plane polarized with the field. During that part of the cycle when the source radiation is polarized similarly,absorption of radiation by the analyte takes place. During the other half cycle, no analyte absorption can occur. Broad-band molecular absorption and scattering by the matrix products occur during both half cycles, which leads to the cyclical absorbance pattern shown in F The data acquisition system is programmed to subtract the absorbance during the perpendicular half cycle from that for the parallel half cycle, thus giving a background corrected value. A second type of Zeeman effect instrument has been designed in which a magnet surrounds the hollow cathode source. Here, it is the emission spectrum of the source that is split rather than the absorption spectrum of the sample. This instrument configuration provides an analogous correction. To date, most instruments are of the type illustrated in Figure 9-15. Zeeman effect instruments provide a more accurate correction for background than the methods. ~escribed earlier. These instruments are particularly useful for electrothermal atomizers and permit the direct determination of elements in samples such as urine and blood. The decomposition of organic material in these samples
leads to large backgrQund corrections (background A 2: 1) and consequent shsceptibility to significant error
Background Correction Based on Source Self-Reversal
A remarkably simple means of background correction is now being marketed and appears to offer most of the advantages of a Zeeman effect instrument. 15 This method, which is sometimes called the Smith-Hieftje background correction method, is based upon the selfreversal or self-absorption behavior of radiation emitted from hollow cathode lamps when they are operated at high currents. As was mentioned earlier, high currents
produce large concentrations of nonexcited atoms,which are capable of absorbing the radiation produced from the excited species. An additional effect of the high currents is to significantly broaden the emission band of the excited species. The net effect is to produce a band that has a minimum in its center, which corresponds exactly in wavelength to that of the absorption peak (see Figure 9-16).