Background over-subtraction

A correction can also be made with a hydrogen or deuterium continuum source with the monochromator set at the same wavelength as the resonance line. Sharp line-absorption of the continuum source by the test element is negligible compared to the background over the bandwidth of the monochromator, so the absorbance of the continuum source can simply be subtracted from the absorbance of the resonance line. Several commercial instruments have arrangements that allow automatic background corrections with continuum sources. 

Figure 5.24(B) shows a line profile extracted from the map of Figure 5.24(A) by averaging over 30 pixels parallel to the boundary direction corresponding to an actual distance of about 20 nm. The analytical resolution was 4 nm, and the error bars (95% confidence) were calculated from the total Cu X-ray peak intensities (after background subtraction) associated with each data point in the profile (the error associated with A1 counting statistics was assumed to be negligible). It is clear that these mapping parameters are not suitable for measurement of large numbers of boundaries, since typically only one boundary can be included in the field of view.

Equilibrium of headspace volatile lipids is influenced by a number of factors including the solubility of the component in the food matrix and the vapor pressure over the food system. This relationship may be very complex and must be determined experimentally by spiking the sample with known quantities of the compound of interest and subtracting background levels (Reineccius, 1993). 

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