After using an atomic absorption spectrophotometer (AAS) for a period of time, its sensitivity will decrease. This not only leads to inaccurate measurement results, but may even exceed the requirements of the calibration procedure, resulting in the instrument being deemed unqualified. In this article, we will explore methods and techniques for improving AAS performance to ensure reliable and accurate results in every analysis.
Tips for improving the sensitivity of atomic absorption spectrophotometers:
Improving analytical sensitivity is almost an eternal topic in analytical chemistry. Instrument manufacturers and analytical workers are always trying to make high-sensitivity instruments and develop high-sensitivity methods. Especially in environmental analysis, drug analysis, and food analysis, regulatory methods have high requirements for sensitivity. Here are some common tips for improving the sensitivity of atomic absorption spectrophotometers:
Lamp current
Flame atomic absorption spectrophotometers typically use hollow cathode lamps as their light source, and the only operating parameter for hollow cathode lamps is the lamp current. Generally, increasing the lamp current can enhance emission intensity but reduces the instrument's sensitivity. Conversely, within a certain range, reducing the lamp current can lower radiation intensity, thereby improving instrument sensitivity, but this also decreases lamp stability and signal-to-noise ratio. Therefore, selecting the appropriate lamp is critical, and the optimal lamp current varies depending on the cathode element and lamp design.
Nebulizer
The nebulizer atomizes the test solution. It is an important component of atomic absorption spectrophotometers, and its performance has a significant impact on measurement sensitivity, precision, and chemical interference. The more stable the nebulizer spray, the smaller and more uniform the mist droplets, and the higher the atomization efficiency, resulting in higher sensitivity.
Sample volume
The size of the sample volume in an atomic absorption spectrophotometer affects sensitivity. Methods to increase the sample volume include:
Increasing the auxiliary gas flow rate. This increases the negative pressure, thereby increasing the sample volume.
Shorten the length of the sample tube. Shorter sample tubes reduce tube resistance, thereby increasing the flow rate of the sample solution. Conversely, to reduce the lift volume, you can decrease the flow rate of the auxiliary gas or lengthen the sample tube.
Spectral line
A hollow cathode lamp for a given element often has multiple spectral lines, and the most sensitive spectral line
should be selected for measurement. Resonance lines typically have the highest sensitivity and are often used as analysis lines.
Burner Position and Cleaning
Adjust the height and front-to-back position of the burner so that the light beam from the hollow cathode passes through the flame region with the highest concentration of free electrons, at which point sensitivity is highest and stability is best. When high sensitivity is not required, such as when measuring high-concentration solutions, the sensitivity can be reduced by rotating the burner angle to facilitate detection.
Additionally, after prolonged use, the burner may accumulate solid substances, including carbon deposits and inorganic salts from the solution.
The presence of these substances can significantly affect flame properties, such as sawtooth flames, flame instability, and uneven flame distribution.
They can also increase signal noise, leading to unstable measurement results.
Therefore, timely cleaning is necessary.
Flame
The type and state of the flame play a crucial role in determining sensitivity levels, and the appropriate flame should be selected based on the characteristics of the element being measured. Currently, flames are classified into three types: air-hydrogen flame, air-acetylene flame, and nitric oxide-acetylene flame.
The air-hydrogen flame has a lower temperature and is used to determine elements that are easily atomized in the flame, such as arsenic and selenium;
The air-acetylene flame is a medium-temperature flame and is used to determine elements that are difficult to dissociate in the flame, such as magnesium, calcium, copper, zinc, lead, and manganese;
Nitrogen oxide-acetylene flames are high-temperature flames and are used to determine elements that are difficult to dissociate in the flame, such as vanadium and aluminum.
Slit
Under constant conditions such as lamp current and negative high voltage, the smaller the slit, the higher the sensitivity. When the element being measured has no adjacent interference lines, such as potassium and sodium, a larger slit can be used. When the element being measured has adjacent interference lines, such as calcium, iron, and magnesium, a smaller slit can be used.
Sample injection capillary tube
The capillary tube is used to draw in the sample solution.
If the sample injection capillary tube is contaminated or blocked, the injection rate will decrease significantly, and a strong signal cannot be generated.
It must be cleared with a fine wire or replaced with a new capillary tube.
Compressed air pressure
Compressed air not only acts as an auxiliary gas in combustion, but also creates negative pressure at the capillary inlet, thereby absorbing the sample solution.
A decrease in compressed air pressure can result in insufficient negative pressure at the capillary inlet, slowing down the sample intake speed.
Therefore, it is necessary to check whether the output pressure of the air compressor is too low, whether there are any air leaks in the air supply system, and whether the air flow rate setting is too low.
Conclusion
The above factors affecting sensitivity are contradictory and complementary. In specific testing work, testers should consider all factors comprehensively and adjust them according to the instrument and the sample being tested to achieve the best working condition.
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