Extra peaks

Raw Measurement Plot In multivariate calibration, it is normally not necessary to plot the prediction data if the outlier detection technique has not flagged the sample as an outlier. However, with MLR, the outlier detection methods are not as robust as with the full-spectrum techniques (e.g., CLS, PLS, PCR) because few variables are considered. Figure 5.75 shows all of the prediction data with the variables used in the modeling noted by vertical lines. One sample appears to be unusual, with an extra peak centered at variable 140. The prediction of this sample might be acceptable because the peak is not located on the variables used for the models. However, it is still suspect because the new peak is not expected and can be an indication of other problems. 

An extra peak is observed at 3 eV for BrU-substituted oligonucleotides in the inset at the bottom of Fig. 22. Since this peak lies at an energy too low to involve electronically... 

Trimethylsilylation is adversely affected by moisture, and therefore, hydrolyzates should be evaporated to dryness as completely as possible. If trimethylsilylation is catalyzed by trifluoroacetic acid, instead of chlorotrimethylsilane, moderate proportions of water may be tolerated,117,127-129 but, even under these conditions, extra peaks may be obtained from partly trimethylsilylated derivatives.130 Catalysis with trifluoroacetic acid is useful when aqueous aliquots from a reaction are to be trimethylsilylated.131 A further advantage of this method, which has been used in the determination of 1,6-anhydro-jS-D-glucopyranose in corn syrup,132 for cycloamyloses,133 and for a series of malto-oligosaccharides,134 is that ammonium trifluoroacetate is soluble in pyridine. 

In the case of this multicomponent mixture, it was known which peaks belonged to which multicomponent. Therefore peak areas of known components could be easily compared to a reference and the average percent difference calculated. This was done for each sample with a simple program. This program was automatically initiated at the time a sample was chromatographed by the HP3354 LAS. An example of the output is shown in Table I. This sample was formulated to contain -100% (or 0%) of a multicomponent C and -50% (or 1/2) the normal concentration of a multicomponent A. From the list of missing peaks and the fraction of individual peaks, this was found to be true. The extra peaks which are listed can be used to pinpoint possible sources of contamination. 

The CrO ion has axial symmetry with S=In Fig. 13 the first derivative of the absorption curve is shown for a powder of KNbOg containing a small amount of chromium. The value of is obtained from the location of the small peak and g from the location of the large peak. The small extra peak near the large peak is due to the hyperfine interaction of Cr53 which is 9.5 per cent abundant. 

Fig. 18. 75.4-MHz 13C Bloch decay MAS spectra showing the dynamics of the toluenium ion. The cation was synthesized by reacting bromomethane-13C with benzene-13Q on AlBr3 at 233 K. The spectrum at 213 K shows all the peaks for the toluenium ion at 32 (methyl), 50 (C-4), 178 (C-3), 139 (C-2), and 201 ppm (C-l). The peak at 129 ppm was the unreacted benzene-13C6. At 243 K, the peaks were much sharper, and the 138 and 50 ppm peaks were NMR invisible. At 273 K, the spectrum shows two extra peaks at 128 and 73 ppm. All these spectral features are rationalized by the chemical exchange between the para and ortho isomers.

Figure 11 shows the differences between synthetic hematite and one from burning an ochre and between a synthetic goethite and a typical yellow ochre (in which the iron is chiefly goethite in character). Note the extra peak or peaks in the center of the hematite spectrum in the natural material. 

In precolumn derivatization, the derivatization process alters the chemical nature of the analytes. Therefore, it may be necessary to develop new chromatographic methods. However, the separation can be optimized for the particular analytes, and any excess reagent can be removed so that it does not interfere with detection. The selection of precolumn derivatization reagents is therefore less restricted than is the choice of postcolumn derivatization reagents, and rapid kinetics are not particularly important. The stability of the derivative is important, however, as is the percent derivatization, which should be as near to 100% as possible. It is also important that the reaction yield only one derivative per analyte, so that coelutions of extra peaks does not occur, and so solute identification and quantitation are accurate. 

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