1. Cathode
The cathode is made of the analyzed element or a substance containing the analyzed element. If the metal is stable in air and has a high melting point, pure metal (such as silver) is generally used as the cathode material. If the metal itself is relatively brittle, sintered metal powders (such as manganese and tungsten) are generally used. If the metal itself is relatively active in air or has a high relative vapor pressure, metal oxides or halides (such as cadmium and sodium) are generally used. Powder technology is also applied to manufacture multi-element lamps containing multiple analyzed metals.
The diameter of the cathode is also very important, as the emission intensity of the lamp depends on the current density.
2. Sealed gas
The sealed gas must be a single-molecule gas to avoid molecular vibrational spectra, so inert rare gases are generally used. Neon or argon gas is generally used as the sealing gas, with neon being the preferred choice. This is because it has a higher ionization potential in order to have a higher emission intensity. Argon gas is only used when the emission line of neon gas is very close to the emission line of the measured element. The low mass number used for helium not only results in significantly smaller sputtering effects, but also shortens the lifespan of the lamp due to rapid gas depletion.
The depletion of the sealed low-pressure gas is caused by the absorption of the surface material of the lamp. When the pressure of the sealed gas is lower than the specified value, the discharge cannot continue, and the lifespan of the lamp reaches its end. Although the lamp can still light up, it can no longer emit resonance lines of the tested element.
3. Anode
The anode is a simple ordinary electrode that can provide discharge bombardment voltage. Zirconium is generally used as an anode material because it is a 'absorber'. This characteristic will be explained in the "5 Processing" section below.
4. Cover
Electrodes are usually enclosed in glass containing optical path windows made of quartz or special borosilicate glass. The material of the light path window is determined by the emission line of the element lamp. Due to the emission lines of most elements being below 300 nanometers, quartz material must be used at this time. Borosilicate glass is generally used for wavelengths higher than this.
5. Handling
The processing steps are key to manufacturing high-performance lamps. The main purpose of processing is to remove pollution and purify it.
The main steps of processing include vacuuming and maintaining a suitable high temperature outside the lamp.
The processing steps can reverse the polarity so that the zirconium anode can transform into a cathode. Zirconium electrode is a good "absorber" for impurity gases such as oxygen and hydrogen, so using this electrode can remove impurity gases. During discharge, a layer of zirconium will remain on the envelope of the lamp.
There will be a black film near the anode. This active membrane can absorb impurity gases and purify the gas in the lamp. Until pure gas fills the entire lamp, then seal it. The processed lamp still needs to be tested for several hours.
Operation of hollow cathode lamp
There are two main parameters that affect the analysis results. respectively
(a) The current of a hollow cathode lamp affects the emission intensity.
(b) Spectral bandwidth (slit) on instruments that control spectral lines
In order to facilitate users' selection of these two parameters, Varian provides recommended operating conditions for each lamp. However, in certain situations, in order to obtain better analysis results, it is necessary to make slight changes to the provided operating conditions. The choice of operating conditions depends on whether the precision to be obtained for the analyzed sample near the detection limit or whether a linear relationship is satisfied within a larger concentration range.
1. Lamp current
The effect of increasing the lamp current is to increase the emission intensity of the lamp, as shown in Figure 2.
The emission intensity of the lamp affects the magnitude of baseline noise (absorption) in the analyzed signal being measured. The stability of the baseline is key to ensuring good precision and detection limits.
Since the magnitude of baseline noise is inversely proportional to the emission intensity of the lamp, the greater the emission intensity of the lamp, the smaller the baseline noise (Figure 3).
On the surface, it is worth noting that the set current must be less than the rated current of the lamp. But in fact, it's not that simple.
When the operating current exceeds the recommended current by a large amount, self-priming occurs, causing the emission line to widen. Due to the atomic cloud in front of the cathode absorbing the resonance lines emitted by the cathode itself, it is like inverting the original emission lines.
Distortion of the transmission line leads to a decrease in sensitivity
This distortion can also affect the linearity of the curve, as shown in Figure 5 for cadmium element, which has very good linearity. It should be noted that this example uses elements with very good linearity. This phenomenon is not obvious or even absent for some other elements
Excessive lamp current can accelerate the sputtering effect and shorten the lifespan of the lamp. This is more pronounced for zirconium volatile element lamps.
When the concentration of the measured sample approaches the detection limit (where baseline noise is crucial), it is recommended to use a higher lamp current. The sensitivity loss caused by increasing the lamp current for certain elements is not significant.
On the other hand, lower lamp currents are beneficial for the linearity of the curve and the expansion of the measurement range, but this must come at the cost of sacrificing baseline noise.
It is obvious that a compromise choice can achieve good sensitivity with high signal-to-noise ratio while also taking into account the lifespan of the element lamp. The Varian user manual provides recommended parameters for each element light to choose from.
2. Lamp intensity
Each analysis line of each hollow cathode lamp has a characteristic intensity related to the signal-to-noise ratio of the atomic absorption spectrometer. The stronger the analysis line, the higher the signal-to-noise ratio. It is normal for the noise levels of lamps with different elements to vary significantly. For example, the noise of silver element lamp at 328.1nm is significantly lower than that of iron element lamp at 248.3nm. Figure 7 lists two types of noise situations.
It is worth noting that the performance of the photocathode in photomultiplier tubes is also one of the reasons that affect noise. The photomultiplier tubes used by Varian have high response over a wide wavelength range.
3. Spectral bandwidth
The spectral bandwidth affects the spectral separation ability of the analytical line. The size of the spectral bandwidth is determined by the proximity of the analysis line (Figure 8).
From the spectral scan of the antimony lamp in Figure 8, it was found that if the strongest 217.6nm is used, the spectral bandwidth must be less than 0.3nm to avoid the interference line at 217.9nm. By studying the spectral bandwidth and analyzing the changes in the absorption signal of the solution, the size of the spectral bandwidth of * can be determined (Figure 9).
4. Preheating time
The stability of the hollow cathode lamp signal is very important. Ordinary hollow cathode lamps require a preheating time after being turned on in order for the lamp to reach a balanced state and output stability.
Preheating is very important for single beam instruments. For the single beam instrument (SpectraAA-110), changing the emission intensity of the lamp will affect the baseline of the instrument, which means that the baseline drift is the lamp drift. Therefore, sufficient preheating must be carried out equally before the measurement. For most elemental lamps, preheating for 10 minutes is sufficient. As, P, Tl, and Cu/Zn multi-element lamps require longer preheating time.
For dual beam instruments, the instrument compensates for the sample beam by continuously comparing the intensity of the reference beam. For instruments used at frequencies of 50 and 60 Hz, the sample beam and reference beam are compared every 20 or 16 milliseconds.
For dual beam instruments, the preheating effect is not significant. However, a short preheating time is required for sample analysis. This is because the emission line contour of the lamp will change during the preheating stage, which will have a relatively small impact on the results. For dual beam instruments, zero calibration must be performed frequently.
It should be noted that although Zeeman atomic absorption has only one optical path, it is a true dual path instrument when analyzing samples.
5. Multi element lamp
Multi element lamps can be composed of six different elements. These elements are made into cathodes through alloy powders. This type of lamp is easy to use, but it also has its own limitations.
Not all multi-element mixtures can be used because the emission lines of certain elements are too close to each other to interfere with each other. The usage conditions of multi-element lamps are generally different from those of single element lamps, and users need to carefully explore. Thanks to the linear advantage of the calibration curve, the analysis results of single element lamps are generally better than those of multi-element lamps. But in comparison, the application range of multi-element lamps is its advantage.