Analysis section: one question per week (October 11 – October 16)

This post was last edited by Cultivator on 2011-10-11 at 13:15. Interferences are classified into chemical interferences, physical interferences, ionization interferences, spectral interferences, and background interferences. Methods to eliminate chemical interferences include changing the flame temperature, adding releasing agents, adding protective chelating agents, and adding buffers. Methods to eliminate background interferences include the dual-wavelength method, deuterium lamp correction, self-absorption method, and Zeeman effect method. Advantages and disadvantages of atomic absorption spectroscopy. 　　(1) Low detection limit and high sensitivity. The detection limit of flame atomic absorption spectroscopy can reach the 10-9 level, while that of graphite furnace atomic absorption spectroscopy can reach 10-14 to 10-10 g. 　　(2) Good analysis accuracy. The relative standard deviation for the determination of medium and high concentration elements by flame atomic absorption spectroscopy can be less than 1%, and its accuracy is already close to that of classical chemical methods. The analytical precision of graphite furnace atomic absorption spectroscopy is generally 3%–5%. 　　(3) Fast analysis speed. An atomic absorption spectrometer can continuously determine 6 elements in 50 samples within 35 minutes. 　　(4) Wide range of applications. Over 70 elements can be measured; not only metal elements can be determined, but also non-metal elements and organic compounds can be measured using indirect atomic absorption spectroscopy. 　　(5) The instrument is relatively simple and easy to operate. (6) The drawback of atomic absorption spectroscopy is that simultaneous determination of multiple elements remains difficult, and the sensitivity for quite a number of elements is not yet satisfactory.

1. Background interference is a special form of spectral interference. It arises from the incomplete separation of the signal of the substance being analyzed from the signals caused by optical interference. During the flame atomization process, absorption is caused by molecules of accompanying components, as well as scattering of the light source radiation by non-vaporizing particles formed within these accompanying components; this represents a type of non-selective absorption. 2. Methods for eliminating background absorption include extraction and separation techniques, as well as optical and chemical methods.

Background interference in atomic absorption spectroscopy mainly refers to the interference effects caused by molecular absorption during the atomization process and light scattering resulting from solid particles. Background interference often increases the absorbance, resulting in a positive error. Suppression of background interference: In practical applications, suppressing molecular absorption interference is often achieved by changing the type of flame, the fuel-to-oxidizer ratio, and adjusting the height of the flame observation area ； In graphite furnace atomic absorption spectroscopy, appropriate matrix modifiers are often used, employing selective volatilization to suppress the interference of molecular absorption.

Methods for eliminating background interference: dual-wavelength method, deuterium lamp correction method, self-absorption method, Zeeman effect method

In atomic absorption spectroscopy, interference effects can be classified into four categories based on their nature and causes: 1. Physical interference; 2. Chemical interference; 3. Ionization interference; 4. Spectral interference. Physical interference refers to the decrease in atomic absorption intensity that occurs during the transfer, evaporation, and atomization of the sample, as a result of changes in any physical properties of the sample such as viscosity, surface tension, density, etc. Physical interference is non-selective interference, and its effect on the various elements in the sample is generally similar. 　　Preparing a standard sample with a composition similar to that of the sample under test is the most common method for eliminating physical interference. When the composition of the sample is unknown or it cannot be matched, the standard addition method or dilution method can be used to reduce and eliminate physical interferences. Chemical interference occurs when atoms of the element being analyzed in the liquid or gas phase form thermodynamically more stable compounds with interfering substances, thereby affecting the dissociation of the compound of the element being analyzed and its atomization. The interference of phosphate with calcium, the formation of poorly dissociable oxides by silicon and titanium, and the formation of poorly dissociable carbides by tungsten, boron, rare earth elements, etc., all prevent the relevant elements from being effectively atomized – these are all examples of chemical interferences. Chemical interference is a type of selective interference. 　　Methods to eliminate chemical interference include: chemical separation ； Use a high-temperature flame ； Add a release agent and a protective agent ； Use matrix modifiers, etc. For example, phosphate does not interfere with the determination of calcium in high-temperature flames; the addition of strontium, lanthanum, or EDTA can eliminate the interference of phosphate on calcium determination. In graphite furnace atomic absorption spectroscopy, a matrix modifier is added to improve the stability of the substance being analyzed or to reduce the atomization temperature of the element in question, thereby eliminating interferences. For example, mercury is highly volatile; by adding sulfides, mercury sulfide with higher stability is formed, and the ashing temperature can be increased to 300℃ ； Cu, Fe, Mn, and As in seawater were determined; NH4NO3 was added to convert NaCl into NH4Cl, which was removed during the ashing stage at temperatures below 500°C prior to atomization. Ionization interference: At high temperatures, atoms get ionized, which reduces the concentration of atoms in the ground state and leads to a decrease in the atomic absorption signal; this type of interference is known as ionization interference. The ionization effect increases with rising temperature and an increasing ionization equilibrium constant, while it decreases as the concentration of the element being measured increases. 　　Adding more easily ionizable alkali metal elements can effectively eliminate ionization interference. Spectral interference includes spectral line overlap, the presence of non-absorbing lines within the spectral bandwidth, DC emission in the atomization cell, molecular absorption, light scattering, and so on. When a sharp-line light source and AC modulation technology are used, the first three factors can generally be disregarded; attention should be focused primarily on the effects of molecular absorption and light scattering, which are the main factors contributing to the spectral background. 　　Effects of molecular absorption and light scattering Molecular absorption interference refers to the interference caused by the absorption of radiation by gas molecules, oxide molecules, and salt molecules generated during the atomization process. Figure 3.10 shows the absorption spectra of sodium halide molecules. Light scattering refers to the scattering of light by solid particles generated during the atomization process, causing the scattered light to deviate from its path and not be detected by the detector, which results in an elevated absorbance value. 　　In addition to wavelength characteristics, the spectral background also has temporal and spatial distribution characteristics. Molecular absorption usually occurs before the atomic absorption signal; with fast-response circuits and recording devices, it is possible to distinguish between molecular absorption and atomic absorption signals in terms of time. The non-uniform distribution of the sample vapor within the graphite furnace leads to a non-uniform spatial distribution of background absorption. 　　Raising the temperature increases the concentration of background substances evaporated per unit time, and it also increases molecular dissociation. These two factors jointly constrain background absorption. In a constant-temperature furnace, increasing the temperature and the rate of temperature rise results in a significant decrease in molecular absorption. Background correction techniques: (1) Dual-wavelength background correction method (rarely used); (2) Continuous light source background correction method; (3) Zeeman effect background correction method; (4) Self-absorption effect background correction method; (5) Continuous light source with stepped monochromator wavelength scanning background correction method (CEWM) (no commercial instruments available yet)

Background interference mainly refers to the interference effects caused by molecular absorption during the atomization process and light scattering resulting from solid particles. In practical applications, methods to eliminate this interference include changing the flame type, the fuel-to-oxygen ratio, and adjusting the height of the flame observation area in order to suppress molecular absorption effects ； In graphite furnace atomic absorption spectroscopy, appropriate matrix modifiers are often used, employing selective volatilization to suppress the interference of molecular absorption.

Reply to 1# *ngguanglueguo: Hehe, I only know that there are physical interference, chemical interference, spectral interference, and ionization interference. As my knowledge is limited, I really don’t know how to eliminate them

The main background interferences include: 1. molecular absorption, 2. light scattering, 3. absorption by flame gases and inorganic acids in the medium. Methods to eliminate these interferences: 1. dilute the sample when the concentration is too high, 2. use less sensitive spectral lines