Atomic Fluorescence Spectrometer (AFS) is a highly sensitive tool for trace element analysis. The core of its performance lies in the unique mechanism of gaseous atomic fluorescence generation and the precisely matched optical path design, which together form the cornerstone of AFS's high sensitivity and low detection limit.
1、 The mechanism of generating gaseous atomic fluorescence
The generation of atomic fluorescence is an excitation relaxation photophysical process, and its mechanism can be decomposed into three key steps:
Atomization and formation of the ground state of gaseous free atoms: Similar to atomic absorption spectroscopy (AAS), the sample solution is atomized and sent to a high-temperature atomizer (usually an argon hydrogen flame or an electric heating/vapor generation atomizer). Here, the tested element is dissociated, forming a large number of gaseous free atoms in the ground state.
Photoexcitation resonance absorption: This is the fundamental difference between AFS and AAS. A specific wavelength sharp light source emitted by high-intensity hollow cathode lamps (HCL) or electrodeless discharge lamps (EDL) precisely illuminates these ground state atoms. When the energy of a photon is exactly equal to the difference between the ground state and an excited state energy level of an atom, the ground state atom will selectively absorb the photon and transition to an unstable excited state.
Fluorescence emission - radiative relaxation: Atoms in an excited state have a very short lifetime (about 10 ⁻⁸ seconds), spontaneously relaxing back to lower energy levels (usually the ground state) and releasing energy in the form of photons. The light emitted during this process is called atomic fluorescence. Its key feature is that the emission wavelength can be the same as the excitation wavelength (resonance fluorescence) or different (non resonance fluorescence), but both carry characteristic information of the tested element.
2、 Optical path design
In order to efficiently implement the above mechanism and maximize signal acquisition, the optical path of AFS adopts two designs, "dispersive" and "non dispersive", which are completely different from AAS. The non dispersive type is more common due to its high light collection efficiency.
The core design concept is "light source and detector arranged at right angles":
Excitation light path (vertical direction): The excitation beam emitted by a high-intensity light source directly passes through the center of the atomizer and interacts with atomic vapor. The goal of this optical path is to maximize excitation efficiency.
Fluorescence collection optical path (horizontal direction): The photomultiplier tube (PMT) detector is placed at a 90 degree angle to the excitation beam. The core advantage of this right angled geometric layout is that it can minimize the entry of strong excitation direct light from the light source into the detector, thereby reducing scattered background interference.
Optical filtering system: In front of PMT, one or more day blind tubes or optical filters will be equipped. Its function is to only allow the characteristic fluorescence wavelength of the tested element to pass through, and firmly block other stray light (especially the scattered light of the excitation light) and the background radiation emitted by the atomizer itself. This is the key to achieving high signal-to-noise ratio.
In summary, atomic fluorescence spectrometers use high-intensity light sources for effective photoexcitation, and cleverly separate weak atomic fluorescence signals from strong background noise using a right angle optical path and filtering system. This collaborative optimization design from the generation mechanism to the detection optical path endows it with sensitivity and anti-interference ability far beyond AAS, especially in the detection of elements such as mercury, arsenic, and selenium that are prone to forming hydrides.