Performance and operation of hollow cathode and xenon lamps
Performance and operation of hollow cathode and xenon lamps
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Hollow cathode lamp
Hollow cathode lamps are mainly used to provide a sharp line spectrum of the measured elements. The spectrum emitted by a hollow cathode lamp for atomic absorption spectroscopy must be sufficiently pure, low noise, and the radiation intensity meets the linear calibration requirements.
The structure of a conventional hollow cathode lamp is as shown in Fig. 1.
When a hollow cathode lamp generates a discharge phenomenon between two electrodes through an internal low-pressure gas, the cathode is subjected to a large amount of electrons, and the bombardment of charged gas ions (i.e., ions filled with gas) that accelerate toward the surface of the electrode. The energy of these ions is so strong that the atoms of the cathode material can be detached or "sputtered" from the surface into the plasma region. The sputtered ions also collide with other energetic substances here. The result of the collision leads to energy transfer, and the metal atom transitions to the excited state. Due to the unstable excited state, the atom spontaneously returns to the ground state while emitting a resonance line of a specific wavelength. Many elements have multiple resonance lines for analysis.
In order to maximize the performance of the lamp, all design parameters must be carefully selected.
Design features of hollow cathode lamps
1. The cathode cathode is made of an element to be analyzed or a substance containing an element to be analyzed. If the metal is stable in air and has a high melting point, the cathode material generally uses a pure metal such as silver. If the metal itself is relatively brittle, sintered metal powders (such as manganese, tungsten) are generally used. Metal oxides or halides (such as cadmium, sodium) are generally used if the metal itself is relatively active in air or has a relatively high relative vapor pressure. Powder technology is also used to make multi-element lamps containing a variety of metals being analyzed.
The diameter of the cathode is also very important because the emission intensity of the lamp depends on the current density.
2. The gas enclosed by the enclosed gas must be a monomolecular gas to avoid the molecular vibration spectrum, and thus an inert rare gas is generally used. Helium or argon is generally used as the enclosed gas, and helium is the best choice. This is because it has a higher ionization potential in order to have a higher emission intensity. Argon is only used when the emission line of helium is very close to the emission line of the element being measured. The lower mass of the helium gas not only causes a significantly smaller sputtering effect, but also shortens the life of the lamp due to the rapid depletion of its gas.
The depletion of the enclosed low pressure gas is caused by the absorption of the surface material of the lamp. When the enclosed gas pressure is lower than the specified value, the discharge cannot be continued, and the life of the lamp reaches the end point. Although the lamp still illuminates, it is no longer possible to emit the resonance line of the measured element.
3. The anode anode is a simple common electrode that provides a discharge bombardment voltage. Zirconium is generally used as the anode material because it is a "getter." This feature is explained in the “5 Processing†section below.
4. Envelope electrodes are typically enveloped using glass containing optical pathways made of quartz or specialty borosilicate glass. The material of the light path window is determined by the emission line of the element lamp. Since most of the elements have emission lines below 300 nm, quartz materials must be used at this time. Borosilicate glass is generally used above this wavelength.
5. Processing steps are the key to making high performance lamps. The main purpose of the treatment is to remove the contamination for purification.
The steps of the treatment mainly include evacuating and maintaining a suitable high temperature outside the lamp.
The processing step reverses the polarity so that the zirconium anode is converted to the cathode. For the impurity gas oxygen and hydrogen zirconium electrode is a good "getter", so the use of this electrode can remove the impurity gas. A layer of zirconium remains on the envelope of the lamp during discharge.
There is a black film near the anode. This layer of active membrane is capable of absorbing impurities and purifying the gas in the lamp. Until the last pure gas fills the entire lamp and then closes. The finished lamp still needs to be tested for several hours.
The operation of the hollow cathode lamp mainly has two parameters affecting the analysis results. They are:
(a) The current of the hollow cathode lamp affects the emission intensity.
(b) Spectral bandwidth (slit) on the instrument that controls the spectral line
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 order to obtain better analytical results in a particular situation, it is necessary to make small changes to the operating conditions provided. The choice of operating conditions depends on obtaining the best precision for the analytical sample near the detection limit or a linear relationship over a large concentration range.
1. The effect of increasing the lamp current by the lamp current is to increase the emission intensity of the lamp, as shown in Figure 2.
The intensity of the emission of the lamp affects the magnitude of the baseline noise (absorption) in the measured analytical signal. Baseline stability is the key to ensuring good precision and detection limits.
Since the magnitude of the 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).
The only thing worth noting on the surface is that the set current must be less than the rated current of the lamp. But in fact it is not that simple.
When the operating current exceeds the recommended current, the self-priming phenomenon causes the emission line to widen. Since the atomic cloud at the front of the cathode absorbs the resonance line emitted by its own cathode, this is like inverting the original emission line.
The distortion of the emission line results in a decrease in sensitivity (Figure 4).
This distortion also affects the linearity of the curve, with a very linear cadmium element such as Figure 5. It should be noted that this example is done with very linear elements. This phenomenon of some other elements is not obvious or even (Figure 6).
Excessive lamp currents accelerate the sputtering effect and shorten the life of the lamp. For zirconium volatile element lamps is more obvious.
A higher lamp current is recommended for the measured sample concentration close to the detection limit (where baseline noise is very important). The sensitivity loss caused by the increase in lamp current for some elements is not significant.
On the other hand, a lower lamp current favors the linearity of the curve and extends the measurement range, but this must be at the expense of baseline noise.
It is obvious that the choice of compromise can achieve better sensitivity with high signal-to-noise ratio and the life of elemental lamps. The Varian User Manual has recommended parameters for each elemental lamp.
2. Lamp Intensity Each analysis line of each hollow cathode lamp has a characteristic intensity associated with the signal-to-noise ratio of the atomic absorption spectrometer. The greater the intensity of the analysis line, the higher the signal to noise ratio. It is normal for the difference in noise levels of different element lamps to be large. For example, the noise of a silver element lamp at 328.1 nm is significantly less than that of an iron element lamp at 248.3 nm. Figure 7 lists two noise scenarios.
It is worth noting that the performance of the photocathode of the photomultiplier tube is also one of the causes of noise. The photomultiplier tube used by Varian has a high response over a large wavelength range.
3. Spectral Bandwidth The spectral bandwidth affects the spectral separation capability 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 xenon lamp in Figure 8, it is found that if the strongest 217.6 nm is used, the spectral bandwidth must be less than 0.3 nm in order to avoid the interference line of 217.9 nm. The optimal spectral bandwidth can be determined by studying the spectral bandwidth and analyzing the change pattern of the solution absorption signal (Figure 9).
4. The stability of the hollow cathode lamp signal during preheating time is very important. A conventional hollow cathode lamp requires a warm-up period after opening so that the lamp reaches an equilibrium state and the output is stable.
It is very important to preheat the single beam instrument. For a single-beam instrument (SpectrAA-110), changing the emission intensity of the lamp affects the baseline of the instrument, that is, the drift of the baseline is the drift of the lamp. Therefore, it is necessary to perform sufficient preheating before the measurement. For most of the element lights preheat for 10 minutes. As, P, Tl and Cu/Zn multi-element lamps require longer time to warm up.
For dual beam instruments, the instrument compensates for the sample beam by continuously comparing the intensity of the reference beam. For instruments using frequencies of 50 and 60 Hz, the sample beam and the reference beam are compared every 20 or 16 milliseconds.
For a two-beam instrument, the effect of preheating is not significant. However, a small warm-up time is required for accurate analysis of the sample. This is because the profile of the emission line of the lamp changes during the warm-up phase and has a small effect on the result. For two-beam instruments, zero correction must be performed frequently.
It should be noted that although Zeeman-type atomic absorption has only one optical path, it is a true dual-path instrument when analyzing samples.
5. A multi-element lamp multi-element lamp can consist of up to six different elements. These elements are made into a cathode by an alloy powder. These lights are easy to use, but they have their own limitations.
Not all multi-element mixtures can be used because the emission lines of some elements are too close to interfere with each other. The use of multi-element lamps is generally different from that of single-element lamps and requires careful user exploration. Thanks to the linearity of the calibration curve, the analysis results of the single element lamp are generally better than the multi-element lamp. However, the application range of multi-element lamps is the advantage.
Xenon lamp
Xenon lamps are a continuous source of radiation used to correct non-atomic or background absorption. This source is a flood-filled discharge lamp that emits a strong continuous spectrum ranging from 190 to 400 nm. This region is where the atomic absorption is often used and the background absorption frequently occurs in the spectral range. The use of diatomic molecules is because it is capable of producing a continuous emission band. Xenon lamps differ in structure and operation from hollow cathode lamps (Figure 10). The lamp incorporates a heated electron-emitting cathode, a metal anode and a confinement aperture between the two poles. Work with hundreds of milliamps of current to excite helium. Current through the aperture forms a high level of excitation in a particular area, resulting in a high intensity emission line. Use a suitable form material so that the emission line passes through to the optical path system of the spectrometer.
In order to obtain excellent background correction, the light path and energy of the xenon lamp must match the hollow cathode lamp. The optical path matching of xenon lamps and hollow cathode lamps is very important. If the match is not complete, the atomic density measured at the two points will be different, resulting in an erroneous result. In order to balance the energy of the xenon lamp and the hollow cathode lamp, it is necessary to increase or decrease the current of the hollow cathode lamp in accordance with the mutual strength of the two. Varian's instrument is equipped with an attenuator (some models are automatic) in front of the xenon lamp to reduce its emission intensity to the balance of the hollow cathode lamp. If the continuous source energy is still too strong, you need to reduce the spectral bandwidth. This is because the energy of the continuous light source increases as the spectral bandwidth increases, whereas the energy of the atomic spectral emission line decreases as the spectral bandwidth increases. Similarly, when the energy of the hollow cathode lamp exceeds the xenon lamp, the spectral bandwidth can be appropriately increased. Through these methods, the balance between the two can be achieved.
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