Two line method

Background absorption can be compensated for or minimized by using sample-like standards or matrix modification, or by moving to an interference-free line if possible. However, the actual background correction methods are (i) Two line method (ii) Continuum source method (iii) Smith-Hieftje method (iv) Methods using the Zeeman effect. 

In the two line method the total absorbance (atomic absorption and background absorption) is measured at the resonance line of the analyte emitted by the hollow cathode lamp and the non-specific absorbance (background absorption) is measured at a nearby line where the analyte atoms do not absorb. Alternatively, another hollow cathode lamp of an element not present in the sample can be used. The line for the background absorbance measurement must be close to the analyte line. One of the problems is to find a convenient nearby line. The maximum permitted distance apart is 10 nm for most of the elements. To ensure there is accurate correction, the background must be constant within the range of these two lines. Another factor which must be considered is that for successive 

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Other methods, based upon two lines, may be used. Two-line methods may be used in absorption, emission or fluorescence. The signal is... 

Because with the two-line method using lines of the same ionization level for the determination of temperatures, it is difficult to fulfil all conditions necessary to obtain highly accurate values [see Eqs. (38) and (39)], a method was developed that enables the plasma temperature to be determined from intensities of lines belonging to different ionization levels. When /, is the intensity of an ion line and I the intensity of an atom line (in general both lines have to belong to two adjacent... 

A two-line method of emulsion calibration has been described by Churchill it makes use of a pair of spectral lines whose intensity ratio is known. The spectral line pair selected must possess certain characteristics, as follows ... 

Relative Spectral Line Intensities of an Iron Line Pair to Use for EmulsionCalibra-tion by the Two-Line Method"... 

FIGURE 8-6. Preliminary curve for the two-line method of spectral plate calibration. 

A variation of the two-line method of emulsion calibration is to use a neutral step filter with an accurately known intensity ratio. The percentage transmittances of the light and dense portions of a series of spectral lines covering a range of transmittances from very light to very dense is obtained. From these data a preliminary curve is obtained from which the calibration data can be determined in the same manner as in the two-line method. The lines selected for the two-step method should lie in the same spectral region as the analysis line. 

The two-line method has been used successfully for nickel in oil, using the resonance spectral line at 2320 A with the nonabsorbing line at 2316 A. Lead also can conveniently be determined by this method. The method cannot be used if the two spectral lines are too far separated in wavelength, since background absorption varies with wavelength. The two-line method also requires two separate measurements taken at separate times or it requires two-channel instrumentation. The method also has limited application because of the difficulty of suitable fixed line pairs for the measurement. 

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