Background offset correction

Fig. 7 The key steps in the analysis of Ln(iii)-induced RE (see Section 3.2). (A) Longitudinal relaxation time traces measured at 80 K for nitroxide radicals in the presence of paramagnetic Dy(iii) and diamagnetic La(iii) centres. The traces are inverted and offset corrected by multiexponential fitting (see text). (B) The result of division of the t wo traces in (A), dotted line indicates the fit of the intermolecular RE background. (C) Intramolecular RE trace obtained from (B) after background correction (see text). Multiexponential fit to obtain the 1/e decay time is plotted as a solid line. (D) The inverted 1/e decay time from (C) is plotted as a point on the AkiD dependence.

Figure 11.13 shows a typical DO AS spectrum measured in air after correcting for atmospheric background light and an electronic offset (Stutz and Platt, 1997). Below the spectrum are shown reference spectra for the gases that contribute to the atmospheric spectrum, scaled by the a, factors determined using Eq. (H). In this case, O, N02, SOz, and HCHO all contribute, leaving a residual spectrum with a peak-to-peak absorbance of 6 X 10 4. 

FIGURE 12.17 Time series of corrected DOC and of POC in the surface layer of station 271 (same data as in Fig. 12.13). DOC was corrected for background concentration using the relation shown in Fig. 12.16. To avoid negative DOC values, an offset of 60 pM C (corresponding to threefold standard deviation of DOC) was added to each value. 

Background measurements of blanks should always be subtracted when making careful diffusion coefficient measurements. Such corrections can often minimize charging current instrumental offsets, and other background contributions to the measured current. Care must be taken, however, that the condition of the electrode does not change between measurements of the blank and the redox species. 

The simplest form of baseline error is an offset. For example, the sample may attenuate the spectrometer beam by the same amount at all wavelengths and consequently raise the absorbance of the sample with respect to the background. As all baseline corrections should be done with the spectrum linear in absorbance, so that the offset manifests itself as a baseline error above or below zero absorbance. To correct this offset, a constant (typically, the minimum value in the absorbance spectrum) is subtracted from all spectral data points. This results in a removal of the offset so that the baseUne is reset to zero absorbance. Peak absorbance may then be read directly. This procedure is often known as one-point baseline correction. 

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