Frequency, working optimum

The role of specific interactions in the plasticization of PVC has been proposed from work on specific interactions of esters in solvents (eg, hydrogenated chlorocarbons) (13), work on blends of polyesters with PVC (14—19), and work on plasticized PVC itself (20—23). Modes of iateraction between the carbonyl functionaHty of the plasticizer ester or polyester were proposed, mostly on the basis of results from Fourier transform infrared spectroscopy (ftir). Shifts in the absorption frequency of the carbonyl group of the plasticizer ester to lower wave number, indicative of a reduction in polarity (ie, some iateraction between this functionaHty and the polymer) have been reported (20—22). Work performed with dibutyl phthalate (22) suggests an optimum concentration at which such iateractions are maximized. Spectral shifts are in the range 3—8 cm . Similar shifts have also been reported in blends of PVC with polyesters (14—20), again showing a concentration dependence of the shift to lower wave number of the ester carbonyl absorption frequency. 

There is no experimentally established optimum frequency for the distribution of samples. The minimum frequency is about four rounds per year. Tests that are less frequent than this are probably ineffective in reinforcing the need for maintaining quality standards or for following up marginally poor performance. A frequency of one round per month for any particular type of analysis is the maximum that is likely to be effective. Postal circulation of samples and results would usually impose a minimum of two weeks for a round to be completed and it is possible that over-frequent rounds have the effect of discouraging some laboratories from conducting their own routine quality control. The cost of proficiency testing schemes in terms of analysts time, cost of materials and interruptions to other work has also to be considered. 

In one of the most common types of photodiodes used for time-resolved work, the p-i-n photodiode (see Figure 12.24), the depletion layer thickness (i for intrinsic) is fabricated to obtain this optimum performance. Manufacturers usually give full specification sheets detailing, active area, time/frequency response, responsivity amps/watt (AAV) at a given wavelength, dark current, depletion layer capacitance, and bias volts such that with minimal external electronics devices can be made operative. 

In order to actually cover 19 decades in frequency, dielectric spectroscopy makes use of different measurement techniques each working at its optimum in a particular frequency range. The techniques most commonly applied include time-domain spectroscopy, frequency response analysis, coaxial reflection and transmission methods, and at the highest frequencies quasi-optical and Fourier transform infrared spectroscopy (cf. Fig. 2). A detailed review of these techniques can be found in Kremer and Schonhals [37] and in Lunkenheimer [45], so that in the present context only a few aspects will be summarized. 

Another factor that may be beneficial is physical activity, since it affects the immune function and antioxidant defenses, transit time of digestion, hormones, and body fat, and it improves energy balance. Therefore, it may have a protective effect on prostate cancer and it may even slow progression and metastasis (G14, H8, K7, 02, 03). In a 9-year follow-up study performed by Hartman et al., the relative risk for physical exercise in prostate cancer was compared with sedentary workers and found to be 0.6 (Cl = 0.4-1.0), 0.8 (Cl = 0.5-1.3), and 1.2 (Cl = 0.7-2.0) for occupational workers, walker/lifters, and heavy laborers, respectively. Except for heavy laborers, an inverse association was observed (RR = 0.7, Cl = 0.5-0.9) compared to men who were sedentary at work and leisure (H8). However, other studies indicate a positive association between vigorous exercise and prostate cancer Cl), and therefore further study is necessary to provide an activity optimum. Frequency, duration, intensity, type of exercise, and the period during a man s lifetime when exercise may be beneficial, must be investigated (02, 03). 

Most of the studies of methane oxidation in the literature utilize simplified feedstreams and fresh catalyst samples, generally in the form of powders or pellets. A recent paper [22] used a laboratory simulated NG vehicle exhaust to study the removal of methane, NO and CO using a Pd-only monolith catalyst. They found that optimum conversion of all three constituents occurred slightly rich of stoichiometry. These results appear to have been obtained over fresh catalyst samples. The present work utilizes monolith catalysts and laboratory simulated NG vehicle exhaust to study the effect of catalyst loading and space velocity, Ce02 addition and variations in hydrocarbon composition. The effect of modulation amplitude and frequency aroxmd the stoichiometric point was also... 

The optimum frequency at which to work is going to be dictated by other considerations as well, availability of a suitable source and detector, signal to noise considerations, working temperature and pressure, and the other species to be determined. Bringing cost into the equation does focus the mind on the expense of both sources and detectors and unless there were a particular operational need the spectral region 40-200 GHz would be a likely compromise for most applications (see Section 6.2). 

The peak absorption coefficient of OCS, 10 m", occurs at 462 GHz. This is by no means, however, the optimum working frequency due to the non-ideal behaviour of most MMW detectors. Commercial Schottky barrier mixer diode detectors show a quadratic roll-off in sensitivity at frequencies >100 GHz. If this is factored into Equation 6.1, the peak sample sensitivity occurs around 300 GHz, and the response is so flat that even at 100 GHz it has only fallen off by a factor of two. What is common to both curves is the dramatic fall-off in sample sensitivity at frequencies <100 GHz, reinforcing the point that the band 26-40 GHz is ill suited to high-sensitivity analytical spectroscopy. 

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