Atomic absorption spectrometry plays an important role in the trace and trace analysis of inorganic elements. It is also the most important analytical instrument in spectral analysis. It is used in geology, metallurgy, environmental testing, medical, commodity inspection and other industries and colleges. It is widely used in schools and research institutes. At present, all manufacturers of atomic absorption have advantages in technology. The accuracy of domestic flame analysis can also be compared with foreign instruments, but in general, foreign manufacturers in instrument automation, background correction technology, graphite furnace atomization, flame atomization atomization The development of improved (atomic capture), continuous light source and instrument innovation technology is better than the domestic momentum . Of course, different levels have different users, different users have different choices, as long as they do their best, people do their best. I think it is not bad. This is my point of view.
For the purchase of atomic absorption, personally think that the following should be understood:
Do you use atomic absorption for general analysis or research (considering the grade of the machine)?
What industry samples do you consider (considering the measured substrate)?
What elements should be analyzed in the sample (considering the way AAS is determined)?
What is the range of measured elements in the sample (considering the accuracy and choice of the assay)?
How much money does the leader have prepared for you (considering the scope of the machine)?
The following focuses on several aspects that users think should be noted when buying an atomic absorption spectrometer for ordinary users:
Note: The following is only a unilateral performance, and a perfect atomic absorption needs to see whether the overall performance of the design is balanced, the level of knowledge of the application personnel. Due to the limited level, it is inevitable that mistakes will be missed. I hope that my friends in the same industry will not be advised.
1. Optical path system: The optical path system should mainly understand the system's light source and light source distribution, monochromator structure, dispersion element performance, wavelength scanning and performance, spectral bandwidth, and detector performance.
1.1 Light source and light source distribution:
The atomic absorption light source is mainly hollow cathode lamp, electrodeless discharge lamp and continuous light source. The technology for manufacturing hollow cathode lamp is relatively mature, and there is no big problem. The electrodeless discharge lamp currently only has arsenic, antimony, cadmium, antimony, bismuth, antimony and mercury. The elements of phosphorus, lead, calcium, barium, strontium, selenium, titanium and zinc have the advantages of small background, high emission intensity and less interference from the light source, but the cost is also high. As for the continuous light source is the latest development technology, in order to cooperate with other components to play its powerful functions. In general, high light intensity, high stability, and low interference are required as the light source. It is necessary to pay attention to the selection of elements such as arsenic, mercury, antimony and antimony which are low in sensitivity when measured by hollow cathode lamps. The light source distribution is simply a structure of a hollow cathode lamp holder (the continuous light source does not consider this problem). Nowadays, the general atomic absorption spectrometer has at least two lamp holders, some as many as eight, with many lamps and one pre-ignition. It can reduce the time to wait for the preheating of the hollow cathode lamp during the measurement process. In fact, this is an advantage, but the VARIANAA280FS uses a fast sequence technology, which is said to achieve the analysis speed of the single-channel scanning ICP. Some of the designs use fixed lamp holders, and some use movable lamp holders. It should be noted that the individual feels that it is better to use the lamp holder to fix it. Because the lamp with low melting point element rotates back and forth in the case of preheating, it may damage the hollow cathode lamp. It is also convenient to adjust the lamp. If there is software that can automatically adjust the optimal position and set the parameters better, this is mainly to consider the funds and the user's own situation to determine, in addition to the analysis needs the electrodeless discharge lamp users, to consider the lamp holder with the electrodeless discharge lamp. BCC: G8
1.2 Monochromator structure:
There are mainly Ebert type (such as thermoelectric S series, GBC, etc.), CT type (should be an improvement of Ebert type ) (such as Shimadzu, ZEEnit60/700, VARIAN, JENAVAVIO 6, etc.), Littrow type (such as PE6/7) /800, etc.), Echelle type (known for its large dispersion, such as JENAContrAA, PE's SIMAA6000, thermoelectric M series, etc.). Among them, the CT type is designed horizontally symmetrically, because the aberration of the collimating mirror is offset by the imaging objective lens, so that the aberration effect can be eliminated; the Ebert type aberration is also relatively small; the Littrow type has few optical components and is compact. However, there is a large aberration; the Echelle type works with a larger diffraction angle and a higher order spectral line, and is used as a high-dispersion step grating homochromator in combination with low-dispersion optics such as other prisms. The array detector combines to receive spectral information over the entire operating wavelength range, so simultaneous analysis of multiple elements can be performed if the source and channel are qualified. In the choice of spectroscopic system, we try to consider less light path and internal materials (coated, total reflection) to absorb less light, so as not to affect the energy loss and instability of light during the analysis, and one consideration is best. The spectroscopic system can be sealed, dustproof and corroded, while minimizing the influence of other stray light. As for the design of the double beam, each manufacturer has its own characteristics for its overall design. Our requirement is to eliminate the unstable source. The effect of the measurement is OK. For the actual use resolution, the Mn 279.5 nm and Mn 279.8 nm can be resolved as long as the spectral bandwidth is 0.2 nm.
1.3 Dispersive components:
At present, gratings are generally used as spectroscopic devices. They are the core devices of optical path systems. What is the function? It is very simple to separate the resonance lines emitted by the elements from other emission lines. Since the hollow cathode lamp itself emits sharp line radiation, in ordinary atomic absorption, only the grating is required to have medium resolution capability (the requirements for atomic absorption of continuous light source can be high, and a high-dispersion echelle or high resolution is required. Monochromator), the line groove density is not less than 1200 strips / mm (except for the echelle grating, I think the worst of all manufacturers are now 1200 / mm, most of which are higher than this), the line The reciprocal rate of the dispersion rate is about 1.5-3.0 nm/mm (see a lot of instrument samples, basically no more than 1.6 nm/mm), the middle step is at 0.xnm/mm, for example: the thermoelectric M series is 0.5. Nm / mm, PE SIMAA6000 is 0.1nm / mm (at 200nm, 113), 0.4nm / mm (at 800nm, 28), this small amount indicates that the dispersion rate is large, that is, the dispersion performance of the grating is good, theoretically The larger the groove density (the smaller the grating constant) and the longer the focal length, the better the dispersion performance. For a grating with blazed characteristics, the diffracted light energy is mainly concentrated in a certain wavelength range centered on the blazed wavelength (a friend of this calculation needs) Can refer to the relevant manual to calculate the relevant wavelength In the surrounding area, compared with the previous ordinary grating, it has a high light collecting efficiency, and can concentrate 80% of the energy to the desired wavelength range. For the double blazed wavelength, it has a wider wavelength range. High luminous flux, and the size of the grating area reflects the output power of the grating wavelength selector: the ability of the optical system to provide a useful radiation beam with as little loss of intensity as possible when the optical system separates the spectral line, in the grating When the inverse line dispersion rate is constant, the output power of the grating wavelength selector is proportional to the grating area. For the performance of the grating wavelength selector, the larger the area, the better, without considering the transmission and reflection losses.
1.4 Wavelength scanning and performance:
It is best to scan it manually, if it is automatic, to facilitate instrument verification and to carry out the background of the line buckle. The general machine has this function, and its wavelength repeatability is required to be no more than 0.3nm, the indication error is not More than 0.5nm is fine.
1.5 spectral bandwidth:
The spectral bandwidth is the width (nm) of the wavelength range of the beam after exiting the slit through the monochromator, which is related to the inverse dispersion rate and the exit slit of the grating, and the specific dispersion rate of the instrument is constant, so it is only limited to the emission. The seam is proportional. If the sample is complex, consider having more adjustable controls to eliminate the adjacent line interference during the analysis and adjust the sensitivity of the measurement, such as: 232.0nm, 231.0nm, 231.6nm of Ni if the spectral bandwidth is 1nm. There is no way to separate the three lines, so that the sensitivity of the measurement is reduced. If the spectral bandwidth is changed to 0.2 nm, it can be separated, and the sensitivity of the measurement will be significantly improved. The general adjustable range is 0.1-2.6 nm, which is mostly The instrument has this function. You can see that some instruments can not only adjust the slit width, but also adjust the height. This can be tested at the time of purchase to see if it has any influence on the measurement. It should be influential if the luminous flux changes according to the reason. I don't have it. I don't know much about it here.
1.6 detector:
At present, the detectors for atomic absorption are mainly based on ordinary PMT detectors of different specifications, and some of them are detectors of CCD (PE6/7/800, some models of JENA, etc.). The detector for atomic absorption should have a spectral response in the range of 190-900 nm. This can be used for edge energy detection with As193.7nm and Cs852.1nm, requiring instantaneous noise less than 0.03A, and its baseline stability (static, ignition). The copper lamp should not exceed ±0.0044A within 30min. The PMT detector detects the received signal by photoelectric conversion, and its spectral response range is limited by the photosensitive material, and there are drift and dark current (the dark current is at least less than 10-10A, and the dark current is smaller, the better the quality of the PMT). The readout noise is relatively large, and the continuous spectrum information cannot be obtained at the same time. However, as a commonly used main detector, he has a glorious history in the development of atomic absorption spectrometer with high gain, high sensitivity, fast response and low cost, and its Technology is also constantly evolving. The CCD detector detects the signal by electronic storage and transfer, and its quantum efficiency is high. Based on the measurement method of the detection signal, it can be improved relative to the PMT when equipped with a continuous light source and a large dispersion echelle. The measured linear range is 5-6 orders of magnitude, and multi-element analysis can also be performed simultaneously. The CCD detector has a relatively high sensitivity over the entire spectral analysis area and is more suitable for the detection of weak light, but his detection of low light is based on long-term integration because he is an integral detector. Because of its lowest distributed capacitance, its readout noise is low, and dark current (affected by temperature, which requires a constant temperature environment) is also significantly lower than that of PMT. Regardless of photon efficiency, dark current, readout noise, multi-element simultaneous analysis, linear range and other aspects, its performance has obvious advantages, which is an inevitable situation in the development of atomic absorption spectrometers (eg Jena, Germany) ContrAA continuous light source AAS "the world's first commercial continuous source atomic absorption").
2. Atomization System: Flame and graphite furnace atomizers are mainly used in common analysis.
2.1 Flame atomization system:
The flame atomization amount should be 3-6ml/min, the atomization efficiency should be no less than 8%, the detection limit of copper should be no more than 0.008 ug/ml, and the RSD of 5 ppm copper should be less than 0.5%. .
The flame atomizer mainly includes a spray chamber, an atomizer, an impact ball, a spoiler, a combustion head, a liquid seal box, and a gas control system. These devices are also the main targets for the optimization of conditions during measurement. The atomization chamber is generally made of organic resin material and has a corrosion-resistant material such as polytetrafluoroethylene or polypropylene. The consideration at the time of purchase is mainly whether the design is reasonable (generally no problem, if there is a problem The accuracy and accuracy of the measurement will not be reached, but it should be noted. As for the atomizer, of course, it is best to choose high-efficiency atomization and adjustable (including the ability to adjust the impact ball). According to my experience, the impact ball is preferably a resin material, and the consumption of glass and ceramic is relatively large (too It is brittle, and it is cut off accidentally. The spoiler is mainly used to filter large droplets and enhance the stability of flame measurement. The burner should be adjustable up and down, and the material is mainly titanium.铌, stainless steel, indium-bismuth alloy, etc., caliber also has 0.5mm*50mm (nitrous oxide-acetylene flame), 0.5*100mm (air-acetylene flame), it is best to choose thermal stability, corrosion resistance, resistance High-salt samples, which should not be blocked, need to be noted that if the elements to be measured are to be nitrous oxide-acetylene flames, a special burner should be selected and must not be mixed to prevent danger. The gas control system is preferably controlled by a computer, and the air compressor has a filtering device. Since acetylene is a flammable gas, it is best to consider a series of safety interlocks and prompts when purchasing.
2.2 Graphite furnace atomizer The determination limit, characteristic quality and precision of cadmium measured by graphite furnace atomizer should be no more than 2pg, 1pg, 5%.
The graphite furnace atomizer has the characteristics of small volume, low detection limit (more than three orders of magnitude), less sample usage and longer analysis time. The disadvantage of graphite furnace atomization is that the matrix evaporation may cause Large molecular absorption, oxidation of the tube itself also produces molecular absorption, background absorption is large, some solid particles cause light scattering to cause false absorption, so the use of graphite furnace atomizer must choose background correction device, and for more complex substrates It is recommended to perform analysis in Zeeman calibration mode. It mainly includes furnace body, power supply, cooling water and gas path system. The commodity instrument furnace body is divided into lateral heating and longitudinal heating, and longitudinal heating (such as: Shimadzu AA63/6800, Agilent3510, etc.) due to the graphite tube Water cooling on the electrode at the end causes a temperature gradient along the optical path, which makes the whole graphite tube have non-equal temperature, which causes serious interference of the matrix and affects the atomization process. For the above problems, the commodity product has been improved several times and the platform has been developed. Atomic (a major focus, a significant contribution to improving the heating of longitudinal graphite furnaces), probe atomization, capacitor discharge, strong pulse heating graphite furnace, which to some extent more or less make up the vertical The shortcomings, but still did not solve the fundamental problem. The transverse graphite furnace (such as: JENA VAVIO 6, novAA 400 Zeenit600/650/700).
In summary, I recommend paying special attention to the atomic absorption spectrometers produced by Jena Company of Germany (AJ) and Shimadzu Corporation of Japan.
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