Smith-Hieftje

In the previous section it has been shown that the measured sample absorbance may be higher than the true absorbance signal of the analyte to be determined. This elevated absorbance value can occur by molecular absorption or by light scattering. There are three techniques that can be used for background correction the deuterium arc the Zeeman effect and the Smith-Hieftje system. 

All instruments should be equipped with a background correction facility. Virtually all instruments now have a deuterium arc background correction. The Zeeman system is also available in instruments marketed by the Perkin-Elmer Corporation and the Smith-Hieftje system by Thermo Electron Ltd. 

Another type of background correction system that has found some use is that developed by Smith and Hieftje. The Smith-Hieftje background correction technique is of especial use when there is strong molecular interference, such as that observed by phosphate on selenium or arsenic determinations. If the hollow-cathode lamp is run at its normal operating... 

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. 

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.)...

Other background correction systems include the Zeeman effect and the Smith-Hieftje background correction. A detailed description of the operational principles of these methods is beyond the scope of this chapter and the required information can be found in the relevant literature.7,13 The advantages of these methods over deuterium lamps are that high background signals (up to 2.0 units) and structured backgrounds can easily be corrected for. 

With this technique, problems may arise with interference, such as background absorption—the nonspecific attenuation of radiation at the analyte wavelength caused by matrix components. To compensate for background absorption, correction techniques such as a continuous light source (D2-lamp) or the Zeeman or Smith-Hieftje method should be used. Enhanced matrix removal due to matrix modification may reduce background absorption. Nonspectral interference occurs when components of the sample matrix alter the vaporization behavior of the particles that contain the analyte. To compensate for this kind of interference, the method of standard addition can be used. Enhanced matrix removal by matrix modification or the use of a L vov platform can also reduce nonspectral interferences. Hollow cathode lamps are used for As, Cu, Cr, Ni, Pb, and Zn single-element lamps are preferred, but multielement lamps may be used if no spectral interference occurs. 

In many modern atomic absorption spectrometers, this correction may be done automatically and simultaneously, time resolution of a few milliseconds being used to separate the two signals. Careful co-alignment of the two source beams is very important. To overcome this need, two other background correction systems have come into use over recent years, the Smith-Hieftje system and the Zeeman system. 

In the Smith-Hieftje system, the lamp power is subjected to short pulses of high current.17 This causes momentary bursts of high atom concentration in the hollow cathode. The emission line profile is broadened as an atom cloud forms just outside the cathode, which causes absorption at the centre of the emitted line profile, as shown in Figure 7. Effectively the single narrow emission line is split into a pair of lines immediately adjacent to the original line centre. Thus, in the normal mode, atomic and molecular absorption are measured, but in the pulsed mode, only molecular absorbance is monitored. The difference between the two signals provides a corrected atomic absorbance signal. 

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. 

The occurrence of molecular absorbance and scatter in AAS can be overcome by the use of background correction methods. Various types of correction procedures are common, e.g. continuum source, Smith-Hieftje and the Zeeman effect. In addition, other problems can occur and include those based on chemical, ionization, physical and spectral interferences. 

Background correction using a pulsed HCL (Smith-Hieftje method)... 

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.

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. 

Deuterium Zeeman Smith-Hieftje Tungsten iodide... 

The Smith-Hieftje background corrector has taken advantage of this self-reversal phenomenon by pulsing the lamp, alternating between high current and low current. At low current, a normal resonance line is emitted and the sample undergoes normal atomic absorption. When the HCL is pulsed to a high current, the center of the emission... 

---