Background self reversal

Background Correction Based on Source Self Reversal... 

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... 

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. 

It is assumed that only the background absorption is measured at high lamp current (which is an only partially valid assumption), however, the broadening of the line profile is limited and self-reversal is not complete. Thus, radiation is still emitted at the centre of the emission line and is absorbed by the analyte, and will subsequently be subtracted from the gross absorption of the analyte. This significantly reduces the sensitivity of the determination, with an average loss of sensitivity of ca. 45 % being observed for the elements most commonly determined by... 

Fig. 12.19 Background correction by source self-reversal, (a) Modulation of the lamp current that drives the hollow cathode lamp (b) resulting hollow cathode lamp emission line profiles generated at high and low current operation.

This technique of background correction makes use of the fact that resonant atomic spectral lines emitted by a hollow cathode lamp may display self-reversal when the lamp is operated at high discharge current. A hollow cathode lamp operates during the first part of the measurement... 

Similar to Zeeman AAS, the Smith-Hieftje technique can be used for lines over the entire spectral range, and uses only one primary radiation source, so that alignment and stability are optimum. The technique is simple and much cheaper than Zeeman AAS. It does not suffer from limitations due to Zeeman splitting of molecular bands. However, as self-reversal is not complete, the technique can be used only for rather low background absorbance, sensitivity is decreased as the... 

In trace analyses, the slope of the analytical evaluation curve (in logarithmic form) is usually I at higher concentrations ij may be > 1 as a result of self-reversal. In trace analysis by AES, the intensity of the spectral background /u can be used as reference signal and cr should be replaced by Cu the concentration at which the line and background intensities are equal (background equivalent concentration or BEC value) ... 

Oppeemann U., Scheam J. and Felkel D. (2003) Improved background compensation in atomic absorption spectrometry using the high-speed self-reversal method, Spec-trochim. Acta, Part B 58 1567-1572. 

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