True threshold

In moderate ketonemia, the loss of ketone bodies via the urine is only a few percent of the total ketone body production and utilization. Since there are renal threshold-like effects (there is not a true threshold) that vary between species and individuals, measurement of the... 

The ZEKE-PFI practitioner relies on the existence of a narrow band of long-lived, high-n Rydberg states ( ZEKE states )15 lying just below each true cation threshold. If the shift of this band of states relative to the true threshold is essentially constant, then spectroscopic information extracted from differences in band frequencies will faithfully reflect the true cation energy levels. This assumption seems to hold to an accuracy of 1 cm-1 or better in the many molecules studied, as judged... 

The kinetic shifts for the immonium a, ion formation from (Gly)H+ (0.4), (GlyNH2)H+ (0.7), (GlyNHCH3)H+ (10), and (Gly-Gly)H+ (21), where the values in brackets give the kinetic shift in kcal/mol (see Table 12), illustrate the very rapid increase of the kinetic shift with increasing size of the precursor ion. For (Gly-Gly)H+ the kinetic shift is close to 50% of the true threshold, E0 = 44 kcal/mol. Obviously unless an accurate evaluation of the kinetic shift is possible, reliable threshold values cannot be obtained with precursor ions of this size. 

The threshold wavelength for the appearance of F+ in (iii) has an energy equal to the ionization energy plus the enthalpy of fluorine. However, because some F+ is generated in (ii) via an excited F2 state, one needs to subtract this quantity of F+ (measured by observing the F-ion) in order to obtain the true threshold at 652.5 A [equivalent to D0(F2) of 157.6 1.0 kJ],... 

Along with other groups, we have given thought to the extrapolation of the rate-energy data from the TRPD technique to determine the true threshold for dissociation. Some of these considerations are discussed in Section VI. Here, we proceed directly to a review and discussion of the results obtained by this technique for specific ionic systems. 

Methylnaphthalene ion presented an interesting case where the dissociation energy was essentially unknown, because a kinetic shift of the order of 2 eV completely masks the true threshold in threshold appearance measurements. TRPD at two wavelengths, with RRKM extrapolation, assigned an Eg of 2.25 eV for the loss of H from the molecular ion, shown in Equation (7). We found that this energy... 

In the methods considered in this chapter, the dissociation rate is typically measured for some known internal energies well above the true threshold. An essential consideration then is how to extrapolate from the observed rates back to the true threshold. This is not a trivial question and has received quite a lot of attention. Three approaches used in recent work are worth description and commentary. 

The most satisfactory situation for making an extrapolation of rate data to the true threshold arises when the threshold is uncertain, but we can confidently calculate the functional form of the rate-energy curve from accurate kinetic theory. For small systems, it is feasible to calculate dissociation rates by quantum methods, but this is not yet feasible for the systems of interest to us. Various approaches to variational transition-state theory (VTST) provide classical or semiclassical calculations that are feasible for large systems and seem to be accurate when carefully... 

These assertions have not been confirmed, and the true thresholds for the above processes are unknown. 

Recognizable difference from control sample not a true threshold concentration mg/L, mg/kg = ppm to convert from liter to kg assume density of liquids % 1.0 g/ml... 

Lutz WK. 2000. A true threshold dose in chemical carcinogenesis cannot be defined for a population, irrespective of the mode of action. Hum. Exp. Toxicol. 19 566-68... 

In addition to the problem of knowing whether a true threshold exists in the dose-response curve, and just where it is, there are at least two other quandries ... 

What can be said about the protective value of the ADI There is no way to be sure how close it is to the true threshold, by which is meant the threshold dose for the most sensitive members of the human population. If the generic safety factors are accurate, or are larger than they need to be, then the ADI should be adequately protective, or even more protective than it needs to be. A recent review of the available... 

It is important to distinguish between factors leading to linearity as dose approaches zero (low-dose linearity) and considerations of Unearity or nonlinearity of the overall dose-response curve. Thus, while consideration of the kinetics of fundamental biological processes (e.g., absorption, DNA reactivity, DNA repair) indicates that these processes tend to be linear as the dose approaches zero, that linear slope may be very different from the slope derived by extrapolation from the animal tumor data. Several authors have also reported nonlinearities in DNA mutation, even for direct-acting mutagens, although issues of assay sensitivity make it difficult to distinguish nonlinearity from a true threshold. 

The Kehe results of a cantilever betim test can depend on how much time elapses before the test is terminated. Recommended test times for establishing the true threshold... 

Chloride stress corrosion cracking is a constant concern when stainless steels are used at elevated temperatures. There is no true threshold of chloride concentration or temperature below which stress corrosion will not occur, but experience has provided some guidelines where stainless steel can be used with conQdence. An example of chlorides in cooling water was discussed eeurlier (Fig. 1). Equipment designed to reduce opportunities for chlorides to concentrate is helpful. The presence of deposits also provides an opportunity for chloride concentration. 

As mentioned previously, the ability to successfully convert VF is a function of dose-response, i.e. the higher the energy the more likely the rhythm will be converted. As such there is no true threshold. However programming a shock energy at least 10 J greater than that from two or more successful defibrillation conversions tends to predict future success. 

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