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

How to Choose Measurement Conditions When Using AA-1800 Atomic Absorption Spectrometry

Key words: atomic absorption spectrometry; measurement conditions; analysis instrument; AA-1800. The atomic absorption method is much more sensitive than atomic emission spectroscopy for several reasons. First, the AAS method uses a sharp line source, typically the resonance absorption line, which results in fewer absorption lines compared to emission lines. This reduces the likelihood of spectral overlap and minimizes spectral interference. Second, AAS relies on ground state atoms, making it less affected by flame temperature. However, in real-world applications, various types of interferences still need to be considered and managed. In atomic absorption spectroscopy, the main sources of interference include physical, chemical, spectral, and background interferences. To ensure accurate measurements, selecting the right measurement conditions is crucial. Here are some key factors to consider: 1. **Selection of Analysis Line**: Usually, the resonance line of the element is chosen as the analysis line due to its high sensitivity. However, this may not always be the best option. For samples with high concentrations of the target element, a non-resonant line with lower sensitivity might be preferred to avoid excessive absorbance values. Also, self-absorption and other line interferences should be taken into account. 2. **Hollow Cathode Lamp Current**: The performance of the hollow cathode lamp depends on the operating current. Too low a current can lead to an unstable discharge and weak light output, while too high a current can cause broadening of the emission line, reducing sensitivity and shortening the lamp’s lifespan. It's generally recommended to use 1/2 to 2/3 of the maximum current specified for the lamp. Experimentation is often necessary to determine the optimal current that provides stable and reliable results. Most lamps also require preheating for 10 to 30 minutes before use. 3. **Flame Selection and Adjustment**: The choice of flame plays a significant role in atomization efficiency. Different elements require different flame types. For example, acetylene-air flames are suitable for elements that form stable oxides at lower temperatures. Acetylene-nitrous oxide flames are ideal for elements that produce difficult-to-dissociate compounds. Hydrogen-air flames are used for elements with absorption lines below 220 nm. Once the flame type is selected, the fuel-to-oxidant ratio must be adjusted experimentally to achieve the desired flame characteristics. A rich flame is useful for elements prone to oxide formation, while a lean or stoichiometric flame is better for unstable oxides. 4. **Burner Height**: Adjusting the burner height ensures that the light beam passes through the region of the flame where free atoms are most concentrated. This directly affects the sensitivity of the measurement. For instance, chromium (Cr) shows reduced absorbance as the flame becomes more oxidizing, while silver (Ag) shows increased absorbance under oxidizing conditions. Magnesium (Mg) exhibits a peak absorbance at a certain flame height before decreasing. Therefore, careful adjustment of the burner height is essential for optimal results. 5. **Slit Width**: The slit width controls the spectral bandwidth and the amount of light reaching the detector. It should be set to separate the absorption line from any interfering lines. If an interfering line enters the spectral passband, the absorbance value will drop. The maximum slit width that doesn’t cause a decrease in absorbance is usually the best choice. Since spectral line overlap is rare in AAS, wider slits can be used to increase light intensity and reduce detection limits. However, the complexity of the element’s spectrum should also be considered. Alkali and alkaline earth metals have simpler spectra, allowing for wider slits, while heavier elements and rare earths require narrower slits. Experimental testing is always recommended to find the optimal slit width.