Threshold, concentration, for

The limit of tolerable error is generally smallest in a minor component assay. It will have been determined that a particular minor component must be at or below a threshold concentration for the product to be usable. Therefore, the decision to accept or reject an entire production batch may depend on the analytical result. Typical batches may contain the contaminant at a concentration very similar to the specification limit. In a minor component assay, the major component may be overloaded and out of the proper range of detection of the assay. Even so, the minor component may be at such low levels that assay noise interferes. 

Female gametes of marine brown algae release and/or attract their conspecific males by chemical signals. The majority of these compounds are unsaturated, nonfunctionalized acyclic, and/or alicyclic Cn hydrocarbons. Threshold concentrations for release and attraction are generally... 

Calcium phosphate precipitation may also be involved in the fixation of phosphate fertilizer in soils. Studies of the uptake of phosphate on calcium carbonate surfaces at low phosphate concentrations typical of those in soils, reveal that the threshold concentration for the precipitation of the calcium phosphate phases from solution is considerably increased in the pH range 8.5 -9.0 (3). It was concluded that the presence of carbonate ion from the calcite inhibits the nucleation of calcium phosphate phases under these conditions. A recent study of the seeded crystal growth of calcite from metastable supersaturated solutions of calcium carbonate, has shown that the presence of orthophosphate ion at a concentration as low as 10-6 mol L" and a pH of 8.5 has a remarkable inhibiting influence on the rate of crystallization (4). A seeded growth study of the influence of carbonate on hydroxyapatite crystallization has also shown an appreciable inhibiting influence of carbonate ion.(5). 

Investigations of pure cultures of bacteria clearly show the existence of a threshold concentration for the carbon source below which replication does not occur. This value is about 18 pg/1 for Escherichia coli and Pseudomonas sp. growing on glucose, 180 pg/1 for Aeromonas hydrophila growing on starch,... 

FIGURE 3 20 Threshold concentration for condensation nuclei formation from various hydrocarbons. Reprinted with permission from Lipelese/ a/. 

Threshold concentrations for Ct+ alkenes are in the parts-per-hundred-million to parts-per-million range. Determination of their gas-phase concentration is required for assessing their contribution to atmospheric organic aerosol. 

Nothing is known about the threshold concentration for aromatic hydrocarbons. No vapor-pressure data on the polyfunctional products are available. However, a possible threshold concentration of several parts per million seems indicated by data of Kopczynski (mesitylene aerosol at 25 ppm, no aerosol at lower concentration), O Brien et al. (mesitylene aerosol at 10 ppm, no aerosol at 2 ppm), Schwartz... 

FIGURE 3-21 Threshold concentration for aerosol formation with 1,6-heptadiene. Reprinted with permission from O Brien et td. 

A graphic expression was developed for pinto bean and Bel tobacco exposure to ozone by Heck and Dunning. Later work with a number of plants permitted the development of a simplistic model derived as an empirical relationship between ozone concentration, time, and response this gave a reasonable interpretation of acute response up through a single 8-h exposure. It also permitted the development of a reasonable acute threshold concentration for a number of species. The equation was a variant of the O Gara equation for sulfur dioxide and is shown as... 

Use exenatide with caution in patients receiving oral medications that require rapid Gl absorption. For oral medications that are dependent on threshold concentrations for efficacy, such as contraceptives and antibiotics, advise patients to take those... 

It is also important to understand that most allelopathic effects apparently result from the combined actions of several allelochemicals, often with each below a threshold concentration for impact. In allelopathic situations which implicate phenolic acids, soil concentrations have ranged from below 10 to above 1000 ppm for each compound. The lower end of the spectrum is below a concentration required for an effect in current bioassays. Additive and synergistic effects have been demonstrated, however, for combinations of cinnamic acids (102), benzoic acids (103), benzoic and cinnamic acids (10 ). and -hydroxybenzaldehyde with coumarin (105). It appears that such combined interactions may be very important under field conditions. 

After allelochemicals have been identified in the substrate, concentrations should be calculated, and threshold concentrations for activity should be determined against test plants using combinations of compounds present in the substrate, in addition to individual ones. Undoubtedly, many important allelopathic effects have been overlooked because of the use of single allelochemicals in determining threshold concentrations for activity. 

In irrigation agriculture this threshold concentration is used as a reference to adjust the concentration of irrigation water so that it exceeds the threshold concentration for the sodicity of the soil and so prevents decreases in permeability. The threshold concentration increases with the degree of sodicity [see Quirk, 2001]. 

At about one-quarter of the threshold concentration,for a given sodicity, dispersed particles appear in the percolate, indicating the start of the dismantling of clay domains. It is noteworthy that this concentration is almost ten times lower, or even more if natural dispersants are present [e.g. organic compounds], than that obtained for the flocculation of a suspension of the soil.This reflects the fact that it is harder to release the crystals from within the clay domains, than to simply flocculate the free crystals. 

Organic acids may exhibit other sensory properties. For example, citric acid possesses sweet-and-sour sensory notes, and succinic acid has a salty-bitter taste. On the other hand, the typical taste and flavor of Emmental cheese can be ascribed to the propionic acid and a few other compounds, such as proline. In fact, taste and flavor result from the combination of different food constituents in definite proportions. Raw meat smells much like lactic acid, which arises from postmortem anaerobic glucolysis and determines the pH of meat, its final properties, and microbial stability. This same organic acid has been related to the inhibition of certain pathogenic bacteria in yogurt (3). Table 1 lists the reported threshold concentrations for various organic acids in different media (4-6). 

Biomass obtained from the wastewater treatment plant of Middle East Technical University (METU) was used as seed in the experiments. In the acclimation of MOs to HMs, maximum concentrations of Cu2+ and Zn2+ reached during acclimation were decided from previous work [17,21], The literature showed that when these metals were present individually in the system, ASP failed to operate at steady state values of 15 mg/L for Cu2+ and 90 mg/L for Zn2+, respectively [5], Therefore, one-third of threshold concentrations of these metals (that is, 5 mg/L for Cu2+, and 30 mg/L for Zn2+) were used to constitute the combined threshold concentration for mixtures of copper and zinc ions. Acclimation experiments were performed by gradually increasing the concentrations of Cu2+ and Zn2+ to two, three and four times their combined threshold concentration. The optimum condition for a successful acclimation providing the growth of healthy MOs, as observed from the increase in turbidity values, was obtained at the concentrations corresponding to twice their combined threshold concentration [21],... 

The Community-Level Aquatic Systems Studies Interpretation Studies (CLASSIC) guidance document, which deals with the interpretation of results of microcosm and mesocosm tests in the risk assessment procedure of pesticides, recommends that regulatory model ecosystem experiments be conducted in spring to midsummer (Giddings et al. 2002). On the basis of the limited number of model ecosystem experiments described above, it seems that threshold concentrations for effects observed in early-season studies are reasonably predictive for threshold concentrations later in the season. Above these threshold concentrations, however, the intensity and duration of the responses (direct and indirect effects) may vary during different periods of the year. Consequently, the extrapolation of NOECcommunity values from one season to another seems to be possible with lower uncertainty than hazard estimates of higher concentrations in which both direct and indirect effects are involved. 

For most pesticides evaluated, an uncertainty factor of 10 to 15 seems to suffice to extrapolate a median acute HC5 to a median chronic HC5, at least when based on toxicity data of sensitive taxonomic groups. In addition, it appears from model ecosystem experiments with pesticides that threshold concentrations for chronic exposures are approximately a factor of 10 lower than those for acute exposure. For a wider generalization, however, more data are required on compounds that differ in toxic mode of action (Section 6.2.4). 

When evaluating the influence of the time of year on responses of aquatic communities to chemical stress, it is convenient to make a distinction in threshold concentrations of direct toxic effects, and in the magnitude of effects that occur above these threshold concentrations. Only a limited number of (model) ecosystem experiments are available that allow a comparison of responses due to treatment with the same chemical in the same type of test system at different periods of the year. These studies indicate that, in freshwater communities, threshold concentrations for direct toxic effects may vary little with the season (within a factor of 2). At higher exposure concentrations, however, the intensity and duration of the responses (direct and indirect effects) may vary considerably between different periods of the year (Section 6.3.2). 

It appears from several model ecosystem experiments with insecticides, where exposure concentrations are similar, that threshold concentrations for effects may be very similar between different types of test systems, at least when they contain representatives of sensitive taxonomic groups (in this case, arthropod populations). For... 

Threshold concentrations for direct toxic effects of the most sensitive endpoints (responses of populations of crustaceans and insects) in model ecosystem experiments that studied the ecological impact of short term exposure to the insecticide chlorpyrifos... 

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