Two line background correction

Two-line background correction The two-line correction method, which was proposed in the late 1970s [22], is based on measuring the absorption at a second, non-resonant line. This line should be close to the resonance Hne of the element that is measured but should not be absorbed by the analyte. If these conditions are met sufficiently well, it can be assumed that the attenuation at this second line is only due to the background absorption in the sample. 

Two-line background correction, which is employed with conventional line sources, uses a second wavelength that is emitted by the light source (or... 

Figure 7.39 The sloping background shown in Fig. 7.38 requires the use of a two-point background correction, with one correction point on each side of the Cd emission hne. The peak is corrected using a straight line fit between the background correction points as shown hy the dotted hne (the new basehne). [From Boss and Fredeen, courtesy of PerkinEhner Inc. (www.perkinelmer.com).]...

Figure C2.10.1. Potential dependence of the scattering intensity of tire (1,0) reflection measured in situ from Ag (100)/0.05 M NaBr after a background correction (dots). The solid line represents tire fit of tire experimental data witli a two dimensional Ising model witli a critical exponent of 1/8. Model stmctures derived from tire experiments are depicted in tire insets for potentials below (left) and above (right) tire critical potential (from [15]).

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. 

Zeeman background correction also depends upon line splitting, but in this instance most commonly the absorption line profile (the n component) is split into two or more components (the a components) by the application of an intense... 

Figure 13.13 Scheme of a AA spectrometer showing deuterium lamp background correction. This double beam assembly includes a deuterium lamp whose broad emission is superimposed, using a semi-transparent mirror, upon the spectral lines emitted by the HCL. Beam path a passes through the flame while beam path b is a reference path. The instrument measures the ratio of the intensities transmitted by the two beams and for the two sources. The domain of correction is limited to the spectral range of the deuterium lamp, being 200-350 nm (reproduced from the optical scheme of model Spectra AA-10/20, Varian). 

When the hollow cathode lamp is selected, the total absorption (background and absorption due to the spectral line of the element) is measured. Absorbances being additive, the difference between the two measurements will yield the absorption due to the background corrected sample, whatever the intensities of the two lamps. 

The measurement should be made at least two bandpasses (Chapter 16) away from the absorption line. The line used for correction can be a filler gas line from the hollow-cathode lamp or a nonresonance line of the element that is not absorbed, or a nearby line from a second hollow-cathode lamp can be used. A solution of the test element should always be aspirated to check that it does not absorb the background correction line. This technique requires two separate measurements on the sample. 

Before analyses were carried out by the two procedures, the optimum HVAA parameters were established empirically using 20-ng/ml aqueous standards. HVAA measurements for chromium were made with a Varian Techtron CRA-63 atomizer other tube furnace atomizers which possibly could have been used, were not investigated. Although the HVAA response was linear between 0-400 ng/ml, a working range of 0-50 ng/ml was utilized. The detection limit (S/N = 2) was calculated to be 1 pg. The absorbance of the neon 359.4-nm line in the chromium hollow cathode lamp was used to make background corrections. 

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