Pulsing hollow cathode lamp

Background correction using a pulsed hollow cathode lamp... 

Figure 14.15—Pulsed hollow cathode lamp background correction, a) Shape of the emission line from a hollow cathode lamp under normal operating conditions, b) the 4000 Smith-Hieftje model from Thermo Jarrell Ash uses the principle of pulsed-source correction. The mercury source and the retractable mirrors are used for calibration of the monochromator. (Reproduced by permission of Thermo Jarrell Ash.)...

From J. D. Ingle Jr. and S. R. Crouch, Spccirocheimail Analvsis, pp. 250-51.300,321. Englewood Cliffs. NJ Premice Hall. 1988. Note Pulsed hollow-cathode-lamp excitation source with ICP atomization. 

Atomic fluorescence is an extremely sensitive technique for determination of elements in samples. We should reiterate that in atomic fluorescence an external Ught source is used to excite the analyte atoms. An ideal light source for AFS must be much more intense than ahoUow cathode lamp to achieve improvements in sensitivity. As a result, pulsed hollow cathode lamps and lasers are frequently used in AFS measurements. Excitation with alight source such as a hollow cathode lamp, which only emits radiation specific for the element of interest, makes AFS virtually completely free from spectral interferences. In addition, AFS is like AES in that a multi-element analysis can be achieved by putting several light sources around the atom cell, as discussed below. 

Modem hollow-cathode lamps require only a very short warm-up period. Lifetimes are measured in ampere hours (usually they are in excess of 5 A h). A starting voltage of 500 V is useful, but operating voltages are in the range 150-300 V. In many instmments, the current supplied to the lamp is modulated. Hollow-cathode lamps may also be pulsed or... 

Where vapour discharge lamp sources exist (for volatile elements such as Hg, Na, Cd, Ga, In, T1 and Zn) they can be used. Hollow-cathode lamps are insufficiently intense, unless operated in a pulsed mode. Microwave-excited electrodeless discharge lamps are very intense (typically 200-2000 times more intense than hollow-cathode lamps) and have been widely used. They are inexpensive and simple to make and operate. Stability has always been a problem with this type of source, although improvements can be made by operating the lamps in microwave cavities thermostated by warm air currents. A typical electrodeless discharge lamp is shown in Fig. 6.3. 

When the intensity of a hollow cathode lamp increases because of a reduction in the shunt resistance, the profile of the emission line changes. As the central part of the cathode becomes very hot, the line is broadened for several reasons. However, vaporised atoms emitted by the cathode will reabsorb in a colder part of the lamp in the form of a very fine line. The net result is that the emission curve dips in the middle because of self-absorption. This observation is the basis of the pulsed lamp technique for correction of background absorption (Fig. 14.15). 

It is necessary that the hollow-cathode lamp source be pulsed or modulated at a certain frequency and for the amplifier to be locked in to this frequency to permit discrimination against the continuous emission signal coming from the atomiser. Only the resonance radiation from the lamp must be seen. In modem digital electronic instruments the lamp cycle is controlled by a sophisticated electronic clock which is sampled to provide the short pulses of... 

Modulation can be accomplished by interposing a motor-driven circular chopper between the source and the flame, as shown in Figure 28-18. Segments of the metal chopper have been removed so that radiation passes through the device half the time and is reflected the other half. Rotation of the chopper at a constant speed causes the beam reaching the flame to vary periodically from zero intensity to some maximum intensity and then back to zero. As an alternative, the power supply for the source can be designed to pulse the hollow-cathode lamps in an alternating manner. 

Smith-Hieftje background correction uses a single hollow-cathode lamp pulsed with first a low current and then a high current. The low-current mode obtains the total absorbance, while the background is estimated during the high-current pulse. Read the interview at the beginning of Part V to learn more about Cary Hieftje and his work. 

Pulsed hollow cathode or electrodeless discharge lamp... 

In the early work on atomic fluorescence, conventional hollow-cathode lamps often served as escitalion sources. To enhance the output inicnsity without destroying the lamp, it was necessary to operate the lamp with short pulses of current that were greater than the lamp could tolerate for continuous operation. The detector was gated to observe the fluorescence signal only during pulses of source radiation. 

Perhaps the most widely used sources for atomic fluorescence have been the EDLs (Section 9B-1), which usually produce radiant jnlensiljes greater than those of hollow-cathode lamps by an order of magnitude or two. EDLs have been operated in both the continuous and pulsed modes. Unfortunately, this type of lamp is not available for many elements. 

A typical setup is shown in Figure 10.12. The two light beams from the lamps are combined by a half-coated mirror (e.g., coated with small reflecting circles so that it will reflect the continuum radiation but will allow space in the mirror for the radiation from the hollow-cathode lamp to pass). Each lamp is pulsed electronically to provide an AC signal, but the two lamps are 180° out of phase. A phase-sensitive detection system then measures the difference of the two signal intensities (which are initially balanced). The sharp-line source measures both atomic absorption and... 

Pulsed lamp background correction A very simple method of background correction has been proposed by Smith and Hieftje [25] and is therefore known as the Smith—Hieftje method. It is based on the self-reversal behaviour of the radiation emitted by hollow cathode lamps when they are operated at high currents. This ef feet is seen when a large number of non-excited atoms are brought into the vapor phase. These atoms absorb the characteristic radiation emitted by the excited species. At the same time, a significant broadening of the emission line is observed. 

Background correction is thus achieved by modulating the lamp current to generate a longer pulse at low current (e. g. 9 ms at 5—10 mA), followed directly by a short pulse at high lamp current (for example, 0.3 ms at 200—300 mA). As the atom cloud persists in the hollow cathode lamp for several milliseconds, a minimum pulse repetition time of typically 50 ms is required to allow the atom cloud to clear before the next measurement cycle is started. 

Hollow-cathode lamps are often used as sources in AFS, as discussed in Section 9E-I. fn this application, the lamps are pulsed with a duty cycle of 1 % to 10%... 

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