Atomic absorption spectroscopy (AAS) is a quantitative analysis technique based on the selective absorption of specific wavelength light radiation by substances. Its core physical and chemical basis can be decomposed into two modules: atomization process and characteristic absorption mechanism. The synergy between the two constructs the technical advantages of high sensitivity and high selectivity of this method.
1、 Atomization process: conversion from sample to free ground state atom
Atomization is a key step in AAS, which aims to dissociate the element to be tested from the sample into free ground state atoms, providing absorbing particles for subsequent characteristic absorption. According to different atomization methods, it can be mainly divided into two categories:
Flame atomization method
The sample is evaporated, dried, and dissociated by high-temperature flames (such as air acetylene flame at a temperature of about 2300 ℃ and nitrous oxide acetylene flame at a temperature of about 2700 ℃), ultimately generating ground state atoms.
Features: Easy to operate, low cost, fast analysis speed, suitable for the determination of most metal elements.
Limitations: The flame temperature is limited, and the sensitivity to refractory elements (such as tungsten and molybdenum) and elements that easily form non-volatile oxides (such as chromium and aluminum) is low.
Non flame atomization method (taking graphite furnace atomization as an example)
Using graphite tubes to generate high temperatures (up to 3000 ℃) under high voltage and high current, the sample undergoes four stages of drying, ashing, atomization, and purification to generate ground state atoms.
Features: High sensitivity (3-4 orders of magnitude higher than flame method), can directly analyze solid and liquid samples, suitable for the determination of trace elements such as lead and cadmium.
Limitations: The precision is slightly lower than that of flame method, and it is susceptible to interference from the substrate.
Factors affecting atomization efficiency:
Temperature: Insufficient temperature can lead to incomplete atomization, while excessive temperature may cause ionization interference.
Sample matrix: Complex matrices may affect atomization efficiency through chemical interference (such as generating non-volatile compounds) or physical interference (such as changing sample viscosity).
Injection method: Excessive injection volume may have a cooling effect on the flame or increase the difficulty of graphite furnace residue removal.
2、 Characteristic absorption mechanism: selective absorption of resonant radiation by ground state atoms
Feature absorption is the quantitative basis of AAS, and its core mechanism is as follows:
Resonance absorption phenomenon
When the frequency of incident radiation is equal to the energy required for electrons in an atom to transition from the ground state to the first excited state, the ground state atom absorbs energy and transitions to the excited state, forming a characteristic absorption spectrum.
Characteristic: Each element has a unique atomic structure, and its resonance absorption line has (such as 589.0nm for sodium and 285.2nm for magnesium), which can be used as a basis for element characterization.
Selectivity: Atoms have a high degree of selectivity in absorbing radiation, only absorbing radiation that matches their energy level difference, thereby avoiding background interference.
The quantitative relationship of Lambert Beer's law is that the absorbance (A) is proportional to the concentration of the element to be measured in the sample (C), i.e. A=K ⋅ C, where K is a constant and includes the influence of all experimental conditions.
Application: By measuring the absorbance of standard and unknown solutions and drawing a standard curve, the concentration of the element to be measured in the unknown solution can be obtained.
Prerequisite: The experimental conditions (such as temperature, atomization efficiency, and optical path length) need to be consistent to ensure
K
The constancy.
Widening of spectral lines and absorption intensity
Atomic absorption lines are not strictly geometric lines, but occupy a certain frequency range (half width of about 10-3-10-2nm), mainly influenced by the following factors:
Doppler broadening: caused by atomic thermal motion, the higher the temperature, the more significant the broadening.
Collision broadening: Inter atomic collisions result in a shortened average lifetime of excited atoms, causing spectral lines to widen (divided into Khruzmak broadening and Lorenz broadening).
Field induced broadening and self absorption effect: In strong electric fields or high concentration samples, spectral lines may further widen.
Impact: Widening of spectral lines will reduce absorption intensity, but by selecting sharp line light sources (such as hollow cathode lamps) and optimizing experimental conditions (such as controlling atomization temperature and pressure), the broadening effect can be minimized and measurement sensitivity can be improved.
3、 Technical Implementation: Instrument System and Interference Suppression
The physical and chemical foundation of AAS needs to be implemented through precision instrument systems, supplemented by interference suppression techniques:
Composition of Instrument System
Light source: emits characteristic resonant radiation of the tested element (such as hollow cathode lamps), requiring high radiation intensity, low background, and high stability.
Atomizer: Achieve sample atomization (flame or graphite furnace).
Spectral splitting system: Separate characteristic spectral lines from interference spectral lines (such as gratings or prisms).
Detection system: Convert optical signals into electrical signals (such as photomultiplier tubes or CCD detectors).
Interference suppression technology
Physical interference: eliminated by preparing standard solutions with similar compositions or using standard addition methods.
Chemical interference: adding release agents (such as phosphate to release calcium), protective agents (such as EDTA to protect iron), or matrix modifiers (such as lanthanum nitrate to improve the determination of aluminum).
Ionizing interference: Adding deionizing agents (such as alkali metal compounds) to suppress the ionization of the tested atom.
Spectral interference: suppressed by reducing slit width, selecting non resonant lines, or using Zeeman effect background correction techniques.