Posted 22 Dec 2010 18:55
Hollow cathode lamp hollow cathode lamp is mainly used to provide a sharp line spectrum measured elements. Atomic absorption spectrometry for the hollow cathode lamp emission spectrum must be sufficiently clean, low noise, radiation intensity required to achieve linear correction. General structure of the hollow cathode lamp as shown in Figure 1. When the hollow cathode lamp by the two internal low pressure gas discharges between electrodes, the cathode will be a large number of electrons accelerated toward the electrode surface of the charged gas ions (that is, the gas filled ion) bombardment. The ion energy is very strong, so that you can make the cathode material sputtering atoms from the surface out of or into the plasma. Sputtering ion energy in the here and other material will collide with each other. Result of collision energy transfer to the excited state transition metal atoms. As excited unstable atoms spontaneously to the ground state, emits a specific wavelength while the resonance line. Many elements have more than one resonance line for analysis. In order to play the best performance of the lamp, all the design parameters must be carefully selected. Hollow cathode lamp design features 1. The cathode was analyzed by the cathode element or substance containing the element being analyzed is made. If the metal is stable in air and has a high melting point, the general use of pure metal cathode materials (such as silver). If the metal itself is relatively brittle, the general use of sintered metal powder (such as manganese, tungsten). If the metal itself is lively in the air, or with a higher relative vapor pressure, the general use of metal oxides or halides (such as cadmium, sodium). Powder technology is being used in manufacturing of metal containing a variety of multi-element lamps. The diameter of the cathode is also very important, because light emission intensity depends on the current density. 2. Enclosing the gas enclosed gas must be a single molecule of gas to avoid the shock molecular spectroscopy, which typically use inert noble gases. Enclosed gas general use of neon or argon, neon is the best choice. This is due to its higher ionization potential to have a higher emission intensity. Argon, neon gas only for the measured emission lines and emission lines of elements very close to the case. For the lower mass of helium is not only significantly smaller effect caused by the sputtering, but also because of its rapid depletion caused by the gas lamp life shortened. Enclosing a low pressure gas depletion is due to absorption of light caused by surface materials. When the enclosed gas pressure less than the specified value can not be sustained when the discharge lamp life at this time is to end. Although the lights still lit, but has not launched a measured element resonance line. 3. Anode is the anode can provide a simple, common electrode voltage discharge bombardment. Zirconium anode materials in general use, because it is a getter. This feature will be handled in the following sections explain 5. 4. Electrodes usually contain quartz envelope or special borosilicate glass window made of optical glass envelope. Optical window materials from elements of the lamp emission line decision. Since most elements are below the 300 nm emission lines, then you must use the quartz material. Higher than the general use of this wavelength, borosilicate glass. 5. Processing steps is to create high-performance processing the key light. The main purpose of treatment is to remove pollution purification. Processing steps including vacuum and in the light of the outside to maintain a suitable high temperature. Processing steps can make the polarity reversal for zirconium anode into the cathode. Oxygen and hydrogen gas for impurities of zirconium electrode is a good getter, so please use this electrode can remove impurities in the gas. There will be a layer of zirconium in the discharge of the lamp envelope to stay. Near the anode will be close to a layer of black film. This layer of active membrane can absorb impurity gas, the lights of the gas purification. Until the last pure gases fill the lamp, and then closed. Done with the lights still need to test a few hours. The operation of hollow cathode lamps are two parameters that affect results. Are: (a) hollow cathode lamp current of emission intensity. (B) control the spectral lines of the instrument spectral bandwidth (slit) in order to facilitate the user to select these two parameters, Varian provides the user with the recommended operating conditions for each lamp. However, in certain circumstances in order to obtain good results, it is necessary to provide the operating conditions of small change. The choice depends on the operating conditions in the analysis of samples near the detection limit to get the best precision, or in a larger concentration range of linear relationship. 1. Lamp current increases, the effect of lamp current is to increase the light emission intensity, shown in Figure 2. Light emission intensity of the analysis is the determination of the baseline signal noise (absorption) in size. The stability of the baseline is to ensure good precision and detection limit of the key. As the size of baseline noise and light emission intensity is inversely proportional, so the greater the emission intensity of light, the smaller the baseline noise (Figure III). On the surface only noteworthy is set lamp current must be less than the rated current. But the fact is not so simple. When the current is more than the recommended operating current, the phenomena of self-absorption occurs emission line broadening. Cathode on the front of the atomic cloud absorbs the resonance line itself cathode, which is like the original transmission lines inverted. Emission lines of the distortion caused reduced sensitivity (Figure IV). This will also affect the linear distortion, very good linear elements such as cadmium Figure 5. Note that this example is the use of linear elements for the good. Some of the other elements of this phenomenon is not obvious or even (Figure VI). High lamp current will accelerate the sputtering effect, shorten the lamp life. For volatile elements zirconium lamp more apparent. For the determination of sample concentration near the detection limit (the baseline noise is important at this time), the higher the lamp current is recommended. For some elements to increase the sensitivity of the loss caused by the lamp current is not obvious. On the other hand, a lower lamp current is conducive to the linear and extended measurement range, but it must be at the expense of the cost of the baseline noise. Clearly the choice of a compromise both with high signal to noise ratio to obtain better sensitivity, while maintaining the elements of lamp life. Varian user manual for each element of light parameters are recommended for selection. 2. Hollow cathode lamp for each light intensity for each analytical line with atomic absorption spectrometer has a signal to noise ratio associated with characteristic strength. The greater the intensity of the line, the higher signal to noise ratio. Different elements of the noise level quite different light is normal. Light elements such as silver at the noise in the 328.1nm to 248.3nm significantly less than at the iron lamp noise, Figure VII shows the two noise conditions. It is noteworthy that PMT photocathode performance is one of the reasons of noise. Varian photomultiplier tubes used in a large wavelength range has a high response. 3. Spectral bandwidth of the spectral bandwidth of the spectral line separation ability. The size of the bandwidth spectrum analysis of the line near the situation by the decision (Figure VIII). Figure VIII light for spectral scanning of antimony found, if you use the strongest 217.6nm, the spectral bandwidth must be less than 0.3nm in order to avoid the interference of 217.9nm line. Through research and analysis solution absorption spectrum bandwidth of the signal changes in plans will be able to determine the optimal size of the spectral bandwidths (Figure IX). 4. Hollow cathode lamp warm-up time signal stability is very important. Common in the open hollow cathode lamp warm-up time required after a period in order to achieve a balanced state of light output and stability. Preheat the single beam instrument is very important. For single beam instrument (SpectrAA-110), the change will affect the light emission intensity of the baseline instrument, that is, the baseline drift is the drift of the lamp. Therefore, such determination must be conducted before the full warm-up. For most of the elements of light warm-up for 10 minutes. And As, P, Tl, and Cu / Zn multi-element light you need more time to warm up. For the double-beam instrument, the instrument will be compared by a continuous reference beam intensity to compensate for the sample beam. For 50 and 60 Hz frequency of the instrument, the sample beam and reference beam every 20 or 16 milliseconds for a comparison. For the double-beam instrument, the effect of preheating is not obvious. However, the precise during the sample analysis, the need for a short warm-up time. This is because in the warm phase of light emission line profiles will change, and the results have a smaller impact. For the double-beam instrument, must always be zero point correction. Note that although only one Zeeman atomic absorption optical path, but in the analysis of samples Shique is a true dual optical instruments. 5. Multi-element multi-element lamps can light up to six different elements. These elements are made of alloy powder through the cathode. These lights easy to use, but also has its own limitations. Not all multi-element mixture can be used, because some elements are too close to transmission lines that interfere with each other. Multi-element lamps with a single element of the general conditions of use of different lights, requiring users to carefully explore. Benefit from the advantages of the linear calibration curve, the analysis of single-element lamps are generally better than the multi-element lamps. But the contrast range of applications multi-element lamps are its advantages. Deuterium lamp is a deuterium lamp radiation be used for correction of non-continuous or background atomic absorption. This light source is a deuterium discharge lamp filled with the launch a strong continuous spectrum ranging from 190 to 400nm. This area is frequently used atomic absorption and background absorption spectrum frequent. Deuterium is used for diatomic molecules can be generated because of its continuous emission band. Deuterium lamp in the structure and operations, and hollow cathode lamps are different (Figure 10). The integrated light emission of a heated cathode, the metal anode and the hole between the two poles of the restrictions. The work of hundreds of milliamps of current when stimulated with deuterium gas. Current through the small hole formed in a particular region highly motivated to produce high-intensity emission lines. Using the appropriate form materials to reach the emission lines through the spectrometer after the optical system. In order to obtain a good effect background correction, the optical path of deuterium lamps and energy must be matched with the hollow cathode lamp. Hollow cathode lamps and deuterium lamp light path matching is very important. If the match is incomplete, then two o'clock there will be differences in the determination of the atomic density, producing erroneous results. To balance the deuterium lamp and hollow cathode lamps of energy, according to the interactions between them need to increase or decrease intensity hollow cathode lamp current. Varian's instrument lights are installed in the deuterium attenuator (some models for the automatic) can reduce the emission intensity reached a balance with the hollow cathode lamp. If a continuous source of energy is still too strong, you need to reduce the spectral bandwidth. This is because the energy of a continuous light source with the larger spectral bandwidth increases, whereas the energy of atomic spectral emission lines, along with the increase of spectral bandwidth becomes smaller. Similarly, when the hollow cathode lamp energy than deuterium lamp to increase the spectral bandwidth can be appropriate. These methods can be achieved through a balance between the two.