The sensitivity of an atomic absorption spectrophotometer (hereafter AAS ) decreases after some time of use. This immediately leads to an increase in the detection limit of the instrument, which even exceeds the requirements of the calibration regulations, so that it is classified as unfit for use. In this article, the reasons for the decrease in sensitivity and the corresponding countermeasures are analyzed of the AAS will be explored.
Optical path system of AAS
1. Is the position of the hollow cathode lamp the best?
The hollow cathode lamp can radiate the resonance line of the element to be measured and has sufficient radiation intensity to ensure a sufficient signal-to-noise ratio.
If the position of the hollow cathode lamp deviates, the light energy is lost in the light path.
The light signal entering the detector is weakened accordingly, and the measurement sensitivity of the instrument is reduced.
Therefore, after each lamp change, the position of the lamp socket should be adjusted to maximize the energy reading of the instrument.
2. Is the most sensitive spectral line selected?
A hollow cathode lamp of an element often has many emission lines.
The most sensitive spectral line should be selected for measurement.
Some resonance spectral lines are relatively close to each other. For example, the manganese lamp has a spectral line at 279.5 nm, 279.8 nm, and 280.1 nm, and the spectral line at 279.5 nm is the most sensitive.
If the wavelength display of the instrument has some error, it can be easily confused if not carefully distinguished.
3. Is the lamp’s current setting appropriate?
The light intensity of a hollow cathode lamp is related to the lamp current.
Increasing the lamp's operating current can increase the emission intensity.
However, if the operating current is too high, abnormal discharge will occur and the light intensity will be unstable.
If the lamp current is too low, the light intensity of the lamp will be weakened, resulting in a decrease in stability and signal-to-noise ratio.
Therefore, it is important to select a suitable lamp.
The optimum lamp current varies depending on the cathode element and lamp design.
AAS atomization system
1. Whether the injection capillary is clogged.
The function of the capillary is to aspirate the sample solution.
If the injection capillary is clogged with impurities, the injection speed will be greatly reduced and no strong signal will be generated.
It must be dredged out with a fine wire or replaced with a new capillary.
2. Is the pressure of the compressed air too low?
Compressed air is not only involved in combustion as an auxiliary gas, but also creates a negative pressure at the capillary orifice to take up the sample solution.
The drop in pressure of the compressed air leads to insufficient negative pressure at the capillary aspiration opening and slows down the aspiration speed of the sample.
Therefore, check whether the output pressure of the air compressor is too low, whether there are leaks in the air line and whether the air flow is set too low.
3. Whether there is a water seal in the waste liquid pipe
If there is no water seal in the waste water line or the water seal is not tight, the liquid outlet line will be connected to the outside atmosphere, the negative pressure in the atomization chamber will be reduced, and the sample suction speed will also be slowed down or even no sample will be sucked.
4. Whether the burner contains too many solid pollutants
After the burner has been in operation for some time, it picks up some solids, including carbon deposits and inorganic salts in the solution.
The presence of these substances significantly affects the characteristics of the flame, such as zigzag flame, unstable flame, uneven flame and so on.
At the same time, they increase the signal noise, which leads to unstable measurement results.
Therefore, it needs to be cleaned in time.
Combination of optical path system and atomization system
The generation of luminosity is caused by the absorption of the characteristic spectral lines emitted from the light source by the atomic absorption spectrophotometer in the ground state.
The greater the number of ground state atoms, the greater the absorption.
When the line of the light source passes through the burner, the line of the light source should pass completely and parallel through the flame, i.e., the atomized layer, to produce a strong enough absorption signal so that the measurement sensitivity is high. Therefore, check and adjust the height and orientation of the burner.
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