For a characteristic mixed gas of fuel gas and auxiliary gas, only when the percentage of gas in the mixed gas is within a certain range, the combustion can start and expand into the mixed gas to form a flame. The upper and lower limits of this gas content are called ignition limits. Within the ignition limit, combustion can spontaneously extend to the lowest temperature of the entire mixed gas, called the ignition temperature. At a certain point of the combustible mixed gas, its temperature will start to burn as soon as it reaches the ignition temperature. Due to the heat conduction, this point of the combustion reaction mixture will be propagated to the adjacent gas layer. In addition to the loss, the temperature of the adjacent gas layer can be increased to its ignition temperature, and the combustion reaction continues and propagates to the entire combustible mixture at a constant speed. Form a flame. This propagation speed is the burning speed of the flame. The three elements of the flame depend on many factors such as the nature and composition of the combustible gas mixture, initial pressure and temperature, the structure of the burning vessel and the nature of the walls.
In actual use, the burning speed of the flame is the most important factor among the three elements, which directly affects the safe use of the flame and stable combustion. The burning speed of the flame is related to the gas composition, initial temperature, humidity and airflow speed. To make the flame burn stably and safely, the combustion speed should be equal to or less than the vertical component of the air flow velocity on the front of the flame. The air flow velocity depends on the gas supply pressure, the structure and shape of the burner. Under the air supply pressure, the airflow speed depends on the opening area of ​​the burner. If the slit is wide and long, the airflow speed is small, and vice versa.
2. Flame temperature
Flame temperature is one of the main characteristics of a flame. It plays an important role in the formation and dissociation of compounds in the flame and the atomization of the elements to be measured. In the flame, on the one hand, the combustible gas mixture generates a lot of thermal energy according to its combustion reaction. On the other hand, due to the dissociation of the compounds in the flame, and in order to raise the equilibrium mixture in the flame to the flame temperature, it requires heat consumption, and flame gas The volume expansion during combustion also consumes part of the energy. The balance of heat energy in these two aspects determines the flame temperature. When the flame is in thermal equilibrium, the temperature can be used to characterize the true energy of the flame. For the above reasons, under normal pressure, the maximum temperature of the chemical flame is only about 3000 ℃.
When the spray test liquid enters the flame, the flame consumes a large amount of heat to evaporate and decompose the test solution solvent, and raise the decomposition products to the flame temperature, which results in a decrease in the flame temperature. If the solvent is water, for low-temperature flames, this cooling effect is not obvious due to the small amount of water decomposed by the flame, but for high-temperature flames, this cooling effect is very significant due to the large amount of decomposed water. If organic solvents such as pyrolysis alcohol are used Solvents, because they can also burn and release a lot of heat in the flame, introducing them into low-temperature flames will help to raise the flame temperature, but for high-temperature flames, they can not significantly increase the flame temperature, but still dominate the cooling effect Therefore, in order to ensure the effect of flame atomization, in practical work, care should be taken to select the appropriate sample solvent and the amount of liquid inflow.
Flames used in atomic absorption spectroscopy generally burn directly in the atmosphere. The dissociation of the gas diffused from the outside into the flame will also affect the flame temperature.
All reactions are strong endothermic reactions. When dissociating, the heat generated by the combustion reaction is consumed to reduce the flame temperature. For atomic absorption spectroscopic analysis, only ground-state atoms are effective for atomic absorption analysis. This requires that the flame must have sufficient temperature to ensure that the sample is fully evaporated and the element compound to be tested is dissociated into free atoms. In this sense, the flame temperature should be as high as possible, but after the flame temperature is increased, the flame emission intensity increases, the Doppler effect is enhanced, the absorption line becomes wider, and the gas expansion factor increases, so that the free atom concentration in the phase Reduced, resulting in reduced sensitivity of the measurement.
In addition, for elements with lower ionization potentials, such as Na, K, Rb, and Cs, the high flame temperature causes them to undergo severe ionization in the flame, and the ground state atomic concentration decreases. Therefore, in actual work, the temperature selection should be completed according to the nature of the sample and the physical characteristics of the measured element.
3. Flame composition
The composition of the flame determines the redox characteristics of the flame, and directly affects the decomposition of the test element compounds and the formation of difficult-to-dissociate compounds, which in turn affects the atomization efficiency and the effective life in the free atomic flame zone. There are many factors that affect the flame composition, such as the type of flame, the fuel-to-fuel ratio of similar flames, and the combustion environment of flames. For the same type of flame, it can be divided into rich combustion flame, stoichiometric flame and lean combustion flame according to the change of combustion assist ratio. The so-called stoichiometric flame means that the ratio of combustion assistance completely conforms to the combustion reaction coefficient ratio of the fuel gas and the combustion gas. This flame has the highest temperature, but the flame itself does not have redox characteristics. Rich combustion flame refers to a flame with a combustion ratio greater than that of a stoichiometric flame. Although the flame temperature is slightly lower than that of a stoichiometric flame, it increases the concentration of carbon atoms in the flame due to the increase of gas, which makes the flame have a certain The reducibility of the gas is conducive to the generation of ground-state atoms; lean burn flames are flames with a gas that is less than the stoichiometric flame-combustion ratio. This flame has a lower temperature and has obvious oxidation. This flame is mostly used for alkali metals Determination of such easily ionizable elements.
In the atomic absorption spectroscopy analysis, the rich combustion flame is used more. It has been shown that in the air-acetylene flame, when the content of acetylene increases, the partial pressure of O, OH and other gases in the flame decreases, and the concentration of carbon atoms increases. The overall flame reduction is enhanced. When the photoatomic ratio of carbon and oxygen is C / O = 1, the flame composition and properties are abruptly changed, and gas molecules such as H2O, CO2, and O2 completely disappear from the flame, and the concentration of free radicals such as O and OH is reduced by 5? Orders of magnitude. If the atom increases by 4 orders of magnitude, the flame will shine. If the amount of acetylene is further increased, the solid carbon particle concentration will increase and the flame will be brighter, but the reduction will remain unchanged and the flame temperature will decrease.
Using organic solvents to spray into the flame can change the composition and characteristics of the flame. For hydrogen flames, the introduction of organic solvents only affects the flame temperature, because the product of the hydrogen flame combustion is water, and water and fire are incompatible. However, if the organic solvent is introduced into the hydrocarbon flame, it can not only be used as an additional heat source to increase the flame temperature, but more importantly, the flame composition and reaction characteristics are changed. According to the difference in the C / O ratio in the organic solvent, the solvent can be divided There are three types. Reducing solvents with a C / O ratio greater than 1 are solvents such as C6H6, C2H5OH, etc. They can increase the C / O ratio of high flames. C / O ratio equal to 1 are neutral solvents such as CH3OH. Its introduction will not change the C / O ratio in the flame. C / O ratio less than 1 is oxidizing solvents, such as HCOOH, H2O, etc., their introduction will reduce the C / O ratio of the flame.
Four, flame transmission performance
Different flame types have different absorption capabilities for different wavelengths, and the flame emission characteristics are also different. Hydrocarbon flames have greater absorption in the short-wave region, while hydrogen flame absorption is smaller. Therefore, for those resonance lines located in the short-wave region Elements, such as As, Se, Pb, Zn, Cd, etc., are preferably air-hydrogen flames to reduce the effects of flame absorption. The air-acetylene flame has different emission signals in the entire visible light region, and these emission signals mostly come from the radiation bands of the excited molecules in the flame. Nitrous oxide-air has N molecular bands. These emission signals increase the noise of the flame and reduce the measurement accuracy.
5. Several common chemical flames
The gas mixtures used for atomic absorption spectrometric analysis are: air-hydrogen, argon-hydrogen, air-propane, air-acetylene, and nitrous oxide-acetylene. The flame temperature of hydrogen gas is not too high (about 2000 ℃), but this hydrogen flame has a relatively low emission background and absorption background, suitable for the analysis of elements (such as As, Se, etc.) whose resonance line is in the ultraviolet region. The air-propane flame temperature is lower (about 1900 ° C), and the interference effect is large. It is only suitable for those elements that are easy to volatilize and dissociate, such as alkali metals and Cd, Cu, Pb, etc. The most widely used flames are the latter two flames, which are currently used for atomic absorption analysis.
1. Air-acetylene flame
Using air-acetylene flame atomic absorption spectrometry analysis can analyze about 35 elements, the temperature of this flame is about 2300 ℃, air-acetylene flame combustion is stable, good reproducibility, low noise, burning speed is not too much, 158cm / sec, but the flame temperature is higher, the highest temperature can reach 2500 ℃, the dissociation energy of MO is greater than 5ev elements such as AL (5.89), Ti (6.9), Zr (7.8), Ta (8.4), etc. Most elements have sufficient sensitivity. Adjusting the flow ratio of air and acetylene can change the combustion assist ratio of this flame so that it has different oxidation-reduction characteristics, which is conducive to the analysis of elements with different properties. The air-acetylene flame is safer to use and simpler to operate. The disadvantage of this flame is that the flame has obvious absorption of radiation with a wavelength of less than 230nm, especially the bright rich flame, due to the presence of unburned carbon particles, the flame emission and self-absorption are enhanced, and the noise increases. Another disadvantage of this flame is that the temperature is not high enough. For the elements B, Be, Y, Sc, Ti, Zr, Hf, V, Nb, Ta, W, Th, u, and Rare earth elements, etc., this flame atomization efficiency is low.
2. Nitrous oxide-acetylene flame is also commonly known as laughing gas-acetylene flame. The temperature of this flame can reach 2900 ℃, which is close to oxygen-acetylene flame (about 3000 ℃) can be used to determine those elements that form refractory oxide . The burning speed of this flame is 160cm / sec, which is close to the air-acetylene flame. The use of this flame greatly expands the application range of flame atomic absorption spectrometry and can measure more than 70 elements.
The argon oxide nitrogen-acetylene flame is strongly reducing, so it can reduce or even eliminate the chemical interference in the determination of certain elements. For example, when measuring Ca with an air-acetylene flame, interference will occur in the presence of phosphate, while Ac will cause interference when measuring Mg, but with nitrous oxide-acetylene flame measurement, all the above interference will disappear, and interference ions over 100 times will not affect the measurement . The atomization efficiency of the nitrous oxide-acetylene flame is extremely sensitive to changes in the flow rate of fuel gas and auxiliary gas. Therefore, in actual work, the fuel-assisted ratio and
Jieyang Huiyi Hardware Products Co., Ltd. , https://www.chinagdhuiyi.com