Negative peaks

FIGURE 13 26 NMR spectra of 1 phenyl 1 pentanone (a) Normal spectrum (b) DEPT spec trum recorded using a pulse sequence in which CH3 and CH carbons appear as positive peaks CH2 carbons as negative peaks and carbons without any attached hydrogens are nulled... 

The curves in Figure 17 show that as the injection volume is increased, so the retention volume of the peak also increases. The retention volume of the small negative peak produced by the smallest charge will be the same as that for a sample... 

A more difficult criterion to meet with flow markers is that the polymer samples not contain interferents that coelute with or very near the flow marker and either affect its retention time or the ability of the analyst to reproducibly identify the retention time of the peak. Water is a ubiquitous problem in nonaqueous GPC and, when using a refractive index detector, it can cause a variable magnitude, negative area peak that may coelute with certain choices of totally permeated flow markers. This variable area negative peak may alter the apparent position of the flow marker when the flow rate has actually been invariant, thereby causing the user to falsely adjust data to compensate for the flow error. Similar problems can occur with the elution of positive peaks that are not exactly identical in elution to the totally permeated flow marker. Species that often contribute to these problems are residual monomer, reactants, surfactants, by-products, or buffers from the synthesis of the polymer. 

Putting together the information from all three spectra makes it possible to tell the number of hydrogens attached to each carbon. The CH carbons are identified in the DEPT-90 spectrum, the CH2 carbons are identified as the negative peaks in the DEPT-135 spectrum, the CH3 carbons are identified by subtracting the CH peaks from the positive peaks in the DEPT-135 spectrum, and quaternary carbons are identified by subtracting all peaks in the DEPT-135 spectrum from the peaks in the broadband-decoupled spectrum. 

Suppose you ran a DEPT-135 spectrum for each substance in Problem 13.33. Which carbon atoms in each molecule would show positive peaks and which would show negative peaks ... 

Figure 3. NMR-DEPT spectra of maitotoxin in CD3CN-D2O (1 1) A, methyls and methines appear as positive peaks and methylenes as negative peaks B, only methines appear C, no quarternary carbons appear and D, a conventional noise-decoupled spectrum.

The VCTTM recorded with the LM cell of Eq. (1) and voltammograms at the Wl/ LM and LM/W2 interfaces are realized as curves 1, 2, and 3, respectively, in Fig. 1. A positive peak and a negative peak exist in voltammograms 1 and 2. The positive peak, the final rise, and the final descent in curve 2 are attributable to the transfer of from Wl to LM facilitated by dibenzo-18-crown-6, TPhB from LM to Wl, and CV+ from LM to Wl, respectively. The negative peak is due to the transfer of that has moved into LM during the positive scan from LM to Wl. The final rise and the final descent in curve 3 correspond to the transfer of CV from LM to W2 and that of TPhB from LM to W2, respectively. 

Comparing curve 1 (VCTTM) with curves 2 and 3 (voltammograms at the Wl/LM and LM/W2 interfaces), it is obvious that (1) the potential window in curve 1 is about twice that in curve 2 or 3, (2) the potential regions where the positive and the negative peaks appear in curve 1 are different from those in curve 2, and (3) the slopes of the positive peak, negative peak, final rise, and final descent in curve 1 are much smaller than those in curves 2 and 3. 

There was no peak in curve 1 recorded with Wl and W2 containing only 0.1 M KCl. When 10 M K TPhB was added into Wl and/or W2, however, a positive peak and a negative peak symmetrical to each other about the origin were clearly observed as shown by curve 2 in Fig. 3. Here, it should be stressed that the peak currents are considerably larger than the background current (cf. curve 1) in spite of the fact that the concentration of K TPhB added into aqueous phases is as small as 10 M. In this regard, the ordinary... 

FIG. 21 The time dependence between positive and negative peaks of action potential induced by Colorado potato beetles on the distance between electrodes. The plants were given water every other day and kept at 20° C. A few electrodes were immersed in potato seed and different parts of the plant stem 2 h before measurements of the potential difference. (From Ref. 7.)... 

Simultaneously, there is the formation of a "negative peak" at about 310 nm in the early stages of exposure. A peak at about 340-360 nm also develops slowly during the exposure. The reasons for the initial decrease of the 285 nm peak and the formation of the negative peak at 310 nm are not completely clear. The phenomena are transitory but quite reproducable and real. An obvious explanation is that some chromophores in the exposed... 

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