Double-peak method

Lakshmanan et al. (1982a,b) observed that an increase in the X-ray photon energy from 29 to 100 keV gives a decrease by a factor of 4 in the sensitivity of the 250"C TL peak of CaF2 Tm, while the ratio of the 250 and 150"C TL peaks decreased only by 16%. This superiority of the double-peak method was found to be kept over a wide range of photon energies. 

A major limitation of CW double resonance methods is the sensitivity of the intensities of the transitions to the relative rates of spin relaxation processes. For that reason the peak intensities often convey little quantitative information about the numbers of spins involved and, in extreme cases, may be undetectable. This limitation can be especially severe for liquid samples where several relaxation pathways may have about the same rates. The situation is somewhat better in solids, especially at low temperatures, where some pathways are effectively frozen out. Fortunately, fewer limitations occur when pulsed radio and microwave fields are employed. In that case one can better adapt the excitation and detection timing to the rates of relaxation that are intrinsic to the sample.50 There are now several versions of pulsed ENDOR and other double resonance methods. Some of these methods also make it possible to separate in the time domain overlapping transitions that have different relaxation behavior, thereby improving the resolution of the spectrum. 

Example 2. This example was first suggested by Chang and Huang (29), and attemped later on by Hamielec and co-workers (l9) The problem is illustrated by Figure 3, which represents the following u(k), the uniform spreading function g(k), the broadened curve z(k), and the recuperated U2(k) by method 2 proposed in (l9). The solution shown in Figure 3 is practically coincident with that of (29), and with that of method 1 in (I9). Clearly, these techniques are unable to appropriately recover the double-peaked input. 

Normally one might expect that if the transition probability vanishes on resonance it also vanishes off resonance. However, such is not the case. When the transition probability is calculated off resonance, by numerically solving Eqs. (14.16) using a Taylor expansion method, it is nonzero for both v E and v 1E.14,16 In Fig. 14.6 we show the transition probabilities obtained using two different approximations for v E, and vlE for the 17s (0,0) collisional resonance.16 To allow direct comparison to the analytic form of Eq. (14.21) we show the transition probabilities calculated with EAA = VBB = 0. For these calculations the parameters ju2l = pLz, = 156.4 ea0, b = 104ao, and v = 1.6 x 10-4 au have been used. The resulting transition probability curves are shown by the broken lines of Fig. 14.6. As shown by Fig. 14.6 these curves are symmetric about the resonance position. The vlE curve of Fig. 14.6(b) has an approximately Lorentzian form, but the v E curve of Fig. 14.6(a), while it vanishes on resonance as predicted by Eq. (14.24), has an unusual double peaked structure. 

According to the above, the electrochemical response in the different differential pulse techniques can be very different, and it is worth analyzing the advantages and disadvantages of each method. Regarding the double pulse methods, in normal mode, DNDPV, this has the inconvenience of presenting asymmetrical peaks that can hinder the experimental determination of the peak current. In addition, the peak... 

Optimal resolution for planar methods are only obtained when the application spot size or width at the origin is as small or narrow as possible. As with any chromatographic procedure, sample and solvent overloading will decrease resolution. Studies show that in most instances automated sample application is preferred over manual application especially when applications are greater than 15 /d [28]. Inadequate manual application of a sample will cause diffusion and double peaking. Depending on the purpose of the analysis, various sample amounts are recommended [29] and listed in Table 3.3. The design of commercially available automatic spotters has been reviewed [30]. 

Because similar retention periods are used for some chlorophenols and polychlorinated gem-dichlorocyclohexadienones a problem of peak times was created and it was necessary to use a double detection method enabling differentiation of both products. In order to obtain the best possible sensitivity and specificity for the polychlorinated gem-dichlorocyclohexadienones, we opted for electrochemical detection based on reduction of gem-dichlorocyclohexadienones at an imposed potential of -0 volt (Fig. 6). 

The second step is to build a calibration curve using the analytical method established previously. Dilutions of the standard are prepared in pure MeOH or MeOH/water mixtures. The cahbration curve is obtained by plotting the peak surface area according to concentration. To minimize carryover, the standards are injected from the lowest to the highest concentration. In the presence of diastereoisomers, double peaks are sometimes recorded. In this case the sum of the two peaks is taken into account. 

The sensitivity of these methods is more than adequate to meet the required detection limits for isocyanates. Thus, the detection limits for hexa-methylene-l,6-diisocyanate (HDI) are quoted to be 0.5, 1, and 0.2pgm when using NMA, MNMA, and MAMA, respectively. However, these methods have some major drawbacks. The NMA method results in the appearance of double peaks in the chromatogram for isocyanates for reasons that are still imclear. Also, some NMA derivatives are difficult to dissolve in ordinary liquid chromatographic solvents. On the other hand, MNMA is relatively imstable, so stock solutions of this reagent must be stored refrigerated and prepared freshly every month. The same is the case with MAMA, solutions of which must be freshly prepared from its hydrochloride. 

Peak widths are also used, although much less often than heights, to construct calibration curves. This method can be especially useful when there are double peaks in order to improve resolution [1]. Thus, the width (W) of a peak at a given height is measured and plotted vs standard concentration (C), usually... 

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