Sloping background

In order to ameliorate the sharply sloping background obtained in an STS spectrum, the data are often presented as di,/dFh vs. Vb, i.e. the data are either numerically differentiated after collection or Vb has a small modulation applied on top of the ramp, and the differential di,/d Vb is measured directly as a function of Vb. The ripples due to the presence of LDOS are now manifest as clear peaks in the differential plot. dt,/dFb vs. Vb curves are often referred to as conductance plots and directly reflect the spatial distribution of the surface electronic states they may be used to identify the energy of a state and its associated width. If V is the bias potential at which the onset of a ripple in the ijV plot occurs, or the onset of the corresponding peak in the dt/dF plot, then the energy of the localised surface state is e0 x F. Some caution must be exercised in interpreting the differential plots, however, since... 

Spectroscopic interferences can also manifest themselves, either as an increase in the continuum background emission or as line overlap, especially if samples with a complex matrix, or organic solvents, are analysed. An increase in the continuum background emission can be easily compensated for by subtraction of the background adjacent to the analytical line. For a sloping background then measurements must be made on both sides of the line and usually the mean value is subtracted. These options are summarized in Fig. 4.18. Line overlap is a particular problem when an element, present in large excess in the matrix, has an emission line close to. 

Fig. 13.4 Diabatic field ionization signals for i changing under increasing incident beam intensities. For clarity the successive curves are displaced upward by one scale unit. Data points are taken from the transient digitizer records, and a small sloping background has been subtracted. Full curves are fits to the model of MacAdam et al. (from ref. 8).

Figure 3-6 Raman spectra of the intramolecular H—H stretching vibration (vibron) in solid hydrogen at 158GPa and 77 (B) and 295 K (A). A sloping background signal has been subtracted from the high-temperature spectrum. The estimated random errors in pressure and temperature are 1 GPa and 2K (low temperature). (Reproduced with permission from Ref. 16.)...

The same original spectrum with a sloping background added is shown in Fig. 5-3. The second derivative with a 15-point smooth was calculated and is shown in the figure, where it is compared with the second derivative with a 15-point smooth of the original spectrum. The two derivatives are identical. This demonstrates the importance of derivatives in matching an unknown with a library spectrum. Frequently, real unknown samples can be plagued with... 

Figure 5-5 Left Four Raman spectra of the same polypeptide at slightly different concentrations but with large sloping background differences. Right Same spectra after applying the SNV transform to all the spectra.

It should be emphasized that any baseline alterations to correct sloping backgrounds should be avoided, especially by one new to the field. Important information may be lost in the flattening process, and important spectral bands may be distorted. 

A common feature of many spectra was the sloped background, despite of the fact that low excitation energy (1.57 eV) was used for measurements. It comes from the high photoluminescence yield of the mbrene [10], In particular, the... 

Ideally, the intensity (/o) of infrared radiation incident upon a sample cell is reduced (to f) by the absorption of the samples. Actually, some of the incident energy is scattered by the sample and this scattered energy makes the Beer-Lambert law inaccurate, especially at high values of absorbance [4]. The baseline method for quantitative analysis is an empirical method used to establish a calibration curve of log (/q//) versus concentration. Infrared absorption bands may overlap neighboring bands or may appear on a sloping background, so transmittance is measured in practice as shown in Figure 8.13. The absorbance. A, is determined from measurements of / and 7o, then a calibration curve of absorbance versus concentration is plotted. 

Figure 8.13. Measurement of lo and I for an infrared absorption band with a sloping background. 

Figure 7.38 An aluminum background spectrum representing a sloping background shift at the Cd 241.438 nm line. This asymmetric background cannot be corrected using the one-point correction shown in Fig. 7.37, because the background intensity is different of each side of the emission line. [From Boss and Fredeen, courtesy of PerkinElmer Inc. (www.perkinelmer.com).]...

Figure 14.24 The direct Auger spectrum of silver (bottom), the direct spectrum with a tenfold enhancement of the signal (middle), and the derivative or differential spectrum (top). The derivative spectrum significantly improves the ability to measure a small signal against a high and sloping background signal. (From Weber.)...

The second adjustment to coal spectra that concerns us, since we believe it may lead to spectral artifacts, is the correction of the baseline for scattering. In his seminal paper on the infrared spectra of coal, Brown ( ) discussed the origin of the sloping background and concluded that scattering was not the only source. Microscopic examination of various KBr pellets... 

Figure 3 A frequency modulation spectrum of the 4z2<-423 transition of HDO near 143 727 MHz, phase coherently detected at twice the modulation frequency and obtained with a synchronously tuned Fabry-Perot cavity cell. This species was present in natural abundance (270 ppm) in a sample of water vapor. The sloping background is due to the nonuniform coupling of the cavity, and may be eliminated by computer analysis. (From Alder JF and Baker JG (2002) Quantitative Millimetre Wavelength Spectroscopy. Cambridge Royal Society of Chemistry reproduced by permission of the Royal Society of Chemistry.)...

Figure 7.38 An aluminum background spectrum representing a sloping background shift at the Cd 241.438 nm line.

Figure 7.39 The sloping background shown in Figure 7.38 requires the use of a two-point background correction, with one correction point on each side of the Cd emission line. The peak is corrected using a straight line fit between the background correction points as shown by the dotted line (the new baseline). [ 1993-2014 PerkinEimer, inc. All rights reserved. Printed with permission.

Temperature dependent dielectrics of Nanocrystalline BaTiOa Ceramics. The dielectric properties with different frequencies for nanocrystalline BaTiOs samples are shown in Figure 6. In the temperature (T) spectra of relative dielectric constant (K) all the samples showed a discernible eubie (C) to T transition. In eomparison, the low temperature transitions were much weaker and broader. At smaller grain size 20 nm, they appeared as diffuse humps on a positively sloping background that is part of the low-temperature tail of the C/T transition. In the range of 20-100 nm, the transition temperatures thus determined show a steady decrease for the C/T transition, in contrast to a steady increase for the T/0 transition and for the 0/R transition. However, even the maximum value of K for 20 nm BaTiOs is only about 930 at IkHz, it still exhibits a broadening and... 

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