How to choose measurement conditions when AA-1800 atomic absorption spectrometry is determined - Database & Sql Blog Articles

How to choose the measurement conditions when AA-1800 atomic absorption spectrometry

Key words: atomic absorption spectrometry; measurement conditions; analysis instrument ; AA-1800 Atomic absorption method is much smaller than atomic emission spectroscopy for the following reasons: 1. Use sharp line source in AAS method The resonance absorption line is applied, and the number of absorption lines is much smaller than that of the emission line, the probability of spectral overlap is small, and the spectral interference is small; 2. In the AAS method, the ground state atoms are involved, so the influence of the flame temperature is small. However, in actual work, interference cannot be ignored, and it is necessary to understand the causes and elimination methods. In atomic absorption spectroscopy, interference mainly includes four types: physical interference, chemical interference, spectral interference, and background interference. Selection of measurement conditions: 1. Selection of analysis line Usually, the resonance line of the element is selected as the analysis line, which makes the measurement have higher sensitivity. But not in all cases. When analyzing a sample with a higher concentration of the measured element, a non-resonant line with lower sensitivity may be selected as the analysis line. Otherwise, the value of A is too large. In addition, consider the problems of self-absorption and interference of the line. Second, the hollow cathode lamp current hollow cathode lamp's emission characteristics depend on the operating current. The lamp current is too small, the discharge is unstable, and the intensity of the light output is small; when the lamp current is too large, the emission line is widened, resulting in a decrease in sensitivity and a shortened lamp life. When selecting the lamp current, try to use a lower operating current while maintaining a stable and suitable light output. General commodity hollow cathode lamps are marked with the maximum current allowed and the current range that can be used. Usually, 1/2 ~ 2/3 of the maximum current is used as the operating current. In actual work, the most suitable current should be determined experimentally. The optimum operating current is selected by measuring the change in absorption value with the lamp current. Hollow cathode lamps generally have to be preheated for 10 to 30 minutes before use. Third, the choice and adjustment of flame flame is an important factor affecting the efficiency of atomization. Which flame to choose depends on the object of analysis. For low temperature and medium temperature flames, suitable elements can use acetylene-air flame; in flames, it is easy to form difficult dissociated compounds and insoluble oxide elements, acetylene-nitrous oxide high temperature flame should be used; elements with line below 220 nm should be analyzed. A hydrogen-air flame can be used. After the flame type is selected, the ratio of gas to combustion gas must be adjusted by experiment to obtain the flame of the desired characteristics. It is easy to form an element that is difficult to dissociate the oxide, and a rich flame is used; if the oxide is unstable, a stoichiometric flame or a lean burn flame should be used. A suitable fueling ratio should be determined experimentally. Fourth, the burner height burner height is to control the light source beam through the flame area. Since the distribution of free atomic concentration varies with flame height in the flame zone, it varies with flame conditions. Therefore, it is necessary to adjust the height of the burner so that the measuring beam passes through a region having a large free atom concentration, and a higher sensitivity can be obtained. As shown in Figure 8-27, for Cr with high oxide stability, as the flame oxidation property increases, the tendency to form oxide increases, and A decreases accordingly. Conversely, for Ag with high oxide instability, its atom The concentration is mainly determined by the dissociation rate of the silver compound, and A increases as the flame potential increases. For Mg with moderate oxide stability, A increases with the height of the flame, and after reaching the maximum, it decreases with the height of the flame. Therefore, the height of the burner must be carefully adjusted during the measurement. V. Slit Width The slit width affects the energy of the spectral passband and the detector receiving radiation. The slit width is chosen to separate the absorption line from the adjacent interference line. When there is a line of interference entering the spectral passband, the absorbance value will decrease immediately. The maximum slit width that does not cause the decrease in absorbance is the appropriate slit width to be selected. In atomic absorption analysis, the probability of spectral line overlap is small, so wider slits can be used to increase light intensity and reduce detection limits. In the experiment, the complexity of the spectral line of the measured element should also be considered. The alkali metal and alkaline earth metal lines are simple, and the larger slit width can be selected. The excessive element and the rare earth element are more complicated, and the smaller narrow is selected. Sew width. A suitable slit width should also be determined experimentally.