On the interference effect in the analysis of atomic absorption spectrometer

Abstract: Atomic absorption spectrometer can measure a variety of elements, flame atomic absorption spectrometry can measure the order of 10-9g / mL, graphite furnace atomic absorption method can measure the order of 10-13g / mL. The hydride generator can measure micro-traces of 8 volatile elements mercury, arsenic, lead, selenium, tin, tellurium, antimony, germanium, etc.

1. Interference effect

In atomic absorption spectrometry, interference effects can be divided into four categories according to their nature and causes:

1. Physical interference

2. Chemical interference

3. Ionization interference

4. Spectral interference

1 Physical interference

Physical interference refers to the effect of a decrease in the atomic absorption intensity due to changes in any physical characteristics of the sample (such as viscosity, surface tension, density, etc.) during the transfer, evaporation, and atomization of the sample. Physical interference is non-selective interference, and the effect on each element of the sample is basically similar.

The preparation of standard samples with similar composition to the tested samples is the most common method to eliminate physical interference. When the sample composition is not known or the sample cannot be matched, the standard addition method or dilution method can be used to reduce and eliminate physical interference.

2 Chemical interference

Chemical interference is caused by the formation of a thermodynamically more stable compound between the atoms of the measured element in the liquid phase or the gas phase and the components of the interfering substance, which affects the dissociation and atomization of the measured element compound. Phosphate interferes with calcium, and silicon and titanium form difficult-to-dissociate oxides, tungsten, boron, and rare earth elements to form difficult-to-dissociate carbides, thus preventing the relevant elements from effectively atomizing. These are all examples of chemical interference. Chemical interference is a selective interference.

Methods to eliminate chemical interference include: chemical separation; use of high-temperature flame; addition of release agent and protective agent; use of matrix improver, etc. For example, phosphate does not interfere with the determination of calcium in a high-temperature flame. Adding strontium, lanthanum, or EDTA can eliminate the interference of phosphate on the determination of calcium. In the graphite furnace atomic absorption method, a matrix modifier is added to increase the stability of the measured substance or reduce the atomic temperature of the measured element to eliminate interference. For example, mercury is very volatile, adding sulfide to produce mercury sulfide with high stability, the ashing temperature can be increased to 300 ℃; determination of Cu, Fe, Mn, As in seawater, adding NH4NO3 to convert NaCl into NH4Cl Before ashing, the ashing stage is lower than 500 ° C.

3 ionization interference

At high temperatures, the ionization of the atoms reduces the concentration of the ground state atoms and causes the atomic absorption signal to decrease. This interference is called ionization interference. The ionization effect increases with increasing temperature and ionization equilibrium constant, and decreases with increasing concentration of the measured element.

The addition of alkali metal elements that are easier to ionize can effectively eliminate ionization interference.

4 Spectral interference

Spectral interference includes overlapping spectral lines, non-absorbing lines in the spectral passband, direct-current emission in the atomization cell, molecular absorption, and light scattering. When using sharp line light source and AC modulation technology, the first three factors can generally not be considered, mainly considering the effects of molecular absorption and light scattering, which are the main factors that form the spectral background.

2. The effect of molecular absorption and light scattering

Molecular absorption interference refers to the interference caused by the absorption of radiation by gas molecules, oxides and salt molecules generated during the atomization process. Figure 3.10 shows the absorption band of sodium halide molecules. Light scattering means that the solid particles generated during the atomization scatter the light, so that the scattered light deviates from the optical path and is not detected.

3. The detector has detected that the absorbance value is too high.

In addition to the wavelength characteristics, the spectral background also has temporal and spatial distribution characteristics. The molecular absorption is usually generated before the atomic absorption signal. When there is a fast response circuit and a recording device, the molecular absorption and the atomic absorption signal can be resolved in time. The uneven distribution of sample vapor in the graphite furnace leads to the uneven distribution of background absorption space.

Increasing the temperature increases the concentration of background material that evaporates per unit time, and also increases the molecular dissociation. These two factors jointly restrict background absorption. In a constant temperature furnace, increasing the temperature and the rate of temperature increase significantly reduces the molecular absorption.

In the graphite furnace atomic absorption method, the effect of background absorption is more serious than that of flame atomic absorption method. If the background is not subtracted, sometimes it cannot be measured at all.

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