Half lives of common

Half-Lives of Common Petroleum Hydrocarbons and Organics in Soil and Groundwater... 

U, 4.5 X 10 y). See Table 17.5 in the text fora list of half-lives of common radioactive isotopes. 

I. Mechanism of toxicity. Calcium antagonists slow the influx of calcium through cellular calcium channels. Currently marketed agents act primarily on vascular smooth muscle and the heart. They result in coronary and peripheral vasodilation, reduced cardiac contractility, slowed (AV) nodal conduction, and depressed sinus node activity. Lowering of blood pressure through a fall in peripheral vascular resistance may be offset by reflex tachycardia, although this reflex response may be blunted by depressant effects on contractility and sinus node activity. Table 11-17 summarizes usual doses, sites of activity, and half-lives of common calcium antagonists. 

The monomers used in the chain polymerization are unsaturated, sometimes referred to as vinyl monomers. In order to carry out such polymerization a small amount of initiator is required. These substances readily fragment into free radicals either when heated or when irradiated with electromagnetic radiation from around or just beyond the blue end of the spectrum. The half lives of common initiators are presented in Table 11.5. The three most commonly used free radical initiators for these reactions are ammonium persulfate (APS, water soluble), 2,2-azobisisobutyronitrile (AIBN, oil soluble), and benzoyl peroxide (BPO, oil soluble). In addition to the heat or light, free radicals can be produced by using y-rays. X-rays or through electrochemical means. In general, however, these methods do not tend to be so widely used. 

Half-lives of radioactive nuclides span an enormous range, from picoseconds ( Fr, 120 ps) to billions of years (238u 4.5 X 10 a or 4.5 Ga). See Table 18.5 in the text for a list of half-lives of common radioactive isotopes. 

If we consider this pair of radioactive isotopes for time scales greater than six half-lives of N2, Equation (3b) can be simplified. Because each decay series starts with a long-lived parent, it is commonly the case that A,2. In this case, after six half lives, e approaches zero and can be removed from the equation. For time scales such that 6T2 [Pg.6]

Decay of the nuclide itself. The conceptually simplest approach is to take a known quantity of the nuclide of interest, P, and repeatedly measure it over a sufficiently long period. The observed decrease in activity with time provides the half-life to an acceptable precision and it was this technique that was originally used to establish the concept of half-lives (Rutherford 1900). Most early attempts to assess half lives, such as that for " Th depicted on the front cover of this volume, followed this method (Rutherford and Soddy 1903). This approach may use measurement of either the activity of P, or the number of atoms of P, although the former is more commonly used. Care must be taken that the nuclide is sufficiently pure so that, for instance, no parent of P is admixed allowing continued production of P during the experiment. The technique is obviously limited to those nuclides with sufficiently short half-lives that decay can readily be measured in a realistic timeframe. In practice, the longest-lived isotopes which can be assessed in this way have half-lives of a few decades (e.g., °Pb Merritt et al. 1957). 

In these situations, addition of a tracer of unique isotopic composition is required, and the nature of the tracers added depends on the measurement technique. For example, short-lived and Th (with respective half-lives of 70 and 1.9 years) are commonly used as a tracer for alpha spectrometric analysis of U and Th, whereas longer-lived... 

Considerable care must be exercised in the selection of the rate constant for each constituent to avoid significantly over- or underpredicting actual decay rates. It is usually most effective to evaluate the results of several analytical events, several weeks apart. Half-lives for common petroleum hydrocarbons and organic compounds encountered in groundwater are presented in Table 13.1. 

Elimination rates are commonly expressed as half-lives, the time required for half the amount of a chemical to leave the body. Half-lives for rapidly eliminated chemicals are typically in the range of a few hours. Highly persistent chemicals (see below) have half-lives of years. 

During each consecutive half-life, the amount of material remaining is one half of the amount present at the start of the half-life (Figure 17.4). The half-lives of radioactive isotopes vary over a wide range, from a fraction of a second to over billions of years. Table 17.3 lists the half-lives of some common isotopes. 

Hydrolysis. Water, often in combination with light energy or heat, can break chemical bonds. Hydrolytic reactions commonly result in the insertion of an oxygen atom into the molecule with the commensurate loss of some component of the molecule. Ester bonds, such as those found in organophosphate pesticides (i.e., parathion Figure 26.1), are highly susceptible to hydrolysis which dramatically lowers the environmental half-lives of these chemicals. Hydrolytic rates of chemicals are influenced by the temperature and pH of the aqueous media. Rates of hydrolysis increase with increasing temperature and with extremes in pH. 

The elimination half-lives of a number of commonly used hypnotics are shown in Table 10.1. It should be noted that many of the drugs in current use have active metabolites which considerably prolong the duration of their pharmacological effect. This is particularly true for the elderly patient in whom the half-life of the hypnotic is prolonged due to decreased metabolism and renal clearance such individuals are also more sensitive to the sedative effects of any psychotropic medication. 

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