Pulsed lamp background correction

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. 

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.)...

Figure 13.16 Pulsed lamp for background correction. Tlie model sliown uses tlie principle of the Smith-Hieftje pulsed source background correction. The mercury source as well as the retractable mirrors are used to cahbrate the monochromator (reproduced courtesy of Thermo Jarrell Ash). Appearance of an emission line of a HC lamp as a function of its voltage.

A spectrometer with rapid response electronics should be used for electrothermal atomization, as it must follow the transient absorption event in the tube. Automatic simultaneous background correction (see Section 2.2.5.2) is virtually essential, as non-specific absorption problems are very severe. It is important that the continuum light follows exactly the same path through the furnace as the radiation from the line source (assuming a deuterium lamp is being used rather than Smith-Hieftje or Zeeman effect). The time interval between the two source pulses should be as short as possible (a chopping frequency of at least 50 Hz) because of the transient nature of the signal. 

Background correction using a pulsed hollow cathode lamp... 

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). 

Figure 7 Schematic representation of how the Smith-Hieftje background correction system works. On the left, the source emits a simple, sharp line at low current, and both atomic and molecular absorption would be measured. On the right, this simple line has effectively been split by a pulse of high lamp current into a pair of lines at either side of the atomic absorption profile, and only molecular absorption or scatter would be detected...

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. 

This self-absorption is the basis of the pulsed lamp technique for correction of the background absorption. Known as the Smith-Hieftje (S-H) method, this application uses a pulsed lamp which enables a comparison of the two measurements. In normal conditions (e.g. 10 mA) and with the sample into the flame, a global measurement representing the sum of the background absorption and the absorption of the element is observed, while under strained lamp conditions (500 mA) only the background absorption is present as the lamp will no longer emit at the wavelength chosen. The comparison of these two absorbance measurements leads, after correction, to the calculation of the absorption due to the sole analyte. 

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. 

Figure 11.2 illustrates the operating principle of the Zeeman 5000 system. For Zeeman operation, the source lamps are pulsed at 100 Hz (120 Hz) while the current to the magnet is modulated at 50 Hz (60 Hz). When the field is off, both analyte and background absorptions are measured at the unshifted resonance line. This measurement directly compares with a conventional atom and absorption measurement without background correction. 

FIGURE 9-14 Schematic of a continuum-source background correction system. Note that the chopper can be eliminated by alternately pulsing each lamp. 

---