True secondary emission

As also pointed out previously in connection with TSC, care must be taken with current transient measurements to distinguish between true thermal emission processes and effects such as leakage currents, thermoelectric currents, and secondary photocurrent (in the case of laser excitation) all of which may also produce current transients. Such effects may often be identified by comparing a series of DLTS spectra obtained using a wide range of measurement parameters. 

Despite the measurement of the emitted radiation by these means it is still possible for scattered or reflected incident radiation to reach the detector. To prevent this, fluorimeters require a second monochromating system between the sample and the detector. Many simple fluorimeters use filters as both primary and secondary monochromators but those instruments that use true optical monochromators for both components are known as spectrofluo-rimeters. Other instruments incorporate a simple cut-off filter system for the emitted radiation while retaining the optical monochromator for the excitation radiation. Because the wavelengths of both excitation and emission are characteristic of the molecule, it is debatable which monochromator is the most important in the design of a fluorimeter. 

Emissions versus Deposition. Several workers have compared estimates of atmospheric emissions with measured deposition fluxes for PCDD/Fs a selection are summarized in Table 8. Whilst the database on which such comparisons are founded is limited, the tentative conclusions which may be derived are of potentially great significance. In particular, it has been observed that most such comparisons, including several of those referred to in Table 8, indicate that current deposition exceeds atmospheric emissions from primary sources.44 Whilst there are significant sources of potential error involved in such calculations (e.g. in addition to those mentioned in earlier sections, like the failure to consider releases from secondary sources, there is the possibility that estimates of representative depositional inputs may overestimate the true figure, owing to the fact that measurements of rural depositional inputs are extremely scarce), the obvious conclusion is that we have yet to discover all sources of the current depositional inputs of PCDD/Fs. 

Activated chemiluminescence is observed from these secondary peroxy-esters as well. When the thermolysis of peroxyacetate [281 in benzene solution is carried out in the presence of a small amount of an easily oxidized substance the course of the reaction is changed. For example, addition of N,N-dimethyldihydrodibenzol[ac]phenazine (DMAC) to peroxyester [28] in benzene accelerates the rate of reaction and causes the generation of a modest yield of singlet excited DMAC. This is evidenced by the chemiluminescence emission spectrum which is identical to the fluorescence spectrum of DMAC obtained under similar conditions. Spectroscopic measurements indicate that the DMAC is not consumed in its reaction with peroxyester 28 even when the peroxyester is present in thirty-fold excess. The products of the reaction in the presence of DMAC remain acetophenone and acetic acid. These observations indicate that DMAC is a true catalyst for the reaction of peroxyacetate 28. The results of these experiments with DMAC, plotted according to (27) give k2 = 9.73 x 10-2 M-1 s-1. 

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